Sodium metaborate (anhydrous)

Between a carbonate developer and a borax one there is a wide gap in alkalinity, and for thirty years photographers had nothing to put in it except a guess at how much carbonate to leave out. Sodium metaborate fills that gap, for a reason visible in the formula: it is what you get when caustic soda acts on borax.

In photography

It is an alkali of adjustable middle strength. Kodak Ltd’s 1949 handbook introduces the trade form under the name Kodalk: “a new alkali, introduced by Kodak Ltd., intermediate in activity between sodium carbonate and borax”, which “permits of greater control of the energy of the developer”. PubChem’s depositor-supplied synonym list for sodium metaborate carries Kodalk directly, which is how the trade name and the chemical species connect — though the synonym sits on the record for the anhydrous salt while the material actually sold is a hydrate, so this course indexes Kodalk under the tetrahydrate.

⚗The chemistry

HSDB gives two routes to sodium metaborate, and the second is the one a photographer should carry: fusing sodium carbonate with borax, or the action of sodium hydroxide on disodium tetraborate. Borax is a tetraborate — four borons sharing two sodiums — so adding two more sodiums as hydroxide splits it into four metaborate units, each with its own sodium. The borate keeps its weak-acid character and so keeps buffering, but the sodium-to-boron ratio has doubled, and a solution of the salt of a weak acid and a strong base is alkaline in proportion (OpenStax’s treatment of salt hydrolysis). That is the whole of “intermediate between carbonate and borax”: more alkali per boron than borax, still a buffer rather than free hydroxide.

Na₂B₄O₇ + 2 NaOH → 4 NaBO₂ + H₂O

HSDB names the route — sodium hydroxide acting on disodium tetraborate; the course has balanced it

Two consequences Kodak singled out. The 1949 handbook gives them in one paragraph. First, a film developed in a Kodalk developer will not blister when placed in an acid fixing bath, even at high temperatures, because the alkali does not evolve carbon dioxide on acidifying — the gas that lifts the emulsion off its base when a carbonate developer meets an acid bath in hot weather. Second, there is less tendency to precipitate aluminium sulfite from fixing baths containing alum, the white sludge that ends the useful life of a hardening fixer.

How much changes what. The metaborate developers of that handbook run from 2 g per litre in the extra-fine-grain DK-20 through 10 g in the general-purpose DK-50 to 22.5 g in the tropical DK-15, and the handbook states the effect of moving within a formula plainly, under DK-50: increasing or decreasing the quantity of Kodalk makes it possible either to raise or lower the contrast obtained in a given development time, or to shorten or lengthen the development time without affecting the contrast. That is a rare thing to have in writing — an alkali described as a control rather than an ingredient. As with the borax developers, the replenisher shows what use consumes: DK-20R carries 20 g per litre against DK-20’s 2 g.

Properties

A white solid, sold as a dry powder or as white hexagonal crystals, melting at 966 °C, density 2.46 g/cm³. HSDB gives its solubility as 20 g per 100 mL of water at 20 °C, rising to 36 g per 100 g at 35 °C — some four times as soluble as borax at room temperature, which is a practical advantage when a formula wants 20 g in a litre. The solution is strongly alkaline.

Anhydrous is not what a formula means. The molar masses do the arithmetic: 65.80 for the anhydrous salt against 137.86 for the tetrahydrate, a ratio of 2.095. Weigh the anhydrous salt where a formula calls for the hydrate and you supply more than twice the alkali it asks for. Haz-Map records that the substance is commercially available as the octahydrate and the tetrahydrate, and it is the tetrahydrate that is generally taken as equivalent to Kodalk. The dihydrate exists too — HSDB gives it a molecular weight of 101.83 and a melting point of 90 to 95 °C — so “sodium metaborate” on a supplier’s list is not a complete specification, and the safety data sheet is where the hydrate is named.

It converts itself in an open tub. HSDB records that the crystals pick up carbon dioxide if exposed to air, forming sodium carbonate and borax. The change is slow, but it is a change of identity rather than only of strength: an old open tub is a mixture of three alkalis, one of which is the carbonate whose carbon dioxide the metaborate was chosen to avoid.

Handling

The aggregated ECHA notifications give the anhydrous salt the signal word Warning, with the exclamation-mark and health-hazard pictograms: H361, suspected of damaging fertility or the unborn child, in 68.5% of the 1,044 company reports that classify it, H319 in 52.6% and H315 in 10.3%. The aggregate is not unanimous, and the disagreement runs in the more severe direction: Safe Work Australia’s Hazardous Chemical Information System classifies the same substance Danger with H360FD, and the ECHA aggregate for the tetrahydrate is also Danger. The course takes the union of those views when choosing controls, and treats every borate in this family the same way.

The course found no ILO-WHO Chemical Safety Card for sodium metaborate, so there is no card-level account of its effects to quote. The nearest sourced description is Card 0567 for the related borax: irritating to eyes, skin and respiratory tract, possible effects on the central nervous system, kidneys and testes, with animal tests showing possible toxicity to human reproduction or development. HSE’s EH40 sets limits for the tetraborates but lists no metaborate, and EH40 states that absence from its list does not mean a substance is without risk.

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”, and a reproductive-toxicity classification — suspected in the ECHA aggregate, asserted outright by Safe Work Australia — is outside it. Level B’s criterion of fine powders that must not be inhaled is what this material is at the balance. It does not reach Level C, whose reproductive-toxin criterion applies where a fume cupboard or specialist disposal is the recognised control; here the controls are dust suppression, gloves, eye protection and hygiene.

⚠Safety

Weigh over a tray without raising dust, powder into water rather than the reverse; nitrile gloves and eye protection throughout, since more than half the notifications carry serious eye irritation and the solution is strongly alkaline; hands washed before eating and no food in the darkroom. The classification driving all of this is reproductive toxicity, so a photographer who is pregnant, breastfeeding or trying to conceive should read the supplier’s safety data sheet for the actual product and take the specific risk assessment ILFORD’s guidance describes for photographic chemicals. Keep it from acids, which turn it back into boric acid and end its usefulness as an alkali.

Waste

Spent metaborate developer is alkaline, its oxygen demand coming mostly from the sulfite and the developing agents rather than the borate. Kodak’s J-52 publication gives the mean limits sewer codes most often set — a pH window of 5.6 to 9.4 and silver at 1.2 mg/L — and a metaborate solution sits above that window, so dilution or neutralisation is the first question a disposal route has to answer. Any bath that has fixed film goes for silver recovery first.

Boron does not break down. Chemical Safety Card 0567 records the related borax as harmful to aquatic organisms and PubChem lists borates among registered herbicides, so spent developer does not go on the garden whatever its pH. ILFORD’s guidance for domestic users in the UK 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 down the drain with plenty of water, unmixed, and never into a septic tank. Check your local regulations, which govern.

History

Kodak’s 1928 primer describes a world of three alkalis: the caustics at one end, the carbonates in the middle, borax newly arrived at the weak end for fine-grain work. Nothing in that list lets a photographer choose an alkalinity; you chose an alkali and accepted its pH. Kodak Ltd’s 1949 handbook is the first source read for this page to describe an alkali as an adjustable control — and it does so under a trade name, with the composition unstated. PubChem’s synonym list is what connects that name to the chemistry: a reminder that a photographic trade name is a commercial object first and a substance second, and that the encyclopaedia’s job is to say which substance it was.