Silver bromide

Almost every photograph made in the last hundred and forty years began as this compound. Kodak’s 1928 primer states it without ceremony: negative-making materials such as plates and films always contain silver bromide, with a small addition of silver iodide.

In photography

It is precipitated in the dark, in gelatin, so the crystals stay suspended instead of settling.

AgNO₃ + KBr → AgBr + KNO₃

Kodak's 1928 primer: making the sensitive salt

The potassium nitrate stays dissolved and is washed out later. Two rules govern the mixing, both from the primer: the silver nitrate goes in a little at a time, so the precipitate is even rather than lumped; and a material that is to be developed must be made with an excess of bromide and no free soluble silver, because free silver fogs the whole surface as soon as it meets the developer.

Speed is made after the precipitation, not during it. The primer gives the lever in one sentence: the degrees of sensitiveness are obtained by varying the temperature and duration of the heating the emulsion undergoes in manufacture, the most sensitive emulsions being heated hotter and longer. That is ripening, and it is the whole difference between a slow contact paper and a fast film made from the same two salts.

🔬Under the microscope

Abney, in 1885, had already seen that the chemistry does not determine the material. Bromide of silver, he says, may be produced in several molecular states of different sensitiveness: the composition is identical and the particles differ in size, “built up of a greater or less number of primary molecules, according to the rapidity of the formation of the solid, and its subsequent treatment by heat, ammonia, &c.” White light through such a film may look ruby, orange, green, purple or grey depending on the state. Crystal size, not formula, is what an emulsion maker controls.

Bromide paper is the same salt coated slowly. The 1928 primer defines it exactly that way — a slow bromide emulsion on paper, used especially for enlargements — against the chloride papers used for contact printing by artificial light. Bromide is the halide fast enough to print through an enlarger lens.

And it is what the sepia bleach makes. Kodak’s G-23 notes that the bleach bath of its sepia toners converts metallic silver in the print back to light-sensitive silver bromide.

Properties

A pale yellow solid, darkened by light, soluble to 0.135 mg per litre at 25 °C — a fourteenth of silver chloride’s already tiny 1.93 mg/L, and about one part in seven million. It is anhydrous; there is no hydrate and no conversion factor.

The colour is the useful property. A white solid absorbs almost nothing in the visible; a yellow one absorbs at the violet-blue end, so the yellower halide can use light that passes straight through the whiter one. That is part of why bromide emulsions outrun chloride ones in blue-rich light — only part, since crystal size and halogen acceptors matter too.

⚗The chemistry

The insolubility is why fixing needs chemistry rather than washing. A LibreTexts general chemistry treatment works the photographic case: silver bromide has a solubility product of 5.35 × 10⁻¹³ at 25 °C, so clearing a single roll of film with pure water would take tens of thousands of litres. Silver ion forms a two-coordinate complex with thiosulfate whose formation constant it gives as 2.9 × 10¹³; adding that equilibrium to the solubility one gives an overall constant of about 15, an increase in solubility of roughly 3 × 10¹³.

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

Fixing: the halide leaves as a soluble complex

Kodak’s 1924 primer adds the detail that matters for an exhausted bath: two compound sodium silver thiosulfates exist, one almost insoluble and one very soluble, and only the soluble one forms while the bath has any appreciable fixing power. A bath worked past that point leaves the insoluble compound in the paper, and washing does not take it out.

Print silver bromide out rather than developing it and the image is a different colour: the Getty atlas records printing-out bromide images as cooler and greyer than the yellow-brown to red of chloride.

Handling

Read the aggregation before the pictogram. PubChem’s ECHA block gives silver bromide the signal word Danger with the health-hazard and environment pictograms — but the whole classification rests on 9 reports from 5 notifications, of which 3 say the substance meets no GHS hazard criteria at all. Of the six reports carrying codes, 66.7 per cent give H410, 55.6 per cent H400 and 22.2 per cent H360D, may damage the unborn child. That last figure is two reports, and the course found no harmonised European classification behind them.

Why Level B. Not because two suppliers out of six flagged reproductive toxicity, and not for the aquatic statements, which alone would sit inside Level A of the course rubric. It is Level B for two reasons the rubric names directly: every route to silver bromide here runs through silver nitrate, which the Level B criteria name in so many words, and emulsion making holds solutions above 50 °C, the Level B temperature criterion. A washed precipitate is also a fine powder that must not be inhaled. The controls follow: splash goggles, gloves and apron; ventilation; particulate protection if the precipitate is handled outside its gelatin. It stops short of Level C because no source the course holds names a fume cupboard as the recognised control and the waste is collectable for recovery.

⚠Safety

Two published exposure limits exist for silver and neither is written for this compound: the NIOSH Pocket Guide’s 0.01 mg/m³ as Ag, with an IDLH of 10 mg/m³, and EH40 Table 1’s identical figure both name silver metal dust and soluble compounds. Silver bromide is not one, so the course quotes no limit for it and controls the dust because it is a dust carrying silver. EH40 states that absence from its list does not indicate a substance is without hazard.

The one incompatibility with a photographic consequence is on no hazard label: Kodak’s 1924 primer warns that a very small amount of hydrogen sulfide will convert enough silver bromide to ruin photographic material. Sulfide toner and unopened paper do not share a cupboard.

Waste

In a conventional darkroom this compound is the silver waste. The developer reduces a small fraction of the crystals; the fixer dissolves all the rest and carries them out as the thiosulfate complex. Kodak’s J-214 lists used fixers among the solutions that typically contain more than 5 ppm of silver, the United States federal threshold at which a liquid waste becomes a characteristic hazardous waste, while reporting that the films and papers themselves did not leach silver at 5 ppm or above under the Toxicity Characteristic Leaching Procedure. Sheet and bath are two different disposal questions.

Recovery is routine industrial chemistry, and Kodak’s J-215 describes both common routes: electrolytic recovery, in which metallic silver plates onto a cathode while sulfite and thiosulfate are oxidised at the anode, and metallic replacement cartridges, in which iron reduces the silver and takes its place in solution — the equation below being bookkeeping for free silver ions, since in a spent fixer the silver is held in the complex.

Fe + 2 Ag⁺ → Fe²⁺ + 2 Ag

Metallic replacement, as bookkeeping for Kodak's description

Spent fixer is collected for recovery, never poured away. ILFORD tells United Kingdom domestic users to bottle wastes separately, label them and take them to a household waste and recycling centre. Check your local regulations; they govern, and they differ.

History

Bromide arrived as a way of making paper faster, not as the halide of the negative. Eder records that Talbot wrote to Biot on 15 March 1839, in a letter read before the French Academy, 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. Ware adds the notebook detail of the same material, Talbot’s “Waterloo” paper: a 10 per cent potassium bromide wash converted the chloride substantially to bromide, “which is the more insoluble of the two”, and a 20 per cent silver nitrate wash supplied the excess silver.

Then the material found its permanent job. The Getty atlas’s timeline records Joseph Swan’s English patent 2,968 of 22 July 1879 for silver bromide printing paper, and the gelatin dry plate carried the same salt into every camera. What Talbot built by hand, one wash at a time, became the thing a factory precipitates in the dark and ripens by the clock.