Sodium selenate

Sodium selenate is the selenium(VI) member of the toning family: two sodium ions, one selenate ion, and one selenium atom to the formula unit. It is here for one reason. A working manufacturer of photographic chemistry names it, by name and by CAS number, as the selenium in a toner he sells today — and until this page was written the encyclopaedia had no entry to link that name to.

In photography

The one thing that is documented, and it is documented twice. Moersch Photochemie’s safety data sheet for MT1 Selentoner, version 3 of 16 February 2025, gives its composition as three declared components: ammonium sulfite at 10 per cent w/w, ammonium thiosulfate at 20 per cent w/w, and sodium selenate, CAS 13410-01-0, EC 236-501-8, at 2 to 3 per cent w/w, with a REACH registration number against that line and against no other. The maker then says the same thing again, unprompted, in running prose on his own shop page, because a regulation changed and customers asked whether his dilutions still held:

The legal regulations have changed: Selenium toner based on sodium selenite may now only be marketed in vol % <1. This leads to questions about changing the dilution ratios of this product. Answer: This product does not contain sodium selenite but sodium selenate, the preparation did not have to be changed.

Two independent statements by the same maker, one in a regulated document and one in a sales description, agreeing on the compound. That is as firm as a selenium toner’s composition gets in this corpus. The formulary entry for MT1 sets out the whole sheet line by line, including the contradiction it prints about its own acute toxicity, and this page does not repeat it.

What it is not. It is not the sodium selenite — selenium(IV) — that HARMAN declare in their toner. It is not the selenosulfate that the wider literature reads into a bath of elemental selenium boiled in sulfite, which is the historical route. And it is not the silver selenide — selenium(−II) — that a toned print ends as. Five selenium species in one small family of products: +VI here, +IV in HARMAN’s bottle, zero in the element the old formulas boiled, −II on the finished print, and the selenosulfate a species this course has not verified. This one is the top of the range.

⚗Why a selenate is a sensible salt for an alkaline bath

The WHO’s monograph on selenium states the property that makes this entry make sense: selenium in the +6 or selenate state is stable under both alkaline and oxidizing conditions. It is the form found in alkaline waters and in alkaline soils, where it stays dissolved and available.

Set that beside the toner. Moersch’s sheet measures his concentrate at pH 9.53 — alkaline, by some way. So the selenium in that bottle is in the one oxidation state the monograph describes as stable there. The same monograph says the neighbouring state does not enjoy that: selenium(IV) in alkaline solution tends to oxidize slowly to the +6 state, if oxygen is present, but not in an acid medium.

Where the reading stops. It is tempting to conclude that an alkaline selenite toner drifts towards selenate on the shelf, and this course does not conclude it. A selenium toner is not a plain aqueous selenite: on the two European composition tables it also holds a sulfite and a thiosulfate at between ten and thirty per cent, and a sulfite is a reducing agent — the monograph itself names sulfur dioxide as one of the reagents that reduces selenium(IV) to the element. Nothing this course holds describes what the selenium species in a part-used toner bottle actually does over a year. The oxidation-state chemistry above is the monograph’s; the temptation to apply it to a stored bottle is the reader’s, and it is marked here as unverified rather than answered.

What it has to do to make an image, and why no equation appears. A selenium-toned print ends as silver selenide, in which selenium is at −II. Starting from a selenate, that is a fall of eight oxidation-state units, in a bath that also contains a sulfite and a thiosulfate, at pH 9.53, acting on metallic silver. No source this course holds describes that pathway, names an intermediate or says which component does the reducing, so no balanced equation for the toning step is printed — here or anywhere else in the course. The size of the gap is stated instead of being papered over, which is what Rule 1 is for. What the bath does to a print — shadows first, always, whatever the dilution — belongs to selenium toning as a process and is taught in Part XX.

Not a photographic chemical anywhere else. PubChem’s use listings for sodium selenate are horticultural insecticide, additive to chromium-plating baths, glass manufacture, veterinary medicine, fertiliser and mineral supplement. ChEBI gives it roles as a fertilizer and as an anticonvulsant. Photography appears in none of them, which is the ordinary condition of a substance this course teaches: the reason to know the compound is that a maker put it on a label, not that any chemical database associates it with a darkroom.

