EDTA disodium salt dihydrate

Six donor atoms on one small molecule, arranged so that they can close around a metal ion from every direction at once. That geometry is the whole of what this salt does in a darkroom, whether the metal it closes around is the iron left in a platinum print or the calcium in the tap.

In photography

Clearing an iron-based print. Every siderotype leaves iron(III) behind in the paper, and residual iron goes on oxidising the image. The nineteenth-century answer was dilute hydrochloric acid, which, as Mike Ware puts it, “tends to dissolve palladium and weaken the cellulose structure of the paper”. The modern answer is disodium EDTA, “effective in binding iron(III) strongly under mildly acid conditions, and removing it from the paper”. The Getty Conservation Institute’s Atlas records the same substitution from outside the darkroom, describing the Ware-Malde process as calling for fixing, clearing and washing with EDTA, Kodak Hypo Clearing Agent and water.

As the first bath, and as the last. Ware’s print-out palladiotype notes give about 50 g of disodium EDTA in a litre of tap water — roughly 5 per cent w/v — as the bath that both develops and first clears the print, with a capacity of fifty to sixty 10 × 8 inch prints, followed by sodium metabisulfite and then the tetrasodium salt. His New Chrysotype notes use the disodium salt weaker, about 1 per cent w/v, and give the criterion that governs the choice: a first bath “should be a non-alkaline, non-reducing, chelating or complexing agent for ferric iron”.

⚗The chemistry

Ware’s §10.9 gives the four dissociation constants of the free tetrabasic acid H₄EDTA as pK₁ 2.0, pK₂ 2.7, pK₃ 6.2 and pK₄ 10.3. Read those backwards and the sodium salts sort themselves out: solutions of the mono- and disodium salts are mildly acidic, pH 3 to 4, the trisodium salt about neutral, the tetrasodium salt alkaline, pH 9 to 10. That is why the two salts are not interchangeable, and why Ware writes “Do not use tetrasodium Edta, which is alkaline, and will cause iron stains”. The ligand carries four carboxylato and two tertiary amino groups, and can occupy up to six coordination positions on one metal ion.

Fe³⁺ + C₁₀H₁₄N₂O₈²⁻ → [Fe(C₁₀H₁₂N₂O₈)]⁻ + 2 H⁺

Iron(III) taken up by the doubly protonated EDTA anion, releasing the two protons that make a working bath acidic

The reaction releases protons, which is why the disodium salt — already carrying two of them — is the form that suits an acid bath.

Sequestering, not clearing. The other job is to hold a metal ion out of the way rather than take it out of the paper. HSDB calls disodium EDTA “a preservative, sequestrant, and stabilizer”, and JECFA gives its functional classes as antioxidant synergist, preservative, sequestrant and synergist: calcium and magnesium from hard water, and iron from a pipe or a tin, are complexed before they can precipitate on a film. Foma’s current data sheet for FOMAPAN R 100 shows the practice in a live formula, its reversal bath being made with “Calgon 1.5 g or Chelaton III 5.0 g” per litre. Chelaton III is a laboratory trade name; the course could not verify from a manufacturer document which EDTA salt it denotes, and says so rather than assuming.

What it cannot do. Measurements Ware cites show that chelating agents alone remove only about 50 per cent of the iron(III) from suspensions of cellulose pulp, even with ligands more powerful than EDTA. Getting the rest out needs the iron reduced to iron(II) first, which is why the modern conservation treatment pairs a reductant with the ligand: Ware’s is an hour at room temperature in a solution 5 per cent w/v in each of sodium dithionite and tetrasodium EDTA at about pH 9, chosen because the formation constant for the Fe(II)EDTA complex peaks at 2 × 10¹⁴ around pH 10.

Properties

A white, odourless crystalline solid. What matters most about it is which article is in the jar. The dihydrate, CAS 6381-92-6, has a relative molecular mass of 372.24; the anhydrous disodium salt, CAS 139-33-3, has a formula mass of 336.21 from the same formula less two waters. A quantity written for the anhydrous salt therefore needs 1.107 times the weight in dihydrate — the course’s own arithmetic from the two masses — and a quantity written for the tetrasodium salt is not convertible by weight at all, because that is a different compound with a different pH.

