Diferent Metod Determination CO2

8/3/2019 Diferent Metod Determination CO2

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2 .1 . D i s s o l v e d C O 2 c o n c e n t r a t i o n

T h e c o n c e n t r a t i o n o f C O 2 i n s o l u t i o n ([C O 2]aq ) is n o r m a l l y e x p r e s s e d b y H e n r y ' s

law [41:

[ C O 2 ]a q = K H P C O 2 ( 3 )

w h e r e K H = H e n r y ' s la w c o n s t a n t ( m o l . a t m -1 ) a n d p C O 2 = t h e p a r t ia l p re s su r e o f

C O 2 i n t h e g a s p h a s e ( a t m ) . F o r c u l t u r e s g r o w n u n d e r a t m o s p h e r i c p r e s s u r e , t h e

p r o p o r t i o n a l i t y o f s o l u b il i ty a n d p a r t i a l p re s s u re s ( H e n r y ' s l aw ) m a y b e a s s u m e d w i t h -

o u t i n t r o d u c i n g a p p r e c i a b l e e r r o r s [ 5 ] .

A t a t e m p e r a t u r e o f 3 7 ° C , K H = 1 0 -1"61 [4 ] w h e r e t h e [ C O z ] is e x p r e s s e d i n m o l a r

t e r m s . T h u s , t o o b t a i n [C O 2] i n m i l l i m o l a r t e r m s , K H = 1 01 39 . H e n c e :

[ C O 2 ] a q = 1 01 "39 x p C O 2 = 24 . 6 x p C O 2 .

I n o t h e r w o r d s, w h e n p C O 2 = l a t m , th e c o n c e n t r a t i o n o f d is s ol ve d C O 2 = 2 4 . 6

m m o l . 1 - l .

2 .2 . B i c a r b o n a t e c o n c e n t r a t i o n in a p H - c o n t r o l l e d c u l t u r e w i t h a c o n s t a n t

t e m p e r a t u r e a n d g a s p h a s e

T h e e q u i l i b r iu m b e t w e e n C O 2 a n d H C O ~ is d e f in e d b y a ' h y b r i d ' e q u i l i b r iu m c o n -

s t an t K 1 [4] w h ere :

I 0 - p H " [ H C O ~ ]K 1 - (4)

[CO2]

F r o m E q n . 4 , i t fo l lo w s th a t :

l o g [H C O ] ] = p H - p K 1 ' + l og [C O z] • ( 5 )

p K 1 is r el a te d t o t h e t h e r m o d y n a m i c p K o f t h e r e a c t io n p K 1° a n d t h e i o n i c s t re n g t h

I by:

p K ~ ' = P K 1 ° - 0 . 5 f ( / ) - b I . (6 )

F r o m D a v i e s ' s E q n . 4 :

f (1) = [11/2 / (1 + 11/2) - 0.211 [2 98 / (T + 273)] 2 /3 (7)

w h e r e T is t h e t e m p e r a t u r e i n ° C . I is t h e i o n i c st r e n g t h o f t h e m e d i u m a n d is g i v e n

by [4, 6] :

l = V2 ECiZi2 ( 8 )

w h e r e c i = t h e c o n c e n t r a t i o n o f i o n i a n d z i = t h e c h a r g e o n i o n i .

T o o b t a i n t h e a p p a r e n t P K a , l f o r t h e C O z / H C O ~ e q u i l i b r i u m , w e u s e [ 4 ] :



157

p K a , 1 = p K a , 1 - f ( I ) - bI

T h e r e f o r e , f r o m E q n . 9 , w e o b t a i n PKa,1, a n d s o f r o m E q n . 5:

l o g [H C O ~ ] = p H -pKa, 1+ o g [ C O 2 ] .

(9 )

I t m a y b e n o t e d t h a t t h e s e e q u a t i o n s a s s u m e t h a t t h e r e i s a n e q u i l i b r i u m b e t w e e n

t h e p C O 2 i n t h e g a s p h a s e a n d t h a t i n s o l u t io n . C l e ar ly , th i s i m p l ie s t h a t t h e r a t e o f

e x c h a n g e o f C O 2 b e t w e e n t h e s e t w o p h a s e s i s ra p i d . I t is , t h e r e f o r e , p e r t i n e n t t o d e t e r -

m i n e t h e e x t e n t t o w h i c h t h i s i s s o .

3 . Rates o f Exchan ge o f CO2 be tween Fermentor Broths and Gas Phas e

F o l l o w i n g a s t e p i n c r e a s e in t h e p C O 2 i n t h e g a s p h a s e , t h e r a t e o f g a s a b s o r p t i o n(R s ) • vo1-1 o f l i qu i d i s g i ven [7 ] by :

dc kLa (cs_C) (10)R s = d - t = h

w h e r e c s = g a s c o n c e n t r a t i o n a t t h e in t e r fa c e , c = c o n c e n t r a t i o n o f t h e g a s in t h e b u l k

l i q u id , a = i n t e r fa c i a l a r e a , k L = a c o n s t a n t d e p e n d e n t u p o n t h e d i f f u s i o n c o e f f i c i e n t

o f t h e g a s a n d h = t h e t h i c k n e s s o f t h e s t a t i o n a r y f i l m .

I n g e n e r a l , b e c a u s e t h e v a l u e o f h i s n o t k n o w n , i t is u s u a l t o c o m b i n e i t w i t h k L

and w r i t e [ 6 , 7 ] :

R s =Kea(Cs -c ). (11)

T h e a b s o r p t i o n r a t e c a n b e e x p r e s s ed i n t e r m s o f p a r t i a p re ss u re s> fi n g a s a n d l i q u id

(p g a n d P l , r e s p e c t i v e l y ) b y m a k i n g t h e s u b s t i t u t i o n s c s = Hpg a n d c = Hpl ( w h e r e H =

H e n r y ' s l a w c o n s t a n t ) i n t o E q n . 1 1 :

Rs = KL aH(pg - Pl) . (12)

T h e m a x i m u m a b s o r p t i o n r a te f o r a g iv e n p a r t ia l p r e ss u r e o c cu r s w h e n c = 0 a n d ,

h e n c e , HPl--0. T h e n :

Rs,max = KLaC = KLaHpg. (13)

T h e r a te o f a b s o r p t i o n o f C O 2 is g iv e n b y th e s lo p e o f p l o ts o f ' C O z ' c o n c e n t r a t i o n

v s . t i m e . F r o m E q n . 1 3 , i t c a n b e s e e n t h a t :

KLa = Rs/Hpg.

A t 3 7 ° C , H = 10 -1'61 w he n [ 'C O 2 ' ] is exp res sed i n M [4] an d H = 101.39 w he n [ 'C O 2 ' ]

is e x p r e s s e d i n m M . H e n c e :

KLa= Rs/IO 1.39 pg.



158

For a detailed review of the diffusi on, deso rpti on an d solubi lity of CO2, in addi-

tio n to some of the reactions in volving CO2, see also Ho et al. [6]. The rate of diffu-

sion of CO2 thro ugh a silicone me mb ra ne may also be used to measu re pCO2 [7a].

Having outl ined CO2-absorption rates and equil ibria, we will now discuss variousmethods by which CO2 may be estimated (Table 1).

4 . T r a d i ti o n a l M e t h o d s f o r E s t i m a t i o n o f C O 2

The detection of CO2 in cultures includes both quali tat ive and quanti tat ive

methods. Prod uct ion of CO 2 from various carbohydrate substrata is a useful diagnos-

tic tool in the ide ntif icat ion of bacteria. Several met hods for detecting CO2 produc-

tion have been used, e.g., Du rh am tubes inserted into broth cultures [8], displa ceme nt

of agar plugs [8, 9] and the appea ranc e of cracks in agar me di um [10]. The Eldredge

tube [11], which is a device for trapping CO2 in barium hydroxide, can be used forboth the qualitative (observation of a precipitate) and quantitative (recovery of the

TABLE 1

METHODS USED IN DETERMINATION OF CO2. METHODS ARE LISTED IN THE ORDER THAT

THEY APPEAR IN TEXT

Method Principle Selected references

Titrimetric A standard alkaline solution is neutralised 20

after absorbing CO2Colourimetric Modification of colour of an indicator 21, 22

solutionGravimetric Isolation and weighing of a compound 12, 20

of CO2 (e.g., barium carbonate)Volumetric Decrease in volume of a gas sample after 27

(Orsat) chemical removal of CO 2Volumetric Increase headspace volume above an 31(Warburg) acidified sampleManometric Increase in pressure of headspace above 31

an acidified sampleKatharometer Differences in thermal conductivities 27

of gasesMass Differences in mass to charge ratios 35spectroscopy of ionsIR Absorption of IR radiation 27Fibre optic Changes in light transmittance of pH 48probes indicatorChromatography Differences in column retention times 36

of gasesPotentiometric Changes in pH induced by CO 2 50Conductiometric Changes in water conductivity caused 70, 71

by CO2Amperometric Flow of electric current due to 75

reduction of CO2Piezoelectric Change in resonant frequency due to 103

adsorption of CO2



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precipitate) detection of C O 2 [12]. However, these procedures depend upon the evolu-

tion of gaseous CO 2 from the growth medium which does not always occur due to

the high solubility of CO2 in water. The insertion of a red-hot loop into cultures

resulted in a copious evolution of gas [13, 14] and this method of CO2 detection isknown as the hot-loop method. Displacement of agar plugs is used in the hot-tube

method which is more sensitive than the hot-loop method [15]. In the hot-tube method,

an agar plug is displaced by the evolution of gaseous CO2 when the culture is heated

to 80°C.

In the estimation of CO2 by the Conway microdiffusion method [16, 17], CO 2 is

liberated by acidifying the sample. There have been a number o f designs of the appara-

tus for the microdiffus ion method [16-18] but they are all based upon the standard

cell illustrated in Fig.1. CO 2, liberated from the sample in the outer compartment, is

allowed to diffuse to the central compartment where it is absorbed by a standard bari-

um hydroxide solution [KOH or NaOH may be used instead of Ba(OH)2] containingthymolphthalein indicator. The contents of the inner compartment are then titrated

with a standard hydrochloric acid until the thymolphthalein ndicator is just colourless

(pH ~-. 9.3) at which point the excess of barium hydroxide is neutralised [16, 17]. CO2

in a liquid sample may be estimated directly. Phenolphthalein is added to the sample

which is then titrated with a standard sodium hydroxide solution. Free CO2 reacts

with the sodium hydroxide to form sodium bicarbonate. Completion of the reaction

is indicated by the development of the pink colour characteristic of phenolphthalein

at the equivalence point of pH 8.3 [19]. Titrimetric methods may also be used for the

determination of CO2 in gases. The CO 2 in the gas sample is absorbed into a standard

barium hydroxide solution which, after the addition o f phenolphthale in indicator, istitrated with standard hydrochloric acid [20].

Colourimetric methods, for the estimation of the CO 2 concentration in a gas, do

not require titration o f the CO2-absorbing solution. The colour intensity of a solution

of phenolphthalein's sodium salt decreases as the concentration of CO2 increases [21,

22]. Similarly, the concentrat ion of CO 2 may be estimated by the modif ication of the

colour of alizarin yellow R in the presence of NaOH [22]. CO2-detection tubes are

also based on a colour change caused by a chemical reaction between the gas and tube

/

C P EDISH

F i g . 1 . C o n w a y ' s s t a n d a r d m i c r o d i f f u s i o n ce l l. T h e c e l l c o n s i s t s o f a P e t r i d i s h in t h e c e n t r e o f w h i c h i sf u s e d a p ie c e o f g l a ss t u b i n g t o f o r m a n i n n e r c e ll o r c o m p a r t m e n t . T h e e d g e o f t h e o u t e r d i s h i s g r o u n d

s o t h a t a l id , i n t h e f o r m o f a s q u a r e o f f l at g r o u n d g l a ss , m a y e n c l o s e c o m p l e t e l y a n y g a s e s w i t h i n t h e

d i s h . A m i x t u r e o f v a s e l i n e a n d p a r a f f i n i s s m e a r e d o n t o t h e li d to a s s i s t t h e s e a l.



