Sodium sulfide

Kodak’s 1928 primer gives sodium sulfide a page of chemistry and then one flat sentence of housekeeping: no photographic materials should be stored in a room where sulfides are kept, or where sulfide toning is done. That is the shape of this substance. It makes the most permanent silver image anyone has found, and it does so by releasing a gas that fogs everything sensitive in the building.

In photography

It is the second half of a bleach-and-redevelop. Kodak’s sequence, unchanged between the 1928 primer and the 2006 toner sheet, is: bleach the developed print in potassium ferricyanide with bromide until only a faint yellowish image is left, rinse thoroughly, then put it in sodium sulfide, which converts the silver bromide directly to silver sulfide. Formula T-7a makes the redeveloper from 45 g of anhydrous sodium sulfide in 500 mL of water as a stock, used at 125 mL of stock to 375 mL of water; toning is complete in about thirty seconds, and Kodak’s instruction is to agitate until no further change occurs and then rinse at once.

⚗Why the sulfide wins the silver

Silver sulfide is, in the 1928 primer’s words, a very insoluble compound of silver: treat a silver image or a silver halide with sulfur or a sulfide and it is at once transformed into it. The bleach has already put the image back into silver bromide, and the sulfide ion takes the silver from the bromide because it holds it far more tightly.

2 AgBr + S²⁻ → Ag₂S + 2 Br⁻

Redevelopment: the bleached image becomes silver sulfide

The solution is strongly alkaline for the same reason it smells. The sulfide ion is the conjugate base of a very weak acid, so it takes a proton from water. CAMEO records the alkalinity; the course supplies the equation.

S²⁻ + H₂O ⇌ HS⁻ + OH⁻

Hydrolysis: why a sulfide bath is alkaline and why it releases hydrogen sulfide

The colour is decided before the toner is mixed. The 1928 primer states that silver sulfide runs from light brown to black according to its state of subdivision, and that the state of subdivision of the toned image follows that of the untoned one. So exposure and development set the hue: a print should be fully developed but not over-exposed to tone well. Nothing you do in the sulfide bath will rescue a print that was wrong before it got there.

Permanence is the reason it survived so long. Kodak’s advice is unambiguous — where great permanency is required, prints should preferably be toned to a silver sulfide image, because experience has shown that this form of silver is one of the most stable. Ilford’s current fibre-base warm-tone paper sheet still recommends polysulphide toners among the two it names as especially suitable, and lists sulphide (sepia) among the toners that give a protective effect.

More or less of it, and the failure it produces. Wall’s 1912 dictionary gives the working strengths — a stock of 2 oz of pure sodium sulfide in 5 oz of water, of which the scan reads as half an ounce taken to ten ounces of water for use — and then the most useful paragraph in the older literature, a taxonomy of failure. As the sulfide deteriorates in solution, hypo is formed; the first sign is a yellow-brown image, the next an apparent failure to act at all, and the last is that the bleached image gradually disappears, because the bath has become a fixing bath. A sulfide toner does not merely stop working. It starts dissolving your print.

Two neighbouring processes are not this one. Direct sepia toning with the hypo-alum bath T-1a gets its sulfur from acidified hypo at 49 °C, not from a sulfide, and is a different chemistry with different faults. And the same bleach-and-sulfide sequence run on a negative is Kodak’s re-development intensifier, which builds density rather than colour.

Properties

White transparent crystals with a strong affinity for water, deliquescing quickly unless sealed; Wall’s 1912 dictionary gives the salt of commerce as the nonahydrate, Na₂S·9H₂O, relative mass 240, and notes the offensive odour, especially on dilution. CAMEO describes the hydrated grade shipped today, containing at least 30 per cent water, as yellow-pink or white deliquescent crystals, flakes or lumps.

!Two numbers this page will not print

The data block above carries no molar mass, and the reason is worth stating. PubChem’s record for sodium sulfide, CID 237873, is indexed as disodium sulfanide, molecular formula HNa₂S⁺, relative mass 79.06 — a hydrogen sulfide anion with two sodium ions, not the Na₂S the formula sheets mean. The formula mass of Na₂S itself is close to but not the same as that figure, and the course does not print a mass whose provenance it cannot state cleanly. Nor is there a sourced water solubility in the record, so none is given.

In practice you rarely need either, because sodium sulfide is bought and used by weight of a grade whose water content is not the same as the formula’s. Kodak’s own conversion is the one to work from: one part by weight of its fused sulfide is equivalent to approximately three parts by weight of the crystals.

Purity was a real problem and Kodak solved it commercially. The 1928 primer records that so much trouble had been caused by impure sodium sulfide that Eastman Kodak had begun supplying a fused grade of definite purity, greyish white, whose colour is not a sign of harmful impurities. The commonest impurity is iron; dissolving in hot water lets the iron sulfide settle as a black sludge and leaves a clear solution to decant. Old stock contains hypo. Wall’s rule for the shop counter was simpler: avoid the greenish-brown crystals, buy white ones.

