Question

Which compound and why would you expect to have a shorter retention time on a reverse-phase HPLC column with a water/acetonitrile mixture adjusted to pH3 with acetic acid — benzylalcohol or benzoic acid

Answer 1

Benzoic acid has a b.p. of 250 degree celcius.This temp is quite close to the limit of most caplillary columns. The peak will be likely to appear at the end of your chromatographic sequence when the tempertaure gradient is at its highest and column bleeding is quite likely. plus it is quite polar and less polar than benzyl alcohol which can form hydrogen bonds with the stationary phase so will result in peak tailing.

In general the more polar the sample is, the more will be the retention time.in this question the water/ acetonitrile is used as the internal standard and the acetic acid is used for maintainging the ph to 3.