Ammonium chloride

Sodium chloride is the chloride the textbooks use to explain salted paper. Ammonium chloride is the one the nineteenth century actually used, and it is not simply a substitute: it brings an acidic counter-ion that does a second job on the egg white.

In photography

Salting albumen. Reilly is explicit about which chloride the trade reached for — “ammonium chloride was the most commonly used in the 19th century” — and about how it goes in: dissolved in a minimum of water and added to the egg white before the beating. His sample preparation is 15 g of ammonium chloride and 2 mL of glacial acetic acid in 30 mL of water, combined with a litre of albumen, beaten to a froth for three minutes, left covered for 24 hours, strained through muslin and refrigerated for a week before use.

⚗The chemistry

The chloride is doing two things at once. It supplies the halide that silver nitrate will convert to silver chloride in the paper — the same double replacement Reilly writes for sodium chloride, with ammonium as the spectator ion. But Reilly also lists the chlorides among the treatments that denature the albumen: one visible effect of adding them is to reduce the volume of froth produced in beating, and the third denaturing treatment is a deliberate lowering of pH, because the forces binding the enormous protein molecules together weaken as the pH falls. Ammonium chloride arrives mildly acidic where sodium chloride does not — PubChem gives pH 5.5 for a 1 per cent solution, 5.1 for 3 per cent and 5.0 for 10 per cent, against 6.7 to 7.3 for sodium chloride. Whether that difference matters beside the glacial acetic acid Reilly adds is not something the course has found measured.

NH₄Cl + AgNO₃ → AgCl + NH₄NO₃

Salting and sensitising, with ammonium as the spectator ion

How much. Reilly’s figures hold for the chloride, whichever one is used: 1 to 1.5 per cent gives a less sensitive paper that tends to slightly more contrasty prints from thin negatives, at the cost of a rich dense image; 1.5 to 2.5 per cent is normal; more than is needed “only results in higher silver consumption without conferring any additional benefit”. One rule is specific to albumen — when the coated sheet is hardened by a brief pull through 70 per cent isopropanol, whatever chloride content is in the albumen must also go into the alcohol, or the chlorides leach out. For plain salted paper the Getty atlas gives a salting solution that “can be sodium chloride but is often sodium citrate and ammonium chloride, usually around 4% by weight”.

Beyond paper. Wall’s 1912 dictionary gives its two uses in one line: salting albumenised paper, and preparing chloride emulsion. Kodak’s 1928 primer adds the period’s purity test — white crystals soluble in water, which “should have no smell, and when evaporated by heat should leave no residue behind” — though the heating half of that is not something this course asks anyone to do. Wall’s 1924 formulary records two more uses. Adding 2.5 to 5 per cent of ammonium chloride to a 20 per cent hypo bath increases the rapidity of fixing, but with a 40 per cent solution it has no effect, and 10 per cent ammonia water does the same job — so the acceleration belongs to the ammonium ion, not the chloride. And 50 to 300 g per litre added to a normal hydroquinone developer was recommended for very fine grained images: exactly the combination the incompatibility list warns about, an acidic ammonium salt in a carbonate or caustic developer. The course records it as history, not as a procedure, and has not found a sourced statement of what such a mixture gives off.

Properties

An odourless, colourless-to-white hygroscopic solid, relative molecular mass 53.49 — the lightest salt on these four pages, so it carries more halide per gram than any of them. It does not melt: it sublimes, at a temperature PubChem’s entries give variously as 338 °C and, from NIOSH, 662 °F. That is why the substance has an industrial fume and a workplace exposure limit when almost no other salt in this encyclopaedia does.

