Sodium Chloride

Sodium chloride (NaCl; CAS 7647-14-5) is common table salt — used in the darkroom as a mild halide restrainer essentially interchangeable with potassium chloride. Its photographic appearances are narrow: occasional paper developer variants, some reversal-processing formulas, and the silver-chloride production step in wet-plate collodion salting baths where a pure chloride source is specified.^[1] The chemistry is otherwise identical to potassium chloride — chloride ion restrains silver-halide reduction.

Photographic uses

• Mild developer restrainer: Small additions (typically 5–10 g/L) to paper developers produce mildly warmer image tone and lower fog without the stronger activity of potassium bromide.
• Silver chloride precipitation: In historical wet-plate collodion work, sodium chloride reacts with silver nitrate on a glass plate to form light-sensitive silver chloride.
• Reversal processing: Some B&W reversal bleach formulas use sodium chloride as a halide source for re-conversion.
• Fixer additive: A small amount accelerates fixer action in certain older formulations by increasing ionic strength.

Practical notes

Use plain non-iodized table salt — iodized salt contains trace potassium iodide, which is a much stronger restrainer that would unpredictably suppress development. Sea salt with mineral impurities is fine. Kosher salt, pickling salt, and canning salt are all suitable; avoid anti-caking additives where formulas call for critical halide balance.

Related compounds

Potassium chloride is the potassium analog, preferred in formulas that use K salts throughout. Potassium bromide is the stronger restrainer for paper developers.