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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
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