Ammonium Bromide

Discussion in 'Silver Gelatin Based Emulsion Making & Coating' started by kb3lms, Sep 25, 2012.

  1. kb3lms

    kb3lms Subscriber

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    Things have been a bit quiet here in the emulsion making forum so here's a general question from thinking about things I have read lately.

    The few batches of SRAD emulsion I have made to date have used Potassium Bromide, KBr. Other formulas call for Ammonium Bromide instead, NH4Br. So, since when using KBr we have

    AgNO3 + KBr -> AgBr + KNO3

    I suppose we'd get

    AgNO3 + NH4Br -> AgBr + NH4NO3

    So what I was wondering is at any point are the ammonium ions (NH4+) available for use as a silver solvent, so a dual purpose, or are they immediately combined with nitrate ions (NO3-) to create ammonium nitrate, NH4NO3?
     
  2. dwross

    dwross Member

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    Jason,

    Thanks for trying to resuscitate this forum! I’d give a lot for a return to the days of good, meaty discussion.

    When I first started emulsion making I read veraciously. I came across a number of references to using ammonium halides in a role as silver solvent. I incorporated that as a given in my subsequent recipe development. The addition of ammonium bromide to the salts in gaslight paper (i.e. Azo-type) eliminates any little “pepper grains” in the emulsion. (That's not the only method, but it's by far the easiest.)

    The only reference I can lay my hands on immediately is good, old Baker (I’ve got his book memorized!) From page 78 of the 1st edition (1941): “A small quantity of ammonia should always be present in either the salts or the first silver solutions, otherwise clots or agglomerates of precipitates may be formed, which will cause black spots on development without exposure…”
     
  3. cliveh

    cliveh Subscriber

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    I would have thought the latter.
     
  4. dwross

    dwross Member

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    Why? Not arguing -- just interested in your reasoning. Also, if so, do you know what implication that would have for an ammonium halide as silver solvent?
     
  5. kb3lms

    kb3lms Subscriber

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    Some old (1880s) emulsion formulas I have read about contain no added ammonia at all. Instead, faster variations claiming greater contrast use more NH4Br. This leads me to think that some of the ammonium ions from the NH4Br are acting in a solvent role to produce larger, less disperse crystals. These emusions likely carry a high excess of halide during preciptation and ripening leaving the excess ammonia that didn't combine to form NH4NO3 available as a solvent. All the excess halide is removed during the washing step.

    I have also read a more modern double-jet formula that did not use ammonia but rather ammonium thiocyanate which served a dual role as a solvent and sensitizer.

    I'd guess that an emulsion ripened without ammonium present would be very slow. Would it even ripen as in growing larger crystals? Or at the end of a ripening phase would you just end up more or less with the crystals you started with when first precipitated?