Question

The developing chamber is not covered with plastic wrap during the development of the chromatogram. Will the Dion value for the cation be greater than or less than the actual value as a result of this technique error? Explain.

Answer 1

For the chromatogram to develop properly and components to show the correct Rf values for the solvent (or solvent mix) used, it is essential that the chamber is saturated with solvent vapour. Otherwise instead of moving up the stationary phase[paper, silica gel etc] solvent will be evaporating along the whole length and at the solvent front. This will give inaccurate Rf values, so you must keep the top firmly on.

Common chromatogaph developers,like butanol:ethanol:ammonia 3:1:1 saturate the surrounding air pretty quickly, but if the top was left off, much of the ammonia would escape more rapidly than the butanol vapour (because its relative molar mass is only 17) and this would actually change the composition of the developing solvent moving up the stationary phase, another reason to keep it sealed!