It decays in air, and the decay explains the failure. CAMEO records that aqueous solutions gradually convert to sodium hydroxide and sodium thiosulfate on exposure to air, and that the crystalline forms produce hydrogen sulfide and sodium carbonate. Thiosulfate is hypo — which is exactly the contaminant Wall blamed for a toner that dissolves the image.

Handling

The aggregated ECHA notifications classify it Danger, with the corrosive, acute-toxicity, irritant and environmental pictograms, and the agreement across 1,100 reports is unusually complete: severe skin burns and eye damage and very toxic to aquatic life in every report, toxic in contact with skin in 96.2 per cent, toxic if swallowed in 65.3 per cent. CAMEO adds the operational summary — burns skin, eyes and mucous membranes; may be toxic by skin absorption; releases toxic hydrogen sulfide gas when mixed with an acid.

The gas is the controlling hazard, and its warning property fails. The NIOSH Pocket Guide gives hydrogen sulfide an IDLH of 100 ppm and a recommended ceiling of 10 ppm over ten minutes, and carries a note that decides how this substance is handled: the sense of smell becomes rapidly fatigued and cannot be relied upon to warn of the continuous presence of H₂S. HSE’s EH40 sets a workplace exposure limit of 5 ppm over eight hours and 10 ppm over fifteen minutes. A darkroom has none of the instruments those numbers assume, and the rotten-egg smell everyone treats as the alarm switches itself off.

Why Level C. Three criteria of the rubric apply, and any one would be enough. The first names reagents classified as acutely toxic at the concentrations used, where engineered extraction is the recognised control: this substance is classified toxic in contact with skin by nearly every notifier. The second is corrosivity in a procedure whose failure mode is a burn rather than a spoiled print — the eye-damage statement is unanimous. The third is waste that cannot enter a domestic drain. Level D would be wrong, because this is not a historical curiosity: Kodak printed T-7a as a current formula in 2006 and Ilford still recommends the toner class. It is a supervised-laboratory procedure, not a kitchen-table one.

⚠The sequence is the hazard

The thing that makes sepia toning dangerous is not the sulfide alone; it is that the bath immediately before it is acidic. Kodak’s own T-7a bleach contains acetic acid, and the print carries some of it. Princeton University’s guidance is explicit that sulfide toners must not be contaminated with acid and that a print must be rinsed thoroughly after an acid bleach before entering the toner. Kodak’s G-23 sheet extends the rule to the sink: sulfide-type toners are never discarded with stop baths or fixing baths, because the combination generates hydrogen sulfide.

Controls: engineered extraction, not an open window; splash goggles and chemical-resistant gloves; separate tongs and dishes for the bleach and the toner, never shared; a rinse between them that is generous rather than nominal; no acid bottle on the same bench; and no unexposed paper or film in the room. Wall’s own advice on cleaning up — flood the dishes, measures and sink with plenty of water afterwards — is still the right first step.

Waste

The stream is alkaline, sulfidic, silver-bearing and, on every notifier’s reading, very toxic to aquatic life. It is collected in its own labelled container, loosely enough that pressure cannot build, and it never shares a bottle with the bleach, the stop bath or the fixer.

The chemistry closes off the usual disposal move. Neutralising an alkaline waste with acid is ordinary practice, and here it is the exact reaction the Environment Agency writes out in its waste guidance — sodium sulfide plus hydrogen ions gives hydrogen sulfide — under the hazard statement for liberating a toxic gas on contact with acid. That guidance calculates a threshold of 0.4 per cent sodium sulfide for a waste to be hazardous on that ground alone, which a spent toner comfortably exceeds. There is no neutralisation this course can offer.

Wall’s 1912 deodorising practice is worth repeating with a modern caution attached. He washes the measures, dishes and sink with plenty of water and finishes with water in which a little permanganate has been dissolved. That works on rinsed vessels; it is not a treatment for the bulk bath, because CAMEO lists strong oxidisers among the substances sodium sulfide reacts with violently. Check your local regulations; they govern, and a sulfide waste is one that most authorities will want to hear about.

History

Sulfur toning was, in the Getty Conservation Institute’s summary, the most common toning of the early twentieth century — brown, matte prints, in vogue with portrait photographers between the wars. The direct hypo-alum bath was used at scale because it lasted; the bleach-and-sulfide route was what smaller users did, because it did not need heating.

The trade improved the material rather than the process: Kodak’s fused sulfide, introduced in the years before 1928, answered both the impure stock and the deliquescence that made an accurate weight impossible. What eventually displaced sodium sulfide was not a better sulfide but a different molecule. Wolfgang Moersch states the position plainly — the odourless toners sold today are thiourea toners. The darkroom traded a stink and a fogging risk for a suspected carcinogen, which is a trade worth knowing you are making.