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c o n t e n t s [ 2 3 ] . W h e n a g a s c o n t a i n i n g C O 2 i s b r o u g h t i n t o e q u i l i b r i u m w i t h s o d i u m

b i c a r b o n a t e s o l u t io n , t h e p H is a m e a s u r e o f t h e c o n c e n t r a t io n o f C O 2 [2 4]. T h e p H

m a y t h e n b e d e t e r m i n e d b y t h e u s e o f i n d i c a t o r s [2 5] o r w i t h a p H e l e c t r o d e ( se e a l s o

S e c t i o n 9.1.1.). W h e n c o m b i n e d w i t h a s p e c t r o p h o t o m e t e r th e p H , a n d h e n c e p C O 2,o f a s o d i u m b i c a r b o n a t e s o l u ti o n , c o n t a i n i n g i n d i c a to r a n d s p a rg e d w i t h a

C O 2 - c o n t a i n i n g g a s , m a y b e e s t i m a t e d o n a c o n t i n u o u s b a s i s [ 2 5 ] .

O t h e r t r a d i ti o n a l m e t h o d s o f m e a s u r i n g C O 2 i n vo lv e t h e p a s s ag e o f a m e a s u r e d

s t r e a m o f g a s t h r o u g h a b s o r b a n t s f o l lo w e d b y g r a v i m e t r ic [ e.g ., 1 2] o r v o l u m e t r i c m e a s -

u r e m e n t s [2 6, 27 ]. S u c h a m e t h o d w a s e m p l o y e d in th e O r s a t a p p a r a t u s i n w h i c h t h e

c h a n g e i n v o lu m e , a t c o n s t a n t p r e s s u re , o f a g a s e o u s s a m p l e , f o l lo w i n g t h e r e m o v a l

o f C O 2 b y it s a b s o r p t i o n i n t o K O H , w a s m e a s u r e d a n d c o r r e s p o n d e d t o t h e pCO 2 [26,

27] .

T o t a l 'C O 2 ' m a y b e m e a s u r e d b y f ir s t s a t u r a t i n g a s a m p l e w i t h C O 2 , t h e n a c i d i f y -

i n g t h e s a m p l e a n d m e a s u r i n g e i t h e r t h e c h a n g e i n t h e v o l u m e o f t h e h e a d s p a c e a ta c o n s t a n t p r e s s u re o r t h e c h a n g e i n th e p r e s s u r e o f t h e h e a d s p a c e a t a c o n s t a n t v o l u m e

[2 6, 2 8 - 3 2 ]. T h e l a t t e r m e t h o d d o e s n o t r e q u i r e g a s m e t e r s f o r t h e a c c u r a t e m e a s u r e -

m e n t o f v o l u m e , a n d a n e x a m p l e o f a n a p p a r a t u s f o r su c h a m e t h o d is sh o w n i n F ig .

2 . I n t e r f e r e n c e f r o m v o l a t i l e a c id s , w h i c h b e c o m e g a s e o u s i n a c id i c c o n d i t i o n s , i s a

p o s s ib i li ty ; h o w e ve r , i n t e r f e r e n c e d u e t o o r g a n i c o r i n o r g a n i c c o m p o u n d s c o m m o n l y

f o u n d i n f e r m e n t a t i o n l i q u o r s a r e e i th e r n e g l ig i b le o r e a s i l y e li m i n a t e d [3 1]. A l t h o u g h

s u c h a m e t h o d r e qu i re s o n l y v e ry s im p l e i n s t r u m e n t a t i o n a n d m a y e a si ly b e a u t o m a t e d ,

i t is n o t a b l e t o p r o v i d e g e n u i n e l y c o n t i n u o u s m o n i t o r i n g o f t h e t o t a l ' C O 2 ' .

E n z y m a t i c m e t h o d s f o r t h e e s t im a t i o n o f v a ri o u s c o m p o u n d s a re s p ec if ic f o r t h e

c o m p o u n d ( s ) i n v o lv e d . O n e s u c h m e t h o d e x is ts f o r t h e e s t i m a t i o n o f ' C O 2 ' a n d i sc o m m e r c i a ll y av a il a be ( e.g ., f r o m t h e S i g m a C h e m i c a l C o m p a n y ) . T h e m e t h o d is b a se d

u p o n t h e r e a c t io n c a t a ly s e d b y p h o s p h o e n o l p y r u v a t e c a rb o x y l a s e w h i c h p r o d u c e s o x -

a l o a c e t at e f r o m p h o s p h o e n o l p y r u v a t e a n d ' C O 2 ' [3 3]. M a l a te d e h y d r o g e n a s e t h e n

SAMPLE I

T U B E

M A N O M E T E R

ACID INLETTUBE

GAS OUTLETTUBE

o

o o

o

° o

JACI~TTI~RMOSTATrED ~ ~ ~ _ PUMP

SAMPLE I ' ~ -~C O2 INLET TUBE

DISCHARGETUBEFig. 2. Designof an apparatus for manom etricdeterminationof CO 2. CO 2 is estimated by measuring the

increase in p ressure of the headspa ce followingacidification of samp le [31].



16 1

c o n ve rt s N A D H a n d t h e s o - f o r m e d o x a lo a c e ta t e t o m a l a t e a n d N A D + . W h e n t h e a b -

s o r b a n c e is r e a d , a t e i th e r 3 4 0 o r 3 8 0 n m , t h e d e c r e a s e i n a b s o r b a n c e u p o n t h e a d d i t i o n

o f P E P is p r o p o r t i o n a l t o t h e o r ig i n a l ' C O i l c o n t e n t o f t h e sa m p l e [3 3].

W i t h t h e e x c e p t i o n o f t h e c o l o u r i m e t r i c m e t h o d , t h e ' t ra d i t i o n a l ' m e t h o d s , d i s -c u s s e d a b o v e , c a n o n l y b e u s e d f o r s p o t c h e c k s a n d a r e t h e r e f o r e n o t s u i t a b l e f o r c o n -

t i n u o u s a n a l y si s . M e t h o d s e m p l o y e d i n t h e g a s a n a l y s is o f c l i n ic a l s a m p l e s ( e. g. , b l o o d )

a r e a l s o u n s u i t a b l e f o r c o n t i n u o u s u s e, si n c e t h e y a re p e r f o r m e d o n s a m p l e s t a k e n f r o m

a pa t ien t [ e .g . , 32 , 34] .

5 . C O 2A n a ly s is yKatharometer

T h e p r i n c ip l e o f CO 2 a n a l y s is b y k a t h a r o m e t e r [2 7] is a s d e s c r i b e d b e lo w . T h e b e -

h a v i o u r o f t h e k a t h a r o m e t e r r e f le c ts th e t h e r m a l c o n d u c t i v i t y o f th e g a s ( m i x t u r e)

w h i c h is p a s s i n g t h r o u g h i t. W i t h i n t h e k a t h a r o m e t e r a r e f o u r s m a l l ce ll s, e a c h c o n t a i n -i n g a g l a s s - c o a t e d P t w i r e i d e n t i c a l w i t h t h e o t h e r s . T h e s e f o u r w i r e s f o r m t h e a r m s

o f a W h e a t s t o n e b r i d g e ( F ig . 3 ) . T w o o f t h e c e ll s (B , D ) a r e e x p o s e d t o a r e f e r e n c e g a s

a n d t h e o t h e r t w o ( A , C ) a re e x p o s e d t o t h e s a m p l e g a s. W h e n t h e b r i d g e c u r r e n t is

c o n s t a n t a n d a ll f o u r c e ll s a r e e x p o s e d t o t h e s a m e g a s , e a c h w i r e w il l a t t a i n t h e s a m e

t e m p e r a t u r e a n d r e si s ta n c e . U n d e r t h e s e c o n d i t i o n s , t h e b r i d g e is b a l a n c e d a n d n o c u r-

r e n t f l o w s t h r o u g h t h e g a l v a n o m e t e r .

I f tw o g a s e s o f d i f f e r e n t t h e r m a l c o n d u c t i v i t i e s , s u c h a s a i r a n d a n a i r / C O 2 m i x -

t u r e, a r e i n t r o d u c e d i n t o t h e c e ll s ( a i r i n t o t h e r e f e r e n c e c e il s a n d t h e m i x t u r e i n t o t h e

s a m p l e c e l ls ), t h e n W i r e s B a n d D w i ll l o s e m o r e h e a t t h a n w i ll W i r es A a n d C . T h i s

is b e c a u s e t h e t h e r m a l c o n d u c t i v i t y o f a i r is g r e a t er t h a n t h a t o f C O 2 . T h e c o n s e q u e n tc h a n g e i n t h e c o n d u c t i v i t i e s o f t h e w i r e s w i ll u n b a l a n c e t h e b r i d g e a n d c a u s e a d e f l e c -

t i o n o f t h e g a l v a n o m e t e r , w i t h t h e s iz e o f t h e d e f l e c t i o n be i n g d e p e n d e n t u p o n t h e

d i f f e r e n c e i n t h e t h e r m a l c o n d u c t i v i t y , a n d i n th i s i n s t a n c e p C O 2 , o f t h e t w o g a s e s.

H o w e v e r , t h e m e a s u r e m e n t o f g a s e s b y k a t h a r o m e t e r s is n o t s p e ci fi c , e. g. , f o r C O 2.

T o m e a s u r e t h e p C O 2 o f t h e e f f l u e n t g a s f r o m a f e r m e n t o r t h e c o n s t i t u e n t g a se s

- - -q l I ' 4A,-

A

I ) B ~ _ _

C

F i g . 3 . A s i m p l i f i e d r e p r e s e n t a t i o n o f a k a t h a r o m e t e r . S a m p l e a n d r e f e r e n c e ce l ls e a c h c o n t a i n a P t w i r e

( A - D ) , e a c h o f w h i c h is i d e n ti c a l, f o r m i n g t h e a r m s o f a W h e a t s t o n e b r id g e . T h e p r i n c i p l e o f C O 2 m e a s -

u r e m e n t u s i n g a k a t h a r o m e t e r i s a s d es c r i b e d i n t ex t.



162

w o u l d f i r st h a v e t o b e s e p a r a t e d , f o r e x a m p l e, b y g a s c h r o m a t o g r a p h y a s d e s c r ib e d i n

S e c t i o n 8 .

