Instant Notes: Analytical Chemistry

Therefore, as the reaction proceeds during titration, the ratio of the concentra-

tions will change and the emf will alter. The potentiometric titration curve will

resemble those described in Topic C5. A summary list of redox reagents is given

in Table 3of Topic C6.

For standard solutions, sodium oxalate and iron(II) ammonium sulfate and

potassium iodate are suitable, but potassium permanganate and iodine solu-

tions decompose on standing and must be standardized before use.

The indicators for redox reactions are reagents whose oxidized and reduced

forms differ in color:

In(Ox) (color 3) +ne-=In(Red) (color 4)

An example of this is 1,10-phenanthroline iron (II)

[Fe (C 12 H 8 N 2 ) 3 ]^3 ++e-=[Fe (C 12 H 8 N 2 ) 3 ]^2 +

oxidized form, pale blue =reduced form, deep red

In several cases, the indicator reaction additionally involves hydrogen ions,

so the change is pH dependent. Table 4 lists commonly used redox indicators.

Table 4. Indicators for redox titrations

Redox Oxidized Reduced EIn/V Solution

color color

1,10-phenanthroline iron(II) complex Pale blue Red 1.11 1 M H 2 SO 4

Diphenylamine Violet Colorless 0.76 Dilute acid

Methylene blue Blue Colorless 0.53 1M acid

Phenosafranine Red Colorless 0.28 1M acid

For a redox indicator where one electron is involved, at 25∞C, the color

change takes place at electrode potentials in the range

E =EIn ±0.059

One further useful indicator employed in redox titrations involving iodine is

starch, or more synthetic equivalent materials. The starch forms a blue-black

complex with iodine, which is rendered colorless when all the iodine has been

removed.

The applications of redox titrations include the determination of metals, with

two well-defined oxidation states, which are present in metallurgical samples

and ores. In order to dissolve the material, it may be necessary to use oxidative

conditions, for example, concentrated nitric acid. This will convert the majority

of the ions into their higher oxidation state, and in order to titrate them they

must first be reduced quantitatively. This may be done by passing the acidified

solution through a Jones reductor,which contains a zinc-mercury amalgam.

The effluent may then be titrated using a suitable oxidant. Some organic

compounds, such as phenols, may be determined by bromination with a

bromate/bromide mixture, followed by back titration of the excess using

thiosulfate.

94 Section C – Analytical reactions in solution