Question

Retention factors for three solutes separated on a${\mathbf{C}}_{\mathbf{8}}$non-polar stationary phase are listed in the table. Eluent was a 70 : 30 (vol/vol) mixture of 50 mM citrate buffer (adjusted to pH with${\mathbf{NH}}_{\mathbf{3}}$) plus methanol. Draw the dominant species of each compound at each pH in the table and explain the behavior of the retention factors.

Short answer

The retention factor is less below pH 7 and more at pH 7.

Step-by-step solution

Explanation for given pH

At all pH values, Acetophenone is neutral and its retention remains unaffected by pH.

In case of Salicylic acid, a neutral molecule HA can be expected. The expected neutral molecule HA might have certain affinity for the${\mathrm{C}}_8$non-polar stationary phase and the ion${\mathrm{A}}^{-}$might have little affinity for${\mathrm{C}}_8$Salicylic acid is mostly HA below pH 2.97 and above pH is likely shows${\mathrm{A}}^{-}$At pH =3 there is 1: 1 ratio mixture of HA and${\mathrm{A}}^{-}$that is soberly retained on the non-polar column. At pH5 and 7 , maximum of molecules (more than 99% ) shows${\mathrm{A}}^{-}$hence the retention is weak(retention factor is small).

Nicotine in its ionic form is subjected to have less affinity for non-polar stationary phase and the neutral molecule shows affinity to certain extent.

Considering Nicotine as B, the form B is most preferable at above$\mathrm{pH}={\mathrm{pK}}_2=7.85{\mathrm{BH}}^{+}$is most likely between pH3.15 and7.85

${{\mathrm{BH}}_2}^{2+}$is most preferable at below 3.15pH

B doesn't become appreciable till pH is approximately equal to 7 , hence the retention factor is less below pH 7 and raise at pH7.