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ESTIMATION OF COPPER BY EDTA METHOD

AIM

To estimate the amount of copper present in 100 ml of the given solution using approximately 0.01 M ethylenediamine tetraacetic acid (EDTA) solution.

PRINCIPLE

The estimation is based on complexometric titration. Copper ions in ammonia solution form a blue colour complex with Fast Sulphon Black-F (FSB-F) indicator. EDTA reacts with the copper ions and releases the free indicator; hence, the colour of the solution changes from blue to dark green.

PROCEDURE

Part A: Preparation of Standard Copper Sulphate Solution

Take approximately 0.25 g of copper sulphate crystals in a clean and dry weighing bottle. Weigh the bottle along with the crystals accurately up to the fourth decimal place by using the rider in a simple balance. Then, transfer the chemical substance completely into a clean 100 ml standard flask with the help of a glass funnel. Then, weigh the empty weighing bottle accurately and carefully. The difference between these two values gives the weight of copper sulphate. Dissolve the copper sulphate crystals in a minimum quantity of distilled water and then make it up to the mark. Shake the standard flask well to obtain uniform concentration and keep aside the solution for further use.

Weight of copper sulphate + weighing bottle (W₁) =_____ g

Weight of empty weighing bottle (W₂) = _____ g

Weight of copper sulphate (W) = (W₁ − W₂) = _____ g

Gram molecular weight (GMW) of CuSO₄ · 5H₂O = 249.69

Part B: Standardisation of EDTA Solution

Clean a burette initially with tap water and then rinse it with distilled water as well as the EDTA solution. Then, fill it with the EDTA solution and note down the initial reading. Pipette out 20 ml of the standard copper sulphate solution into a clean conical flask. Add 20 ml of ammonia solution and two to three drops of the FSB-F indicator. Titrate this solution mixture against the EDTA solution taken in the burette. The end point is the change of colour from blue to dark green. Repeat the titration until two consecutive concordant values are obtained. Note down the titration values in a tabular column and calculate the strength of EDTA 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. Fill the burette with the EDTA solution and note down the initial reading. Pipette out 20 ml of the unknown copper sulphate solution into a clean conical flask. Then, add 20 ml of distilled water, 5 ml of liquor ammonia and two to three drops of the FSB-F indicator. The solution turns to blue colour immediately. Titrate this against the EDTA solution until the colour changes from blue to dark green, which indicates the end point of titration. Repeat the titration to get two consecutive concordant values. Calculate the molarity of the copper ion solution and the amount of copper present in 100 ml of the given solution from the titre values.

RESULT

Molarity of the copper sulphate solution (M₁) =_____ M

Molarity of the EDTA solution (M₂) = _____ M

Molarity of the unknown copper sulphate solution (M₃) = _____ M

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

VIVA QUESTIONS

1. What is the role of the indicator in this experiment?
2. Why does the colour change at the end point?
3. What is GMW?
4. What is the unit of molarity?
5. Why does EDTA act as a good chelating agent?
6. What is the GMW of CuSO₄ · 5H₂O?