Define EDTA Titrimetric Method for Estimation of Calcium?

The amount of calcium in any sample can be estimated by EDTA titrimetric method. The principle behind this method is described next.

Principle

Calcium at pH I0 in presence of dye eriochrome black T (indicator) is wine red. The affinity of ethylene diamine tetraecetate (EDTA) for calcium is greater than the affinity of the indicator for calcium and so the indicator releases its metal to EDTA. When completely complexed (chelated) with EDTA, the indicator is released from its combination with calcium. This free indicator gives a blue colour to the solution indicating that the end point is reached. Magnesium must be present for satisfactory end point and is added as Mg EDTA. End point sharpness increases with pH but high pH may cause precipitation of Ca(OH)₂ and cause colour changes of dye. A pH of 10.0 + 0.1 is a satisfactory compromise: A NH₄OH - NH₄CI buffer is used to maintain this pH. Limit of 5 minutes for titration minimizes precipitation.

The reaction involved in this estimation is presented herewith:

So in the estimation of calcium, the solution containing calcium is titrated against EDTA solution. EDTA can form a chelate complex with calcium. It is a six-coordinate Mg- EDTA complex. A small amount of indicator (In) is added to the Ca²⁺ to form a red complex as indicated in the reaction above. As EDTA is added, it reacts first with free colourless Ca²⁺ and then with the red CaIn complex. The EDTA binds to Ca²⁺ better than the indicator binds to Ca²⁺. The indicator thus releases its metal to EDTA. The change from the red CaIn to the blue of unbound indicator (In) signals the end point of titration. It is important to note that the colour of the free indicator is pH dependent. Hence most indicators can be used only in certain pH range. In order to maintain pH we therefore use buffers. A metal ion indicator is a compound which changes its colour on binding to metal ion.