woelen
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Easy way of distinguishing Na2SeO4 and Na2SeO3?
I have some sodium selenite and sodium selenate (at least, that is how they were sold to me). Both chemicals certainly contain selenium. A solution of
these in HCl is very easily reduced to red elemental selenium.
I have tried many different ways to see any difference:
- both give a green precipitate with CuSO4-solution
- both reduce KMnO4 to some green solution
- both form a red complex of SeBr6(2-) with HBr-solution, and on heating this decomposes, giving Br2.
The only difference I see is when I add the solids to concentrated HCl. With the selenite, the liquid remains colorless (and solid NaCl forms at the
bottom). With the selenate, the liquid becomes light green. But, no smell of chlorine appears.
I'm stuck with this now. I really suspect that the seller sent me two batches of sodium selenite. The powders, however, also are slightly different.
The sodium selenite is a purely white powder. The supposed to be sodium selenate has a light beige color.
Is there a test, which clearly distinguishes between selenate and selenite ion?
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YT2095
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have you tried it with Ascorbic acid (vit C)?
this may be an interesting read: "One problem with sodium selenITE is that you can't take it when you take vitamin C, zinc, copper, and other minerals
because it combines or interacts in a manner such that the selenium becomes useless. Sodium selenATE, however, doesn't combine readily with minerals
and doesn't react with vitamin C! "
taken from: http://www.nutri.com/wn/sel.html
\"In a world full of wonders mankind has managed to invent boredom\" - Death
Twinkies don\'t have a shelf life. They have a half-life! -Caine (a friend of mine)
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not_important
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From Sidgwick's Chemical Elements
| Quote: | | ...even dilute selenic acid will oxidize hydriodic acid quantitatively, so that the reaction may be used for its analysis, and will oxidize
hydrobromic acid fairly easily; while concentrated selenic acid will oxidize hydrochloric acid, so that a mixture of the two acids behaves like aqua
regia... |
Also see
http://books.google.com/books?id=g98JAAAAIAAJ&pg=PA383&a...
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woelen
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@not_important: Nice link and nice book. I downloaded it. It did not provide me with the info I want, but it is interesting to read anyways.
@YT2095: The link about nitrition value of selenate seems a commercial link to me with little scientific value. I did the test with addition of
ascorbic acid to a solution of sodium selenite and to a solution of sodium selenate. Both solutions are reduced quickly to elemental selenium, and
this is what I expected to happen.
I also have done tests with very dilute acidified bromide solutions. Both with the selenite and selenate, nothing happens. The solution remains
colorless and no bromine is formed with the selenate.
Finally, I did a test with adding cerium(IV)sulfate in 2M H2SO4 to solutions of Na2SeO3 and Na2SeO4. In both cases I obtained a bright yellow
flocculent precipitate.
Altogether, I'm inclined more and more to think that the seller simply sold me two times Na2SeO3, the one sample being purer than the other. The only
differences I've found so far is that the supposed to be Na2SeO4 gives a light green solution with concentrated HCl and the supposed to be Na2SeO4 is
light beige instead of white. These differences probably are due to some slight impurity. I did not pay very much for these chemicals (EUR 9 per
chemical, free shipping), but still, it is not good if a seller sells chemicals and these in reality are different ones (with the same seller I also
had, that I purchased 250 grams of Ba(NO3)2, and I received instead 250 grams of KBrO3, but labeled as Ba(NO3)2  ).
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kmno4
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Solubility in H2O?
Na2SeO3 89.8g/100g
Na2SeO4 58.5g/100g
(25 deg)
Use 1g H2O (for example) and add 0,5g - at this moment it should be clear solution - then add 0,2g ... 
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not_important
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Woelen,
pages 385 - 390 in the book describe conditions in which SeO4(-2) will convert chloride or bromide to the free halogen, while SeO3(-2) will not.
While the actual procedure is run on a large scale it should be possible to perform it on a small scale and simply estimate the amount of halogen
released to determine the approximate amount of selenate present.
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woelen
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I tried this test with the bromide, but no bromine is formed at all, not under any condition, except in concentrated (40+ %) HBr and heating. But in
the latter case, bromine is formed with the selenite as well. Hence my conclusion that the seller made a mistake and has sent me two jars with sodium
selenite (or I made some mistakes, performing the procedure in a wrong way). But I'm getting more and more confident that the seller made a mistake. I
will contact him about this.
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