Hydrochloric acid

Almost every photographic use of hydrochloric acid is a use of a very dilute solution of it. The clearing baths that finish a platinum print are under two per cent v/v; the pre-bath that strips chalk from a watercolour paper is one or two. The hazard belongs to the bottle it is diluted from, and the gap between the two is the whole of the handling problem.

In photography

Clearing an iron-based print. A platinum or palladium print leaves the developer carrying iron salts that were never converted to metal, and if they stay in the paper they yellow it and shorten its life. Ware’s Platinomicon records the traditional remedy: black platinotypes were cleared for about ten minutes in each of three successive baths of hydrochloric acid, the concentrated 36 per cent w/w acid diluted 1 in 60 by volume, which is about 1.7 per cent v/v and, in Ware’s figures, 0.2 mol/L at a pH below 1; sepia platinotypes took a bath of half that strength. Irving Penn used 1 in 64. The point of three baths rather than one is that each removes a share of what is left, so the last bath finishes the job the first could not.

What more of it does. Ware is careful to record that these procedures “were not always successful”, and that some workers used much longer times. Penn, who coated heavily to get dense blacks, found he needed six to ten hours in the dilute acid to shift the residual iron — and that these very prolonged treatments caused density loss in his palladium-containing images. More acid and more time clear better and etch the image; that is the trade the printer is making.

Destroying chalk in a paper. Modern art papers are often buffered with calcium carbonate, which is exactly the wrong thing under an iron sensitiser: it raises the pH and hydrolyses the iron(III). Ware’s pre-treatment is a bath of dilute acid — 1 to 2 per cent v/v hydrochloric acid for ten to twenty minutes, or 5 to 10 per cent w/v sulphamic acid — followed by a long wash, and he notes that the paper’s pH should preferably end up below 7 for coating even though that runs contrary to conservation practice, because the wet processing that follows will de-acidify it again.

In a chromium intensifier. Kodak’s 1924 primer describes bleaching the silver image with a solution of bichromate containing a very little hydrochloric acid — one to two drams of the concentrated acid in 32 ounces — then washing until the yellow stain has gone and redeveloping after a short exposure to diffused daylight. The primer’s note on quantity is the useful part: the more hydrochloric acid, the quicker the bleaching takes place and the less intensification is obtained.

Making other reagents. The same primer records that gold chloride is made by dissolving gold in a mixture of hydrochloric and nitric acids and evaporating, giving brownish deliquescent crystals containing 65 per cent metallic gold; and it uses hydrochloric acid as the bench test for silver nitrate, which gives a white precipitate that dissolves in ammonia and reappears on adding sulfuric acid. That test is the whole of salted-paper chemistry in a test tube.

AgNO₃ + HCl → AgCl + HNO₃

Chloride meeting silver: the bench test, and the reason a chloride waste is never poured into a silver one

Properties

A colourless watery liquid with a sharp, irritating odour that fumes in air; the fume is hydrogen chloride gas coming out of solution, which is why the smell reaches you across a room. Solubility falls as the acid warms — HSDB gives 82.3 g of hydrogen chloride per 100 g of water at 0 °C against 56.1 g at 60 °C — so a bottle stored warm fumes more than a bottle stored cool. CAMEO records that dilution generates heat.

⚗The chemistry

Hydrochloric acid is a strong acid: in water it is completely ionised, so there is no reserve of undissociated molecules and no buffering. That is the opposite of what a stop bath or an acid fixer wants, and it is exactly what a clearing bath wants — the job there is to hold the pH low enough to keep iron(III) from hydrolysing and to dissolve what has already precipitated. OpenStax adds a detail worth knowing: water levels the strong acids, because the strongest acid that can exist in aqueous solution is the hydronium ion, so hydrochloric, nitric and sulfuric acid all present the same species to the solution and their differences show up only in a less basic solvent. In ethanol, HCl is a weak acid, and weaker than HBr and HI.

