DO Experiment

8/8/2019 DO Experiment

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ABSTRACT / SUMMARY

An exper iment ca l l ed B iochemica l Oxygen Demand (BOD) tes tis conducted in order to determine the amount o f d isso lved oxygen

conta ins in water sample co l lec ted f rom Tas ik Seksyen 7 , Shah

Alam . The method compr ises main ly three s tages, wh ich are

co l lec t ion o f water sample , s tab i l iza t ion o f water sample w i th

manganous su l fa te and a lka l i iod ide az ide reagent powder p i l lows,

and t i t ra t ion o f the sample w i th sod ium th iosu l fa te car t r idge p lus

add i t ion o f s ta rch so lu t ion . Besides that , th is exper iment a lso

d iscusses the impor tance o f d isso lved oxygen for aquat ic

ecosystem, the reasons o f reduct ion l eve l o f d isso lved oxygen which

is temperature , pressure, sa l in i ty and e levat ion o f water bod ies as

wel l as the c i rcumstances o f the dep le t ion o f concent ra t ion o f

d isso lved oxygen to the surv iva l o f aquat ic ecosystem . Based on the

exper iment resu l t , the amount o f d isso lved oxygen in the water

sample is 6 .62 mg/L . The exper iment is comple ted and conducted

successfu l ly.

INTRODUCTION

Oxygen is indeed a wel l kno wn fac t as the a i r that both humans

and an imals need for resp i ra tory sys tem . Disso lved oxygen is

oxygen in the form of water wh ich is f ree ly ava i lab le to aquat ic

p lants and an imals inc lud ing f ish , inver tebra tes , p lants and aerob ic

bacter ia . The ex is tence o f d isso lved oxygen is by the oxygen that


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are be ing t ransfer red f rom the a tmospheres in to water sur faces

a long wi th the oxygen produced by aquat ic p lants , a lgae and

phytop lankton as a by-product o f photosynthes is . Once the oxygen is

d isso lved in water , i t d i f fuses very s lowly throughout a water body .

The d is t r ibu t ion is s low due to the dependency on the movement o f

aera ted water , wh ich is the water that is over f lown wi th carbon

diox ide, by turbu lence and cur rents , water f low as wel l as thermal

upwe l l i ng .

The leve l o f d isso lved oxygen is an ind icator fo r water body s

ab i l i ty to suppor t des i rab le aquat ic l i fe.

The spec ies and the s tage o f

l i fe o f a spec ies are the var iab les that determine the amount o f

oxygen . As for growth and act iv i ty , the requ i red amount are between

range o f f ive to s ix par ts per mi l l i on (5 6 ppm) . In add i t i on ,

d isso lved oxygen leve ls be low three par ts per mi l l ion (3 ppm) is

d icey and means harm to most aquat ic organ isms . I t ge ts even worse

i f the amount o f d isso lved oxygen is be low two par ts per mi l l ion (2

ppm) as the aquat ic l i v ings espec ia l l y f ish may not surv ive . Oxygen

is par t icu lar ly sens i t ive and hard ly ava i lab le as over per iod o f t ime,

i ts ex is tence is depending upon the temperature , pressure, sa l in i ty

e f fec t , o ther chemica ls present in water and b io log ica l processes .

Temperature indeed p lays a major ro le in in f luenc ing the amount o f

d isso lved oxygen in water . Water a t a tempera ture o f 31C on ly

conta ins ha l f o f d isso lved oxygen contents in co ld a tmosphere a t

1C . This shows that co ld water has the ab i l i ty to d isso lve oxygen

greater than in warm water .

Oxygen f rom the a tmosphere is t ransfer red in to sur face water

a t the po in t o f contac t where i t in ter faces wi th the a i r . Poin ts o f


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contact are e levated cor responding to the w ind behav iour through a

process ca l led phys ica l aera t ion . The oxygen is a lso compr ises o f

the by-product o f photosynthes is process . Likewise, the leve ls o f

d isso lved oxygen are increased by rooted and f loa t ing aquat ic p lants

through th is process . The process o f produc ing oxygen usua l ly can

occur near the sur face o f p lants or in sha l low water where l igh t is

permiss ib le due to the fac t that p lants depends on ava i lab i l i ty o f

l igh t to l i ve and surv ive . Apar t f rom that , oxygen is cons idered to be

harmfu l fo r consumpt ion a t n ight or on c loudy days where oxygen

produc ing v ia photosynthes is process cou ld not occur because o f

lack ing o f sun l igh t . Hence, near ly a l l aquat ic p lants organ isms use

the d isso lved oxygen to move and susta in in l i fe.

