Citric acid (anhydrous)

Three carboxylic acid groups and one hydroxyl on a three-carbon backbone: a shape that can grip a metal ion from several directions at once. That is why the same jar of white powder does two jobs that look unrelated — stopping a print in its tray, and holding iron(III) in solution so it can be coated on paper or washed back out of it.

In photography

The modern stop bath. ILFORD’s sundries sheet describes ILFOSTOP as a low odour citric acid stop bath, used at 1+19, ten seconds at 20 °C, over a working range of 18 to 24 °C. The concentrate reads pH 2.1 with a specific gravity of 1.101 to 1.111. Unreplenished capacity is fifteen 135-36 films per litre, sixty 20.3 × 25.4 cm resin-coated prints, or thirty fibre-based ones. Its purpose is the one Kodak gave for acetic acid a century earlier: stop development immediately and neutralise the developer, which maintains the activity of the fixer by reducing the carry-over of alkaline developer into an acidic bath. An indicator dye shifts from yellow to purple as the bath exhausts, and the sheet warns against machine processing, where short fix and wash times can leave that dye as a stain. Where a stop bath cannot be used, ILFORD says a water bath must be changed at very frequent intervals, and that the risk of processing marks and stains rises.

The sensitiser for an iron process. Ware’s Simple Cyanotype builds its light-sensitive salt in the beaker: 13 g of citric acid monohydrate, or 12 g of the anhydrous acid, dissolved in 35 cc of water, then 12 g of iron(III) nitrate nonahydrate added in portions with stirring, then ammonia and potassium ferricyanide. What forms is ammonium dicitratoferrate. Ware explains the reasoning on his own site: the ferric nitrate is dissolved straight into a solution already holding twice its molar quantity of citrate, as the free acid and at low pH, so that hydrolysis and polymerisation of the iron(III) never get started, and the ammonia then neutralises the excess. The product he expects in solution is the monomeric dicitratoferrate(III) anion — a species isolated and structurally characterised only in 1998, and quite unlike the polymeric, variable ammonium ferric citrate of the classic formula. The same acid then develops the print: half a minute to a minute in 1 per cent w/v citric acid until Prussian blue starts to run off. Leave the acid out and process in water alone and the exposure scale collapses from about 2.7 to about 1.3, with higher contrast and a weakened maximum density; use the acid too strong and the highlights blue.

⚗The chemistry

Citric acid is triprotic. The IUPAC compilation records pK_a1 2.87, pK_a2 4.35 and pK_a3 5.68 at 20 °C in 0.1 mol/L sodium perchlorate, all assessed as approximate. Three protons close together in pK means all three come off within a narrow pH span, so a citric solution buffers broadly rather than sharply — useful in a stop bath, which must swallow alkali without swinging. Strip the protons and what is left is the citrate ion, and PubChem’s ChEBI description names its other identity outright: a chelator, and specifically a calcium chelator. Several oxygen atoms on one small molecule can close a ring around a metal ion. That single property explains why citrate both carries iron(III) onto the paper and lifts it off again.

Clearing an iron print. Ware’s Platinomicon records Willis recommending, for his own Palladiotype papers, development in trisodium citrate dihydrate at 20 per cent w/v acidified with 2.2 per cent w/v citric acid, and clearing in the same stock with 9 per cent w/v citric acid, diluted eightfold for use to about 2.5 per cent w/v citrate and 1.1 per cent w/v citric acid, in three successive baths of ten, fifteen and twenty minutes followed by a thirty-minute wash. The 1917 account of the process to the Camera Club gives 5 per cent w/v potassium citrate acidified with 0.26 per cent w/v citric acid for both jobs. Ware also notes that where only tetrasodium EDTA is available it can be converted to the disodium salt with 31 g of citric acid per 100 g, “and the presence of the citrate ion can only assist the clearing”.

Why not acetic acid. The same book gives the reason citric acid is preferred: iron(III) and acetate form a deep red acetato-oxo-triiron(III) cation, so an acetic bath colours the print instead of clearing it.

Printing-out papers. Reilly’s account of the albumen and salted-paper era classes citric acid among the “active” organic substances, which facilitate the more complete reduction of silver chloride and themselves form light-sensitive compounds with silver nitrate — silver citrate among them. Starch is not active, so arrowroot paper carries citric acid instead: 35 g of sodium chloride and 3 g of citric acid boiled with the arrowroot cream into a litre of salting solution, sensitised by floating on 12 per cent w/v silver nitrate containing 4 to 5 per cent w/v citric acid. Without the acid, Reilly writes, the prints would be grey and flat; with it they take on a warm purple. Citric acid in the silver bath also keeps sensitised paper usable for several weeks, and he records 5 per cent w/v — 50 g in the litre, by his own replenishment arithmetic — as the concentration giving the maximum preservative effect, with as little as 1 per cent w/v extending the life noticeably.

