Sciencemadness Discussion Board

Qualitative V/Cr detection

chornedsnorkack - 23-3-2021 at 13:21

What are good reactions to spot V and Cr in aqueous solution?

They have a lot of similarities:
In acid reducing environment, they form V2+ and Cr2+ - both are blue, slowly reduce water, on addition of alkali form precipitate that rapidly reduces water
In mildly reducing environment, both V(III) and Cr(III) are green.
In oxidizing environment, both form orange H3VO4 and H2CrO4. In basic conditions, oxidation is easier, but the vanadate and chromate colour pales.

There are points of difference, too.
Cr(OH)3 dissolves in alkali, and V(OH)3 does not.
V has rich chemistry of stable V(IV), which Cr lacks.

But which of the divergent reactions provide for reliable detection of oe element in presence of the other?

unionised - 23-3-2021 at 14:37

https://en.wikipedia.org/wiki/Chromium(VI)_oxide_peroxide

DraconicAcid - 23-3-2021 at 16:14

I believe vanadium gives a red colour under the same conditions as the peroxide test, so they may interfere with each other.

Bedlasky - 23-3-2021 at 21:43

Draconic acid: Yes, vanadium give red peroxide complex.

Chornedsnorkack: Orthovanadate is colourless (and it is formed at pH>13), chromate is yellow - so you can test chromium presence by oxidizing in alkaline solution.

V(IV) make very intensively purple complex with molybdate in slightly acidic solution. Chromium give pink complex, but vanadium complex is much more intense, so Cr doesn't interfere (practically any metal doesn't interfere). Look at Metal heteropolymolybdates section in this article: https://colourchem.wordpress.com/2020/08/27/molybdenum-chame...

So proposed procedure: Oxidation in 2M NaOH by H2O2 --> yellow colour is positive test for Cr. Boiling for a while to destroy excess H2O2, than acidify to pH around 3, add excess of metabisulfite and add sodium molybdate or ammonium heptamolybdate --> purple colour is positive test for V.

[Edited on 24-3-2021 by Bedlasky]

clearly_not_atara - 24-3-2021 at 06:00

Ammonium metavanadate has a particularly low solubility in water, while ammonium chromate is much more soluble.

S.C. Wack - 24-3-2021 at 14:36

Quote: Originally posted by DraconicAcid  
I believe vanadium gives a red colour under the same conditions as the peroxide test, so they may interfere with each other.


Perchromic acid is soluble in the more polar organics. But that's not aqueous solution is it? OP and every other thread here for years seem to be constructed so that only impossible answers are possible. e.g. it's easy to separate Cr from V, but then that's not a test of one in the presence of the other. Tired of it.

DraconicAcid - 24-3-2021 at 15:32

I didn't say it was impossible, just pointing out something that might complicate things. If you're so tired, take a nap.

S.C. Wack - 24-3-2021 at 17:42

Quote: Originally posted by clearly_not_atara  
Ammonium metavanadate has a particularly low solubility in water, while ammonium chromate is much more soluble.


PS I suspect the opposite is true for ammonia plus whatever remains in solution after reduction from the high oxidation state is applied...I don't have any V to test this though...

Quote: Originally posted by DraconicAcid  
I didn't say it was impossible, just pointing out something that might complicate things. If you're so tired, take a nap.


Obviously I have something against clearly_not_atara as well? Yeah I'm the one who needs to nap.

Bedlasky - 25-3-2021 at 00:48

S.C. Wack: I don't understand where is the problem. Draconid acid just pointed out, that vanadium also form strongly coloured peroxide complex, which interfere with blue [CrO(O2)2]. Yes, you are right, that [CrO(O2)2] can be extracted in to ether, ester or higher alcohol, but I suspect that V-peroxide complex behave the same (I can test it).

However, you can use peroxide for test for V in the presence of Cr in acidic solution. Any reduced state of V is oxidized by peroxide in to V(V)-peroxide complex, while Cr remain at III oxidation state.

Quote: Originally posted by Bedlasky  
So proposed procedure: Oxidation in 2M NaOH by H2O2 --> yellow colour is positive test for Cr. Boiling for a while to destroy excess H2O2, than acidify to pH around 3, add excess of metabisulfite and add sodium molybdate or ammonium heptamolybdate --> purple colour is positive test for V.


I correct myself - I didn't realize, that peroxide form with vanadate in alkaline solution yellow complex. I don't know if it is stable (from my experience, peroxide complexes in alkaline solutions aren't stable and heating destroy them, but I didn't play with vanadium peroxide complexes, so I can't tell with sure, that this is also true for vanadium). So maybe would be better using bleach instead of peroxide for testing the presence of Cr.

S.C. Wack - 25-3-2021 at 15:43

Quote: Originally posted by Bedlasky  
S.C. Wack: I don't understand where is the problem. Draconid acid just pointed out, that vanadium also form strongly coloured peroxide complex, which interfere with blue [CrO(O2)2].


Yes in retrospect it's obvious how pointing out that perchomic acid is soluble in organic solvents is antisocial in all situations. I'm like Hamburger Man at the end of Nice Dreams. I would not have mentioned it however if the V was also soluble, since that would make it pointless.

If thousands of question-threads (that always come with conditions and fine print or are otherwise unreasonable, seemingly designed for masturbators by masturbators, with no correct answer possible) are what you want, what can I say.

