Sodium selenite

This page begins with what the course could not establish, because that is the most useful thing on it. Sodium selenite is the soluble selenium(IV) salt of the toning family. Not one of the manufacturer sheets this course holds names it as the active ingredient of a selenium toner, and the page says so before it says anything else.

In photography

The verified chain, and where it breaks. Three things are documented. A selenium-toned print ends as silver selenide, which Kodak’s toner sheet states outright. The Getty Conservation Institute’s atlas records why the trade used it — first for a dark brown-orange tone, later for the chemical and environmental stability of the toned silver. And the element itself will not dissolve in water, so any toner must start from a soluble selenium compound.

What is not documented, in anything this course has read, is that the compound is this one.

!Three sheets, three different selenium species

• Kodak, on Rapid Selenium Toner: “the active ingredient is a sulfite salt that has a concentration of less than 2 percent”, and working solutions “generally contain less than ½ percent selenium sulfite”. Selenium sulfite is not a name with a CAS number behind it; it describes a selenium species held in a sulfite bath, and Kodak does not identify it further. The species usually named in the wider literature for selenium dissolved in sulfite is a selenosulfate; the course has not verified that identification against a source it holds, and records it here as the reading to test rather than as a fact.
• Moersch, in the safety data sheet for MT1 Selentoner: 2 to 3 per cent w/w sodium selenate, CAS 13410-01-0 — selenium(VI), not the selenium(IV) of a selenite — with 10 per cent w/w ammonium sulfite and 20 per cent w/w ammonium thiosulfate.
• Wall’s 1924 handbook, for the historical baths: sodium selenide, selenium(−II), made by dissolving the element in sodium sulfide or fusing it with caustic soda.

Three sources, three oxidation states of selenium, and no sodium selenite among them. PubChem’s own use listings for sodium selenite name glass manufacture, ceramics, bacteriology, seed testing, porcelain decoration, soil and feed supplementation and medicine — and not photography.

So what is this entry for? Selenium(IV) is the oxidation state between the historical selenide and Moersch’s selenate, so a reader following the family needs its identity, its solubility and its hazards to make sense of either end. And anyone who meets a home-brew selenium toning formula will meet this name — its aqueous solubility of about 85 g per 100 mL is exactly what a toner formulation needs — and will want to know what the classification says before deciding whether to weigh it. The course records the classification so that the decision to have nothing to do with the solid can be made on evidence.

What the family does to a print, quantitatively. Because the mechanism belongs to selenium rather than to any one salt, the behaviour documented for the bought toners applies whatever the species. Toning reaches the higher densities first, whatever the dilution or temperature, so a strong bath used briefly intensifies the shadows and leaves the highlights alone; a very dilute bath used longer will reach the highlights but risks losing shadow density first. That is Moersch’s observation, and it is the single most useful thing to know about selenium toning. Harman’s dilutions put numbers on the same idea: 1+3 for a tone change, 1+20 where the purpose is protection of the image with minimal colour shift, complete in two to four minutes.

Properties

A white crystalline solid, denser than water; the safety card calls it a hygroscopic solid in various forms, and Haz-Map records a beige odourless powder among the commercial descriptions. Its relative molecular mass is 172.95 and it decomposes at 320 °C rather than melting cleanly.

Solubility is the whole reason a selenium salt exists in this story. Elemental selenium is insoluble in water; this salt dissolves at 85 g per 100 mL at 20 °C on the safety card’s figure, 89.8 g per 100 g of water on HSDB’s, and at least 100 mg/mL at 21.5 °C on CAMEO’s. It is insoluble in alcohol. A bath of a fraction of a per cent is therefore nowhere near saturation, which is why selenium toners keep their strength through many prints.

The solution is weakly basic, according to the safety card, and that matters twice over. It places a selenium bath on the alkaline side, where Moersch’s MT1 sheet measures its own toner at pH 9.53, and it means that acid is chemically as well as procedurally out of place: CAMEO’s entire reactivity profile for this substance is the sentence that it is incompatible with strong acids, and Moersch’s sheet records violent reaction with strong acids and the release of toxic materials with acids.

⚗A dose-response lesson in one substance

ChEBI gives disodium selenite a role as a nutraceutical, and PubChem’s use listings include dietary supplementation, soil additives for selenium-deficient areas, and use as a feed additive against selenium deficiency in livestock and poultry. The same compound carries “fatal if swallowed” from every notifier that classifies it.

Both are true, and the gap between them is quantity. CAMEO puts a number on where the margin closes: in humans, a concentration of 5 ppm in food, or 0.5 ppm in milk or water, has been estimated to be dangerous. An essential trace element and an acute poison are not two different substances; they are two different doses, and the interval between them is narrower for selenium than for almost anything else a darkroom might hold.