Properties

Colourless odourless crystals with a strongly acid taste, melting at 153 °C and decomposing above 175 °C. Chemical Safety Card 0855 gives 59 g per 100 mL of water at 20 °C, and HSDB the curve from 54 per cent w/w at 10 °C to 84 per cent at 100 °C — this is a substance that dissolves in almost its own weight of water. The card calls the aqueous solution a medium strong acid and records that it reacts with oxidants and bases and attacks metals.

Two practical points about which powder is in the jar. The monohydrate effloresces in dry air, so an old, part-used container has lost water and no longer matches its label. And the two forms are not interchangeable by weight: the anhydrous acid is 192.12, the monohydrate is that plus one water, 210.14, so a formula written for the anhydrous acid needs 1.094 times the weight in monohydrate — the course’s own arithmetic from the two molar masses. Ware’s sensitiser sidesteps the problem by printing both figures, 13 g of the monohydrate or 12 g of the anhydrous acid.

Handling

The aggregated ECHA notifications classify it Warning with the exclamation-mark pictogram alone: H319, serious eye irritation, in 84.7 per cent of the 4,373 company reports that carry hazard codes, and H335, respiratory irritation, in 23 per cent — while 359 of those 4,373 reports say it does not meet GHS hazard criteria at all. That spread is itself informative: notifiers disagree about a substance that is also a food additive. Safe Work Australia’s HCIS entry adds skin irritation, H315. Chemical Safety Card 0855 records irritation of the eyes, skin and respiratory tract on short exposure and possible erosion of the teeth on repeated exposure, and sets a MAK of 2 mg/m³ for the inhalable fraction. HSE’s EH40 lists no British workplace exposure limit for it, and states in its introduction that the absence of a substance from the list does not mean it is without hazard.

Why Level A. The course rubric admits to Level A substances “whose GHS classification is at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled, and in the quantities of a home darkroom”. Citric acid is an irritant and nothing worse, the strongest bath the course uses is around 9 per cent w/v, and the Level A controls — nitrile gloves, eye protection, an openable window or extractor, dedicated labelled containers — are the controls the safety card asks for. What is not a hazard here is worth naming: the card records that a dust explosion is possible if the powder is mixed with air, and gives explosive limits of 0.28 to 2.29 vol%, but those figures describe an industrial dust cloud, not ten grams weighed carefully over a tray in still air.

⚠Safety

Goggles rather than spectacles when the concentrate or a 9 per cent w/v clearing bath is poured, because the eye is the organ the classification agrees about. Weigh without raising dust. Keep it dry and away from strong bases and from metals, both of which Chemical Safety Card 0855 names. One incompatibility deserves care from anyone mixing a cyanotype: CAMEO lists metal nitrates among the substances citric acid reacts with, and says those reactions are potentially explosive, while Ware’s sensitiser deliberately combines citric acid with iron(III) nitrate. Ware wants the nitrate there — at low pH it keeps iron(II) from forming in the bottle, and so acts as the preservative that dichromate provides in his New Cyanotype — and the two are reconciled in practice by how it is done: solid nitrate added to a concentrated acid solution a little at a time, stirred, at room temperature, never heated and never mixed dry. The course has found no source that draws the boundary explicitly. Follow a published order and quantity exactly, do not scale up, and do not improvise with a nitrate.

Fe³⁺ + 2 C₆H₄O₇⁴⁻ → [Fe(C₆H₄O₇)₂]⁵⁻

The monomeric dicitratoferrate(III) anion, isolated and characterised only in 1998 and made in the beaker by Ware's sensitiser

Waste

A citric stop bath leaves as an acidic solution carrying developer oxidation products; a clearing bath from an iron process leaves carrying iron, and from a platinum or palladium print, traces of the noble metal as well. Kodak’s J-300 guidance for amateurs treats an acidic stop bath by neutralising it with sodium bicarbonate solution before it goes to the sewer, adding the bicarbonate slowly because the mixture foams, and Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most frequently set. ILFORD’s guidance for domestic users is to bottle wastes separately, label them and take them to a household waste and recycling centre’s chemical cupboard; failing that, small amounts flushed with plenty of water, unmixed, and never into a septic tank. The indicator dye in a proprietary stop bath is an extra reason not to let it near anything it can stain. Check your local regulations, which govern.

History

Citric acid entered photography as a preservative before it was ever a stop bath. Reilly dates the practice of adding organic acids to the silver bath to the working generation of the 1860s and 1870s, when the alternative was to sensitise, print and process on the same day; citric acid bought the printer several weeks. Ware’s Chrysonomicon places it in a wider family, noting that the whole class of iron-based printing processes rests on iron(III) salts of “certain organic acids, such as citric, tartaric, and oxalic acids” — the acids nineteenth-century chemists called the vegetable acids, because citric came from citrus, tartaric from grapes and oxalic from rhubarb and sorrel. Of the three, citric turned out to be the one that also made a comfortable stop bath, which is why a twenty-first-century darkroom is more likely to smell of nothing than of vinegar.