Quantitative Inorganic Analysis Laboratory
Formal Report
Date Performed: 20 ctober 20!" Date #ubmitted: $0 ctober 20!" Institution%Department: Department of &'emical (ngineering &ollege: (ngineering Instructor: Cris Angelo Pagtalunan
Quan Quantit titat ativ ivee Dete Determ rmin inat atio ion n of Disso Dissolv lved ed Oxyg Oxygen en Content by Winkler Redox Titration M. K. Coo Dissolved Dissolved oxygen oxygen (DO !ontent !ontent is an essentia essentiall tool in determin determining ing t"e #ater $uality and ability to sustain life. T"e ex%eriment #as done to determine t"e DO !ontent and to evaluate its ability to sustain life and its level of %ollution. Winkler met"od #as used to determine t"e DO !ontent of a #ater sam%le from a %ond near &in'ons )all. *n t"is met"od+ Mn(*** be!omes Mn(** on!e a!idifies (#it" t"e %resen!e %rese n!e of * ,. -iven t"e rea!tion MnO / 0) / / * , Mn / / * , / 1) O+ t"e * is a%%roximately e$uivalent e$uivalent to t"e DO !ontent #"i!" #as titrated #it" t"e sodium t"iosulfate solution. T"e DO !ontent #as !al!ulated from t"e volume readings. T"e ex%eriment ex%eriment yielded 2.3 %%m O. O. 4rom t"is DO !ontent+ !ontent+ it !an be !on!luded t"at t"e %ond is "ig"ly %olluted and !annot sustain life for larger organisms su!" as fis"es. 5ossible sour!es of error in!lude volatili'ation of iodine+ ex%osure of t"e sam%le to air #"ile t"e Mn / and * , solution solutionss #ere being being added added and t"e t"e %resen!e %rese n!e of t"e t" e bubbles bub bles in t"e t" e sam%le sa m%le (faulty (faul ty in i n sam%lin sa m%ling. g.
Quantitative Determination of Dissolved Oxygen Content by Winkler Redox Titration
Co o ,
201!
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rganic &'emistry Laboratory
Formal Report Introduction
8
−9
&lean good -ater
Dissolved oxygen (DO !ontent is essential to DO !ontent is affe!ted by #ater tem%erature+ t"e #ater $uality of an a$uati! system be!ause of its salinity+ atmos%"eri! %ressure and altitude+ sus%ended !a%ability to determine or estimate life sustainability. *t matter+ and a$uati! %lants and animals. 8$uati! %lants also indi!ates t"e %resen!e of %ollutants in #ater. in!rease DO in #ater t"roug" %"otosynt"esis. T"is Organi! #aste dis!"arges from !ooling to#ers dire!tly re$uires lig"t and so+ DO also varies #it" t"e !larity of affe!t dissolved oxygen !ontent in #ater. Organi! t"e #ater+ t"e time of day+ and t"e seasons. 6 T"e lo!ation materials are broken do#n by mi!roorganisms. T"is also affe!ts DO !ontent. *n stagnant systems su!" as %ro!ess re$uires oxygen and as a result+ de!reases lakes and %onds+ t"e DO !ontent varies #it" de%t" and dissolved oxygen !ontent. 6 7e!ause of t"is+ it !an be a in river systems+ DO !ontent varies along its lengt". good indire!t indi!ator of t"e "ealt" of a$uati! systems 9o!ations #it" "ig" turbulen!e tend to "ave more sin!e "ig" levels of %ollution or organi! material #ill dissolved oxygen t"an stagnant lo!ations. 2 in!rease t"e oxygen !onsum%tion of t"e system. T"is is One met"od used to measure dissolved oxygen im%ortant be!ause if t"e total amount of dissolved !ontent is t"e Winkler met"od. *n t"e Winkler met"od+ oxygen is lo#er t"an #"at is ideal+ a$uati! life may manganese (** is added to t"e sam%le. T"is rea!ts #it" be!ome stressed "ydroxide ions to form Mn ( OH )2 (:$uation 6 7e!ause of t"is+ !an be a good indire!t indi!ator #"i!" rea!ts #it" t"e dissolved oxygen as s"o#n in of t"e "ealt" of a$uati! systems sin!e "ig" levels of Mn ( OH )3 %ollution or organi! material #ill in!rease t"e oxygen :$uation 6+ %rodu!ing t"e bro#n !onsum%tion of t"e system. T"is is im%ortant be!ause if %re!i%itate. Manganese (** sulfate #as added before K* t"e total amount of dissolved oxygen is lo#er t"an #"at be!ause t"e oxidation,redu!tion rea!tion bet#een −¿¿ is ideal+ a$uati! life may be!ome stressed+ i.e. ex"ibit and O2 is too slo#. 3 I errati! be"avior+ move a#ay+ or die. -enerally+ #arm, −¿ → Mn( OH )2 #ater fis" re$uire less oxygen t"an !old,#ater fis". T"e ¿ ;6< 2 +¿+ 2 OH dissolved oxygen levels needed by a$uati! life is s"o#n ¿ in Table 6 and t"e e$uivalent %ollution levels in Table . Mn ;<
Table 1.
