PRECIPITATE-BASED ION-SELECTIVE...

PRECIPITATE-BASED ION-SELECTIVE ELECTRODES

E. PUNGOR and K. TOni

Inst itute for General and Analytical Chemistry, Technical University,Budapest, Hungary

ABSTRACT

A critical and wide-ranging revicw of the whole field of precipitate-basedion-selective electrodes is presented. The current theories of the mechanismof the electrode response are discussed and related to the experimental data.The most important analytical and physicochemical applications of the elec-trode are summarized, and, in particular, some examples of the use of elec-

trodes to study complex formation are treated in detail.

INTRODUCTIONWhen Kolthoff79 first proposed fused silver chloride and silver bromide

discs for the detection of chloride and bromide in 1937, and later, in 1961,when we122 pointed out the possibility of highly selective direct measure-ment of iodide ion, using silver iodide precipitate incorporated in paraffinwax, it could not have been foreseen how great a development had beeninitiated. In contrast to the lack of success in the fifties in the search forion-selective electrodes, in the sixties many such electrodes, based onprecipitates, were developed (Table 1) and many studies of their mostimportant parameters were carried out.

Parallel with the discovery of other types of electrodes not based onprecipitates. great efforts have been put into clarifying the mechanism of theelectrode response of the precipitate-based type, and in finding new lines ofdevelopment for novel ion-selective electrodes. Unfortunately, in thisrespect we are still very much in the early stages and so it is understandablethat there has been practically no increase in the range of selective electrodes(with respect to the ionic species) available since the middle of the sixties.

In this paper the conclusions and data are drawn partly from the literatureand partly from our findings for the precipitate-based electrodes. Opinionsgiven in the literature are contradictory in some cases and we propose toadd our own comments on some of them later. Furthermore, on the basisof up-to-date results we will offer our opinion about further developments.

We use the nomenclature 'precipitate-based electrodes' instead of solid-state electrodes because there are other types of solid-state ion-selectiveelectrodes such as ion-exchange electrodes. In all solvents the properties ofthe precipitate-based electrodes depend on the solubility of the activecomponent of the electrode in the appropriate solvent, whereas the propertiesof other types of solid-state electrode will depend on other parameters.

105

Tab

le I.

Pre

cipi

tate

-bas

ed

ion-

sele

ctiv

e el

ectr

odes

Sens

itive

to

ion

Solid

stat

e A

ctiv

e m

ater

ial

Mat

rix

Sele

ctiv

ity (

Com

men

ts)

Wor

kers

R

efer

ence

s

Ca2

+

Het

erog

eneo

us

Ca-

oxal

ate a

nd

Para

ffin

+ n

on-i

onic

Fa

irly

acc

epta

ble'

T

ande

loo

and

Kri

ps

167,

168

othe

r Ca-

salts

de

terg

ent

on g

auze

(1

957)

Ba2

' B

a504

SO

j H

eter

ogen

eous

B

aCrO

4 Pa

raff

in w

ithou

t gau

ze

Not

per

fect

ly s

elec

tive

Fisc

her

and

Bab

cock

46

to

eith

er a

nion

s or

(195

8)

catio

ns.

(Slo

w to

atta

in

Fisc

her

and

Yat

es (

1960

) 47

equi

libri

um)

K

Het

erog

eneo

us

K-t

etra

phen

ylbo

rate

Poly

styr

ene

+ ga

uze

Not

sel

ectiv

e T

ande

loo a

nd K

ripp

s (1

959)

16

9

Ca-

+

Het

erog

eneo

us

Ca-

stea

rate

Pa

raff

in a

s ab

ove

Stro

nger

res

pons

e to

Tan

delo

o an

d va

n de

r 17

0

Ca2

+ th

an o

xala

te. N

o V

oort

(196

0)

resp

onse

to K

+

Z

Ca2

+

Het

erog

eneo

us

Ca-

oxal

ate

Para

ffin

+ de

terg

ent

Non

-spe

cifi

c, p

orou

s C

loos

and

Frip

iat (

1960

) 28

('Exh

ibit

mem

ory'

) H

eter

ogen

eous

A

g!

Para

ffin

K

CI

does

not

inte

rfer

e Pu

ngor

and

Hol

lós-

12

2 >

. (±

SmV

rep

rodu

cibi

lity)

R

okos

inyi

(196

1)

Mg2

', Sr

2 M

ultil

ayer

C

a-st

eara

te

Gre

gor,

Gla

tz a

nd

55

Mg-

stea

rate

Sc

honh

orn (

1963

) r

F

Het

erog

eneo

us

Agi

Si

licon

e ru

bber

10

M

KC

I do

es n

ot

Pung

or, T

óth

and

Hav

as

130

, in

terf

ere

(196

6)

SO

Het

erog

eneo

us

BaS

O4

Silic

one

rubb

er

Phos

phat

e in

terf

eres

Pu

ngor

, Tót

h an

d H

avas

11

9, 1

20,

(196

4)

121

Ag'

,X

AgX

PO

H

eter

ogen

eous

M

n P0

4 Si

licon

e rub

ber

Hig

h se

lect

ivity

(th

e Pu

ngor

, Hav

as a

nd T

óth

126,

131

Al3

t A

l-ox

ine

elec

trod

es fo

r P0,

(1

965)

N

i2

Nic

kel-

DM

G

Al3

', N

i2',

Bi3

' are

B

i3 , P

0 —

B

iPO

4 no

t sel

ectiv

e)

Co2

' H

eter

ogen

eous

C

o3(P

04)2

C

ollo

dion

, pa

raff

in

Lev

elle

d by

KC

I, L

i250

4 M

oraz

zani

, Pel

letie

r and

10

5

(por

ous m

embr

ane)

B

affl

er (1

965)

Hom

ogen

eous

R

are

eart

h —

(S

ingl

e cr

ysta

l)

Fran

t and

Ros

s (19

66)

49

fluo

ride

s H

t OH

- T

i02,

Fe2

03, 5

n02

Poly

ethy

lene

N

a Z

r02

Pol

ypro

pyle

ne

Al3

+

Het

erog

eneo

us

A12

03

Para

ffin

(T

itrat

ions

only

stu

died

) G

eyer

and

Syri

ng (1

966)

53

SiFt

, K

K2S

iF6

Aga

r or p

aper

B

oth

ions

A

g4Fe

(CN

)6

Aga

r B

oth

ions

Pb

WO

4 Pa

raff

in

Ca2

H

eter

ogen

eous

C

a-ox

alat

e Pa

raff

in+

non-

ioni

c N

ot c

ompl

etel

y Sh

atka

y (19

67)

12,

155

dete

rgen

t + ga

uze

perm

sele

ctiv

e; n

ot

spec

ific

. (G

auze

nece

ssar

y fo

r con

duct

ion.

No

en

diff

eren

ce b

etw

een

pure

pa

raff

in a

nd p

araf

fin +

C

a-ox

alat

e)

F-

Hom

ogen

eous

L

aF3

(For

mic

roch

emic

al

Dur

st an

d T

aylo

r (19

67)

40, 4

1 an

alys

is)

SO

BaS

O4

3 .

3t

.. .

. P

04

Het

erog

eneo

us

BiP

O4

Silic

one

rubb

er

Rec

hnitz

, L

in an

d Z

amoc

hnic

k 138

en

(196

7)

a A

g, S

2 H

omog

eneo

us

Ag2

S L

ight

and

Sch

war

tz (

1968

) 92

5

BaS

O4.

R

espo

nd to

tota

l fre

e Z

N

i-D

MG

ca

tions

I

Div

alen

t ion

s H

eter

ogen

eous

C

o3(P

04)2

Pa

raff

in

(Con

ditio

ning

very

. B

ucha

nan a

nd S

eago

23

r

Ni3

(P04

)2

Sili

cone

rubb

er

impo

rtan

t cry

stal

line

(196

8)

CuH

PO4

form

; hyd

ratio

n and

M

nHPO

4 an

ion

not i

mpo

rtan

t)

J M

nCO

3 H

eter

ogen

eous

C

aF2

Sili

cone

rubb

er

Sel

ectiv

e res

pons

e to F

T

hF4

Not

rep

rodu

cibl

e M

cDon

ald a

nd T

óth

103

LaF

3 Se

lect

ive

resp

onse

to

F (1

968)

A

gt s2

H

eter

ogen

eous

A

g25

Sili

cone

rubb

er

Una

ffect

ed by

C1,

Pu

ngor

, Sch

mid

t an

d T

óth

125

BC

, F

(196

8)

a C

r A

gC1

BC

H

omog

eneo

us

AgB

r Sm

, Pr,

Eu,

Ho

dope

d V

esel

y an

d Ji

ndra

(196

8)