Properties

Colourless rhombic crystals, denser than water at a specific gravity of 3.098, with no melting point to give: both the melting and the boiling entries on the CAMEO datasheet read “decomposes”. Relative molecular mass 188.95. It dissolves freely — 58.5 g per 100 g of water at 25 degrees C on HSDB’s figure — and the solution is basic, which is why CAMEO files it under the reactive group Salts, Basic. A toner concentrate holding two to three per cent of it is nowhere near saturation, which is part of why such a bath keeps working through many prints.

Which jar, and why the number on it matters. The anhydrous salt is CAS 13410-01-0 and weighs 188.95 per mole. There is also a decahydrate, CAS 10102-23-5, at 369.10 per mole. Both carry one selenium atom per formula unit, so the same weight of the hydrate holds a little over half the selenium of the anhydrous salt — the ratio of those two masses is 0.51. Moersch’s sheet names the anhydrous number. A formula that says “sodium selenate, 5 g” without saying which is not a formula yet. And the numbers themselves invite a slip worth naming: 10102-23-5 is the selenate decahydrate, while 10102-18-8 is sodium selenite — a different oxidation state of the element, in a different bottle. Read all nine digits.

!Two words in a spill database, and what they do not license

CAMEO’s reactivity profile for this substance is one sentence and a fragment: SODIUM SELENATE is a weak oxidizing agent, and may react with strong or weak reducing agents. Light sensitive.

In a photographic chemistry course those last two words are a trap. CAMEO gives no wavelength, no product, no rate and no threshold; the note sits in an emergency-response database whose purpose is telling a first responder how to store and handle a drum. It is evidence that the salt should be kept in the dark. It is not evidence that a selenate is photographically light-sensitive, and this course does not treat it as such: nothing here exposes a selenate to light on purpose, and every substance this encyclopaedia gives the light-sensitive-salt role to is a silver halide, an iron(III) salt or a uranyl salt. The line is recorded because leaving out an inconvenient sentence would be worse than printing it with its limits attached.

The oxidation state is the whole of this page’s chemistry. Two sodium ions carry +1 each; four oxygens carry −2 each; the selenium is left at +6, which is the same place sulfur sits in a sulfate. That analogy is worth keeping, because it explains the shape of the ion and its solubility, and it explains why a selenate behaves in alkaline water the way the WHO monograph describes. It explains nothing at all about the toning reaction, where the selenium has to travel from +6 to −II and the analogy runs out.

Handling

Why Level C. Argued against the rubric rather than asserted. The first Level C criterion names reagents classified as acutely toxic at the concentrations used, where a fume cupboard or specialist disposal is the recognised control: every notifier that classifies this substance gives it an acute-toxicity statement by both the oral and the inhalation route, the two routes splitting only over whether the word is toxic or fatal, and CAMEO’s non-fire response for it begins “avoid breathing dust”. The fourth criterion names waste that cannot lawfully enter a domestic drain, and the Waste section below is that criterion in full. Level B does not reach it, because Level B’s own criterion tops out at reagents that are toxic or corrosive with a splash as the failure mode, and a salt whose failure mode is an acute poisoning is a category above that.

A second criterion may apply, and the sources disagree about it. The same Level C criterion also names mutagens and reproductive toxins. Japan’s NITE-CMC classified this substance in FY2016 as Muta. 2, H341, suspected of causing genetic defects, and Repr. 2, H361, suspected of damaging fertility or the unborn child, together with STOT RE 1 at the more severe category and both aquatic statements. No ECHA notifier lodges either of those above PubChem’s printing threshold, and Moersch’s sheet for the diluted mixture states flatly that it shall not be classified as germ cell mutagenic or as a reproductive toxicant. Those three statements are not in contradiction — a national authority classifying a pure salt, European notifiers classifying the same salt, and a maker classifying a product that is more than two-thirds water are answering three different questions — but a reader weighing whether to keep a jar of this at home should know that one competent authority has put a suspected-mutagen category on it. The classification is Level C on the acute toxicity alone, so the mutagenicity question changes nothing about the handling; it is recorded because a reader deciding whether to own the jar is entitled to the whole of what has been said about it.