Dissolving it is strongly endothermic, so the beaker goes cold while you stir. Published solubilities cluster in a narrow band: 39.5 g per 100 g of water at 25 °C from HSDB, 28.3 g per 100 mL on the ILO-WHO card, and 37 per cent from NIOSH and CAMEO; Wall’s 1912 dictionary says 1 in 3 of cold water and 1 in 55 of alcohol, the second being the figure that matters for an alcohol hardening bath. Both hydrochloric acid and sodium chloride decrease its solubility in water, so a mixed salting solution is not the sum of its parts. It is hygroscopic with a tendency to cake in store, a concentrated solution may crystallise if it gets cold, and the solid may volatilise and condense on cool surfaces.

Handling

The aggregated ECHA notifications held by PubChem classify ammonium chloride Warning, irritant: harmful if swallowed from 99.9 per cent of the 3,389 reports that carry hazard codes, and serious eye irritation from more than 99.9 per cent. Only 2 reports out of 3,391 say it meets no GHS criterion. After the wide disagreements on the other three pages in this family, that near-unanimity is worth noticing: this is a substance the notifiers agree about.

It is also the only one of the four with an exposure limit. NIOSH sets a recommended limit for ammonium chloride fume of 10 mg/m³ over eight hours with a 20 mg/m³ short-term limit, and HSE’s EH40 lists the same two figures as the British workplace exposure limit. A fume is what sublimation or a soldering flux produces, not what a salting tray produces — but the limit is the reason the course never heats it.

Why Level A. Against the course rubric, the classification stops at harmful if swallowed and eye irritation, which is inside Level A’s ceiling; the concentrations handled are a 2 to 4 per cent salting bath or 15 g in a litre of albumen; nothing is heated; and the waste needs no specialist neutralisation. One Level B control is added by preference — splash goggles rather than glasses for the solid and for concentrates — because effectively every notifier who classifies it gives the serious-eye-irritation statement.

⚠Safety

Do not heat it, and keep it dry: PubChem records that it decomposes on heating to toxic and irritating fumes of nitrogen oxides, ammonia and hydrogen chloride, and CAMEO notes that it slowly releases hydrogen chloride and that ammonia and hydrogen chloride form in a fire. Store it away from ammonium nitrate and potassium chlorate, with which the ILO-WHO chemical safety card reproduced by PubChem records a violent reaction and a fire and explosion hazard. NIOSH also lists alkalis and their carbonates, lead and silver salts, strong oxidisers and bromine trifluoride among its incompatibilities and notes that it corrodes most metals at fire temperatures, while that same card adds that it attacks copper and its compounds — so it belongs in glass or polyethylene, and nowhere near the carbonate side of a developer.

Waste

Ammonium chloride leaves a printing bench in three streams. The spent salting solution is dilute and is the least of them. The first rinses after sensitising carry silver nitrate and silver chloride, and those are collected. The third is particular to albumen: the 70 per cent isopropanol hardening bath, which carries the same chloride content as the albumen and is a flammable solvent. Kodak’s J-300 guidance lists solvents and flammable materials among the things that must not go to the sewer, even though it sends developers, stop baths, fixers after silver recovery and wash water there, and it is explicit that septic systems are not designed to treat photographic processing solutions. ILFORD tells domestic users in the United Kingdom to bottle wastes separately, label them and take them to a household waste and recycling centre’s chemical cupboard. Check your local regulations; they govern.

History

Ammonium chloride is on both ends of the first proof that light turns silver chloride into silver. In the experiments Scheele published in 1777, he precipitated a solution of silver by sal ammoniac, washed and dried the precipitate, and left it on paper in the sun for a fortnight until the surface grew black. A different reagent, his caustic spirit of sal ammoniac, then dissolved the horn silver light had not acted on and left a black residue; that residue dissolved in nitric acid, and the solution, precipitated “again by means of sal ammoniac”, gave horn silver back. The chloride made the sensitive salt at the start and identified the product at the end.

Seventy years later it was the working chloride of the printing trade. Reilly names it as the one most commonly used in the nineteenth century for albumen, and Wall’s 1912 dictionary still gives salting albumenised paper as its principal photographic use. On those two accounts, a large share of the albumen prints that survive from that century began as sal ammoniac stirred into egg white.