Add a few grams of ammonic nitrate and 10
c.c. of ammonium molybdate solution, heat nearly to boiling, and allow
to settle; filter off, and wash the yellow precipitate. Dissolve with
dilute ammonia, add "magnesia mixture," and allow to stand overnight.
Filter, wash with dilute ammonia, dry, ignite, and weigh as
pyrophosphate of magnesia. The weight, multiplied by 0.6396, gives the
weight of phosphoric oxide.
~Soluble Silica.~--Some silicates are acted on by hydrochloric acid, and
leave on evaporation a residue; which, when the soluble salts have been
washed out, consists generally of the separated silica with perhaps
quartz and unattacked silicates. It should be ignited, weighed and
boiled with a solution containing less than 10 per cent. of caustic
soda: this dissolves the separated silica. The liquor is diluted,
rendered faintly acid, and filtered. The residue is washed, ignited and
weighed. The loss gives the soluble silica.
~Estimation of Silica in Slags~ (Ferrous silicates).--Take 1 gram of the
powdered slag, treat with aqua regia, evaporate to dryness, extract with
hydrochloric acid, filter, dry, ignite, and fuse the ignited residue
with "fusion mixture," then separate and weigh the silica in the usual
way. Slags are for the most part decomposed by boiling with aqua regia,
but it will be found more convenient and accurate to first extract with
acids and then to treat the residue as an insoluble silicate.
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