Question

When compounds move through a column (in a column chromatography experiment), an ideal scenario involves the compounds separating into distinct (tight) bands, and traveling at different speeds through the column. This doesn’t always happen, though, for a variety of reasons. Is it possible for the stationary phase particles (silica or alumina particles in our case) to be too large? If so, what impact would these too-largeparticles have on the separation? Briefly explain your answer

Answer 1

Column chromatography is a purification technique. In column chromatography a vertical glass column is filled with adsorbents like silica(SiO₂) or alumina (Al₂O₃) this constitutes the stattionary phase and the mobile phase a solvent is added to the top of the column. The mobile phase flows down the column either by gravity or external pressure. The seperation of various components is by varying adsorption between mobile phase and stationary phase. Since silica and alumina are both polar, polar compounds are adsorbed more strongly by them and eluted from the column last.

Adsorbent particle size affects the elution of various compounds through the column. Alumina and silica is available in different sizes. Size is indicated by mesh value which is the number of holes in the mesh available for sieving the adsorbent. There are typeI,II and III of alumina (indicates water content in alumina), typeI with least content and are suitable to bind more number of polar sites are available. so large number of polar compounds will get adsorbed to silica and remain in the column for longer time. Type III alumina is having large water conent.

Silica with 230-400 mesh is have more holes per unit area and smaller size. It will have more binding sites and these types of silica are suitable for flash column. Silica having a mesh size 70-230 is suitable for gravity column.Also larger the mesh size smaller particles are easily adsorbed.