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Introduction:

The separation and estimation of copper and zinc from a mixture involves two

steps-

(a) Determination of copper volumetrically in presence of zinc

(b) Separation of copper and determination of zinc gravimetrically

In the process of estimation of copper volumetric titration is used!In

volumetric titration, a known volume of a standard solution (one whoseconcentration is known) reacts with a known volume of a solutionwhose concentration is unknown. This procedure standardizes

thelatter solution, by allowing a calculation of its concentration.

Titration of copper solution is done by standard solution ofsodium thiosulphate ("a#S#$%)! &or this purpose first we have to standardize sodium

thiosulphate by standard solution of potassium dichromate ('#r#$)! This is an

oxidation-reduction titration!

Determination of zinc is done by gravimetric analysis! *ravimetric analysis is aprocess of producing and weighing a compound or element in as pure a form as possible

after some form of chemical treatment has been carried out on the substance to be

examined! Traditional gravimetric determinations have been concerned with thetransformation of the element ion or radical to be determined into a pure stable

compound which is suitable for direct weighing or for conversion into another chemical

form that can be readily +uantified!

(a) Determination of Copper volumetrically in presence of Zinc-

&or determining the amount of copper Iodometry is used! Iodometry is anapplication of iodine chemistry to oxidation-reduction titrations for the +uantitative

analysis in certain chemical compounds in which iodine is used as a reductant and the

iodine freed in the associated reaction is titrated usually in neutral or slightly acidmediums with a standard solution of a reductant such as sodium thiosulfate or sodium

arsenite, examples of chemicals analyzed are copper(III) gold(I) arsenic()antimony() chlorine and bromine!

&or determining copper in the solution the liberated iodine ions are titrated by

standard sodium thiosulphate ("a#S#$%)! &or this first we have to standardize "a#S#$%!

.otassium dichromate is used to titrate "a#S#$%! Starch is used as a indicator here!

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#u#/ / 0'I 1 u#I# / I# / 0'/

#u#/ 2 I# 2 #"a#S#$%

&or the determination of copper liberated iodine ions are produced by adding

potassium iodide ('I)into the copper salt solution! Sometimes free mineral acid is

present in the solution for that a few drops of dilute sodium carbonate ("a#$%) solution

or 343 ammonia solution must be added until a faint permanent precipitate remains! Thisprecipitate is removed by means of a drop or two of ethanoic acid! 'I is then added and

the titration continued! &or accurate results the solution should have a p5 of 0 to 6!6!

7hen titrating with "a#S#$%the color of the solution becomes faded from brown! Then

3or # ml of starch is added and titration is continued until the blue color begins to fade!This time 3 ml of 389 ammonium thiocyanate ("50"S)is added and immediately the

blue color of the solution will become intense! "ow the titration is completed as +uic:lyas possible! The precipitate possesses a light green color and a distinct permanent end

point is readily obtained!

(b) Separation of Copperand Determination of Zinc gravimetrically-

5ere first we have to separate copper from the solution then we can determine

zinc gravimetrically!

&or separating copper 3 or # g of sodium sulphite ("a#S$%)is added which

reduces u#/

to u/

! Then 36 ml of freshly prepared ammonium thiocyanate ("50"S)solution is slowly added from a burette with constant stirring! "50"S reacts with u

/

to precipitate u"S! The precipitate should be white and the mother li+uor should be

colorless!

"a#S$% "50"S

u#/ u/ u"S

(7hite precipitation)

&inally the precipitation is filtered through an ordinary filter paper washed with39 "50"S and discarded!

&or gravimetric analysis the filtrate is first evaporated to about 388 ml! ;inc isprecipitated in the form of zinc pyrophosphate (;n#.#$)by adding 36 ml of 389

diammonium hydrogen phosphate(("50)#5.$0)!

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to neutralize the solution by adding ammonia solution and chec: the neutrality by methyl

orange indicator!"ow the precipitate is filtered through 7hatman "o!08 and transferred to a

previously ignited and weighed porcelain crucible along with the filter paper! &inally the

crucible is ignited ta:ing usual precautions! "ow from the residual of the ignition we cancalculate the amount of zinc present in one liter of supplied sample!

Chemicals:

"a#S#$% '#r#$, 'I,"50$5 "50"S,"a#S$%,"50"S,("50)#5.$0, methyl

orange!

Calculations:

(a) Determination of Copper volumetrically in presence of Zinc-

Standardization of "a#S#$%by standard '#r#$ solution

"o! of

$bs!

olume of

'#r#$

ml

#3!? @!>

@!>? # 38 #3!? %3!6 @!@

% 38 %3!6 03!0 @!@

5ere olume of '#r#$ 31 38 ml

olume of "a#S#$% #1 @!>? ml "ormality of '#r#$ "31 8!3 "

3 x "3

"ormality of "a#S#$% "# 1

#

38 x 8!3

1@!>?

1 8!3830 "

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Determination of copper

"o! of $bs! olume of opper

Solution ml

g

7eight of ;n#.#$! 8!886 g

3 g of ;n#.#$contains 8!%3%# g of zinc

8!886 g of ;n#.#$contains 8!%3%# x 8!886 1 3!6?? x 38-% g