Instant Notes: Analytical Chemistry

Section C – Analytical reactions in solution

C7 TITRIMETRY II:

COMPLEXATION,

PRECIPITATION AND REDOX

TITRATIONS

During complexation reactions the concentration of the analyte ion (for

example, a metal ion) changes most rapidly at the end point. As noted in Topic

C6, the most widely used complexing agent is ethylenediaminetetracetic acid or

EDTA, and Table 1gives a selection of metal EDTA formation constants.

Using the values of a 4 given in Topic C6, Table 1, we may calculate the

practical, or conditional, formation constant at a particular pH

K’MY = KMYa 4

From the data in the tables, it can be calculated that magnesium could be

titrated at pH 10, but not at low pH. This has already been discussed in Topic

C6.

Complexation
titrations

Key Notes

The techniques of titrimetry, using both visual and potentiometric end

point detection, are used to measure species, particularly metal ions, in a

wide range of samples.

Reactions producing an insoluble product are valuable analytical tools for

the titrimetric determination of halide and other anions.

Oxidation and reduction titrations may be used to measure many species,

especially metals in high or low valency states, iodine and iodides, and

easily oxidized organic compounds.

Related topics Complexation, solubility and redox equilibria (C6)

Redox titrations

Complexation

titrations

Precipitation

titrations

Table 1. Metal-EDTA formation constants at 25∞C

Cation KMY log (KMY)

Ag+ 2.0¥ 107 7.3

Mg^2 + 4.9¥ 108 8.7

Ca^2 + 5.0¥ 1010 10.7

Fe^2 + 2.1¥ 1014 14.3

Fe^3 + 1.0¥ 1025 25.1

Zn^2 + 3.2¥ 1016 16.5

Cd^2 + 2.9¥ 1016 16.5

V^3 + 8.0¥ 1025 25.9