Tartaric acid

Tartaric acid came into photography from wine. It is the acid of grapes, and it belongs to the small group of what nineteenth-century chemists called the vegetable acids — citric from citrus, oxalic from rhubarb and sorrel, tartaric from the crust in a wine cask — whose iron(III) salts turned out to be sensitive to light. That family relationship is the whole reason it is in a darkroom.

In photography

The Van Dyke brown sensitiser. The Photographers’ Formulary kit for the process supplies three weighed chemicals and a starch: ferric ammonium citrate (green) 9.0 g, tartaric acid 1.5 g, and silver nitrate 3.8 g. They are dissolved as three separate solutions in 33 mL of distilled water each — B is the tartaric acid — and only then combined, in the order A, B, C, because the separate solutions are not light-sensitive and the mixture is. That is where most photographers meet the substance, and the kit does not say what it is doing there.

Ware’s Chrysotype Manual supplies the principle. All the iron imaging systems rest on the same photochemistry: light acting on an iron(III) salt of an organic acid transfers an electron from the acid’s anion to the metal, giving iron(II), and the key compound is usually an iron(III) salt of either citric, tartaric or oxalic acid. The iron(II) so formed is a reducing agent strong enough to throw a noble metal out of solution as the metal itself.

⚗The chemistry

Ware writes the light reaction for ferric oxalate, because it is the simplest of the three: under ultraviolet light two iron(III) ions and one oxalate ion give two iron(II) ions and two molecules of carbon dioxide, two electrons having crossed from the organic anion to the metal. The image-forming step that must follow, while the paper is still damp enough for the molecules to meet, is the reduction of a noble-metal complex by that iron(II) — silver, in a Van Dyke print. Tartrate plays the same structural part as oxalate: two carboxylate groups and two hydroxyls on four carbons give it several oxygen atoms to close around an iron(III) ion, and a chelated iron(III) is both soluble and photoactive where a bare one would hydrolyse and precipitate. The acid is not a spectator that happens to be acidic; it is half of the light-sensitive compound.

2 Fe³⁺ + C₂O₄²⁻ → 2 Fe²⁺ + 2 CO₂

Ware's dissected form of the light reaction in an iron(III) carboxylate

What the course could not verify. No source read for this page states why the Van Dyke formula uses tartaric acid rather than more citrate, when the iron is already supplied as ferric ammonium citrate; the kit instructions give the weight and say nothing about the reason. Practitioners’ notes circulate claiming that varying it shifts contrast and image colour, but the course found no manufacturer, standard text or conservation source that tests the claim. Treat the acid’s presence as well attested and its precise function as open.

As a developer for chrysotype. Ware lists tartaric acid among the alternative first baths for a gold print, used at 1 to 2 per cent w/v, and describes what it gives: redder tones, but still fairly subdued — where citric acid produces rose-pink hues crossing into blue highlights, and oxalic acid the most intense red-blue colour splits and the longest tonal range. He adds a warning that applies to all three: because they are acids, overlong treatment tends to block up the shadow tones by coagulating the gold, and their sodium, potassium or ammonium salts often work better.

In gold toning. The Chrysonomicon records that the stability of a gold-thiocyanate toning bath could be improved by the presence of a weak acid such as tartaric acid, and gives Namias’s formulation: gold chloride 0.02 per cent, ammonium thiocyanate 2.5 per cent, tartaric acid 0.2 per cent and sodium chloride 0.5 per cent, all four given without a basis in the source and reproduced here as written. The acid survived into twentieth-century manufacture. Kodak’s Gold Protective Solution GP-2, intended for the preservation of microfilm, contains 0.05 per cent gold chloride, 0.5 per cent thiourea, 0.1 per cent tartaric acid and 1.5 per cent anhydrous sodium sulfate; Kodak’s blue toner T-26 for Panalure paper carries the same 0.1 per cent alongside gold chloride and ferric EDTA.

Properties

Colourless or translucent crystals, or a white crystalline powder, odourless, with an acid taste; melting at 206 °C. Chemical Safety Card 0772 gives a water solubility of 1400 g per litre at 20 °C and calls the aqueous solution a medium strong acid. PubChem’s other entries are less generous — HMDB gives 582 mg/mL at 20 °C, JECFA one gram in 0.8 mL of water — and the course has not resolved which figure belongs to which isomer; every source agrees it dissolves freely, which is all a working formula needs.