(
2
addition of
0)2 ppm *+"),+, ppm /)!! ppm
Pollution degree
2
2
2
(
)
O → 4 Mn OH
3
*f t"e system does not !ontain oxygen+ t"e
Dissolved oxygen and a$uati! life
Dissolved oxygen
) + O + 2 H
4 Mn OH
+¿
¿
Mn
!auses t"e #"ite
(
Mn OH
)
2
%re!i%itate to form. 6 7y a!idifying t"e solution+ manganese (*** "ydroxide dissolves and rea!ts #it"
−¿¿
to form iodine ( I 2 . (=ee :$uation &annot sustain life I &an sustain life for 2. *odine t"en forms a !om%lex #it" ex!ess iodide to −¿ -arm)-ater .s' form I ¿ . T"is is s"o#n by :$uation 1. *odine is Ideal for cold)-ater .s' 3 iodide (
volatile so ex%osure to air !an redu!e its amount. Table 2.
Dissolved oxygen and %ollution degree
−¿¿ I 3
Dissolved oxygen (ppm) below 4.5
−6.6 6.7 − 7.9 4.5
Pollution degree
is less volatile and its formation im%roves
a!!ura!y. 6
+¿+ I + 6 H O +¿ → 2 Mn ¿ −¿+ 2 Mn ( OH ) + 6 H ¿ 2
ig'ly polluted 1oderately polluted #lig'tly polluted
C"em #1$1 %e&tion, Room 'o$, Time
;2<
2
2
3
¿
2 I
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rganic &'emistry Laboratory
Formal Report ;1<
−¿
−¿+ I
2
→ I 3
¿
¿
I
T"e amount of oxygen in t"e system is dire!tly related to t"e amount of of
−¿¿ I 3
−¿¿
%rodu!ed. T"e titration
I 3
#it" t"iosulfate !an t"en be done to
determine DO !ontent in t"e sam%le. T"e rea!tion is s"o#n by :$uation 3. 6
;3<
−¿ ¿ 2−¿+ 3 I −¿ → S O ¿ ¿ 2 −¿+ I 4
6
*t forms a blue !om%lex #it"
3
2 S2 O 3
T"is rea!tion must be done in a!idi! !onditions. =ulfuri! a!id #as added be!ause :$uation 0 #ill not %ro!eed under neutral or basi! !onditions. K* #as added first in order for t"e rea!tion to %ro!eed to !om%letion. A 8fter addition of t"ese reagents+ t"e solution #as immediately titrated #it" Na2 S2 O3 solution until it #as %ale yello#. =tar!" #as t"en added. T"is !aused t"e solution to turn dark blue and t"e titration #as !ontinued until t"e blue !olor disa%%eared. T"is #as done #it" t"e solution in ea!" :rlenmeyer flask. =tar!" #as added as an indi!ator in t"is ex%eriment.
−¿¿ I 3
and t"e !olors
disa%%earan!e marks t"e end%oint of titration. A T"is reagent must be added near end of titration be!ause t"e !om%lex disso!iates slo#ly #"en it #as formed in t"e
¿
T"e goal of t"is ex%eriment #as to measure t"e DO !ontent and assess t"e ability of t"e system to sustain %resen!e of numerous life and its level of %ollution.
