In the preparation of HBr or HI,NaX(X = Br,I) is treated with H₃PO₄ and not by conc.H₂SO₄ since,

 HBr and HI further get oxidized with H₂SO₄ and release Br₂ and I₂ respectively. Thus H₃PO₄ is used to prepare HBr, HI from NaBr and NaI

 I${\mathrm{X}}^{-}+ {\mathrm{SO}}_4^{-2}\stackrel{{\mathrm{H}}^{+}}{\to }{\mathrm{X}}_2+{\mathrm{SO}}_2, \mathrm{where} \mathrm{X}=\mathrm{Br}, \mathrm{I}$

H₃PO₄ is a non oxidizing acid.