Sodium bicarbonate

Kodak’s 1928 primer disposes of this salt in a clause: bicarbonate is practically useless as an alkali in photography, and the manufacture of photographic sodium carbonate is timed so that practically none of it survives. That is accurate. It is also the reason it has the jobs it has: every one of them is a job a stronger alkali would do badly.

In photography

It is too weak to develop with, and the numbers say by how much. A carbonate solution buffers near pH 10.3, because the carbonate and bicarbonate ions are the pair around the second ionisation of carbonic acid. A bicarbonate solution sits far lower.

⚗The chemistry

OpenStax’s Chemistry 2e works this exact salt. The sodium ion is inert; the hydrogencarbonate ion is amphiprotic, able to give a proton or take one. Its acid ionisation constant is 4.7 × 10⁻¹¹ and its base ionisation constant 2.3 × 10⁻⁸, and because the base constant is much the larger the solution comes out basic — but only just. CAMEO measures it: pH 8.3 for a 0.1 molar solution at 25 °C, and 8 to 9 saturated. Set that beside the pH 10.3 a carbonate holds and the two-order-of-magnitude gap in hydroxide concentration is the whole difference between an alkali that drives a developing agent and one that does not.

Where mildness is the point: toning. Eder’s history records that Waterhouse of Halifax used, in 1858, a gold chloride bath made mildly alkaline by adding sodium carbonate or sodium bicarbonate; Wall’s 1912 dictionary gives the salt’s photographic use in four words — it is used for toning. The reason a weak base is wanted is in Reilly’s account of alkaline gold toning: a gold chloride stock is acid, and one nineteenth-century approach neutralised it with chalk precisely because chalk will not make the gold chloride alkaline but will take out the free acid. Bicarbonate is a different weak base doing the same thing — removing the acid without pushing the bath hard alkaline.

Where it appears without being added. In a one-solution developer carrying sulfite, bisulfite and carbonate, Kodak’s primer says, the bisulfite converts to sulfite at the carbonate’s expense and sodium bicarbonate is the by-product — and experiments showed that the bicarbonate so formed acts as an antifogging agent, which is why such a formula does not simply use sulfite alone.

Where it is the modern working reagent: neutralisation. Kodak’s J-300 environmental guidance for amateurs states that KODAK Indicator Stop Bath contains acetic acid and has a pH of 1.0, and directs that the solution be neutralised with sodium bicarbonate solution before it is sent to the sewer, to help protect the drain pipes.

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

Neutralising an acetic acid stop bath: the fizz is the reaction finishing

The operating detail matters more than the equation: add the bicarbonate slowly, because the mixture foams, in a well ventilated area, wearing gloves, goggles and an apron. A weak base is right here for the same reason as in the toning bath — it climbs towards neutral and stops, instead of overshooting into a waste that is out of range in the other direction.

As a bench alkali it is counted in twos. Mike Ware’s Chrysotype Manual lists alkalis that neutralise 12.5 g of an organic acid interchangeably: 7.4 g of anhydrous sodium carbonate, 8.7 g of the monohydrate, 20.0 g of the decahydrate, or 11.8 g of sodium hydrogen carbonate — close to twice the molar quantity of the first, because each carbonate ion takes two protons and each bicarbonate ion takes one.

Properties

An odourless white crystalline powder, density about 2.1 g/cm³, dissolving to 9.3 g per 100 mL of water at 20 °C on the Chemical Safety Card and 10.3 g per 100 g at 25 °C in HSDB — roughly a third of the carbonate’s solubility, which is why a strong bicarbonate stock is not a practical thing to keep. HSDB adds a detail worth knowing wherever the two salts share a solution: the bicarbonate’s solubility is lower in the presence of sodium carbonate.

Its instability is its other working property. CAMEO records it as stable in dry air but slowly decomposing in moist air, and Kodak’s primer gives the reaction that photographic carbonate is manufactured by:

2 NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

Bicarbonate to carbonate: half the carbonic acid driven off by heat

The sources do not agree on where that begins. Chemical Safety Card 1044 puts the start of decomposition at 50 °C and completion at 270 °C; CAMEO gives a single decomposition figure of 228 °F, about 109 °C. The course reports both rather than choosing. Either way, a tub that has sat open for years is part carbonate, and it will neutralise more acid per gram than the label implies.

Handling

The aggregated ECHA notifications are the unusual case in this encyclopaedia: not classified. Of 6,838 reports across 9 notifications, 6,796 — 99.4% — state that the substance does not meet GHS hazard criteria, and only 0.6% of companies supplied any GHS information at all; Japan’s NITE-CMC returns the same verdict. That is a statement about where GHS sets its thresholds, not a description of the powder. CAMEO’s health-hazard entry records irritation of skin, eyes, nose and throat with coughing and gastrointestinal disturbance, effects of high doses including alkalosis and renal injury, and toxic fumes when it is heated to decomposition; Chemical Safety Card 1044 notes that dispersed powder reaches nuisance airborne levels quickly.

Why Level A. Against the course rubric, the Level A criteria are met with room to spare: no classification at all in the aggregate, nothing heated, and a waste stream that is collectable. The course nevertheless works it under full Level A controls, and the reason is worth stating. A darkroom runs one standard at one sink. The bicarbonate is on the bench beside the stop bath it is there to neutralise, the gloves are already on, and a substance whose classification is empty is still a fine powder that irritates an eye. Dropping controls for one container is how controls get dropped for the next one.

⚠Safety

Gloves and eye protection; weigh without raising dust; label the container and keep it out of the food cupboard. The larger hazard is not the powder but the reaction you use it for: neutralising an acid stop bath is exothermic and evolves carbon dioxide, so use a vessel with several times the headroom you think you need, add the bicarbonate in small portions, and wait for each to subside. Kodak’s J-300 asks for a well ventilated area and for gloves, goggles and an apron for exactly this operation.

Waste

The stream is usually the one it created: a neutralised stop bath, carrying sodium acetate, dissolved carbon dioxide and whatever the bath had picked up from the prints. Kodak’s J-52 publication gives the parameters sewer codes most often regulate, including a pH window of 5.6 to 9.4 — which is the target a bicarbonate neutralisation is aiming at, and the reason a weak base is the right tool: it is hard to overshoot into alkalinity with it.

The neutralisation is the step to do carefully; once neutral, the solution is still photographic waste. 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, and remember that any bath that has touched film or paper carries silver and belongs in the recovery stream first.

History

The name is a fossil of an argument. Wall’s 1912 dictionary explains that the salt takes its name from a period when it was regarded as the normal carbonate, the salt now looked on as the normal carbonate being then called the subcarbonate — so “bi-” records a nomenclature that has since been inverted. The same dictionary carries the practical consequence for a photographer of that period: the white powder ordinarily sold as carbonate of soda by the pharmaceutical chemist is a bicarbonate, and is not used in ordinary photographic operations. A darkroom worker who bought “carbonate of soda” over a counter and mixed it into a developer got a bath that would barely develop, which is why the entry says so.

Its own photographic career is short and specific. Eder puts it in a toning bath in 1858, when separate gold baths were being worked out; Wall still lists toning as its use in 1912. A century later Kodak reaches for it not to make a picture but to make a waste stream fit to leave the darkroom — a fair description of what a mild alkali is for.