Page 75 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

The use of masking and selective demasking agents permits the
successive titration of many metals. Thus, a solution containing Mg2+,
Zn2+ and Cu2+ can be titrated as follows:

1. Add excess of standard EDTA and back-titrate with standard Mg2+
solution using Solochrome Black (Eriochrome Black T) as
indicator. This gives the sum of all the metals present.

2. Treat an aliquot portion with excess of KCN and titrate as before.
This gives Mg2+ only.

3. Add excess of formaldehyde-acetic acid solution to the titrated
solution in order to liberate the Zn2+ from the cyanide complex, and
titrate until the indicator turns blue. This gives the Zn2+ only. The
Cu2+ content may then be found by difference.

(b) Suitable control of the pH of the solution
This makes use of the different stabilities of metal-EDTA

complexes. For example, trivalent cations can usually be titrated without
interference from divalent species by maintaining the solution at pH
about 1. At this pH, the less stable divalent chelates do not form to any
significant extent, but trivalent ions are quantitatively complexed. A
mixture of bismuth and lead ions can be successfully titrated by first
titrating the bismuth ion at pH 2 with xylenol orange as indicator and
titrating the lead ion at about pH 5.

(c) The proper choice of indicator

Calcium could be selectively titrated in presence of magnesium when using
murexide indicator at pH 12.

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