Chromium hydroxide is amphoteric (or should be - there is a ton of literature of it)
But, trying to use this, to separate iron hydroxide from chromium hydroxide does not work (or at least for me)
Original solution : stainless steel + HCl.
Making alkaline with
a)NaOH
b)Na2CO3
till ph 8-9. (best ph for precipitating chromiun hydroxide)
washing 10 times by decantation the mixed hydroxides.
this is the test:
Taking an aliquot of 5ml of the hidroxides and adding NaOH 6M til Ph 14+ (few drops)
So by hypothesis, iron hydroxide should not dissolve and chromium should be dissolved.
filtering (by decanting, becasue its difficult to filter iron hidroxide) and then the filtrate (which should contain the chromium) is acidicified with
HCl till ph 8. So chromium should precipitate.
No precipitate, but clear solution...
So, why????
few ideas:
1) when precipitating a solution which could give iron and chromium hydroxides, you get no "pure" metal hidroxide, but a mixture hydroxide in which
chromium is not amphoteric, or iron hinders amphoterism of chromium.
(reference:
In the absence of iron the final product is typically of the form Cr(OH)3·xH2O whereas in the presence of iron the precipitate is a mixed
Fe(1−x)Crx(OH)3 phase. In this study, we report on the synthesis, characterisation and stability of mixed (Fex,Cr1−x)(OH)3 hydroxides as compared
to the stability of Cr(OH)3. We established that the plain Cr(III) hydroxide, abiding to the approximate molecular formula Cr(OH)3·3H2O, was
crystalline, highly soluble, i.e. unstable, with a tendency to transform into the stable amorphous hydroxide Cr(OH)3(am) phase. Mixed
Fe0.75Cr0.25(OH)3 hydroxides were found to be of the ferrihydrite structure, Fe(OH)3, and we correlated their solubility to that of a solid solution
formed by plain ferrihydrite and the amorphous Cr(III) hydroxide
Synthesis, characterization and stability of Cr(III) and Fe(III) hydroxides
N. Papassiopi, K. Vaxevanidou, C. Christou, E. Karagianni, G.S.E. Antipas https://doi.org/10.1016/j.jhazmat.2013.09.058
Regretfully I dont have pure Cr(OH)3 to test its amphoterism with/without iron hydroxide to test my hypotesis....
Also, can someone provide this document? (could not get access)
To understand better chromium hydroxide amphoterism in highly alkaline media:
Über die Alterung des Chromhydroxydes, sowie über Alkalichromite und ihre Lösungen
R. Fricke, O. Windhausen
I'm led to believe Cr(OH)3 is of mediocre amphotericity, but if you want to separate iron from chromium, add some hydrogen peroxide to turn the
chromium into chromate.RU_KLO - 20-11-2024 at 10:14
yes, thats the ultimate goal. To get the chromate, but Im not getting good results.
thats why im searching for ways to separate chromium (III) from other metals before oxidizing.
the best separation was with oxalate (as chromium oxalate is soluble and other metal oxalates are not - there are exceptions but are seldom)
But oxidizing or decomposing chromium oxalate is difficult. did not find a way...
(read somewhere that KOH could decompose chromium oxalate, but did not get good result) DraconicAcid - 20-11-2024 at 10:40
yes, thats the ultimate goal. To get the chromate, but Im not getting good results.
Really? We use that for separation of iron from chromium in a qualitative analysis lab, and it seems to work beautifully.AvBaeyer - 20-11-2024 at 19:02
Chromium (iii) hydroxide is amphoteric, but probably takes too long to dissolve.
If chromium oxalate is soluble, what about a soluble copper or calcium salt? Both copper and calcium oxalates are very insoluble and drop out, leaving
chromium (iii) sulfate or chloride, for example.