It is not Level D. Level D is for substances the course teaches and will not have in a darkroom at all — the mercury, cyanide, uranium, cadmium and chromium(VI) families. Selenium toning is current, useful and taught here; it is the weighing of a selenium salt from a jar that belongs in a laboratory, not the process. A bought, dilute concentrate is a different object with a different classification, and the Part XX lessons work from the bottle.

⚠The gas has a name, and the rule is still absolute

The sodium selenite page records that its sources warn about acid and never say what comes off. For the selenium family this course can now name it. HSDB’s incompatibility entry, written for selenium compounds as a class, is an instruction: do not use metal and acid to reduce selenium compounds, because this will produce toxic gaseous hydrogen selenide.

Read that sentence precisely. It describes reduction by a metal in acid — the classical laboratory route to a volatile hydride — and not acidification on its own. It therefore does not establish what an acidified toning bath releases, and the course still does not claim to know. What it does establish is why the precaution has no proportionate version. HSE’s EH40 controls dihydrogen selenide, CAS 7783-07-5, at 0.02 ppm — 0.07 mg/m³ — over eight hours, and 0.05 ppm or 0.17 mg/m³ over fifteen minutes, among the very lowest limits in that table. A limit at that level cannot be judged by smell, by ventilation or by eye.

So: no acid near a selenium salt or a selenium bath, ever. No acid stop bath in a toning sequence without a thorough rinse before the toner. No acid in the waste bottle this substance goes into. And no metal — no steel spatula left in an acidified spill, no aluminium tray.

The skin, and what nobody has measured. Moersch’s own symptom list for the diluted toner includes, under if on skin, the phrase risk of absorption via the skin. The WHO monograph is the place to check that, and what it says is a gap: apart from a single 1972 study in which 10 per cent of a 0.1 mol/L solution of sodium selenite applied to rat skin was absorbed in an hour, there are no quantitative data on the dermal absorption of water-soluble selenium compounds. A maker warns of a route nobody has measured. The response to an unquantified route is a glove with a stated breakthrough time — Moersch gives 0.11 mm nitrile at 480 minutes for his product — changed the moment it is contaminated, and not a judgement about how much splash is acceptable.

Where the exposure numbers come from, and what they cover. There is no workplace exposure limit for sodium selenate as such; Moersch’s section 8.1 records control parameters as not available. What exists is set for the element and read across to its compounds. NIOSH’s recommended limit and the OSHA permissible limit are 0.2 mg/m³ as an eight-hour average, stated to apply to selenium compounds as Se; HSE’s EH40 is stricter at 0.1 mg/m³ on the same basis; the concentration immediately dangerous to life or health for selenium is 1 mg/m³. The U.S. Department of Energy’s Protective Action Criteria, quoted on the CAMEO datasheet, are the only air numbers in this corpus attached to this compound by name — 1.4, 1.6 and 2 mg/m³ for PAC-1, PAC-2 and PAC-3 — and those are emergency-planning thresholds for a release, not workplace limits, and must not be used as though they were. EH40’s own introduction supplies the sentence that governs all of this: the absence of a substance from the list does not indicate that it is safe.

A difference between the two salts that is not on either label. The WHO monograph measured what happens to a milligram of each in a human volunteer, and the two are not interchangeable. Selenate is absorbed at about 94 per cent from solution against about 60 per cent for selenite; 81 per cent of a selenate dose was excreted in urine within five days against 23 per cent for selenite, and total excretion was 90 per cent against 63. The selenate is the more completely absorbed of the two and also the more completely cleared. Neither of those facts makes a jar of it safer to open, and the course records them because a reader who has met “selenium toner” as one undifferentiated thing should know that the two European products are not the same substance in the body any more than they are in the bottle.

