Borax (sodium tetraborate decahydrate)

D-76 contains 2 g of borax per litre. Kodak’s D-72 paper developer, in the same 1928 primer, carries 67.5 g of sodium carbonate per litre of stock, and even at its weakest working dilution — one part stock to four of water for bromide papers — it still holds 13.5 g. That difference is not an accident of taste: it is the whole design of a fine-grain developer, and borax is what makes it possible.

In photography

It is the alkali that will not hurry. Kodak’s 1928 primer introduces borax as an alkali that “has come into extensive use recently”, recommended for a developer suited to fine-grained motion picture negatives, and explains why that developer needs a mild one. Graininess grows partly by clumping: several silver halide crystals close together, even unexposed ones, may develop and fuse through contact with an exposed crystal. D-76 answers that with a very high sulfite concentration — 100 g per litre — acting as a silver halide solvent that dissolves a little of each grain as development proceeds, so grains shrink apart instead of merging. Then comes the sentence that explains the borax: adding carbonate to such a developer speeds development and accentuates the graininess of the resulting negative. Borax holds the pH where the solvent action has time to work.

⚗The chemistry

Borax is the sodium salt of a very weak boron oxyacid, so it hydrolyses in water and the solution is alkaline: HSDB records an aqueous borax solution at about pH 9.5, and Chemical Safety Card 0567 calls it a weak base. That figure sits close to the ionisation constant OpenStax gives for boric acid, 5.4 × 10⁻¹⁰, a pK_a of 9.27 — the signature of a conjugate acid-base pair, which is what OpenStax’s buffer section describes: such a pair holds the pH near the pK_a and moves only slowly as acid is added. Development releases acid; a borax bath walks down that plateau instead of falling off it.

The quantity, and what the replenisher tells you. Kodak Ltd’s 1949 handbook gives D-76 as Elon 2.0 g, sodium sulfite (anhydrous) 100 g, hydroquinone 5.0 g and borax 2.0 g per litre, dissolved in that order and used without dilution. Its replenisher D-76R, in the same handbook and “to maintain volume and activity of tank developer”, carries the same 100 g of sulfite but 20 g of borax — ten times as much, while nothing else changes by more than half. What use consumes is the alkalinity, and the replenisher is largely a device for putting it back; without it, Kodak’s current data sheet allows only 4 rolls or sheets per litre, with times raised 15 per cent after every four. For scale, ILFORD measures its ID-11 stock at pH 8.60 to 8.70 — the sheet gives no formula, so the course attributes that to no single ingredient, but it locates this family of film developers a full unit of pH below a carbonate paper developer.

In toning it makes gold deposit quickly. Kodak’s 1928 primer explains that finely divided gold is red and coarser gold blue, so the colour depends on the rate of deposition, and that to ensure rapid deposition the bath must be kept alkaline — for which borax, sodium acetate or sodium phosphate is added to the gold chloride. Reilly’s conservation monograph gives the Borax Bath for albumen and salted paper prints as 10 g of borax with 40 mL of 1% gold chloride solution per litre. Mike Ware supplies the mechanism: the alkali provides hydroxide ions that displace chloride from the tetrachloroaurate(III) ion in a pH-dependent series, the dihydroxo complex dominating above pH 8.5.

[AuCl₄]⁻ + OH⁻ → [AuCl₃(OH)]⁻ + Cl⁻

Ware's first step: hydroxide displaces chloride from gold(III), and borax supplies the pH

Wall’s 1912 dictionary records the older and slipperier observation that borax “is a restrainer when used with pyrogallol and pyrocatechine, but an accelerator with eikonogen and hydroquinone” — an alkali’s effect depends on which developing agent it meets.

Properties

White odourless crystals or crystalline powder, relative density 1.7. Published solubilities disagree by about a factor of two, between 3.17 and 5.93 g per 100 g at 25 °C, and the course reports the spread rather than choosing. All agree on the steep temperature dependence: 1 g dissolves in 16 mL of cold water but 0.6 mL of boiling water, which is why D-76 is mixed with the borax going into hot water.

The hydrates are the trap. Borax is sold as the decahydrate, the pentahydrate and the anhydrous salt, and formulas that say “borax” mean the decahydrate. The molar masses set the conversions: 381.4 for the decahydrate, 291.4 for the pentahydrate (both from NIOSH) and 201.2 for the anhydrous salt (PubChem), so 2 g of the decahydrate is 1.53 g of the pentahydrate or 1.06 g of the anhydrous salt — and a gram-for-gram substitution of the anhydrous form would make a D-76 nearly twice as alkaline as intended. HSDB adds a slower version of the same trap: borax effloresces in warm dry air, losing water of crystallisation between 75 °C and 320 °C, so a warm tub becomes stronger per gram.

Kodak’s 1924 primer gives a bench test that still works: mix the solid to a paste with strong sulfuric acid, pour alcohol over it and light it, and the flame shows unmistakable yellow-green fringes. Borax gives an alkaline solution, boric acid a feebly acid one.

Handling

The aggregated ECHA notifications classify it Danger on H360, may damage fertility or the unborn child, in 93.2% of the 2,865 company reports that classify it, with H319 in 12.6%. Chemical Safety Card 0567 fills in the rest: irritating to eyes, skin and respiratory tract, possible effects on the central nervous system, kidneys and testes, animal tests showing possible toxicity to human reproduction or development, and a harmful airborne concentration reached quickly if the material is powdered. The limits are set for the dust: HSE’s EH40 gives 5 mg/m³ long-term for the decahydrate and 1 mg/m³ for the anhydrous and pentahydrate forms.

Why Level B. Borax is mild in every respect except one, and that one decides it. The course rubric caps Level A at a classification “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”, and reproductive toxicity is outside that ceiling; Level B’s criterion of fine powders that must not be inhaled is what the safety card describes. It does not become 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 it without raising dust, over a tray; nitrile gloves and eye protection throughout; wash hands before eating and keep food out of 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. Keep it from acids, which liberate boric acid and destroy the alkalinity the developer depends on.

Waste

A used borax developer is a dilute alkaline solution whose oxygen demand comes from the sulfite and the developing agents rather than from the borate. Kodak’s J-52 publication gives the mean limits sewer codes most often set — pH 5.6 to 9.4, a five-day biochemical oxygen demand of 350 mg/L — and a borax solution at about pH 9.5 sits just outside that window, so dilution is usually the whole of the answer at darkroom volumes. Any bath that has fixed film goes for silver recovery first.

Two things about borate specifically: Chemical Safety Card 0567 records that it is harmful to aquatic organisms, and PubChem’s use list records borax as a registered pesticide for total control of vegetation on non-crop land — 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

Borax reached photography as an industrial commodity rather than a laboratory reagent. Wall’s 1912 dictionary has it found native or made by neutralising boric acid with soda, already used in toning and only suggested as a developer additive. Kodak’s 1928 primer describes the manufacture of its day — calcium borate ores roasted, boiled in carbonate solution, crystallised in large iron vats — and records the change that made cheap pure borax available: a new source discovered in Kern County, California, in 1926. Its next sentence is the one that matters to a photographer: only the pure salt should be used for photographic purposes. The fine-grain borax developer appears in that same primer as a recent arrival, at the moment a clean supply of the material does, and it is still in the catalogue a century later.