−¿¿ I 3
. T"e end%oint may be
delayed be!ause t"e !olor !"ange #ill not be as sudden. 8lso+ star!" breaks do#n in a!idi! !onditions so t"is affe!ts t"e indi!ation of end%oint #"i!" is #"y t"e boiling is maintained all t"roug"out. 1 8 glass bottle #as !overed #it" aluminum foil for sam%le !olle!tion. T"is #as !ondu!ted at a %ond in &in'ons )all. T"e bottle #as submerged in t"e #ater and tilted at around 13B+ and #as !a%%ed under#ater only #"en t"ere #ere no more bubbles !oming from t"e bottle. Ca%%ing under#ater %revents air from getting in. T"is !an !"ange t"e DO !ontent of t"e sam%le. Taking !are not to %rodu!e air bubbles in t"e solution+ >.3 m9 of 1.> M Mn SO 4 #as added before adding >.3 m9
Materials and Methods
T"e Winkler met"od is an iodometri! met"od. *odometry is a titration t"at involves t"e use of iodide as a redu!ing agent. *n !ontrast+ iodimetry is a titration t"at uses iodine as an oxidi'ing agent. One limitation of iodimetry is t"at iodine is a #eak oxidi'ing agent so some rea!tions+ es%e!ially t"ose #it" #eak redu!ing agents+ may not be !om%lete. 1+3 8%%roximately >.63 g of KI O3 #as dissolved in 3> m9 of distilled #ater in a beaker. T"e solution #as t"en $uantitatively transferred into a 6>>,m9 volumetri! flask and diluted to mark. T"ree 6>,m9 ali$uots #ere Na N 3 . T"e bottle #as t"en !losed+ t"en taken and %la!ed into 2 se%arate :rlenmeyer flasks. of t"e aO),K*, T#enty milliliters of #ater + about 6 gram of K* and 6> taking !are not to in!lude air bubbles+ and t"en s"ook t"oroug"ly. T"e !over #as removed and !on!entrated m9 of >.3 M H 2 SO4 #ere t"en added to ea!" flask. %"os%"ori! a!id #as added. T"e bottle #as !overed and :x!ess %otassium iodide #as added to t"e KI O 3 s"aken. 8 3>,m9 sam%le #as taken from t"e bottle and −¿¿ to !onvert it to I as s"o#n in :$uation 1 and 0. %la!ed in an :rlenmeyer flask. T"e solution #as t"en 3 Na2 S2 O3 titrated #it" t"e solution until %ale T"e volatility of t"e iodine %rodu!ed in :$uation 0 is yello#. =tar!" #as t"en added and !aused t"e solution redu!ed by its rea!tion #it" ex!ess iodide. T"is makes to turn blue and t"e titration #as !ontinued until t"e t"e iodine more stable and "el%s redu!e t"e loss due to !olor disa%%eared. T"is #as done t"ree times. volatility.0 T"e titration of t"e iodine #it" t"iosulfate is s"o#n ;1< * , / *? @ ;* 2<, by :$uation 3.
+¿ → 3 I + 3 H −¿+ 6 H ¿ −¿+5 I ¿ 2
;0<
I O3
C"em #1$1 %e&tion, Room 'o$, Time
2
O
¿
Au th or ’s Su rn am e ,
201! (age 'umber
rganic &'emistry Laboratory
Formal Report ;3<
−¿ ¿ 2−¿+ 3 I −¿ → S O ¿ ¿ 2 −¿+ I 4
-iven t"ese values+ t"e !on!entration of dissolved oxygen !an be !al!ulated using t"e stoi!"iometry of t"e Winkler titration. 8lso+ using Tables 6 and + t"e %ollution level and ability to sustain life of t"e system !an be assessed. T"ese are s"o#n on Table 1.
6
3
2 S2 O 3
¿
Table 4.
T"e stoi!"iometry of t"e standardi'ation %ro!edure is one mole of iodate is e$uivalent to 0 moles of t"iosulfate and in t"e Winkler titration+ four moles of t"iosulfate is e$uivalent to one mole of oxygen. Results and Discussion
*n t"is ex%eriment+ one of t"e aims #as to measure t"e DO !ontent using t"e Winkler met"od. T"e sam%le #as mixed #it" manganese (** sulfate and a solution of sodium "ydroxide+ %otassium iodide+ and sodium a'ide ( Na N 3 . T"e resulting solution #as t"en a!idified #it" %"os%"ori! a!id and titrated #it" sodium t"iosulfate. *n t"e %ro!edure+ t"e manganese (** sulfate #as added before t"e aO),K*, Na N 3 solution be!ause t"e overall rea!tion t"at forms t"e iodine #ill be faster t"is #ay. T"e rea!tion bet#een iodide and oxygen is slo# and is "am%ered by t"e %resen!e of aO)+ #"i!" #as added to %romote t"e %rodu!tion of Mn ( OH )2 + as t"is %ro!ess %refers a lo# %). T"e aO),K*, Na N 3 must be added immediately after t"e manganese (** sulfate be!ause of Mn ( OH )3 dis%ro%ortionation. T"e !an dis%ro%ortionate into
2
+¿
¿
Mn
and
MnO2
. T"is
makes t"e !on!entration of Mn( OH )3 de!rease so less of t"at s%e!ies #ill be available for t"e formation of iodine. T"is de!reases t"e !al!ulated DO !on!entration. 6 Table 2 s"o#s t"e volumes of t"iosulfate used %er trial. Table 3. Measured volumes for Winkler titration
3olume sample 3olume titrant
TRIAL 1
TRIAL
TRIAL !
of
"0+0 mL
"0+0 mL
"0+0 mL
of
!+, mL
!+, mL
!+" mL
C"em #1$1 %e&tion, Room 'o$, Time
Measured Dissolved Oxygen Content (%%m
D content Average D content A4uatic life Pollution level
TRIAL 1
TRIAL
TRIAL !
