Potassium iodide

Iodide is the halide that is never used on its own. In every place photography uses it — the emulsion, the developer, the reducer — it is the minority partner, present at a few per cent or a few tenths of a gram, doing something the majority halide cannot.

In photography

In the emulsion. Kodak’s 1928 primer states it flatly: negative-making materials such as plates and films “always contain silver bromide with a small addition of silver iodide”. Denise Ross’s Light Farm negative emulsion adds 1 mL of a 10 per cent potassium iodide solution — a tenth of a gram — against 7 g of ammonium bromide and 10.8 g of silver nitrate. Davey and Knott’s 1952 Kodak patent asks for far more: at least 6 per cent of the total silver halide as silver iodide, better at 10 to 20 per cent, brought up if necessary with potassium iodide.

⚗The chemistry

Bromide added to a developer works in the solution; iodide added to an emulsion works inside the crystal. It goes down with the bromide as a mixed silver iodobromide, so what it buys is a property of the grain rather than of the bath — which is why it is measured as a percentage of total halide and not in grams per litre. Abney’s 1885 treatise gives the spectral half: silver iodide in its pure state is sensitive only to the ultra-violet, the violet and the blue, so adding iodide to a bromide emulsion must alter the range of spectral sensitiveness. The modern account of why a few per cent of iodide raises speed is not in the documents this course holds, and it does not guess at it.

What iodide bought, argued in public. Abney’s chapter on iodide in emulsions records a dispute: its usefulness was, he writes, energetically disputed by Eder and others, so he published the comparison. Two emulsions to the same formula, one carrying about 10 grains of potassium iodide against 93 grains of potassium bromide, gave plates “a little slower in coming out; but, on the other hand, they were certainly much brighter and cleaner”; boiled with ammonia present, the bromide-only emulsion fogged and the iodide one “remained quite bright”. Slower, cleaner: that is the trade iodide still offers.

In the developer it is a restrainer. Kodak’s 1928 primer names the restrainer as “bromides and iodides of sodium or potassium”, and its tropical process developer D-13 takes 2.1 g of potassium iodide per litre where most of its formulas take bromide. Iodide is also a by-product: as a developer is used, sodium bromide and sodium iodide accumulate, and “the bromide and iodide and developer oxidation products restrain development”. Wall’s 1924 formulary puts it to a narrower job — 2 g per litre is suggested as obviating the black hair-like abrasion marks glossy development papers show, with the note that “the iodide is the active agent”.

🕰The Lainer effect

Eder records that Alexander Lainer, studying developers, found that under certain conditions the action of the developer is accelerated by adding potassium iodide — in contrast to potassium bromide — and that the technical literature named this the Lainer effect. The same ion that restrains can therefore also speed development up. The course has not found a sourced statement of which conditions, and gives the effect without an explanation.

It dissolves iodine. Iodine will not dissolve in water but dissolves readily in a solution of potassium iodide, which is how every iodine bath in the literature is made: Kodak’s 1928 primer prepares its cutting reducer by dissolving iodine crystals in a 10 per cent potassium iodide solution. That reducer then adds potassium cyanide, which puts it at Level D and out of this course as a procedure; Wall’s 1924 iodide-and-hypo reducer, 10 g with 250 g of hypo per litre for eight to ten hours on a generally fogged negative, stays in. Iodide is also one of the three halides Wall gives for an indirect sulfide bleach — 18.6 g against 35 g of ferricyanide — where he notes that iodides give more yellowish tones than bromides.

Properties

An odourless white solid, slightly deliquescent, and among the most soluble salts in the darkroom: 148 g per 100 g of water at 25 °C, or 1 g in 0.7 mL; Wall’s 1912 dictionary gives 138 per cent in cold water and 220 per cent in hot. There is no hydrate. The relative molecular mass is 166, which is 47 more than potassium bromide’s 119, so a gram of iodide carries fewer halide ions than a gram of bromide: formulas that swap one for the other are not swapping equivalents. The solution is reported as neutral or usually alkaline, pH 7 to 9.

The salt is stable in dry air, but light and moisture accelerate its decomposition, and Kodak’s 1928 primer gives the visible sign: potassium iodide solutions often turn deep yellow from liberated free iodine, so both solid and solution belong in dark brown bottles. A yellowed stock is no longer only potassium iodide — it is iodine dissolved in iodide, and that mixture attacks a silver image.

Handling

The aggregated ECHA notifications held by PubChem classify potassium iodide Danger, with the irritant, health-hazard and environmental pictograms. Of the 627 reports carrying hazard codes, 43.6 per cent give damage to organs through prolonged or repeated exposure, 30.1 per cent serious eye irritation, 28.4 per cent skin irritation, 20.8 per cent allergic skin reaction and 11.7 per cent allergy or asthma symptoms if inhaled; 14 per cent give toxicity to aquatic life. Ninety-four of the 721 reports (13 per cent) say it meets no GHS criterion, and CAMEO’s health-hazard line is milder still: “May irritate eyes or open cuts”.

Why Level B. Against the course rubric, Level A is capped at substances classified at most as irritant, harmful if swallowed or corrosive. Potassium iodide goes past that — a fifth of the classifying notifiers call it a skin sensitiser, one in eight a respiratory sensitiser, and nearly half give organ damage on repeated exposure — and Level B’s first criterion names sensitisers and fine powders that must not be inhaled. It is not Level C: that criterion is qualified by the words “where a fume cupboard or specialist disposal is the recognised control”, and for weighing a fraction of a gram of a non-volatile solid it is not. Neither EH40 nor the NIOSH pocket guide sets an exposure limit for it, and EH40 states that absence from its list does not indicate that a substance is without hazard.

⚠Safety

Level B controls: everything Level A asks, plus chemical splash goggles and an apron, a particulate mask when weighing where the supplier’s sheet requires one, and work at an open window or under local extraction. Because a fifth of the classifying notifiers call it a skin sensitiser, gloves are changed rather than rinsed, and repeated contact is the exposure to avoid, not the single splash. PubChem and CAMEO both call the salt a weak reducing agent that may react with oxidising agents, strong or weak.

Waste

Iodide leaves the darkroom in spent developer, in the rinse after an iodide bleach or reducer, and in wash water; a reducer bath carries the silver it took off the negative as well. The environmental statement decides the route: 14 per cent of the notifiers who classify potassium iodide give H411, toxic to aquatic life with long lasting effects, which is reason enough to collect iodide-bearing solutions rather than dilute them away. Kodak’s J-300 guidance is explicit that septic systems are not designed to treat photographic processing solutions, and 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

Iodide came into photography as a disappointment. Eder records that Talbot’s attention was drawn in 1834 to Davy’s report that silver iodide is more sensitive to light than silver chloride, and that his own experiments gave the opposite. What he found instead was the more useful discovery: an excess of potassium iodide annuls the light sensitivity of the silver salt altogether, so he could fix his silver chloride images in a solution of potassium iodide — a fixative Eder says he found as satisfactory as common salt, and which he named to Biot on 20 February 1839 beside a strong salt solution and the still-secret preparation Herschel had given him. Excess halide as a desensitiser is the restrainer’s ancestor, and it was found as a fixer.

The productive use came with paper, then with plates. Eder gives Blanquart-Évrard’s printing paper as a litre of water carrying 10 g of gelatine, 10 g of potassium iodide and 2.5 g of potassium bromide, dried, held in hydrochloric acid vapour and sensitised in a 7 per cent silver bath. Thirty years later Abney and Eder were still arguing about whether a few grains of it belonged in a gelatine emulsion; the bromo-iodide films of the following century settled that.