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My chemistry textbook suggests that "acids which contain a P-H bond have strong reducing properties". Both hypophosphorous acid (H3PO2) and pyrophosphorous acid (H4P2O5) contain two P-H bonds; a P-O-P bond is present additionally in pyrophosphorous acid (as in the figures below). Which of these two turns out to be a stronger reducing agent, and why? Or do they have equal reducing properties?

Hypophosphorous Acid Pyrophosphorous Acid

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  • $\begingroup$ IMHO, pyrophosphorous acid has comparable reducting properties as phosphorous acid, not as hypophosphorous acid. $\endgroup$ – Poutnik Apr 25 at 9:15
  • $\begingroup$ @Poutnik Phosphorous acid has three OH groups, while pyrophosphorous acid has two. Phosphorous acid is a stronger reducing agent in general, hence? (I understand the reducing strength will change in acidic, neutral and basic media) $\endgroup$ – Kartik Apr 26 at 7:47
  • $\begingroup$ AFAIK, phosphoric acid had 3 OH groups, phosphorous acids has 2 OH and 1 H. $\endgroup$ – Poutnik Apr 26 at 8:11
  • $\begingroup$ IMHO, condensation of acids does not affect reduction ability much. What does affect it is number of H on P. $\endgroup$ – Poutnik Apr 26 at 8:14
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    $\begingroup$ See en.wikipedia.org/wiki/… -0.499V versus -0.276V $\endgroup$ – Poutnik Apr 26 at 10:02

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