Bedlasky - 25-3-2021 at 20:10

Quote: Originally posted by S.C. Wack  
Quote: Originally posted by Bedlasky  
S.C. Wack: I don't understand where is the problem. Draconid acid just pointed out, that vanadium also form strongly coloured peroxide complex, which interfere with blue [CrO(O2)2].


Yes in retrospect it's obvious how pointing out that perchomic acid is soluble in organic solvents is antisocial in all situations. I'm like Hamburger Man at the end of Nice Dreams. I would not have mentioned it however if the V was also soluble, since that would make it pointless.

If thousands of question-threads (that always come with conditions and fine print or are otherwise unreasonable, seemingly designed for masturbators by masturbators, with no correct answer possible) are what you want, what can I say.


You behave like insulted little child. Did you read my whole post? Can you on 100% say, that [VO(O2)]+ complex can't be extracted with organic solvent? I think not, because you have capability to just insult SM members which say something constructive.

Because you are not capable to find a truth, I did it for you. I poured in to the test tubes small amount of Na3VO4 solution, add some peroxide, little bit sulfuric acid and ethyl acetate/diethyl oxalate. I tried to extract complex in to the organic layer, but it didn't work in both cases. So yeah, S.C. Wack, you are right, if you add peroxide and ester in to acidic solution containing dichromate + vanadate, you can prove presence of both, V and Cr, as they peroxide complexes (V in aqueous layer, Cr in organic layer). You could tell that you read about it or even did it yourself or that you will wait for my results. Instead of it you are just spitting insults. "Excellent scientific" discussion.

[Edited on 26-3-2021 by Bedlasky]

chornedsnorkack - 26-3-2021 at 02:36

Quote: Originally posted by Bedlasky  
Draconic acid: Yes, vanadium give red peroxide complex.

Chornedsnorkack: Orthovanadate is colourless (and it is formed at pH>13), chromate is yellow - so you can test chromium presence by oxidizing in alkaline solution.


Would alkaline reducing conditions be better? V would precipitate as V(OH)3, Cr would be green. Which is a better and brighter indicator for Cr - the yellow of CrO42- or the green of [Cr(OH)6]3-?

Bedlasky - 26-3-2021 at 02:49

I think that chromate is better, because V(OH)3 is slowly oxidized by air to form brown soluble hypovanadates, which affect the colour.

clearly_not_atara - 26-3-2021 at 06:24

Quote: Originally posted by S.C. Wack  
Quote: Originally posted by clearly_not_atara  
Ammonium metavanadate has a particularly low solubility in water, while ammonium chromate is much more soluble.


PS I suspect the opposite is true for ammonia plus whatever remains in solution after reduction from the high oxidation state is applied...I don't have any V to test this though...

I don't think I understand your objection. The methodology I had imagined, for a solution known to contain Cr and suspected of containing V, should look something like this:

- oxidize under alkaline conditions, pH ~10-12, to convert V and Cr to VO43- and CrO42-

- (maybe) buffer/neutralize chromate solution so it will not deprotonate ammonium

- add dropwise to concentrated/buffered NH4+ solution to observe possible precipitation of NH4VO3

- if necessary, characterize the precipitate, if any

So yes, reduction products would not behave nicely, but you should be able to get a qualitative is-V-present without them. Wikipedia says:
https://en.wikipedia.org/wiki/Ammonium_metavanadate
"[Ammonium metavanadate] is prepared by the addition of ammonium salts to solutions of vanadate ions, generated by dissolution of V2O5 in basic aqueous solutions, such as hot sodium carbonate."

Now, as for detecting Cr in a mostly-V solution, I am much less sure how to do that, but CrO3 decomposes at ~250 C while V2O5 is stable to ~1500 C, so that seems like a clue...

[Edited on 26-3-2021 by clearly_not_atara]

S.C. Wack - 26-3-2021 at 15:52

Quote: Originally posted by Bedlasky  
You behave like insulted little child.


I see.

Quote: Originally posted by Bedlasky  
Did you read my whole post?


I quoted the part of your post relevant to what I had to say.

Quote: Originally posted by Bedlasky  
Can you on 100% say, that [VO(O2)]+ complex can't be extracted with organic solvent?


There's a thousand quotes in analysis lit over a hundred years old saying Cr and V can be told apart by ether solubility. Did not have to google that.

Quote: Originally posted by clearly_not_atara  
I don't think I understand your objection.


Wondering in the course of this if a reduced V soln. is insoluble in conditions in which Cr+3 et al. is completely precipitated, is an "objection"?

It wasn't even implied that something else might also precipitate from oxidized solution of the elements + ammonium. Now, then? Many things precipitate V...and Cr...and a bunch of other stuff. Just 1 element though? Not even in every oxidation state? Something someone actually has around? Ammonium, sure...as a starting point.

Wouldn't immediate success depend on what is present and how much? I suspect that in general, analytical routines using separations (of maybe many things) are needed at some point, for detection of one element in mysterious hypothetical solution in masturbatory/testq mysterious hypothetical solutionland.

With the OP mating such prominent exceptions then, (the peroxide and ether extraction was in fact mated from the beginning, Barreswill tested both V and Cr in 1847) with or without caveats (aqueous) and uh everything yes it makes me think, not so much about Cr and V.