This process,

ca l led resp i ra t ion can in turn reduce the concent ra t ion o f oxygen, in

th is contex t , to the r isky low leve ls .

There is however a lo t more fac tors that cont r ibutes to the

deple t ing o f d isso lved oxygen in water . One of i t i s due to the

presence o f organ ic wastes . The decay ing process o f dead

organ isms or organ ic wastes consumes oxygen and is o f ten

concent ra ted in summer. This is because the aquat ic an imals requ i re

more oxygen for h igher metabo l ism . Deple t ions o f d isso lved oxygen

leve l can cause major sh i f ts in the k inds o f aquat ic l i v ings . This

s i tuat ion may cause catas t rophe or fu r ther d i f f icu l t ies for human

beings espec ia l ly , as aquat ic l i v ings are one o f our source o f food .

The decrease in leve l o f d isso lved oxygen is a lso re la ted to

pressure . As the barometr ic pressure o f a tmosphere decreases, the

concent ra t ion o f d isso lved oxygen a lso decreases . This occurs due

to the increase in the a l t i tude or e levat ion o f water body . As an


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example , cons ider the barometr ic pressure in the mounta ins , wh ich

apparent ly is less than the barometr ic pressure in the oceans . Thus ,

a t h igher a l t i tudes, water ho lds less d isso lved oxygen compared to

the cond i t ion a t lower a l t i tudes .

OBJECTIVE

The main ob jec t ive o f th is exper iment is to determine theamount o f d isso lved oxygen in water sample . Besides that , a few

minor ob jec t ives are a lso cons idered, wh ich inc lude re la t ing the

concept o f oxygen wi th leve ls o f d isso lved oxygen in the aquat ic

ecosystem, d iscuss ing the impor tance o f measurement d isso lved

oxygen as water qua l i ty as wel l as s tudy ing the reasons and

c i rcumstances or e f fec t cor responding to the change in leve l o f

d isso lved oxygen .

THEORY

The concent ra t ions o f d isso lved oxygen may vary dras t ica l ly

depending upon depth and d is tance o f the water bod ies f rom shore .

There are three d i f fe rent methods o f measur ing the leve l o f

d isso lved oxygen . The f i r s t one i s the Wink le r t i t ra t ion method ,

where i t requ i res lo ts o f use o f s t rong chemica ls . Thus, i t i s not

o f ten used for c i t izen moni tor ing programs . The second method is by

bas ica l ly us ing a d isso lved oxygen probe and meter . Never the less,


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the probes are eas i ly ru ined as the membrane deter iora tes , bes ides

the t rapp ing o f a i r bubb les under the membrane and contaminat ion

of the sens ing e lement . I t i s a lso not easy to assure whether the

probe is funct ion ing proper ly . The th i rd method is by us ing d isso lved

oxygen f ie ld k i ts . The k i ts are user - f r iend ly , conven ient and come

wi th s imp le i ns t ruc tions . Di f ferent k i t manufacturer however has

d i f fe rent ins t ruc t ion o f us ing i t . This tes t a lso requ i res shor t per iod

of t ime . However , th is f ie ld k i ts method most ly does not meet the

requ i rements for prec is ion and accuracy needed for pro fess iona l

qua l i ty data .

The decrease in leve l o f d isso lved oxygen is re la ted to

pressure . As the barometr ic pressure o f a tmosphere decreases, the

concent ra t ion o f d isso lved oxygen a lso decreases . This occurs due

to the increase in the a l t i tude or e levat ion o f water body . As an

example , cons ider the barometr ic pressure in the mounta ins , wh ich

apparent ly is less than the barometr ic pressure in the oceans . Thus ,

a t h igher a l t i tudes, water ho lds less d isso lved oxygen compared to

the cond i t ion a t lower a l t i tudes .

The exper iment conducted is a lso known as b iochemica l

oxygen demand (BOD) tes t , and is no d i f fe rent w i th Wink l er method.

I t i s genera l ly a techn ique used in order to determine and measure

the leve l o f d isso lved oxygen in water bod ies . Disso lved oxygen is

impor tant to be measured as i t ind icates the cond i t i on or product iv i ty

o f the water source as h igh concent ra t ion o f d isso lved oxygen g ives

a cor responding resu l ts that the water is o f h igh product iv i ty and

l i t t l e po l lu tan t .


