Some of the chemicals we use (sodium sulphite and sodium carbonate would
be examples of this) come in different forms according to whether or not
they are hydrated. In use, we would use different amounts by weight;
more of the hydrated forms is used in order to account for the water
contained in the crystals. This can be a significant source of error.
Some of my developers have contained trisodium phosphate which is (or
used to be) readily available from paint stores; the only trouble is,
they don't tell you which hydration state it is, and the only way to get
the formula right for sure is to buy the real (expensive) stuff from the
chemical dealers. Dry forms of some of these chemicals could possibly
over time and under certain storage conditions change to hydrated forms.
This would not make them useless, but would certainly change the way we
use them. Perhaps a chemist could help us with this.
Some chemicals will absorb moisture from the air and these may be a
problem. Ferric Ammonium Citrate becomes a solid mass of green concrete
up in this northern maritime climate if left in opened jars, even if you
crank the lid on tight. Very discouraging.
Skagit Valley College