Page 106 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES
correction is necessary in these cases. However, where absolute weights are
required, as in the calibration of graduated glassware, corrections for the
buoyancy of the air must be made (compare Section 3.16). Although an
electronic balance does not employ any weights, the above remarks apply
to weights recorded by the balance because the balance scale will have been
established by reference to metal (stainless steel) weights used in air.
Now consider the general case. It is evident that the weight of an object
in oacuo is equal to the weight in air plus the weight of air displaced by the
object minus the weight of air displaced by the weights. It can easily be shown
that if Wu = weight in oacuo, Wa = apparent weight in air, da = density of air,
d, = density of the weights, and d, = density of the body, then:

The density of the air will depend upon the humidity, the temperature, and
the pressure. For an average relative humidity (50 per cent) and average
conditions of temperature and pressure in a laboratory, the density of the
air will rarely fa11 outside the limits 0.001 1 and 0.0013 g mL-'. It is therefore
permissible for analytical purposes to take the weight of 1 mL of air as
0.0012 g.
Since the difference between Wu and Wa does not usually exceed 1 to
2 parts per thousand, we may write:

where

If a substance of density d, weighs Wa grams in air, then Wa. k milligrams are
to be added to the weight in air in order to obtain the weight in oacuo. The
correction is positive if the substance has a density lower than 8.0, and
negative if the density of the substance is greater than 8.0.
3. The correct reading of weights is best achieved by checking weights as they
are added to the balance and as they are removed from the balance. In the
case of electronic balances any digital displays should be read at least twice.

GRADUATED GLASSWARE

3.7 UNlTS OF VOLUME
For scientific purposes the convenient unit to employ for measuring reasonably
large volumes of liquids is the cubic decimetre (dm3), or, for smaller volumes,
the cubic centimetre (cm3). For many years the fundamental unit employed was
the litre, based upon the volume occupied by one kilogram of water at 4 OC (the
temperature of maximum density of water): the relationship between the litre