Stage 2The efficiency of the acid rinse is controlled by Xray
fluorescence: the SiO2 content of the remaining
solid must be at least 99%. Measurements of the
remaining solid by 29Si solid state NMR spectroscopy
showed residual SiO2 Q4 tetrahedra and SiO5/2H Q3
tetrahedra and confirmed the absence of silica gel
precipitation after the acid treatment [9].
Stage 3: After thermal treatment of the residual
solid at 1000 8C, the silanol groups are condensed to
give Q4 and lose water: