c. of hydrochloric acid; add a solution of
sulphurous acid in quantity sufficient to reduce 2 or 3 per cent. of the
iron to the ferrous state; allow to stand a week; and then boil, to
remove the hydrochloric acid added. Nitric acid, which is prejudicial,
is also removed by this treatment.
[107] When the amount of arsenic to be estimated is small (as in refined
coppers), it is better to use a weaker solution of iodine. This is made
by diluting 200 c.c. of the standard solution with water to 1 litre.
Each c.c. will equal 0.1 per cent., if 1 gram of the metal has been
taken for the assay.
[108] The constitution of these phosphates may be thus illustrated--
Magnesic meta-phosphate MgO.P_{2}O_{5}.
Magnesic pyro-phosphate 2MgO.P_{2}O_{5}.
Magnesic ortho-phosphate 3MgO.P_{2}O_{5}.
[109] The composition of which is--
MoO_{2} 90.74,
P_{2}O_{5} 3.14,
(NH_{4})_{2}O 3.57,
H_{2}O 2.55 = 100.00.
[110] This is made by adding 27 grams of magnesium carbonate (a little
at a time) to a solution of 270 grams of citric acid in 350 c.c. of warm
water; and, when dissolved, adding 400 c.c. of dilute ammonia, and
making up the bulk to 1 litre; 20 c.c. of the solution is sufficient for
0.1 gram of P_{2}O_{5}, although more will be required if much iron or
alumina is present.
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