Experiment Name:
Separation and Estimation of Copper and Zinc from a Mixture Theory: To separate and estimate Copper and Zinc from a mixture it is needed to perform three steps. a) Standar Standardiz dizati ation on of sodium sodium thiosu thiosulf lfate ate (Na (Na2S2O3) with standard potassium dichromate ( 2Cr 2O!) solution. ") #etermination of Copper (Cu) $olumetricall% in presence of Zinc (Zn). c) Separation Separation of Copper (Cu) (Cu) and determinat determination ion of Zinc (Zn) (Zn) &ra$imetric &ra$imetricall% all%.. The determination of Cu from a solution is prefera"l% done "% idometric titration. In
volumetric titration, a known volume of a standard solution (one whose concentration is known) reacts with a known volume of a solution of unknown concentration. his procedure procedure standardi!es standa rdi!es the latter solution, "# allowin$ a calculation of its concentration. he stren$th of %a&S&' is determined "# titratin$ it with standard &Cr&'*. +etermination +etermination of Zn is done "# $ravimetric anal#sis. he uantitative determination determination of a su"stance "# precipitation followed "# isolation and wei$hin$ of the precipitate precipitate is called $ravimetric anal#sis.
a) Standardization of Sodium Thiosulfate with standard Potassium dichromate solution: 'n acidic media excess ' reduces 2Cr 2O! and li"erates eui$alent amount of '2. The Na2S2O3 is oxidized "% this li"erated iodine. 'n acidic media 2Cr 2O! is a stron& oxidizin& a&ent that*s wh% +Cl is added. The reactions that occur are mentioned "elow, 2Cr 2O! - / +Cl - 0 '
1 2 CrCl3 -3 '2 -! +2O-Cl
0Na2S2O3 -3'2
1 3 Na2S/O0 - 0 Na'
This reactions is idometric "ecause in idometric reaction '2 li"erates with an oxidizin& a&ent (here 2Cr 2O!) comes from '. '2 is less sta"le in + 2O so excess amount of ' solution is used.
'2
(a)
- ', (a)
1 '3 (a) (deep "rown)
'2 has hi&her atmospheric pressure as it melts awa% in a ir and oxidized "% O2 . This phenomena causes wron& calculation in experiment due to less amount of ', . So the conical flas is co$ered with watch &lass . /',
- O2
- / +-
1 2'2
(&)
- 2+2O
To mae inert atmosphere CO2 la%er is produced upon the conical flas( ', solution) for pre$entin& oxidizin& of ', from Na+CO3 salt solution. Na+CO3 - +Cl 1 NaCl - +2O - CO2 +ere 2Cr 2O! wors as an oxidizin& a&ent and it can oxidize properl% in presence of +Cl acid. Cr 2O!2, - /+- -0 ', 1 2Cr 3- - 3'2 - ! +2O 4rom the a"o$e en. we o"ser$e ' 5 is the reducin& a&ent that comes from ' . This reducin& a&ent "ecomes oxidized into '2 . This li"erated iodine reacts with Na2S2O3 . '2 - 2Na2S2O3
1
Na2S/O0 - 2Na'
6fter completion of this reaction starch is used as an indicator. 7ecause of followin& the a"o$e steps there remains some '2 into the conical flas when starch is added it maes an a"sorption complex with iodine distin&uished "% deep "lue color. The iodine of this starch,iodine complex reacts with Na2S2O3 . Starch
- '2 1 Starch,iodine a"sorption complex
Starch,iodine a"sorption complex - 2Na2S2O3 1 starch - 2Na' - Na 2S/O0 So a&ain the "lue color will disappear and the solution will ha$e li&ht "lue color or li&ht &reen due to the presence of Cr 3-. 7% this specific color chan&e the end point is identified.
b) Determination of Cu olumetrically in presence of !n: The determination of Cu from a solution is prefera"l% done "% idometric titration. 'odometr% is an application of iodine chemistr% to oxidation,reduction titrations for the uantitati$e anal%sis in certain chemical compounds in which iodine is used as a reductant and the iodine feed in the associated reaction is titrated usuall% in neutral or sli&htl% acid mediums with a standard solution of a reduc tant such as sodium thiosulfate or sodium arsenite8 examples of chemicals anal%zed a re copper (''') &old (9') arsenic (9) antimon% (9) chlorine and "romine. Since the stoc solution is acidic8 it has to "e neutralized "% alaline solution. Then ' is added and the iodine li"erated "% Cu is then titrated with Na2S2O3 solution which was
standardize "efore. 4rom the $olume of Na2S2O3 of nown stren&th we can determine the amount of Cu :resent in stoc solution.
