11. Estimation of iron content of the given solution using potentiometer

11. ESTIMATION OF IRON CONTENT OF THE GIVEN SOLUTION USING POTENTIOMETER

Expt. No.

Date:

AIM

To estimate the amount of Iron (ferrous iron (Fe²⁺)) present in 100 ml of the given solution using potentiometer. A standard solution of potassium dichromate of strength .......... N is provided.

PRINCIPLE

Potentiometric titrations depend on measurement of emf between reference electrode and an indicator electrode. When a solution of ferrous iron is titrated with a solution of potassium dichromate, the following redox reaction takes place.

During this titration Fe²⁺ is converted into Fe³⁺, whose concentration increases. At the end point, there will be a sharp change due to sudden removal of all Fe²⁺ ions.

The cell is set up by connecting this redox electrode with a calomel electrode as shown below.

Hg / Hg₂Cl_2(s) KCI // Fe²⁺, Fe³⁺ / Pt

A graph between emf measured against the volume of K₂Cr₂O₇ added is drawn and the end point is noted from the graph.

MATERIALS REQUIRED

1. Potentiometer 2. Pt electrode 3. Saturated Calomel electrode 4. Standard K2Cr207 solution 5. Given ferrous iron solution 6. Burette and pipette 7. Dil. H₂SO₄ 8. Distilled water

PROCEDURE

The given iron (ferrous iron) solution is transfered into 100 ml standard flask and made upto the zero mark using distilled water. 20 ml of this made up solution is pipetted out into a clean 100 ml beaker. About 10 ml of dil H₂SO₄ and 20 ml of distilled water are added in it. A platinum electrode is dipped into the solution. This electrode is then coupled with a saturated calomel electrode and the cell is introduced into potentiometric circuit.

TITRATION - I

The burette is washed well with water and rinsed with the given K₂Cr₂O₇ solution. It is then filled with the same upto zero mark and titrated against the ferrous iron solution taken in the connical flask.

The addition is carried out by adding std. K₂Cr₂O₇ solution in portions of 1 ml and the emf of the cell is measured after each addition. The addition of K₂Cr₂O₇ is continued even after the end point and the range at which end point lies is found out by plotting volume of K₂Cr₂O₇ added against emf (graph - 1).

TITRATION - 11

Another titration is carried out by adding std. K₂Cr₂O₇ solution in portions of 0.1 ml near the end point and the emf of the cell is measured after each addition. The addition of K₂Cr₂O₇ is continued even after the end point for further 1 ml. The accurate end point is determined by plotting ΔE/ΔV Vs Volume of K₂Cr₂O₇added (graph - 2). From the end point, the strength of ferrous iron solution and hence its amount is calculated.

Step II

Calculation of strength of ferrous iron solution

Volume of ferrous iron solution, V₁ = 20 ml

Strength of ferrous iron solution, N₁ = ........?

Volume of K₂Cr₂O₇, V₂ = ............... ml

Strength of K₂Cr₂O₇, N₂ = ........ N

According to the law of volumetric analysis V₁N₁ = V₂N₂

RESULT

The amount of ferrous iron present in 100 ml of the given solution is = …….. gms.