4

ESTIMATION OF COPPER BY IODOMETERY

AIM

To estimate the amount of copper present in 100 ml of the given solution by using approximately 0.05N sodium thiosulphate (hypo) solution.

PRINCIPLE

The estimation is based on iodometric titration. Hypo is standardised by titrating with standard potassium dichromate solution, which liberates iodine from potassium iodide in an acidic medium.

Copper sulphate reacts with potassium iodide liberating iodine.

The liberated iodine is titrated against hypo using starch as the indicator.

The starch indicator forms a violet blue colour with iodine. The disappearance of the violet blue colour and the appearance of a light green colour indicate the end point of titration.

PROCEDURE

Part A: Preparation of Standard Potassium Dichromate Solution

Take approximately 0.245 g of potassium dichromate in a clean and dry weighing bottle. Weigh the bottle accurately up to the fourth decimal place using the rider in a simple balance. Then, transfer the substance into a clean 100 ml standard flask with the help of a glass funnel. Then, weigh the empty bottle accurately. The difference between the two values gives the weight of potassium dichromate. Dissolve the substance in a minimum quantity of distilled water and make it up to the mark. Shake thoroughly to obtain uniform concentration and keep aside for further use.

Weight of potassium dichromate + weighting bottle (W₁) = _____ g

Weight of empty weighing bottle (W₂) = _____ g

Weight of potassium dichromate W = (W₁ − W₂) = _____ g

Gram molecular weight (GMW) of potassium dichromate = 294.16

Gram equivalent weight (GEW) of potassium dichromate = 49

Part B: Standardisation of Hypo Solution

Clean a burette initially with tap water and then rinse it with distilled water as well as the hypo solution. Then, fill it with the hypo solution and note down the initial reading. Pipette out 20 ml of the standard potassium dichromate solution into a clean conical flask. Add 20 ml of dilute sulphuric acid and 10 ml of 10% potassium iodide solution. The solution turns to a deep brown colour. Titrate this solution mixture against the hypo solution taken in the burette. When the solution forms a light brown colour, add 1 ml of 1% starch solution and continue the titration until the colour changes from brown to green. The end point of titration is the change of colour from violet brown to light green. Repeat the titration to get two consecutive concordant values. Calculate the normality of hypo from the titre values.

Part C: Estimation of Copper

Make up to the mark the given unknown copper ion solution by adding distilled water. Shake the resulting solution well to get uniform concentration. Pipette out 20 ml of the made-up copper sulphate solution into a clean conical flask. Add a few drops of ammonium hydroxide solution to get a blue colour precipitate, and then add a few drops of acetic acid to dissolve the formed precipitate. Add 10 ml of 10% potassium iodide solution and titrate against hypo taken in a burette. When the solution turns to a light brown colour, add 1 ml of 1% starch solution and continue the titration. The end point of titration is the change in colour from violet blue to dirty white or off-white. Repeat the titration to get two consecutive concordant values. Calculate the normality of the copper sulphate solution and the amount of copper present in 100 ml of the given solution from the titre values.

RESULT

Normality of the potassium dichromate solution (N₁) =_____ N

Normality of the hypo solution (N₂) = _____ N

Normality of the copper sulphate solution (N₃) = _____ N

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

VIVA QUESTIONS

1. What is the principle involved in this experiment?
2. What is the indicator used in this experiment?
3. What are the main differences between complexometry and iodometry?
4. What is normality?
5. What is the end point of titration in this experiment?
6. What is the importance of the addition of sulphuric acid in this experiment?
7. What is the chemical name of hypo?
8. Give the formula of hypo.
9. What is the oxidation state of chromium in potassium dichromate?
10. What is the importance of iodometry?