9 P40 + 16 NaOH + 48 H2O 20 P-IVH4 + 16 NaH2PVO4

This is an oxidation-reduction (redox) reaction:

20 P0 + 80 e- 20 P-IV (reduction)

16 P0 - 80 e- 16 PV (oxidation)

P4 is both an oxidizing and a reducing agent (disproportionation).