PHYSICAL cHEMISTRY - Ii

Experiment 5

Aim of the Experiment

Estimation of Iron present in Mohr's salt using Potassium Dichromate.

Principle

Ferrous ammonium sulphate [FeSO4, (NH4)2SO4,6H2O] is a stable double salt with FeSO4 being its active constituent. Acidic potassium dichromate (K2Cr2O7) solution is a strong oxidizing agent and is rapidly reduced by ferrous ion at the ordinary temperature to a green chromic salt when added to ferrous ammonium sulphate or Mohr’s salt [FeSO4, (NH4)2SO4,6H2O] solution containing dilute H2SO4. In this reaction ferrous sulphate is oxidized to ferric sulphate, while, ammonium sulphate remains unreacted.

K2Cr2O7 + 4 H2SO4 → K2SO4 + Cr2(SO4)3 + 4 H2O +3 [O]

6 FeSO4 + 3 H2SO4 + 3 [O]→ 3 Fe2(SO4)3 + 3 H2O

K2Cr2O7 + 6FeSO4 + 7H2SO4 = 3 Fe2(SO4)3 + K2SO4 + Cr2(SO4)3 + 7 H2O

Or, (Cr2O7) 2- + 6Fe2+ + 14H+ ----> 6Fe3+ + 2Cr3+

N-phenyl anthranilic acid is used as an indicator. Indicator is not oxidized as long as Fe2+ ions are there in the solution. The slight excess amount of dichromate will oxidize the indicator when all of the Fe2+ ions have been converted to Fe3+ ions resulting in colour change of the solution from greenish (due to Cr3+ ) to purple.

The green colour thus forms during the reaction is due to the reduced Cr3+ ions. Since, in this redox reaction there is no prominent colour change to indicate the end point of the titration, the addition of indicator is necessary. Nphenylanthranilic acid (Figure 1) is used as internal indicator here. Oxidized form of the indicator is of purple colour. As long as the solution contains the Fe2+ ions, the dichromate solution oxidizes only the Fe2+ to Fe3+. When all the Fe2+ ions are consumed, dichromate oxidizes the N-phenylanthranilic acid and the solution become purple coloured. The colour change is sharp and hence it is easy to detect the end point.

In this way,Potassium Dichromate ,K2Cr2O7 is a very powerful oxidizing agent in acid medium.

Dichromate ion reduces to two chromium(III) ions. This reaction requires 6 electrons and 14 (!) hydrogen ions:

Cr₂O₇^2-+ 14H⁺ + 6 e^- ---> 2Cr³⁺ + 7H₂O

In acid medium it is reduced to the green chromic ion,Cr+3

It can be obtained pure, therefore it can be used as a primary standard.

Only one electron is necessary to reduce Fe(III) to Fe(II)

Fe³⁺ + e^- ---> Fe²⁺

Therefore, 1 mole of Cr₂O₇^2-(the oxidizing agent) reacts with 6 moles of Fe²⁺ (the reducing agent) to form 6 moles of Fe³⁺ and 2 moles of Cr³⁺. Thus, in net ionic form:

Cr₂O₇^2-+ 6Fe²⁺ + 14H⁺ ---> 6Fe³⁺ + 2Cr³⁺ + 7H₂O

The molecular form of the reaction equation can be written as:

K₂Cr₂O₇ + 6Fe(NH₄)₂(SO₄)₂ + 7H₂SO₄ ---->

3Fe₂(SO₄)₃ + Cr₂(SO₄)₃ + K₂SO₄ + 6(NH₄)₂SO₄ + 7H₂O

The 1:6 mole ratio with respect to the amounts of Cr₂O₇^2-and Fe²⁺ consumed will provide the stoichiometric basis for all of the calculations in this experiment.

Materials required

1. Pipette (10 ml), burette (25 ml), conical flask (100 ml), beakers

2. Ferrous ammonium sulphate [FeSO4, (NH4)2SO4,H2O] (known as Mohr’s salt) solution of unknown strength

3. Standard potassium dichromate (K2Cr2O7) solution (1.0 g/L )

4. Diphenyl amine or (N-phenylanthranilic acid) as indicator

Procedure

A. Preparation of a Solution of K₂Cr₂O₇.

Weigh out 1.0-1.2 grams of K₂Cr₂O₇ , transfer into a 250 mL volumetric flask, dissolve this sample in distilled water, and carefully dilute to the mark with additional distilled water. Mix the solution thoroughly by stoppering the flask and inverting several times.

B. Titration of unknown Fe(II) solution

You receive a solution of unknown concentration in 100 mL volumetric flask. Dilute it carefully to the mark.

1. Using a 10 mL pipette, transfer exactly 10.00 mL of an unknown solution into an Erlenmeyer flask.

2. Using a graduated cylinder, add 25 mL of 1 M H₂SO₄ to each flask. Then add 10 mL of the l phosphoric acid solution and 8 drops of sodium diphenylamine sulfonate indicator to the flask. Swirl each flask gently to mix the contents.

3. Fill your burette with the K₂Cr₂O₇ solution and drain out enough so that the liquid level is just below the upper calibration mark and the burette tip is full. Read the initial volume from the calibration scale on the burette. This reading and all other burette readings should be estimated to the nearest 0.01 mL.

4. Titrate the iron solution in the flask. The intense purple color produced by the first drop of excess K₂Cr₂O₇ signals the end point for the titration. Obtain the final volume reading from the calibration scale on the burette.

Observation table

Calculations

Normality of the potassium dichromate solution = N2 = Given

Let us say the strength of ferrous ammonium sulphate solution = N1

We know,

N1V1 = N2V2

Where, V1= volume of ferrous ammonium sulphate solution taken = 10 ml

V2= volume of potassium dichromate solution required.

Strength of ferrous ammonium sulphate solution = Normality * Equivalent wt. of FAS (392.13 g/mol)

Result

The strength of ferrous ammonium sulphate solution was found to be ………… g/L.

Notes

1) Before the end point, colour of the solution is light green and at the end point, purple colour appears. The depth of the purple colour should not be considered, once the purple colour of the solution persists, the titration should be stooped.

2) For this titration, instead of Mohr’s salt, only FeSO4.7H2O is not used for the following reason: Ferrous sulphate salt is not stable since it gets oxidized by the oxygen (in air), when it is exposed to air. On the other hand, ferrous sulphate in Mohr’s salt is quite stable.

3) The benzenoid form of N-anthranilic acid is colourless. On oxidation it gets converted to quinoid form which after dimerization forms a compound that resulted in violet-red colour solution at the end point.

4) In acid solution, the reduction of dichromate is: Cr2O7 2- + 14 H+ + 6 e → 2 Cr3+ + 7 H2O, it follows that the equivalent weight is one-sixth of the molecular weight, 294.22/6 = 49.037 g.

Reference Material

1. Practical Physical Chemistry Google EBOOK Link.

Questions

1.Explain the colour change that takes place at the end-point of the titration from greenish to violet-red colour.

2. Why only ferrous sulphate is not used to make a primary standard solution instead of ferrous ammonium sulphate?

3. Explain that the equivalent weight of potassium dichromate in acidic medium is one sixth of its molecular weight.