$+$2
$+$2 $+2"
$+!2
&annot sustain life for large organisms i+e+ .s'es ig'ly polluted
Waste #ater usually !ontains !ontaminants ot"er t"an organi! material. One of t"ese !ontaminants is nitrogen in t"e form of nitrite. T"is s%e!ies is toxi! to a$uati! life and is usually oxidi'ed into nitrate. *n !ases #"ere t"e nitrate !on!entrations are "ig"+ nitrite may still remain in t"e #ater. T"is interferes #it" t"e titration by rea!ting #it" iodine in a redox rea!tion. T"is #ould de!rease t"e volume of t"iosulfate used and #ould de!rease t"e !al!ulated DO. =odium a'ide #as added to rea!t #it" t"e nitrite to form N 2 and N 2 O . 6 8not"er ty%e of !ontaminant is metals. Co%%er and iron !an interfere #it" t"is met"od. T"e %resen!e of !o%%er !ataly'es t"e oxidation of t"iosulfate #it" air. T"is #ould in!rease t"e volume of titrant used+ and #ould in!rease t"e !al!ulated DO !ontent. A 5resen!e of iron also interferes #it" t"e Winkler met"od and is a %ossible sour!e of error. Ot"er !ontaminants may also interfere #it" t"e titration rea!tion by rea!ting #it" oxygen+ iodine or iodide. E T"e %resen!e of mi!roorganisms and organi! material in t"e sam%le also !auses error. 8fter !olle!tion+ t"ese mi!roorganisms !ontinue to de!om%ose t"e organi! material+ !onsuming oxygen in t"e %ro!ess. T"is de!reases t"e !al!ulated DO !ontent. *t is im%ossible to obtain a sam%le from t"e environment #it"out t"e %resen!e of t"ese so one #ay to lessen t"e effe!t is to immediately add Mn SO 4 and aO),K*, Na N 3
. T"e in!lusion of fis" and ot"er organisms t"at !onsume oxygen in t"e sam%le also !auses t"e same ty%e of error. )o#ever+ t"e in!lusion of %lants or algae #ill !ause t"e DO !ontent to in!rease t"roug" %"otosynt"esis. T"is #as %revented by !overing t"e
Au th or ’s Su rn am e ,
201! (age 'umber
rganic &'emistry Laboratory
Formal Report bottle in foil to kee% t"e sam%le in darkness. 5lants absorb oxygen in order to break do#n t"e !arbo"ydrates %rodu!ed during %"otosynt"esis and maintain t"eir metabolism. )o#ever+ #"en %"otosynt"esis !annot o!!ur+ %lants absorb more oxygen t"an t"ey release and t"is !auses t"e DO !ontent to de!rease. *f t"e sam%le #as left to stand overnig"t+ t"e organisms t"at are in t"e sam%le #ould de!rease t"e DO !ontent. Fnder a!idi! !onditions+ ex!ess iodide !an be oxidi'ed by oxygen in t"e air+ forming additional
−¿¿ I 3
. T"is is #"y t"e titration "as to be !arried out
immediately+ to minimi'e error resulting from t"is. T"is error in!reases t"e !on!entration of t"iosulfate in t"e standardi'ation and in!reases t"e !al!ulated DO !ontent.E 8not"er %otential sour!e of error is t"e %resen!e of dissolved !arbon dioxide in t"e t"iosulfate solution. T"is %romotes t"e dis%ro%ortionation of t"iosulfate into bisulfate and sulfur. T"is redu!es t"e !on!entration of t"e titrant over time and !an in!rease t"e volume used for t"e titration. One #ay to "el% %revent t"is is to boil t"e distilled #ater before use. A 7a!teria !an also !ause de!om%osition in solutions t"at #ere not fres"ly %re%ared. 8 #ay to !ountera!t t"is is to use fres" solutions+ boil t"e #ater t"at #ill be used in %re%aring t"e solution or add !"loroform to a bottle of t"iosulfate. 