The quantity of substance
taken for assay must not contain more than 0.1 or 0.2 gram of magnesia.
After precipitating as ammonic-magnesic phosphate with sodium phosphate,
and well washing with ammonia, it is dissolved in dilute hydrochloric
acid, neutralised with ammonia, and sodic acetate and acetic acid are
added in the usual quantity. The solution is boiled and titrated.
EXAMINATION OF A LIMESTONE.
~Silica and Insoluble Silicates.~--Take one gram of the dried sample and
dissolve it in 10 c.c. of dilute hydrochloric acid; filter; wash, dry,
and ignite the residue.
~Organic Matter.~--If the residue insoluble in hydrochloric acid shows
the presence of organic matter, it must be collected on a weighed filter
and dried at 100°. On weighing, it gives the combined weights of organic
and insoluble matter. The latter is determined by igniting and weighing
again. The organic matter is calculated by difference.
~Lime.~--Where but little magnesia is present, this is determined by
titration with standard acid. Take one gram, and dissolve it in 25 c.c.
of normal hydrochloric acid. Tint with methyl-orange and titrate with
semi-normal ammonia. Divide the quantity of ammonia used by 2, deduct
this from 25, and multiply the remainder by 2.8. This gives the
percentage of lime. Where magnesia is present, the same method is
adopted, and the magnesia (which is separately determined) is afterwards
deducted.
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