Ammonium Chloride

Ammonium chloride (NH₄Cl; CAS 12125-02-9) is a mild ammonium salt used as an accelerator in some fixing-bath formulations and as a component of salted-paper emulsions.^[1] The chloride ion provides halide chemistry for silver-chloride-based paper emulsions; the ammonium cation can accelerate fixing reactions by coordination chemistry, somewhat similar to the accelerating effect that makes ammonium thiosulfate fixers faster than their sodium counterparts.

Photographic uses

• Fixer accelerator: Small additions (5–10 g/L) to a sodium thiosulfate fixer shift behaviour somewhat toward ammonium-fixer kinetics. Less effective than simply using ammonium thiosulfate outright.
• Salted-paper emulsion: Paper is floated on an ammonium chloride solution, then on silver nitrate, forming silver chloride in the paper fibres as the light-sensitive compound.
• Collodion salting (chloride variant): Some collodion formulas use ammonium chloride alongside potassium iodide to form a mixed silver halide on the plate.
• Mercury intensifier clearing: Dilute ammonium chloride solution dissolves excess mercury compounds after intensifier bleach-redevelopment.

Practical notes

Supplied as white crystalline powder. Highly soluble in water (37 g/100 mL at 20 °C), producing mildly acidic solutions (pH ~5). Shelf-stable indefinitely in a closed container.

Mildly hygroscopic: dry powder absorbs atmospheric moisture over months, developing a faint crust but remaining useful. Store sealed for best weight accuracy.

Related compounds

Ammonium thiosulfate is the ammonium-cation fixer that provides the acceleration effect natively. Sodium chloride is the simpler chloride salt without the ammonium contribution.