>And speaking of bichromate (or dichromate) solubility -- Cassells (1911,
>as I recall) says one part ammonium bichromate dissolves in 4 parts water
>(which if I'm figuring correctly would indeed make the 33% solution spoken
>of), but a 1908 BJP Annual table says one part in 5, which would make a 25
>% solution. (I mix 26% & see no precipitate, even in cold studio.) Both
>sources agree, BTW, that sodium bichromate makes a 100% solution -- one
>part bichromate, one part water.
There has been so much misinformation on the "dichromate" (called
"bichromate" in older documents) situation, that it might be good to put the
facts down to prevent further speculation:
Compound Solubility at 0 degC 20 degC 40 deg C
pH @ 10%
Ammonium dichromate 15.16% 26.67% 46.14%
3.45
Potassium dichromate 4.30% 11.70% 20.90%
3.57
Sodium dichromate 70.60% 73.18% 77.09%
3.50
Note that the solubility of ammonium and potassium compounds is very
sensitive to temperature, while the sodium salt with its high solubility is
less so. There are a few problems with the sodium dichromate: (a) it is
hygroscopic/deliquescent (absorbs moisture from the air) so may have
variable moisture content, and (b) the crystals or powder normally are
formed with water within the molecule and all the figures for weight must be
normalized for the specific compound - this is not necessary when preparing
saturated solutions (which is probably why the "ancients" used all of these
compounds that way).
All dichromates are toxic: the solid forms are pulmonary irritants, and
destroy mucous membranes (such as the nasal passages, not to mention more
intimate areas). The solutions are cutaneous irritants, and the authorities
recommend using rubber gloves or equivalent when handling. In addition, some
people are allergic to the stuff.
Ammonium dichromate is used in pyrotechnics (fireworks) as it is quite
flammable.
Note the pH factors in the table: they are all quite close, showing there
should be little difference in buffering required. A pH of 3.5 is acidic;
compare with weak acetic acid solutions, which are about 3. Dichromates are
incompatible with all metal salts, as metal chromates are all insoluble (and
usually colored - many pigments are made from metal chromates).
You can see from the table that the high solubility of the sodium dichromate
could lead to serious errors if you try to use it interchangeably with the
potassium compound, with its low solubility. Percentage by percentage, all
three should produce the same effect; by ml of a saturated solution, they
would all be quite different. Another factor is always the effect of the
anion (ammonium, potassium, or sodium) on the other ingredients of the
working solution and the base material.
As to ambient temperature and its effect on the solutions, you can see there
is little problem with the sodium, and considerable with the potassium. To
correct this, you could stabilize the temperature (not hard to do; it's done
all the time in color processing), or work out a table showing the molar
content of the compound at the various temperature you might encounter.
But, then, sometimes the odd effects of unstabilized chemicals may be very
creative!!
Have fun.
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net