177

I—

AgF

si

ngle

cry

stal

L

aF3

NO

; H

eter

ogen

eous

N

i(N

03)2

Ph

OH

+ H

CH

O+

NH

3 R

espo

nsiv

e to

uni

vale

nt

Dob

beis

tein

and

Die

hl

36

anio

ns a

nd H

; (1

969)

un

resp

onsi

ve to

m

ultiv

alen

t ani

ons a

nd

catio

ns

Tab

le 1

(con

tinue

d)

Sens

itive

to

ion

Solid

sta

te

Act

ive

mat

eria

l M

atri

x Se

lect

ivity

(Com

men

ts)

Wor

kers

R

efer

ence

s

F Cu2

+

Cu2

+

s2-

Cu2

+

—

2+

o °°

Cl0

NH

PO -

Het

erog

eneo

us

Inor

gani

c Si

licon

e ru

bber

G

uilb

ault

and

Bri

gnac

58

phos

phat

e sal

ts

(196

9)

1, B

C. C

1 H

eter

ogen

eous

A

gI

AgB

r, A

gCl

Poly

ethy

lene

M

asci

ni a

nd L

iber

ti (1

969)

10

0

Cd2

+

Hom

ogen

eous

C

dS +

Ag2

S B

rand

, Mili

tello

and

Rec

hnitz

(19

69)

14

Hom

ogen

eous

E

u'1'

dop

ed s

ingl

e cr

ysta

l W

illia

ms

(196

9)

180

Het

erog

eneo

us

Cu2

S Si

licon

e ru

bber

H

irat

a an

d D

ate

63

r1

Hom

ogen

eous

Het

erog

eneo

us

Cu2

S

Ag2

S Po

lyet

hyle

ne

Cer

amic

mem

bran

e (1

970)

H

irat

a, H

igas

hiya

ma a

nd

Dat

e (19

70)

Mas

cini

and

Lib

erti

(197

0)

65

101

a

Hom

ogen

eous

A

g2S +

MeS

(P

aten

t)

Fran

t and

Ros

s (1

970)

51

Hom

ogen

eous

Pb

2, B

i3t Y

3 Sc

3 or

L

anth

anid

e F

Non

-pol

ariz

ed

ion-

se

lect

ive

elec

trod

e (S

emic

ondu

ctor

m

embr

ane a

nd a

Pb

and'

or B

i met

al

cont

act)

(pa

tent

)

Farr

en a

nd S

taun

ton

(Per

kin—

Elm

er) (

1970

) 45

' Z H

eter

ogen

eous

A

liquo

t 336

E

poxy

tre

ated

gla

ss fr

it T

ated

a, F

ritz

and

Itan

i (1

970)

16

5

Het

erog

eneo

us

K-s

alt b

iolo

gica

l m

ater

ial

Litt

le in

terf

eren

ce f

rom

ot

her c

omm

on ca

tions

(B

eckm

an M

odel

396

22)

Kru

ll, M

ask

and

Cos

grov

e (1

970)

83

Het

erog

eneo

us

NH

4-sa

lt bi

olog

ical

m

ater

ial

Litt

le in

terf

eren

ce f

rom

ot

her c

omm

on ca

tions

(B

eckm

an M

odel

396

26)

Cos

grov

e, M

ask

and

Kru

ll (1

970)

32

F H

omog

eneo

us

LaF

3 Sm

, Pr,

Eu,

Ho,

Nd

dope

d L

aF3

sing

le

crys

tal

Ves

ely

(197

1)

175

Pb2

+

Hom

ogen

eous

Pb

S C

eram

ic m

embr

ane

Hir

ata

and

Hig

ashi

yam

a 67

(1

971

Pb2t

H

eter

ogen

eous

Pb

S Si

licon

e ru

bber

H

irat

a an

d D

ate (

1971

) 64

Cu2

t H

eter

ogen

eous

C

uS +

Ag2

S

Poly

ethy

lene

M

asci

ni a

nd L

iber

ti 10

2

(197

1)

S2

Ag2

S Pp

t. on

the

surf

ace

of

Hal

ides

A

g-ha

lides

a g

raph

ite ro

d R

uzic

ka an

d L

amm

14

5, 1

46

Hg2

+

HgS

(S

elec

trod

e—th

e uni

vers

al (

1971

)

Agt

H

omog

eneo

us

Ag2

S io

n-se

lect

ive

solid

-sta

te

Cu2

+

CuS

el

ectr

ode)

Pb

2t

PbS

Cd2

CdS

Cu2H

Ag

Hom

ogen

eous

T

CN

Q s

alts

Sh

arp

and

Joha

nsso

n 15

4

NH

: (1

971)

6

e H

omog

eneo

us

CuS

G

ordi

evsk

ii et

al.

(197

1)

163

v205

C

N

Hom

ogen

eous

A

g ((

3-gl

ucos

idas

e in

Rec

hnitz

and

Lle

nado

13

9

acry

lam

ide

gel)

(1

971)

Cu2

H

omog

eneo

us

Cu2

S G

ordi

evsk

ii et

a!.

(197

1)

164

Pb2

+

Hom

ogen

eous

Pb

S C

eram

ic m

embr

ane

Hir

ata

and

Hig

ashi

yam

a 66

Ag2

S w

ith s

inte

red m

etal

(1

971)

Cu2

5 su

lphi

des

CN

H

omog

eneo

us

AgI

+ A

g2S

Flee

t and

Sto

rp (

1971

) 48

Cl

Het

erog

eneo

us

AgC

l M

ethy

l m

etha

cryl

ate

(Pat

ent)

N

eff,

Sam

buce

tti a

nd C

arlo

s 10

7

(197

1)

c C

st

Het

erog

eneo

us

Cs 1

2-m

olyb

do-

Silic

one

rubb

er

Coe

tzee

and

Bas

son (

1971

) 29

phos

phat

e C

t

Cu2

t H

omog

eneo

us

Cu1

, Se

(S

ingl

e cr

ysta

l)

Ves

ely

(197

1)

176

Hg2

t Cu2 +

Cd2

t Pb2t

Hom

ogen

eous

A

g2S

+ M

eS

On

the

surf

ace

Anf

ält a

nd J

agne

r (19

71)

4

Zn2

t of

a s

ilver

wir

e

Ni2

t C

o2t

Tab

le I

con

tinue

d)

Sens

itive

to

ion

Solid

stat

e A

ctiv

e m

ater

ial

Mat

rix

Sele

ctiv

ity (C

omm

ents

) W

orke

rs

Ref

eren

ces

Cu2

A

g H

omog

eneo

us

Sele

ctiv

ity,

sens

itivi

ty,

Ada

met

zova

and

Gre

gr

1

S2

infl

uenc

e of

pH. r

espo

nse

(197

1)(C

rytu

r m

odel

) C

N

time

are

disc

usse

d Z

C

a2

Hom

ogen

eous

C

aF2

+ La

F3

Mg2

, Sr2

, Pb2

, Ba2

, Fa

rren

(197

1)

44

Na,

K io

ns in

terf

ere

(Per

kin—

Elm

er)

Ca2

+

Het

erog

eneo

us

Ca-

dide

cylp

hos-

5 °.

PV

C i

n N

a, M

g, N

i, B

a, S

r,

Cat

tral

l and

Fre

iser

27

phat

e in

dio

ctyl

- cy

cloh

exan

one

Pb, Z

n in

terf

ere.

(1

971)

0'

ph

osph

onat

e C

oate

d w

ire

elec

trod

e

Cu2

H

eter

ogen

eous

C

u2 _

S Si

licon

e ru

bber

V

ery

high

sel

ectiv

ity

Pung

or, T

óth

and

Pick

13

2

for t

he c

oppe

r (1

972)


The precipitate electrodes can be prepared in three ways: from singlecrystals; from pressed crystals, and from crystals embedded in a suitablematerial. The first two kinds of electrodes form the group of homogeneouselectrodes, while the third type gives the heterogeneous electrodes. However,no basic difference exists between these two types of electrodes (except forsome effects, which have not yet been explained). There are of course,differences between the mechanical properties of the electrodes, their life-time and so forth, but from the electrochemical point of view they shouldbe treated together.

With regard to the construction of the electrodes, the electrode membranes,either homogeneous or heterogeneous, are sealed onto the end of glass orplastic tubes, or screwed on with the help of an inert material which doesnot swell or cause a short circuit between the two sides of the membranelayer.

THEORETiCAL SECTIONMechanism of the electrode response

The correlations and ideas derived from glass and the so-called ion-exchange electrodes greatly helped in the theoretical description of theprecipitate-based electrodes. From these guidelines the electrode behaviourmay be interpreted in two ways. The first is based on ion transport throughthe membrane, derived for ion-exchange electrodes. The second is relatedto the theory given for the mechanism of the glass electrode, in which it isproposed that different ions take part in the ion-transport phenomenonthan in the surface equilibria.

