Potassium carbonate (anhydrous)

Many an alkaline developer in the old formularies is printed with either carbonate, and some give both on the same page. The chemistry hardly changes. What changes is how much you can get into a litre, what it costs, and whether the tub is still dry when you open it.

In photography

It is the same buffer with a heavier counter-ion. The carbonate ion does the work: carbonic acid is weak, so a dissolved carbonate is alkaline, and the carbonate–bicarbonate pair holds the pH near the pK_a of 10.33 that follows from the second ionisation constant OpenStax gives, 4.7 × 10⁻¹¹. Potassium changes none of that. It changes the weight of salt you need and how much of it will dissolve.

⚗The chemistry

Two carbonates, one job, and the balance is where they differ. From the PubChem molar masses, K₂CO₃ is 138.21 and Na₂CO₃ 105.99, so a mole of carbonate costs 30% more grams as the potassium salt. Wall’s 1924 Photographic Facts and Formulas prints exactly that arithmetic without deriving it: in the eikonogen developer, the alkali solution of 75 g of dry sodium carbonate may be replaced by “an equivalent weight of potassium carbonate, namely 98 g”. That is a ratio of 1.31 against a molar ratio of 1.30 — a period formulary and a modern molar mass agreeing to within one per cent.

Why anyone bothers: solubility. Chemical Safety Card 1588 gives 112 g per 100 mL of water at 20 °C, against 30 g for sodium carbonate — HSDB’s figures, 111 g per 100 g at 25 °C rising to 155.7 g at 100 °C, say the same. Kodak’s 1924 primer records the practical consequence in one line: potassium carbonate is more soluble and a somewhat stronger alkali than sodium carbonate, but is more expensive and absorbs water very readily. Nearly four times the solubility is what lets a concentrated one-solution developer carry its alkali without crystals settling out of the stock bottle in a cold darkroom.

What more of it does. Wall’s 1924 formulary is blunt where it matters: given the choice of a 5 per cent solution of either carbonate in a two-solution edinol developer, “the potash works more rapidly and gives denser negatives”. Kodak’s D-11 process developer names its alkali “Potassium Carbonate (or Sodium Carbonate)” — the 1924 primer’s wording, which settles a damaged line in the 1928 revision — and gives one weight for either, 25 g per litre in 1928, with no further comment. Wall’s 1912 dictionary records the cost in timing: in factorial development, where the total time is a multiple of the time the image first appears, substituting potassium carbonate for sodium carbonate requires a factor from one-fourth to one-half larger.

A use outside the tray. Mike Ware’s Platinomicon lists salts whose saturated solutions hold a known relative humidity in a closed box, for conditioning paper before a platinum or palladium print: potassium carbonate holds 44% RH at 20 °C, and sodium carbonate decahydrate, in the same table, 92%. The property being used there is the deliquescence that makes the salt a nuisance on the balance.

Properties

Colourless hygroscopic crystals or a hygroscopic white powder, melting at 891 °C, density 2.29 g/cm³, relative molecular mass 138.21. It is insoluble in ethanol and acetone. Chemical Safety Card 1588 describes the aqueous solution as a medium-strong base, which is the same phrase the card for sodium carbonate uses.

Deliquescence is its defining working property. Wall’s 1912 dictionary calls it extremely deliquescent — absorbing moisture from the air until it becomes a pasty mass — and adds that it nearly always contains about 16 per cent of water of crystallisation as sold. Kodak’s 1924 primer says it absorbs water very readily and must be kept in well-sealed bottles. Haz-Map records that it is supplied anhydrous, as a sesquihydrate, and as a 47% solution, so the identity of what is in the jar is a real question. Two consequences follow. A tub that has gone sticky is being weighed partly as water, and the developer comes out weak by an amount you cannot calculate. And a formula written for “potassium carbonate” without a hydrate is ambiguous in the same way the sodium formulas are — Wall’s own entry writes it as K₂CO₃·3H₂O with a relative molecular mass of 192, which is neither the anhydrous salt nor the sesquihydrate the modern trade sells — and his 16 per cent of water does not fit that formula either, since three waters on 192 is 28 per cent. The course reports both figures as the entry gives them and does not reconcile them.