6 . M a s s S p e c t r o m e t r y

T h e b a s ic p r i n c i p l e o f m a s s s p e c t r o m e t r y is t h e s e p a r a t i o n a n d r e g i s tr a t io n o f io n i c

m a s s e s [ 35 ]. T h e s a m p l e i s i o n i se d , e .g ., b y e l e c t r o n b o m b a r d m e n t , a n d t h e i o n s , w i th a

m a s s - t o - c h a r g e r a t i o o f m/z, a r e a c c e l e r a t e d b y a n e l e c t r i c f i e l d t h r o u g h a s e r i e s o f

s li ts t o f o r m a n i o n b e a m . T h i s i o n b e a m is t h e n d e f l e c t e d b y a m a g n e t i c f i el d , H , a c -

c o r d i n g t o m/z = ( H 2 r 2 ) / 2 V w h e r e H i s t h e s t r e n g t h o f t h e m a g n e t i c f i e l d , r t h e c u r v a -

t u r e o f t h e p a t h w a y ( i n s t r u m e n t a l l y d i c ta t e d ) a n d V t h e v o l t a g e u s e d t o a c c e l e ra t e t h e

i o n s [ 35 ]. B y c o n t i n u o u s s c a n n i n g o f e i t h e r t h e m a g n e t i c f i el d o r t h e a c c e l e r a ti n g v o l t-

a g e , i o n s o f d i f f e r e n t m/z v a l u e s c a n b e f o c u s s e d o n t h e d e t e c t o r a n d s u b s e q u e n t l y

m e a s u r e d . I n a d d i t i o n t o t h e id e n t i f ic a t i o n o f c o m p o u n d s , t h e m a s s s p e c t r o m e t e r c a nb e a d a p t e d f o r s e l e c t e d i o n m o n i t o r i n g t o p e r m i t h i g h l y s e l e c t i v e d e t e r m i n a t i o n s o f

v e r y h i g h s e n s i ti v it y [ 36 ]. H o w e v e r , m a s s s p e c t r o m e t r y c a n n o t a lw a y s b e u s e d t o e x a m -

i ne a n u m b e r o f s u b s t an c e s s i m u l t a n e o u s l y w i t h o u t a d d i t i o n a l s e p a r a t io n t e c h n i q u e s

s u c h a s g a s c h r o m a t o g r a p h y ( se e S e c t i o n 1 .5 ). T h e r e a s o n f o r th i s is t h a t m a s s s p e c t r a

t e n d t o b e c o m p l i c a t e d a n d e x a m i n a t i o n o f s ev e ra l c o m p o u n d s s i m u l ta n e o u s l y c a n l e ad

t o s i g n i f ic a n t o v e r la p , s e r io u s l y c o m p r o m i s i n g t h e q u a n t i t a t i o n o f t h e i n d i v i d u a l s pe -

c ie s [ 37 ]. D e s p i t e th i s p r o b l e m , g a s e s p r o d u c e d b y f e r m e n t a t i o n p r o c e s s e s c a n , i n f a c t ,

b e e s t i m a t e d s i m u l t a n e o u s l y b y m a s s s p e c t r o m e t r y [ 3 8 - 4 0 ] , b y e x p l o it i n g t h e f a c t t h a t

d i f f e r e n t g a s es h a v e d i f f e r e n t i s o t o p i c c o m p o s i t i o n s . C O 2 is m e a s u r e d a t m/z=44.

T h e o n l y p o s si b le i n t e r fe r e n c e is f r o m N 2 0 ( f o r m e d b y m a n y d e n i t ri fi e rs ) a n d f r o mv o l a t i l e f a t t y a c i d s ( w h i c h m a y b r e a k d o w n t o C O 2 w i t h i n t h e m a s s s p e c t r o m e t e r

[39]) .

T h e t y p e o f m a s s s p e c t r o m e t e r d e s c r ib e d a b o v e is th e m a g n e t i c m a s s s p e c t r o m e t e r

w h i c h i s p e r h a p s m o r e o f t e n u s e d f o r th e a n a l y s is o f f e r m e n t o r e x h a u s t g as e s w h i l st

q u a d r u p o l e m a s s s p e c t r o m e t e r s a r e u s e d f o r t h e e s t i m a t i o n o f d i s s o l v e d g a s e s [ 3 9 ] .

T h e s e n o n m a g n e t i c m a s s s p e c t r o m e t e r s e m p l o y a s a m a s s f il te r f o u r e l e c tr i c a ll y c o n -

d u c t i n g p a r a l le l r o d s p r e c i se l y l o c a t e d i n a c e r a m i c h o l d e r. T h e o p p o s i t e r o d s a r e c o n -

n e c t e d e l e c t ri c a ll y a n d a r a d i o - f r e q u e n c y ( R F ) v o l ta g e s u p e r i m p o s e d o n a DC v o l t a g e

is a p p l i e d t o t h e t w o p a i rs o f r o d s , 1 80 ° o u t o f p h a s e w i t h e a c h o t h e r [3 5, 3 8 - 4 0 ] .

I o n s e n t e r i n g a t o n e e n d o f t h is a r r a y a r e c o n s t r a i n e d t o m a k e o s c i ll a ti o n s i n t h e t ra n s -ve r se e l ec t r i c f i e l d . A t a g i ven RF/DC v o l t a g e , o n l y i o n s o f a s p e c i f i c m/z v a l u e w i ll

a v o i d c o l l is i o n w i t h o n e o f t h e r o d s a n d w i ll , t h u s , p a s s t h r o u g h t h e f il te r . S c a n n i n g

t h e RF a n d DC v o l t a g e s m a y b e p e r f o r m e d r a p i d l y w i t h t h e i o n s b e i n g t r a n s m i t t e d

s e q u e n t i a l l y i n o r d e r o f t h e i r m/z r a t i o s w i t h c o n s t a n t r e s o l u t i o n [ 3 5 , 3 8 - 4 0 ] .

S a m p l i n g d i s s o l v e d g as e s is a c h i e v e d b y a m e m b r a n e i n l e t t o t h e m a s s s p e c t r o m e t e r

[ 3 9 - 4 1]. A p e r f o r a t e d c a p i l l a r y o r s h e e t, o r p o r o u s m a t e r i a l , i s c o v e r e d w i t h a p l a s t ic

m e m b r a n e a n d i m m e r s e d i n t o t h e f e r m e n t a t i o n m e d i u m . A v a c u u m i s a p p l i e d t o

t h e i n si d e o f t h e p r o b e a n d t h e d i s s o l v e d g a s e s d i f f u s e t h r o u g h t h e m e m b r a n e . S i n c e

t h e d i f f u s i o n c o e f fi c ie n t is d e p e n d e n t u p o n b o t h t h e g as a n d t h e m e m b r a n e u s ed , t h e

m e m b r a n e i n le t is se le c ti ve . A n o t h e r m e a n s b y w h i c h t h e d i s s o lv e d g a s es i n f e r m e n t a -t i o n m e d i a c o u l d b e s a m p l e d i s b y a c a r ri e r g a s tu b i n g m e t h o d [3 , 4 2 ]. T h i s m e t h o d

i n v ol v es p a s s i n g a c a r r i e r g a s t h r o u g h a t u b e , o r a c r o s s a m e m b r a n e , m a d e o f a g a s -



16 3

p o r o u s m a t e r i a l , im m e r s e d in t h e f e r m e n t a t i o n m e d i u m . T h e e x h a u s t g a s f r o m t h e t u b -

i n g i s t h e n a n a l y s e d .

U n f o r t u n a t e l y , m e t h o d s s u c h a s m a s s s p e c t r o m e t r y a n d i n f r a r e d a n a l y s e s ( se e t h e

f o l l o w i n g s e c t i o n ) a r e n o r m a l l y p r o h i b i t i v e l y e x p e n s iv e f o r r o u t i n e u s e o n s i n g l e f er -m e n t o r s a t t h e l a b o r a t o r y s c a l e [ 4 3 ] .

7 . O p t i c a l M e t h o d s o f C O 2 D e t e r m i n a t i o n

7.L IR absorbance

T h e a b s o r b a n c e o f I R r a d i a t i o n b y a g a s (m i x t u r e ) is p r o p o r t i o n a l t o t h e m a s s o f

a b s o r b i n g m o l e c u l e s i n t h e l ig h t p a t h a n d i s, th e r e fo r e , p r o p o r t i o n a l t o t h e a b s o l u t e

p r e s s u r e o f t h e g a s i n a n a n a l y s i s c e ll [ 27 ]. T h e p r i n c i p l e o f I R s p e c t r o s c o p y f o r e s t i m a t -

i n g t h e p a r t i a l p r e s s u re s o f g a s e s i s a s f o l l o w s [ 27 , 4 4] . W h e n a n a l y s i n g a s in g l e c o m p o -

n e n t i n a g a s s t r e a m , a s o - c a l le d ' n o n d i s p e r s i v e a n a l y s e r ' i s u s e d . T h i s e x a m i n e s a b s o r b -a n c e i n t h e w a v e l e n g t h r a n g e o f 3 - 1 5 /z m . T h e l i g h t b e a m , a f t e r p a s s in g t h r o u g h t h e

a b s o r b a n c e c e ll t h r o u g h w h i c h t h e g a s i s fl o w i n g , p a s s es t o a d e t e c t o r t h a t i s s el e ct iv e

f o r t h e w a v e l e n g th a t w h i c h t h e c o m p o n e n t b e i n g a n a l y s e d a b so r b s . T h e i n t e n s i ty o f

t h e t r a n s m i t t e d l i g h t is c o m p a r e d w i t h t h e i n t e n s i t y o f t h e i n c i d e n t l ig h t b y u s i n g a

' b l a n k ' o r r e f e r e n c e c e ll f il l e d w i t h C O 2 - f r e e g a s . T h e d i f f e r e n c e b e t w e e n t h e t w o s ig -

n a l s i s a m e a s u r e o f t h e pCO 2. I n a n u l l - b a l a n c e i n s t r u m e n t , t h i s d i f f e r e n c e i s u s e d

t o a c t u a t e a s e r v o s y s te m . T h i s a d j u s t s a s h u t t e r s i t u a t e d i n t h e li g h t p a t h o f t h e r e f er -

e n c e c el l u n t i l t h e s a m p l e a n d r e fe r en c e r a d i a ti o n s a r e m a t c h e d . T h e s h u t t e r p o s i t i o n

is a m e a s u r e o f t h e p C O 2 .

W a t e r a b s o r b s s t r o n g l y i n t h e I R [ 4 5 ] a n d , h e n c e , t h e p r e s e n c e o f w a t e r v a p o u r i nt h e g a s s a m p l e m a y i n te r f e re w i t h t h e I R s p e c t r o p h o t o m e t r i c m e a s u r e m e n t o f C O 2.

S i n c e t h i s a b s o r p t i o n i s c h a r a c t e r i s t ic o f - O H b o n d s , v o l a t i l e a l c o h o l s m a y a ls o in t e r -

f er e. S o m e o t h e r c o m p o u n d s f r o m b a c t e r i a l cu l tu r e s, w h i c h a b s o r b i n t h e I R a n d t h o s e

m a y , t h u s , i n te r f e re , a r e a c e t o n e , a c e t a l d e h y d e a n d s u b s t a n c e s w i t h - S H b o n d s [4 4] .

I R g a s a n a l y s i s i s, t h e r e f o r e , n o t s e le c ti v e f o r C O 2 u n l e s s t h e g a s s a m p l e h a s b e e n

t r e a t e d t o r e m o v e i n t e r f e r i n g s u b s t a n c e s p r i o r t o t h e pC02 m e a s u r e m e n t a n d , a s st a t-

e d a b o v e , t h i s t e c h n i q u e m a y b e r a t h e r e x p e n s i v e .

