Page 290 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

10 TITRIMETRIC ANALVSIS

In practice, an ideal primary standard is difficult to obtain, and a compromise
between the above ideal requirements is usually necessary. The substances
commonly employed as primary standards are indicated below:
(a) Acid-base reactions - sodium carbonate Na,CO,, sodium tetraborate
Na, B407, potassium hydrogenphthalate KH(C, H404), constant boiling
point hydrochloric acid, potassium hydrogeniodate KH(IO,),, benzoic
acid (C,H,COOH).
(b) Complex formation reactions - silver, silver nitrate, sodium chloride,
various metals (e.g. spectroscopically pure zinc, magnesium, copper, and
manganese) and salts, depending upon the reaction used.
(c) Precipitation reactions - silver, silver nitrate, sodium chloride, potassium
chloride, and potassium bromide (prepared from potassium bromate).
(d) Oxidation-reduction reactions - potassium dichromate K, Cr, O,,
potassium bromate KBrO,, potassium iodate KIO,, potassium hydrogen-
iodate KH(IO,),, sodium oxalate Na2C204, arsenic(II1) oxide As,O,,
and pure iron.
Hydrated salts, as a rule, do not make good standards because of the difficulty
of efficient drying. However, those salts which do not effloresce, such as sodium
tetraborate Na, B40 ,, 10H2 O , and copper sulphate CuSO,, 5H2 O, are found
by experiment to be satisfactory secondary standards.'
A secondary standard is a substance which may be used for standardisations,
and whose content of the active substance has been found by comparison against
a primary standard. It follows that a secondary standard solution is a solution
in which the concentration of dissolved solute has not been determined from
the weight of the compound dissolved but by reaction (titration) of a volume
of the solution against a measured volume of a primary standard solution.

NEUTRALISATION TlTRATlONS

10.7 NEUTRALISATION INDICATORS
The object of titrating, Say, an alkaline solution with a standard solution of an
acid is the determination of the amount of acid which is exactly equivalent
chemically to the amount of base present. The point at which this is reached
is the equivalence point, stoichiometric point, or theoretical end point; the resulting
aqueous solution contains the corresponding salt. If both the acid and base are
strong electrolytes, the solution at the end-point will be neutral and have a pH
of 7 (Section 2.17); but if either the acid or the base is a weak electrolyte, the
salt will be hydrolysed to a certain degree, and the solution at the equivalence
point will be either slightly alkaline or slightly acid. The exact pH of the solution
at the equivalence point can readily be calculated from the ionisation constant
of the weak acid or the weak base and the concentration of the solution (see
Section 2.19). For any actual titration the correct end-point will be characterised
by a definite value of the hydrogen-ion concentration of the solution, the value
depending upon the nature of the acid and the base and the concentration of
the solution.
A large number of substances, called neutralisation or acid-base indicators,
change colour according to the hydrogen-ion concentration of the solution. The