No abstract

Consideration of how an instructor's response can encourage a student to think or discourage a student from thinking.

In a general chemistry experiment, students have achieved good results in separating compounds with handmade chromatography columns constructed from disposable pipets. Directions for preparation of suitable inexpensive columns are readily found in the literature {1, 2). However, at times columns made by a few students tend not to perform adequately as air bubbles can spontaneously form and block the elution process. When this happens, students often inappropriately feel personally responsible, and the level of frustration experienced by students can ruin an otherwise good laboratory experience. However, with appropriate instructor intervention, it is possible to rejuvenate a poorly functioning column. By placing a regular disposable pipet bulb (normally used to draw solution into the pipet) over the end of the column and gently applying pressure, the eluting solvent and the offending air bubbles are forced through the column. The students can then resume normal operation with their column. Care must be taken with this procedure as air can also be forced through the column if the level of solvent drops below the top of the column while pressure is being applied. In addition, the bulb must be removed carefully or the partially contracted bulb will pull air back up through the column, and damage the column irreparably. So, it is not recommended that the students be instructed to try this. In addition, this method is not recommended for cellulose-based resins as the applied pressure greatly compacts the columns to the point that the column no longer functions properly. When this simple technique is used appropriately by instructors, all students can experience success with their handmade chromatography columns.