GENERAL OPERATIONS

35

Weighing.—From all of the foregoing discussion it will be seen
that every analytical determination involves, at some point,
obtaining an accurate estimation of weights. Even the volumet-
ric process requires weighing the sample and a weight is usually
involved, directly or indirectly,, in the standardization of the
solutions used for the titrations. It is obvious from this that an
accurately constructed weighing apparatus is a necessary part of
the equipment of the analytical laboratory.

Methods of Weighing.—Any method that depends upon the
attainment of equilibrium between the force of gravity and the

PIG. 15.—Essential parts of the balance.
resistance to distortion of a spring is necessarily subject to
considerable and variable errors. These are chiefly due to varia-
tions in (a) elasticity of the spring and (5) the value of gravity
for different altitudes. The only method that is free from these
errors is weighing on a balance, a standard mass being compared
with the object to be weighed and the former being varied until
equilibrium is attained.
The Balance.—The analytical balance should be so constructed
as to provide means for accurate weighing to one ten-thousandth