Ammonium bromide

Every bromide in an emulsion does the same thing: it puts bromide ions in the water so that silver nitrate, added to them, precipitates silver bromide rather than staying in solution. What separates one bromide from another is the ion that comes along with it, and here that ion is ammonium — which is not a spectator.

In photography

As the halide in a plain-silver emulsion. Denise Ross’s emulsion tutorials give the substitution in the plainest possible form: her modification of Kodak’s AJ-12 uses 5 g of potassium bromide, and the ammonium-bromide version is the same recipe with 4.1 g of ammonium bromide in its place. Nothing else changes. The two weights are the same number of moles of bromide — 5.00 g of KBr is 5.00/119.00 = 0.0420 mol, and 0.0420 mol of NH₄Br is 4.12 g — so the emulsion precipitates the same quantity of silver bromide from the same quantity of silver nitrate.

NH₄Br + AgNO₃ → AgBr + NH₄NO₃

The precipitation; the by-product is ammonium nitrate rather than potassium nitrate

What the ammonium ion changes. Ross’s account is that a plain-silver emulsion cannot normally reach the speed of one made with ammonia-treated silver, and that putting the ammonium ion in by way of the bromide gives “something of a hybrid beast”. Her evidence is a side-by-side comparison: the same recipe made both ways, coated on the same film, exposed and developed together. The potassium-bromide negatives come out a little more contrasty; the ammonium-bromide ones hold more shadow detail and less dense highlights for the same exposure; and — the practical difference — the ammonium-bromide emulsion tolerates developers that wreck the other, sodium carbonate as an accelerator and Rodinal at 1:10, where AJ-12 made with potassium bromide gives “a disaster of pepper and fog”. She says plainly that her research is not exhaustive or conclusive, and the course repeats that rather than rounding it into a rule.

The mechanism is not given by any source the course has read. Ross quotes Baker’s 1941 emulsion manual for the nearest thing to one — “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” — but that is an argument about clotting at the moment of precipitation, not about the developer tolerance she measured. The course reports both and joins them with nothing.

In the nineteenth-century emulsion literature it is the default bromide. Abney’s 1885 treatise runs on it: his gelatino-bromide method calls for 140 grains of ammonium bromide “or its equivalent in zinc, potassium, or any other bromide”, the equivalence understood as a matter of course, and Burton’s process specifies 122 grains of “pure and well-dried ammonium bromide”. It survives in wet plate as one bromide of three — Bostick & Sullivan’s Old Workhorse bromo-iodizer combines it with cadmium bromide and potassium iodide — a process the course does not teach, and cadmium bromide is why.

⚗The chemistry

Ammonium bromide dissolves into an ammonium ion and a bromide ion, and only one of them is inert. The bromide ion is the conjugate base of a strong acid and does nothing to the pH. The ammonium ion is the conjugate acid of ammonia, and OpenStax gives its acid ionisation constant as K_a = 5.6 × 10⁻¹⁰ — weak, but not zero, so a solution of ammonium bromide in pure water is faintly acidic where a solution of potassium bromide is neutral.

That is the chemistry behind Abney’s warning, and it is the reason his warning is stronger than the chemistry alone would predict. A salt that is genuinely pure is only faintly acid. Abney’s complaint is about a commercial article that is “usually more or less acid, even though otherwise pure, and frequently becomes strongly acid by keeping”, and about what free acid does to an emulsion: it “has in some way a powerful effect in retarding the conversion of the silver bromide into the sensitive form”. Neither he nor the course can say what that mechanism is.

Properties

White odourless crystals or granules, slightly hygroscopic, yellowing slowly in air — CAMEO, the Merck Index by way of Haz-Map, and Abney all note the yellowing, which is bromide oxidising to bromine at the surface. Relative molecular mass 97.94, against 119.00 for potassium bromide: a gram of ammonium bromide carries 21% more bromide than a gram of the potassium salt. Replacing ammonium bromide with potassium bromide therefore means multiplying the weight by 119.00/97.94 = 1.21; going the other way, multiply by 0.82, which is Ross’s 5 g to 4.1 g.

