EDTA disodium salt (anhydrous)

This page and the one for the dihydrate describe the same ligand doing the same job. What separates them is 36.03 grams per mole and a labelling problem that runs through the whole of alternative-process printing.

In photography

Everything this salt does, it does as the same anion. Lifting residual iron(III) out of a platinum, palladium or kallitype print; holding calcium and magnesium out of the way in hard water; keeping iron from a pipe or a tin out of a developer. The chemistry — six donor atoms closing around one metal ion, the four dissociation constants that make the disodium salt mildly acidic where the tetrasodium salt is alkaline, the 50 per cent ceiling on what chelation alone can remove from cellulose — is set out in full on the dihydrate’s page and is not repeated here. In solution the two articles are indistinguishable.

What is not indistinguishable is the jar. Anhydrous Na₂H₂EDTA, CAS 139-33-3, has a formula mass of 336.21. The dihydrate, CAS 6381-92-6, has 372.24. The difference is two waters, 36.03 g/mol, which is 10.7 per cent of the anhydrous mass. Weigh the dihydrate where a formula meant the anhydrous salt and you have put in about 10 per cent less active ligand than intended; weigh the anhydrous salt where the dihydrate was meant and you have put in about 12 per cent more.

🧮When the 10.7 per cent matters, and when it does not

A quantity written for the anhydrous salt needs 1.107 times the weight in dihydrate; a quantity written for the dihydrate needs 0.903 times the weight in anhydrous salt. Both are the course’s own arithmetic from the two formula masses.

Now ask where that error lands. Ware’s palladiotype clearing bath is about 50 g in a litre of tap water — 5 per cent w/v — with a capacity of fifty to sixty 10 × 8 inch prints, discarded when it colours. A 10 per cent error in that bath changes its capacity slightly and changes nothing else. Bostick & Sullivan’s kit sheets do not weigh it at all: two tablespoons of EDTA clearing agent with two tablespoons of sodium bisulfite in a quart of water, and their own sheet says “this is not a critical measurement; less will just take longer to clear, more will clear faster”.

The error matters when a molarity is specified, because a molarity is a statement about moles and a balance only knows about grams. Ware’s New Cyanotype sensitiser calls for 2.4 g of tetrasodium EDTA and immediately writes “(0.06 M)” beside it, which is a formulation whose author expects you to know which hydrate you have. Anywhere a formula gives a concentration in mol/L, the CAS number on the jar is part of the recipe.

One effect of hard water is visible in the tray, and the suppliers say so. Bostick & Sullivan’s platinum/palladium and Ziatype sheets both carry the same remark about their EDTA clearing bath — “water hardness will sometimes affect clearing time” — and recommend distilled water where you can. That is the sequestering job and the clearing job competing for the same ligand: every calcium ion the bath picks up from the tap is a molecule of EDTA that is no longer available to lift iron out of the print. It is also the clearest practical link between this page and calcium carbonate, whose milligrams per litre are how hardness is reported. A clearing bath in a hard-water district is weaker than its label says, and the fix is distilled water rather than more salt.

The wider labelling problem is worse than two hydrates. At least four articles are sold to photographers as “EDTA”: the free acid H₄EDTA, this anhydrous disodium salt, the disodium dihydrate, and the tetrasodium salt. They are not interchangeable, and the difference between the disodium and tetrasodium salts is not a conversion factor at all but six units of pH — Ware’s instruction for the New Chrysotype first bath is “do not use tetrasodium Edta, which is alkaline, and will cause iron stains”.

The suppliers’ own sheets show how easily this slips. Bostick & Sullivan’s platinum/palladium and Ziatype kits list “250 g EDTA clearing agent” without naming the salt; their kallitype sheet names EDTA tetrasodium for its clearing bath. A printer who reads both and assumes one product is making an assumption the sheets do not support. Where the course cannot establish which salt a published formula meant, it says so rather than choosing quietly, and it prefers formulas that name the article.

Properties

C₁₀H₁₄N₂Na₂O₈, relative molecular mass 336.21, CAS 139-33-3, PubChem CID 8759. White, odourless, crystalline. The record ends there: no melting point, no density, no measured water solubility, no classification.

That emptiness is itself a fact about the substance rather than an accident of the database. The naming is worth one note. PubChem resolves the name “edetate disodium” to CID 636371 — which is the dihydrate — so a search by name lands on the other article, and the course’s research record for this page pins the CID explicitly to avoid it. If a database can be caught by the ambiguity, a darkroom can.

Handling

There is nothing to quote and the absence is thinner here than on the dihydrate page. That record carries an ECHA C&L entry reporting the substance as not meeting GHS criteria in all six of its six company reports, which the course already treats as weak evidence. This record carries no classification block at all. HSE’s EH40 has no entry for EDTA or edetate under any spelling, and CAMEO holds no datasheet for it. The course found no sourced chemical incompatibility for the compound either, which is a gap in the evidence rather than a finding of none, and the incompatibilities list on this page is empty for that reason and no other.

⚠Why Level A, and the one operation that is not

Against the rubric, Level A admits 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 there is no exotherm on dissolving. The residual risks are those of a fine powder and of a bath that has taken up iron and, sometimes, palladium or gold.

The one operation that takes it out of Level A is substituting the wrong salt. Not because the tetrasodium salt is more dangerous — it is not — but because the level is assigned to a procedure, and a first clearing bath at pH 9 to 10 does not clear the print: it stains it, and the failure is a ruined print rather than an injury. This is the rare case where the course’s level and the printer’s risk point at different things, and the label is what protects both.

Waste

A clearing bath leaves carrying the metal it removed, held in solution as a soluble complex — which is precisely what makes the stream different from a plain acid rinse, and precisely why it is bottled rather than poured. The reasoning is the dihydrate page’s and applies unchanged: the noble metal is worth recovering, and the iron is worth keeping out of a septic system. ILFORD tells United Kingdom domestic users to bottle wastes separately, label them and take them to a household waste and recycling centre. Check your local regulations; they govern, and they differ.