9ig"t also !ataly'es t"e de!om%osition so it is best to kee% lig"t ex%osure to a minimum. *n t"e measurement of DO !ontent+ t"e lo!ation of t"e sam%le origin is also very im%ortant. *n rivers and streams+ t"e turbulen!e+ vegetation+ and !urrent of t"e river !"anges along its lengt" and t"ese fa!tors affe!t DO !ontent.1 *n order to "ave an a!!urate %i!ture of DO !ontent+ t"e sam%les must be taken from different lo!ations around t"e river. 8not"er sour!e of error #ould be t"e sam%ling. *n our sam%le obtained+ t"e bottle "ad bubbles %resent in it indi!ating t"e %resen!e of air. T"is #ould result to an in!rease in DO !ontent sin!e air !ontains more oxygen+ t"us making it a %ositive error.A Conclusions and Recommendations
T"e goal of t"e ex%eriment #as to determine DO !ontent of a #ater sam%le and using t"at value+ assess t"e #ater $uality and t"e a$uati! systems ability to sustain life. To measure DO !ontent+ t"e Winkler met"od #as used by titrating t"e iodine %rodu!ed by
C"em #1$1 %e&tion, Room 'o$, Time
t"e rea!tion of
2
+¿
¿ Mn
+ oxygen+ and
−¿¿ I
. T"e DO
!ontent !al!ulated #as averaging 2.2 %%m O2 . T"is oxygen !on!entration indi!ates "ig" level of %ollution and t"at t"e stream !annot su%%ort large #ater organism life. T"e ex%eriment goals "ave been met sin!e t"e DO !ontent "as been identified and t"e degree of #ater %ollution and life,sustaining ability of t"e stream "as been assessed. To im%rove a!!ura!y+ it is re!ommended t"at t"e ex%eriment be re%eated multi%le times+ taking sam%les from different %arts of t"e stream to "ave a more a!!urate %i!ture of t"e "ealt" of t"e a$uati! system. 8lso+ it is re!ommended t"at t"e time bet#een sam%le !olle!tion and sam%le analysis be redu!ed to avoid DO !"anges !aused by mi!roorganisms and !ontaminants. Make sure to follo# !orre!t %ro!edure in sam%le !olle!tion to redu!e errors from air bubble %rodu!tion. *t is also im%ortant to mind t"e order of addition of reagents be!ause any !"ange from t"e %ro!edure !an !ause errors and t"at t"e titration of iodine must be done $ui!kly to redu!e errors from iodine volatility.
Re"erences ;6< *bane'+ G.-.+ )ernande',:s%ar'a+ M.+ Doria,=errano+ C.+ 4regoso,*nfante+ 8.+ =ing"+ M.M. :nvironmental C"emistrHyI =%ringer =!ien!eJ e# ork+ >>E. ;< =#edis" :nvironmental 5rote!tion 8gen!y. :nvironment Quality CriteriaJ 9akes and Water!ourses. "tt%JLL###.naturvardsverket.seLDo!uments L%ublikationerL0>,0>2,3.%df (a!!essed O!tober 0+ >63. ;2< Fnited =tates -eologi!al =urvey. Water %ro%ertiesJ Dissolved Oxygen. "tt%JLL#ater.usgs.govLeduLdissolvedoxygen."tml (a!!essed O!tober 0+ >63. ;1< C"ristian+ D.-. 8nalyti!al C"emistry+ 0t" ed.I Go"n Wiley =onsJ )oboken+ >>1. ;3< 5ier!e+ W.C.+ )aenis!"+ :.9.+ =a#yer+ D.T. Quantitative 8nalysis+ t" ed.I Go"n Wiley =onsJ e# ork+ 61E.
Au th or ’s Su rn am e ,
201! (age 'umber
rganic &'emistry Laboratory
Formal Report ;0< )arris+ D.C. Quantitative C"emi!al 8nalysis+ Et" ed.I W.). 4reeman Com%anyJ e# ork+ >6>.
;< O"io =tate Fniversity. Common Mis!on!e%tions about 5lants. ;A< =koog+ D. 8.+ West+ D. M.+ )oller+ 4. G.+ Crou!"+ =. "tt%JLLbeyond%enguins.e"e.osu.eduLissueL%olar, R. 4undamentals of 8nalyti!al C"emistry+ t" ed.I %lantsL!ommon,mis!on!e%tions,about,%lants (a!!essed 7rooksLColeJ 7elmont+ >61. O!tober A+ >63 ;E< 5atnaik+ 5. Deans 8nalyti!al C"emistry )andbook+ nd ed.I M!-ra#,)illJ e# ork+ >>1.
C"em #1$1 %e&tion, Room 'o$, Time
Au th or ’s Su rn am e ,
201! (age 'umber