CAMEO’s clinical summary is worth having in plain words for anyone who thinks a selenium compound is a nuisance rather than a poison. Its health hazard entry reads that sodium selenate causes damage to liver and kidneys and resembles arsenic in its effects; its first-aid section lists garlic odour on the breath, drooling, nasal irritation, nausea, abdominal pain, vomiting and poor reflexes among the signs of acute exposure. That is the pure substance, in an emergency-response document, and it is the reason the answer to “can I weigh out five grams of this at home” is no.

Waste

Everything on this page converges here, and the argument is chemistry before it is regulation.

The classification is as close to unanimous as this encyclopaedia ever sees. Aquatic Chronic 1 — very toxic to aquatic life with long-lasting effects — is carried by 100 per cent of the ECHA notifications that classify, and Aquatic Acute 1 by 97.7 per cent. Notifiers who cannot agree whether the substance is fatal or merely toxic to swallow agree completely about the water.

The WHO monograph says why the selenium(VI) form in particular. Selenate is stable under alkaline and oxidising conditions; it is the most common form of the element found in alkaline waters; it is soluble and easily available to plants; and the monograph concludes that, because of that stability and solubility, it may potentially be the most environmentally dangerous form of the element. The contrast with the other salt is exact and it is in the same section: selenium(IV) binds tightly to iron and aluminium oxides, so it is quite insoluble in soils and generally not present in waters in any appreciable amount. Pour a selenite away and much of it sorbs onto mineral surfaces; pour a selenate away and it travels. Dilution is not a disposal method for either, and it is least of all a disposal method for the one that stays dissolved.

The maker of the only product this course can name it in reaches the same place from the regulatory side. Moersch’s section 13 states that the material and its container must be disposed of as hazardous waste and that it must not be emptied into drains, and classifies the waste under the European hazardous properties HP5 (specific target organ toxicity), HP6 (acute toxicity) and HP14 (ecotoxic). His section 8.2 adds, under control of environmental exposure, the single line do not let product enter drains.

There is nothing to neutralise it with. Acid is the one reagent that must never come near it, and acidifying a selenium salt does not destroy the selenium in any case — it only changes its form, and in the wrong direction. So: collect the spent bath in a labelled bottle, dated, with the substance named on the label; keep acids out of that bottle; treat it as part of the silver-bearing stream as well, because a toning bath that has worked on prints carries dissolved silver and thiosulfate along with the selenium; and route it through a licensed hazardous waste service. The course’s reading of the British position is set out under disposal and the containers and labelling under general chemical waste. Check your local regulations; they govern, they differ between authorities inside one country, and they change. The chemistry does not.

History

Sodium selenate has no photographic history that this course can trace, and saying so is more useful than implying one. The nineteenth- and early twentieth-century selenium toners started from the element and a sulfide or a sulfite; the mid-century commercial products named a species without a formula, as the Part XX lesson sets out from the sheets themselves; and the twenty-first-century European sheets name two different salts. Where in that sequence a selenate first went into a photographic bottle, and why, is not in any document this course holds. Moersch’s own statement is the closest thing to a date, and it points the other way: the preparation did not have to be changed when the marketing rules for selenite-based toners tightened, so the selenate was already in his formulation before the regulation that made it convenient. Why it was chosen in the first place, nobody outside Hürth has published.

This page was written late, and the reason is worth recording. The chemical encyclopaedia planned four selenium entries — the element, the selenite, the selenosulfate and the silver selenide — which is selenium(0), selenium(IV), the species the literature reads into a sulfite-dissolved bath, and the selenium(−II) that ends up on the print. Selenium(VI) was the state nobody planned for, and it is the state a living manufacturer names by CAS number. The register learned of it on 6 September 2026 from two written Part XX lessons, selenium toning chemistry and selenium toning and the print curve, both of which named sodium-selenate in their dependencies and neither of which could link it. The planned count moved from 137 to 138. A substance two written lessons depend on is not optional, and a reader who arrives at “sodium selenate” on a safety data sheet should not find the encyclopaedia silent.