Co-crystallization of benzoic acid and o-chlorobenzoic acid from toluene solutions.

概要

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When o-chlorobenzoic acid (OCBA) is recrystallized from toluene solution containing benzoic acid (BA) as impurity, BA co-crystallizes with OCBA because of formation of solid solution. The distribution coefficient of BA between crystal and toluene solution was estiamted by the differential crystallization method. The coefficient was observed to remain almost constant independently of solution composition for XLBA less than 0.6. However, it decreased with crystallization temperature. If the concentration of BA in solutions increased more than some limiting value during crystallization, not only the solid solution but also BA crystals precipitated simultaneously and the purity of OCBA crystals suddenly decreased. Batch crystallization was carried out by evaporation at constant temperature. The evaporation rate was controlled using an electro-controller, but no influence of evaporation rate on average composition of crystals was observed. The average composition of crystals was calculated using the distribution coefficient obtained by differential crystallization. The calculated values agreed well with experimental data and it became clear that the distribution coefficient for differential crystallization hardly differs from that in batch evaporative crystallization.
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