Page 103 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

CARE AN0 USE OF ANALYTICAL BALANCES 3.5

With these top pan balances it is not necessary to shield the balance pan from
gentle draughts, and weighings can be accomplished very rapidly and with the
usual facility of the results being recorded with a printer.

3.4 WEIGHTS, REFERENCE MASSES
Although with modern balances it is not necessary to make use of a box of
weights in the weighing process, as indicated in Section 3.2 a set of weights is
desirable for checking the accuracy of a balance.
For scientific work the fundamental standard of mass is the international
prototype kilogram, which is a mass of platinum-iridium alloy made in 1887
and deposited in the International Bureau of Weights and Measures near Paris.
Authentic copies of the standard are kept by the appropriate responsible
authorities* in the various countries of the world; these copies are employed
for the comparison of secondary standards, which are used in the calibration
of weights for scientific work. The unit of mass that is almost universally
employed in laboratory work, however, is the gram, which may be defined as
the one-thousandth part of the mass of the international prototype kilogram.
An ordinary set of analytical weights contains the following: grams, 100, 50,
30,20,10,5,3,2,1; milligrams, 500-100 and 50-10 in the same 5,3,2,1 sequence.
The weights from 1 g upwards are constructed from a non-magnetic nickel-
chromium alloy (80% Ni, 20% Cr), or from austenitic stainless steel; plated
brass is sometimes used but is less satisfactory. The fractional weights are made
from the same alloys, or from a non-tarnishable metal such as gold or platinum.
For handling the weights a pair of forceps, preferably ivory-tipped, are provided
and the weights are stored in a box with suitably shaped compartments.
Analytical weights can be purchased which have been manufactured to
'Class A' standard; this is the only grade of laboratory weights officially
recognised in the United Kingdom. In 'Class A' weights the following tolerances
are permitted: 100g, 0.5mg; 50g, 0.25mg; 30g, 0.15 mg; 20g, O.Iomg;
10 g-100 mg, 0.05 mg; 50-10 mg, 0.02 mg.
The National Bureau of Standards at Washington recognises the following
classes of precision weights:
Class M For use as reference standards, for work of the highest precision,
and where a high degree of constancy over a period of time is
required.
Class S For use as working reference standards or as high-precision
analytical weights.
Class S-1 Precision analytical weights for routine analytical work.
Class J Microweight standards for microbalances.

3.5 CARE AND USE OF ANALYTICAL BALANCES

No matter what type of analytical balance is employed, due attention must be
paid to the manner in which it is used. The following remarks apply particularly
to electronic balances.

*The National Physical Laboratory (NPL) in Great Bdtain, the National Bureau of Standards
(NBS) in USA, etc.