Potassium bromide

Less than two grams of it per litre decides whether a print has clean whites. Potassium bromide is the restrainer: the salt whose anion is already the by-product of development, put into the developer before any developing has been done.

In photography

In the developer. Kodak’s 1928 primer lists four ingredients — the developing agent, the alkali, the preservative, and “the restrainer (bromides and iodides of sodium or potassium)” — and gives the fourth one sentence: bromides and iodides are added “to compensate for any chemical fog produced by the developer, or inherent in the emulsion”. Omitting the bromide is listed among the mixing faults that make a developer fog.

⚗The chemistry

Development reduces exposed silver bromide to metallic silver, and every grain reduced sets a bromide ion free in the gelatin. Hurter and Driffield stated the consequence as ordinary chemical dynamics: the products of a reaction generally retard it. Bromide added at the start is the developer’s own exhaust, supplied in advance. They tested it against ferrous oxalate and against alkaline pyrogallol up to a 12 per cent solution and never found the reaction stop — “It is simply retarded, and, if sufficient time be allowed, the image will make its appearance in full force.”

What more or less of it does. Their Experiment 15 measured it: 128 g per litre of pyro-soda almost entirely prevented development in three minutes, while 2 g per litre pulled the highest density from 2.50 to 2.00. Far enough, and the image changes character — at 128 g their deposit was yellowish grey and held 2.43 times as much silver for a given measured density. The 1928 primer adds that a little bromide affects hydroquinone strongly and Elon much less.

In print developers bromide is a control rather than a trace. Kodak’s primer takes a single paper stock, D-52, and gives four different additions of a 10 per cent bromide solution to it — 4, 8 or 16 mL per litre of working solution, or 0.4 to 1.6 g of salt per litre — one for each grade of Vitava paper. Wall’s 1924 formulary gives the tonal consequence for chloride papers: the longer the exposure, with a corresponding increase of bromide, the warmer the tones.

In emulsion making bromide is not a modifier at all but the halide itself.

AgNO₃ + KBr → AgBr + KNO₃

Kodak's 1928 primer: precipitating the sensitive salt

The primer adds the rule that governs how much: the emulsion must carry an excess of bromide or chloride, because free soluble silver fogs the whole surface as soon as it meets the developer. Davey and Knott’s 1952 Kodak patent puts a figure on that excess — above 0.03 g of potassium bromide per litre in the finished emulsion, preferably 0.03 to 0.3 g/L, or the equivalent of another bromide or chloride, where equivalent means the same suppressing action on the silver ion.

In the sepia bleach it decides what the oxidised silver becomes. Kodak’s G-23 gives the bleach stock as 75 g of potassium ferricyanide, 75 g of potassium bromide and 195 g of potassium oxalate with 40 mL of 28 per cent acetic acid in 2 litres, diluted 1+1 for use, and says the bath “converts metallic silver in the print to light-sensitive silver bromide”. The ferricyanide oxidises; the bromide chooses the halide.

⁇Contested

Does bromide cost film speed? Photographers commonly say it does. Hurter and Driffield measured otherwise: matching densities by extending development, they wrote that the speed of the plate is not really altered by the addition of bromide. Kodak’s 1928 primer sounds opposite — in an old developer the restraint of accumulated bromide and iodide “is analogous to cutting down the exposure”. The course takes them as reconcilable: fresh bromide retards and time compensates; an exhausted developer carries oxidation products too, and time does not. What the course cannot verify against a source it holds is why the restraint falls harder on the unexposed grain, which is what makes bromide an antifoggant rather than a brake.

Properties

Colourless cubical crystals, a white crystalline powder, or the white granular solid photographic suppliers sell; odourless, with a bitter saline taste. It is very soluble — 67.8 g per 100 g of water at 25 °C, or 1 g in 1.5 mL — which is why a 10 per cent stock is trivial to make and why Kodak doses it by the millilitre. Wall’s 1912 dictionary gives 58 per cent in cold water and 102 per cent in hot.

Two properties matter for the way it is used. Its aqueous solution is neutral, about pH 7, so adding bromide changes a developer’s restraint without touching its alkalinity. And there is no hydrate in photographic use: the salt is anhydrous, its relative molecular mass is 119, and nothing stands between the formula and the balance. PubChem calls it hygroscopic, so a tub that has caked has been taking up water and its weighed grams are partly water.

Handling

The aggregated ECHA notifications held by PubChem classify potassium bromide Warning, irritant. Of the 521 reports carrying hazard codes, 91.4 per cent give serious eye irritation, 12.6 per cent skin irritation and 12.3 per cent possible respiratory irritation; 17 of the 538 reports (3.2 per cent) say it meets no GHS criterion. CAMEO calls it toxic by ingestion and inhalation and an irritant of the skin, eyes and respiratory tract.

Why Level A. Against the course rubric this handling meets the Level A criteria and no more: the classification tops out at irritant at the concentrations actually handled — a 10 per cent stock, 0.4 to 1.6 g per litre in a working developer; nothing is heated above 50 °C; and the waste is spent developer and rinse water, both collectable. HSE’s COSHH essentials sheet P1 supplies the glove specification. One Level B control is added by preference — splash goggles rather than glasses for the dry salt and the concentrate — because the eye statement is near unanimous among the notifiers who classify it at all. Neither EH40 nor the NIOSH pocket guide sets an exposure limit for potassium bromide, and EH40 states that absence from its list does not indicate that a substance is without hazard.

⚠Safety

Weigh over a tray, container away from the face, powder into water rather than water into powder. Gloves and eye protection throughout; goggles for the solid and the stock. PubChem’s reactivity profile calls potassium bromide a weak reducing agent incompatible with oxidisers, records a violent reaction with bromine trifluoride, and notes that concentrated sulfuric acid generates fumes of hydrogen bromide from it — keep it away from bleaches, peroxides and strong acids. It is also listed as incompatible with salts of silver and of mercury: that reaction is the one that makes an emulsion, and it will just as readily ruin a sensitised sheet.

Waste

Bromide leaves a darkroom in spent developer, in the rinse after a sepia bleach and in wash water. It is not what makes those streams a problem — the silver in the fixer and the ferricyanide in the bleach are — but it travels with all three, so the route is the same. Kodak’s J-300 guidance for amateur photographers sends developers, stop baths, fixers after silver recovery and wash water to the sewer where a household has one, and 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, and they differ.

History

Bromide entered photography as the sensitive salt, not the restrainer. Eder records that Talbot wrote to Biot on 15 March 1839 announcing the great light sensitivity of silver bromide paper: he saturated paper with silver nitrate, then potassium bromide, then silver nitrate again, and made a camera picture of a window in six or seven minutes.

The restraining use arrived with alkaline development. Eder credits Major C. Russell, in 1862, with the pyro-ammonia developer used with additions of potassium bromide, and notes that the second edition of Russell’s The Tannin Process (1863) describes the part the bromide plays as a retarding agent. Sixteen years later the same salt made the fast plate: Charles Bennett published on 29 March 1878 that gelatine silver bromide gains greatly in sensitivity when prepared with an excess of potassium bromide and held for five to ten days at 32 °C. Excess bromide, ripening and the modern emulsion all follow from that sentence.