On solubility the record disagrees with itself and the page says so. PubChem carries two HSDB entries: 78.3 g per 100 g of water at 25 °C, and 97 g per 100 cc at 25 °C rising to 145.6 g at 100 °C. Those are not one measurement expressed two ways — 78.3 g per 100 g of water is about 79 g per 100 cc, not 97 — and nothing says which is preferred, so both are quoted. The spread does not matter at emulsion weights, which dissolve easily either way; it is named so that no single figure is taken from this page as settled. Freely soluble in methanol, ethanol and acetone.

Handling

This is where ammonium bromide parts company with potassium bromide, and the difference is not one of degree. Potassium bromide carries a notified classification that tops out at irritant. Ammonium bromide carries a harmonised classification under Regulation (EC) No 1272/2008 — the binding entry, not an aggregate of what companies chose to report — giving Danger, two pictograms and five statements, among them H360FD, may damage fertility and may damage the unborn child, H362 for breast-fed children and H372 for organ damage through prolonged or repeated exposure. The 704 aggregated ECHA notifications give the same five; Japan’s NITE-CMC adds more. Neither EH40 nor the NIOSH Pocket Guide sets a workplace exposure limit, and EH40 states that absence from its list does not indicate that a substance is safe.

Why Level B. The course rubric caps Level A at a classification “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”, and a harmonised reproductive-toxicity entry is outside that ceiling. Level B fits: its criterion of fine powders that must not be inhaled describes exactly the control problem, and its controls — goggles, apron, dust discipline, an eyewash within reach — are the ones the classification calls for. It does not reach Level C, whose reproductive-toxin criterion applies where a fume cupboard or specialist disposal is the recognised control; here the recognised controls are dust suppression, gloves, eye protection and hygiene, and the same reasoning the course applied to boric acid applies here.

⚠The one operation that takes this out of Level B

Reaching for it as a general-purpose restrainer when potassium bromide would do. That is not a step up in level — it is a step the course would rather you did not take at all. Where a formula asks for a soluble bromide and does not care which, potassium bromide does the same job with no reproductive, lactation or organ-damage statement anywhere in its record. Choose ammonium bromide when the ammonium ion is the point, as it is in Ross’s emulsion, and not otherwise. Weigh it over a tray without raising dust, keep it off skin, wash hands before eating, and treat H362 as what it says: this is a substance to keep away from anyone who is pregnant or breastfeeding, and HSE’s HSG262 is the reason gloves are not the whole answer.

Waste

The stream is an emulsion make’s wash water and spent developer where it has served as a restrainer. Any bath that has touched silver carries silver, and recovery comes first. Kodak’s J-300 guidance for amateur photographers puts developers, stop baths, fixers after silver recovery and wash water into the sewer where there is one, and states that septic systems are not designed to treat photographic processing solutions.

ILFORD’s guidance for domestic users in the United Kingdom is to bottle wastes separately, label them and take them to a household waste and recycling centre. Because this substance carries a harmonised reproductive-toxicity classification, that is the route the course recommends rather than the drain, even for the small quantities one emulsion make involves; the disposal caveat governs, and so does your local authority.

History

Abney’s 1885 book, written while gelatine emulsions were still being argued about in the photographic press, treats ammonium bromide as the ordinary bromide and the choice between ammonium, potassium and zinc as a substitution by equivalent weight. What he does not treat as ordinary is its quality: the passage insisting the salt “should be as nearly as possible neutral” is one of the few places where he stops describing a process to describe a supply problem, and a reminder that a nineteenth-century formula assumed a reagent the reader had to test.

The modern return runs the other way. Potassium bromide is what a twentieth-century formula means by “bromide”, and Ross’s tutorial is explicitly a variation from it: one substitution, everything else held, the result photographed rather than asserted.