1. EDTA should be dried be remove moisture in the preparation of its solution.
2. Redistilled water should be employed for preparing the EDTA solution.
3. Polyvalent cations, if present, interfere and hence should be removed as hydroxides.
4. The colour change near the end point is very slow. Care should be taken to avoid over titration.
To determine calcium against EDTA using Murexide as an indicator. Materials required
(i) 0.01 M EDTA solution:
(ii) Murexide indicator solution:
(iii) NaOH solution: Prepare 1M aqueous solution.
(iv) Calcium ion solution (0.01 M): Weigh accurately 0.50 g A.R. CaCO3 and transfer the salt in a 500 ml measuring flask. Dissolve the salt in minimum quantity of 6M HCl adding drop by until effervescences stop and the solution become clear. Make up the mark.
Procedure
Talk 25 ml of the calcium solution, dilute to 100 ml with water and add 2 ml of 1 (M) NaOH solution to have a solution of pH about 12. Add few drops of murexide indicator and titrate against standard EDTA solution. The colour chang at the end point is from red to purple.
Calculations
1 ml of 0.01 M EDTA = 0.4008 mg Ca
To determine Ca2+ and mg+ ions in a given solution and estimate total hardness of water