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

HBr or HI cannot be prepared by heating Br⊖ or I⊖with conc. H2SO4 because HBr and HI are strong reducing agents and reduces H2SO4 to SO2 and Br⊖ or I⊖are oxidized to Br2 or I2 respectively.NaX+H2SO4⟶NaHSO4+HXH2SO4+2HX⟶SO2+X2+2H2O(X=Br or I)Hence HBr and HI are prepared by heating Br⊖ or I⊖respectively with H3PO4.3NaBr+H3PO4⟶Na3PO4+3HBr3NaI+H3PO4⟶Na3PO4+3HI