Boric acid

An acid fixing bath has to solve two problems that pull in opposite directions. It needs a great deal of acid, because alkaline developer is carried into it on every film. It must not be strongly acid, because acid decomposes thiosulfate and throws sulfur out of solution. Boric acid is one of the answers to that, and understanding why is understanding what “weak” means in chemistry.

In photography

It is acid capacity without acid strength. Kodak’s 1928 primer sets out the difficulty exactly: the strength of an acid depends on the fraction of its hydrogen that dissociates, but the quantity of alkali it can neutralise depends on the total hydrogen present, dissociated or not. So a large quantity of a very weak acid neutralises a great deal of carried-over developer while keeping the solution only mildly acid. The primer’s own answer is acetic acid; boric acid takes the same argument further, because it is weaker still.

⚗The chemistry

OpenStax’s table of ionisation constants gives boric acid a single value of 5.4 × 10⁻¹⁰, a pK_a of 9.27. In the same table acetic acid is 1.8 × 10⁻⁵ and the first ionisation of carbonic acid 4.3 × 10⁻⁷, so boric acid is about three thousand times weaker than the acid Kodak’s primer recommends and some eight hundred times weaker than carbonic. HSDB’s measured pH values show what that buys you: a 0.1% solution reads pH 6.1, a 1% solution 5.1 and a 4.5% solution 3.7. Nearly all the hydrogen is still on the molecule, waiting; only a tiny fraction is in solution setting the pH. That is the whole trick of a buffer reservoir — capacity and strength are different quantities, and a fixer needs a lot of the first and very little of the second.

Where you meet it. Kodak’s F-5 tropical acid hardening fixing bath, in Kodak Ltd’s 1949 handbook, is sodium thiosulfate 240 g, sodium sulfite (anhydrous) 15 g, glacial acetic acid 17 mL, boric acid 7.5 g and potassium alum 15 g per litre, dissolved in that order; films fix in ten minutes in a fresh bath, and the handbook warns that prolonged immersion at high temperatures is harmful. The separate hardener stock F-5a, still in Kodak’s 2006 toning publication, is far more concentrated: sodium sulfite 75 g, 28% acetic acid 235 mL, boric acid crystals 375 g and potassium alum 75 g in a litre — with the footnote that powdered boric acid should be avoided because it does not dissolve easily.

Why it is in there with the alum. Mike Ware gives the reason alum makes the bath acidic at all: the hexaaquaaluminium ion hydrolyses, shedding protons. Kodak’s primer describes what happens when developer neutralises that acidity — a white sludge forms, the bath turns alkaline and prints fixed in it stain brown — and Kodak Ltd’s 1949 handbook names the precipitate as aluminium sulfite. So the fixer lives in a narrow window: too acid and the thiosulfate sulfurises, too alkaline and the alum sludges out and hardening stops.

H₂S₂O₃ → H₂SO₃ + S

The lower wall of the window: thiosulfuric acid decomposing, from Kodak's 1928 primer

What the course could not verify. No source read for this page states why Kodak put boric acid rather than more acetic acid into F-5, or by what mechanism it lengthens the life of an alum bath. What is established is that it is a very weak acid present in quantity, in a bath that is pH-critical at both ends. Treat the buffering role as well supported and the detailed mechanism as open.

An older use. Wall’s 1924 collection records that neutralising a selenium toning bath with boric acid stops the selenium precipitating, and gives a combined selenium toning and fixing bath of hypo 66 g and boric acid 40 g per litre, dissolved hot with the selenium solution added cold.

Properties

Odourless colourless crystals or a white powder, slightly unctuous to the touch, relative density 1.5. Chemical Safety Card 0991 gives 5.6 g per 100 mL of water at 20 °C, and HSDB the curve: 2.52% w/w at 0 °C, 4.72% at 20 °C, 10.27% at 50 °C, 27.53% at 100 °C. A stock carrying 375 g in a litre is therefore mixed hot and holds only while the bath stays warm — and that is why Kodak asks for crystals rather than powder, which cakes instead of dissolving. The card adds that it decomposes above 100 °C to water and irritant boric anhydride and attacks metals with evolution of hydrogen; CAMEO that it is incompatible with alkali carbonates and hydroxides, which convert it to a borate.

One honest oddity: HSDB’s measured pH falls faster with concentration than a simple weak monoprotic acid of pK_a 9.27 would predict — pH 5.1 for a 1% solution is about right, 3.7 for a 4.5% solution is well below it. The course could not verify the reason and does not guess one; practically, concentrated boric acid solutions are more acidic than the tabulated constant suggests.

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

Handling

The aggregated ECHA notifications classify it Danger with the health-hazard pictogram alone, on a single hazard class: reproductive toxicity, H360 in 88.7% of the 2,123 company reports that classify it and the more specific H360FD in a further 11.2%. There is no acute toxicity, skin corrosion or eye damage entry. Chemical Safety Card 0991 adds what the codes omit — irritation of the respiratory tract, mechanical irritation of the eyes, possible effects on the central nervous system and kidneys, no acute skin symptoms expected — and sets a TLV of 2 mg/m³ for the inhalable fraction with pregnancy risk group B. CAMEO records that dust and solutions are absorbed through burns and open wounds but not through unbroken skin, which makes broken skin the thing to cover.

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 reproductive toxicity is outside it; Level B’s criterion of fine powders that must not be inhaled is what the safety card describes. 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 it over a tray without raising dust; nitrile gloves, eye protection, hands washed before eating and no food in the darkroom. Cover broken skin, the one route CAMEO says the substance takes that intact skin does not. Store it separated from strong bases, which Chemical Safety Card 0991 specifies and which is also how it is ruined as an acid. And note the company it keeps in a fixer: CAMEO’s weak-acid group records gas-generating reactions with thiosulfates and sulfites, and F-5 is stable only because it is buffered well above the pH at which those reactions run.

Waste

Spent acid hardening fixer is the stream this substance leaves in, and it is silver-bearing, so recovery comes before any question about the boron. Kodak’s J-52 publication gives a pH window of 5.6 to 9.4 as the limit sewer codes most frequently set, alongside a silver limit of 1.2 mg/L; a used fixer sits below that window.

Boron does not degrade. Chemical Safety Card 0567 records the closely related borax as harmful to aquatic organisms, and PubChem lists boric acid among registered pesticides — herbicides, wood preservatives, insect baits — so spent fixer is not poured on a garden or into a soakaway 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

The idea is older than the Kodak formula. The 1906 British Journal Photographic Almanac — the archive copy is catalogued 1909, but its running heads read 1906 and its abstracts cite 1905 journals — carries a note from R. Namias: if boric acid is added to a fixing bath until saturated, troubles caused by small traces of developer are avoided, and a 30% hypo solution saturated with boric acid shows no decomposition even after some weeks.

The almanac’s editor added a bracketed correction on the spot, citing Valenta’s 1896 work: the hypo is decomposed, only much more slowly than by a stronger acid. That is the honest version and the one the modern formula rests on. Nothing stops thiosulfate decomposing in acid; boric acid slows it enough that the bath outlives its useful fixing capacity. The same volume records Namias recommending saturated boric acid in 10% potassium bromide, ten parts to a hundred of a metol-hydroquinone-carbonate developer, as a restrainer said to save over-exposures of forty and a hundred times — reported here as period practice the course has not verified.