Protective action of gelatine on silver chromate

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