At the beginning of our research we tried to interpret the membranephenomena by ion-transport122. However, later on, experimental resultswere obtained which were contradictory. No electrochemical difference wasfound experimentally between electrodes made as a sandwich (silver orplatinum plates between two membrane layers assuring electrical contactonly through the metal) and normal electrodes' 18

Our opinion about the surface ion-exchange reaction was strengthenedby radioactive measurements of the exchange rate of the iodide ions on thesilver iodide precipitate-based electrodes' 18 The rate of the exchangereaction was fast and was dependent on the surface charge of the precipitateembedded in the electrode membrane. Of course the charge transportthrough the crystal and its mechanism are also very important in theinterpretation of the behaviour of the electrodes. In this context we wouldlike to mention the paper of Durst and Ross42, describing the application ofan LaF3 electrode doped with Eu for the coulometric generation of F,with an efficiency of 99.2 per cent. Sher and co-workers' showed that theLaF3 electrode has a conductivity of iO ohm' cm' which can beexplained by the following reaction:

LaF3 + vacancy —* LaF + F

Silver halides and silver sulphide are also ionic conductors (silver suiphideis partly an electric conductor). By analogy with LaF3, Ross'42 has stated

111

E. PUNGOR AND K. TOTH

that the theory of solid-state electrodes is simple because no ions other thanthose composing the crystal go through the membrane, and the potential isa result of the ion transport through the membrane. This statement showsthat he has not taken into consideration the ion-exchange theory describedin the literature24' 1 18, Brand and Rechnitz'5, using impedance measure-ments, have provided further evidence for the surface-exchange theory.However, their detailed discussion seems to be scarcely justified by thesmall amount of experimental data. In their work they classify four types ofelectrodes, according to electrical analogies to the response mechanism.The first group, to which the LaF3 electrode belongs, have, in the analogy,two frequency-dependent impedances in series, one of which is accountedfor by the La(OH)3 film on the surface of the electrode, while the other isdue to the LaF3 itself. Brand and Rechnitz assume that there is no differencebetween the fluoride transport in the two layers. They put the silver chlorideelectrode into the second group, where there is no surface film on the silverchloride. According to them, in both groups the halide is transportedthrough the membrane and ion-exchange equilibria are established on thesurfaces. In the third group, consisting of Br, 1, S2, Pb24 and Cd2electrodes and in the fourth group, the copper electrode, ion transportthrough the membrane has not been proved, and in an electric field chargeaccumulates on the surface. The authors are of the opinion that measurementof the electrode impedance is a good way of evaluating the rate of the ion-exchange reaction on the surface.

A new contribution has recently been made by Hirata and co-workers6466'67 to the study of the electrical conductivity of the membranephase. They found that electrically conducting sintered PbS has no responseto Pb2 In spite of this the electrode functions if the sintered phase containsa large amount of Ag2S in addition to PbS. In this case ion transport throughthe membrane and the exchange reaction on the surfaces are affected bydifferent ions. It is interesting to mention that they found a Pb2 responseif the PbS was incorporated into silicone rubber. This phenomenon so far,however, cannot be explained.

The importance of surface ion-exchange reactions in the response mecha-nism has been underlined by our experiments carried out studying theadsorption of various anions on the surfaces of Ag! precipitates' 17,124,149From these experiments, it was concluded that halide and pseudo-halideions, after losing their coordinated water, were adsorbed on the surface ofthe silver iodide, while other ions such as PO and SO are adsorbedwithout loss of the coordinated water. Furthermore, the investigationshowed that the anions forming slightly soluble silver salts are exchangedin a very fast reaction by other anions which form less soluble salts withsilver.

Since the development of the precipitate-based ion-selective electrodes ithas been stated unambiguously that electric conductivity is a necessarycondition for the electrode response in addition to the surface-exchangeequilibria. The conductivity can be assured by crystal defects or doping ofsingle crystals as suggested by Frant and Ross49 who used Eu for dopingLaF3. The last method needs further investigation as emphasized by thework of Vesely' who found that there is no unambiguous correlation in

112

PRECIPITATEBASED ION-SELECTIVE ELECTRODES

the case of the LaF3 electrode between the doping and the electrochemicalbehaviour. His finding is in accordance with the opinion of Lingane94 whobelieves that doping is unnecessary.

From our research98 we have concluded that there is a correlation betweenthe number of crystal defects and the standard potential of the electrode.Table 2 shows the difference between the standard potentials of electrodesof the second kind and the corresponding ion-selective electrodes, the activecomponent of which has been exposed to light for different time intervals.

Table 2. Effect of exposure time on the differences between the potentials of membrane electrodesand electrodes of the second kind

Membrane electrode Exposure time (mm) EsE (mV)

AgCI 51020

807260

AgBr 5102030

15713913173

AgI 102030

177151104

As the slopes remained constant the calibration curves of the electrodeswere shifted parallel to each other. In a simple case when the membraneprecipitate consists of univalent ions, there is a correlation between theshift of the standard potential and the number of crystal defects. Thiscorrelation is as follows:

AE=Jln(ax) (ic)lwhere K is the solubility product of the precipitate, K is the equilibriumconstant for the formation of the defect in the crystal, and (ax). is the activityof the component X in the crystal and varies between 1 and K.

The electrode function, however, is not influenced by the standardpotential. This was proved not only by us but also by Loshkarev andYudina95, who found this for the sulphide electrodes.

The migration of the crystal defects can be described by the Nernst—Plank equation. Buck24 has applied this equation for the derivation of anexpression for the lower detection limit of ion-selective electrodes andreached basically the same conclusion which we reached using the theory ofDonnan potentials established on the surfaces131.

E — 21ln [CAg+(_L) + [Cg+(_L) + 4K9(y±Y2]1 YAg+(_L)—F [ CAg+(L) + [Cg+(L) + 4K59(y)2] i YAg+U)

where (L) and (—L) denote the two sides of the membrane and is thesolubility product.

113


For the calculation of the lower detection limit Covington33 gave asimpler equation, which also takes into account the solubility of silver halidesin halide solutions.

Naturally this lower detection limit varies from electrode to electrode andwith the medium used for measurements. Furthermore, the limit shifts tolower concentrations with higher ionic strength. Shifts of several decadescan be achieved by alteration of some parameters—as will be discussedlater on.

In solutions containing more than one potential-determining ion, themembrane potential of the precipitate-based electrodes can be described onthe basis of the ion-exchange equilibrium by the following equation'18,which is analogous with the Nikolsky equation

E = E0 + Tlna1 EKIkF k a1

where KIk is the selectivity constant of the electrode. (We have derived anequation for the calculation of Kk, which is given later.) Accordingly theselectivity constant may be determined unequivocally from the solubilityproducts of the precipitates built into the membrane and also those formedduring the exchange reaction.

Buck24 has modified this concept and stated that it is valid only for suchimmiscible precipitates as silver chloride and silver iodide, and he hasproposed a more complex expression used for mixed crystals such as mixedLa(OH)3 and LaF3, for which the selectivity constant is:

UOH Y0H1'OH.F X

UF Yr

where u is the mobility of the ion in the crystal, and y is the activity co-efficient in the crystal.

For the practical determination of the selectivity constant the methodused by Eisenman43 for glass electrodes was first employed. This calculationis based on the potential difference measured with the electrode in solutionscontaining only the reversible ion, and then in solutions containing onlythe interfering ion, both solutions containing the same counter-ions withactivity 0.1 M. The equation for the calculation is

AE = !JlnK

This method has a fundamental error. It ignores the fact that the concentra-tion of the potential-determining ion, when in a pure solution of the inter-fering ion, is determined solely by the dissolution of the precipitate. So themeasuring conditions do not relate to the required conditions and are notwell defined: these defects in the measuring conditions are admirablyexemolified by the poor experimental results'36.

114

Tab

le 3

. Se

lect

ivity

con

stan

ts o

f hal

ide i

on-s

elec

tive

elec

trod

es f

or so

me a

nion

s.

K de

note

s val

ues

mea

sure

d by

dir

ect

met

hod;

K8

deno

tes v

alue

s m

easu

red b

y in

dire

ct m

etho

d

(M

Chl

orid

e-se

lect

ive

elec

trod

e B

rom

ide-

sele

ctiv

e el

ectr

ode

Iodi

de-s

elec

tive

elec

trod

e

Ion

KIk

cal

c.

K, m

eas.

K

8 m

eas.

K

k ca

lc.

K m

eas.

K

* m

eas.

K

1k c

aic.

K

mea

s.

K*

mea

s.

C1

1 1

2.0

x iO

1.

8 x

iO

6.0

x iO

9.

6 x

1O

3.7

x B

r 1

1 2.

0 x

101

2.1

x 10

1.

8 x

SCN

1.

5 x

10°

0.2

x 10

° 3.

0 x

t04

2.4

x to

0H

1.

0 x

10-8

0.

9 x

10_8

C

rO

5.2

x 1O

4.

5 x

1O

2.5

x iO

1.

! x iO

5.

0 x

10"

6.6

x 10

_li

CO

6.

