chornedsnorkack
National Hazard
Posts: 521
Registered: 16-2-2012
Member Is Offline
Mood: No Mood
|
|
Qualitative V/Cr detection
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
International Hazard
Posts: 5102
Registered: 1-11-2003
Location: UK
Member Is Offline
Mood: No Mood
|
|
https://en.wikipedia.org/wiki/Chromium(VI)_oxide_peroxide
|
|
|
DraconicAcid
International Hazard
Posts: 4278
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
I believe vanadium gives a red colour under the same conditions as the peroxide test, so they may interfere with each other.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
|
|
|
Bedlasky
International Hazard
Posts: 1219
Registered: 15-4-2019
Location: Period 5, group 6
Member Is Offline
Mood: Volatile
|
|
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
International Hazard
Posts: 2692
Registered: 3-11-2013
Member Is Offline
Mood: Big
|
|
Ammonium metavanadate has a particularly low solubility in water, while ammonium chromate is much more soluble.
[Edited on 04-20-1969 by clearly_not_atara]
|
|
|
S.C. Wack
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
Member Is Offline
Mood: Enhanced
|
|
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
International Hazard
Posts: 4278
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
I didn't say it was impossible, just pointing out something that might complicate things. If you're so tired, take a nap.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
|
|
|
S.C. Wack
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
Member Is Offline
Mood: Enhanced
|
|
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...
Obviously I have something against clearly_not_atara as well? Yeah I'm the one who needs to nap.
|
|
|
Bedlasky
International Hazard
Posts: 1219
Registered: 15-4-2019
Location: Period 5, group 6
Member Is Offline
Mood: Volatile
|
|
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
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
Member Is Offline
Mood: Enhanced
|
|
| 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
International Hazard
Posts: 1219
Registered: 15-4-2019
Location: Period 5, group 6
Member Is Offline
Mood: Volatile
|
|
| 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
National Hazard
Posts: 521
Registered: 16-2-2012
Member Is Offline
Mood: No Mood
|
|
| 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
International Hazard
Posts: 1219
Registered: 15-4-2019
Location: Period 5, group 6
Member Is Offline
Mood: Volatile
|
|
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
International Hazard
Posts: 2692
Registered: 3-11-2013
Member Is Offline
Mood: Big
|
|
| Quote: Originally posted by S.C. Wack |
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]
[Edited on 04-20-1969 by clearly_not_atara]
|
|
|
S.C. Wack
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
Member Is Offline
Mood: Enhanced
|
|
I see.
I quoted the part of your post relevant to what I had to say.
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.
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.
|
|
|
DraconicAcid
International Hazard
Posts: 4278
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
I happened to be checking this (revamping a qualitative analysis lab for first year), and the peroxide test for vanadium works very well even in the
presence of chromium. It makes it tougher to see the blue flash from the chromium, but the vanadium does not extract into the organic solvent (unlike
the chromium).
ETA: And before anyone pipes up with "I told you so", I know that- I'm just publicly confirming.
[Edited on 13-11-2021 by DraconicAcid]
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