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This exper iment uses t i t ra t ion method where water sample is

f i l led comple te ly in a spec i f ic bot t le ca l led B iochemica l Oxygen

Demand (BOD) bot t le . There must be no a i r bubb les conta in in the

water sample and the d isso lved oxygen content is then added wi th a

ser ies o f reagents that fo rm an ac id compound, which is then t i t ra ted

wi th neut ra l iz ing compound by cons ider ing any co lour change .

Th is tes t techn ica l ly invo lves three main s tages; co l lec t ion o f

water sample , s tab i l iza t ion o f the water sample , as wel l as t i t ra t ion

the water to determine the va lue o f d isso lved oxygen . The contact

between the water sample and the a tmosphere w i l l no t a f fec t much

of the resu l ts , so i t i s not necessary to per form the procedure

immedia te ly a f ter the water sample is co l lec ted .

However , th is tes t does not g ive any in format ion on the amount

of the d isso lved oxygen the water is capable o f ho ld ing a t the

temperature o f when i t i s tes ted . When water ho lds a l l the poss ib le

d isso lved oxygen a t a g iven temperature , i t i s cons idered to conta in

hundred percents (100%) o f sa tura ted oxygen .

EXPERIMENTAL PROCEDURE

1 . ) A water sample is co l lec ted in a c lean 300-mL BOD Bot t le .

2 . ) The contents o f the Manganous Sul fa te Powder P i l low and one

A lka l ine Iod i de-Az ide Reagent Powder P i l low are added .


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3 . ) The s topper is inser ted immedia te ly so a i r is not t rapped in the

bo t t le . The s topper is inver ted severa l t imes to mix .

4 . ) The f loc in the so lu t ion is wa i ted to set t le . The bo t t le i s inve r ted

again severa l t imes and the f loc is wa i ted to set t le.

5 . ) The s topper is removed and the content o f one Sul famic Ac id

Powder P i l low is added . The s topper is rep laced wi thout t rapp ing a i r

in the bot t le and inver ted the prepared sample severa l t imes to mix .

6 . ) A sample vo lume and Sodium Thiosu l fa te T i t ra t ion Car t r idge are

se lec ted cor responding to the expected d isso lved oxygen (D .O)

concent ra t ion .

7 . ) A c lean de l ivery tube is inser ted in to the t i t ra t ion car t r idge . The

car t r idge is a t tached to the t i t ra tor body .

8 . ) The de l ivery knob is tu rned to e jec t a few drops o f t i t rant . The

counter is reset to zero and the t i p is w iped .

9 . ) A graduated cy l inder is used to measure the sample vo lume f rom

Tab le1 . The sample is t ransfer red in to a 250-mL Er lenmeyer f lask .

10 . ) The de l ivery tube t ip is p laced in to the so lu t ion and the f lask is

swi r led whi le t i t ra ted wi th sod ium th iosu l fa te to a pa le ye l low co lour .

11 . ) Two 1-mL droppers o f Starch Ind icator So lu t ion are added and

swi r led to mix .

12 . ) The t i t ra t ion is cont inued to a co lour less end po in t . The number

o f d ig i ts requ i red is recorded .

13 . ) The va lue is ca lcu la ted us ing the equat ion: -

mg/L Dissolved Oxygen = Digi ts Required x Digi t Mult ip l ier .


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APPARATUS / REAGENTS

~ Manganous Sul fa te Powder P i l low, A lka l ine Iod ide-Az ide ReagentPowder P i l low, Su l famic Ac id Powder P i l low, Sod ium Thiosu l fa te

T i t ra t ion Car t r idge, 0 .2000N, Sodium Thiosu l fa te T i t ra t ion Car t r idge,

2 .0000N, Starch Ind icator So lu t ion, 300-mL BOD bot t le , graduated

cy l inder , de l i ve ry tube , con ica l f l ask , and d ig i ta l t i t ra to r .

RESULT

Range

(mg/L)

Sample

volume (mL)

Ti t ra t ion car t r idge

(N Na 2S 2 O 3 )

Dig i t

mul t ip l ie r

D ig i t

requi red

Resul t

(mg/L O 2 )

2 - 10

100 0 .200 0 .02 331 6 .62

Observat ionThe f loc appears soon a f ter MgSO 4 powder p i l low and a lka l ine

iod ide-az ide reagent powder p i l low is mixed, and d isappears .

