Photo Engineer
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This action takes on many forms that I hope to cover in this brief description. We often put Iodide into the beaker (kettle) when we start to make a bromo-iodide emulsion. This is usually in the range of 0.3% to 3.0 Mole % in published formulas. In many modern formulas that I am familiar with though, this can range up to 10%. I’ll get back to this!
Due to the huge difference in solubility between Bromide and Iodide, the Iodide ends up in the center, or core, of the emulsion that is made when it is used at the lower range. After all, the approximate ratios of their solubility in water is about 6000:1 in favor of Silver iodide precipitating. So, you end up seeing only a tiny effect from Iodide if used in the kettle from the start of the precipitation. If used on the surface, you are limited in use do to the fogging effect of Iodide if used at high concentration. In either case, you see a tiny increase in speed and contrast. The contrast effect is more pronounced if you add it to the emulsion after precipitation, as it adsorbs to the finer grains (which have more surface area) and these gain speed, thus increasing contrast. This latter method is used to control contrast in many photographic products. An example was Brovira paper.
However, adding Iodide smoothly during a precipitation could give us much more in both speed and contrast, but it is harder to do.
You can add it in a stream with the Silver, side-by-side and you can either keep the Iodide flow rate constant, ramp it up, or ramp it down. This has the effect of producing constant Iodide, increasing Iodide, or decreasing Iodide in the forming crystals. You can also add it with a stream of Bromide or mixed with the stream of Bromide. Doing this requires either 2 or 3 delivery systems, either pumps or syringes for example.
This can produce a high Iodide content AgBrI emulsion giving very high speeds.
The highest speed though, might be obtained as follows: Run AgNO3 into pure Iodide, thus giving AgI nuclei, and then, when you have run just enough Silver, stop and dump in Bromide and continue running Iodide. This way you get a core-shell emulsion with a smooth gradation of iodide out to the surface. This method is very complex in actual practice but produces some very fine grained, high speed films with ISO values of 800 or higher.
Perhaps the most interesting method of addition of Iodide, is based on the fact that KI is soluble in concentrated AgNO3. This allows the precise placement of the Iodide in the forming crystals. Unfortunately, this method cannot be used in production, because the equipment cannot be cleaned. Once you add water, AgI precipitates all over the surfaces of the equipment. AFAIK, no method has ever been found that allows one to use this unless the equipment is considered to be expendable.
It also limits one to a fixed maximum Iodide amount, and a fixed amount of Iodide placement in the emulsion.
I hope that this stimulates some of you to try some new things.
Have fun.
PE
Due to the huge difference in solubility between Bromide and Iodide, the Iodide ends up in the center, or core, of the emulsion that is made when it is used at the lower range. After all, the approximate ratios of their solubility in water is about 6000:1 in favor of Silver iodide precipitating. So, you end up seeing only a tiny effect from Iodide if used in the kettle from the start of the precipitation. If used on the surface, you are limited in use do to the fogging effect of Iodide if used at high concentration. In either case, you see a tiny increase in speed and contrast. The contrast effect is more pronounced if you add it to the emulsion after precipitation, as it adsorbs to the finer grains (which have more surface area) and these gain speed, thus increasing contrast. This latter method is used to control contrast in many photographic products. An example was Brovira paper.
However, adding Iodide smoothly during a precipitation could give us much more in both speed and contrast, but it is harder to do.
You can add it in a stream with the Silver, side-by-side and you can either keep the Iodide flow rate constant, ramp it up, or ramp it down. This has the effect of producing constant Iodide, increasing Iodide, or decreasing Iodide in the forming crystals. You can also add it with a stream of Bromide or mixed with the stream of Bromide. Doing this requires either 2 or 3 delivery systems, either pumps or syringes for example.
This can produce a high Iodide content AgBrI emulsion giving very high speeds.
The highest speed though, might be obtained as follows: Run AgNO3 into pure Iodide, thus giving AgI nuclei, and then, when you have run just enough Silver, stop and dump in Bromide and continue running Iodide. This way you get a core-shell emulsion with a smooth gradation of iodide out to the surface. This method is very complex in actual practice but produces some very fine grained, high speed films with ISO values of 800 or higher.
Perhaps the most interesting method of addition of Iodide, is based on the fact that KI is soluble in concentrated AgNO3. This allows the precise placement of the Iodide in the forming crystals. Unfortunately, this method cannot be used in production, because the equipment cannot be cleaned. Once you add water, AgI precipitates all over the surfaces of the equipment. AFAIK, no method has ever been found that allows one to use this unless the equipment is considered to be expendable.
It also limits one to a fixed maximum Iodide amount, and a fixed amount of Iodide placement in the emulsion.
I hope that this stimulates some of you to try some new things.
Have fun.
PE