Salts of silver, zinc, and nickel react with cyanide just as copper
does, and consequently interfere. Ferrous salts are sure to be absent,
and ferric salts yield ferric hydrate with the ammonia, which is not
acted on by the cyanide, but, owing to its bulkiness, it settles slowly;
this lengthens the time required for titration, and so modifies the
manner of working. _An assay should not be worked with ferric hydrate
present, unless the standard contains about the same amount of it._ On
mines it is often inconvenient to separate the copper by means of
sulphuretted hydrogen; hence it is customary to titrate without
previous separation. In this case, instead of standardising the cyanide
with electrotype copper, a standard ore should be used. This should be
an ore (of the same kind as those being assayed) in which the copper has
been carefully determined.
~Effect of Varying Copper.~--In these experiments 10 c.c. of nitric
acid, 30 c.c. of ammonia, and water to 200 c.c. were used.
Copper nitrate present 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.
Cyanide required 0.7 " 11.2 " 21.7 " 54.5 " 108.1 "
These results show that under the conditions laid down the various
causes of disturbance nearly neutralise one another, and the results
within a fair range are practically proportional.
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