Abstract
The object of this research was to investigate
the condition of silver chromate formed by the interaction of equivalent solutions of silver nitrate and
potassium chromate in the presence of gelatine.
It has frequently been observed that the
presence of a small quantity of a hydrophilic colloid
will render a hydrophobic solution more stable in the:
presence of electrolytes. This is attributed to
"protection" of the colloidal particles by the formation
of envelopes of the hydrophilic colloid around them,
preventing coalescence. Further, if reactions between
dissolved salts,which normally would yield a precipitate,
are carried out in the presence of a hydrophilic colloid
o visible precipitation may occur. It is generally
considered that the insoluble substance separates in
the colloidal condition and is "protected" from being
precipitated" by the other product of double decomposition in the manner indicated.
It must however be emphasised that the
formation of a solid phase by the interaction of two
salts in aqueous solution is a complex process. At
least the following four processes are to be
recognised.
1.) Union or ions to form molecules.
2.) Union of molecules to form amicronic
aggregates which may act as crystallisation,
centres or nuclei,
3.) Growth of the nuclei by the addition of
molecules. This may not proceed beyond
colloidal dimensions,
4.) Union of nuclei or larger aggregates
(coagulation).
How the presence of a substance like gelatine
may affect 4) has already been considered. A fair
amount of experimental evidence can "be adduced to
show that it may influence 3.). The extent to which,
however, it is able to modify 1.) and 2.) has been very
little studied. Ostwald in his explanation of
Liesegang rings supposes that supersaturation precede®
precipitation, i.e. that the gelatine or similar
substance is able to Intervene at either or both of the
earlier stages.
In this research, to a solution of each of
the reactants was added the same quantity of gelatine.
On mixing these solutions there resulted either clear
yellow systems or turbid red ones. It was considered
that in the latter precipitation had taken place.
The former will be designated "protected" solution
for convenience in description. They contain much
more silver and chromate than does a saturated
aqueous solution or silver chromate.
