Sodium carbonate (anhydrous)

A developing agent left in plain water does almost nothing. Kodak’s 1928 primer gives the rule in a clause: developing agents must be in an alkaline solution to work, and the energy of the developer depends on how much alkali is present. Which alkali you pick decides how steadily the bath behaves across a session, and for a century the answer was this salt.

In photography

It is a reservoir, not a dose. Carbonic acid is very weak, the primer explains, so a carbonate in solution is not neutral but alkaline: the strong base predominates over the weak acid, and the carbonate is split up, to some extent, into the bicarbonate and the caustic alkali. Using a carbonate therefore represents a sort of reservoir of alkali — little free alkali at any moment, but more generated as it is used up. A proportional quantity of caustic soda would give that same small quantity with nothing behind it, and would soon be exhausted.

CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻

The reservoir: carbonate takes a proton from water and releases hydroxide

⚗The chemistry

Modern language calls that a buffer. OpenStax’s Chemistry 2e gives the second ionisation constant of carbonic acid — the one governing the carbonate and bicarbonate ions — as 4.7 × 10⁻¹¹, a pK_a of 10.33. A conjugate pair holds the pH near its own pK_a, so a carbonate solution that has picked up acid parks near pH 10.3 and stays there while the two forms remain within about a factor of ten of each other. Development consumes alkali; the ratio walks across that range instead of falling off a cliff.

The quantity sets the energy. Too much alkali, the primer warns, and the developer tends to produce chemical fog; too little and it is slow. Alkali also softens gelatin, so an over-alkaline bath over-swells the emulsion and gives frilling or blisters in warm weather. The 1928 formulas show the range, counted in the solution as mixed and before any dilution for use: 67.5 g per litre in D-72 for Velox, Azo and bromide papers, 52.5 g in the undiluted D-19 X-ray developer, 45 g in D-49, 15 g in D-52 for Vitava, 11.3 g in D-61a for film. More carbonate is faster, more contrasty and colder-working; less is slower and, on a chloro-bromide paper, warmer. It is also what a fine-grain developer leaves out: carbonate added to a high-sulfite developer such as D-76, the primer says, speeds development and coarsens the grain. And where a formula carries sodium bisulfite, some of the carbonate is destroyed on mixing.

Outside the developer it makes a fixer alkaline. Reilly’s conservation monograph fixes albumen and salted paper prints in 150 g of sodium thiosulfate pentahydrate per litre with 2 g of sodium carbonate: the slight alkalinity stops acid decomposing the thiosulfate and liberating sulfur, and an acid fixer would attack the finely divided image silver.

Properties

A white hygroscopic powder, melting at 851 °C, density 2.5 g/cm³, decomposing above 400 °C to sodium oxides and carbon dioxide. Chemical Safety Card 1135 gives 30 g per 100 mL of water at 20 °C, and HSDB a steep curve with temperature: 6% w/w at 0 °C against 28% at 30 °C. Cold water is a poor place to make a strong stock.

The hydrates are the trap. Kodak’s primer describes three commercial forms: crystals with ten parts of water, Na₂CO₃·10H₂O, containing 37% of the carbonate; crystals with one part, Na₂CO₃·H₂O, containing 85%; and the dry powder containing about 98%. Weigh the wrong one and the developer is wrong by a factor approaching three. Mike Ware’s Chrysotype Manual gives the cleanest modern equivalence, listing four alkalis that do one job interchangeably: 7.4 g of the anhydrous salt, 8.7 g of the monohydrate, 20.0 g of the decahydrate or 11.8 g of sodium hydrogen carbonate. Those ratios are 1 : 1.18 : 2.70 : 1.59, and they track the molar masses (105.99, 124.00, 286 and twice 84.01) closely enough to use as working factors. Kodak Ltd’s 1949 handbook recommends the anhydrous salt wherever a formula says carbonate and directs that crystals be increased; its own factor is illegible in the scan the course read, so only the direction is reported here.

Wall’s 1912 dictionary adds the commercial caution: ordinary washing soda is of indefinite strength, usually carries much sulfate, and is not the salt to use. Photographic carbonate is made by heating the bicarbonate — too little leaves bicarbonate, practically useless as an alkali; too much makes caustic soda. No measured pH for a working-strength carbonate developer has turned up in the course’s sources, and it does not guess one.

Handling

The aggregated ECHA notifications on PubChem give Warning, the exclamation-mark pictogram and one statement — serious eye irritation, from 99.8% of the 8,784 reports that classify it — and repeat it under the European CLP Regulation. It is not unanimous: Japan’s NITE-CMC and Safe Work Australia both call it Danger and give H318, serious eye damage. Chemical Safety Card 1135 adds what the classification omits: harmful airborne concentrations of the powder are reached quickly on dispersion, and repeated exposure can cause dermatitis and perforate the nasal septum.

Why Level A. Against the course rubric, this handling meets the Level A criteria and no more: the classification tops out at irritant, or corrosive to the eye on the severest reading, at the tens of grams per litre used; nothing is heated above 50 °C; and the waste is spent developer or spent alkaline fixer, both collectable. Two controls come from Level B by preference — splash goggles rather than glasses for powder and concentrates, because two national authorities put eye damage on the record, and dust discipline, because the card says the airborne concentration climbs fast. Neither EH40 nor the NIOSH Pocket Guide sets an exposure limit for sodium carbonate, and EH40 states that absence from its list does not indicate that a substance is without risk.

⚠Safety

Gloves and eye protection throughout; goggles for the dry salt; weigh over a tray, powder into water and not water into powder. HSE’s COSHH essentials sheet P1 asks for more than five air changes an hour from a powered fan with a through draught and takes single-use nitrile gloves 0.2 mm thick as splash protection. Keep it from every acid: carbonate meeting an acid stop bath fizzes, and the fizz is carbon dioxide from a reaction the Chemical Safety Card calls violent. In a closed bottle that is pressure.

Waste

Spent developer leaves alkaline, and carbonate is most of the reason. Kodak’s J-52 publication gives a pH window of 5.6 to 9.4 as the limit sewer codes most often set; a concentrated carbonate stock is well above it, so dilution or neutralisation is the question a disposal route has to answer. The same publication puts the biochemical oxygen demand of typical photographic effluent at 100 to 1,000 mg/L against a usual regulated mean of 350 mg/L.

Carbonate is also a treatment reagent: Kodak’s J-215 publication lists 200 mL of 10% sodium carbonate per litre of EKTACOLOR RA-4 bleach-fix overflow as one way of raising its pH to 7.5 to 8.0 before silver recovery. Reilly records an older economy — silver in the first wash waters of a salted-paper session thrown down with sodium chloride or sodium carbonate, as insoluble silver chloride or silver carbonate.

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. Do not combine carbonate waste with acid waste believing you are neutralising it: that reaction foams and evolves gas, and needs an open vessel with room to rise.

History

The alkaline developer arrives with Major C. Russell, whom Eder credits with the alkaline pyrogallol developer used with ammonia and potassium bromide and calls the most important improvement in development; Eder adds that alkaline carbonates were in use in pyro developers at that time, and dates the account to the second edition of Russell’s The Tannin Process, 1863. Ammonia lost for the reason Kodak’s primer gives: being a solution of a gas, its strength is uncertain and variable, and it dissolves silver bromide and tends to produce coloured stains. The carbonates won by being solids you could weigh, and what improved after that was not the ion but the certainty about what was in the tub.