306; this gives the weight of potassium chloride.
Deduct this from the weight of the mixed chlorides first got; the
difference will be the sodium chloride, which weight, multiplied by
0.53, will give the weight of soda (Na_{2}O).
~Ferrous Oxide.~--When a qualitative test shows both ferric and ferrous
oxide to be present, the proportion of the ferrous oxide must be
separately determined. The finely ground mineral mixed with dilute
sulphuric acid is treated on a water bath with hydrofluoric acid.
Solution is best effected in an atmosphere of carbonic acid. In about an
hour the decomposition is complete, and the solution is diluted with
cold water, and titrated with the solution of bichromate or of
permanganate of potassium. The iron found is multiplied by 1.286, and
reported as ferrous oxide. To find the proportion of ferric oxide, the
ferrous iron found is multiplied by 1.428, and this is deducted from the
weight of ferric oxide obtained by precipitation with ammonia. The
ammonia precipitate contains the whole of the iron as ferric oxide;
hence the necessity for calculating the ferrous oxide as ferric, and
deducting it.
~Phosphoric Oxide (P_{2}O_{5}).~--Weigh up 5 grams of the finely-divided
and dry sample, and digest with 10 or 20 c.c. of nitric acid; evaporate
to dryness on the water-bath; take up with a little dilute nitric acid;
dilute with water; and filter.
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