Silver iodide

Silver iodide is the halide of the daguerreotype, the calotype and the wet collodion plate: the most insoluble of the three, the slowest to darken on its own, and the one that most needed help to finish the job.

In photography

The daguerreotype. A copper plate electroplated with silver and polished to a mirror is held over iodine in a fuming box, and the vapour converts the surface itself to silver iodide. Nothing stands between viewer and image, which is why the plate resolves detail nothing else of its century approaches; conservators note that it is sensitive only to blue and ultraviolet light.

The calotype. Ware’s account: silver iodide is deposited within the paper fibres, and the sheet sensitised with gallic acid and excess acidified silver nitrate. A short exposure leaves an invisible latent image, which more gallic acid and silver nitrate develop to a grey negative in metallic silver — raising the speed of negative-making about a hundredfold and dropping Talbot’s exposures from an hour to about a minute.

Wet collodion, and the reason cyanide entered the darkroom. Kodak’s 1928 primer states it directly: cyanide solutions dissolve the silver halides by forming soluble double compounds, and potassium cyanide was used for fixing wet collodion plates, “which, being made from silver iodide, are not easily fixed in hypo” — followed at once by the warning that the cyanides are extremely poisonous and a few grains swallowed will cause death. That sentence is why this course puts cyanide work at Level D of the classification rubric.

In a modern emulsion it is a trace additive. The 1928 primer describes negative materials as “silver bromide with a small addition of silver iodide”, and Abney’s seventh chapter is his side of the argument about why. In paired gelatin emulsions made to Eder’s formulas, one plain and one with about 10 grains of potassium iodide against 93 of potassium bromide, the iodide plates were “a little slower in coming out; but, on the other hand, they were certainly much brighter and cleaner”, and the ammonia-boiled emulsion without iodide fogged while the one with iodide “remained quite bright”. A little iodide costs a little speed and buys clean highlights. It shows in the print too: the Getty atlas records that many professional portraits with a greenish tonality were made on paper carrying a large concentration of silver iodide, while cautioning that no iodide survives processing, so only the colour suggests it.

Properties

A light yellow solid, gradually darkened by light, and the least soluble compound on any of these four pages: 28 × 10⁻⁷ g per litre at 25 °C, or 0.0028 mg/L, against 0.135 for the bromide and 1.93 for the chloride. Ware states the consequence: iodide added to a photogenic-drawing paper converts the excess silver ions, and probably the unexposed silver chloride too, to silver iodide, “as a consequence of the insolubility of the iodide being relatively greater than that of the chloride”.

AgCl + I⁻ → AgI + Cl⁻

Ware: the more insoluble halide displaces the less

Being yellow, it absorbs at the violet and blue end. Abney puts the limit plainly: iodide of silver in its pure state is sensitive only to the ultraviolet, the violet and the blue rays — so adding it to a bromide emulsion must modify that emulsion’s spectral range.

⚗The chemistry

Silver iodide is the clearest demonstration in the course that a halide alone is not a photographic material. Ware writes that it is quite insensitive to light without halogen acceptors, and Abney had shown it experimentally sixty years earlier: a washed silver iodide emulsion made with excess iodide, coated and exposed, gives no developable image, but dip half the plate in tannic acid or beer and that half develops while the other does not. The iodine has to have somewhere to go — which Abney took as one of the strongest proofs that photolysis is a chemical change rather than a rearrangement.

What dissolves it. HSDB names potassium iodide, potassium cyanide, ammonium hydroxide and thiosulfate among its solvents, and concentrated halide and thiocyanate solutions besides, but gives no concentration for any of them, so the course quotes none. The ordering in practice can be sourced: hypo clears a chloride paper quickly and, in Kodak’s words, does not easily fix a collodion iodide plate. Ware adds the complex formed in excess iodide, AgI₃²⁻, which is why a strong iodide bath redissolves some of what it just precipitated.

Handling

Of the four substances in this family, silver iodide carries the narrowest classification: the aggregated ECHA notifications held by PubChem give it Warning and the environment pictogram alone, and no health statement reaches the 10 per cent threshold PubChem displays.

Why Level B, despite that. The classification alone would place it inside Level A of the course rubric; it is Level B because of how the course reaches it. Every route runs through silver nitrate, which the Level B criteria name in so many words, or through iodine, and volatilising iodine generates vapours under the same criteria. A dry precipitate is also a fine powder that must not be inhaled. The controls are the silver nitrate controls plus ventilation wherever iodine is volatilised. It stops short of Level C on the iodide’s own account: no source the course holds names a fume cupboard as the recognised control for it.

⚠Safety

Do not read the mild classification as a mild page. The processes silver iodide belongs to are the most dangerous in the whole course, for reasons that have nothing to do with the iodide: the daguerreotype was developed in mercury vapour and the wet collodion plate fixed in potassium cyanide. Both are Level D here — chemistry, workflow and history, no procedure — and the rubric names mercury vapour development and cyanide fixing in its Level D criteria.

Neither the NIOSH Pocket Guide’s 0.01 mg/m³ for silver as soluble compounds nor EH40’s identical Table 1 figure is written for silver iodide, which is not a soluble compound; the course quotes no exposure 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.

Waste

The insolubility cuts both ways. Less of it leaves a coating in the fixer than would leave a chloride or bromide one, so a poorly fixed iodide material keeps its silver and its problem; what does dissolve joins the same silver-bearing bath as everything else and is collected for recovery. The environmental statement is where the notifiers agree most strongly of the three halides, at 95.8 per cent of the reports carrying codes. Kodak’s J-214 sets the United States federal line at 5 ppm of silver in a liquid waste, EPA Hazardous Waste Number D011, while noting that state and local agencies may be stricter; 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

Silver iodide had to be discovered twice before anyone believed it was photographic. Eder credits Humphry Davy with producing it and recognising its sensitiveness to light in 1814, and says the fact matters because both Niépce and Daguerre later worked with it and Talbot learned of it in 1834. In the same year Steffens, Link and Fischer — given a rare pinch of iodine by Gay-Lussac — reported the opposite: that the precipitated and the melted compound alike “retain their color under light”.

⁇Both results were right

Eder’s reconciliation is the chemistry of this page in a sentence. Davy, he suggests, probably prepared his iodide with an excess of silver nitrate, “since his preparation changed rapidly in light, which, as is well known, occurs only in that case”, while the three chemists in Paris evidently used an excess of potassium iodide. Excess silver supplies the halogen acceptor; excess iodide removes it.

Talbot used it as a fixer before anyone used it as a sensitiser. Ware describes the mechanism: potassium iodide converts the excess silver ions, and some or all of the unexposed silver chloride, into insoluble silver iodide, giving the pale primrose-yellow highlights that identify an iodide-fixed print. It does not fog, which is why Talbot favoured it at first. Its failure mode is the opposite one: with excess iodide present the nanoparticle silver of the image is easily oxidised back to pale silver iodide by the oxygen of the air, so the picture fades even in the dark, and the yellow iodide in the highlights absorbs exactly the blue and ultraviolet a negative needs to print with. That is why Herschel’s thiosulfate won.