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Abstract
A working hypothesis, based on evidence reported in literature, concerning the stabilizing action of 7-hydroxy-5-methyl-s-triazolo (I, 5-a) pyrimidine (MTP) and I-phenyl-tetrazoline-5-thione (PTT) on emulsion ripened with sodium thiosulphate is presented. It is suggested that the sodium thiosulphate, which has not reacted during digestion, is displaced by MTP or PTT from the most active sites on the silver halide surface and thus prevented from reacting into fog nuclei (Ag2S). This is confirmed by adsorption measurements, proving that both MTP and PTT can displace sodium thiosulphate. To investigate further that these products really retard Ag2S-format ion, experiments were carried out with radioactive sodium thiosulphate. It was shown that MTP effectively inhibits Ag2S-formation, while PTT acts as a catalyst for Ag2S-formation although at the same time it retards chemical sensitization. Further it was shown, as already known, that PTT inhibits development, while MTP has almost no influence on development. These experiments confirm the proposed hypothesis for MTP On the other hand, only a small part of the retarding action of PTT on sulphur sensitization can be ascribed to development inhibition. Since Ag2S-formation is catalyzed by PTT, some other, still unknown interaction, between PTT and sodium thiosulphate must account for the greater part of the inhibiting action of PTT.
Notes
Research made with the support of the “Instuut lot Aanmoediging van het Wetenschappelijk. Onderroek in Nijverheid en Landbouw” of Belgium. Paper presented at a Symposium on “Adsorption at Silver Halide Surfaces”, organised by the Scientific and Technical Group on 31 March- 1 April 1965, in Bristol.