Page 73 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

A small amount of indicator (In) is added to the Mg2+ to form a red
complex. As EDTA is added, it reacts first with free, colorless Mg2+ and
then with the small amount of red MgIn complex. (The EDTA must
therefore bind to Mg2+ better than the indicator binds to Mg2+). The
change from the red of MgIn to the blue of unbound In signals the end
point of the titration.

In many titrations an auxiliary complexing agent, such as ammonia,
tartarate, citrate, or triethanolamine, is employed to prevent the metal ion
from precipitating in the absence of EDTA.

II. Back titration

In a back titration a known excess of EDTA is added to the analyte. The excess
EDTA is then titrated with a standard solution of a second metal ion. The metal
ion used in the back titration should not displace the analyte metal ion from its
EDTA complex. Magnesium or zinc ions are usually used to back-titrate EDTA.
A back titration is necessary if the analyte

(a) precipitates at the pH necessary for the titration with EDTA e.g.
Al3+

(b) reacts too slowly with EDTA under titration conditions e.g. Al3+

III. Displacement titration
Displacement titration is applied when no indicator is available

to detect the end point. In this procedure, the analyte usually is treated
with excess MgY2- chelate to displace Mg2+, which is later titrated with
standard EDTA.

Mn+ + MgY2- → MYn-4 + Mg2+

An interesting application is the titration of calcium, because in the
direct titration of calcium ions, Eriochrome black T gives a poor end
point. Yet, if magnesium is present, it is displaced from its EDTA
complex by calcium and an improved end point results.

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