Crystallization in Gels

Crystallization in gels is an excellent (and unfortunately seldom used) technique for obtaining single crystals of compounds with low solubility. The crystallization process in gels is dominated by diffusion; convection and sedimentation is diminished. So you can get crystallization conditions in your laboratory, which are otherwise only found in the weightlessness of space missions.

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 potassium tartrate.
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.

Recipe for gel preparation

1. Prepare a 2 molar solution of sodium metasilicate Na2SiO3·9H2O in water. This process is rather time consuming, but after a few hours it will be finished.

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.

Easier recipe for gel preparation

An easier recipe for gel preparation is described by H. Arend, J.J. Connelly, J. Cryst. Growth (1982) 56, 642. If you hydrolize tetramethoxysilane with water you get silica gel and methanol.

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.

Further informations

Further informations about crystallization in gels can be found in the following books:

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 £

Page maintained by Martin Lutz. Last change 2-OCT-98.
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