24 QUANTITATIVE AGRICULTURAL ANALYMH
Because of the excess of sulphur trioxide in the pyrosulphute,
this readily reacts with metallic oxides when heated with the
latter:
Fe203 + 3Na2S207 -» 3Na2S04 + Fe,(S< )4)«.
The biborates likewise combine with metallic oxides because
of their excess of boric anhydride.
FejOa + 3Na2B407"-»2Fe(BO,),'} + GNuRO,.
Precipitation.—The process of precipitation is usually a
chemical reaction between substances in solution, the result bring
the production of another substance of relatively small solubility.
The actual precipitation is always preceded by a condition of
supersaturation (with respect to the precipitating substance)
and this breaks down at different rates with different precipitates.
In some cases equilibrium between the precipitate and the satu-
rated solution of the same substance is attained only after Uw
lapse of considerable time, while in other cases such equilibrium
results very quickly. An example of the first- class of precipitates
is found in magnesium ammonium phosphate. In order to
obtain the greatest possible amount by precipitation the solution
must be allowed to stand for some hours in contact with the
crystals that have already been precipitated. Stirring usually
serves to hasten the attainment of equilibrium by promoting
contact of solid with the supersaturated solution.
It is important to note that there is no definite relation between
the degree of persistence of nuperxatumtion and the degree of
solubility at final equilibrium.
Solubility Product.—In order to diminish the solubility of the
precipitate to the lowest possible figure, use in made* of the princi-
ple that in any saturated solution the product of the concentra-
tions of the constituent ions is a constant (the "solubility
product") which cannot be exceeded without reproducing the
abnormal condition of supcrsaturation. By adding an excrnn
of the precipitating reagent (and therefore of one of the ioiw of
the substance precipitating) the concentration of this ion in
largely increased. There must then be a eorroKpoiidhig derreawt
in the concentration of the ion that is being determined and thin
results from increased precipitation.