[wildebeestdivision]

Hello,

I am working on an undergraduate honors thesis. I need to reduce arsenate(V) to arsenite(III). The catch is that I need to reduce this while not reducing phosphate (V) to phosphine gas (-III). Is it reasonable that I should suspect hydroxylamine hydrochloride to reduce phosphate to phosphine gas? I checked out the literature, and tsang 2007 did not mention hydroxylmine hydrochloride as a reducing agent of arsenate, but my advisior told me to use it.

Another important tidbit is that I am reducing a bunch of arsenate (around .3-.8 mg/L) and using about 2 orders of magnitude less phosphate. No phosphate can be reduced to phosphine gas or it renders the exercise useless and provides a significant health hazard.

Any ideas?

Thanks

[DevaDevil]

well, hydroxylammonium can be a powerful reducing agent:

N₂ + H₂O + 2 e^- <-->2 NH₃OH⁺;   E⁰ = -1.87 V
(for comparison, BH₄^-, a commonly used strong reducing agent, has an E⁰ of -1.24V only.)

plenty reduction potential for:
AsO₄^3- + 2 H₂O + 2e^- <--> AsO₂^- + 4 OH^-; E⁰ = -0.71V   (-0.08V if in NaOH)


Still, I do not know if this can prevent phosphite formation...
P + 3H₂O + 3e^- <--> PH₃ + 3 OH^-; E⁰ = -0.87V
H₃PO₂ + H⁺ + e^- <--> P + 2 H₂O; E⁰ = -0.51V
H₃PO₃ + 2H⁺ + 2e^- <--> H₃PO₂ + H₂O; E⁰ = -0.5V
PO₄^3- + 2 H₂O + 2e^- <--> H₃PO₃ + 3 OH^-; E⁰ = -1.05V

[wildebeestdivision]

well, what should i do?

N2 + 4H2O + 2e- <---> 2NH2OH + 2OH-  E^o= -3.04

is the reducing agent.

I need a water with

0.8 mg/L AsO4^3-
0.01 mg/L PO4^3-

to be reduced to having only
AsO2^-
and PO4^3-

[AndersHoveland]

I do not actually know, but can offer my opinion:
Phosphates are actually fairly difficult to reduce. One relevant parallel exists with sulfuric and selenic acids. Selenous acid H2SeO3 is a much stronger reducing agent than sulfurous acid H2SO3, while contrastingly concentrated selenic acid H2SeO4 is a much stronger oxidizer than concentrated sulfuric acid H2SO4. Thus the selenium oxyacids can be seen to be more reactive (both as oxidizing and reducing agents) than the oxyacids of sulfur. This is due to the larger atomic radius.

In the same way, arsenites are stronger reducing agents than phosphites, while arsenates can act as mildly reactive oxidizers, whereas phosphate is usually inert in most reactions. I highly doubt that any phospite is formed by reaction of zinc in excess phosphoric acid, and neither would sulfur dioxide be formed by dilute sulfuric acid on zinc.

As a side note:
I also know that even dilute perchloric acid is not reduced by zinc metal, the reaction only generates hydrogen gas, but if a nitrate salt is present, the nitrate ion will get reduced, somewhat unexpectedly showing that perchlorate ions are often not reactive.


Perhaps my posted topic about "Preparation of Hydroxylamine" has inspired this topic?