CHAPTER II
GENERAL OPERATIONS
Preparation of Samples.—The object of all preliminary work
with samples is to make it possible to obtain, for the actual analy-
sis, a portion that shall truly represent the average composition
of the entire material at hand. This matter is likely to be treated
lightly by the beginner, but proper sampling is often one of the
most difficult problems of quantitative analysis. It is often
necessary to use a quantity of 1 gm or less and if the substance
is not homogeneous this small quantity may have an average com-
position that is very different from the average composition of
the entire material being investigated. No matter how carefully
an analysis may be performed or how accurate the results ob-
tained, if the substance used does not represent the average of
the substance originally at hand the results become nearly or en-
tirely valueless. If the substance is practically homogeneous the
operation of sampling involves nothing more difficult than grind-
ing down to a degree of fineness required for the work. This is
the case when the substance is an approximately pure chemical
compound, such as will be used for the earlier exercises.
The gross sample, as the analyst receives it, may be in the
form of lumps, as is frequently the case with minerals, or it may
be in the form of small pieces, crystals, powder, or solution. In
any case except that of liquid samples, the object is to reduce the
size of pieces to that required for the analysis (usually a rather
fine powder) and at the same time to select from the total mass
such a quantity as is required for the experimental work. The
original sample is often quite large. It is obviously unnecessary
and practically impossible to grind the entire amount into a fine
powder. The operation then resolves itself into a thorough mix-
ing and progressive grinding and dividing. Many forms of
both hand and power grinders are in common use. For the first
exercises nothing more complicated than a porcelain mortar and
pestle will be required.