Ammonium citrate (dibasic)

Two salts are sold as “ammonium citrate”, they look identical in the jar, and only one of them is what the platinum and palladium literature means. That is the first thing this page has to say, because everything else about the substance is mild.

In photography

It develops a palladium print. In an iron-based printing process the exposed sensitiser holds iron(II); the developer’s job is not to reduce anything but to dissolve that iron out of the paper fast enough that the palladium salt beside it is reduced in situ. A citrate does that by chelating the iron. Mike Ware’s Platinomicon records that William Willis, who invented the platinotype, specified for his Palladiotype papers a developer of trisodium citrate at 20 per cent w/v with citric acid at 2.2 per cent, and clearing baths of the same citrate with more acid — and that he would not have gone to the trouble of a different chemistry for palladium if his existing potassium oxalate developer had served.

Ammonium citrate is the same argument made with the ammonium salt, and Ware records the modern reason for preferring it: on the lightest Japanese washi papers, around 10 gsm, alkaline tetrasodium EDTA damages the fibres, so “the use of mildly acidic diammonium citrate as developer and clearing agent is preferred”. The finding is attributed to one printer, Gilles Lorin, communicated to Ware privately; it is a practitioner’s result reported by a Tier 1 author, not a published comparison, and the course marks the difference.

What it costs against oxalate. The Platinomicon also carries a printer’s step-wedge test in which ammonium citrate “yielded equal response with the potassium oxalate and a very neutral black which is so hard to get from all other papers I have used” on one particular Hahnemühle paper. Ware’s own comparison of the two developer families, made for palladium, is that oxalate gives slightly higher densities and a warmer image colour but “does cause a perceptible chemical fog in the high values, which is worsened by humid conditions”. Those are two different observations about two different questions and the course does not merge them into a recommendation.

⚗The chemistry

Citric acid is triprotic. The dibasic ammonium salt has two of its three carboxyl groups neutralised and one still carrying its proton, which is why a solution of it is mildly acidic rather than neutral — the word Ware chose. The tribasic salt has all three neutralised and its solution is faintly alkaline.

That single proton is the whole difference between the two articles, and it decides the thing the developer is being chosen for. On a paper that a base attacks, an acid citrate bath is a control; a tribasic citrate bath is not the same control, and substituting one for the other because the label says “ammonium citrate” changes the pH of the bath without changing the printed formula.

Historically it is a solution, not a solid. Wall’s 1912 dictionary gives both formulas — NH₄H₂C₆H₅O₇ and (NH₄)₂HC₆H₅O₇ — under the one heading, notes that the salt “is usually met with in the form of solution, the salt itself being so deliquescent that it is an extremely difficult matter to keep it”, and gives the British Pharmacopoeia preparation: neutralise 12 parts of citric acid with 11 parts of strong ammonia solution. Abney’s 1905 manual uses a saturated solution of it as the solvent for a ferrous citrate developer, “adding citric acid if required to give a clear picture” — the same acid-citrate combination Willis specified, arrived at from the other direction. Wall’s 1924 formulary carries it as an ingredient of Ashman’s printing-out emulsion at 15 g per 500 ccm.

Properties

White odourless granules; colourless or white as a solid. Relative molecular mass 226.18 against 243.22 for the tribasic salt, a difference of 7.5 per cent that matters if a formula gives a weight rather than a percentage. Soluble in about one part of water, so a 20 per cent bath is nowhere near saturation and mixes cold. Slightly soluble in alcohol.

Deliquescent, and that is the practical property. Wall’s warning is not historical: a citrate that has picked up water from the air weighs more per mole of citrate than the formula assumed, and the error is invisible. Keep it closed, or keep a dated stock solution and work from that.

The identity check. The dibasic salt is CAS 3012-65-5, molar mass 226.18, EC 221-146-3. The tribasic salt is CAS 3458-72-8, molar mass 243.22, EC 222-394-5. They are different substances with different ECHA entries. If a supplier’s label gives only “ammonium citrate”, the CAS number on the safety data sheet settles it, and the course’s position is that a bath whose pH is the reason for choosing it should not be made from an unidentified grade.

Handling

The aggregated ECHA notifications classify the dibasic salt Warning, irritant: serious eye irritation from 92.1 per cent of the 359 classifying reports, skin irritation and possible respiratory irritation from 44.4 per cent each, with 31 of 390 reports (7.9 per cent) saying it meets no criterion. There is no harmonised entry. CAMEO’s datasheet describes a white granular solid whose primary hazard is environmental. Neither EH40 nor the NIOSH Pocket Guide sets an exposure limit for ammonium citrate or for citrates generally, and EH40 states that absence from its list does not indicate that a substance is without risk.

Why Level A. Against the course rubric this meets Level A on every criterion: the classification tops out at irritant at the 20 per cent working strength actually used; nothing is heated; there is no mains work; and the waste is a collectable bath. One Level B control is added by preference — splash goggles rather than glasses for the powder and the stock — because the eye statement is the one nearly all the classifying notifiers agree on.

⚠The one operation that takes this out of Level A

Warming the developer. Platinum and palladium printers routinely run an oxalate developer hot, and the habit transfers. The Level A criterion is explicit that nothing is heated above 50 °C; a hot tray of any developer also raises a vapour and turns a splash into a scald. Willis’s citrate developer was specified for use at room temperature and there is no sourced reason to heat this one. If a session does heat it, the assessment is Level B and the controls are Level B: goggles, apron, a tray in a water bath rather than on a hotplate, and nothing carried across the room while hot.

Waste

The bath that comes off a cleared print is carrying iron, and in a platinum or palladium session a trace of the noble metal as well; the citrate itself is a food-grade organic salt and the ammonium ion is fertiliser. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most often set, and a citrate bath sits inside it, which is exactly why the pH is not the deciding question — the dissolved metal is. Collect the spent developer and the three clearing baths together, label the container, and follow the disposal caveat and your local authority. ILFORD’s guidance to domestic users in the United Kingdom is a household waste and recycling centre’s chemical cupboard.

History

The name is older than the photography. Ammonium citrate reached the darkroom from the pharmacy, where the British Pharmacopoeia’s neutralisation of citric acid with ammonia was a standard preparation, and the same pharmacy origin gave photography its ferric ammonium citrate — a different, ill-defined substance which this course files as ammonium iron(III) citrate and which is not related to this salt except by name. Wall’s dictionary printing both the mono- and the diammonium formula under one heading is the earliest form of the confusion this page exists to settle: even in 1912, “ammonium citrate” named a family rather than a compound.