Trisodium citrate dihydrate

Citrate has three carboxyl groups and a hydroxyl on a three-carbon backbone, arranged so that it can wrap around a metal ion from several directions at once. Every job this salt does in photography is a consequence of that shape, and the jobs look nothing like each other.

In photography

It develops a palladium print. Mike Ware’s Platinomicon records that William Willis specified for his Palladiotype papers a developer of trisodium citrate dihydrate at 20 per cent w/v with citric acid, used for 4 to 5 minutes, and clearing baths made from a stock of 20 per cent citrate with 9 per cent citric acid, diluted eightfold for use to about 2.5 per cent citrate, in three sequential baths of at least ten minutes each followed by a thirty-minute wash. Ware gives the developer’s acid strength twice and not quite identically — 2 per cent in his historical chapter, 2.2 per cent in the chapter where he converts the original imperial units — and the clearing bath’s as 1.2 per cent in one place and 1.1 per cent in the other. The course quotes both and does not split the difference; the converted figures are the fuller statement.

Willis’s reason for a different chemistry is on the record and it is not about the image. The Palladiotype developer and clearing baths, his distributor’s notice says, “have no tendency to injure the beautiful surface of the paper employed for coating”, where prolonged immersion of a matt platinotype in the oxalate and acid baths does destroy its bloom.

It develops a kallitype. Photographers’ Formulary’s New Kallitype kit uses a 20 per cent solution of sodium citrate as its developer, made by stirring a 300 g packet into 1100 mL of distilled water, with a reserve added to the working bath in 100 mL portions as it is used up. Same concentration, same salt, different metal.

It goes into a salting solution, and there it is doing something else entirely. Ware’s account of the printing-out papers is that they were “silver chloride in a binder of gelatine or collodion, with excess silver nitrate and a halogen absorber such as sodium citrate”, and that in the earlier albumen process the albumen itself “acts as a scavenger for the chlorine”.

⚗The chemistry

A printing-out paper darkens because light splits silver chloride into silver and chlorine. That reaction is reversible: chlorine left in the layer recombines with the silver and the image goes backwards. A halogen absorber is a substance that takes the chlorine out of circulation as it is released, so the reaction runs one way only and the print keeps building in the frame.

That is the citrate’s job in a salting solution, and it is not the same job as the chloride’s beside it. The Formulary’s printing-out kit puts 11 g of sodium citrate with 11 g of ammonium chloride and 11 g of gelatin in 500 mL; the Getty’s conservation atlas records that a salting solution “can be sodium chloride but is often sodium citrate and ammonium chloride”. Reilly gives a plain salted-paper variant of 20 g of sodium chloride, 20 g of sodium citrate and 2 g of gelatin per litre and reports the visible consequence: “the addition of a neutral citrate will cause the prints to be more reddish in color and slightly more ‘brilliant’.”

Why absorbing the halogen should shift the colour toward red is not explained by any source the course has read, and the page does not supply a reason. Reilly’s own caveat is worth keeping with the observation: the porosity of the paper stock remains the largest single factor in the result.

Properties

A white solid in various forms — crystals, granules or powder. Relative molecular mass 294.10 for the dihydrate, against 258.07 for the anhydrous salt, CAS 68-04-2: a gram of the anhydrous salt carries 14 per cent more citrate, and converting a formula from anhydrous to dihydrate means multiplying by 294.10/258.07 = 1.14. That is a smaller trap than the carbonates’ factor of 2.7, but a 20 per cent developer made from the wrong grade is a 17.5 or a 22.8 per cent developer, and Ware’s figures are given to three significant figures for a reason.

Solubility is 77 g per 100 mL on Chemical Safety Card 1219, which gives no temperature; HSDB puts it at one gram in 1.3 parts of water and one in 0.6 part of boiling water, and records the solution as slightly alkaline to litmus, pH about 8. That last is the number that matters for the salting solution and for the developer, and it is why Willis acidified his: the working developer is a citrate buffer pulled down from pH 8 by added citric acid, not a citrate solution as supplied.

The card records that the salt decomposes at 150 °C, and adds a note the course likes: that this is an apparent melting point caused by the loss of the water of crystallisation, not a real one.

Handling

The ECHA entry for the dihydrate says not classified: 520 of 536 company reports find no criterion met, on the usual thin participation of 3 per cent. A second entry, filed against the anhydrous record under the looser name “Citric acid, sodium salt”, disagrees — Warning and serious eye irritation in 72.3 per cent of its 148 reports — and although that entry covers a less well defined article, it is why this page asks for eye protection rather than treating an empty box as an answer. Chemical Safety Card 1219 does the same thing from the other direction: its classification box is empty and its text records that the substance is irritating to the eyes and respiratory tract. 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 on the harsher of the two ECHA readings the classification is eye irritation, well inside the ceiling; the developer is used at room temperature; there is no mains work; and the waste is a collectable bath. Splash goggles for the 20 per cent stock is a Level B control taken by preference.

⚠The one operation that takes this out of Level A

Sensitising the salted sheet. In its developer role the citrate stays at Level A from first weighing to final wash. In its salting role it does not, because the sheet it has salted is then floated on or brushed with silver nitrate, which the rubric names explicitly among the Level B criteria and whose spills are permanent. From that point the assessment belongs to the silver and to the ultraviolet exposure that follows. The same is true of the printing-out kit that supplies this salt: nothing about the gelatin-salt solution is difficult, and everything that raises the level of the session arrives afterwards from another bottle.

One thing the course removes rather than raises: that kit’s contrast-control solution is potassium dichromate. Chromium(VI) is not used here at any level, in any process — see the chromium policy.

Waste

A spent citrate developer carries dissolved iron and, in a palladium session, a trace of the noble metal; a spent salting or first-wash stream carries silver. Those are what the collection is for. The citrate itself is the food additive E331 and its solution sits at about pH 8, comfortably inside Kodak’s J-52 window of 5.6 to 9.4 — which again makes pH the wrong test. Collect the developer, the three clearing baths and the first wash together, label the container, 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’s chemical cupboard.

History

The citrate developer arrived with the metal. Ware dates the switch from platinum to palladium to 1917, when the British government’s wartime embargo on platinum photography forced it, and records that the Platinotype Company’s technical manager demonstrated the new paper to the Camera Club of London on 8 March that year, showing that “both developing and clearing of Palladiotype were accomplished simply with baths of 5 per cent potassium citrate solution acidified with 0.26 per cent citric acid”.

That is not the formula in the published instructions, and the difference is worth naming rather than smoothing: a demonstration used the potassium salt at 5 per cent with a trace of acid, and Willis & Clements printed the sodium salt at 20 per cent with about 2 per cent acid. Ware gives both without reconciling them, and so does this page. Ware’s argument for why any citrate at all is a small masterpiece of reading a source properly: Willis was “a shrewd and observant chemist, painstaking in his perfection of process”, and he would not have recommended a new developer and a new clearing agent if his existing potassium oxalate had served. The reason turns out to be the paper’s surface rather than the print’s tone.

What happened next is the part worth remembering. American printers, Ware records, largely ignored the instruction and developed their palladiotypes in the used oxalate baths they already had, then cleared them in hydrochloric acid — two errors that partly cancelled. The argument about whether Stieglitz followed Willis’s citrate route or his own oxalate one is still open.