Wall’s dictionary also carries the warning that still applies: it should not be confounded with the bicarbonate or acid carbonate of potash, KHCO₃, which is a much less active salt.

Handling

The aggregated ECHA notifications on PubChem give Warning and the exclamation-mark pictogram, with four statements: serious eye irritation from 97.7% of the classifying reports, skin irritation from 45%, respiratory irritation from 42.3% and harmful if swallowed from 15.4%. Twenty-five of the 2,657 reports say it meets no criterion. Two national authorities are harsher — Japan’s NITE-CMC gives Danger with serious eye damage and adds aquatic-hazard statements, and Safe Work Australia gives Danger with eye damage and respiratory irritation. Chemical Safety Card 1588 puts it simply: irritating to the eyes, skin and respiratory tract, with sore throat and cough on inhalation and redness and pain on skin or eye contact.

Why Level A. Against the course rubric, this meets the Level A criteria: the classification tops out at irritant, or eye-corrosive on the severest reading, at the tens of grams per litre used; nothing is heated above 50 °C; and the waste is spent developer, which is collectable. Its skin and respiratory statements pull two controls up from Level B. Splash goggles rather than glasses, because eye damage is on the record from two authorities; and a particulate mask if weighing raises visible dust, because a respiratory-irritation statement appears in more than four notifications in ten, which is a far stronger signal than the sodium salt carries. Neither EH40 nor the NIOSH Pocket Guide sets a workplace exposure limit for potassium carbonate, and EH40 states that absence from its list does not indicate that a substance is without risk.

⚠Safety

Gloves, goggles and a tray under the balance; close the tub between scoops, because the salt is taking water from the room while it is open. Add it to water rather than water to it. Keep it from every acid: Chemical Safety Card 1588 records a violent reaction, and CAMEO’s carbonate-salts group notes that the acid-base reaction generates heat and releases carbon dioxide. The card also records reactions with chlorine trifluoride and with powdered metals, neither of which belongs anywhere near a darkroom.

Waste

Spent developer, alkaline. Kodak’s J-52 publication gives a pH window of 5.6 to 9.4 as the limit sewer codes most often set, along with a usual regulated biochemical oxygen demand of 350 mg/L against 100 to 1,000 mg/L for typical photographic effluent; a concentrated carbonate stock is above that pH window.

One classification is a disposal instruction in itself. NITE-CMC’s 2023 revision gives H402 and H412, harmful to aquatic life with long-lasting effects, which the ECHA aggregate does not carry. Where two sources disagree on an environmental statement, this course takes the stricter one: collect rather than dilute. Kodak’s J-215 publication uses the same salt in the other direction, listing 300 mL of 10% potassium carbonate per litre of EKTACOLOR RA-4 bleach-fix overflow as a way of raising its pH to 7.5 to 8.0 — half as much again as the sodium carbonate in the same table, a little more than the 1.30 the molar masses alone would ask for.

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

Its names come from the apothecary rather than the darkroom. Wall’s 1912 dictionary lists what a Victorian photographer would have found on a label — Pearlash, Subcarbonate of Potash, Salt of Tartar, Salt of Wormwood, Potash — and describes its preparation by lixiviation and purification of the ashes of wood and vegetable matter. Pearlash, the dictionary says elsewhere, is simply a synonym for the impure salt.

Its photographic career is the story of a substitution that never quite took. Wall’s own 1924 formulary offers it as an alternative in developer after developer, often with a note that it works faster; Kodak’s primers of 1924 and 1928 each give it a single paragraph, as something sometimes used instead of sodium carbonate, and both dismiss it on the same two grounds — it costs more and it takes up water. The sodium salt won the formularies because it is cheap and stays dry, and the potassium salt kept the jobs where its solubility earns the price: liquid concentrates, and a bottle of saturated solution holding the humidity steady in a printing box.