3 x

iO

4.6

x i0

3.

1 x

10

1.0

x 10

PO

1.

3 x

1O

0.5

x i0

6.

3 x

iO

3.1

x i0

1.

2 x

10- 10

0.

2 x

10_b

A

sO

3.3

x i0

" 2.

0 x i0

1.

6 x

10-6

1.

2 x

t0_6

3.

2 x o

10

2.6

x o

10

Fe(C

N)r

2A

x 1

06

3.5

10

I—

1 1

C

-l

H

Ct

rn C 2 r rn

C

H r rn

C

H

C

rn

Ct

E. PUNGOR AND K. TOTFI

Therefore we have worked out methods'28, on a theoretical basis, for thedetermination of the selectivity constant. In this calculation the activities atthe coprecipitation point have been used. The expression for the selectivityconstant is as follows:

s 1/a ( 'tb/a— —- j7 -

jk kJSand the activities at the coprecipitation point are substituted in the calcula-tions. In the formula, S and SJk are the solubility products of precipitates,('j)a (Ii)b. and (I) (Ik)m, while (a,)5 and (ak). are the activities of theprincipalion and the interfering ion at the coprecipitation point. The determinationof the selectivity constants can be carried out either by direct or indirectmethods. Some results obtained by such methods are collected in Tables 3,4 and 5.

Table 4. The selectivity constants of the sulphide-selective electrode for some anions

pK5Interfering anion (X) Measured Calculated*

7.8 7.85-8.60Br' 11.6 11.51—12.09(TV 14.2 14.10—14,500H 16.2 I.00—1(.73SCN 12.1 11.86—12.13

SO 21.2 22.00

PO 16.1 14.75—18.45

CN 3.20 3.30 5.96

S2O 10.70 10.47-11,42

* The values were calculated using the highest and lowest solubility constant data of Ag2S found in the literature.

In these tables the calculated and measured selectivity constants arecompared and they are in good agreement. Furthermore, it can also be seenthat the selectivity of some electrodes to many ions is so good that thereare practically no interferences to the electrode response.

Naturally, the electrode potential is a result of the difference in the electro-chemical potentials of solvated and desolvated ions, which are bonded in

Table 5. The selectivity constants of the sulphide-selective electrode for some cations

PKAg. YInterfering cation (Y) Measured Calculated*

TI 24.2 24.48—29.25Cu2 13.7 10.91-13.30Pb2 21.8 20.93—21.80Cd2 22.4 20.46—22.30Ni2 23.3 21.70-28.90

Zn2 27.9 22.27-26.80Fe2 31.5 26.82-31.20Mn2 36.2 33.24—38.80La3 40.3 44.17

* The values were catculated iiiig he highest an d lowest soluhility constant data of Ag2S found in the literature.

116


the crystal lattice. In this respect the research of Morf and Simon'°6 isvery valuable; they proposed a rather simple method for the calculation ofthe hydration enthalpy of the cations. A similar method for anions wouldbe welcome for the theoretical interpretation of anion-selective electrodes.

Many authors have investigated the effect of complexes on the precipitate-based electrodes. The studies have been carried out in two directions. Thefirst was to clarify the nature of the electrode response in solutions containingcomplexes of the appropriate ion. Secondly, it was fascinating to study thebehaviour of the electrodes in solutions of ions which dissolve the precipitateas a complex.

A further complication in the measurement of ions which form complexesis that they may react with the solvent, for example basic anions, such ascyanide, will form acids. Using sulphide and cyanide anion-selective elec-trodes it was clearly demonstrated that they measure only freeanions'25' 148, 173

pH7 8 9 10 11 12 13

I I I I I II /,I /

pkd?d2.65 1

toW2

Ar,,, ,A' /Ar o,Ar/ .3

Ar,C' 'Ar,ot U)Ar' " a0/',Ar,

Ar,0,,0/,'1/Ar',, -6

/,/.7//

Figure 1. Relation between pH and the sulphide concentration measured with a sulphide-selective electrode in Na2S solutions

For sulphide the results, shown in Figure 1, give good agreement betweenthe theoretical and experimental values. Wemeasured, at 25°C at an ionicstrength of 0.1 M, a value of pK2 = 12.65 compared to the literature valuepK2 = 12.92. Hseu and Rechnitz69 using the Orion sulphide electrode

117


measured the actual suiphide concentration at various pHs. On the basisof their experimental results, they calculated a value for pK2 which agreedwith one other determination (pK, 14.92) made more than forty yearsago20' 21 78, Since the difference between the pK value of Hseu and Rechnitzand that found by us is quite large, and because we have repeated suchmeasurements and calculations with different systems with unexceptionableresults, we concluded that their results must remain open to doubt.

In the case of cyanide we determined the stability constant and found avalue of 9.20 which is in agreement with the literature value'73.

Various authors investigated the association and dissociation problemsof fluoride. Shrinivasan and Rechnitz'57 and later on, Vanderborgh'74published results for the stability constant of hydrogen fluoride using afluoride ion electrode, which agree well with literature data publishedpreviously. For example, Broene and de Vries'9 using a glass electrodefound pK = 3.173 at 25°C and Vanderborgh using an Orion fluorideelectrode found a pK value of 3.189 at 25°C.

If any ion present in the solution is a complex ing agent, then it may reactwith the membrane. A good example of this is cyanide ion and silver halidemembrane electrodes. Due to the formation of complex silver cyanide, thecyanide ion liberates halide on the surface of the electrode, which diffusesin the opposite direction to the cyanide ion reaching the surface from thebulk of the solution. From this the actual cyanide concentration at thesurface may be assumed to be virtually zero, except at very high cyanideconcentrations. This was the basis for our deduction of the dependence ofthe potential on the cyanide activity.

E = E0 + 1n (a + KaN)

where a,( is the activity of halide liberated and present at the surface of theelectrode this will equal aCN in the absence of the appropriate halide inthe solution. K is the dissolution constant, for which we173 have foundexperimental values of between I and 0.1. The electrode behaviour, indicatedby the equation, is illustrated in Figure 2.

Concerning the details of the mechanism, it is doubtful whether the reaction

AgX + 2CN Ag(CN) + Xalone determines the electrode response. Fleet and Storp48, who investigatedthe iodide-based cyanide electrode, propose that the simple precipitation-exchange reaction also plays a role:

AgX + CN AgCN + X

Silver halides can be completely dissolved in a cyanide solution when thecyanide present is equivalent to that of the halide. However, if complexformation needs a large excess of complexing agent, the electrode alsoresponds to that kind of complexing ion. This is the situation when an iodideelectrode is used for the determination of In this case the K valuelies between 102 and iO. The same is also true when measuring cyanidewith a sulphide electrode.

118


E(mV)

1001

0 1 2-log aCN-

Figure 2. Calibration curve for the determinationpoint.

of cyanide; —,calculated curve ;Q, measured

Whatever basic halide electrode is used for the determination of cyanidethe electrode responds with the same efficiency to the cyanide as to theanion composing the electrode. The selectivity constant for other ionsremains the same as it would be in the presence of the anion correspondingto the appropriate electrode. This was one of the fundamental experimentson the basis of which we deduced the potential equation mentioned pre-viously. This is demonstrated in Table 6, where an iodide-based electrodewas used and selectivity constants for various ions in the presence of bothcyanide and iodide ions in the solution were determined. It is worth notingthat the complexing agent destroys the electrode, and so the lifetime ofelectrodes is limited.

If the anion investigated forms complexes with metal ions, then the elec-trode potential allows us to observe the complex formation and to getinformation about the stability data. Szabót62 has been able to demonstrate,by using an iodide-selective electrode, the formation of the I complex.This was also shown by Liberti and Mascini88. There are now many resultsof complex formation studies in the literature obtained from work withion-selective electrodes. It is generally found that complexes of low stability

Table 6. Potentiometric selectivity of iodide membrane electrodes

IonSelectivity constants in

CNthe presence of

L

CL 10_5_106 10_bBr io—io' iO'1 I ——

CN — 1

NH 10_s_b_b 10°SO bo_5._lo_6 io

119

I

3 4


produce no interference with the electrode response. For example Rech-nitzt36' 137 has shown that metal ions forming complexes of low stabilitywith halides do not influence the calibration curve of halides. In the presenceof stable complexes the free ligand activity or that of complex-formingcations can be measured. In our laboratory173 we have investigated cyanidecomplexes and found that one can distinguish, in the case of silver the twocomplexes, Ag(CN) and Ag(CN); for copper the Cu(CN) ion, andfor mercury and nickel the Hg(CN) and Ni(CN)ions. No cyanidecomplexes of Cd and Zn could be observed. Fleet and Storp48 explainedthis finding by saying that the hydroxide complexes were formed preferentiallyto the cyanide complexes.

