The same solution
standardised with white arsenic gave a standard of which 100 c.c. =
0.5333 gram arsenic. On this standard the 0.6 gram of sodic phosphate
should have required 23.5 c.c.
Experiments in which 0.1 gram of bismuth and 0.1 gram of antimony were
present with 0.1 gram of arsenic, showed no interference on the
titration. Ferric or aluminic salts would remove their equivalent of
arsenic, and, consequently, must be removed before titrating.
~Effect of Varying Arsenic.~--Varying amounts of metallic arsenic were
weighed up and dissolved in nitric acid, &c., and titrated:--
Arsenic taken 0.010 gram 0.050 gram 0.100 gram 0.200 gram
Arsenic found 0.010 " 0.050 " 0.100 " 0.197 "
These experiments show that the method yields good results within these
limits.
~Determination of Arsenic in Mispickel.~--Weigh up 1 gram of the dried
and powdered ore, and evaporate to near dryness with 20 c.c. of dilute
nitric acid. Make up to 100 c.c. with water, and pass sulphuretted
hydrogen to reduce the ferric iron to the ferrous state, then add 20
c.c. of dilute ammonia, and again pass sulphuretted hydrogen. Warm,
filter, and evaporate the filtrate to drive off the excess of ammonia;
then add 10 c.c. of nitric acid, and boil down till the sulphide of
arsenic at first precipitated is dissolved; neutralise; add 5 c.
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