Cooling Water Chemical-43

To determine magnesium against EDTA using Eriochrome Black T as an indicator. 

Material required 

(i) 0.1 M EDTA solution: Dissolve 37.225 g of EDTA into 1000 ml of volumetric flask, in redistilled water and make upto the mark with the same water. 

(ii) Eriochrome Black T (EBT) indicator solution. 

(iii) Buffer solution (pH 10): Add 57 ml of concentrated ammonia solution (sp. gr. 0.88) to 7 g of NH4Cl (A.R.) and dilute to 100 ml with distilled water. 

(iv) Magnesium ion solution (0.1M): Weigh accurately 0.61 g of magnesium turning and dissolve in HCl. Neutralise the resulting solution with M-sodium hydroxide. Transfer in to a 250 ml volumetric flask and dilute the solution with distilled water up to the mark. MgSO4.7H2O can be employed for preparing the solution. Weigh accurately 7.10 g magnesium sulphate heptahydrate and dissolve in water and dilute to 250 ml. 

MgSO4 + Na2C10H14O8N2  MgC10H14O8N2 + Na2SO4 

Procedure 

Take 250 ml of the magnesium solution in a conical flask, dilute to about 100 ml with distilled water. Add 2 ml of the buffer solution (pH10). Add 4-5 drops of indicator solution and titrate against 0.1M EDTA solution with constant solution stirring until the colour changes from red to blue at the end point. 

1 ml of 0.1M EDTA = 2.432 mg of Mg. 

Precautions: 

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 

The mixture having ca2+ & mg2+ ions is titrated in one aliquot of pf the sample solution and ca2+ ions are titrated in a further aliquot in strong alkaline medium in which Mg2+ ions get precipitated. Mg2+ ions are then evaluated from the difference. 

Materials required 

0.01M Ca2+ and Mg2+ ions solution, 0.01M EDTA, 2M NaOH, buffer pH. 10, Murexide and Erio T indicators. 

Procedure 

(i) Pipette out 25 ml of the solution (Ca2+ and Mg2+ ions) into a conical flask. Add 5 ml of buffer (pH = 10) and dilute to about 50 ml of redistilled water. Now add 3 – 4 drops of Erio T indicator. Warm the solution upto 600C and titrate with EDTA until the colour of the solution changes from wine red to clear blue. Titrate slowly near the end point. Suppose A ml be the volume (in ml) of EDTA used. 

(ii) Pipette out 25 ml of the solution (Ca2+ and mg2+ ions) into a conical flask. To this add 5 ml of 2M NaOH solution and 2 – 3 drops of Murexide indicator. After diluting the solution to about 25 ml with distilled water, titrate with EDTA until the colour changes from red to purple. Suppose B be the volume (in ml) of EDTA used.