Self-indicating Reagents and Specific Indicators
Potassium permanganate and iodine, which are important redox reagents, are both self-indicating, i.e.
the colour of the reagent in each case is intense and will impart a perceptible colour to a solution when
present in very small excess. One drop of a solution of potassium permanganate (0.02 mol dm–^3 ) can be
detected in a titrand solution of 100 cm^3 , and a similar amount of iodine by shaking the titrand with 5
cm^3 of chloroform or carbon tetrachloride to produce an intense purple colour. Specific indicators react
in a specific manner with one participant in the reaction. The best examples are starch, which produces
an intense blue colour with iodine and potassium thiocyanate, which forms an intense red compound
with iron(III).
Applications of Redox Titrations
Titrimetric methods based on the direct use of redox reactions are widely used. Their application to the
determination of metals which have two well-defined oxidation states is well known. Analysis is often
carried out either by first converting all the analyte metal ions to a higher oxidation state with oxidizing
agents such as sodium peroxide and sodium bismuthate, or by reduction to a lower state using sulphur
dioxide or sodium bisulphite. In all cases an excess of reagent is required which is then destroyed or
removed before the sample is titrated. A more elegant means of quantitative reduction is to allow an
acidified analyte solution to percolate through a column containing a metal reductor. The silver
reductor contains powdered or granulated silver metal whilst the Jones reductor uses a zinc-mercury
amalgam. The former is the milder reducing agent and is hence the more selective. An effluent from a
column reductor, which is free from excess reducing agent, may be titrated directly with a suitable
oxidizing agent. Table 5.6 compares the behaviour of these two reductors.
Table 5.6 A comparison of silver and Jones reductors
Reduction effected by silver reductor Reduction effected by Jones reductor
Fe3+ + e– = Fe2+ Fe3+ + e– = Fe2+
Cu2+ + c– = Cu+ Cu2+ + 2e– = Cu (metal)H 2 MoO 4 + 6H+ + 3e–^ = Mo3+ + 4H 2 O
TiO2+ not reduced TiO2+ + 2H+ + e– = Ti3+ + H 2 OCr3+ not reduced Cr3+ + e– = Cr2+Where the reduction potentials of two analytes are sufficiently different a mixture may be analysed.
Titanium(III), , may be titrated with cerium(IV) in the presence of iron(II), using
methylene blue as indicator. Subsequently the total, iron plus titanium, may be determined using
'ferroin' as indicator. The determination of iron is illustrative of some practical problems which are
encountered in direct titration procedures.