Ammonia solution (ammonium hydroxide)

Everything difficult about this reagent follows from one fact: it is a gas that happens to be dissolved in water. It leaves the bottle, it leaves the bath, it reaches your eyes before you have finished pouring, and the strength printed on the label is a claim about the day it was made.

In photography

Making the emulsion. Abney’s 1885 treatise gives a chapter to Eder’s ammonia processes, introduced as allowing “ammonia to re-act upon the bromide of silver in its original condition”. Eder’s ammonio-nitrate method dissolves 370 grains of potassium bromide with 520 to 700 grains of gelatine in 10½ ounces of water, then drops strong liquor ammonia into 460 grains of silver nitrate in the same volume “till the precipitate is just re-dissolved” before adding that drop by drop in a ruby light and digesting at 95 °F for a quarter to half an hour — for plates six or seven times as sensitive as a wet plate. Two remarks matter more than the numbers. The strength of the ammonia is “a secondary consideration”, because the quantity is “defined sharply enough by the words — ‘as much ammonia as will re-dissolve the precipitate produced in the nitrate of silver solution’”: the reaction is its own measure. And the digestion “should never exceed 105 °F”, “otherwise fog is sure to result”.

⚗The chemistry

Ammonia does two jobs at once, and confusing them is the commonest error about it. It is a base, and it is a ligand. Ware gives the sequence for Talbot’s ammonio-nitrate of silver, made by adding “caustic ammonia” to about 20 per cent w/v silver nitrate until the brown precipitate that forms first just redissolves:

2 Ag⁺ + 2 OH⁻ → Ag₂O + H₂O

First: the brown precipitate of silver oxide

Ag₂O + 4 NH₄OH → 2 [Ag(NH₃)₂]⁺ + 2 OH⁻ + 3 H₂O

Then: the colourless solution of the diammine silver complex

The strength of the second step is what makes ammonia a silver-halide solvent. OpenStax’s table of formation constants gives K_f for Ag⁺ + 2 NH₃ ⇌ [Ag(NH₃)₂]⁺ as 1.7 × 10⁷ — large enough to pull silver into solution, small beside the thiosulfate complexes that displaced it in fixing, and given by LibreTexts as 1.1 × 10⁷, so treat it as an order of magnitude. Reilly’s survey lists ammonia first among the substances with “some fixing or stabilizing action”, alongside potassium cyanide, strong chloride solutions, thiocyanates, thiourea, sodium sulfite and the thiosulfates, then explains why only one survived: “of all these sodium thiosulfate has the fewest drawbacks”.

The ammonia-nitrate era. Reilly dates it: “In 1840 it was first noticed that adding ammonia to a silver bath until the brown precipitate which forms is finally re-dissolved causes a more brilliant, vigorous image in salted papers.” The process was widely used for salted paper from 1840 to 1860 and was useful for weak negatives, but the prints yellowed quickly, the solution discoloured, and when albumen paper arrived the alkaline bath dissolved the albumen off the sheet. Photographers neutralised it with nitric acid, which, as Reilly points out, merely “form[ed] ammonium nitrate, which builds up in use anyway, so the utility of adding ammonia then acidifying with nitric acid is doubtful”.

Fuming, and hypersensitising. The alternative was to expose the dried, sensitised sheet to the vapour instead: Reilly describes fuming as hanging sheets in a closed box over a dish of strong ammonia for five to ten minutes, common in the United States, rarely used in Europe, and in his judgement “troublesome and unpleasant” and unnecessary for modern practice. Fifty years later Carroll and Hubbard measured the same trick on panchromatic plates and found it a selective increase in sensitivity for the region the dye sensitises, obeying the Donnan membrane equation, and that “bathing with ammonia and drying leaves an emulsion with an excess of silver over halogen”, confirmed by analysis of plates and baths. Their conclusion is worth setting against the folklore: “for many commercial panchromatic emulsions, water is preferable to ammonia solutions for practical use”, and the hypersensitised material has “a useful life of a few weeks or even days”.

On a modern bench. Ware’s Simple Cyanotype uses ammonia to neutralise the excess citric acid, added slowly with stirring until “the yellow solution turns green”. His quantities show how a formula copes with a reagent of variable strength: 33.0 cc of 10 per cent w/w solution, or 14.5 cc of 24 per cent made up to 33 cc with water, or 13.9 of 25, 12.6 of 28, 11.8 of 30, 11.1 of 32, 10.3 of 35. He warns against household ammonia, whose additives “may spoil the chemistry”.