7.2. Fibre optic measurements

S t i m u l a t e d b y r e ce n t d e v e l o p m e n t s i n f i b r e o p t ic s t e c h n o l o g y , a n u m b e r o f n e w u s e s

o f f i b r e o p t i c s i n s e n s i n g h a v e b e e n p r o p o s e d . T h e d e v e l o p m e n t o f f i b r e o p t i c p H

p r o b e s [ 4 6 ] h a s l e d t o t h e p r o d u c t i o n o f f i b r e o p t i c C O 2 p r o b e s [ 47 , 4 8]. I n s u c h f i b r e

o p t i c C O 2 p r o b e s, a m b i e n t p C O 2 c o n t r o l s th e p H o f a b i c a r b o n a t e b u f f e r s o l u t i o n ,

c o n t a i n e d w i t h i n a m e m b r a n e , w h i c h t h e n i n f l u e n c e s t h e o p t i c a l t r a n s m i t t a n c e o f a

c o l o u r i m e t r i c p H i n d i ca t o r . O n e f ib r e ca r ri e s l ig h t t o t h e b u f f e r / i n d i c a t o r s o l u ti o n , a n d

a s e c o n d f i b r e c a r ri e s t h e t r a n s m i t t e d s i g n a l (F i g . 4 ) t o a r e c e i v er w h i c h c o n v e r t s t h e

s i g n al to a n e l e c t ri c al o u t p u t . T h e o p t i c a l a p p r o a c h a v o id s p r o b l em s o f m a k i n g m i n i a -

t u r e g l a ss o r l i q u i d m e m b r a n e p C O 2 e l e c t ro d e s [ 4 9 ] w i t h a d e q u a t e r e f e r e n c e e l e c t r o d e

s t a b i l i t y . H o w e v e r , t h e c h a n g e s i n p H , t h a t a r e m e a s u r e d w i t h s u c h a d e v i c e , a r e n o t

n e c e s s a r i l y d u e s o l e l y t o a c h a n g e i n P f O 2 , a q s i n ce o r g a n i c a c i d s o r b a s e s t h a t a r e a b l e

t o p e n e t r a t e t h e m e m b r a n e m a y a l so c a u s e s u c h a c h a n g e i n p H [5 0, 51 ].



164

TEFLON SH EA TH INDICATORN SILICONE UBE

~-~ ; ....... . ~ . . . . . .

TO DETECTOR

OPTIC HBRE EPOXY " SIL ICONE DItESIVE

Fig. 4. Con struction of sensor tip of a fibre optic probe. Two fibres extending from end of app ropriatelyopaqu e an d inert tubing are sealed into it w ith epoxy. Epo xy is also layered onto c ut end of fibres to preventbinding o f indicator dye. Clear silicone adhesive s coated o ver epoxy except at fibre ends. A piece of siliconetubing is then plugged at one end with w hite silicone adhesive and filled with a solution of phenol red,

KHCO3 and KC1 [48].

8 . C h r o m a t o g r a p h i c C O 2 D e t e r m i n a t i o n

I n g a s l iq u i d c h r o m a t o g r a p h y , a s a m p l e is i n t r o d u c e d i n to a s t r e a m o f c a r r ie r g a s

( th e m o b i l e p h a s e ) a n d is s w e p t t h r o u g h a c h r o m a t o g r a p h i c c o l u m n w h i c h c o n t a in s

a s t a t i o n a r y p h a s e. A f t e r in t r o d u c t i o n o n t o t h e c o l u m n , t h e s a m p l e c o m p o n e n t s a r e

a l lo w e d t o d i s t ri b u t e b e t w e e n t h e s t a t i o n a r y a n d m o b i l e p h a s e s a c c o r d i n g t o t h e i r p a r -

t i t i o n c o e f f i c i e n t s . K , t h e p a r t i t i o n c o e f f i c i e n t , is d e f i n e d [3 6, 5 2] a s :

K=Cs/Cm

w h e r e C s is th e c o n c e n t r a t i o n o f s o l u t e i n th e s t a t i o n a r y p h a s e a n d C m is th e c o n c e n -

t r a t i o n o f s o l u t e i n t h e m o b i l e p h a s e . T h e l o w e r t h e v a l u e o f K , i.e., t h e l o w e r th e s o l u b i l -

i t y o f t h e s o l u t e i n t h e s t a t i o n a r y p h a s e , t h e q u i c k e r i t w i ll p a s s t h r o u g h t h e c o l u m n .

V a r i o u s c o m p o n e n t s o f a s a m p l e c a n b e s e p a r a t e d o n l y i f t h e y h a v e d i f fe r e n t K v a lu e s.

T h e t i m e t a k e n t o p a s s t h r o u g h a c o l u m n , t h e r e t e n t i o n t im e , w i ll b e a ff e c t e d b y t h e

t y p e o f c o l u m n , t e m p e r a t u r e a n d c a r r i e r -g a s f lo w r at e. O n c e s e p a r a t e d , t h e c o m p o -

n e n t s e n t e r a d e t e c t o r ( e .g ., t h e r m a l c o n d u c t i v i t y d e t e c t o r , m a s s s p e c t r o m e t e r ) c o n n e c t -

e d t o a r e c o r d e r o r a p r i n t e r / p l o t t e r a n d a r e c o n s ec u t i v e ly r e gi st e re d a s c h r o m a t o g r a p h -

ic p e a k s . P r o v i d e d t h a t t h e d e t e c t o r a n d s e p a r a t i o n p r o c e d u r e a r e a p p r o p r i a t e , e a c h

p e a k w i ll c o r r e s p o n d t o o n e o f th e c o m p o n e n t s o f t h e o r i g in a l s a m p l e . A n i n t e rn a ls t a n d a r d m a y b e a d d e d t o th e s a m p l e b e f o r e an a l y s is t o c o m p e n s a t e f o r p o s s i b l e v a r i a -

t i o n s i n a n a ly t i c a l c o n d i ti o n s . T h e r e t e n t i o n t i m e s o f t h e c o m p o n e n t s a r e t h e n c a l c u l a t-

e d r el a ti v e t o t h a t o f th e i n t e r n a l s t a n d a r d . F o r a m o r e d e t a il e d d i s c u s s io n o f c h r o m a t o -

g r a p h i c s e p a r a t i o n t h e o r y , s e e , e .g ., L a r s s o n a n d O d h a m [ 36 ], P e r r y [ 5 2] , G r o b [5 31 ,

C r a m e r s a n d M c N a i r [54 ] a n d W i ll e tt [ 5 5]. T h e a r e a b e n e a t h e a c h p e a k c a n t h e n b e

c o r r e l a t e d t o t h e c o n c e n t r a t i o n o f t h e c o r r e s p o n d i n g c o m p o n e n t i n t h e i n j ec t e d s a m p l e

[ 36 , 5 2 , 5 4, 5 5] . H o w e v e r , s i n ce c h r o m a t o g r a p h i c t e c h n i q u e s r e ly u p o n t h e s p a t i a l s e p a -

r a t i o n o f th e c o m p o u n d s t h a t a r e b e i n g q u a n t i f ie d , t h e y a r e o n l y o f u se o n a n o n c o n -

t i n u o u s b a s i s .



165

9 . C O 2 E l e c t r o d e s

9 .1 . P o t e n t i o m e t r i c

I o n - s e l e c ti v e e l e c t r o d e s m a y b e u t il is e d f o r th e e s t i m a t i o n o f a w i d e r a n g e o f c o m -p o u n d s , i n c lu d i n g g a se s, e .g ., a m m o n i a , 0 2 a n d C O 2 [ 5 6 - 5 8 ] . T h e d e t e r m i n a t i o n o f

p C O 2 , a q b y i o n - s e l e c t i v e p o t e n t i o m e t r i c e l e c t r o d e s , s u c h a s t h e S e v e r i n g h a u s - t y p e

( F ig . 5 ) , i n v ol v es t h e m e a s u r e m e n t o f t h e p H o f a t h i n l a y e r o f a b u f f e r s o l u t i o n i n

e q u i l i b r i u m w i t h a m b i e n t C O 2 [ 57 , 5 9 ]. T h e f e r m e n t a t i o n m e d i u m a n d t h e b u f f e r s o -

l u t i o n w i t h in t h e e l e c t r o d e ar e s e p a r a t e d b y a m e m b r a n e w h i c h is ' f r e e ly ' p e r m e a b l e

t o C O 2 . H o w e v e r , t h e u s e o f t h e S e v e r i n g h a u s - t y p e p o t e n t i o m e t r i c e l e c tr o d e f o r

m e a s u r i n g t h e l ev e ls o f d i ss o l v e d C O 2 is h a m p e r e d b y it s sl ow r e s p o n s e t im e ; c o m p l e x

r e l a t io n s h i p s a m o n g m u l t ip l e c h e m i c a l s p ec i es d u r i n g u n s t e a d y d i f f u s i o n w e r e s h o w n

t o c a u s e h y s te r es i s a n d p H - d e p e n d e n t r e s p o n s e r a t es [5 9]. D i g i ta l s i m u l a t i o n s m a y b e

u s e d t o p r e d i c t t h e d y n a m i c r e s p o n s e o f s u c h e l e c t r o d e s [6 0] a n d e l e c t r o d e s ta b i li tym a y b e s a c r i f i c e d t o i m p r o v e r e s p o n s e t im e . A s w i t h t h e f i b re o p t i c p r o b e s m e n t i o n e d

a b o v e , t h e c h a n g e s i n p H t h a t a r e m e a s u r e d w i t h s u c h a d e v ic e a r e n o t n e c e s s a r i ly d u e

s o l e l y t o a c h a n g e i n p C O 2 , a q [ 5 0]; a d d i t i o n a l l y , t h e e l e c t r o d e s ar e b y n o m e a n s c h e a p .

A n o t h e r p r o b l e m w i t h d e v ic e s, s u c h a s t h e S e v e r i n g h a u s - t y p e e l e c tr o d e , is th a t t h e y

m u s t b e i m m e r s e d i n th e f e r m e n t a t i o n b r o t h a n d a re , t h e r e fo r e , 'i n v a s iv e ' . T h e u s e o f

s u c h i n v a si v e d e v ic e s l e a d s t o a n u m b e r o f p o t e n t i a l p r o b l e m s : a n a d d i t i o n a l p o r t o n

t h e f e r m e n t a t i o n v e ss e l w i ll b e n e c e s s a r y l e a d i n g t o a n i n c r e a s e d r i sk o f c o n t a m i n a t i o n :

t h e d e v i c e m u s t b e r o b u s t e n o u g h t o w i t h s t a n d s t e ri li s at io n : t h e d e v i c e is r e q u i re d t o

b e c a l i b r a t e d e a s il y w i t h o u t h a v i n g t o b e r e m o v e d f r o m t h e f e r m e n t a t i o n v e s se l. S u c h

a s t e a m - s t e r i li z a b l e S e v e r i n g h a u s - t y p e p C O 2 e l e c t r o d e t h a t c a n b e c a l i b r a t e d i n s it u ,h a s , i n f a c t , b e e n d e s c r i b e d [ 6 1 ] .

K 2 C O 3 m a y b e u s e d a s a s o l id e l e c t r o l y t e f o r t h e p o t e n t i o m e t r i c m e a s u r e m e n t o f

g a s e o u s c a r b o n o x i d e s . I t w a s s h o w n t h a t t h e t r i p l e c o n t a c t s : K 2 C O s ( s ) , C O 2 ( g ), P t ( s )

a n d : K 2 C O 3 ( s ) , C O 2 ( g ) , Z r O 2 - C a O ( s ) c a n b e u s e d f o r C O 2 d e t e r m i n a t i o n i n a i r - o r

i n O 2 - b e a r in g g a s es a t t e m p e r a t u r e s r a n g i n g f r o m 4 5 0 t o 7 5 0 ° C [6 2].

pH ELECTRODE - -

BUFFER SOLUTI01~

CAP

~CE

~RODE

Fig. 5.

SILICONE RUBBE R ~INGMEMBRANE

Design of Severinghaus-typeelectrode.CO 2 is absorb ed into o r desorbs from buffer solution andcorresponding chang e in p H is m easured by pH electrode [50].



