Spherical silica

The Preparation of Spherical Silica Gel Spherical particles of silica can be prepared by spraying a neutralized silicate solution (the colloidal silica sol.) into fine droplets before gelling has taken place and subsequently drying the droplets in a stream of hot air. It has also been shown possible to disperse a silica sol in the form of an emulsion in a suitable organic solvent where the droplets gel in spherical form (12). Unfortunately, details for the preparation of spherical silica have tended to be kept very confidential for commercial reasons and so information is a little sparse. One of the first methods reported was that of Le Page et al (13,14). A stable silica sol. (generated at low pH so that gelling does not take place) is passed through a non aqueous solvent in such a manner as to produce

covers the molecular size of most of the solutes separated on LC columns. It follows, that silica gel will act as an exclusion media and be capable of separating substances on the basis of molecular size. The smaller molecules will enter the majority of the pores and thus, be retained the most. The larger molecules may be excluded from all the pores and, consequently, will be retained the least. Contemporary silica gel particles that are used per se or for the production of bonded phases are spherical in shape and made differently. However, the chemical process of gel formation is very similar and leads to the same porous material of high surface area with very similar physical properties.� Spherical particles of silica can be prepared by spraying a neutralized silicate solution (the colloidal silica sol) into fine droplets before gelling has taken place and subsequently drying the droplets in a stream of hot air. It is also possible to disperse a silica sol in the form of an

a firm, almost rigid gel is produced which is called the hydrogel. The compact hydrogel is then well washed under controlled conditions to eliminate the last of the sodium chloride and then heated for a few hours at 120oC. The resulting product is a hard amorphous mass called the xerogel. The xerogel, ground and graded is the material that is used for packing LC columns and manufacturing bonded phases. The product, prepared in this way, is called irregular silica gel, to differentiate it from spherical silica gel which is prepared by employing an entirely different synthetic procedure. Irregular silica gel is also the basic material from which bonded phases can be prepared

activated form, is used to separate the permanent gases and hydrocarbons up to about pentane. Alumina is usually activated by heating to 200˚C for about an hour. A common particle size is about 100/120 mesh and the pore size range from about 1  to 100,000. Silica gel in spherical form (prepared by spraying a neutralized silicate solution (a colloidal silica sol) into fine droplets, allowing the silica gel to be formed, and subsequently drying the droplets in a stream of hot air). Silica is produced with a wide choice of surface areas and porosity’s, which can range from about 750 m2/g and a mean pore size of 22 , to a material having a surface area of only 100m2/g and a mean pore diameter of 300 . It is used for the separation of the lower molecular weight gases and some of the smaller hydrocarbons. In a specially prepared form, silica can be used for the separation of the sulfur gases, hydrogen sulfide, sulfur dioxide and carbon disulfide. Molecular sieves

nbsp; Employing equation (1) the volume standard deviation for three popular, contemporary, LC columns eluting solutes of different (k’) values can be calculated. The type of packing need not be defined and may be silica, bonded silica or polymeric in character, but all will be assumed to be spherical in form. In fact, the particle diameter of the stationary phase controls the magnitude of column dispersion and not its chemical character. A value of 0.6 is taken for (e) which is generally accepted (2). The properties of the three columns are shown in table 1 including the peak volume, (4sv), which has more significance to the practicing chromatographer It is seen that the dead volume peak widths are very narrow (assessed as (4sv(k’=0)) and range from about 5 ml for the peak

This toroidal mirror focuses the portion of incident light onto the reference photocell, providing an output that is proportional to the strength of the incident light. The path of the fluorescent light is largely on the right hand side of the diagram. Fluorescent light from the cell is focused by an ellipsoidal mirror on to a spherical mirror at the top right-hand side of the diagram. This mirror focuses the light onto a grating situated at about center right of the figure. This grating can scan the fluorescent light and provide a fluorescent spectrum. Light from the grating passes to a photoelectric cell which monitors its intensity. The instrument is quite complex and, as a result, rather expensive; however, from the point of view of measuring fluorescence it is extremely versatile. Again the cell and all conduits