Formation of Silver Halide Crystals in Double-Jet Precipitations: AgCl
Ingo H. Leubner, Journal of Imaging Science, 29:219-225 (1985)
The BNG theory of crystal nucleation of insoluble salts was tested with silver chloride; these results were compared with silver bromide. In controlled double-jet precipitations, the number of AgCl crystals formed is directly proportional to the reactant addition rate, and decreases with increasing solubility and temperature. These results agree with the theory. The ratio of critical nucleus size r* to average crystal size r during precipitation was determined. For AgCl at 60C, r*/r=0.54 and independent of addition rate and solubility. With constant solubility and addition rate, at pAg <5.2, r*/r decreases from 0.88 at 35C to 0.65 at 70C; at pAg >5.7, it varied from 0.55 at 35C to 0.65 at 80C. For AgBr, r*/r is independent of addition rate (R>= 16 mmole/min, 70C, cubic morphologyaaa0. It is a function of crystal morphology an increases with solubility and decreases with temperature; it ranges from 0.51 to 0.90. Thus, r*/r falls in the same range for AgCl and AgBr. The correlations were quantitative within ht context of the theory. The experimental r*/r-values give a basis for the calculation of critical supersaturation ratios, where spontaneous renucleation is observed.
The experimental results indicate for octahedral AgBr that r*/r approaches the maximum, limiting value of 1.0. This predicts that at this condition (pAg>9.4) octahedral AgBr is no longer stable. Indeed, it is known that at high pAg other morphological forma are formed.