wg48temp9 - 27-4-2021 at 11:09
for a good price £7.19 500g including delivery Item: 152381325518
Its a green solid soluble in water forming a very dark green solution.
Probably Cr2(SO4)3·6H2O, that's my best guess based on the Cas number see https://www.chemicalbook.com/CAS_10101-53-8.htm
[Edited on 4/28/2021 by wg48temp9]
woelen - 27-4-2021 at 23:17
I assume it is this one:
https://www.ebay.co.uk/itm/152381325518?hash=item237aa280ce:...
This is not normal chromium sulfate, but it is so-called basic chromium sulfate. Also interesting, but different. It is a soluble mixed
hydroxide/sulfate of chromium(III). I myself purchased one of these a few years ago, from the same seller. Every now and then it appears on eBay.
At Sigma-Aldrich's website it is written as Cr4(OH)2(SO4)5.
I actually have four different "chromium sulfate" chemicals:
- potassium chrome alum, KCr(SO4)2.12H2O, a dark purple solid, giving greyish/blue/purple solutions in water.
- chromium sulfate, Cr2(SO4)3.xH2O (x has a somewhat variable value). A purple solid, giving the same color in its solutions as potassium chrome alum.
- chromium sulfate, Cr2(SO4)3.yH2O (y has a different value than x). A dark green solid, giving greyish/green solutions in water. This chromium
sulfate, in fact, is a sulfate-complex and contains both hydrated Cr(3+)-ions and sulfato-chromate(III) ions. These grey/green solutions turn like the
standard greyish/blue/purple solutions of trivalent chromium on standing for a few days.
- basic chromium sulfate, the chemical offered now on eBay. This also is green, but this is a real deep bright green, while the previously mentioned
compound is very dark gray/green. Solutions of this basic chromium sulfate remain green on standing, but when they are acidified with dilute H2SO4,
then over several days, they also turn greyish/blue/purple.
[Edited on 28-4-21 by woelen]
unionised - 27-4-2021 at 23:54
The bottle on the ebay auction you cited says it is Cr2(SO4)3.H2O
There's not much guarantee of it being right, but assuming they are then it's presumably a more finely powdered (thus lighter coloured) version of the
stuff you describe as "A dark green solid, giving greyish/green solutions in water. This chromium sulfate, in fact, is a sulfate-complex and contains
both hydrated Cr(3+)-ions and sulfato-chromate(III) ions. These grey/green solutions turn like the standard greyish/blue/purple solutions of trivalent
chromium on standing for a few days."
wg48temp9 - 28-4-2021 at 01:12
woelen: yes that that is the item.
My statement about what it probably was should have been "my best guess based on the cas number see
https://www.chemicalbook.com/CAS_10101-53-8.htm".
Unionised: yes the formula ,Cas no, Ec no and molecular weight are not coherent. The powder is an intense green colour. It is not a fine powder. It
is similar to table salt in particle size.
After two days the very dark green (the colour can only be seen through the meniscus at the beaker walls) solution has not changed colour. I will
check what effect sulphuric acid has on the solution color which may support woelen's formula.
[Edited on 4/28/2021 by wg48temp9]
woelen - 28-4-2021 at 05:12
The name of the compound and the formula are not consistent. The label says chromium sulfate (basic). You can see that on the eBay picture of the
bottle. The color of the solid also is like the color of basic chromium sulfate. I am sure that the compound is basic chromium sulfate and not the
third option of my previous post. That third option has a much darker green color for similar particle size and its solutions also are different.
CAS 10101-53-8 is anhydrous chromium sulfate. I once made some of that. The anhydrous compound is insoluble in water, and also insoluble in dilute
acid. You can make that by gently heating purple chromium sulfate or dark green chromium sulfate (second and third options of my previous post) in
concentrated sulfuric acid. If you do that, you get a green solid, having a color like powdered Cr2O3, suspended in the concentrated sulfuric acid.
When the acid is added to water, then it mixes (producing a lot of heat), but the green solid does not dissolve. On allowing it to stand for many
days, it slowly dissolves. The green solid, offered on eBay, however, dissolves quickly, just by swirling a minute or so with cold water.