bismuthate - 31-5-2014 at 05:35
When I mixed equal amounts of sodium thiosulfate, iodate and CrCl3 in dilute solution a fantastic red/orange color can be observed. I believe that
this is due to the formation of a chromium-thiosulfate complex. With excess thiosulfate in acidic media the solution turns green again after around a
minute. What do you people think about this?
bismuthate - 31-5-2014 at 08:49
Well after more experimentation I found that the color is iodine. How is the I2 produced?
woelen - 31-5-2014 at 10:58
Thiosulfate is a reductor and iodate is a strong oxidizer.
Iodate is easily reduced to iodine:
2IO3(-) + 12H(+) + 10e --> I2 + 6H2O
Thiosulfate is oxidized to sulfate by strong oxidizers and this reaction produces a lot of H(+):
S2O3(2-) + 5H2O --> 2SO4(2-) + 10H(+) + 8e
I leave it to you to derive the total reaction between iodate and thiosulfate. The reaction produces some H(+), the liquid becomes acidic and you get
a deep red/brown color of iodine. Iodine can further oxidize thiosulfate if the latter is present in excess amount.
The Volatile Chemist - 1-6-2014 at 12:34
Though woelen's probably right, thiosulfate complexes that I've always made have seemed very unstable, but this is just an observation, not a fact.