The best method is that which involves the least number of separations.
The reactions must be sharp and complete, and yet not be liable to error
under varying conditions.
To bring the richer and poorer materials under the same conditions for
the assay, a small weight, say 1 gram of the richer, and a larger weight
(5 or 10 grams) of the poorer, substance is weighed up. A method is then
adopted which will concentrate the whole of the metal (either during or
after solution) in a product which need not necessarily be pure. The
work on this product is comparatively easy. In separating small
quantities of a substance from a large bulk of impurities, the group
separations must not as a rule be too much relied on. Very large
precipitates carry down small quantities of bodies not belonging to the
group, more especially when there is a tendency to form weak double
compounds. The re-dissolving and re-precipitating of bulky precipitates
should be avoided.
When a large number of assays of the same kind have to be carried out, a
plan something like the following is adopted:--The samples, after having
been dried, are placed in order on a table at the left hand of the
assayer. He takes the first, marks it with a number, samples and weighs
up the quantity required, and transfers it to a flask, which is
similarly marked.
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