Question

If you switched to a 50/50 mixture of ethyl acetate/acetic acid eluent (originally 95% ethyl acetate, 5% acetic acid) the Rf value will increase, decrease or unchanged? Explain your answer briefly.

Answer 1

Here acetic acid is the polar solvent and ethyl acetate somewhat non-polar and silica gel is also polar.
  When polarity of the solvent is 95% ethylacetate and 5% acetic acid, the Rf value for the polar spot will be very less as the polar part of the compound binds with silica whereas when polarity of the solvent is increased the Rf value also increases as now the polar spot moves upward with the help of the acetic acid or we can say more distance is covered by the polar spot . For the non-polar spot the distance will be almost same hence less change in the Rf value for non-polar spot.