166

T h e f a b r i c a t i o n a n d p e r f o r m a n c e o f a v a r i e t y o f i o n - s e n s it iv e f i e ld - e f f e c t t r a n s is t o r s

( I S F E T ) , w h i c h a r e i n e f f e c t s o l i d - s ta t e d e v i c e s ( n o i n t e r n a l r e f e r e n c e e l e c t r o l y t e ) , h a v e

b e e n d e s c r i b e d [ 6 3 - 6 5] . T h e I S F E T h a s a t t r a c t e d c o n s i d e r a b l e i n te r e st b e c a u s e it is

e n v i s a g e d t h a t a s in g le m i n i a t u r i s e d s o l id - s t at e c h i p c o u l d c o n t a i n m u l t ip l e s e n s o r sa n d b e u s e d t o s e n se s ev e ra l i o n s s i m u l t a n e o u s ly . I t h a s b e e n s h o w n t h a t I S F E T s b a s e d

o n i o n - s e le c t iv e m e m b r a n e s w e r e s u b j e c t t o p o s i t i v e in t e r f e r e n c e b y e i t h e r C O 2 o r o r -

g a n i c a c i d s [5 1]. W h e n f i e l d - e f f e c t t r a n s i s t o r s w e r e c o a t e d w i t h P V C o r s i li c o n e r u b b e r

m e m b r a n e s , l a rg e r r e s p o n s e s t o C O 2 t h a n w i t h io n - s e le c t iv e m e m b r a n e s w e r e o b -

s e r v e d a n d t h e r e s u l ti n g I S F E T s b e h a v e d v e r y m u c h l i ke S e v e r i n g h a u s - t y p e p o t e n t i o -

m e t r i c C O 2 s e n s o r s [ 5 1 ] . T h e r e s p o n s e t i m e s , h o w e v e r , w e r e m u c h s l o w e r t h a n w i t h

S e v e r i n g h a u s - t y p e e le c t r o d e s a n d t h e I S F E T s s ti ll re s p o n d e d t o o r g a n i c a c id s . I S F E T s ,

l ik e S e v e r i n g h a u s - t y p e e l e c t r o d e s , a r e in v a s i v e d e v i c e s a n d , t h e r e f o r e , p o s s e s s a l l o f t h e

p r o b l e m s a s s o c i a t e d w i t h i n v a s iv e d e v i c e s l i st e d a b o v e . H o w e v e r , f o r g a s a n a l y s is , r a t h -

e r t h a n t h e d e t e r m i n a t i o n o f t h e c o n c e n t r a t i o n s o f p a r t i c u l a r i on s i n a s o l u ti o n , I S F E T sc o u l d b e u s e d a s n o n i n v a s i v e d ev i c e s b y s i m p l y p l a c i n g t h e m i n t h e e f f l u e n t g a s f lo w .

9.1.1. Invasive and noninvasive electrode methods for determination of CO2. T h e

m e a s u r e m e n t o f p C O 2 i n t h e e f f l u e n t g a s is sa i d t o g i ve a n e x c e l le n t a p p r o x i -

m a t i o n o f t h e p C O 2 ,a q [ 3, 6 6 b u t c f . 6] , t h u s , e l i m i n a t i n g t h e n e e d f o r t h e m o r e

t e c h n i c a l l y d e m a n d i n g m e a s u r e m e n t o f p CO 2,a q a n d a l lo w i n g th e u t i l i sa t i o n o f

n o n i n v a s i v e d e v i c e s . I t is p o s s i b l e , h o w e v e r , to u s e a n i n v a s iv e d e v i c e in a n o n i n v a s i v e

w ay . A p o t e n t i o m e t r i c m e t h o d w a s p r o p o s e d w h e r e b y c h a n g e s i n p H , c a u s e d b y t h e

a b s o r p t i o n o f C O 2 w h e n t h e e f f l u e n t g a s w as b u b b l e d t h r o u g h a s o l u t io n o f s o d i u m

c a r b o n a t e , w e r e m e a s u r e d [2 3]. T h i s m e t h o d i s s ti ll h a m p e r e d b y th e p r o b l e m s a s s o c i a t -e d w i t h S e v e r i n g h a u s - t y p e e l e c tr o d e s li s te d a b o ve . N o n i n v a s i v e m e t h o d s f o r d e t e r m i n -

i n g p C O 2 h a v e re c e i v ed a t t e n t i o n f o r c li n ic a l u se s in t h e t r a n s c u t a n e o u s a n a l y si s o f

a r t e r ia l p C O 2 [e .g ., 6 7 - 6 9 ] . T h e p r i n c i p l e i n v o lv e d is o n c e a g a i n t h e m e a s u r e m e n t o f

t h e p H c h a n g e w i t h i n a b u f f e r s o l u t i o n a n d , d e s p i t e t h e s l o w r e s p o n s e t i m e , s u c h a

t e c h n i q u e i s o f u s e in c o n t r o l s y s te m s f o r m a i n t a i n i n g a n o r m a l a r t e r ia l p C O 2 b y a r-

t i f i c i a l r e s p i r a t i o n s i n c e s u c h s e n s o r s a r e c a p a b l e o f c o n t i n u o u s m e a s u r e m e n t .

9.2. Conductimetric

C o n d u c t i m e t r y h as a ls o b e e n ap p l i e d t o th e m e a s u r e m e n t o f p C O 2. A c o n t i n u o u s

c o n d u c t i m e t r i c s e n s o r f o r C O 2 ( F ig . 6 ) w as d e s c r i b e d i n w h i c h C O 2 d i f f u s e d t h r o u g ha h y d r o p h o b i c g a s - p o r o u s m e m b r a n e i n t o a t h in l a y e r o f p u r e w a t e r [ 70 , 7 1]. T h e b a c k

w a l l o f t h e t h i n w a t e r l a y e r ( s e e F i g . 6 ) w a s a p o r o u s s c r e e n s e p a r a t i n g t h e w a t e r f r o m

a m i x e d - b e d i o n -e x c h a n g e c o l u m n w h i c h c o n t i n u o u s l y r e m o v e d i o n ic s p e ci es f r o m t h e

w a t e r la y er . E l e c t r o d e s p o s i t i o n e d i n t h e w a t e r l a y er c o n t i n u o u s l y m e a s u r e d i ts c o n d u -

c t a n c e. T h e d i f f u s i o n o f C O 2 i n t o t h e w a t e r l a y e r a n d t h e r e m o v a l o f i o n i c s p e c ie s b y

t h e m i x e d - b e d i o n i c e x c h a n g e r e s t a b l i s h e d a s t e a d y - s t a t e c o n c e n t r a t i o n g r a d i e n t o f

C O 2. T h e g r a d i e n t w a s p r o p o r t i o n a l t o p C O 2 i n th e g a s p h a s e a n d t h e c e ll c o n d u -

c t a n ce w a s p r o p o r t i o n a l to (pC O 2) 1/2 [70, 7 1]. H o w e v e r , th i s d e v i c e w a s e v a l u a t e d f o r

u s e i n t h e d e t e r m i n a t i o n o f pC02 o n l y u p t o 0 .0 1 a t m .

9.3. Amperometric

I n c o n t r a s t t o CO2, 0 2 i s r o u t i n e l y a n d m o r e - o r - l e s s r e l i a b l y m e a s u r e d e i t h e r



167

M I X E D - B E D I O N EXCHANGERLH--ITHIN WATER

GLASS TUBING

WATER-POROUS_ _ SPA CE R SCREEN

G OR E-T EX - - W AT ER -P OR OU SAS-POROUSMEMBRANE - - SUPPORTING SCREEN

PLATINISED, GOLDCONDUCTIVITY ELECTRODES

F i g. 6 . S c h e m a t i c r e p r e s e n t a t i o n o f a c o n t i n u o u s c o n d u c t i m e t r i c CO 2 sensor. E l e c t r o d e s p o s i t i o n e d i n

t h i n w a t e r l a y e r m e a s u r e i ts c o n d u c t a n c e . D i f f u s i o n o f C O 2 t h r o u g h t h e m e m b r a n e i n t o w a t e r l ay e r a n d

r e m o v a l o f i o n i c s p e c i e s t h r o u g h s c r e e n b y i o n e x c h a n g e r e s t a b l i s h a s t e a d y - s t a t e g r a d i e n t o f C O 2 [ 7 0, 7 1] .

p a r a m a g n e t i c a l l y [ 2 7 ] o r b y m e a n s o f t h e C l a r k - t y p e a m p e r o m e t r i c e l e c tr o d e [ 4 3, 7 2]

w h i c h c o n s i s ts t y p i c a l ly o f a n A g r i n g a n o d e s u r r o u n d i n g a P t c a t h o d e ( F i g . 7). W h e n

a n a p p r o p r i a t e p o t e n t i a l is a p p l i e d a c r o s s t h e e le c t ro d e s , 0 2 i s r e d u c e d a t t h e c a t h o d ei n t h e r e a c t i o n :

2 H + + 2 e - + 1A0 2 - - H 2 0 .

3M KC1 _ TEFL ON MEMBRANE

P t CATHODE

- - " 4 -

F i g . 7 . D e s i g n o f t h e C l a r k - t y p e el e ct r od e . A n A g r i n g a n o d e s u r r o u n d s a P t c a t h o d e a n d t h e c i r cu i t i s c o m -

p l e t e d b y a K C 1 s o l u t i o n . S u c h e l e c t r o d e s a r e u s u a l l y c o n s t r u c t e d i n P e r s p ex .



168

T h e c i rc u i t is c o m p l e t e d b y a K C l -s a l t b r i d g e b e t w e e n t h e a n o d e a n d t h e c a t h o d e a n d

t h e a n o d e r e a c t i o n i s :

A g + C 1 - - - A g C I + e - .

T h u s , i f 0 2 i s p r e s e n t , a c u r r e n t f l o w s a n d A g C 1 i s d e p o s i t e d o n t h e a n o d e . A s t h e

p o t e n t i a l o f t h e c a t h o d e i s m a d e m o r e n e g a t i v e , O 2 i s r e d u c e d m o r e r a p i d l y u n t i l a

p o t e n t i a l ( d i f fe r e n c e ) is r e a c h e d w h e r e t h e r e d u c t i o n r a te is e q u a l t o t h e r a te a t w h i c h

O 2 c a n d i f f u s e t o t h e c a t h o d e , p r o d u c i n g a p l a t e a u r e g i o n ( d i f f u s i o n - l i m i t e d c u r r e n t )

( F i g . 8 ) [7 3 ]. I f t h e p o t e n t i a l d i f f e r e n c e i s i n c r e a s e d b e y o n d t h e p l a t e a u r e g i o n t h e e v o l u -

t i o n o f H c o m m e n c e s a t th e c a t h o d e a n d t h e r e i s a la r g e in c r e a s e i n c u r r e n t . I f le ss

O 2 i s p r e s e n t i t s d i f f u s i o n i s s l o w e r a n d t h e p l a t e a u c u r r e n t i s , t h e r e f o r e , l o w e r . T h e

a m o u n t o f c u r r e n t is , t h u s , l i n e a r w i th t h e O 2 c o n c e n t r a t i o n a t t h e e l e c t r o d e s u r fa c e .

C O 2 m a y a l s o b e r e d u c e d a t s u c h a c a t h o d e i n t h e f o l l o w i n g w a y [7 4]:i n a n h y d r o u s c o n d i t io n s ,

C O 2 q- e - - - C O ~ ,

2 C 0 2 - - - C O + C O 2 - ;

i n t h e p r e s e n c e o f w a te r ,

C O 2 -b e - - - C O 5 ,

C O 5 + H 2 0 - - H C O 5 + O H . ,

O H - + C O 5 - - H C O ~ .

