It was found that there are two types of the high speed photographic emulsion grains; one of them (I) may be desensitized by soaking in Na
2SO
3 solution before development and another (II) was not affected.
By using the fact that the solubility of AgBr in 10% Na
2SO
3 solution decreases to about 1/100 by addition of KBr (12 g/1), it was shown that the phenomenon in the former paragraph can not be attributed to the activation or inactivation of sensitive development centres but to the dissolving action of SO
3= on the silver halide grains.
Assuming that the silver halid grains are homogeneous in composition, it was estimated that the depth of the dissolved surface region of the grains belonging to larger group which occupied the one half of the total projecting area but was composed of only 15% in numbers, reached
(I) to 382Å and 1290Å from the surface, respectively at 10 and 30 min. treatment (G-film, the mean grain radius in larger group γ=0.765μ) and (II) to 176Å and 590Å, respectively at 10 and 30 min. treatment (A-film, γ=0.798μ). Group (I) showed a remarkable decrease in sensitivity with the dissolved depth up to about 100Å.
Electronmicroscopic observation showed that (II) was dissolved much more irregularly than (I). The similar effect was observed by soaking the emulsion in KCN solution. The ratio of desensitization by prebath in KCN solution was (II) >(I), and. when X
- was added to KCN solution, then I
- had the greater effect in changing the ratio than Br
-.
As the result, the author would like to say that in case of (II) the silver halidegrain is inhomomogeneous in composition, such that the AgI content on some parts of the surface is higher than in (I) and that the development centres are formed on such parts. By presoaking in Na
2SO
3 solution, parts of the surface (II) having development centres are not dissolved, and so they keep the sensitivity.
Such a stable development centre could be formed by adding a cationic surface active agent, such as cethyloxymethyl-α-picolinium chloride, during chemical digestion, or better in the beginning of it, in the emulsion containing silver halide grains inhomogeneous in their composition.
As the mechanism of this effect the author supposes that: AgI which is more negative than AgBr reacts less remarkably with anions such as S
2O
3= or SO
3= than the latter during the chemical digestion. So in case of (I), development centres may be found in the surface parts which are dissolved by SO
3=, while in (II), when the emulsion is treated by a cationic surface active agent, the latter reduces the negative charge on the stable parts of the higher AgI concentration and let react them with S
2O
3= or SO
3=. And also, the adsorption of such a substance on centres may make easier the trapping of electrons released by light.
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