2Cu2- - /' 1 2Cu' - '2 - / 2Cu2- ; '2 ; Na2S2O3
Sometimes free mineral acid is present in the solution for that a few drops of dilute sodium car"onate (Na2CO3) solution or < ammonia solution must "e added until a faint permanent precipitate remains. This precipitate is remo$ed "% means of a drop or two of ethanoic acid. ' is then added and the titration continued. 4or accurate results the solution should ha$e a p+ of / to =.=.
c) Separation of Copper "Cu) and determination of !inc "!n) #raimetrically: The determination of Zn from a solution is p refera"l% done "% &ra$imetric titration. +ere Na2S2O3 is a reducin& a&ent . 't reduced Cu-- to Cu- .This Cureacts with N+/CNS and creates the white precipitate of Cu CNS. The +2SO/ in solution is neutralized "% N+3 solution. Zn-- reacts with (N+/)2+:O/ (diammonium h%dro&en phosphate) and form Zinc p%rophosphate (Zn+:O/). The precipitate of Zn+:O/ is i&nited in a porcelain cruci"le and it dri$es up to Zn2:2O!. 4rom the amount of Zn2:2O! we can calculate the amount of Zn.
Cu 2-
Cu Na2SO3
CuCNS (>hite precipitation) N+/CNS
Zn2- - (N+3)2+:O/ 1 Zn(+:O/) - 2N+3 Zn(+:O/) , +2O 1 Zn2:2O!
Chemicals $noled: Na2S2O3 2Cr 2O!, ', N+/O+ N+/CNS, Na2SO3, N+/CNS, (N+/)2+:O/, meth%l oran&e.
Procedure:
4irst we ha$e to standardize Na2S2O3. :otassium dichromate is used to titrate Na2S2O3. Starch is used as indicator here. 4or the determination of copper li"erated iodine ions are produced "% addin& potassium iodide (') into the copper salt solution. Sometimes free mineral acid is present in the solution for that a few drops of dilute sodium ca r"onate (Na2CO3) solution or < ammonia solution must "e added until a faint permanent precipitate remains. This precipitate is remo$ed "% means of a drop or two of ethanoic acid. ' is then added and the titration continued. 4or accurate results the solution should ha$e a p+ of / to =.=. >hen titratin& with Na2S2O3 the color of the solution "ecomes faded from "rown. Then or 2 ml of starch is added and titration is continued until the "lue color "e&ins to fade. This time ml of ?@ ammonium thioc%anate ( N+/CNS) is added and immediatel% the "lue color of the solution will "ecome intense. Now the titration is completed as uicl% as possi"le. The precipitate possesses a pale pin color and a distinct permanent end point is readil% o"tained. 4or separatin& copper or 2 & of sodium sulphite ( Na2SO3) is added which reduces Cu2- to Cu-. Then = ml of freshl% prepared ammonium thioc%anate ( N+/CNS) solution is slowl% added from a "urette with constant stirrin&. N+/CNS reacts with Cuto precipitate CuCNS. The precipitate should "e white and the mother liuor should "e colorless. 4inall% the precipitation is filtered throu&h an ordinar% filter paper washed with @ N+/CNS and discarded. 4or &ra$imetric anal%sis the filtrate is first e$aporated to a"out ?? ml. Zinc is precipitated in the form of zinc p%rophosphate (Zn2:2O!) "% addin& = ml of ?@ diammonium h%dro&en phosphate((N+/)2+:O/). 7ut the reaction "etween zinc and (N+/)2+:O/ tae place in neutral condition. 4or this "efore addin& (N+/)2+:O/ we ha$e to neutralize the solution "% addin& ammonia solution and chec the neutralit% "% meth%l oran&e indicator.
Data Tables: a) Standardization of Na%S%&' by standard (%Cr%& with starch as a indicator: %o of -olume o"serv of ation &Cr&' * (ml)
Initial "urette eadin $ (ml)
/inal -olume 0vera$e "urette of (ml) eadin %a&S&' $ (ml) (ml)
Stren$th of %a&S&' (M)
2 3
?.? 2.3 2/.0
2. 2/.0 30.A
?.?!0
? ? ?
