Pure silica was extracted by refluxing with
6 N HCI for 4 h and then washed repeatedly
using deionised water to make it acid free. It
was then dissolved in 2, 2.5 and 3.0 N NaOH
by continuous stirring for 10 h on a magnetic
stirrer and then concentrated H2SO4 was
added to adjust pH in the range of 7.5-8.5.
The precipitated silica was washed repeatedly
with warm deionised water until the filtrate
became completely alkali free. The washing
process continued by deionised water
repeatedly and dried at 50 oC for 48 h in the
oven.