Handling

The aggregated ECHA notifications classify it Danger, with the acute-toxicity, irritant and environmental pictograms. Fatal if swallowed is carried by every report that classifies it. The allergic-skin-reaction statement is carried by 92.9 per cent — this is a sensitiser, and HSE’s approach to sensitisers is that once a person has reacted, further exposure must be prevented rather than managed. The inhalation statements split between toxic and fatal, at 71.6 and 28 per cent, which is a disagreement about severity rather than about kind.

The gas the sources do not name. Both CAMEO and the safety card record a reaction with strong acids that generates a toxic hazard, and neither says what the gas is. The reason the rule is nonetheless absolute is visible one line away in HSE’s EH40, which sets dihydrogen selenide at 0.02 ppm over eight hours and 0.05 ppm over fifteen minutes — among the lowest limits in the whole table, and more than two hundred times lower than the one EH40 sets for hydrogen sulfide. The course states plainly that it has not verified which species an acidified selenite bath releases; it has verified that the family’s volatile hydride is controlled at a level a darkroom cannot measure.

The exposure limits belong to selenium, not to this salt in particular. NIOSH’s recommended limit and the OSHA permissible limit are 0.2 mg/m³ as an eight-hour average and are stated to apply to selenium compounds as Se; HSE’s EH40 sets 0.1 mg/m³ on the same basis; the safety card gives a TLV of 0.2 mg/m³ and a German MAK of 0.02 mg/m³ for the inhalable selenium fraction. The IDLH for selenium is 1 mg/m³. Those are milligram-per-cubic-metre numbers, and a domestic room has no way of knowing whether it meets them.

Why Level C. The rubric’s first Level C criterion names reagents classified as acutely toxic or as sensitisers at the concentrations used, where a fume cupboard or specialist disposal is the recognised control. This substance is both, unanimously acutely toxic by ingestion and near-unanimously a skin sensitiser, and the safety card’s own prevention entry asks for respiratory protection and for dust dispersion to be avoided. The fourth criterion — waste that cannot lawfully enter a domestic drain — applies as well. It is not Level D, because selenium toning is current, useful and taught: it is the preparation from a weighed solid that belongs in a laboratory, not the process.

⚠The solid and the bottle are different problems

Weighing a hygroscopic powder classified fatal if swallowed and fatal or toxic if inhaled, at a scale where the workplace limit is a fifth of a milligram per cubic metre, is a fume-cupboard operation. This course gives no procedure for it, and treats any formula that begins “sodium selenite, 5 g” as a laboratory formula rather than a darkroom one.

The bought toner is a different object with a different risk. Harman states that its product is toxic if swallowed and may cause sensitisation by skin contact, calls it a toxic chemical, and requires rubber gloves in all use and disposal, eye or face protection, and a well-ventilated area preferably with air extraction. Moersch’s sheet gives glove data a reader can act on — nitrile rubber, minimum thickness 0.11 mm, breakthrough time 480 minutes — and specifies eye protection and adequate ventilation. Neither sheet permits an open tray in a sealed room.

And whichever you have: no acid near it, ever, and no acid stop bath in the toning sequence without a thorough rinse first.

Waste

Every route is closed except collection. Three-quarters of notifications give the long-lasting aquatic hazard and roughly three in ten give the acute one; the safety card records the substance as toxic to aquatic organisms and warns that bioaccumulation may occur along the food chain, then specifies storage in a place with no access to a drain. Kodak’s environmental guidance for amateur photographers reaches the same conclusion from the other direction: in its table of waste routes, the selenium toner is the one product for which sewer discharge, discharge to a nearby treatment works and household refuse are all withheld, leaving a household hazardous waste collection facility or a licensed hazardous waste hauler.

There is no neutralisation to offer. Acidifying a selenite solution is the reaction the safety card and CAMEO both warn about, and it does not destroy the selenium in any case — it only changes its form. Bioaccumulation is the reason dilution is not a disposal method: a substance that concentrates up a food chain is not made safer by being spread thinner. Collect it in a labelled bottle, keep it away from acids, and use a licensed hazardous waste route; in England and Wales the government’s household hazardous waste service finder is the starting point. Check your local regulations; they govern.

History

The history of this salt in photography is a gap the course is content to leave visible. Selenium toning arrived as one of a crowd of metal toners, and Wall’s 1924 handbook shows the formulas of the day starting from the element and a sulfide. By the second half of the twentieth century the Getty atlas can describe it as a standard archival treatment for the art and museum market — and by then the chemistry had moved into a bottle whose composition the buyer is mostly not told.

That transition is why the identification is hard to pin down. Kodak names a species with no formula; Moersch names a selenate; the older literature names a selenide. Somewhere among the patents, the trade formularies and the supply catalogues, sodium selenite has a documented place in that story, and this course has not yet found the document that establishes it. Until it does, the page says so. A page with a gap and an honest note is better than a page with a confident error, and a reader who can supply the citation will improve this entry.