THEORY AND GENERAL PRINCIPLES

11

In case the primary standard is a solution already standard-
ized in the normal system the normalities of the solutions
are inversely as the respective volumes that are equivalent to
each other.

5. Thirty cubic centimeters of y~ sodium thiosulphate solu-
tion is found by titration to be equivalent to 29.8 cc of iodine
solution. The normality of the latter is required.

300 N = __ N

29.8* 10

This is

If solutions are to be standardized in the decimal system
the calculations involve nothing more than finding the weight of
the substance in terms of which the standardization is to be ex-
pressed, equivalent to 1 cc of the solution which is being stand-
ardized, always using as the starting point the known weight of
the primary standard.

In many cases the standardization is to be expressed in
terms of the primary standard itself. For example, iodine solu-
tion is to he standardized against pure arsenic trioxide and
expressed in terms of the same substance. Here we have the
very simple method of weighing a- suitable amount of arsenic
trioxide, then dissolving and titrating by the iodine solution.

Then

1 cc iodine solution

gm As203.
cc I-solution

Other familiar examples of this class of methods are the
standardization of permanganate solutions against oxalates or
against elementary iron or antimony for obtaining the weights
of these elements equivalent to 1 cc of the solution.
The following example will serve to illustrate the first case
just discussed:
6. A solution of potassium permanganate was standardized
against sodium oxalate as follows: 2.5340 gm of sodium, oxalate
was dissolved and the solution was diluted to 1000 cc. Twenty-
five-cubic centimeter portions were titrated and gave an average
of 24.25 cc of potassium permanganate solution equivalent to
the oxalate solution used. Required the weight of iron and of
salcium equivalent to 1 cc of permanganate solution.