The separation of the sodium from the mixed chlorides is
effected in the following way:--The chlorides are dissolved in a little
water and the potassium separated as platino-chloride. The soluble
sodium platino-chloride, with the excess of platinum, is boiled, mixed
with sulphuric acid, evaporated to dryness, and ignited. On extracting
with water, filtering, evaporating, and igniting, sodium sulphate is
left, and is weighed as such.
It is more usual, and quite as satisfactory, to calculate the weight of
the sodium chloride by difference from that of the mixed chlorides, by
subtracting that of the potassium chloride, which is separately
determined. For example, 1 gram of a rock gave--Mixed chlorides, 0.0266
gram, and 0.0486 gram of potassic platino-chloride. This last is
equivalent to 0.0149 gram of potassium chloride.
Mixed chlorides found 0.0266
Deduct potassium chloride 0.0149
------
Leaves sodium chloride 0.0117
The weight of sodium chloride found, multiplied by 0.5302, gives the
weight of the soda (Na_{2}O).
GRAVIMETRIC DETERMINATION.
The solution, which must contain no other metal than sodium, is
evaporated in a weighed platinum crucible or dish. Towards the finish an
excess, not too great, of sulphuric acid is added, and the evaporation
is continued under a loosely fitting cover.
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