Page 289 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

PRlMARY AN0 SECONOARV STANOAROS 10.6

state of high purity and are therefore suitable for the preparation of standard
solutions: sodium carbonate, potassium hydrogenphthalate, benzoic acid,
sodium tetraborate, sulphamic acid, potassium hydrogeniodate, sodium oxalate,
silver, silver nitrate, sodium chloride, potassium chloride, iodine, potassium
bromate, potassium iodate, potassium dichromate, lead nitrate and arsenic(II1)
oxide.
When the reagent is not available in the pure form as in the cases of most
alkali hydroxides, some inorganic acids and various deliquescent substances,
solutions corresponding approximately to the molar strength required are first
prepared. These are then standardised by titration against a solution of a pure
substance of known concentration. It is generally best to standardise a solution
by a reaction of the same type as that for which the solution is to be employed,
and as nearly as possible under identical experimental conditions. The titration
error and other errors are thus considerably reduced or are made to cancel out.
This indirect method is employed for the preparation of, for instance, solutions
of most acids (the constant boiling point mixture of definite composition of
hydrochloric acid can be weighed out directly, if desired), sodium hydroxide,
potassium hydroxide and barium hydroxide, potassium permanganate,
ammonium and potassium thiocyanates, and sodium thiosulphate.

10.6 PRlMARY AND SECONDARY STANDARDS
In titrimetry certain chemicals are used frequently in defined concentrations as
reference solutions. Such substances are referred to as primary standards or
secondary standards. A primary standard is a compound of sufficient purity
from which a standard solution can be prepared by direct weighing of a quantity
of it, followed by dilution to give a defined volume of solution. The solution
produced is then a primary standard solution. A primary standard should satisfy
the following requirements.
1. It must be easy to obtain, to purify, to dry (preferably at 110-120 OC), and
to preserve in a pure state. (This requirement is not usually met by hydrated
substances, since it is difficult to remove surface moisture completely without
effecting partial decomposition.)
2. The substance should be unaltered in air during weighing; this condition
implies that it should not be hygroscopic, oxidised by air, or affected by
carbon dioxide. The standard should maintain an unchanged composition
during storage.
3. The substance should be capable of being tested for impurities by qualitative
and other tests of known sensitivity. (The total amount of impurities should
not, in general, exceed 0.01-0.02 per cent.)
4. It should have a high relative molecular mass so that the weighing errors
may be negligible. (The precision in weighing is ordinarily 0.1-0.2 mg; for
an accuracy of 1 part in 1000, it is necessary to employ samples weighing at
least about 0.2 g.)
5. The substance should be readily soluble under the conditions in which it is
employed.
6. The reaction with the standard solution should be stoichiometric and
practically instantaneous. The titration error should be negligible, or easy to
determine accurately by experiment.