Inclusion Mechanism of Impurity in Crystals

-o-Chlorobenzoic acid-benzoic acid-toluene system-

Abstract

The inclusion mechanism of impurity and habit modification for o-chlorobenzoic acid (OCBA) and benzoic acid (BA) crystals precipitated from OCBA-BA-toluene system were investigated.
The relationship between crystal and solution compositions was obtained by differential crystallization. As the result of experiments, a large amount of BA was found to be included in OCBA crystals, although in BA crystals only a small amount of OCBA was included.
The washing effect of toluene on crystal impurities was examined by using benzyl alcoholas a tracer for mother liquor. It was clarified that BA is included in OCBA crystal structure and cannot be washed by toluene. However, OCBA in BA crystal may be due to adhesive mother liquor on the crystal surface.
Furthermore, powder X-ray diffration measurement was carried out for OCBA crystal obtained by differential crystallization and the diffraction pattern was compared with that of a mechanical mixture of OCBA and BA. As the result of comparison, it is suggested that BA is included in OCBA crystals by forming solid solution with OCBA. The shape of OCBA crystals (pillar-shaped) was deformed by inclusion of a large amount of BA. On the other hand, the shape of BA crystals (also pillar-shaped) was largely changed to scalelike form, even if only a small quantity of OCBA is present in the solution. This change may be due to the habit modification effect owing to the adsorption of OCBA on BA crystal surface.