The filtrate from this is evaporated to
dryness with an excess of hydrochloric acid, and the residue, consisting
of the mixed chlorides, is gently ignited and weighed. In many cases
(such as the analysis of slags and of some natural silicates where the
percentage of alkalies is small) the percentage of soda and potash
(which most commonly occur) need not be separately determined. It is
sufficient to report the proportion of mixed alkalies; which is thus
ascertained:--Dissolve the ignited and weighed chlorides in 100 c.c. of
distilled water, and titrate with the standard solution of silver
nitrate (using potassic chromate as indicator) in the manner described
under _Chlorine_. The c.c. of silver nitrate used gives the weight in
milligrams of the chlorine present. Multiply this by 0.775, and deduct
the product from the weight of the mixed chlorides. This will give the
combined weight of the alkalies (Na_{2}O and K_{2}O) present. For
example, 0.0266 gram of mixed chlorides required on titrating 14.2 c.c.
of silver nitrate, which is equivalent to 0.0142 gram of chlorine. This
multiplied by 0.775 gives 0.0110 to be deducted from the weight of the
mixed chlorides.
Mixed chlorides 0.0266 gram
Deduction 0.0110 "
------
Mixed alkalies 0.0156 "
Assuming this to have been got from 1 gram of a rock, it would amount to
1.
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