Silver(I) oxalate

Nobody sells this and nobody in this course makes it. Silver oxalate arrives on its own, as a white cloud in a bottle of kallitype sensitiser, the moment the silver nitrate meets the ferric oxalate — and the sheet that tells you to expect it also tells you to disregard it. Both instructions are correct, and the reason the compound has a page is the third instruction, which the sheet does not give: it must not be allowed to dry.

In photography

It is the precipitate a kallitype sensitiser is expected to throw. Photographers’ Formulary’s kallitype sheet has the reader stir 2 g of solid silver nitrate into 30 mL of 20 per cent ferric oxalate solution, and then says plainly what usually happens next: “It is very common for a precipitate of silver oxalate to form. This precipitate does no harm. If your sensitizer does form a precipitate, just be sure you do not transfer it to the paper you will be coating.”

2 Ag⁺ + C₂O₄²⁻ → Ag₂C₂O₄

Silver(I) meets free oxalate, and the silver leaves the solution

Where the oxalate comes from is the interesting half. A concentrated ferric oxalate solution is not a bottle of one species. The siderotype lesson works it out from Ware’s stepwise formation constants: the third oxalate ligand of the trisoxalatoferrate(III) anion is held about a thousand times less tightly than the second, so a roughly 1 molar solution of the complex carries on the order of 0.01 mol/L of free oxalate in equilibrium with it. That free oxalate is what the silver finds. It is not an impurity, not a sign of a bad batch and not something a better supplier would remove — it is the chemistry of the complex, and it is why the two solutions of a modern kallitype kit are kept in separate bottles until the moment of coating.

⚗Why the same reaction does not happen in a Van Dyke

The Van Dyke sensitiser also puts silver nitrate in the same bottle as an iron(III) salt, and it keeps for months. The difference is the anion. Its iron salt is ammonium iron(III) citrate, and citrate holds silver(I) in solution rather than throwing it down; tartaric acid keeps the pH where it needs to be. Change the anion to oxalate and the silver has somewhere to go. The kallitype’s answer is two bottles and no keeping time at all; Mike Ware’s answer, a century later, was to change the silver salt instead, and that is the argyrotype and its silver sulfamate.

What it costs the print. Every milligram of silver that goes down as the oxalate is a milligram that will not be reduced to image silver, and — worse for anyone trying to work repeatably — the solution left above the precipitate is no longer the concentration its label claims. That is the argument the kallitype sensitiser page makes for mixing only what one sheet needs. The Formulary’s own remedy for its one-bottle version is mechanical rather than chemical: ripen the mixture for two or three days, stir it occasionally, and keep the solid out of the brush.

Two nineteenth-century uses, both abandoned

Willis whitened a negative with it, on the way to a platinum image. Pizzighelli and Hübl record the method in 1886, and it is a startling piece of chemistry: treat a developed gelatine plate with ferric oxalate and the dark silver image is converted into a white one of silver oxalate; wash; then flow over it a dilute solution of potassium chloroplatinite, which decomposes the oxalate and leaves silver chloroplatinite in its place. Pizzighelli and Hübl print the double decomposition as Ag₂C₂O₄ + K₂PtCl₄ → Ag₂PtCl₄ + K₂C₂O₄; it is set here as text rather than as a checked reaction because silver chloroplatinite is a substance this course does not otherwise teach and has no encyclopaedia page for.

They also record the variant with no platinum in it at all — bleach the image to silver oxalate, wash for two or three minutes, and redevelop with the ferrous oxalate developer, which reduces the oxalate back to metallic silver at greater density and without the yellow stain a pyrogallic developer would leave. Their own comment on it is worth keeping, because it is a nineteenth-century author admitting a mechanism was not understood: how Willis really obtains a denser negative “by thus working in a circle, as it were, is not clear to us”.

Namias substituted it for silver nitrate in an intensifier. Wall’s 1924 collection gives the recipe — silver nitrate solution mixed with neutral potassium oxalate, the precipitate allowed to settle, the water decanted, and the solid suspended in a litre of water and shaken before each application to a bleached negative. The stated reason for preferring it is that silver nitrate “may cause stains”. The course reproduces neither method and gives no procedure for either, and the classification section below says why.

Cassell’s 1911 verdict on the compound is one clause long and it is the fairest summary anyone has written: it “has been suggested for printing-out emulsions, but is rarely used”.

Properties

Ag₂C₂O₄, relative molecular mass 303.76 on PubChem’s computed value; Cassell rounded it to 304 in 1911 and was right to three figures. Two silver ions to one oxalate ion, which makes it 71.0 per cent silver by weight on the course’s own arithmetic from the atomic weight of silver, 107.87, against that molecular mass — a higher proportion than silver nitrate carries, which is one reason a precipitate that looks trivial is not.

A white crystalline powder, practically insoluble in water and in alcohol, soluble in nitric acid. That single sentence from Cassell is the whole of the physical description the course could source, and the gap is a real one rather than an oversight: there is no solubility figure at a stated temperature in any document read for this page, and none is invented here. PubChem’s record carries the computed identity and the hazard classification and nothing experimental at all.

It darkens in light, and it has done so in the literature since before photography existed. Bergman’s Opuscula physica et chemica of 1779 states it in one sentence, as Eder’s history translates it — “the rays of the sun darken the oxalate of silver” — and Robert Hunt put it in a camera in 1844 and got “an exceedingly faint representation” after ten minutes. Hunt’s summary is the more careful statement: the carbonate, sulphate, acetate, citrate and oxalate of silver all change considerably and fairly rapidly in light, and in some proportions the oxalate shows a very high degree of susceptibility. That qualification is the point. Its light sensitivity was real and was known, and it never became a process: a salt that will not dissolve cannot be coated as a solution, and the reason nobody worked around that by coating it as a powder is set out under Handling.