Properties

A very volatile colourless liquid with a pungent odour. Nothing about it is fixed except the water: HSDB records that ammonium hydroxide “exists only in solution”, so every quantity must be stated as a concentration, which is why Ware prints seven alternative volumes for one step.

For a 25 per cent solution, Chemical Safety Card 0215 gives a boiling point of 38 °C, a relative density of 0.9 and a vapour pressure of 48 kPa at 20 °C — roughly half an atmosphere, which is why the smell arrives instantly. The relative vapour density of the gas is 0.6: ammonia is lighter than air and rises, unlike the solvent vapours that pool at floor level, so extraction placed low will not catch it. CAMEO notes that diluting it with water generates a small amount of heat.

The strength falls with time and with every opening, so Ware’s table of specific gravities (given under Storage above) is the practical check. The old British name for the strongest grade, “.880 ammonia”, is simply the last figure in that table used as a label.

Handling

The aggregated ECHA notifications classify ammonia solution Danger, with the corrosion, exclamation-mark and environment pictograms: H314 in 99.9 per cent of the 6,271 classifying reports and H400 in 99.8 per cent, with H335 in 16.7 per cent and H318 in 10.9. HSE’s EH40 sets ammonia at 25 ppm (18 mg/m³) over eight hours and 35 ppm (25 mg/m³) over fifteen minutes; NIOSH recommends the same and gives an IDLH of 300 ppm — an order of magnitude tighter than either alcohol on the neighbouring pages. Chemical Safety Card 0215 records that harmful contamination of the air “can be reached very quickly on evaporation of this substance at 20 °C”.

Why Level B. The course rubric puts at Level B the “handling of concentrated acids or alkalis”, “procedures that generate mists or vapours”, and “procedures whose failure mode is a splash, a burn or a spill of something corrosive”. Ammonia solution meets all three, and Level B’s controls — splash goggles and an apron, local extraction or an open window with a fan, eyewash within reach — are the ones Ware prescribes for his own workshop. It is not Level C, because alt-process quantities are tens of millilitres neutralised into a beaker, not open dishes of concentrate. Ammonia fuming is a different matter, and this course gives no fuming procedure: a dish of 25 per cent ammonia evaporating in a closed box is a deliberate vapour generator.

⚠Safety

Goggles before the cap comes off, ventilation running before that, gloves and an apron for the concentrate. The reaction to take seriously is with silver. CAMEO records that ammonium hydroxide “reacts with aqueous silver nitrate sodium hydroxide to give a black precipitate of silver nitride. Such a precipitate can explode on stirring”, and Ware’s caution is that solutions of diammine silver “can, with time, deposit a black solid precipitate of silver nitride — also called ‘fulminating silver’ — which, as its name suggests, is a highly sensitive contact explosive, detonating at a touch, and sometimes even when wet. Storing such solutions is not recommended.” Reilly records the same hazard from the other direction, refusing to describe the “boiling down” of an exhausted ammoniacal silver bath that nineteenth-century manuals recommended. Make an ammoniacal silver solution only for the session that uses it; never store it, never evaporate it, and never let one dry out in a bottle.

Waste

The classification decides this one. H400 is given by 99.8 per cent of classifying notifiers, and Chemical Safety Card 0215 asks for storage “in an area without drain or sewer access” and states that the substance is very toxic to aquatic organisms. Concentrate is not poured. Dilute working solutions are neutralised with a dilute acid first — slowly, with stirring, because the neutralisation is exothermic and ammonia is driven off if it runs hot — and then heavily diluted. ILFORD’s route for domestic users in the United Kingdom is to bottle wastes separately, label them and take them to a household waste and recycling centre’s chemical cupboard, and Kodak’s J-300 guidance excludes prohibited materials from the sewer route it allows an amateur. Anything that has held ammoniacal silver is dealt with in the session that made it. Check your local regulations, which govern.

History

Ammonia’s photographic career is one reagent used for three unrelated properties in succession. From 1840 it was a ligand, redissolving silver oxide to make the ammonio-nitrate sensitiser that gave salted paper its vigour; Ware credits the formulation to Alfred Swaine Taylor early in 1839 and records Talbot testing it “with evident approval”. From 1880 it was a ripening agent, the fastest route to a sensitive gelatine emulsion. From the 1920s it was a hypersensitiser applied after the plate was made, until Carroll and Hubbard showed in 1932 that plain water often did as well. What links the three is one equilibrium: ammonia takes silver into solution, and the brilliance, the ripening and the excess of silver over halogen all follow from how much it takes and how quickly it leaves again.