Grain growth in silver halide precipitates

NOTES FROM THE RESEARCH LABORATORY, EASTMAN KODAK COMPANY.* GRAIN G R O W T H IN SILVER HALIDE PRECIPITATES. l S. E. Sheppard and R. H. Lambert.

PARTICULARS are given of an experimental study of the grain growth of silver bromide precipitates which shows that the process is regulated by definite quantitative laws. Two principal periods are distinguished, the first consisting in nucleation of the new solid phase; the second, in the growth of grains. Precipitation was accomplished by mixing silver nitrate and potassium bromide, each containing gelatin, at the juncture of a Y-tube; the mixed material was then ripened at constant temperature by digestion in a solution containing excess of potassium bromide. Samples were removed from time to time and grain sizes were measured photomicrographically. Following a statistical analysis, examination now indicates that the grains in the first classes (small grains) decrease proportionately to the number present at any given time. A break in the curve, however, occurs at a time independent" of excess soluble bromide but depends on the size of grains. Growth before the break is attributed to Ostwald ripening. After the break, the growth is attributed to collision and recrystallization. A theory of grain growth is suggested involving a double layer about individual grains through hydration of imperfectly oriented ions of silver bromide and of the solubilizing potassium bromide. This explains the effective dispersion of grains during Ostwald ripening and subsequent coalescence, orientation, and recrystallization. * Communicated by the Director. 1Communication No. 357 from the Kodak Research Laboratories and published in the Colloid Symposium Monograph, 6 : 2 6 5 . I928. I37