The influence of physical and chemical factors on the preparation of silica gel media.

The need of an inorganic gel for the preparation of microbiological media arises frequently in nutritional studies. The isolation of fastidious microorganisms, such as the nitrifying bacteria, may be facilitated by the use of solid media prepared with silica gel rather than with agar. Until recently, the procedures available for the preparation of silica gel were time-consuming and inconsistent in the result they produced. As a consequence, silica gel media were not employed in many investigations where their use was otherwise desirable. In 1950, Taylor described a greatly improved method for the preparation of silica gel. A stable silica sol was produced from sodium silicate solution by use of a cation-exchange resin. The silica sol was heat sterilized and converted to a gel by adjusting the pH and adding inorganic and organic nutrients. Smith (1951) improved the method of Taylor and presented information regarding the properties and use of the silica sols obtained. The ease with which silica sol is prepared by ion-exchange methods is reflected in a recent increase in the use of silica gel media (Augier, 1954; Kingsbury and Barghoorn, 1954; Leathen et al., 1956; Selkon and Mitchison, 1957). The preparation of silica gel and the nature of the medium obtained are subject to numerous influences, including silica content, pH, temperature, amount and type of ions present, and the procedure for sterilization. To obtain a satisfactory medium it is necessary to vary empirically one or all of the above factors until the desired result is acquired. The present study was undertaken to evaluate systematically and quantitatively the influence of various physical and chemical factors on the preparation of silica gel. The results permit a rational approach to the development and production of silica gel media to satisfy individual requirements.