Stock solution crystallised in cold storage

Lifted from chemical storage and incompatibilities, with the mechanism from solubility and precipitation.

What you see

A layer of crystals in the bottom of a concentrated stock bottle, and a liquid above them that looks entirely normal. The bottle has usually been somewhere cold — a garage in January, an unheated shed, the back of a fridge.

Poured off and used, the clear liquid gives a bath that is weak in a way the label cannot explain, and weak in whichever constituent came out of solution rather than weak in all of them equally.

Likely causes

1. Storage below the temperature at which the least soluble constituent of a concentrated stock stays dissolved. Kodak’s guidance for amateurs asks for unmixed concentrates to be kept between 5 and 30 °C, and the lower bound is there for exactly this reason.
2. An unheated outbuilding used as chemical storage. A garage in a British January goes well below 5 °C and can go below freezing.
3. A stock mixed close to saturation at room temperature and then cooled. Nothing was done wrong at the bench; the margin was simply not there.

What is happening: the chemistry and physics

A saturated solution is not full — it is at a dynamic balance where dissolving and crystallising run at equal rates. For most solids that balance moves with temperature, and the amount that can stay in solution falls as the solution cools. A concentrated stock is by definition close to that limit, so it is the first thing in the darkroom to feel a cold night.

The consequence that matters is one Kodak Ltd’s 1949 handbook states directly and that is easy to get backwards — the precipitate often contains the most important constituents of the solution. The crystals are not an impurity that has helpfully separated itself out. They are, very often, the developing agent or the alkali, which is why the clear liquid above them is not a weaker version of the stock but a differently composed one.

✕The wrong instinct is to filter it

Filtering the bottle and using the clear liquid is the natural response and it is precisely the wrong one. It removes the constituent that dropped out, keeps the ones that did not, and leaves you with a solution whose composition you cannot state.

Diagnostic questions

• Where has the bottle been, and how cold did that place get? A minimum-reading thermometer in the storage space answers this once and for good.
• Do the crystals redissolve on warming? That is the confirmation that this is a solubility effect rather than a decomposition or a reaction with something that got into the bottle.
• Is the supernatant weak in everything, or in one thing? A developer whose alkali has crystallised out behaves like a low-pH developer, which is a different fault from a developer that has simply aged.
• Was the stock recently mixed and never previously stored cold? A stock that has survived a year of ordinary shelf temperatures and crystallised in one cold week is telling you about the week.

Corrective action

Warm the whole bottle gently and evenly until the crystals redissolve, then mix it thoroughly before use. Kodak’s own instruction is to redissolve by warming rather than to discard, precisely because the precipitate carries the constituents you need.

Do not decant the clear liquid. Do not filter. Do not top the bottle up with water to compensate for what appears to be missing.

If the crystals will not redissolve on warming to ordinary room temperature, the fault is not crystallisation and the bottle should be retired rather than forced with heat — a stock that has decomposed is a different problem and warming it will not restore it.

Prevention

Store unmixed concentrates and stocks between 5 and 30 °C, away from direct sunlight and away from sources of heat. That range is a specification, not a suggestion, and both ends of it do work.

Do not use an unheated garage, shed or outbuilding as chemical storage. If there is no alternative, keep a minimum-reading thermometer with the bottles and read it, so that a bath which behaves oddly in February can be explained rather than guessed at.

Record the storage location on the container’s own label alongside the dates, so that a bottle which has moved can be traced.