Handling

The aggregated ECHA notifications classify hydrogen chloride Danger, with H314, severe skin burns and eye damage, in 99.9 per cent of the 5,109 company reports that carry hazard codes, H335 in 59 per cent, H331, toxic if inhaled, in 49.4 per cent, H290 in 22.8 and H318 in 20. HSE’s EH40 sets a British workplace exposure limit for the gas and its aerosol mists of 1 ppm (2 mg/m³) over eight hours and 5 ppm (8 mg/m³) over fifteen minutes; the NIOSH pocket guide sets a ceiling of 5 ppm rather than an average, with an IDLH of 50 ppm. Chemical Safety Card 0163 adds that inhalation may trigger asthma-like reactions, that high concentrations risk lung oedema, and that repeated exposure erodes teeth and inflames the airway; it records that acid mists are carcinogenic to humans.

Why Level B. The course rubric reaches Level B on three of its criteria at once: the handling of a concentrated acid, a procedure that generates mists or vapours, and a failure mode that is a splash or a burn rather than a spoiled print. Level A is out of the question for the concentrate. It stops short of Level C because the acid the course actually stands over is a one to two per cent v/v bath in an open tray, whose recognised controls are gloves, goggles, an apron and work at an open window with a fan rather than an engineered fume cupboard. The dilution step is the part that needs the ventilation, and a printer without extraction is better served by buying the acid ready diluted, or by using sulphamic acid for the decalcifying job, than by opening a 36 per cent bottle in a small room.

⚠Safety

Acid into water, slowly, stirring, and never water into acid — Princeton’s guidance for photographic work states the rule for cleaning acids without qualification, and CAMEO gives the reason: dilution generates heat. Open the concentrate outdoors or under extraction, in goggles, a face shield if you have one, gloves and an apron, with eyewash within reach. Three things must never come near it. Hypochlorite bleach, because the card records violent reaction with oxidants and the production of toxic chlorine gas, and Princeton says plainly not to add acid to hypochlorite bleaches. Spent fixer, because Kodak’s 1928 primer records that a few drops of hydrochloric or sulfuric acid decompose thiosulfate and throw out sulfur. Metal, of any kind — NIOSH names copper, brass and zinc, CAMEO adds aluminium, zinc, calcium, magnesium, iron, tin and the alkali metals, and the product is hydrogen.

Waste

What leaves the darkroom is dilute: three clearing baths and a decalcifying rinse, carrying iron and, from a platinum or palladium print, traces of the noble metal. Its problem is pH. Kodak’s J-52 publication gives 5.6 to 9.4 as the window sewer codes most frequently set, and a clearing bath at pH below 1 is a long way outside it, so neutralisation is deliberate: a carbonate or bicarbonate solution added slowly, in a ventilated place, wearing what was worn to mix the bath, with the fizzing expected and allowed for.

The rule that matters more than the pH is what it must not be mixed with. Acid waste and fixer waste do not share a bottle, and neither do acid waste and anything containing hypochlorite, sulfide or cyanide. ILFORD’s guidance for domestic users 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 with plenty of water, unmixed, and never into a septic tank. That word unmixed is doing real work here. Check your local regulations, which govern.

History

The acid is old and its old names survive on bottles: muriatic acid, from the Latin for brine, and spirit of salt, because it was made by pouring oil of vitriol on common salt. Kodak’s 1924 primer introduces it as the type case for the whole halogen family — chlorine combines with hydrogen to form hydrochloric acid, and if the hydrogen is replaced by metals you get the chlorides, “of which the best known is sodium chloride, Na Cl, which is common salt” — and then runs the same pattern for bromine and iodine. Every silver halide in the course is a member of the series that primer sets out on one page.

Its photographic career has been mostly in the background: the acid that made the gold chloride, the acid that cleared the platinum print, the acid in the intensifier’s bleach. Ware’s judgement on the clearing baths is worth keeping, because it is the honest version of a nineteenth-century routine that reads as settled practice: they were not always successful, workers argued about times, and the process left iron in prints that survive today with the yellowed highlights to prove it.