⚗Two acids, one formula

Tartaric acid exists as mirror-image forms that are chemically identical in almost every respect but not quite. The IUPAC compilation separates them: for the optically active acid it records pK_a1 3.03 and pK_a2 4.46 at 25 °C and 0.001 mol/L, and 2.89 and 4.52 at 20 °C from a different determination; for the meso isomer, which is a distinct substance rather than a mirror image, 3.17 and 4.91, assessed as reliable. Photographic suppliers sell the natural L-(+) acid, CAS 87-69-4, which is the one PubChem’s record and Ware’s chemicals list name. The ILO-WHO safety card, confusingly, covers the racemic DL-acid of CAS 133-37-9; its physical and hazard information is the best available, but the number on the bottle will not match the number on the card.

Handling

The aggregated ECHA notifications classify it Danger, with both the corrosion and exclamation-mark pictograms, but the striking thing is the disagreement: across 4,731 company reports, H319 appears in 57.8 per cent, H335 in 55.4, H315 in 54.3, H302 in 42.5, H317, an allergic skin reaction, in 42.5 and H318 in 41.2, while 44 reports say it meets no GHS criteria at all. Chemical Safety Card 0772 is firmer: corrosive to the eyes, irritating to the skin and upper respiratory tract, with a harmful concentration of airborne particles reached quickly when the powder is dispersed. It sets a MAK of 2 mg/m³. 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 B. The course rubric caps Level A at a classification “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”. Two entries here sit outside that: serious eye damage, H318, and skin sensitisation, H317. Level B’s criterion of fine powders that must not be inhaled is what the safety card describes. It does not reach Level C, whose sensitiser criterion applies where a fume cupboard or specialist disposal is the recognised control; the recognised control for a skin sensitiser is a glove.

⚠Safety

Goggles, not spectacles, and gloves that stay on — a sensitiser is a substance you can become allergic to by repetition, so the aim is zero contact rather than tolerable contact. Weigh it without raising dust. Keep it away from strong oxidants and hydrogen peroxide, and note the card’s inclusion of silver among the substances it reacts with: a Van Dyke sensitiser is built by bringing tartaric acid and silver nitrate together, and the kit’s separate stock solutions, combined only in the published order and only when wanted, are the control that makes that reasonable.

Waste

Tartaric acid leaves the darkroom in the first wash of a print, and it is not the part of that wash that matters. In a Van Dyke print the first bath carries the unreduced silver; in a chrysotype, gold. Ware’s instruction for those first baths is that they should be treated as one-shot and not stored, because they accumulate the excess iron and metal and quickly acquire the colour of colloidal metal. Bottle them for the metal’s sake. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most frequently set. Chemical Safety Card 0772 records that the environmental effects of the racemic acid are inadequately investigated while the L-isomer shows no significant effects — a reason for caution, not for confidence. 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. Check your local regulations, which govern.

History

The light sensitivity of iron salts of the vegetable acids was known before photography existed, and Ware’s Chrysonomicon traces the first observation to Count Bestuscheff’s Tinctura tonico-nervina of 1725. Tartaric acid’s own moment came in 1842, when Herschel described what he obtained by adding silver nitrate to ferrotartaric acid: a paper that, exposed wet, showed nothing at first but “develops itself spontaneously” in the dark until the image becomes very intense — a developing-out material two years before Talbot’s calotype was widely understood. Herschel’s own recipe for the reagent survives only in a postscript to a letter to Talbot of 13 September 1844: precipitate tartrate of ammonia and iron with lead, then decompose the lead salt with weak sulphuric acid. Ware notes that the chemistry of ferrotartrate complexes is still not fully elucidated.

The acid arrived on Herschel’s bench through the same channel as the citrate. Alfred Smee wrote to him on 10 May 1842 offering two salts newly “vamped up by the Chemists and Druggists” — the ammonio-citrate and ammonio-tartrate of iron, “which are perfectly soluble and give very dark solutions”. Herschel took the citrate and built an infinity of processes on it; the tartrate waited until the end of the century, when Herschel’s argentotype fathered the family of iron-silver processes — Van Dyke brown, kallitype, sepiaprint, brownprint — in which the small bottle of grape acid still sits.