Sodium sulfate

Sodium sulfate is on this course’s list for a reason that has nothing to do with what it does and everything to do with what it is mistaken for. It is what sodium sulfite becomes when the preservative does its job, and the two names differ by one letter.

In photography

First, as the thing that has already gone wrong. Kodak’s 1924 and 1928 primers put it plainly: oxygen readily attacks sodium sulphite, especially in the presence of moisture, converting it into sodium sulphate, “which is useless as a preservative”.

2 Na₂SO₃ + O₂ → 2 Na₂SO₄

The oxidation the sulfite exists to undergo, happening in the jar instead of the developer

The primer then gives the two cases separately, and the distinction is the practical heart of this page. With crystallised sodium sulphite the sulfate forms on the outside of the crystals as a powder, “which may be washed off and the crystals dried” — visible, and recoverable. With the desiccated (anhydrous) salt it is “not easy to detect sodium sulphate in desiccated sulphite except by chemical tests” — invisible, and unrecoverable. A tub of anhydrous sulfite that has been open for a year weighs the same and works less, and there is nothing on the balance to tell you.

Second, as a real ingredient of a tropical developer. Kodak’s Tropical Process Developer D-13, a Kodelon–hydroquinone formula in the 1928 primer, carries 105 g of desiccated sodium sulphate per litre alongside 52.5 g of sodium sulphite, 52.5 g of sodium carbonate, 10.5 g of hydroquinone, 5.2 g of Kodelon and 2.1 g of potassium iodide. It develops in 6 to 7 minutes at 85 °F, and is followed by a 30-second rinse, three minutes in a 5 per cent formalin bath, a one-minute wash, an acid hardening fix and a 15- to 20-minute wash. The sulfate is there in quantity — more of it than of anything else in the formula — and the primer does not, in the passage the course has read, say what it is for. The course does not guess. What can be said is that this is a tropical formula built around a hardening step, and that concentrated neutral salts are a classical way of restraining the swelling of gelatin; that is a general statement about gelatin, not a claim about Kodak’s reasoning.

Third, in a blue toner. Kodak’s G-23 gives Blue Toner T-26 as 937 mL of water at 52 °C, 40 mL of a 1 per cent gold chloride stock, 1 g of thiourea, 1 g of tartaric acid and 15 g of anhydrous sodium sulfate. Kodak notes that the contrast and density of toned prints appear to increase, which the printer compensates for by reducing exposure slightly, and that a soft grey-blue is more common than a saturated blue. This course does not offer T-26 as a procedure, because thiourea is a Level C substance under the course rubric.

Properties

A white hygroscopic solid in various forms. Relative molecular mass 142.04, against 126.05 for sodium sulfite: a gram of the sulfate contains 11 per cent fewer sulfur atoms, which is of no use to anybody because none of them can be oxidised further. Chemical Safety Card 0952 gives a melting point of 884 °C and a relative density of 2.7.

A solubility curve that runs backwards. HSDB’s figures do not agree with each other — 28.1 g per 100 g of water at 25 °C in one entry, 445.47 g per litre at 20 °C by the shake-flask method in another, which is about 44.5 g per 100 mL — and the course quotes both without reconciling them. What the entries do agree on is the shape, and it is unusual: soluble in about 3.6 parts of water, reaching a maximum solubility of 1 part in 2 parts of water at 33 °C, and falling back to 1 in 2.4 parts at 100 °C. Above about 33 °C, hotter water dissolves less sodium sulfate, not more. Anyone who has met this salt only through the rule that solids dissolve better hot should note the exception.

Handling

The ECHA aggregate says not classified: 4,127 of 4,277 company reports state that sodium sulfate meets no GHS hazard criterion, and two further entries agree. The same caveat applies as on the potassium chloride page — only 3.5 per cent of companies supplied information, and seven notifications covering 150 reports do carry codes below PubChem’s display threshold — so the honest description is unclassified on a broad but shallow evidence base. Japan’s NITE-CMC dissents hard, giving Danger, the health-hazard pictogram, eye irritation and damage to organs; nothing else in the record supports that, and the course reports it rather than adopting it. Chemical Safety Card 0952 leaves its classification box empty and records only that ingestion could affect the gastrointestinal tract and that a nuisance dust is reached quickly. Neither EH40 nor the NIOSH Pocket Guide sets an exposure limit, and EH40 states that absence from its list does not indicate that a substance is safe.

✕Common mistake

The real hazard on this page is the label. Sodium sulfite is CAS 7757-83-7; sodium sulfate is CAS 7757-82-6. The names differ by one letter, the CAS numbers by one digit in the middle block, and both are white crystalline solids. Substituting the sulfate for the sulfite in a developer removes the preservative entirely: the developing agents oxidise, the bath goes brown, and nothing about the appearance of the powder warned you.

Write the full name on every container, keep the two jars apart, and check the CAS number rather than the name when you buy. Kodak’s primers listed both in the same solubility table in 1928 and the trap has not changed since.

Why Level A. Against the course rubric this meets Level A on every criterion with room to spare: no classification, no heating required by the salt, no mains work, and a waste stream that is neutral and already part of a spent developer. Gloves, eye protection and dust discipline are the whole of it.

⚠The one operation that takes this out of Level A

Making Blue Toner T-26. The sulfate is the innocent ingredient in that formula. The bath is mixed at 52 °C, above the rubric’s Level A ceiling, and it contains thiourea, which this course classifies Level C as a suspected carcinogen and reproductive toxin for which engineered extraction is the recognised control. No amount of care with the sodium sulfate changes that assessment; the level belongs to the company it keeps.

Waste

Sulfate leaves in spent developer and in the first wash, and it is where the sulfite in that developer was always going to end up. It is not the reason the bath is collected — the silver and the developing agents are. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most often set, and a sulfate solution is neutral; the developer it is dissolved in is not. Route the bath as a whole according to the disposal caveat, which is jurisdictional, and ILFORD’s advice to domestic users in the United Kingdom — bottle separately, label, and take to a household waste and recycling centre.

History

Glauber’s salt, the decahydrate, is named for the seventeenth-century chemist who prepared it from common salt and sulfuric acid and sold it as a medicine, sal mirabile. Photography met it in the industrial form: sodium sulfate is the by-product of the Leblanc process that made sodium carbonate, and Wall’s complaint that commercial washing soda “usually contains much sulfate” is a complaint about exactly this substance turning up uninvited in another one.

That is its whole career in the darkroom, and the two formulas above are the exceptions that prove it. Sodium sulfate is mostly the residue of something else: of a Leblanc furnace, of a sulfite that has oxidised, of a preservative that has finished doing its job. The reason it has a page is that a photographer needs to recognise the residue.