Colloidal silica is silica particles that are suspended in a liquid. The liquid is denser than water and has been stabilized electrostatically to allow the particles to stay suspended in the solution. The silica particles are also very small and do not have a large density. The range has to be approximately from thirty to a hundred nanometers. If the particles are too large, they will settle out of the solution. If the silica particles are too small, they are difficult to stabilize in a solution.
Preparation of colloidal silica is a multi-step process. Beginning with an alkali-silicate solution, the solution is neutralized which will cause silica nuclei to form. These particles are extremely small, usually just a few nanometers in diameter. The greatest control over the solution happens in this period.
If the pH is reduced below 7 or if salt is added, then the units tend to fuse together in chains. These products are often called silica gels. If the pH is kept slightly on the alkaline side of neutral, then the subunits stay separated, and they gradually grow. These products are often called precipitated silica or silica sols. Hydrogen ions from the surface of colloidal silica tend to dissociate in aqueous solution, yielding a high negative charge. Substitution of some of the Si atoms by Al is known to increase the negative colloidal charge, especially when it is evaluated at pH below the neutral point. Because of the very small size, the surface area of colloidal silica is very high.
Technical specification :
|
TEST
|
ACCEPTABLE STD.
|
TESTING MEANS TO USE
|
|
SIO2
|
30.00% MIN
|
wt. ANALYSIS
|
|
Na2O
|
0.35 – 0.50 %
|
TITRATION
|
|
SIO2 / Na2O
|
60 – 120
|
BY CALCULATION
|
|
Specific Gravity
|
1.200 to 1.350 gm/cc
|
Wt. ANALYSIS
|
|
pH
|
9.50 – 10.50
|
pH – METER / pH – paper
|
|
Surface Area
|
225 – 375 M2 / gm
|
BY CALCULATION
|