chornedsnorkack - 26-3-2024 at 10:41
How stable are hydroxochromate solutions?
Production of chromates is commonly done with strong oxidants in aqueous solution - like hypochlorite or peroxide.
Yet production of chromates is also accomplished in molten salts with air dioxygen alone!
Do alkaline hydroxochromate solutions react with air oxygen at any appreciable rate? Like
4Na3Cr(OH)6+3O2=4Na2CrO4+4NaOH+10H2O?
There are alternative reactions for hydroxochromate solutions to react, too. Like
2Na3Cr(OH)6+3CO2=3Na2CO3+2Cr(OH)3
"Cr(OH)3" is often described as an amorphous gel of indefinite stoichiometry. Is this still the case when it precipitates from a solution
where it has good solubility, over a period of years, as CO2 diffuses in and water out of the bottle headspace?
woelen - 27-3-2024 at 00:21
The reaction between oxygen and alkaline solutions of chromium(III) indeed does occur, but very very slowly. On geological time scales, this process
led to the formation of chromate-based minerals. For making chromates in an industrial or even home lab setting, this is not a sensible process
though.
So, no, with air alone in practice you cannot prepare chromates or dichromates from chromium(III) in decent amounts with aqueous chemistry. Heated
solutions of chromium(III) in molten alkalies are another story. the reaction rate then is sufficiently high to make chromium(VI) with oxygen from
air. But working with larger quantities of molten alkalies and bubbling air through these does not sound like a safe and amateur-friendly thing. Using
a molten mix of alkali, Cr2O3. and an oxidizer like KNO3 is less of a hassle, but still not something most amateurs would like to do.