Handling

The aggregated ECHA notifications held by PubChem classify it Danger, with the exploding-bomb, exclamation-mark and environment pictograms. There is no harmonised entry behind that: the substance’s own ECHA CHEM record, EC 208-568-3, has an empty index number, and its only regulatory processes are the EC Inventory, a CLP notification and a 2008 pre-registration. So the classification is the notifiers’ own: 104 of the 109 company reports carry hazard codes, five say the substance meets no GHS criteria at all, and every one of the five statements above appears in 95.4 per cent of the reports that do.

The statement that governs this page is the explosive one. PubChem prints it as “(Deleted) Explosive; mass explosion hazard”; the deletion marker belongs to the H-code’s status in the GHS revision PubChem indexes against and not to the notifiers’ claim, and the corroboration is in the precautionary codes filed alongside it. P210, P230, P240, P250, P370+P380 and P373 is the set written for explosives and for nothing else, and P250 in words is do not subject to grinding, shock or friction while P230 is keep wetted with. Read together they describe a dry solid sensitive to heat, shock and friction that is stored wet. The International Chemical Safety Card for oxalic acid, which is a tier-1 document and independent of the notifiers, says the same thing from the other direction: oxalic acid forms explosive silver oxalate with certain silver compounds, and is stored separated from silver compounds for that reason.

What the course does not have, and will not supply, is a decomposition temperature, an impact sensitivity, a friction sensitivity or a detonation velocity. No document read for this page publishes one. The honest position is that the substance is classified as an explosive by nearly every company that has notified it, that the precautions those companies filed are the precautions for a friction- and shock-sensitive solid, and that this is sufficient to establish a handling rule without pretending to a number the course has not read.

☠Why Level D

The rubric puts a substance at Level D when the course teaches its chemistry, history and significance and gives no actionable home procedure, and that is exactly the position here. There is no preparation on this page, no isolation, no drying step and no quantity, because every published route to the dry compound ends with a friction-sensitive powder in a domestic room, and because nothing in photography needs one — the two historical uses are an intensifier and a bleach-and-redevelop step that the course teaches as history and does not reproduce.

What is not Level D is the situation a reader will actually be in, which is a wet white cloud at the bottom of a kallitype bottle. That is handled inside the kallitype lab’s own Level B assessment, whose controls are the silver nitrate’s rather than this compound’s — splash goggles, nitrile gloves, no metal touching anything, and every vessel rinsed at once. The one rule this page adds to that lab is negative and absolute: nothing containing both an oxalate and a silver salt is left to dry out. A tray put aside wet is a rinse away from being nothing; the same tray found dry a week later is the case the safety card is describing.

On exposure limits, the page has to say that there are none. HSE’s EH40 carries two silver entries, metallic silver at 0.1 mg/m³ and soluble silver compounds as Ag at 0.01 mg/m³, and the NIOSH Pocket Guide’s entry covers silver metal dust and soluble silver compounds at the same 0.01 mg/m³ with an IDLH of 10 mg/m³ and argyria among its symptoms. Neither names this compound and neither is transferred to it here, because it is practically insoluble and the soluble-compound limit was not written for it. EH40’s own introduction states that absence from the list does not indicate that a substance is without risk. Since the course asks nobody to weigh, dry or disturb the solid, it specifies no airborne control for it and specifies instead that the solid is never dry in the first place.

Waste

Wet, labelled, and in the silver bottle. The mixed sensitiser, the rinses from the graduate and the rod, the blotting paper under the coating pool and any filter that has caught the precipitate all go into the same silver-bearing waste container, and they go in wet. That container is then a silver waste rather than an oxalate waste, and it is kept apart from the print-clearing and developer wastes for the reason the oxalic acid card gives.

The environmental half of the classification is as strongly agreed as the explosive half — H400 and H410, very toxic to aquatic life with long lasting effects, in 95.4 per cent of reports. Kodak’s J-52 lists silver among the effluent parameters municipalities most often regulate, at a mean limit of 1.2 mg/L, and J-214 sets the United States federal line at 5 ppm of silver in a liquid waste, EPA number D011. At 71 per cent silver by weight, a gram of this precipitate is 0.71 g of silver, which is enough to carry more than half a cubic metre of water past that 1.2 mg/L limit. ILFORD tells domestic users in the United Kingdom to bottle wastes separately, label them and take them to a household waste and recycling centre. Check your local regulations; they govern.

History

The compound is older than photography and its first recorded observation is a light-sensitivity one. Torbern Bergman, in the Opuscula physica et chemica of 1779, records that the sun’s rays darken the oxalate of silver, in the same passage in which he describes the mercury oxalates blackening. Eder’s history, which the course reads in the 1945 English edition, treats Bergman as the observer to whom we are indebted for the light sensitivity of both.

Robert Hunt tested it as a picture-making salt in 1844 and again in the 1854 edition, and reported honestly that it lost: ten minutes in the camera gave an exceedingly faint representation, where the phosphate gave a well-marked picture and the carbonate a tolerable one in five. It appears in his general list of the silver salts that change under light and never in his list of the ones worth using.

Its one genuine period of usefulness was as an intermediate rather than an image. Willis, having invented the platinotype, used it in the 1880s as a white way-station in a plate intensifier, and Namias later preferred it to silver nitrate in another intensifier because it stained less. Both are on the shelf of nineteenth-century negative treatments the course studies and does not reproduce, and both disappeared with the plate.

What survived is the sentence in the kit instructions. A hundred and forty years after Willis, the compound’s only remaining appearance in photography is as a nuisance in the bottom of a kallitype bottle — and the one thing worth carrying away from all of the above is that the nuisance is a classified explosive when it is dry, and that the whole of the control is to keep it from ever getting there.