Oxygen is present as the so lu t ion turn orange-brown . Pale ye l low

colour is fo rmed af ter su l famic ac id powder p i l low is mixed .


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SAMPLE CALCULATION

The to ta l number o f mi l l i l i t res o f t i t rant used equals the to ta ld isso lved oxygen in the sample in mg/L . In order to ca lcu la te the

content o f d isso lved oxygen in water sample the fo l lowing

express ion is used;

Dig i t requ i red x D ig i t mul t ip l i e r = mg/L o f d isso lved oxygen

311 x 0 .02 = 6 .62 mg/L

DISCUSSION

From the exper iment , a f loc is observed to appear in the water

sample soon a f ter the add i t ion o f Manganous Sul fa te and A lka l ineIod ide Az ide powder p i l lows are added . The f loc is in orange-

brown co lour , ind icat ing the presence o f d isso lved oxygen . The

stab i l ized water sample is then t i t ra ted wi th sod ium th iosu l fa te and

tu rns pa le ye l low . When the s ta rch so lu t ion i s added , the so lu t ion

turns co lour less , ind icates the endpoin t o f the t i t ra t ion process . The

amount o f d isso lved oxygen is ca lcu la ted to be a va lue o f 6 .62 mg/L,

which fa l ls in the range requ i red for 0 .200 N of sod ium th iosu l fa te ,

which is between 2 to 10 mg/L.

Never the less, there must have been some er rors made dur ing

the exper iment . In my op in ion, the va lue o f d isso lved oxygen


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obta ined f rom the water sample co l lec ted f ro m Tas ik Seksyen 7 ,

Shah A lam is not the ac tua l va lue, due to the presence o f a i r

bubb les ins ide the water sample when i t i s f i l led in the BOD bot t le .

There is a lso l i t t le a i r bubb les present in the de l ivery tube, wh ich

resu l ts in the inaccuracy o f the amount o f d isso lved oxygen . Besides

that , the water sample is not co l lec ted a t near the midd le o f the lake

or a t arms length be low the water sur face . The s tarch so lu t ion

added may a lso not be o f accurate ly 1 mi l l i l i t re for each drop, wh ich

a lso adds to reasons o f my perspect ive that the amount o f d isso lved

oxygen obta ined is not the prec ise va lue .

CONCLUSION

The amount o f d isso lved oxygen in the water sample is 6 .62

mg/L, hence the water sample is cons idered to have h igh

concent ra t ion o f d isso lved oxygen and there fore , h igh product iv i ty

and less po l lu ted .

RECOMMENDATION

There are a few recommendat ions and precaut ions that have to

be taken account in th is exper iment .


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Fi rs t ly , the water sample is best co l lec ted a t near the midd le o f

the water s t ream and a t least a few inches or a t arms length be low

the water sur face . This is to ensure that the water is co l lec ted r ight

near the presence o f aquat ic organ isms .

Secondly , the d i f fe rence between the t ime the water sample is

co l lec ted and the t ime the exper iment is conducted shou ld not be too

long, as the temperature and barometr ic pressure o f a tmosphere

dur ing the co l lec t ion o f water sample w i l l change over a t ime per iod .

Th i rd ly , in order to avo id or e l iminate the presence o f a i r

bubb les ins ide the water sample , i t i s a must to cap the BOD bot t le

immedia te ly a f ter the water sample is f i l led in . I f a i r bubb les are

detected, remove the water sample and re f i l l w i th another sample .

The water sample shou ld be a l lowed to over f low the BOD bot t le a t

least 2 minutes or unt i l the water in the bot t le has rep laced i tse l f

two or th ree t imes . The bot t le may a lso be inver ted or tu rned ups ide

down to observe any bubble movement . Besides that , the bot t le may

be squ i r ted w i th d is t i l l ed water a long the s topper , capped wi th

a lumin ium fo i l and rubber band .

The reagents used in th is exper iment are most ly s t rong

chemica ls and hazardous to one s hea l th . Therefore , caut ions are

requ i red when handl ing these reagents.

Gloves and goggles are

used in order to prevent any unexpected contact w i th the reagents .


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APPENDICES


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Hach DREL/2400 Basic Water Quality Laboratory

Digital titrator BOD bottle

Delivery tube

Titration cartridge

DO Experiment

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