We should also like to mention a very detailed study of the copper(ti)complexes of ethane- 1-hydroxy- 1, 1-diphosphoric acid, which was made witha copper electrode by Wada and Fernando' 78 They compared their resultswith those obtained by spectrophotometry and by potentiometry using aglass electrode. This study underlined the advantages of the copper electrodefor such purposes.

Kinetic studies of the formation of FeF2 + and A1F2 + complexes havebeen performed by Shrinivasan and Rechnitz'5 .Theywere able to distinguishthe kinetic steps of A1F2 + complex formation in the presence of aqueousand hydroxo-aluminium complexes.

A further example of a study of metal—fluoride complex formation isprovided by Bond and Hefter'3 who calculated the formation constants oflead fluoride complexes using a fluoride electrode.

Very important results concerning the lower detection limit of electrodeshave been obtained by the investigation of complexes. Our investigationswith cyanide lend weight to the fact that the lower detection limit shiftsfurther down in the presence of complexes. At the same time Mesmer'°4 hasdemonstrated similar phenomena with beryllium complexes, with which thelowest detectable fluoride concentration in 1 M sodium chloride solutionwas7 x 108M.

Aziz and Lyle5 successfully investigated fluoro-complexes of the metalsgadolinium, europium, scandium, iron, magnesium, and calcium: the fivalues are all lower than 1O I mol -' and this was found to be the limit ofthe method. Their investigation therefore excluded a study of the thoriumcomplexes. In spite of this, Baumann'° reports that she investigated thezirconium, thorium and lanthanum fluoro-complexes in solutions of ionicstrength of 0.5 to 2 M. Among the various complexes, she was able to dis-tinguish the ZrF3 + and ZrF + complexes for zirconium, the ThF3 + andThF + complexes for thorium and the LaF2 + complex in the case oflanthanum.

One problem encountered in this work is that lanthanum fluoride influ-ences the fluoride measurement by the dissolution of its surface, whichreleases fluoride ions into the solution. In this respect the polycrystallineelectrodes have a further disadvantage by having a higher dissolution ratethan the single crystals.

Baumann also reports that the Nernstian response of the fluoride electrodeextends down to 5 x iO M in the presence of thorium and zirconium,8 x iO M in the presence of lanthanum and 3 x iO M in the presence

120


of perchioric acid. At the same time, however, in pure fluoride solutions theresponse is linear down to only 8 x 10 M.

Buffers

Unfortunately, among the ion-selective electrodes, we only have buffersof high capacity for pH electrodes. Because of this lack of suitable buffersfor other ions, the calibration of electrodes responsive to these other ionsis difficult. Moreover, junction potentials are a further problem which hasbeen discussed in a paper by Bates and Alfenaar7. However, following theneed for further scales of ionic activity, some practical methods have beendevised for their construction. For concentrations greater than i0 M thecalibration can be carried out directly with solutions of the appropriate ion,the ionic strength being maintained by electrolytes for whose componentsthe selectivity constants are lower than

Another calibration method which is available uses the complexes orprecipitates which are in equilibrium with the appropriate ion. It should bementioned that such systems generally have low-buffer capacities. To thisgroup belong those buffers suggested by Havas and his co-workers61; theyhave used saturated solutions of silver halides for calibration points. Thedisadvantage of this method is the great susceptibility to change of thesolubility of the silver halides by any one of a number of parameters.

There are many papers published in the literature on the application of thefluoride electrode. Because the fluoride ion takes part in an acid-baseequilibrium and is also a good complexing agent for various metal ions, it isnecessary to ensure reproducible conditions for the fluoride measurements.Frant and Ross5° suggested the TISAB buffer (Total Ionic Strength AdjustingBuffer). This has a pH of 5.0 and an ionic strength of 1.75 M: it consists of1.0 M sodium chloride, 0.25 M acetic acid, 0.75 M sodium acetate and 0.001 Msodium citrate. In this medium fluoride determination can be carried outdown to 106 M fluoride ion activity without interference.

The behaviour of the electrodes in non-aqueous solvents

A systematic study of the behaviour of the precipitate-based electrodeshas been carried out by Kazarjan and Pungor74' The silver halide elec-trodes were used in alcohols, ketones and other solvents such as dimethyl-formamide. In this investigation aqueous and aqueous/non-aqueous solventmixtures in different ratios were used. The solubility product of the silverhalides and the selectivity constants of the electrodes were measured in thedifferent solvent mixtures. The calculated and measured selectivity constantsare compared in Tables 7 and 8.

It may be seen from the tables that the theoretical values agree with themeasured values and thus one may conclude that the ion-exchange conceptis also valid in these systems.

In these experiments we found that it was necessary to first conditionthe electrode in the appropriate solvent mixtures as the membrane is hetero-geneous; during the soaking we observed a swelling effect.

As expected the theory for the lower detection limit of the electrodesderived for aqueous solvents is also valid for other media. So, for example,

121


Table 7. Selectivity constants of an iodide-selective electrode to bromide in mixtures of distilledwater and an alcohol

Ratio of Mole pKSolvent solvent fraction

(vv 0)) of solvent

CHOH 10:90 0.04740:60 0.22860:40 0.40090:10 0.802100: 0 1.00

10:90 3.20 3.62 3.7040:60 16.20 3.50 3.5560:40 29.80 3.34 3.3090:10 67.70 3,00 2.76100: 0 100.00

10:90 2.60 3.70 3.7420:80 5.60 3.58 3.5430:70 920 3.58 3.4640:60 13.70 3.58 3.42

in 90 per cent ethanol, the lower detection limit of the iodide electrode liesat about iO M, this decrease being produced by a difference of approxi-mately 1.5 units between the solubility product exponent in this mediumand in water.

For measurements with the fluoride electrode, Lingane94 reported thatin alcoholic solvents the lower detection limit of the fluoride electrodeshifts downward by nearly one decade.

Another study in non-aqueous solvents, using the lead electrode, con-sisting of lead sulphide plus silver suiphide, has been carried out by Rechnitzand Kenny134. They used this electrode with success in methanol, dioxane,acetonitrile and dimethylsuiphoxide.

TahleS. Selectivity constants of an iodide-selective elec1rde to bromide in non-aqueous solvents

Ratio of pKDMFtoH2O

Experimental (v/v %) Theor. Experimental

3.75 10:90 3.74 3.753.60 40:60 3.26 3.20

60:40 2.64 2.60

Theor. Caic. ApK

3.64 3.78 —0.143.48 3.60 -0.123.34 3.38 —0.043.02 3.02 —0.002.93

C,HsOH

n-C3H -OH

iso-CH OH

—0.08—0.05—0.04+0.24

10:9020:8030:7040:60

—0.04+ 0.04+ 0.12+0.16

2.60 3.745.60 3.649.20 3.56

13.70 3.50

3.78 —0.043.60 + 0.043.49 + 0.073.16 +0.34

In mixtures of distilled water andIn mixtures of distilled water and acetone

dimethylformamide

Ratio of acetone pKto 1-120 .

(vv Theor.

10:90 3.8220:80 3.68

30:70 3.62 3.56

40:60 3.56 3.55

122


Transient phenomena of the electrode

The transient phenomena of the precipitate electrodes are very importantfrom both theoretical and practical points of view, but primarily becauseof the information afforded concerning continuous measurement tech-niques129. Despite this importance, there has been little research in this field.This may be explained partly by the intrinsic measurement difficulties andpartly by the complexity of the interpretation of the results.

The immersion and injection techniques for producing transient pheno-mena are equally useful. In our review paper summarizing the state of theart, we concluded that, for the halide electrodes, the literature data werenot always useful because the results were sometimes influenced by the res-ponse lag of the measuring systems. Our results172 and those of Rechnitz1have indicated that the response time is, for halide electrodes, less than onesecond.

Our most important conclusions, reached with our heterogeneous elec-trodes, are as follows:

(i) The response time can be described by one exponential equation.(ii) The response time is only slightly dependent on the thickness of the

membrane layer.(iii) The response time decreases if the solution contains non-interfering

ions in large excess.(iv) The response time of the iodide-based cyanide electrode in cyanide

solutions is the same as in iodide solutions. This strongly suggests that thepotential-determining processes are identical for the two responses.

Using a sulphide electrode, rather than a halide electrode, Light92 founda response time of about one millisecond.

PRACTICAL SECTION

Measuring techniquesThe measuring techniques applied to the precipitate-based electrodes are

the same as those used for every other kind of electrode.The measurements may be carried out either in cells with liquid—liquid

junctions or without liquid—liquid junctions. In the latter case we mustchoose a reference electrode which is reversible to one of the components inthe system—one which does not interfere with the ion-selective electrode.At the same time this component may also be used to maintain the constancyof the ionic strength. We have used in this way, as a reference electrode, theion-exchange based perchlorate electrode1 23 Such an experimental systemis especially suitable for the investigation of complex equilibria, the ionicstrength being maintained by perchlorate.

Cells with liquid-liquid junctions have to be constructed in such a waythat the electrolyte in the salt bridge does not interfere with the measuringelectrode.

