TITRATION OF IODINE WITH STANDARD THISULPHATE SOLUTION GENERAL DISCUSSION: Oxidation and reduction titration involving Iodine is known
as Iodo etric titration !"#e indirect iodo etric titration et#od deals wit# t#e titration o$ iodine li%erated in c#e ical reactions !In t#is ex&eri ent iodide ions are oxidi'ed into iodine !"#e li%erated iodide (I)* +w#en titrated wit# standard sodiu t#iosul&#ate oxidi'es t#e latter w#ile itsel$ gets reduced to iodide ions! )Cu), , -I. I) , )S)O/) . )I. )CuI) (unsta%le* , S-O0). Cu)I) , I)
"#e latter reaction can %e used to deter ine t#e a ount o$ iodine li%rated in c#e ical reaction! In iodide.t#iosul&#ate titration t#iosul&#ate solution is run $ro %urette! In t#ese circu stances+ t#e colour c#anges $ro intense 1ellow or %rown (colour I )* to a colorless (colour o$ I.* at t#e end &oint! "#e end &oint is ade visi%le %1 t#e use o$ a solution o$ starc# as indicator! Starc# reacts wit# iodide in t#e &resence o$ iodide ions to $or an intensel1 %lue.colored co &lex! At t#e end &oint i!e !w#en t#ere is no iodine (I )* starc# gives colorless solution! "#oug# starc# is inex&ensive its ain disadvantage is t#at it adsor%s iodine and gives a water.insolu%le co &lex and titration %eco es less accurate! 2or t#is reason+ in titration o$ iodine t#e starc# s#ould not %e added until 3ust %e$ore t#e end &oint w#en t#e colour %eings to $ade i!e! t#e solution %eco es lig#t 1ellow! Anot#er &oint to re e %er is t#at evolved iodine is lost due to its a&&recia%le volatilit1! "#ere$ore titration s#ould not %e &rolonged unnecessaril1! "itration s#ould %e &er$or ed in cold solution in conical $lasks and not in o&en %eakers! 4out# o$ t#e conical $lasks containing iodine s#ould %e covered wit# watc# glass to c#eck t#e loss o$ iodine %1 eva&oration and at t#e %eginning o$ t#e titration eva&orated iodine s#ould %e trans$erred %ack to conical $lask %1 rinsing t#e watc# glass wit# water and collecting t#e water in t#e conical $lask! Anot#er i &ortant source o$ error in t#is titration is t#e oxidation o$ acid solution o$ iodide %1 ox1gen $ro t#e air: -I. , O) ,-5, )I) , )5)O
"#is reaction is catal1'ed %1 certain etal ions o$ varia%le valenc1 (&articularl1 co&&er* %1 NO). and also %1 strong sunlig#t! "#ere$ore+ iodine solution s#ould %e ke&t in dark and titration s#ould not %e &er$or ed in excess lig#t!
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EXPERIMENT: TO ESTIMATE THE AMOUNT OF COPPER IN A GIVEN SAMPLE OF COPPER (II) SULPHATE BY IODOMETRIC TITRATION. A66ARA"US: 7urette+ 6i&ette+ Conical $lask+ 8atc# glass "5EOR9: A known weig#t o$ su&&lied cu&&er (II* sul&late cr1stals is dissolved in water and certain volu e o$ t#is solution is treated wit# excess &otassiu iodine solution! "#e unsta%le co&&er (II* iodide (CuI)* $or ed deco &oses to give co&&er (I* iodide (Cu )I)* and $ree iodine! "#e li%erated iodine is titrated wit# standard sodiu t#iosul&#ate solution! )Cu), , -I. I) 2ro ) i!e , )S)O/). )CuI) S-O0). Cu)I) , I) , )I.
a%ove reactions it $ollows t#at: oles o$ Cu), : : ole o$ I)
oles o$ S)O/).
oles o$ Cu), :
ole o$ S)O/).
71 calculating t#e nu %er o$ oles o$ t#iosul&#ate ions consu ed in t#e titration t#e nu %er o$ oles o$ co&&er and t#e a ount o$ co&&er also can %e deter ined! (Relative ato ic PROCEDURE: :! 2ill u& t#e %urette wit# standard (=!: 4* Na)S)O/ solution! )! 8eig# out accuratel1 0!)-g o$ CuSO- cr1stals and dissolve it in water in )<=c /volu etric $lask! Add a%out ) c / o$ dil! Sul$uric acid to t#e solution (to re ove tur%idit1 due to #1drol1sis o$ t#e salt* and add ore water to ake u& t#e volu e u&to t#e ark! S#ake t#e solution well to ake it #o ogeneous! /! "ake :=c / o$ co&&er (II* sul&#ate solution in a conical $lask> add a onia solution dro& %1 dro& until little tur%idit1 is seen! Add acetic acid dro&wise to redissolve t#e tur%idit1 !"#is is done to re ove ineral acid (5)SO-* &resent in t#e solution ot#erwise in t#e &resence o$ ineral acid iodide ions are oxalised into iodine! -! Add <!=c / o$ (:!= 4* ?I solution to t#e $lask+ cover its out# wit# watc# glass and kee& t#e $lask in dark $or at least < inutes !Li%erated iodine is volatile !"#ere$ore+ it condenses on t#e watc# glass sur$ace! "rans$er t#is iodine into t#e $lask %1 was#ing it wit# water and collecting t#e water in t#e $lask! <! Now run t#e standard t#iosul&#ate solution $ro t#e %urette until t#e dee& %rown colour o$ iodine turns into &ale 1ellow colour!"#en add a%out :c / o$ starc# solution (as an indicator *to o%tain %lue colour! Again run t#e t#iosul&#ate solution dro&wise until t#e %lue colour o$ t#e solution 3ust disa&&ears! ass o$ Cu;0/!<-0*
0! Re&eat t#e &rocess (&rocedure no!/ to <* to o%tain at least two concurrent reading!
OBSERVATION: Concen !" #on o$ %o&#'( )#o%'*+)" e (N",S,O-) %o*' #on. No. Vo*'(e o$ C'So/ %o*' #on "0en In# #"* $#n"* B'!e e Re"&#n1 &#$$e!ence Conc'!!en !e"&#n1
"itration su ar1: Iodine li%erated $ro @@@@@@@@@@@@ c / co&&er (II* sul&#ate solution consu ed @@@@@@@@@@@@@@@ c / o$ =!:4 sodiu t#iosul&#ate (Na)S) O/* solution!
RESUL": a ount o$ co&&er in @@@@@ g o$ Co&&er (II* sul&#ate cr1stals is@@@@@@@@@@ g!
6ercentage A ount o$ co&&er (%1 weig#t* in co&&er (II* sul&#ate is @@@@@@@@@@@@
S6ACE 2OR CALCULA"ION:
