Page 58 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 ANDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

With the salts of certain weak acids, such as carbonic, sulphurous, and nitrous
acids, an additional factor contributing to the increased solubility is the actual
disappearance of the acid from solution either spontaneously, or on gentle
warming. An explanation is thus provided for the well-known solubility of the
sparingly soluble sulphites, carbonates, oxalates, phosphates(V), arsenites(III),
arsenates(V), cyanides (with the exception of silver cyanide, which is actually
a Salt of the strong acid H[Ag(CN),]), fluorides, acetates, and salts of other
organic acids in strong acids.
The sparingly soluble sulphates (e.g. those of barium, strontium, and lead)
also exhibit increased solubility in acids as a consequence of the weakness of
the second-stage ionisation of sulphuric acid (K, = 1.2 x 10-'mol L-'):

Since, however, K, is comparatively large, the solvent effect is relatively small;
this is why in the quantitative separation of barium sulphate, precipitation may
be carried out in slightly acid solution in order to obtain a more easily filterable
precipitate and to reduce CO-precipitation (Section 11.5).
The precipitation of substances within a controlled range of pH is discussed
in Section 11.10.

2.10 EFFECT OF TEMPERATURE ON THE SOLUBlLlTY OF A PREClPlTATE
The solubility of the precipitates encountered in quantitative analysis increases
with rise of temperature. With some substances the influence of temperature is
small, but with others it is quite appreciable. Thus the solubility of silver chloride
at 10 and 100 OC is 1.72 and 21.1 mg L - ' respectively, whilst that of barium
sulphate at these two temperatures is 2.2 and 3.9 mg L-' respectively. In many
instances, the common ion effect reduces the solubility to so.smal1 a value that
the temperature effect, which is otherwise appreciable, becomes very small.
Wherever possible it is advantageous to filter while the solution is hot; the rate
of filtration is increased, as is also the solubility of foreign substances, thus
rendering their removal from the precipitate more complete. The double
phosphates of ammonium with magnesium, manganese or zinc, as well as lead
sulphate and silver chloride, are usually filtered at the laboratory temperature
to avoid solubility losses.

2.11 EFFECT OF THE SOLVENT ON THE SOLUBlLlTY OF A PREClPlTATE
The solubility of most inorganic compounds is reduced by the addition of
organic solvents, such as methanol, ethanol, propan-1-01, acetone, etc. For
example, the addition of about 20percent by volume of ethanol renders the
solubility of lead sulphate practically negligible, thus permitting quantitative
separation. Similarly calcium sulphate separates quantitatively from 50 percent
ethanol. Other examples of the influence of solvents will be found in Chapter 11.