H e n c e , a n a m p e r o m e t r i c C O 2 p r o b e i s, t h e re f o r e , p o s s ib l e [7 5]. T h i s e l e c t r o d e f o r

a m p e r o m e t r i c C O 2 m e a s u r e m e n t w a s a t h r e e - e l e c tr o d e sy s t e m (F i g. 9 ), c o n s is t in g o f

A I R - S A T U R A T E D

/

1 / 2 A I R - S A T U R A T E DWATER

0 -0 -5 -1~0POTENTtAL DIFFERENCE iV )

Fig. 8. Curr ent- volt age curve for reduction of 0 2 in Clark-type amperometric electrode (Fig. 7). As

potential difference between Pt cathode and Ag anod e is increased, rate o f O 2 reduction at cathode in-creases. A plateau is reached wh en reduc tion rate of 0 2 is equal to rate at which O z can dif fuse to cathode.

If potential difference is increased above plateau region, H evolution commences and there is a large increase

in current.



TEFLON MEMBRANE

I / DM SO + ELECTROLYTE

" 0 " - R I N G P ~ I ~ R E N C EELECTRODE

N O R K / N G C O U N T E R

ELECTRODE ELECTRODE

169

ELECTRODES

RE FE RE NCE COUNT E RELECTRODE ELECTRODE

Fig. 9. Design of an am perom etricCO 2 electrode[102] based upo n the design of an O2/CO 2 electrodebyAlbery and Barron [75]. Since Perspex dissolves in DM SO, the electrode was constructed from nylon.

a w o r k i n g e l e c t r o d e , a c o u n t e r e l e c t r o d e a n d a r e f e r e n c e e l e c tr o d e , a s c o m p a r e d t o t h e

C l a r k - t y p e e l e c t r o d e w h i c h i s g e n e r a l l y a t w o - e l e c t r o d e s y s te m ( s e e F i g . 7 ). I n s u c h t w o -

e l e c t r o d e sy s t em s , c u r r e n t p a s s in g t h r o u g h t h e r e f e r e n c e e l e c t r o d e m a y c a u s e i ts p o t e n -

t i a l t o d e v i a t e a n d g i v e r i s e t o i n t e r n a l p o l a r i s a t i o n . T h i s , a n d r e s is t iv e d r o p s d u e t o

t h e p r e s e n c e o f j u n c t i o n p o t e n t i a l s a c r o s s s o lu t i o n s a n d t h e i n t e r fa c e b e tw e e n d i f f e r e n t

m a t e r i a l s, l e a d s to a d e c r e a s e in t h e a b s o l u t e p o t e n t i a l d i f f e r e n c e b e t w e e n t h e w o r k i n g

e l e c t r o d e a n d t h e r e f e r e n c e e l e c tr o d e . A d d i t i o n o f t h e t h i r d e l e c t r o d e m i m i m i s e s t h e s e

p r o b l e m s a s th e c u r r e n t n o w f l o w s b e t w e e n t h e w o r k i n g e l e c t ro d e a n d t h e c o u n t e r e le c -

t r o d e w i t h t h e p o t e n t i a l o f t h e w o r k i n g e l e c t r o d e b e i n g s e t r el a ti v e t o t h a t o f t h e r e f e r -

e n c e e l e c t r o d e [ 7 6 - 7 8 ] .

T o r e d u c e t h e r e s i s t i v e d r o p s t o a n i n s i g n i f i c a n t l e v e l , i n e r t s u p p o r t i n g e l e c t r o l y t e s

a r e u s e d [7 6, 7 9 ]. P o t a s s i u m c h l o r i d e is c o m m o n l y u s e d a s a s u p p o r t i n g e l e c t ro l y t e

s in c e it is e a si ly a v a il a b le i n h i g h p u r i t y f o r m a n d t h e m o b i l i t y o f t h e p o t a s s i u m i o n

a n d t h e c h l o r i d e i o n a r e a l m o s t e x a c t l y e q u a l . P o t a s s i u m n i t r a te is a n a l t e rn a t i v e w h e n

c h l o r i d e c a n n o t b e u s e d . H o w e v e r , w h e n a h i g h c o n c e n t r a t i o n o f p o t a s s i u m i o n s is

p r e s e n t , t h e f o o t o f i ts p o l a r o g r a p h i c r e d u c t i o n w a v e m a y e x t e n d t o v a l u es s u f f i c ie n t l y

p o s i t iv e a s t o c a u s e u n w a n t e d i n t e r f e r e n c e [7 9 ]. T h i s c a n b e a v o i d e d b y t h e u s e o f

t e t r a a l k y l a m m o n i u m s alts . I n t e r f e r e n c e b y th e c h l o r id e i o n o f t e n a r is e s w h e n m e r c u r y

e l e c t r o d e s a r e u s e d a n d m a y b e a v o i d e d b y u s i n g n i t r a t e o r p e r c h l o r a t e s a l t s .

A n u m b e r o f p r o c e ss e s h a s b e e n r e p o r t e d f o r t h e r e d u c t i o n o f C O z a l t h o u g h t h e se

h a v e b e e n d i r e c t e d p r i m a r i l y t o a q u e o u s s o l u t i o n s [e.g ., 8 0 - 8 3 ]. H o w e v e r , a t t h e p o t e n -

t ia l r e q u i r e d f o r t h e r e d u c t i o n o f C O 2 ( a t a n A g w o r k i n g e l e c t r o d e t h e ½ - w a v e p o t e n -

t i a l is - 2 . 1 3 V vs . t h e s t a n d a r d c a l o m e l e l e c t r o d e [ 7 5 ] ) w a t e r i s e l e c t r o l y s e d [8 4 ], t h u s ,

c o n t r a - i n d i c a t i n g t h e u s e o f a q u e o u s s o l u t i o n s w i t h i n th e e l e c t r o d e f o r a n a l y ti c a l p u r -

p o s e s . 0 2 is m u c h m o r e r e a d i ly r e d u c e d t h a n is C O 2 a n d , i f p r e s e n t, w i ll g iv e ri se t o

t h e p r o d u c t i o n o f s u p e r o x id e w h i c h a c c u m u l a t e s a n d i n te r fe r e s w i th t h e m e a s u r e m e n t



170

o f p C O 2 [7 5 ]. I n th e e l e c t r o d e d e s c r i b e d b y A l b e r y a n d B a r r o n [7 5], t h is p r o b l e m w a s

c i r c u m v e n t e d b y t h e u s e o f a m e t a l li s e d m e m b r a n e a s a n e l e c t r o c h e m i c a l f i lt e r f o r t h e

r e m o v a l o f 0 2 . T h e p r i n c i p l e o f th i s is t h a t 0 2 is r e d u c e d , a t t h e m e t a l l is e d m e m b r a n e

i n a n a q u e o u s s o l v e n t to p r o d u c e w a t er , a t a lo w e r p o t e n t i a l t h a n t h a t w h i c h i s r e q u i r e dt o r e d u c e C O 2 . H o w e v e r , i n th e c a s e o f a n e l e c t r o d e i n t e n d e d f o r t h e m o n i t o r i n g o f

p C O 2 i n a n a e r o b i c f e r m e n t a t i o n s t h e p r e s e n c e o f O z w i l l n o t b e a p r o b l e m .

D e t e r m i n a t i o n o f p C O 2 w i t h a n a m p e r o m e t r i c el e c tr o d e sh o u l d b e a b le t o g iv e c o n -

t i n u o u s m e a s u r e m e n t s , h a v e a l i n e a r r e l a t io n s h i p b e t w e e n o u t p u t a n d p C O 2 ( as c o m -

p a r e d w i t h t h e S e v e r i n g h a u s - t y p e e le c t r o d e w h i c h h a s a l o g a r i t h m i c r e l a t i o n s h i p b e -

t w e e n t h e v a r ia b l e a c t u a l l y m e a s u r e d , p H a n d p C O 2 [ 50 ]) a n d b e b o t h s i m p l e a n d

c h e a p t o c o n s t r u c t , e s p e c i a l l y , i f i t i s t o b e u s e d u n d e r a n a e r o b i c c o n d i t i o n s .

R e c e n tl y , a C O 2 s e n s o r u s i n g i m m o b i l i s e d t h e r m o p h i l i c b a c t e r i a a n d a n a m p e r o -

m e t r i c 0 2 e l e c t r o d e w a s d e s c r i b e d [ 8 5 ] . T h e c o n s t r u c t i o n o f t h i s C O 2 s e n s o r i s a s

s h o w n i n F ig . 1 0. T h e i m m o b i l i s e d b a c t e r i a w e r e s u p p l ie d w i t h O 2 - s a tu r a t e d b u f f e rc o n t a i n i n g v a r i o u s m e t a l i o n s a n d 2 0 0 /~ M g l u co s e . C O 2 p e r m e a t e s t h r o u g h t h e d i a ly -

s is m e m b r a n e a n d is a s s i m i l a t e d b y t h e b a c t e r i a , c a u s i n g t h e i r r e s p i r a t o r y r a t e t o in -

c r e a s e [ 8 6 ] . T h e ' C O 2 ' c o n c e n t r a t i o n w a s e s t i m a t e d f r o m t h e d e c r e a s e i n t h e o u t p u t

c u r r e n t f r o m t h e 0 2 e l e c t r o d e , c a u s e d b y t h e d e c r e a s e i n t h e 0 2 c o n c e n t r a t i o n c o r -

r e s p o n d i n g t o t h e i n c re a s e d b a c t e r i a l r e s p i ra t i o n . A l t h o u g h t h e e l e c tr o d e p r o d u c e d a n

o u t p u t w i t h a l i n e a r r e l a t i o n s h i p t o ' C O 2 ' , t h e r a n g e o f C O 2 c o n c e n t r a t i o n s t e s t e d

w a s s m a ll , t h e t e m p e r a t u r e r a n g e w a s l i m i te d a n d t h e r e s p o n s e t im e w a s 5 - 1 0 m i n .

C O 2 m a y b e d e t e c t e d b y a n o t h e r m e t h o d w h i c h u ti li se s a n 0 2 e l e c t r o d e [8 7 ]. A s w i t h

t h e i m m o b i l i s e d b a c t e r i a l s e n s o r, t h e p r e s e n c e o f C O 2 r e su l ts i n a r e d u c t i o n , p r o p o r -

t i o n a l to t h e p C O 2 , o f t h e c u r r e n t o u t p u t o f t h e 0 2 e le c tr o d e.

9 .3 .1 . R e d u c t i o n o f C O 2 m e d i a t e d b y a n e l e c t r o c a ta l y s t. R e c e n t l y , a g r e a t d e a l o f

a t t e n t i o n h a s b e e n d e v o t e d t o t h e u t i l i s a t i o n o f C O 2 a s a s o u r c e o f C i n t h e e l e c -

A N O D E - - - -

DIALYSIS

M E M B R A N E

E ~ C ~ O L ~ E

C A T H O D E

B U F F E R - * BUFFER

G A S ]M E M B R A N E 0 - R I NG

IMMOBILISEDBACTERIA

Fig. 10. Cross -sect ion of a CO 2 sens or which utilises immobilised therm ophil ic bacteria [85]. The bacteriaassimilate C O 2 and, in doing so, their r espirat ion rate increases resulting in a decreased 02 concen tratio n.

It is a decrease in 0 2 conc entr atio n tha t is measu red by the electrode. Such electrodes generally exhibit a

rather poor dynamic range.



