Page 71 - Pharmaceutical Analytical Chemistry II - Pharm D Clinical- 07-PA202

The success of an EDTA titration depends upon the precise
determination of the end point. The requisites of a metal ion indicator for
use in the visual detection of end points include:

(a) The metal-indicator complex must possess sufficient stability but less
stable that the metal-EDTA complex to ensure that, at the end point,
EDTA removes metal ions from the metal indicator-complex.

(b) The color contrast between the free indicator and the metal-indicator
complex should be such as to be readily observed.

(c) The above requirements must be fulfilled within the pH range at
which the titration is performed.

Types of EDTA titrations

I. Direct titration
In a direct titration, the analyte is titrated with standard EDTA. The

analyte is buffered to an appropriate pH at which the formation constant
for the metal-EDTA complex is large enough to produce a sharp end
point and at which the free indicator has a distinctly different color from
the metal-indicator complex.

A typical analysis is illustrated by the titration of Mg2+ with EDTA
using Eriochrome black T (Erio T) as the indicator. We can write the
reaction at the end point as follows:

MgInd + EDTA → MgEDTA + Ind

(red) (colorless) (colorless) (blue)

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