Equivalent weight of H₃PO₂ when it disproportionate into PH₃ and H₃PO₃ is:

The disproportionation reaction is 3H₃PO₂ → PH₃ + 2H₃PO₃.

Now the conversion of H₃PO₂ to PH₃ involves the addition of 4 electrons.

     n factor = $\frac{\mathrm{no}. \mathrm{of} \mathrm{electrons} \mathrm{involved}}{\mathrm{total} \mathrm{no} \mathrm{of} \mathrm{molecules} \mathrm{involved}}$

=$\frac{4}{3}$

Hence, equivalent weight= $\frac{\mathrm{m}.\mathrm{wt}}{\mathrm{n}-\mathrm{factor}}$

=$\frac{\mathrm{M}\times 3}{4}$