171

trochemical synthesis of organic products [e.g., 80, 88]. One possibility would be to

use an electrocatalyst, developed for such CO2 utilisation, in an amperometric CO2

electrode, to reduce the potential required for the reduction of CO2. However, few

catalytic systems are known despite the development of a number of different strategies(electro-, photo- and photo-electro-chemical). The purely photochemical systems may

be classified as either heterogenous, making use of semiconductor suspensions

[89-92], or homogenous, employing aqueous solutions of metal ions [82, 93-95] ,

organic dyes or transition metal complexes [96-99] . In a system based on

Re(bipy)(CO)3C1, high yields of CO were photogenerated from CO 2 [100]. When

Re(bipy)(CO)3C1 was used as a catalyst in the electrochemical reduction of CO2 to

CO [83], the system displayed high current efficiency and long-term stability (high

overall turnover). Such catalytic properties would appear to make Re(bipy)(CO)3Cl

an attractive catalyst for use in an amperometric CO2 electrode for the improvement

of selectivity and specificity. Direct noncatalysed reduction of C02 follows a monoe-lectronic pathway (equations given above), requiring potentials as negative as - 2 V

[74, 101]. On the other hand, polyelectronic reduction of CO2 may occur at much less

negative potentials, e.g., the E01 for the dielectronic reduction of CO2 to CO in aque-

ous solutions at pH 7 is only -0.52 V [83, 96] and occurs as follows:

CO2 + 2H ÷ + 2e- .-. CO + H20.

Re(bipy)(CO)3C1 was reported to reduce CO2 to CO at a potential of -1.25 V, sub-

stantially below the potential for monoelectronic CO2 reduction.

Although the use of Re(bipy)(CO)3C1 in an electrode may in theory appear to bebeneficial, in practice, this electrocatalyst offered no added benefit and, in fact, wor-

sened matters by reducing the rate o f current decay after a reduction in pCO2 [102].

That the use of the electrocatalyst in an electrode caused an increase in the time taken

for the current to decrease following a decrease in the pCO 2, was possibly due to in-

teraction between CO 2 and the electrocatalyst. Such interactions would 'delay' CO2

desorption from the solvent giving rise to higher concentrations of 'CO2' than would

normally be encountered and reflected in the increased current. However, the use of

other electrocatalysts in amperometric electrodes may improve this method of CO2

measurement.

10. Concluding Remarks and Summary

As with any set of analytical methods, there is likely to be a trade-off between reso-

lution, precision, accuracy, response time, ease of use and expense. The methods dis-

cussed herein, and summarised in Table 2, cover the spectrum of simplicity. At present,

mass spectrometry is probably the most widely used method industrially and/or where

money is no object. Fibre optic probes seem to offer many advantages whilst continu-

ing technological development work may be expected for potentiometric devices in

particular. IR methods probably suffer too many interferences to be of general use

in fermentations though the microwave region does not yet seem to have been explored.



1 7 2

T A B L E 2

S U M M A R Y O F M E T H O D S F O R E S T I M A T I O N O F ' C O z ' D E S C R IB E D I N T E X T

M e t h o d A p p l i c a ti o n s C o m m e n t s

T i t r i m e t r i c L i q u i d s a m p l e s

C o l o u r i m e t r i c L i q u i d o r g a s e o u s s a m p l e s

G r a v i m e t r i c E x h a u s t g a s e s

V o l u m e t r i c D i s s o lv e d ' C O 2 '

E n z y m a t i c D i s s o lv e d ' C O 2 '

K a t h a r o m e t e r D e t e c t i o n o f g a s e s

M a s s D i s s o lv e d g a s e s o r f e r m e n t o rs p e c t r o m e t r y e x h a u s t g a s e s

I R E x h a u s t g a s e s

F i b r e o p t i c D i s s o lv e d ' C O 2 '

p r o b e s O v e r c o m e s p r o b l e m s o f m a k i n g

m i n i a t u r e g l a s s e l e c t r o d e s

G a s E x h a u s t g a s o r s m a l l l i q u id

c h r o m a t o g r a p h y s a m p l e s

P o t e n t i o m e t r i c D i s s o lv e d ' C O 2 '

C o n d u c t i o m e t r i c E x h a u s t g a s e s

A m p e r o m e t r i c E x h a u s t g a se sP i e z o e l e ct r ic E x h a u s t g a s e s

U n s u i t a b l e f o r c o n t i n u o u s u s e



N o t a b l e to p r o v i d e g e n u i n e ly c o n -

t in u o u s m o n i t o r i n g . C u m b e r s o m e

Sp e c i f ic f o r ' CO 2 ' b u t u n s u i t a b l e f o r

c o n t i n u o u s u s e

N o t s p e c i f ic f o r CO 2 . Sh o u l d i d e a l l y

b e c o m b i n e d w i t h a s e p a r a t i o n

m e t h o d , e . g . , G C

E x p e n s i v e b u t p r e c i se a n d a c c u r a t e

E x p e n s i v e . N o t s p e c i f i c f o r CO 2

M e a s u r e s p H - o r g a n i c a c i ds o r

b a s e s m a y i n t e r fe r e


S l o w r e s p o n s e t i m e , e l e c t r o d e s n o t

c h e a p , o r g a n i c a c i d s / b a s e s m a y i n -

t e r fe r e , l o g a r i t h m i c r e s p o n s e

E v a l u a t e d o n l y u p t o 0 .0 1 a t m

S i m p l e a n d c h e a p .S i m p l e a n d c h e a p . I n t e r fe r e n c e s n o t

k n o w n b u t v o l a t i l e i n t e r fe r e n c e s

l ike ly

A c k n o w l e d g e m e n t s

W e a r e g r a t e f u l to t h e B i o t e c h n o l o g y D i r e c t o r a t e o f t h e S c i e n c e a n d E n g i n e e r i n g

R e s e a r c h C o u n c i l , U K , f o r f i n a n c i a l s u p p o r t .

N o t e d a d d e d i n p r o o f

F a t i b e l l o - F i l h o e t a l . [ 1 0 3] h a v e r e c e n t l y d e s c r i b e d a C O z - s e n s i t i v e p i e z o e l e c t r i c

c r y s t a l w h i c h g a v e a l i n e a r r e s p o n s e t o p C O z i n t h e r a n g e o f 0 . 0 1 8 - 0 . 1 6 a t m u n d e r

f e r m e n t a t i o n c o n d i t i o n s .

R e f e r e n c e s

1 K n o c h e , W. ( 1 98 0 ) Ch e m i c a l r e a c t i o n s o f CO 2 i n w at er . I n : B i o p h y s i c s a n d Ph y s i o l o g y o f C a r b o n D i -

ox ide (Baue r , C. , Gros , G . and Bar re l s , H . , eds .) , pp . 3 - 11 , Springe r -Ver lag , Be r l in .

2 Co v i n g t o n , A .K . ( 1 98 5 ) Po t e n t i o m e t r i c t i t r a t i o n s o f a q u e o u s c a r b o n a t e s o l u t io n s . Ch e m . So c. Re v.1 4 , 2 6 5 - 2 8 1 .

3 Y a g i , H . a n d Y o s h id a , E (1 97 7) D e s o r p t i o n o f C O / f r o m f e r m e n t a t i o n b r o t h . B i o te c h n o l . B io e n g . 1 9,

8 0 1 - 8 1 9 .



1 7 3

4 Bu t l e r, J .N . ( 19 8 2) CO 2 E q u i l i b r i a a n d T h e i r A p p l i c a t i o n s , A d d i s o n W e sl ey , L o n d o n .

5 Sc h u m p e , A . , Q u i c k e r , G . a n d D e c kw e r, W. -D . ( 1 98 2 ) G a s s o l u b i l i t ie s i n m i c r o b i a l c u l t u r e m e d i a . A d v .

B i o c h e m . E n g . 2 4 , 1 - 3 8 .

6 H o , C . S . , S m i t h , M . D . a n d S h a n a h a n , J . E ( 19 87 ) C a r b o n d i o x i d e t r a n s f e r i n b io c h e m i c a l re a c t or s .

A d v . B i o c h e m . E n g . B i o t e c h n o l. 3 5, 8 3 - 1 2 5 .

7 P i r t , S . J . (1 9 75 ) P r i n c i p l e s o f M i c r o b e a n d Ce l l Cu l t i v a t i o n , B l a c k w e l l Sc i e n t i f ic Pu b l i c a t i o n s , O x f o r d .

7 a C a n o n g a t e T e c h n o l og y L t d (1 98 3) T h e d i f f u s i o n o f g as e s t h r o u g h a s i li c o n e r u b b e r m e m b r a n e , a n d

i t s a p p l ic a t i o n t o a n i n - li n e c a r b o n a t i o n m e t e r . P ro c . M e e t . M B A A S a n D i e g o .

8 H a y w a r d , A . C . ( 19 5 7) D e t e c t i o n o f g a s p r o d u c t i o n f r o m g l u c o s e b y h e t e r o f e r m e n t a t i v e l a c t i c a c i d b a c -

t e r ia . J . G e n . M i c r o b i o l . 1 6 , 9 - 1 5 .

9 G i b s o n , T . a n d A b d e l - M a l e k , Y . ( 19 45 ) T h e f o r m a t i o n o f c a r b o n d i o x i d e b y l ac t i c a c i d b a c t e r i a a n d

Bacillus licheniformis a n d a c u l t u r a l m e t h o d o f d e t e c t i n g t h e p r o c es s . J . D a i r y Res . 1 4 , 3 5 - 4 4 .

1 0 Ro g o s a , M . , Wi s e m a n , R .E , M i t c h e l l , J .A . , D i sr ae ly , M .N . a n d Be a u m a n , A . J . ( 1 95 3 ) Sp e c i e s

d i f f e r e n t ia t i o n o f o r a l l a c to b a c i l li f r o m m a n i n c l u d in g d e s c r i p t i o n s o f Lactobacillus cellobiosus nov .

spec . J . Bac te r io l . 65 , 681-699 .

1 1 E l d r e d g d e , E . E . a n d Ro g e r s , L . A . ( 1 91 4 ) T h e b a c t e r i o l o g y o f c h e e s e o f t h e E m m e n t h a l t y p e . Z e n t r a l b l .

Ba k t e r i o l . Pa r a s i t e n k d . I n f e k t i o n s k r . H y g . A b t . 2 . 4 0 , 5 - 2 1 .

1 2 W i l l ia m s , O . B . a n d C a m p b e l l , L . L . (1 9 52 ) T h e d e t e c t i o n o f h e t e r o f e r m e n t a t i o n b y l a c ti c a c i d b a c t e r i a .

Food Technol . 5 , 306 .

1 3 H a m m e r , B .W. a n d Ba k e r , M . E ( 19 2 3) S t u d i e s o n Streptococcus paracitrovorus g r o u p . I o w a A g r i c.

H o m e E c o n . E x p . S t n . Re s . Bu l l . 8 1 .

1 4 Sp e r b e r , W .H . a n d Sw a n , J . ( 19 7 6) H o t - l o o p t e s t f o r t h e d e t e r m i n a t i o n o f c a r b o n d i o x i d e p r o d u c t i o n

f r o m g l u c o s e b y l a c t i c a c id b a c t e r i a . A p p l . E n v i r o n . M i c r o b i o l . 3 1, 9 9 0 - 9 9 1 .

1 5 D i c k s , L . M . T . a n d v a n V u u r e n , H . J . J . (1 9 87 ) A m o d i f i c a t i o n o f t h e h o t - t u b e m e t h o d f o r t h e d e t e c t i o n

o f c a r b o n d i o x i d e p r o d u c e d b y h e t e r o f e r m e n t a t i v e Lactobacillus s t r a i n s . J . M i c r o b i o l . M e t h o d s . 6 ,

273 - 275.

