[ressorgimento]

Can H₃PO₂ reduce Na₂SO₄ with the formation of SO₂ or H₂S?

Kind regards,

Ressorgimento

[AWK]

Do you mean H₃PO₂?

http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=AD0711884
http://pubs.acs.org/doi/pdf/10.1021/ac60311a018

[ressorgimento]

Yes sorry, Indeed H3PO2

[DevaDevil]

Relevant reactions in aqueous media:

HPO₃^2- + 2H₂O + 2e^- <--> H₂PO₂^- + 3 OH^-
E⁰ = -1.65 V (alkaline environment)


H₃PO₃ + 2H₂O + 2e^- <--> H₃PO₂ + H₂O
E⁰ = -0.59 V (neutral environment)

------------------

SO₄^2- + H₂O + 2e^- <-->  SO₃^2- + 2OH^-
E⁰ = -0.92 V (neutral environment)

SO₄^2- + 4H⁺ + 2e^- <-->  H₂SO₃ + H₂O
E⁰ = +0.20 V (acidic environment)


-

So H₃PO₂ is most easily oxidized in alkaline environment, whereas sulfate is most easily reduced in acidic environment... on the neutral middle ground, the reducing power of H₃PO₂ is not enough to reduce sulfate.



Now as for generating H₂S or SO₂, I do not have data of formation enthalpies, I am sorry (I am electrochemist so could help with these potentials). Someone else may be able to assist there

[ressorgimento]

But it seems to me if I compare the redox potentials in neutral media,
H3PO2  + H2O <-> H3PO3 + 2 H+ + 2 e- E0= +0,59 V neutral
SO42- + H2O + 2e- <-->  SO32- + 2OH- E0 = -0.92 V (neutral environment)

=> sum is negative so reaction doesn't occur in neutral environment?

But in acidic media you give me this:
SO42- + 4H+ + 2e- <-->  H2SO3 + H2O
E0 = +0.20 V (acidic environment)

So redox potential of second couple gets higher,
you didn't give me redoxpotential of other couple in acidic media,
but it also looks like it got higher, from -1,65 in alkaline media to -0,59 in neutral media.

So to me it looks possible that in acidic media H2SO3 can be formed?

[DevaDevil]

I don't give the potentials because the data is not available to me. So I have no idea how much higher the potential will be.

If it gets higher than +0.2 V you have to assume the reaction will not occur in any appreciable amount.
But again, I have no data, so cannot make a claim with certainty

[ressorgimento]

I found this value;
H₃PO₂ + H₂O <-> H₃PO3 + 2H⁺ + 2e^-   E0=+0.499V
SO₄^2-+ 4 H⁺ + 2e- <-> H₂SO₃ + H₂O E0= +0.2V

That would give
H₃PO₂ + H₂SO₄ <-> H₃PO₃ + H₂SO₃ E0= +0.7V

So that would mean H2SO3 can form.
Correct?

[DevaDevil]

I find that value hard to believe, considering it is almost the same as the value for neutral media. But if this value is indeed true, then yes, the reaction could occur.