Before leaving the discussion on silica chemistry, the question of colloidal silica must be explored. Since colloids are generally smaller than 0.5 micron, a simple filtration test can be performed with a standard 0.45-micron filter such as those used to determine silt density indexes (SDI’s) ahead of RO systems. Anything smaller than 0.01 micron is probably dissolved. The difference in silica, after digestion, reflects the concentration of colloidal silica.