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Bisulfite adduct and reagents?
I suppose that the decent and proper way to purify aldehydes is to form adduct with them. The process is described around (for ex http://chem2all.blogspot.com/2009/03/addition-of-bisulfite-t... ).
But when I looked for sulfite, I found various sites selling all kind of sulfates, sulfites, di- bi- pyro- meta- hydrogen and other sulfonous
products. It turns out, sodium bisulfite per se seems to be specialty chemical that does not appear to have any otc use.
But there are campden tablets, that contain potassium or sodium pyro- or metabisulfite. It is stated in wiki, that:
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Upon dissolution in water, bisulfite is generated:
Na2S2O5 + H2O → 2 Na+HSO3-
In this way, sodium metabisulfite is equivalent to sodium bisulfite.
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So, is campden product sold as pure powder a viable source for bisulfite to be used as an adductant?
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woelen
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The bisulfite ion does not exist in the solid state. Sodium bisulfite is not a chemical you can buy.
In aqueous solution you have the bisulfite ion, HSO3(-). On evaporation and crystallization, this ion, however, loses water and the solid you get is
the metabisulfite, S2O5(2-):
2 Na(+) + 2 HSO3(-) --> H2O + Na2S2O5
The above is true for both the sodium and potassium salt. Both metabisulfites you can buy, Na2S2O5 and K2S2O5.
When you dissolve a metabisulfite in water, you again get the bisulfite ion, the reverse of the reaction, given above.
Normal sulfites also can be purchased, Na2SO3 as anhydrous salt, but also Na2SO3.7H2O.
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Sulfates are completely different and have very different chemical properties. You need sulfite. Sulfates exist in different forms, e.g. for the
sodium salts there is the following:
Na2SO4. sodium sulfate
NaHSO4, sodium hydrogen sulfate, a.k.a. sodium bisulfate
Na2S2O7, sodium pyrosulfate --> with water this gives sodium bisulfate. The reverse does not occur. Getting sodium pyrosulfate requires heating of
sodium bisulfate to several hundreds degrees Centigrade.
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Refinery
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Thanks, appreciate it. Times ago I got so confused on all these sulf-whatevers that they actually scared me away from adductions. Now that I've
studied it for a while, it does seem a bit more clear. Gotta order some and perform the adduction. Funny how the purification of aldehyde was all the
time right in front of me, but I didn't realize it but just a moment ago. I was totally frustrated on how I was gonna separate it from impurities that
have almost exactly the same solubilities and boiling points.
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woelen
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The sulphur world is a complicated one. There are many compounds which you can obtain, all of them of the form SxOy(2-), with varying x and y:
S2O3(2-) : thiosulfate, a fairly strong reductor, unstable in acid. The sodium salt Na2S2O3.5H2O can be purchased. Sometimes called hypo or sodium
hyposulphate.
S2O4(2-) : dithionite, a very strong reductor. You can buy the sodium salt, Na2S2O4, sodium dithionite, sometimes called sodium hydrosulfite.
S2O5(2-) : metabisulfite, a fairly strong reductor. With water it gives bisulfite HSO3(-). Both the sodium and potassium salts can be purchased.
S2O6(2-) : dithionate. Rather inert, not reducing, nor oxidizing. Can sometimes be purchased as potassium salt or sodium salt. Has no real application
in the home lab.
S2O7(2-) : pyrosulfate. Acidic sulfate, gives HSO4(-) with water irreversibly. Not interesting, the bisulfate is equally interesting and much cheaper.
SO3(2-) : sulfite, a mild reductor, somewhat basic. Available as sodium salt, Na2SO3 or as heptahydrate, Na2SO3.7H2O.
SO4(2-) : sulfate, quite inert. Many salts of the sulfate ion are available, because it is non-reactive and hence the cation-part of the compound
mainly determines its properties.
HSO4(-) : bisulfate, acidic sulfate. Available as sodium salt, a.k.a. pH-minus, chemical name is NaHSO4. The usual pH-minus is the monohydrate,
NaHSO4.H2O.
S2O8(2-) : peroxodisulfate, a very strong but somewhat slowly acting oxidizer. Available as sodium, potassium and ammonium salts. Used for etching PCB
in electronics hobbies. Sometimes simply called persulfate.
SO5(2-) : peroxosulfate, a strong oxidizer, not as strong as peroxodisulfate, but faster acting. Available as oxygenator for swimming pools as
"oxone", a triple salt of K2SO4, K2SO5 and KHSO4.
There are more sulfur oxoanions, but these are quite exotic and have no application besides the use as research chemicals.
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DavidJR
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Quote: Originally posted by woelen  |
S2O3(2-) : thiosulfate, a fairly strong reductor, unstable in acid. The sodium salt Na2S2O3.5H2O can be purchased. Sometimes called hypo or sodium
hyposulphate.
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Minor correction, thiosulfate is sometimes called hyposulfite, not hyposulfate.
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