If both the measuring and reference electrodes have high internal resist-ances, then the leads connecting them to the measuring instrument shouldbe shielded and both instrument inputs should have high impedances,rather than just one as with a normal pH meter. To meet these requirements,

123

F. PUNGOR AND K. TOTH

Brand and Rechnitz' ' have designed a device with two, very high impedance,matched and symmetrical inputs using operational amplifiers.

Measurement with ion-selective electrodes can be carried out in severaldifferent ways. The activity of the ion can be estimated directly, as whenusing a glass electrode for pH determination, or alternatively the standardaddition technique8 16 22, 39, which is also a direct method, can be used.In many cases this latter technique gives better results than the simple direct-measuring procedure. A further elaboration is the so-called multi-additiontechnique. This technique has the advantage, compared to the simple calibra-tion procedure, that it is not necessary to know anything other than that theelectrode response is linear with the logarithm of the ionic activity in themeasurement range. This multiple standard addition technique gives veryaccurate results, particularly when combined with a computer.

Precipitate-based electrodes can be used as indicator electrodes in potentio-metric titrations. One method where they may be particularly used toadvantage is null-point potentiometry. Durst37 has devised an arrangementusing this technique for the measurement of nanogramme quantities involumes of 50 tl.

ErrorsThe errors arising from the direct application of membrane electrodes

have recently been studied. Krull82 studied generally the influence of errorsof the selectivity constant on the analytical results and his findings are alsovalid for the precipitate-based electrodes, if the measurement of the selec-tivity constant is incorrect.

In direct potentiometry an error of 1 mV leads to an error of nearly 4 percent in the determined concentration. Moreover, this error increases if themeasurement is carried out in a non-Nernstian part of the calibration curve.

If precipitate-based electrodes are used for end-point indication in atitration, the precision of the determination is higher than in direct measure-ment. Schultz1 ° has suggested that in ideal conditions the titration curveshould be symmetrical about the end-point. However, these ideal conditionsare only assured if, among other things, the system to be titrated does notcontain ions for which the selectivity constant is appreciable (< 108).Schultz has written the equation of the titration curve when the titrandcontains interfering ions, as follows:

n(E—E0) / c.V(= — logc +

If the interfering ion is in the titrant:

n(E — E0) ( cV— log ci + k 0 + tand if both the titrand and titrant contain interfering ions, the expressionbecomes

n(E—E0) / cl'0 cV= — log + k 01++ k

v0z)124


Carr26 discusses the question of errors in detail and points out that theend-point error of a precipitation titration is only negligible if interferingions are absent. Schultz'5' studied the same question for chelatometrictitrations and found that the titration error increases as a function of theselectivity constant.

The difficulties increase if the composition of the titration product isuncertain, as was found, for example, by Lingane93 when using metal—fluoride complexes in determinations.

Anfält and Jagner2 studied various titration methods using ion-selectiveelectrodes from the point of view of the errors, and concluded that the mostprecise data were achieved by use of the Gran54 method, corrected byIngman and Still70. Liberti and Mascini87 have published a paper describingan application of the Gran method.

A further measuring error arises if the electrode becomes polarized, ashas been pointed out by Buck25. Hence, because of the high resistance ofprecipitate-based electrodes, the input impedance of the measuring devicemust be high, normally not less than 1012 ohms.

SOME ANALYTICAL APPLICATIONS OFPRECIPITATE-BASED ION-SELECTIVE ELECTRODES

Some of the more recent analytical applications of precipitate-basedelectrodes are collected in Table 9. We should like to briefly discuss someof these applications.

One type of application is in the determination of very low concentrations.As examples we can mention the determination of cyanide in sewage, someresults of which are presented in Table 10, and of cyanide in leaves, as shownin Table 11. In this estimation of cyanide in leaves, a particular advantageis that the usual preliminary distillation step is unnecessary. A further appli-cation is shown in Table 12, which contains results of chloride determina-tions in potassium hydroxide; such determinations are of especial importancein semiconductor technology. In this method the sample is initially passedover a cation-exchange resin, and the eluted chloride measured with theelectrode.

Another type of application is in the titration of such compounds thatreact to produce components to which the electrode is reversible, as forexample in the determination of thiourea' Thiourea is only slightlyhydrolysed in aqueous solution, as may be demonstrated by potentiometricmeasurements using a sulphide-selective electrode. The approximate con-centration of sulphide ion, as calculated by the Nernst equation, in alkaline10' M thiourea solution is extremely small—about iO M. On titrationof the solution with silver nitrate in the presence of 0.1 N sodium hydroxide,the following reaction takes place:

H2N—C—NH2 + 2AgNO3 H2N—CN + Ag2S + 2HN03S

H2N—CN + 2AgNO3 Ag2N—CN + 2HN03125

Tab

le 9

. Som

e re

cent

app

licat

ions

of p

reci

pita

te-b

ased

elec

trod

es

CN

D

irec

t C

N

pCN

= 1

-6

Hg2

C

u2

Ni2

Was

te w

ater

pH

> 1

1 17

3

Ag

Dir

ect

Ag

pAg=

0—17

—

—

H

g2

Cm

pise

nt

Mea

suri

ng

Inte

rfer

ing

Non

- R

efer

- or

ion

Met

hod

Ele

ctro

de

rang

e T

itran

t E

rror

io

ns

inte

rfer

ing

Sam

ple

Med

ia

Com

men

ts

ence

s ill

s

92

('I —

2°3

Res

pons

e tim

e S

Oj -

and

tem

pera

ture

de

pend

ence

dat

a H

alid

es

Indi

rect

A

g0

0.l—

l0tc

q.

AgN

O3

0.35

—

—

In

orga

nic

mat

eria

ls

80, a

cetic

aci

d A

fter

bur

ning

, ha

lf-a

utom

atic

Hg2

0 In

dire

ct

Ag0

0.

l—4O

ppm

D

ithio

oxam

ide

0.3°

, C

o2

Ag0

Zn2

C

d20

Pb2

0 Fe

30

—

Buf

fer

—

CN

In

dire

ct

Ag0

m

mol

A

gNO

3 0.

4 —

—

—

- —

C

onse

cutiv

e C

l —

pote

ntio

met

ric

titra

tion

CL

Dire

ct

Cl

pCI

= 1—

5 —

±

0.05

pC

I —

—

U

rine,

blo

od

—

In th

e pr

esen

ce

Indi

rect

C

L

pCi

= I

-3

AgN

O3

<1

seru

m

of p

rote

ins

Cl

Indi

rect

C

1 pC

I =

1—

3 A

gNO

3 2.

1 B

r, L

—

P

lant

s A

cidi

c D

irect

CL

dete

rmin

atio

n ca

n no

t be

use

d

CL

D

irect

C

L

pCi =

1—

4 —

±

0.05

pC

i B

r, I.

S2

—

Phar

mac

eutic

als

Cal

cula

ted

activ

ity

indi

rect

co

effic

ient

A

myg

dalin

D

irect

C

N -

pAm

= 3

-.5

—

Cu2

C

d20

Hg2

0

—

Buf

fer

pH =

10—

11

Imm

obili

zed -

gluc

osid

ase

on

theC

N

elec

trod

e su

rfac

e,

lifet

ime

200h

C

yano

- D

irect

C

N

pB1,

> 6

±

0.O

4pC

N

—

Pha

rmac

eutic

als

Alk

alin

e pH

> 7

C

N

was

co

bala

min

lib

erat

ed b

y (v

itam

in

redu

cing

age

nts

B12

) an

d lig

ht

CN

D

irect

C

N

pCN

5

—

Fru

its,

bran

dy

60 alc

ohol

—

in

dire

ct

CN

pC

N

4 A

gNO

3 pH

> 1

1

pCI =

I

3 \g

NO

81

73

31

112

85

113

139 35

59

Indl

r.ct

C

N

pCN

>4

AjN

O1

1%

—

—

—

—

CN

D

irect

C

N

>ip

pm

2%

Plan

t it

extr

actio

ns

CN

in

dire

ct

CN

N

i-sa

lt —

K

3P04

N

aOH

or

At th

e en

d-po

int

166

KSC

N

NH

3 so

lns

is {

Ni(

CN

)4]2

N

aCl

Na2

SO4

at

4 x•

102M

P1

so

ins

CN

D

irec

t C

N

pCN

= 5

—

S2,

Cl

Air

, dri

nkin

g In

the

pres

ence

30

w

atcr

, m

d. w

aste

, or

cr

biol

. m

edia

H

CN

mus

t be

isol

ated

C

N

Dir

ect

CN

pC

N=

2-6

—

—

--

—

Cl

Dir

ect

Cr

pCI

= 1

—5

Hg2

,Cu2

tNi2

t-

Was

te w

ater

—

t2

7 In

dire

ct

Cl -

pCI =

1—3

AgN

O3

1—13

1 D

irec

t 1

p1 =

1—

6 M

ilk

18

Hg2

In

dire

ct

1 pH

g2 <

6 N

at

±0.

