Sodium acetate (anhydrous)

Sodium acetate is the alkali in the oldest gold toning bath still described in the conservation literature, and it behaves in a way no other reagent on this page does: mix it and it will not work. It has to stand for a day first.

In photography

The acetate gold bath. Reilly’s conservation monograph gives it in three lines — fused sodium acetate 20 g, 1 per cent gold chloride solution 40 to 50 mL, water to make 1 litre — and then the sentence that explains the whole family: a gold solution “retains a yellow color while still in the relatively inactive acidic state; when it has passed into the more active state through contact with alkaline substances, it becomes colorless. This decolorization is the best guide to the state of the toning bath.” The acetate bath is the slowest of them all to make that transition: it “generally requires 24 hours to ‘ripen’ before it can be used.”

Towler’s 1864 manual gives the same bath in the proportions of its day — water 1000 parts, chloride of gold 1 part, acetate of soda 30 parts — and the same instruction, “prepared at least twenty-four hours before use, with the addition of a few drops of the usual solution of chloride of gold immediately before immersing the prints.” Two sources a century apart, one waiting time.

Why the wait, and what the course cannot say. Reilly explains the observation — the colour change tracks the shift from an acidic, inactive gold species to an alkaline, active one — and neither he nor any other source read for this page names the species involved or gives the reaction. The course does not guess. What Reilly does add is a practical consequence: acetate baths, unlike most alkaline toners, “can be used repeatedly if strengthened with additions of gold stock solution”, whereas most alkaline baths are one-use and go inactive spontaneously after a few hours. A bath that takes a day to start is a bath that keeps.

⚗The chemistry

Acetate is the conjugate base of acetic acid, and the pair is the textbook buffer: OpenStax uses “a solution of acetic acid and sodium acetate” as its first example of a solution that resists a change in pH, because added base is converted to acetate and added acid to acetic acid, and both of those are weakly ionised.

That is what a toning bath is buying. A gold bath’s activity is set by its pH; an unbuffered alkali drifts as the print carries acid into it and as carbon dioxide dissolves from the air; an acetate bath holds still. It is also why the bath is destroyed rather than merely diluted by an acid — Chemical Safety Card 0565 records that sodium acetate decomposes on contact with strong acids to give acetic acid fumes, which in a tray means the alkali has simply been spent.

The course has found no measured pH for any of the acetate toning baths above, and does not calculate one, because the buffer ratio depends on how much free acetic acid the “fused” salt carries and no source states it.

Two footnotes from the wider literature. Eder’s history records that Parr added sodium acetate as an accelerator to the iodo-bromine salting of calotype paper, and later concluded it was best to treat the silvered paper with sodium acetate, forming silver acetate. And Mike Ware’s Platinomicon records that Edward Steichen treated Stieglitz’s yellowed palladiotypes with “a solution of sodium acetate” — then argues that the reagent “could not have cleared the prints of iron(III): sodium acetate solution is quite alkaline, and would have promoted hydrolysis”, that XRF and re-creation experiments show very little iron is actually removed although the stain does fade, and that “it has also been suggested that the name of this salt may have been mistaken for that of sodium citrate”, which is Willis’s recommended clearing agent. A hundred years of conservation history may turn on a misread word.

Properties

A white hygroscopic crystalline powder. Relative molecular mass 82.03; Chemical Safety Card 0565 gives a melting point of 328 °C, a density of 1.5 g/cm³ and an auto-ignition temperature of 607 °C, and records that the substance is combustible — unusual for a salt, and worth remembering near a flame.

“Fused” is a grade instruction, not decoration. Reilly’s formula specifies fused sodium acetate, which is the anhydrous salt: melted to drive off its water and re-solidified. The commercial crystal is the trihydrate, CAS 6131-90-4, relative molecular mass 136.08 against 82.03 — so a gram of the trihydrate carries 60 per cent of the acetate a gram of the anhydrous salt carries, and substituting it weight for weight makes the bath two-thirds strength. To go the other way, multiply by 136.08/82.03 = 1.66. This is the same trap the course documents on the three sodium carbonate pages, and the same answer applies: write the grade on the jar.

Solubility, where the sources will not agree. Chemical Safety Card 0565 gives 46.5 g per 100 mL at 20 °C. HSDB gives 119 g per 100 mL at 0 °C and 170 g at 100 °C. Those cannot both be measurements of the same grade on the same basis — a salt cannot be more soluble at 0 °C than at 20 °C — and neither source says which grade or which basis it means. The page names the spread and does not resolve it. Nothing here turns on it: the strongest bath on this page is 2 per cent.

Handling

This is a page where the two authorities disagree, and the disagreement is small but real. The ECHA aggregate says not classified — 1,320 of 1,383 company reports find no GHS criterion met, on the usual thin base of 4.6 per cent participation. Chemical Safety Card 0565 does classify it, filling in its own box according to UN GHS criteria with Warning and “Causes eye irritation”, and adding that the substance is mildly irritating to the eyes and skin. The card is the more considered document and the course works to it: eye protection is not optional here. 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.

Why Level A. Against the course rubric this meets Level A on every criterion even taking the card’s classification rather than the empty aggregate: eye irritation is well inside the Level A ceiling, the toning bath is cold, there is no mains work, and the waste is a collectable toner. Gloves, goggles and dust discipline are the whole of it.

⚠The one operation that takes this out of Level A

Making an acetate buffer from glacial acetic acid. Sodium acetate is half of a buffer, and the other half is acetic acid, which this course assesses at Level B: a concentrated corrosive whose failure mode is a splash and whose vapour needs ventilation. Weighing the salt is Level A; adding glacial or 28 per cent acid to it is not, and the assessment for that step belongs to the acid — goggles, apron, acid into water, eyewash within reach. The toning baths on this page need no acid at all.

Waste

Spent gold toner carrying dissolved silver and unspent gold, plus the wash after it. Recover the metals first; that is what the collection is for. The acetate itself is the food additive E262, biodegradable, and its solution is a weak base sitting inside Kodak’s J-52 pH window of 5.6 to 9.4. Bottle the toner separately, label it, and follow the disposal caveat and your local authority; ILFORD’s route for domestic users in the United Kingdom is a household waste and recycling centre.

History

The acetate bath belongs to the first generation of gold toners, the ones worked out in the 1850s and 1860s when a printer’s problem was that an untoned salted-paper print faded and a badly toned one went an unpleasant yellow-brown. Towler gives it as the bath that “answers best in my hands”, and the twenty-four-hour ripening was simply part of the routine: you made the toner the day before you printed.

What is striking, reading Reilly against Towler, is how little the practical advice changed in a century and how much the explanation did. Towler tells you to wait. Reilly tells you to wait, tells you what to watch for while you do — the yellow going to colourless — and tells you what that colour change signifies in terms of an equilibrium. Neither can tell you what the gold is actually doing, and neither pretends to.