The sample of tap water is titrated with standard ca 0.01 M EDTA solution, and the total harness calculated as parts per million of CaCO3. The total hardness of water is generally due to dissolved Ca and Mg salts. Both of these react with EDTA to form a complex. The Ca complex is more stable than the Mg complex. During the titration, the Ca2+ will therefore combine with added EDTA first, followed by complexing for the Mg2+ after which the indicator endpoint is reached.
It should be noted, however, that the complex of Ca2+ with Eriochrome Black T is less stable than that of Mg2+.
(A) STANDARDIZATION OF CA 0.01 M EDTA SOLUTION
PROCEDURE
• 250 mL of a 0.01 M solution of MgSO4.7H2O was prepared by weighing an accurate mass. Calculate the mass weighed. Assuming 0.6241 g was weighed, calculate the exact concentration in 250 mL (concentration to be used in calculations).
• 25.00 mL of this solution was pipetted into a conical flask and a measuring cylinder was used to add 25 mL distilled water.
• Then 2 mL of a buffer 10 solution and a spatula tip of the solid indicator (Eriochrome BlackT) was added.
• The solution was titrated with the EDTA until the endpoint was reached.
• A series of titrations were done. Choose the best three volumes from below and calculate the molarity of the EDTA solution.
• EDTA volumes: 25,68; 26,92; 28,23; 25,15; 25,56; 25,72 mL