At HARMAN technology Ltd we are applying the technology we have learnt in decades of experience in the photographic field in other areas.
The techniques used for controlled precipitation of silver halide photographic emulsions (technically, dispersions) has now been applied to other precipitations from aqueous systems.
We show here some results of precipitation of water-soluble dyes with polyvalent metal salts, concentrating on control of the particle size and morphology of the precipitates. This can be achieved by varying the concentration of the reagents and , temperature of the reaction, control of addition rate using ion-selective electrodes, and addition of dispersants or colloid stabilisers.
The Scanning Electron Microscope picture above shows particles of a red metal salt pigment precipitated in the presence of a dispersant. The crystals are platelets of a fairly tight size distribution, around 1 - 2 microns (µm) across and approximately 0·1 µm thick.
View through an optical microscope between crossed polarisers of a precipitated metal salt of a copper phthalocyanine blue dye. Some of the particles are amorphous, the rest are needle-shaped crystals. The average particle size is less than 1 µm.
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