THEORY AND GENERAL PRINIPLES

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Standardization may be accomplished Ibji pne ^
general methods: . ^ \
Direct Weighing. — The active substariib^/''of;Cthe
accurately weighed and dissolved so as to
of solution. The method is applicable to only
as may be obtained in a pure state or in a state of uniform and
accurately known composition. Most of such materials are
crystallized salts or acids, or soluble gases.
Weighing a Substance Produced by a Measured Volume of
the Solution. — Sulphuric acid solution may be standardized by
adding an excess of barium chloride to a measured volume of
the solution. From the weight of barium sulphate found the
weight of sulphuric acid may be calculated. Similarly hydro-
chloric acid solution may be standardized by adding silver
nitrate to a known volume of solution and weighing the silver
chloride produced.
Measuring the Volume of Solution Required to React with
a Known Weight of a Substance of Known Purity. — An acid may
be allowed to react with a pure carbonate and the required
volume noted. Sodium thiosulphate may likewise be titrated
against a weighed quantity of iodine or (indirectly) against
a weighed quantity of arsenic trioxide.
Titration against Another Solution Which Has Already Been
Standardized. — This method is very much used in the laboratory.
Primary Standards. — It will be noticed that in each of these
cases there is some substance of known composition which is
measured or weighed and the solution is somehow compared
with this for standardization. This substance of known com-
position is called the " primary standard/7 whether it be the
substance dissolved in the solution, something produced by the
solution or something reacting with the solution.
The following examples will illustrate the methods of calcula-
tion in each of the cases discussed.
1. The method of calculation for the first method of stand-
ardization is self-evident. The normality is equal to the ratio
of the number of grams dissolved in 1000 cc to the number of
grams in 1000 cc of a normal solution. That is,
,.. imi per 1000 cc
normality = -- .— ~r-Ğr- ------------ .---y-r
- equivalent weight