Judy Seigel wrote:
<<That is, potassium dichromate is the least soluble in water (15%?),
so it has the least dichromate per unit & the least sensitivity; ammonium
dichromate will make about a 26% (29%?) solution, and is more sensitive;
sodium dichromate I believe goes to about 50% and so is the most
sensitive.>>
Although it will make very little difference to the results I would like to
point out that equal masses of the three solids do not contain equal amounts
of dichromate:
sodium dichromate is normally Na2Cr2O7.2H2O, molar mass 298.
The percentage of chromium in this is thus 2*52/298 = 35%
Solubility is, as Judy states about 1 part in 1 of water
potassium dichromate is K2Cr2O7, molar mass 294,
The percentage of chromium is thus very similar at 2*52/294 = 35%
The rough figure I have to hand for solubility is 1 part in 10.
ammonium dichromate is (NH4)2Cr2O7, molar mass 252
The percentage of chromium is higher at 41%
Rough figure for solubility is 1 part in 4
You would get equal concentrations of dichromate in solution by taking
10.00 g of sodium dichromate or
9.87 g potassium dichromate or
8.46 g of ammonium dichromate.
Just a little nit-picking from an ex-chemist. Hope I got my sums right!
Peter Marshall
petermarshall@cix.compulink.co.uk