4%

—

—

Hgc

ompl

exin

g 6

ligan

ds c

an b

e 2

mas

ked

with

m

etal

s fr

i Ph

arm

a-

Dir

ect

Cl

pCI =

1—4

—

0.O

t—0.

04

Bas

ic m

ater

ials

O

rgan

ical

ly

34

ceut

ical

s pX

of

pha

rmac

euti-

bo

nded

hal

ogen

s In

dire

ct

1 p1

= 1—

3 A

gNO

3 ca

ls

can

be m

easu

red

—

afte

r bu

rnin

g,

half

-aut

omat

ic-

IT

ally

H

alid

es

Dir

ect

Cl —

Dire

ct

62

Br

pX =

1—6

—

52

dete

rmin

atio

n —

1

of h

alid

es

Indi

rect

1

AgN

O3

—

S2

p.a.

KO

B

(a)

part

ial

C

oxid

atio

n nI

(b

) ion

-exc

hang

e H

alid

es

Dir

ect

X

pX =

1—6

—

± 0.0

5—

—

Stu

dy o

f "p-

sel"

96

0.

15 p

X

NaB

Ph4

Indi

rect

H

alid

es

0.2

mM

A

gNO

3 0.

38—

0.49

%

—

pH =

5

K +

In

dire

ct

Ag

or

4—20

mg

AgN

O3

—

—

—

Bac

k titr

n.

159

hahd

es

exce

ss N

aBPh

4

NT

A

Indi

rect

C

u2

pCu

= 3—

4.3

NT

A

Al3

In

dire

ct

Cu2

pA

l =

2.5

—3.

5 D

CT

A

Ca2

+ M

g2In

dire

ct

Cu2

0.

03—

0.41

M M

e

Cs

Indi

rect

C

s 6.

5—72

.4 m

g C

s F

0.03

8-19

ng

ml

met

ry

F In

dire

ct

F pF

= 5

—6

La(

N03

)3

Non

-aqu

eous

m

etha

nol

acet

one

acet

onitr

ile

Zn,

Cd,

Ni,

Pb,

Ca.

Mg

indi

rect

de

term

inat

ion

with

cop

per-

ch

elat

e in

dica

tor

Zr,

Th,

La,

Sm

, Fe

, C

a, H

g in

dire

ct de

ter-

m

inat

ion

with

C

u-E

DT

A

indi

cato

r

Indi

rect

po

tent

iom

etri

c tit

ratio

n W

ater

anal

ysis

C

u-E

DT

A

indi

cato

r met

hod

The

end-

poin

ts

wer

e de

term

ined

fr

om G

ran

plot

s W

ater

anal

ysis

The

sol

ubili

ty

prod

ucts

of

LaF

3, E

uF3

have

be

en m

easu

red

PKH

Y h

as b

een

dete

rmin

ed

Tab

le 9

(con

tinue

d)

Com

pone

nt

or io

n M

etho

d M

easu

ring

E

lect

rode

ra

nge

Titr

ant

Inte

rfer

ing

Err

or

ions

N

on-

Ref

er-

inte

rfer

ing

Sam

ple

Med

ia

Com

men

ts

ence

s io

ns

Me2

In

dire

ct

Cu2

pM

e =

3—

5 T

EPA

, ED

TA

, —

C

l, pE

, pH

= 3

—11

E

GT

A, 1

, 10

- N

a ph

enan

thro

line

Cu2

+

Indi

rect

C

u2 +

pC

u =

2—5

ED

TA

, T

EPA

, 5,

6-di

met

hyl-

1,

10-p

hena

nth-

ro

line

Me4

In

dire

ct

Cu

5—7J

.tM M

e F

DT

.\ pH

= 5

Na-

acet

ate

Me3

bu

ffer

M

e2

pH=

10

0.1M

NaO

H

—

—

Por

, SO

R

-S0

Na5

P3O

10

—

Mg2

N

a,N

H

Ca2

+

—

—

NaC

l

F L

inea

r nu

ll-po

int

pote

ntio

-

ED

TA

H3P

Mo1

2O40

—

1%

Wat

er

pH

9.6

dete

rgen

ts

0.1

M

NH

3-N

H4N

O3

buff

er

—

pH=

5.5

Wat

er

144

133

C) 0 >

135

-I

0'

160

- 99

29

37

94

174

Li

Na

NaB

Ph4,

C

sNO

3,

LiN

O3

—

Wat

er

Dtr

ect

F pF

=3—

5

pH =

9.2

bor

ate

buff

er p

H =

10.

0

phen

olat

e buf

fer

Aqu

eous

6% al

coho

l

Aci

dic

F D

irec

t F

pF <

6

±10

%

Prot

eins

B

iolo

gica

l 17

1 sa

mpl

es

F D

irec

t F

>0.

1 pp

m

—

—

—

Rep

rodu

cibi

lity

90

0.05

ppm

D

irec

t F

1 m

g m

l' 1%

N

atur

al w

ater

D

istil

led w

ater

F

dete

rmin

a-

91

Indi

rect

F

—

0.00

5 M

4%

—

W

ater

—al

coho

l tio

n af

ter i

on-

, T

h(N

03)4

ex

chan

ging

pr

oces

ses

F D

irec

t F

pF <

6.3

in

1.7%

N

aCI

TIS

AB

Se

wag

es

179

IMN

aCI

Indi

rect

F

pF =

1—

6 2.

3%

Nat

ural

wat

er

F D

irec

t F(

mic

ro)

pF =

2—5.

3 —

In

mic

ro-s

ampl

es

38

F

Dire

ct

F 2 p

pm

—

Fe2

, B

iolo

gica

l A

fter

dist

illat

ion

76

D

Fe3

÷

sam

ples

F -

S

tand

srd

F -

0.07

34 m

ss

>0.

2%

Stab

le F

- S

ea w

ater

H

alf-

auto

mat

ic

3 v,

ad

ditio

n kg

1 co

mpl

exes

tit

ratio

n, en

d-

poin

t is

calc

ulat

ed by

6

com

pute

r Z

D

irect

F

2—6%

0.

05—

0.1%

A

l34

—

Oxi

des,

T

ISA

B

Fast

met

hod

115

cn

Fe34

fl

uors

par,

C

a2 +

op

al g

lass

rn

Dir

ect

F 0.

1—1%

F

—

Pho

spha

te

pH =

6.5

—

11

6

rock

, ap

atite

N

H4O

AC

buff

er

F D

irec

t F

l—lO

ngF

—

B

Org

anic

H

OA

C +

NaC

I +

In b

urnt

sam

ple

68

I fl

uori

ne

+ N

a-ci

trat

e +

+N

aOH

pH=

5.5

r In

dire

ct

F 0.

1 pp

m

Pollu

ted

air

Na-

mon

ocitr

ate

Aut

omat

ic

89

NaO

H b

uffe

r co

ntro

l, th

e ai

r be

ing f

ilter

ed

thro

ugh

a su

itabl

e m

ediu

m

rn

F Po

tent

io-

F 10

—20

00 p

pm

5.7%

In

veg

etat

ion

Aci

d ex

ts. f

rom

71

'

met

ric s

td.

9.3

%

pulv

eriz

ed p

lant

tis

sue

F D

irec

t F

0.01

—1.

0 pp

m

—

Cya

nam

ide

Co,

Mg,

Fe

ln fe

rtili

zers

—

F

was

dis

tr.

182

H3P

O4,

A

l, C

r, T

i an

d pl

ant

(cal

ibra

tion f

or

silic

tc a

cid,

si

licof

luor

ide)

H

2CO

1

Tab

le 9

(co

ntin

ued)

Com

pone

nt

or io

n M

etho

d E

lect

rode

M

easu

ring

ra

nge

Titr

ant

Inte

rfer

ing

Non

- E

rror

io

ns

inte

rfer

ing

ions

Indi

rect

Pb

24

pPb

= 3

P

b2 +

Dire

ct

PlY

÷

pPb

= 1

—5

Indi

rect

Pb

24

K2S

04

K2C

rO4

PO

Indi

rect

Pb

24

0.5—

1.5m

g

H2 S

O4

Aqu

eous

in

the

pres

ence

of

com

plex

ing

ions

, T

ISA

B

In th

e pr

esen

ce

pH =

5.3

of

com

plex

ing

ions

det

n. F

in

op

al g

lass

—

pH

=3,

8 /2

= 0.

3

Was

te C

aSO

4 an

d ga

ses

Was

te w

ater

R

iver

wat

er

TIS

AB

Aft

er di

still

atio

n In

terp

reta

tion o

f th

e re

sults

usin

g a c

ompu

ter

Th(

N03

)4

Sam

ple

Med

ia

Indi

rect

F

—

F

Dir

ect

F pF

= 6—

11

pF

5 or

0.

