The residue is ignited over
the blowpipe, a fragment of ammonic carbonate being added towards the
end, when fumes of sulphuric acid cease to be evolved. This ensures the
removal of the excess of acid. The crucible is cooled in the desiccator,
and weighed. The substance is sulphate of soda (Na_{2}SO_{4}), and
contains 43.66 per cent. of soda (Na_{2}O), or 32.38 per cent. of sodium
(Na).
VOLUMETRIC METHODS.
There are various methods used for the different compounds of sodium.
There is no one method of general application. Thus with "common salt"
the chlorine is determined volumetrically; and the sodium, after
deducting for the other impurities, is estimated by difference.
With sodic carbonate and caustic soda, a given weight of the sample is
titrated with standard acid, and the equivalent of soda estimated from
the alkalinity of the solution.
With sodium sulphate, a modification of the same method is used. To a
solution of 3.55 grams of the salt contained in a half-litre flask, 250
c.c. of a solution of baryta water is added. The volume is made up to
500 c.c. with water. The solution is mixed and filtered. Half of the
filtrate is measured off, treated with a current of carbonic acid, and
then boiled. It is transferred to a half-litre flask, diluted to the
mark, shaken up, and filtered.
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