Sodium carbonate monohydrate

One water molecule per formula unit is the whole difference between this page and sodium carbonate (anhydrous). In the beaker there is no difference at all. On the balance there is about a sixth — enough to make a developer that does not match its own instructions.

In photography

It does what the anhydrous salt does: carbonic acid is very weak, so a dissolved carbonate is alkaline, and the ion 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 water of crystallisation takes no part in it.

⚗The chemistry

Water of crystallisation is water built into the crystal lattice, not water bound to the carbonate ion chemically. Dissolve 124.00 g of Na₂CO₃·H₂O and you release one mole of carbonate ions and 18 g of water into a solution that already contains far more; dissolve 105.99 g of Na₂CO₃ and you release exactly the same mole of carbonate. The only consequence is arithmetic:

• 1 g of the monohydrate = 1/124.00 = 0.008065 mol of carbonate.
• 1 g of the anhydrous salt = 1/105.99 = 0.009435 mol of carbonate.

So a gram of the monohydrate carries about 17% less alkali, and replacing anhydrous with monohydrate means multiplying the weight by 124.00/105.99 = 1.17.

The grades, and what a formula means by “carbonate”. Kodak’s primers of 1924 and 1928 give the same 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%. Between the extremes lies a factor of 2.6, so a formula that says only “sodium carbonate” is ambiguous in a way that matters more than almost any other ambiguity in a developer. The two Kodak primers both prefer the dry powder, on the ground that the composition of the crystals is uncertain, and Kodak Ltd’s 1949 handbook says plainly that where a formula specifies carbonate, the anhydrous salt is the one to use.

A modern statement of the equivalence. Mike Ware’s Chrysotype Manual neutralises 12.5 g of 3,3′-thiodipropanoic acid with whichever alkali is to hand, and prints the four weights side by side: 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. The monohydrate against the anhydrous is 8.7/7.4 = 1.18, which is the molar ratio of 1.17 rounded to the nearest tenth of a gram. Ware’s list is the most useful sourced conversion the course has found, because it was written to be used with a balance.

Why a hydrate can be the steadier thing to weigh. Kodak’s primer records that the decahydrate, left in air, loses nine of its ten waters and falls to the monohydrate, the last water being removed only with difficulty by heating; the anhydrous powder is hygroscopic and travels the other way. The monohydrate is therefore what ordinary room air tends to settle on — an inference from those two statements, not a claim the primer makes.

Properties

Colourless to white odourless powder or granules, hygroscopic. The relative molecular mass PubChem computes is 124.00 against 105.99 for the anhydrous salt, and 18.02 of that is water.

On solubility the course has a gap and states it. The PubChem record for the monohydrate carries no solubility value at all, and the figures on the anhydrous page — 30 g per 100 mL at 20 °C from the ILO-WHO Chemical Safety Card, HSDB’s curve from 6% w/w at 0 °C to 28% at 30 °C — are quoted for the anhydrous salt. A saturated solution reaches the same carbonate concentration whichever grade you start from, but the weight of monohydrate needed is about 1.17 times larger, and the course will not silently move a number between grades. Make a saturated stock by adding solid until no more dissolves.

pH behaviour, stability and reactivity follow the anhydrous entry: alkaline in water, a reservoir rather than a dose, and a solid that fizzes when it meets an acid.

Handling

The aggregated ECHA notifications on PubChem give Warning, the exclamation-mark pictogram and one statement: serious eye irritation, from 97.7% of the 219 reports that classify it, with 5 saying it meets no criterion. Six notifications is a thin base beside the seventeen behind the anhydrous salt, and the authorities that take a harsher view — Japan’s NITE-CMC and Safe Work Australia, both giving Danger and serious eye damage — filed against the anhydrous entry. The ion is the same, so this course applies the harsher view here as well, and takes the ILO-WHO Chemical Safety Card written for the anhydrous salt as the best description available: severe eye irritation, mild skin and respiratory irritation, airborne dust climbing quickly on dispersion, dermatitis on repeated contact.

Why Level A. Against the course rubric, this handling meets the Level A criteria: 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, collectable. Splash goggles rather than glasses for the powder is a Level B control taken by preference. 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.

⚠Safety

Gloves and eye protection throughout, goggles for the dry salt, and enough ventilation to keep dust out of the air: HSE’s COSHH essentials sheet P1 asks for more than five air changes an hour from a powered fan with a through draught. Keep it from acids. The label is the other control on this page — write the grade, not just the name, on every container and every stock bottle, because “sodium carbonate” on a tub tells the next person nothing they can weigh with.

Waste

The stream is the developer it was mixed into, and it leaves alkaline. Kodak’s J-52 publication gives a pH window of 5.6 to 9.4 as the limit sewer codes most often set, and 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; 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 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 put a carbonate waste into a container that has held an acid: the reaction foams and evolves carbon dioxide, and a closed bottle is the wrong place for it.

History

The three grades are the history. Wall’s 1912 dictionary takes sodium carbonate to mean the decahydrate — washing soda, sal soda, soda crystals — and warns that the commercial article is of indefinite strength and usually contains much sulfate. A decade later Kodak’s primer lists all three, explains that the decahydrate crumbles in air to the monohydrate, and concludes that because the composition of crystals is uncertain it is better to prepare the pure dry carbonate. By 1949 Kodak Ltd was simply telling readers to use the anhydrous salt and to scale up if they had crystals.

The monohydrate survives in the middle of that argument as the honest compromise: a definite composition, unlike weathered decahydrate, and one that already carries the water the anhydrous powder would otherwise take from the room. What it demands in exchange is that the formula and the tub agree on which grade is meant — which is why the course writes the hydrate into the name of every entry, and why an undifferentiated “sodium carbonate” in an old formula is a question rather than an instruction.