19 p

pm

F D

irec

t U

1.

84m

g

Al3

D

irec

t U

pA

l =

4—6

—

HF

D

irect

U

0.

41—

7.72

%

CaF

, C

N

CN

0.

lmgC

N-r

'—

U

Dir

ect

U

Cl -

Dire

ct

Pb24

Ref

er-

Com

men

ts

ence

s

—

Cry

olite

A

1F,

—

—

0.6—

2.5%

B

e. F

e M

g24.

Cu2

P0

4 >

0.2

Pb

24

/2M

mr

0.01

-0.1

9%

—

H2S

04,

BaS

O4

SO,,

Cu5

04

0.05

—I

mg

U

pPb=

2—6

—

—

Ni2

1

Zn2

4 E

DT

A

Bio

logi

cal

sam

ples

B

iolo

gica

l sa

mpl

es

—

Ag%

TP

, N

H:

Mic

ro a

nd

sem

imic

ro

dete

rmin

atio

n

In b

lood

un

succ

essf

ul

Sele

ctiv

ity

dete

rmin

ed

140

52 6

72

84

109

tb

C 2

134

-i

86

z

152

153

143

Pb(C

l04)

2 1-

2%

C1,

U.

Cr0

C

20 -

Indi

reet

Pb

24

1—25

mg

Pb(C

104)

2 0.

06%

C

ompl

exin

g C

20

and

ppt-

ing

spec

ies

S0

Indi

rect

Pb

24

pPb

<4.

3 Pb

(C10

4)2

±1%

A

g4.

Cu2

4 H

g2 P

O

TIS

AB

Wat

er al

coho

l

dim

ethy

l su

lfox

ide

Buf

fer

pH =

8.25

—8.

75

40 %

p-di

oxan

e pH

= 3

.5—

10.5

SOy/

v % d

ioxa

ne—

w

ater

S0, N

0

Ac,

form

ate

prop

iona

te,

phth

alat

e

Dir

ect

S2

p5 =

0-2

0 —

S

2O -,

—

A

lkal

ine

Res

pons

e tim

e 92

SO

. SO

pH

13

sn

d te

mpe

ratu

re

Indi

rect

52

A

gNO

3 I,

CIT.

—

depe

nden

ce da

ta

Br,

F, C

N

Indi

rect

52

pS

<4

Na2

Pb(O

H)4

—

—

C

IT B

r, IT

—

—

—

10

8 SC

N, S

O

Inor

gani

c In

dire

ct

S2

AgN

O3

Alk

alin

e 1

M

Dir

ect m

etho

d 16

1 Q

pu

lpin

g liq

uor

NH

4OH

w

as a

lso

stud

ied

Org

anic

B

ack

S2 -

—

AgN

O3

-—

-1

titra

tion

S2

Indi

rect

S2

Sp

pm

AgN

O3

<lp

pm

S2O

C

O, S

O

—

Aqu

eous

soln

. —

57

rn

SS

gro

up

Indi

rect

52

A

gNO

3 7%

In

pro

tein

s -—

60

ex

cept

lys

osym

e T

hiol

s In

dire

ct

52

—

AgN

O3

—

Hg2

—

56

rn

H

alid

es

indi

rect

S2

0.

04—

1.Sg

eq.

AgN

O3

1%

-—

75%

ace

tic ac

id

Aut

omat

ic

80

NH

4_

Dir

ect

S2

—

±0.

1 N

a2S2

O3

Red

ox-c

oupl

ei

flot

atio

n m

ill

—

Met

allu

rgic

al

141

diet

hyl

pS2

- so

lns

indu

stry

; z

dith

io-

sele

ctiv

ity is

ph

osph

ate

disc

usse

d T

hiou

rea

Dir

ect

52

pS =

1—

6 —

—

-—

—

In

vest

igat

ion

and

114

Indi

rect

52

pS

= 1

—3

AgN

O3

—

—

—

Alk

alin

e an

d id

entif

icat

ion

C

of th

e pr

oduc

t by

elem

enta

ry

anal

ysis

and

i.r.

N

a2S

Indi

rect

52

pS

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17

AgN

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e pr

esen

ce

—

Mea

sure

d an

d 14

7 of

diff

eren

t ca

lcul

ated

sel

ectiv

ity

ITt

anio

ns an

d co

nsta

nts s

how

14

8 ca

tions

go

od ag

reem

ent

H2S

—

—

—

A

lkal

ine

Sel

ectiv

e gas

- 77

0

HC

I —

C

IT

l05M

—

eh

rom

atog

raph

ic

Me2

S2

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)

F. PUNGOR AND K. TOTH

In the first step, one sulphur splits off to form silver sulphide and at thesame time cyanamide is formed. The resulting cyanamide reacts with a furthertwo molecules of silver nitrate and a silver cyanamide precipitate is formed.

2

The products of the titration were separated by filtration and were analysedgravimetrically and also for their elemental composition. In addition theirinfrared spectra were taken. For comparison the spectra of pure cyanamideand thiourea were also recorded. These experiments proved the proposedreaction path. The titration allows an accurate determination of thioureaand a fully detailed method has been worked out with the help of ourco-worker Mrs Kucsera. The reaction, which takes place rapidly as the titra-tion proceeds, is followed conveniently by a suiphide ion-selective electrodein 10 '--iO M thiourea solutions.

Table 11. Determination of cyanide in various materials by direct potentiometric measurementwith the pCN electrode

Type of Designation Cyanidematerial or variety content (ppm)

Bitter almond 280Spicy almond 92

9598

86

Sweet almond 2231

Bt 1355

Bt 1362

Bt 13 65

Bt l366

Bt 70

Bt I

lit 11

I. A

BC

2. ABC

Type of Designation Cyanidematerial or variety content (ppm)

Peach leaves Alexander 39Sunbeam 63May Flower 72Mariska 74Champion 76Elberta 91Ford 120

Sudan grasses 5. A 130B 49C 16

6.A 71B 45C 36

7.A 123B 62C 34

175B 70C

132

successive developmeniai stages (capiial

Table 10. Determination of cyanide in sewage

Cyanide content of sewageSample Dilution (ppm) Direct method*

1:1 13.11:5 14.61:10 13.1

24.71:1 23.4

* The values are calculated back to the undiluted solution,

Sudan grasses*54

29886

89

27

14

231270

12

17

27

2

3. ABC

4. ABC

* Sudan grasses are altered by the fertili7aiion practices lsample numbers) andletter.sj ul the plants in the respective sample.


Table 12. Direct determination of chloride in KOl-! using a chloride membrane electrode

[Cl ], (M) CLcontent

(Q/)Error(%)

- -Caic. Found

1.OOx i0' 1.02x i0 7.24x 10 21.OOxlO 1.04x104 7.38x103 4l.OOx iO 1.05x iO 7.45x iO 5

Another case where ion-selective electrodes may be used indirectly forthe determination of compounds to which there is no direct response, is incombustion analysis. In organic microanalysis, combustion is frequentlyemployed and ion-selective electrodes may be conveniently used to deter-mine halides in the combustion products. Some results are collected inTable 13.

Table 13. Potentiometric determination of pharmaceuticals using a chloride ion-selectiveelectrode

Compound

2,6-Dinitro-chlorohenzene

Sample taken(mg)

Halide coCaIc.

ntent (%)Found A( %)

18.105 17.21 17.43 + 1.224.084 17.21 17.42 + 1.221.074 17.21 17.67 +2.624.018 17.21 17.71 +2.920.757 17.21 17.42 +1.2

Chlorpromazine UCI 20.20320.78717.29019.95017.782

19.96199619.9619.9619.96

20.44200319.9819.9920.03

+ 2.4+0.3+0.1+0.1+0.3

SOME OTHER APPLICATIONS OF ION-SELECTIVEELECTRODES

As well as the above analytical applications, ion-selective electrodes canbe used in the field of physical chemistry for investigating equilibria andkinetics of various reactions. In this article we have referred to many paperswhere the electrodes have been used for determining acid-base dissociationconstants and formation constants of complexes. Another interesting newapplication, which is furnished by the short response time of the electrodes,is the study of oscillating reactions. Woodson and Liebhafsky18' used theiodide electrode in a study of the ClOH2O2 reaction and Körös et a!.(personal communication) are using our bromide electrode for investigatingsuch reactions as that between BrO and malonic acid.

CONCLUSION

In the last decade the field of ion-selective electrodes has undergone anastonishingly rapid development with many new sensors being developed

I 33


and applied in a wide range of analytical and physicochemical research.But can we predict future developments in this branch of chemistry? In ouropinion the number of precipitate-based electrodes will be limited, but inthe application of these electrodes to the monitoring of chemical processesthere will be a rapid expansion. However, future major developments cannotbe expected unless we can discover the exact relations between the propertiesof the precipitates used for the electrodes and their electrochemical behaviour.

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