In the most simple experiment you fill the gel containing component A into
a test tube. Thereafter the gel is overlayered with a solution of
component B. The compound generated by the reaction of A with B should be
of low solubility. If the gel for example includes a solution of tartric
acid and the gel is overlayered with a solution of potassium chloride,
you will get wonderful crystals of the sparingly soluble compound
Compound B can also be an organic solvent like ethanol if you want to precipitate an inorganic compound like ammonium dihydrogen phosphate contained in the gel.
Another arrangement can be a U formed tube filled with the pure gel. A solution of compound A will be filled in one arm, a solution of compound B in the second arm.
You can minimize the equipment and even use a melting point capillary as a crystallization vessel.
2. Filtrate the metasilicate solution by membrane filtration. Most optimally
the filtration should not be done by suction, because of enrichment of
carbon dioxide. Much better is a filtration with pressure. The pressure of
approximately 3 bar is generated with nitrogen or argon gas. Pressure filter
funnels (stainless steel) for membrane filtration are commercially available.
If you store the metasilicate solution, you must filter it each time before use.
3. Titrate under continous stirring
a 2 molar solution of nitric acid
with the 2 molar sodium metasilicate solution until pH 5.5. Please control
the pH very carefully, because if the pH reaches pH 7 the gel will solidify
at once. If you want to have compound A in the gel, you can add it to the
nitric acid before the titration or just before pH 5.5.
The solution of pH 5.5 should be transfered immediately to the final crystallization vessel, because it takes only a few moments until the gel solidifies.
4. Close the crystallization vessel and store the gel for at least one night, before you put in the solution of compound B. Close the crystallization vessel again and wait for crystals. This can take a few days or weeks. HAVE SUCCESS!!!
The concentrations of compound A and B should be tried out for optimal conditions. In most cases 0.1 - 1 molar solutions are good starting conditions. You can also try out crystallization at different temperatures.
Tetramethoxysilane and water are nearly immiscible. Therefore a 5-10 vol% mixture of tetramethoxysilane (toxic!) in water is stirred heavily with a magnetic stirrer (closed vessel, highest stirring speed). If the solution is getting clear (no phase separation anymore), it must be immediately transferred to the final crystallization vessel, because it takes only a few moments until the gel solidifies.
H.K. Henisch: Crystals in Gels and Liesegang Rings,
Cambridge University Press (1988)
ISBN 0521345030; 55.00 £
H.K. Henisch: Crystal Growth in Gels,
Dover Publications (1996)
ISBN 0486689158; 7.95 £