Sodium carbonate decahydrate

This is the third of the course’s three sodium carbonate pages, and the three exist for one reason: a formula that says only “sodium carbonate” is ambiguous by a factor of 2.7, and this grade is the weak end of that range. Ten molecules of water sit in the crystal lattice for every carbonate ion, so only about 37 per cent of what you weigh is alkali.

In photography

It does what the other two grades do. Carbonic acid is very weak, so a dissolved carbonate is alkaline, and the carbonate sits in equilibrium with bicarbonate and hydroxide as a reservoir that releases alkali as development consumes it. OpenStax gives the second ionisation constant of carbonic acid as 4.7 × 10⁻¹¹, a pK_a of 10.33, and that is where the pair holds the pH. The full account is on the anhydrous page. The water of crystallisation takes no part in it whatever.

What it changes is the balance, and only the balance.

🧮Do the maths

Relative molecular masses: anhydrous 105.99, monohydrate 124.00, decahydrate 286.14. Per gram of salt:

• 1 g anhydrous = 1/105.99 = 0.009435 mol of carbonate
• 1 g monohydrate = 1/124.00 = 0.008065 mol
• 1 g decahydrate = 1/286.14 = 0.003495 mol

So the decahydrate carries 37.0 per cent of the carbonate a gram of the anhydrous salt carries, and 43.3 per cent of the monohydrate’s. To replace anhydrous with decahydrate, multiply the weight by 286.14/105.99 = 2.70. To replace the monohydrate, multiply by 286.14/124.00 = 2.31.

Kodak’s 1928 primer reaches the same place independently, quoting the three commercial forms as containing 37 per cent, 85 per cent and about 98 per cent carbonate. And Mike Ware’s Chrysotype Manual prints the equivalence as four weights that do one job — 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 — of which 20.0/7.4 = 2.70, the molar ratio to two figures. Three sources, one number.

What a nineteenth-century formula means. Wall’s 1912 dictionary takes sodium carbonate to mean this grade — washing soda, sal soda, soda crystals — and it is the grade a Victorian formula in ounces and grains assumes unless it says otherwise. It is also the grade Wall distrusts: he warns that the commercial article is of indefinite strength and usually contains much sulfate. A decade later Kodak’s primers list all three forms and prefer the dry powder on the ground that the composition of the crystals is uncertain; by 1949 Kodak Ltd was simply telling readers to use the anhydrous salt wherever a formula says carbonate, and to scale up if all they had was crystals.

Properties

Transparent or colourless crystals that effloresce in air. The record’s two melting points disagree — 30 °C from the Merck Index, 32 to 34 °C from an Alfa Aesar sheet — and the course quotes both rather than picking one. Either way the practical fact is the same and it is unusual: these crystals melt in a warm room, dissolving in their own water of crystallisation, which is why a jar left on a windowsill can turn to slush.

Efflorescence is the property that matters. Kodak’s primer records that the decahydrate, left in air, loses nine of its ten waters and falls back to the monohydrate, the last water being removed only with difficulty by heating. That is not a slow degradation to a slightly different material: it is a migration from 37 per cent carbonate to 85 per cent, a change of a factor of 2.3, happening invisibly inside a tub in an ordinary room.

✕Common mistake

A partly effloresced jar of washing soda is not a hydrate at all. It is a mixture of decahydrate and monohydrate in unknown proportion, and its carbonate content is somewhere between 37 and 85 per cent with nothing on the label to say where. That is exactly Wall’s “indefinite strength” and exactly Kodak’s “the composition of the crystals is uncertain”, and it is why both of them, seventy years apart, told readers to buy the anhydrous salt instead.

If a formula specifies crystals and crystals are what you have, use a jar that has been kept tightly closed, and if the surface is powdery rather than glassy, do not weigh from it.

On solubility this page has a gap and says so. The PubChem record carries no solubility figure for the decahydrate. The numbers on the anhydrous page are for the anhydrous salt and are not transferred here. A saturated solution reaches the same carbonate concentration whichever grade it was made from; the weight of decahydrate required is 2.70 times larger, and to make a saturated stock, add solid until no more dissolves.

Handling

Two ECHA entries cover this hydrate — 186 company reports between them — and both give Warning, the exclamation-mark pictogram and one statement, serious eye irritation, at 100 per cent. That is unanimous and it is thin: the anhydrous salt has 8,784 reports behind it. Safe Work Australia, filing against this CAS number, takes the harsher view — Danger, H314 for severe skin burns and eye damage, and H335 — and the same authority files the same view against the anhydrous entry. Since the ion is identical in all three grades, the course applies the harsher reading here as it does there, and treats Chemical Safety Card 1135, written for the anhydrous salt, as the best available description: severe eye irritation, mild skin and respiratory irritation, dust concentrations climbing quickly on dispersion, dermatitis on repeated contact. Neither EH40 nor the NIOSH Pocket Guide sets an exposure limit for sodium carbonate in any grade, and EH40 states that absence from its list does not indicate that a substance is without risk.

Why Level A. Against the course rubric this meets the Level A criteria at the strengths a developer uses — the classification tops out at irritant, or corrosive to the eye on the severest reading, at tens of grams per litre; nothing is heated; the waste is spent developer, collectable. Splash goggles rather than glasses for the solid is a Level B control taken by preference.

⚠The one operation that takes this out of Level A

Making and handling a concentrated stock. Level B’s first criterion is the handling of concentrated acids or alkalis, and a near-saturated carbonate solution is a concentrated alkali by any reading — more so on Safe Work Australia’s, which calls the substance capable of severe skin burns and eye damage. Because this grade is 2.7 times heavier per mole, a “concentrated stock” made from crystals looks reassuringly dilute on the balance and is not. Goggles, apron, eyewash within reach, and a labelled bottle that names the grade as well as the strength.

Waste

The stream is the developer it was mixed into, and it leaves alkaline. Kodak’s J-52 publication puts the pH window sewer codes most often set at 5.6 to 9.4 and the biochemical oxygen demand of typical photographic effluent at 100 to 1,000 mg/L against a usual regulated mean of 350; a concentrated carbonate stock sits above the pH window. Any bath that has touched film or paper carries silver, and recovery comes first. ILFORD’s guidance for domestic users in the United Kingdom is to bottle wastes separately, label them and take them to a household waste and recycling centre; the disposal caveat governs. Never into a bottle that has held an acid: the reaction foams and evolves carbon dioxide, and a closed bottle is the wrong place for it.

History

Washing soda is the oldest of the three grades in photographic use simply because it is the one the Leblanc and Solvay processes delivered to the household — soda crystals, sold by the pound for laundry, and picked up by photographers because it was on every shelf. Every complaint the trade literature makes about it follows from that origin: it was an industrial commodity of variable purity, sold with sulfate in it, and it did not keep. The move to the monohydrate and then to the dry powder is photography deciding that a reagent whose composition drifts on the shelf is not a reagent, and it is the same decision the course records on all three pages: write the grade on the container, because “sodium carbonate” on a tub tells the next person nothing they can weigh with.