Hydrate substituted for an anhydrous salt at the same mass

Lifted from concentration and dilution, whose hydrates section works both substitutions in full.

What you see

A bath that is dramatically wrong in a direction the formula cannot explain. Substituting the anhydrous carbonate for the decahydrate at the same mass puts 2.70 times the intended alkali into a developer, and pH governs developing-agent activity — so the negatives come back contrasty, dense and fogged, and nothing about the developing agent or the time will account for it.

The reverse substitution is quieter and therefore worse. Weighing the pentahydrate of sodium thiosulfate where the formula asked for the anhydrous salt puts only 63.7 per cent of the intended fixing agent into the bath, which does not fail outright; it fixes, slowly, and it exhausts early.

Likely causes

1. The jar on the shelf is a different hydrate from the one the formula names, and neither the jar nor the formula was read carefully enough to notice.
2. The formula is older than about 1950. The British Journal Photographic Almanac states its convention explicitly — “sodium carbonate” and “sodium sulphite” name the crystallised forms, and the dry or anhydrous forms are named in qualification. A modern reader’s default is the opposite.
3. The formula names the salt with no hydrate state at all, and the ambiguity was resolved by guessing.

What is happening: the chemistry and physics

The chemistry wants a number of ions, not a mass. Water of crystallisation is part of the mass you weigh and none of the chemistry you want, so the two masses stand in the ratio of the relative molecular masses.

mass of form B = mass of form A × (M_B / M_A)

Substituting one form of a salt for another

M is the relative molecular mass of each form. Sodium carbonate is 105.99 anhydrous and 286.14 as the decahydrate; sodium thiosulfate is 158.11 anhydrous and 248.19 as the pentahydrate.

🧮The two substitutions, worked

Carbonate. A formula asks for 100 g of the decahydrate crystals and you have the anhydrous powder - 100 × (105.99 ÷ 286.14) = 37.0 g. Weigh 100 g of the anhydrous salt instead and you have put in 2.70 times the sodium carbonate the formula wanted.

Thiosulfate. A formula asks for 100 g of the anhydrous salt and you have the pentahydrate - 100 × (248.19 ÷ 158.11) = 157 g. The extra 57 g is water, and it goes into the solution, which is one more reason the make-up-to-volume habit is not fussiness.

Diagnostic questions

• What does the jar say? Not “sodium carbonate” but the full name with the hydrate state and, ideally, the CAS number, because the two forms have different ones.
• What does the formula’s own source say its convention is? A publication older than about 1950 that names a salt without qualification is likely to mean the crystals.
• Is the error a factor of about 2.7, 1.57 or 0.637? Those three numbers are the signatures of the commonest substitutions in this course and none of them looks like an ordinary weighing error.
• Was the developer more active than expected, or less? An excess of carbonate raises the pH and therefore the activity, the contrast and the fog; a deficit of thiosulfate lowers the fixing capacity and lengthens the clearing time.

Corrective action

Recompute the mass with the ratio above and remake the bath. There is no salvage, because you cannot add “the missing water of crystallisation” back to a solution in a way that restores the intended concentration of everything else at the same time.

Anything already processed in the bath should be recorded as processed in a solution of a stated, different composition. That is a usable record; “developer, D-76-ish” is not.

Prevention

Label every solid with its full name and its hydrate state, and keep the two forms of the same salt apart on the shelf where you have both.

Before weighing from a formula older than about 1950, find the source’s own statement of its conventions. If it has none, write that in the notebook and treat the first batch as a test rather than as a production mix — which is also the reason an undated, unattributed formula off the internet is not usable at all.

Prefer anhydrous salts where the formula’s source allows it, because an anhydrous salt cannot quietly change its water content on the shelf the way an efflorescing hydrate can.