6

ESTIMATION OF FERROUS IRON BY DICHROMETRY

AIM

To estimate the amount of ferrous iron present in 100 ml of the given solution by using approximately 0.05N potassium dichromate solution.

PRINCIPLE

The estimation is based on redox titration. Ferrous iron is estimated by dichrometry using diphenylamine indicator. Potassium dichromate oxidises ferrous iron to ferric iron in an acidic medium.

The addition of orthophosphoric acid is to reduce the redox potential of the ferrous to ferric iron and to obtain the sharp change of the end point of titration from a colourless to a permanent violet blue colour solution.

PROCEDURE

Part A: Preparation of Standard Mohr’s Salt Solution

Take approximately 2 g of crystalline ferrous ammonium sulphate (Mohr’s salt) in a clean and dry weighing bottle and weigh the substance. Transfer the chemical substance into a clean 100 ml standard flask with the help of a glass funnel. After transferring the substance, weigh the empty bottle accurately. The difference between the two values gives the weight of Mohr’s salt. First add 5 ml of dilute sulphuric acid to Mohr’s salt to avoid the oxidation of ferrous iron to ferric iron. Then, dissolve it in a minimum quantity of distilled water and finally make it up to the mark. Shake the standard flask well to obtain a standard solution of uniform concentration and keep it aside for further use.

Weight of weighing bottle + Mohr’s salt (W₁) = _____ g

Weight of empty weighing bottle (W₂) = _____ g

Weight of Mohr’s salt W = (W₁ − W₂) = _____ g

Gram equivalent weight (GEW) of Mohr’s salt = 392.13

Gram molecular weight (GMW) of Mohr’s salt = 392.13

Part B: Standardisation of Potassium Dichromate Solution

Clean a burette with tap water and then rinse it with distilled water as well as the potassium dichromate solution. Fill it with the potassium dichromate solution and note down the initial reading. Pipette out 20 ml of the standard Mohr’s salt solution into a clean conical flask. Add 20 ml of dilute sulphuric acid, 5 ml of phosphoric acid and two to three drops of diphenylamine indicator. Titrate the solution mixture against the potassium dichromate solution. The end point of titration is the appearance of a permanent violet blue colour. Repeat the titration until two consecutive concordant values are obtained. Note down the titration values. Calculate the normality of the potassium dichromate solution from the titre values.

Part C: Estimation of Ferrous Iron

The given unknown ferrous iron solution is make up to the mark by adding distilled water and shake well to get uniform concentration. Pipette out 20 ml of the made-up ferrous iron solution in a clean conical flask. Add 20 ml of dilute sulphuric acid, 5 ml of orthophosphoric acid and three drops of diphenylamine indicator. Titrate this solution mixture against the potassium dichromate solution. The end point of titration is the appearance of a permanent violet blue colour. Repeat the titration until two consecutive concordant values are obtained. Calculate the normality of the ferrous iron solution and the amount of ferrous iron present in 100 ml of the given solution from the titre values.

RESULT

Normality of the Mohr’s salt solution (N₁) = _____ N

Normality of the potassium dichromate solution (N₂) = _____ N

Normality of the unknown ferrous iron solution (N₃) = _____ N

Amount of ferrous iron present in 100 ml of the given solution (W) = _____ g

VIVA QUESTIONS

1. What is the difference between ferrous and ferric iron?
2. What is redox titration?
3. What is the end point of titration in this experiment?
4. What is Mohr’s salt?
5. What is the oxidation number of chromium in potassium dichromate?
6. Which indicator is used in this experiment?
7. What is the unit of Mohr’s salt concentration?