Question

In a water Hardness complexometric micro-titration, EDTA was used as a standard. The formula is : Ca ^2+ Y^4- - CaY^2 where (Y is EDTA) Suppose the EDTA used was not as dry as it should have been a. Would [EDTA] in the EDTA solution be higher or lower than the actual value?

b. Would the [Ca] determined in the unknown be higher or lower than the actual value?

Answers:

a) Due to presence of water, the actual mass of EDTA taken will be less than required mass. Hence, [EDTA] concentration in the EDTA solution will be lower than the actual value.

b) The [Ca] determined in the unknown will be higher than the actual value.

Can someone provide more guidance/ feebdack/ background on this?

Answer 1

A. First the mass of EDTA is measured to make a solution of it. If the EDTA is wet the mass of water is included in it, so the actual mass of EDTA is less than the required mass. When you dissolve this in water to make a solution amount of EDTA in the solution will be less than required. Hence EDTA concentration will be lower than required.

B. For example, let EDTA concentration required is 0.05 M, but the solution of yours is 0.04 M (and you are not aware of this!).

When you titrate the Ca solution with the EDTA solution, more EDTA is required because it is actually dilute; let the amount of EDTA required to titrate is y mL .   You take the [EDTA] 0.05 M when you calculate Ca concentration. But as the actual concentration is 0.04 M, [Ca] determined in the unknown will be higher than the actual value because the amount of Ca equivalent to y mL of 0.04 M EDTA is less than the amount of Ca equivalent to y mL 0.05 M EDTA.