ANALYTICAL cHEMISTRY - II

Experiment 4

Principle

• The reduction potential of any redox system is given by the Nernst equation, 𝐄 = 𝐄° + (𝐑𝐓 / 𝐧𝐅) 𝐥𝐨𝐠 ([𝐑𝐞𝐝𝐮𝐜𝐞𝐝 𝐒𝐭𝐚𝐭𝐞] / [𝐎𝐱𝐢𝐝𝐢𝐬𝐞𝐝 𝐒𝐭𝐚𝐭𝐞])

• For a Ferrous – Ferric redox system, Fe²⁺ ↔ Fe³⁺,

𝐄 _(𝐅𝐞^𝟑+_/𝐅𝐞^𝟐+₎ = 𝐄°_(𝐅𝐞^𝟑+_/𝐅𝐞^𝟐+₎ + (𝐑𝐓/𝐧𝐅) 𝐥𝐨𝐠 ([𝐅𝐞^𝟐+] / [𝐅𝐞^𝟑+])

where, 𝐄°_(𝐅𝐞^𝟑+_/𝐅𝐞^𝟐+₎ is the redox potential of the Ferrous–Ferric redox system.

• The cell constructed is, 𝐏𝐭/𝐇𝐠_𝟐𝐂𝐥_𝟐/𝐊𝐂𝐥(s𝐚𝐭𝐮𝐫𝐚𝐭𝐞𝐝) II 𝐅𝐞^𝟑+/𝐅𝐞^𝟐+/𝐏𝐭

• The observed EMF of the cell is given by, 𝐄_o𝐛𝐬 = 𝐄_red – 𝐄_oxi

𝐄_obs = 𝐄°_(𝐅𝐞^𝟑+_/𝐅𝐞^𝟐+₎ + (𝐑𝐓/𝐧𝐅) 𝐥𝐨𝐠 ([𝐅𝐞^𝟐+] / [𝐅𝐞^𝟑+]) – 𝐄_oxi

where, 𝐄_oxi is the standard electrode potential of calomel electrode, which is equal to 0.2422 V.

• The addition of potassium dichromate increases the observed EMF. At the endpoint, there is a sharp increase in EMF due to the complete oxidation of ferrous ion (Fe²⁺) to ferric ion (Fe³⁺).

Materials required

Potentiometer, Beakers, Burette, Pipette, Glass rod, Potassium dichromate, FAS, Sulphuric acid, Distilled water, etc.

Procedure

1. The burette is filled with the given standard potassium dichromate solution, following usual precautions.

2. The given ferrous ion solution is made up to 100 mL in a standard measuring flask.

3. 20 mL of the prepared ferrous ion solution is pipette out into a clean 250 mL beaker.

4. The equal volume of the dilute H₂SO₄ solution, and 100 mL distilled water are added to enable the electrodes to immerse well in the solution.

5. A platinum electrode (indicator/working electrode) and a standard calomel electrode (reference electrode) from the potentiometer are dipped into the beaker.

6. The solution in the beaker is stirred using a magnetic stirrer/glass rod and the initial EMF is noted.

7. A 0.5 mL of potassium dichromate solution is added from the burette, at regular intervals, stirred well, and the EMF is measured.

8. The volume of the solution added and the corresponding EMF readings are noted.

9. At the endpoint, there is a sharp increase in EMF due to the complete oxidation of ferrous ion to ferric ion.

10. The addition of potassium dichromate solution is continued until the equivalent point is crossed by at least 5 mL.

11. From the observed values, ΔE = E2 − E1, ΔV = V2 − V1 and ΔE / ΔV is calculated and a graph is obtained by plotting ΔE / ΔV (Y–axis) versus the volume of potassium dichromate added (X–axis).

12. A smooth curve is obtained by joining the points.

13. The peak in the graph indicates the endpoint.

14. From the endpoint, the amount of the ferrous present in the given solution is calculated.

Result

• The amount of the ferrous ion in the given solution is _________ gm.

Reference Material

1. G H Jeffery, J Bassett, J Mendham and R C Denney, Vogel's Textbook of Quantitative Chemical Analysis, 5th Edition