Sodium metaborate tetrahydrate (Kodalk)

A photographic formula that says “Kodalk” means this: the hydrated sodium metaborate Kodak sold under that trade name, whose chemistry is set out on the anhydrous page and whose weight is set out here. The distinction is not pedantry. The tetrahydrate is 2.1 times the molar mass of the anhydrous salt, so getting the two confused is a two-fold error in the alkalinity of the bath.

In photography

It is the middle alkali, and it is adjustable. Kodak Ltd’s 1949 handbook describes Kodalk as an alkaline accelerator “intermediate in activity between sodium carbonate and borax”, which “permits of greater control of the energy of the developer”. The formulas span an order of magnitude: 2 g per litre in the extra-fine-grain DK-20, 10 g in the general-purpose Elon-hydroquinone developer DK-50, and 22.5 g in the tropical DK-15, which also carries 105 g of sodium sulfate per litre — the handbook gives the quantity without explaining it, and this course does not supply a reason it has not verified.

⚗The chemistry

The 1949 handbook’s note under DK-50 is the clearest statement in this course’s sources of what an alkali actually controls. Increasing or decreasing the quantity of Kodalk makes it possible either (a) to increase or decrease the contrast obtained in a given time of development, or (b) to decrease or increase the time of development without affecting the contrast. Two knobs from one ingredient: more alkali drives the developing agent into its active anionic form faster, so in a fixed time development runs further up the curve and contrast rises — or, holding contrast fixed, the same degree of development arrives sooner. Which of the two you get depends on whether you hold the clock or the contrast constant.

Why it does not blister a film. This is the property Kodak led with. A carbonate developer carried into an acid fixing bath fizzes: the acid liberates carbon dioxide from the carbonate, and in warm weather the gas forms inside the swollen gelatin and lifts it off the base. The 1949 handbook states that films 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. That is why the tropical developer DK-15 uses it and not carbonate.

And why it is kinder to a hardening fixer. The same paragraph adds that there is less tendency to precipitate aluminium sulfite from fixing baths containing alum. Kodak’s 1928 primer describes the failure this refers to: when enough alkaline developer has been carried over, a white sludge forms, the bath turns alkaline and prints fixed in it stain brown. Kodak’s F-5 answers the same problem from the other side, by loading the fixer with acid reserve in the form of boric acid; the metaborate answers it by putting less carbonate alkalinity into the developer in the first place.

The replenisher, again. DK-20 carries 2 g per litre and its replenisher DK-20R carries 20 g, ten times as much, “to maintain volume and activity of tank developer”. Exactly the same ratio holds between D-76 and D-76R with borax. Whatever the alkali, what a tank of developer runs out of first is its alkalinity.

Properties

White odourless crystalline granules, specific gravity 1.74, melting at 53.5 °C. That low melting point is worth knowing before the tub goes on a shelf above a radiator: this is not a salt that tolerates warmth. HSDB gives its solubility as 41.9% at 20 °C, rising to 109.8% at 100 °C — around eight times borax at room temperature, so a metaborate developer dissolves in cold water where a borax one wants hot.

The pH is the number that matters. HSDB gives the aqueous pH of the tetrahydrate against concentration at 20 °C: 10.52 at 0.1% w/v, 11.0 at 1.0%, 11.4 at 4.0%, 11.8 at 10% and 12.0 at 15%. Set that beside borax, whose saturated solution HSDB puts at about pH 9.5, and beside a carbonate bath, and the description “intermediate in activity between sodium carbonate and borax” becomes a measured statement rather than a marketing one. Notice too how slowly the pH climbs — a hundred and fifty-fold change in concentration moves it by 1.5 units. That is buffering: the borate holds a plateau where a strong alkali would run away.

Weighing. The molar masses are 65.80 for the anhydrous salt, 101.83 for the dihydrate and 137.86 for the tetrahydrate. So 10 g of the tetrahydrate is 4.77 g of the anhydrous salt or 7.39 g of the dihydrate, and substituting gram for gram in either direction throws the alkalinity of a DK-50 out by a factor approaching two. Where a supplier’s label says only “sodium metaborate”, the hydrate is on the safety data sheet and nowhere else.

Handling

The aggregated ECHA notifications classify the tetrahydrate Danger: H319 in 82.6% of the reports, H360 in 45% and the weaker H361 in 26.2%. Two cautions belong with those numbers. The first is that the sample is small — 149 reports from 7 notifications, against 1,044 for the anhydrous salt and 2,865 for borax — so the percentages are thin. The second is that the notifiers disagree with one another about whether reproductive toxicity is established or suspected, and Safe Work Australia classifies the same CAS number Danger with H360FD. The course treats the more severe reading as the one to design controls around.

The course found no ILO-WHO Chemical Safety Card for any sodium metaborate. The nearest sourced account of effects is Card 0567 for borax: irritating to eyes, skin and respiratory tract, possible effects on the central nervous system, kidneys and testes, and a harmful airborne concentration reached quickly if the material is powdered. HSE’s EH40 sets long-term limits for the tetraborates but lists no metaborate at all, and states that absence from its list does not indicate that 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 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, which is also what ILFORD’s guidance prescribes, with a specific risk assessment for an expectant or breastfeeding worker.

⚠Safety

Weigh over a tray without raising dust, solid into water rather than the reverse; nitrile gloves and eye protection throughout, since a 1% solution is already pH 11; hands washed before eating and no food in the darkroom. 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. Keep it from acids, which turn it back into boric acid, and keep it cool: it melts at 53.5 °C.

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 a pH window of 5.6 to 9.4 as the limit sewer codes most frequently set, with a silver limit of 1.2 mg/L. Even a 0.1% metaborate solution reads pH 10.5, so a working developer sits above that window and dilution or neutralisation is the first question. 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’s use list records borates among registered herbicides and defoliants, 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 Ltd’s 1949 handbook still calls Kodalk “a new alkali, introduced by Kodak Ltd.”, and its short list of what the alkali does reads as a list of the complaints it was designed to answer: blistering in tropical processing, alum sludge in hardening fixers, and the coarse control a photographer had over developer energy when the only choices were carbonate and borax.

The name outlived the product. Kodalk left the catalogue, the DK-series formulas did not, and the alkali they call for is still bought and sold as sodium metaborate — usually the tetrahydrate, sometimes the octahydrate, occasionally the anhydrous salt. That is what makes hydrates worth a page of their own: a trade name hides a hydrate, a hydrate hides a factor of two, and a factor of two in the alkali is a different developer.