Page 108 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES

sodium dichromate (Na2Cr20,,2H20) is much more soluble (about 70 g per
litre); for this reason, as well as the fact that it is much cheaper, the latter is usually
preferred for the preparation of 'cleaning mixture'. From time to time it is
advisable to filter the sodium dichromate-sulphuric acid mixture through a
little glass wool placed in the apex of a glass funnel: small particles or sludge,
which are often present and may block the tips of burettes, are thus removed.
A more efficient cleaning liquid is a mixture of concentrated sulphuric acid and
fuming nitric acid; this may be used if the vessel is very greasy and dirty, but
must be handled with extreme caution.
A very effective degreasing agent, which it is claimed is much quicker-acting
than 'cleaning mixture' is obtained by dissolving 100 g of potassium hydroxide
in 50mL of water, and after cooling, making up to 1 litre with industrial
methylated spirit.6a

3.9 TEMPERATURE STANDARD
The capacity of a glass vessel varies with the temperature, and it is therefore
necessary to define the temperature at which its capacity is intended to be
correct: in the UK a temperature of 20 OC has been adopted. A subsidiary
standard temperature of 27 OC is accepted by the British Standards Institution,
for use in tropical climates where the ambient temperature is consistently above
20 OC. The US Bureau of Standards, Washington, in compliance with the view
held by some chemists that 25 OC more nearly approximates to the average
laboratory temperature in the United States, will calibrate glass volumetric
apparatus marked either 20 OC or 25 OC.
Taking the coefficient of cubical expansion of soda glass as about 0.000 030
and of borosilicate glass about 0.000010 per 1 OC, Part A of Table 3.1 gives the
correction to be added when the sign is +, or subtracted when the sign is -,
to or from the capacity of a 1000 mL flask correct at 20 OC in order to obtain
the capacity at other temperatures.
In the use of graduated glassware for measurement of the volume of liquids,
the expansion of the liquid must also be taken into consideration if temperature
corrections are to be made. Part B of Table 3.1 gives the corrections to be added
or subtracted in order to obtain the volume occupied at 20 OC by a volume of
water which at the tabulated temperature is contained in an accurate 1000 mL
flask having a standard temperature of 20 OC. It will be seen that the allowance
for the expansion of water is considerably greater than that for the expansion

Table 3.1 Temperature corrections for a 1 L graduated flask
Temperature (OC) (A) Expansion of glass (B) Expansion of water
Correction (mL) Correction (mL)
Soda glass Borosilicate glass Soda glass Borosilicate glass