Question

You work in a small-scale manufacturing facility that uses phthalic acid as the starting material for a multi-step synthesis. Your company buys ~98% pure phthalic acid from a major manufacturer and purifies it in-house prior to its use in the multi-step synthesis. You are given the job to purify 50 g of the impure phthalic acid by recrystallization from water. Approximately how much water will you need to use to prepare the hot saturated solution? Upon cooling in an ice-bath, and subsequent filtration, how much pure phthalic acid would you expect to recover?

Answer 1

Recrystallization is done by adding a small portion of the solvent to the beaker containing impure sample and heating the sample till boiling. The contents should be stirred gently. Sufficient solvent should be added to dissolve the solute and obtain a saturated solution at the boiling point of the solvent. One should be careful that if too much solvent is used, the amount of the recovery of the substance may be decreased.

Further, the crystallization point is obtained when the volume of the solution containing impure sample is reduced to one half. A glass rod dipped in the concentrated solution when shows the formation of a thin crust on cooling indicates the crystallisation.

Phthalic acid is very soluble in boiling water (18 g/100 mL).

i.e., 280 mL for 50 g. I would suggest taking double the amount so that half the volume could be boiled off.

Therefore, approximately, 560 mL water will be needed to prepare the hot saturated solution.

Since, the impure sample contained 98% pthalic acid. Less than 49 g of pure phthalic acid can be expected to recovered.