Table 5.3 Basic solvents. Reagents: benzoic acid (acid) and tetra-n-butyl ammonium hydroxide (base)
Solvent Analyte Indicator
n-butylamine carboxylic acids, phenols thymol blue
ethylenediamine amine hydrochlorides, phenols azo violet
N,N-dimethyl formamide phenols, salts and sulphonamides azo violet thymol blueexample is the Kjeldahl method for nitrogen determination in organic compounds. Subsequent to the
destruction of carbonaceous material with sulphuric acid, the mixture is made strongly alkaline with
sodium hydroxide. The ammonia produced from the original nitrogenous sample is then distilled off and
absorbed in an excess of hydrochloric or boric acid. Finally the excess acid is back titrated to complete
the analysis.
Redox Titrations
The concept of 'reduction potential' is introduced in Chapter 6. When the reduction potentials of two
species differ by 0.1 V or more, the resulting redox reaction will proceed rapidly and stoichiometrically
so that it may be used as the basis for a titrimetric procedure. The end point of a redox titration may be
observed by following the potential of the titrand with an indicator electrode or with a visual indicator.
In two special cases, the reagent (potassium permanganate and iodine) is self-indicating (vide infra).
The properties and behaviour of some important redox reagents are summarized in Table 5.4. When
assessing these data three main points should be borne in mind.
(1) The division into oxidizing and reducing reagents has been made only on the basis of the common
ways in which the reagents are employed. In principle it is possible to define oxidizing and reducing
agents only for a specified reaction.
(2) Most half reactions involve large numbers of hydrogen ions and are therefore pH dependent.
(3) Reduction potentials are thermodynamic quantities and cannot be used to predict the rate at which a
redox reaction will occur.
Visual Indicators for Redox Titrations
The behaviour of a reversible visual indicator in a redox titration may be represented by
(where Inox and Inred represent the oxidized and reduced forms of the indicator). Nearly all indicator
reactions involve hydrogen ions and the