The course holds no measured water solubility figure for this salt and does not invent one. What it can say is a lower bound from practice: Ware dissolves about 50 g in a litre of tap water with stirring at room temperature, so 5 per cent w/v goes into solution without heat.

Ware’s notes give the conversions for a shelf that holds the wrong article. Tetrasodium EDTA becomes the disodium salt with 31 g of citric acid per 100 g, and “the presence of the citrate ion can only assist the clearing”. From the free acid H₄EDTA, about 35 g per litre partially neutralised with 9.6 g of sodium hydroxide gives the disodium salt and 19.2 g the tetrasodium, with a table of equivalent weights for the carbonates and the bicarbonate.

Handling

The aggregated ECHA entry for the dihydrate reports it as not meeting GHS hazard criteria in all six of the six company reports behind it, from two notifications, with no hazard statement code anywhere in the aggregation. This page treats that as the thin evidence it is. Six companies declining to classify a substance is not a measurement, and EH40’s paragraph 6 says in terms that absence from its list does not indicate that a substance is without hazard. EH40 lists no exposure limit for EDTA or edetate; CAMEO holds no datasheet for the disodium salt; and the course found no sourced chemical incompatibility for it, which is a gap in the evidence rather than a finding of none.

Why Level A. The course rubric admits to Level A substances whose classification is “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”. An unclassified salt used at 1 to 5 per cent w/v in an open tray at room temperature clears that easily: nothing is heated, nothing is volatile, and the controls the rubric asks for are the ones this substance needs. What is not a hazard here is worth naming as well — no vapour, no exotherm on dissolving, no reaction with the other trays in the sequence. The residual risks are those of a fine powder and of a bath that has taken up iron and, sometimes, palladium or gold.

⚠Safety

Weigh over a tray without raising dust, goggles on for the powder, gloves on for the bath. Keep the disodium and tetrasodium salts in separate, clearly labelled jars and never substitute one for the other: Ware’s instruction that tetrasodium EDTA “will cause iron stains” in the first bath is the mildest consequence of confusing them, and their pH differs by six units. Do not read the empty hazard block on this page as a statement about the product you have bought — read that product’s own safety data sheet, which is written for the grade and the impurities actually in the container.

Waste

A spent bath leaves carrying what it was there to remove, and a chelated metal stays dissolved instead of settling out, so a clearing bath from a platinum or palladium print holds noble metal in a form that will travel. Kodak’s J-300 guidance sends an amateur’s developers, stop baths, fixers after silver recovery and wash water to the sewer where a household has one, while listing solvents and other prohibited materials among what must not go; ILFORD tells domestic users in the United Kingdom to bottle wastes separately, label them and take them to a household waste and recycling centre’s chemical cupboard. This course bottles clearing baths rather than pouring them, for reasons that have nothing to do with the ligand’s own classification: the metal in them is worth recovering, and a septic system is not designed to receive it. Check your local regulations, which govern and which differ.

History

Ware dates the arrival of chelating agents such as EDTA to the 1930s, and the change is easy to state: before them, getting iron out of a print meant acid. Hydrochloric acid clears, but it dissolves palladium and weakens cellulose, and generations of platinum prints carry the yellow stain of the iron that survived it. The conservation literature Ware cites — Helen Burgess on chelating agents in conservation treatments, then Jacquelyn Rees and Megan Gent at the Victoria and Albert Museum — worked out that reduction and chelation together do what chelation alone cannot, and lifted iron stains from historic platinotypes with sodium dithionite and tetrasodium EDTA. Matthew Clarke and Dana Hemmenway at the National Gallery of Art then tested the next generation, finding 0.01 M DTPA with 2 per cent dithionite at pH 8 more effective than EDTA. A darkroom tray and a conservation bench turn out to be doing the same chemistry with the same reagent.