P₂H₄ can be removed from phosphine containing traces of it

When the PH₃ mixture containing P₂H₄ is passed through the freezing mixture, then pure PH₃ is obtained as P₂H₄ gets condensed.

PH₄I forms when PH₃ is passes HI. PH₄I is later treated with KOH (aq), to obtain free PH₃, which is free from P₂H₄.
$\begin{array}{l}{\mathrm{PH}}_3+\mathrm{HI}⟶{\mathrm{PH}}_4\mathrm{I}\\ {\mathrm{PH}}_4\mathrm{I}+\mathrm{KOH}\left(\mathrm{aq}\right)⟶\mathrm{KI}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{PH}}_3\uparrow \end{array}$

This concludes that both given methods are used to obtain pure PH₃ (free from P₂H₄).