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Expt. No: 5555 Determination of Strength of Iron and the number of Water

molecules of Crystallization in Mohr’s salt using Standard Potassium Dichromate solution

Aim

To determine the strength of iron and the number of water molecules of crystallization in

Mohr’s salt provided standard potassium dichromate solution (N/20), using diphenyl amine as

internal indicator.

Theory

Mohr’s salt is ferrous ammonium sulphate [FeSO4.(NH4)2SO4.6H2O]. For determination of

the amount of iron in the given solution of Mohr’s salt, a known volume of this solution is titrated

with standard potassium dichromate solution (K2Cr2O7) in a medium acidified with

dilute sulphuric acid. Potassium dichromate oxidises ferrous sulphate (FeSO4) present in Mohr’s salt

into ferric sulphate [Fe2(SO4)3].

����� ���� �������� ���� + ��� × �

������� + ���� + ��� ��� � ������������� + ����

!����� � �:����� + ������� + ���� ⟶ ����� + ����� + ����

For finding out the end point, internal indicator diphenyl amine is used. At the end point, all

the ferrous ions present in the solution get completely oxidised to ferric ions by chromate ions and as

soon as a slight excess of potassium dichromate solution is added. It leads to the oxidation of

diphenyl amine which results in the formation of a blue coloured complex. This indicates the end

point of the titration.

NH

Diphenyl amine

Oxidation

with K 2Cr2O7

Blue Coloured complex

The number of water of water molecules of crystallization in Mohr’s salt can be calculated

from the following equation,

%����&�' (')����*�+�%����&�' (��')����*�+� =

�-� + �-��-�

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Observations and Calculations:

S.No

Volume of given

solution of Mohr’s salt

in ml

Burette Readings in ml Volume of

K2Cr2O7 solution in ml Initial Final

1

2

3

4

Concordant Volume of K2Cr2O7 solution, V1 =………ml

Normality of standard K2Cr2O7 solution, N1 = ��. = .. .0!

Volume of standard K2Cr2O7 solution, V1 = ……………ml

Volume of given solution of Mohr’s salt, V2 = ………….....ml

Normality of given solution of Mohr’s salt, N2 can be calculated from the normality formula,

i.e., N1 x V1 = N2 x V2

Normality of given solution of Mohr’s salt, N2 = !� × 1�

1�

= ………………..N

The strength of iron in the given sample of

Mohr’s salt = !� × 23.4� (5� � = !� × 00. -0 g/Lit

= ………………g/Lit

The strength of anhydrated Mohr’s salt = !� × 23.4� = !� × �-� g/Lit

= ……………….. g/Lit

The strength of hydrated Mohr’s salt = 20 g/Lit

StrengthofhydratedsaltStrengthofanhydratedsalt =

20g/Lit…… . g/Lit =

�-� + �-��-�

The number of water molecules of crystallization in Mohr’s salt, x

= ………………..

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Apparatus:

Burette, Conical flask, Pipette, Measuring flask

Reagents:

Standard potassium dichromate solutionJ !0.K , dilute sulphuric acid, 1:1 phosphoric acid,

Indicator diphenyl amine

Procedure:

Pipette out 20 ml of given solution of Mohr’s salt into a conical flask, add 5 ml of dilute

sulphuric acid, 2-3 ml of 1:1 phosphoric acid and then two drops of diphenyl amine to this solution.

Run the potassium dichromate solution in small lots from the burette, shaking the conical flask after

each addition and also stirred at regular intervals. At the end point, the colourless solution becomes

deep blue. Note down the volume of the titrant used. The titration is repeated until a concordant

volume is obtained.

Result:

(i) The strength of iron in the given sample of Mohr’s salt is………………….g/Lit

(ii) The number of water molecules of crystallization in Mohr’s salt………….