1 6 M i l t o n , R . E ( 19 5 5) I l l u s t r a t iv e e x a m p l e s o f m i c r o v o l u m e t r i c p r o c e d u r e s . I n : M e t h o d s o f Q u a n t i t a t i v e

M i c r o - A n a ly s i s ( M i l t o n, R . E a n d W a te r s, W . A . , e d s. ), p p . 1 7 0 - 2 2 4 , E d w a r d A r n o l d , L o n d o n .

1 7 Co n w a y , E . J . ( 19 6 2) M i c r o d i f f u s i o n A n a l y s i s a n d V o l u m e t r i c E r r o r , C r o s b y L o c k w o o d & So n ,L o n d o n .

1 8 O b r i n k , K . J . ( 19 5 5) A m o d i f i e d Co n w a y u n i t f o r m i c r o d i f f u s i o n a n a l y s is . B i o c h e m . J . 5 9, 1 3 4 - 1 3 6 .

1 9 G r e e n b e r g , A .E . , C o n n o r s , J . J . , J e n k i n s , D . a n d F r a n s o n , M . A . H . ( 1 98 1 ) S t a n d a r d M e t h o d s f o r th e

E x a m i n a t i o n o f W a t e r a n d W a ste w ate r, A P H A - A W W A - W P C F , W a s h i n g to n .

2 0 V o g el , A . I. (1 9 59 ) A T e x t - Bo o k o f Q u a n t i t a t i v e I n o r g a n i c A n a l y s i s T h e o r y a n d P r a c t i c e , L o n g m a n s ,

G r e e n & C o ., L o n d o n .

2 1 M i l t o n , R . E ( 19 5 5) Co l o r i m e t r i c a n a l y s i s. In : M e t h o d s o f Q u a n t i t a t i v e M i c r o - A n a l y s i s ( M i l t o n , R . E

a n d W a t e r s , W . A . , e d s . ) , p p . 2 2 9 - 4 2 0 , E d w a r d A r n o l d , L o n d o n .

2 2 Sn e l l , E D . a n d S n e l l, C .T . ( 1 9 5 9 ) C o l o r i m e t r i c M e t h o d s o f A n a l y s is , V o l. I I A , D . V a n N o s t r a n d Co . ,

P r i n c e t o n , N e w J e r s e y .

2 3 R o z z i, A ., B u r t o n , K . W . a n d H a w k e s, D . L . (1 98 3) P o t e n t i o m e t r i c m e t h o d f o r th e d e t e r m i n a t i o n o f

c a r b o n d i o x i d e i n b i o g a s . J . A g r ic . E n g . Re s . 2 8, 5 0 5 - 5 1 2 .2 4 Sn e l l , E D . a n d Sn e l l , C .T . ( 1 96 1 ) Co l o r i m e t r i c M e t h o d s o f A n a l y s i s , V o l. I I, D . V a n N o s t r a n d Co .,

P r i n c e t o n , N e w J e r s e y .

2 5 M a x o n , W . D . a n d J o h n s o n , M . J . (1 95 2) C o n t i n u o u s p h o t o m e t r i c d e t e r m i n a t i o n o f c a r b o n d io x i d e

i n g a s s t r e a m s . A n a l . C h e m . 2 4 , 5 4 1 - 5 4 5 .

2 6 W i l s o n , K . M . ( 19 55 ) G a s o m e t r i c m e t h o d s o f m i c r o -a n a l y si s . I n : M e t h o d s o f Q u a n t i t a t i v e M i c r o -

A n a l y s i s ( M i l t o n, R . E a n d W a te r, W . A . , e d s. ), pp . 5 2 1 - 6 0 5 , E d w a r d A r n o l d , L o n d o n .

2 7 E l s w o r t h , R . ( 1 97 0) T h e m e a s u r e m e n t o f o x y ge n a b s o r p t i o n a n d c a r b o n d i o x i d e e v o lu t i o n i n s t ir r ed

d e e p c u l t u r e s . I n : M e t h o d s i n M i c r o b i o l o g y 2 ( N o r r is , J . R . a n d R i b b o n s , D .W . , e d s. ), p p . 2 1 3 - 2 2 8 ,

A c a d e m i c P r e s s , L o n d o n .

2 8 V a n S ly k e, D .D . a n d N e i l l, J .M . ( 19 2 4) T h e d e t e r m i n a t i o n o f g a se s i n b l o o d a n d o t h e r s o l u t i o n s b y

v a c u u m e x t r a c ti o n a n d m a n o m e t r i c m e a s u r e m e n t . J . B i ol . C h e m . 6 1, 5 2 3 - 5 7 3 .

2 9 D i x o n , M . (1 95 2) M a n o m e t r i c M e t h o d s a s A p p l i e d t o th e M e a s u r e m e n t o f C el l R e s p i r a t io n a n d O t h e rP r o c es s e s, T h i r d E d i t i o n , C a m b r i d g e U n i v e r si t y P r e ss , C a m b r i d g e .

3 0 U m b r e i t , W.W. , B u r r is , R . H . a n d S t a u f f e r , J . E ( 19 5 7) M a n o m e t r i c T e c h n i q u e s , Bu r g e s s Pu b l i s h i n g ,

M i n n e a p o l i s , M i n n e s o t a .



1 7 4

3 1 Ro z z i , A . a n d L a b e l l a r t e , G . (1 9 84 ) D i r e c t b i c a r b o n a t e d e t e r m i n a t i o n in a n a e r o b i c d i g e s t e r l i q u o r s b y

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g a l l iu m p h o s p h i d e p h o t o c a t h o d e i n a q u e o u s e l ec t ro l yt e s. B u l l. C h e m . S o c. J p n . 5 7 , 5 8 3 - 5 8 4 .

9 2 I k e d a , S ., Y o s h i d a , M . a n d I t o , K . (1 9 85 ) Ph o t o e l e c t r o c h e m i c a i r e d u c t i o n p r o d u c t s o f c a r b o n d i o x id ea t m e t a l c o a t e d p - G a P p h o t o c a t h o d e s in a q u e o u s e l e c t r o ly t e s . Bu l l . Ch e m . So c . J p n . 5 8, 1 3 5 3 - 1 3 5 7 .

9 3 T i n n e m a n s , A . H .A . , K o st er , T . P . M . , T h e w i s s e n , D . H . M .W . a n d M a c k o r , A . ( 1 98 2 ) Fo r m a t i o n o f

m e t h a n o l a n d o t h e r C 1 -C 3 c o m p o u n d s i n t h e p h o t o a s s i st e d re a c t i o n o f f o r m a l d e h y d e a n d w a t e r ov e r

s t r o n t i u m t i t a n a t e s u s p e n s i o n s c o n t a i n i n g t r a n s i t i o n m e t a l o x id e d e p o s i t s . N o u v . J . Ch i m . 6 , 3 7 3 - 3 7 9.

9 4 U l m a n , M . , T i n n e m a n s , A . H . A . , M a c k o r , A . , A u r i a n - B l a j e n i , B . a n d H a l m a n n , M . (1 98 2) P h o -

t o r e d u c t i o n o f c a r b o n d i o x i de t o f o r m i c a c i d , f o rm a l d e h y d e , m e t h a n o l , a c e t a l d e h y d e a n d e t h a n o l u s -

i n g s u s p e n s i o n s o f s t r o n t i u m t i t a n a t e w i t h t r a n s i t i o n m e t a l a d d i t i v e s . I n t . J . So l. E n e r g y 1 ,2 1 3 - 2 2 2 .

9 5 T h a m p i , K . R . , K iw i, J . a n d G r a t ze l , M . ( 19 87 ) M e t h a n a t i o n a n d p h o t o - m e t h a n a t i o n o f c a r b o n d io x i de

a t r o o m t e m p e r a t u r e a n d a t m o s p h e r i c p r e s s u r e . N a t u r e ( L o n d o n ) 3 2 7 , 5 0 6 - 5 0 8 .

9 6 F i s h e r , B . a n d E i s e n b e r g , R . ( 19 8 0) E l e c t r o c a t a l y i c r e d u c t i o n o f c a r b o n d i o x i d e b y u s i n g m a c r o c y c l e s

o f n i c k e l a n d c o b a l t . J . A m . Ch e m . So c. 1 0 2 , 7 3 6 1 - 7 3 6 3 .

9 7 L e h n , J . - M . a n d Z i e ss e l, R . (1 98 2) P h o t o c h e m i c a l g e n e r a t io n o f c a r b o n m o n o x i d e a n d h y d r o g e n b yr e d u c t i o n o f c a r b o n d i o x i d e a n d w a t e r u n d e r v i s i b le l i g h t ir r a d i a t i o n . P r o c . N a t l . A c a d . Sc i. U .S.A .

7 9 , 7 0 1 - 7 0 4 .

9 8 T i n n e m a n s , A . H . A . , K o ste r, T . P . M . , T h e w i s s e n , D . H . M . W . a n d M a c k o r , A . ( 19 84 ) T e t ra a z a -

m a c r o c y c l i c c o b a l t ( I I ) a n d n i c k e l ( I I ) c o m p l e x e s a s e l e c t r o n - t r a n s f e r a g e n t s i n t h e p h o t o ( e l e c t r o ) c h e m i -

c a l a n d e l e c t r o c h e m i c a l r e d u c t i o n o f c a r b o n d i o x id e . J . R . N e t h . C h e m . S oc . 1 0 3 , 2 8 8 - 2 9 5 .

9 9 G a m b a r o t t a , S . , S t r o l o g o , S ., F l o r i a n i , C . , Ch i e s i -V i l la , A . a n d G u a s t i n i , C . ( 1 98 5 ) S t e p w i s e r e d u c t i o n

o f c a r b o n d i o x id e t o f o r m a l d e h y d e a n d m e t h a n o l : r e a c t i o n s o f C O 2 a n d C O s - li k e m o l e c u le s w i t h

h y d r i d o c h l o r o b i s ( c y c l o p e n t a d i e n y l ) z i r c o n i u m ( I V ) . J . A m . Ch e m . So c . 1 0 7 , 6 2 7 8 - 6 2 8 2 .

1 0 0 H a w e c k e r , J ., L e h n , J . - M . a n d Z i e s s e l, R . (1 9 83 ) E f f i c i e n t p h o t o c h e m i c a l r e d u c t i o n o f CO 2 t o CO b y

v i s i b l e l i g h t i r r a d i a t i o n o f s y s t e m s c o n t a i n i n g Re ( b ip y ) ( CO ) 3 X o r Ru (b ip y )3 2 + - C o s + c o m b i n a t i o n s

a s h o m o g e n o u s c a t a l y s t s . J . C h e m . S o c . , C h e m . C o m m . 5 3 6 - 5 3 8 .

1 01 Ro b e r t s , J . L . a n d Sa w y e r, D .T . ( 1 96 5 ) V o l t a m m e t r i c d e t e r m i n a t i o n o f c a r b o n d i o x i d e u s i n g d i m e t h y l -s u l f o x i d e a s a s o l v e n t . J . E l e c t r o a n a l . Ch e m . 9 , 1 - 7 .

1 0 2 D i x o n , N .M . ( 19 8 8) E f f e c t s o f CO 2 o n a n a e r o b i c b a c t e r i a l g r o w t h a n d m e t a b o l i s m . Ph .D . T h e s i s ,

U n i v e r s i t y Co l l e g e o f Wa l es , A b e r y s t w y t h .

1 03 Fa t i l l o - F i lh o , O . , d e A n d r a d e , J . E , Su l e i m a n , A .A . a n d G u i l b a u l t , G .G . ( 1 9 89 ) P i e z o e l e c t r i c c r y s t a l

m o n i t o r f o r c a r b o n d i o x i d e in f e r m e n t a t i o n p ro c e ss e s. A n a l . C h e m . 6 1, 7 4 6 - 7 4 8 .

Diferent Metod Determination CO2

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