Calculation:
2. 2.3 2.3
2.23
+ere 9olume of 2Cr 2O! 9 1 ? ml 9olume of Na2S2O3 92 1 2.23ml ?./= x ?. Normalit% of 2Cr 2O! N 1
1 ?.?AA
?./A x N
9 Normalit% of Na2S2O3 N2 1
∴
92 ?
x
.?AA
1 2.23 1 ?.?!0
b) Estimation of Cu olumetrically in presence of !n* Titratin# with standard Na%S%&': %o of -olume '"serv of ation mixture( ml)
Initial 1urette eadin$(m l)
/inal 1urette eadin$( ml)
-olume of %a&S&'( ml)
0vera$e volume of %a&S&'
2 3
?.? 2.! 2=.3
2.= 2=. 3!.0
2.= 2./ 2.3
2./=
Calculation:
? ? ? B
2./=ml ?.?! N
∴
N
Na2S2O3
Na2S2O3
; ?.?03=/ & Cu
; ?.?03=/ x 2./= x ?.?! 1 ?.?0/03 & Cu
'n ? ml solution remains ?.?0/03 & Cu ∴
'n B solution remains
?.?0/03 x ??? ? 1 0./03& Cu B
c) Separation of Copper "Cu) and determination of !inc "!n) #raimetrically: >ei&ht of the empt% cruci"le 1 /.0/= & >ei&ht of the cruci"le with Zn2:2O! 1 /.!0!2 & ∴>ei&ht of Zn2:2O! = ?.?!2& & of Zn2:2O! contains ?./2A2 & of zinc ?./2A2 x ?.?!2 1 33.=03x ?,3 & ∴?.?!2 & of Zn2:2O! contains
+esult: he amount of Copper ( Cu) 2 3.43$5l he amount of Zinc (Zn) 2 .63x789$
Discussion: Not onl% this experiment "ut also all experiment must "e done $er% carefull%. >hen we estimated Cu and Zn we should "e $er% careful from the "e&innin& of the experiment "ecause, >hen we standardizin& Na2S2O3 "% standard 2Cr 2O! then the standardization must "e correct if we don*t standardize Na2S2O3 then the estimation of Cu would not "e perfect. 'n standardization of Na2S2O3 Na+CO3 D +Cl is added to create inert en$ironment otherwise iodide ', mae reaction with air. 2', - O2 -/+- 1 '2 - +2O so to create inert en$ironment this reaction happens Na+CO3 -+Cl 1 NaCl - +2O - CO2
>hen Cu is estimated then Na2S2O3 D C+3COO+ is added for same purpose which descri"ed in Na2CO3 - C+3COO+ 1 C+3COONa - +2O - CO2 To indicate the end point starch is added at last . when Na2S2O3 is standardized and Cu is determined. Starch mae reactions with the remainin& iodide ion in the solution. 'f starch is added at first then it maes reaction with iodine D iodide ion will mae wron& result. >hen Cu is estimated N+/CNS is added after addin& starch "ecause it maes reactions with Cu2'2 D '2 is produced . Otherwise the correct result will not achie$es 2N+/CNS - Cu2'2 1 2N+3' - 2CuCNS - '2
The mass of cruci"le and Zinc :%rophosphate is wei&hted carefull% for &ettin& proper result. Cu is separated "% filtration it should "e done carefull% if Cu remained in solution the correct wei&ht of Zn would not achie$e
,erits: The merit of a $olumetric anal%sis is that it can "e done in a short time. 'f we "e careful to trace the end point then the result would "e uite accepta"le. 'n a industr% time is a $er% important factor . when approximate result is accepta"le then we can use $olumetric anal%sis to determine Cu. On the other hand &ra$imetric anal%sis has it*s own merits. 7% this process we can determine the exact amount of a su"stance. 'n a la"orator% of science when we need to determine the exact amount then &ra$imetric anal%sis is mostl% used.
Demerits: Thou&h $olumetric anal%sis can "e done within a short time there is more pro"a"ilit% of &ettin& wron& result. The wron& result can "e caused "% not usin& "urette and pipette correctl%. 6&ain &ra$imetric anal%sis needs much time. >e should heat the solution and wait to o"tain a &ood precipitate 'f we mae mistae in wei&htin& of cruci"le there would occur a discrepanc% in result.
