Crystallise the acid from water. Also, dissolve it in dilute NaOH and add AcOH to pH 5-6 and allow it to crystallise (m 304o dec). Alternatively dissolve the acid in aqueous NH3 and boil; when the NH3 has evaporated, the acid separates, filter it off and dry it at room temperature in a vacuum. In another procedure 1g of acid is dissolved in 10mL of conc HCl + 15mL of H2O at 80o, filter immediately, dilute with 20mL of H2O and allow to stand for 24hours. When the monohydrochloride (0.7g, m 175-156o dec) crystallises out, filter and dry it. It has also been purified by dissolving it in the minimum volume of 10% HCl, filtering, and diluting with 5volumes of H2O when the crystals separate slowly on standing. The acid is filtered off after 24hours and dried (m 306-306o dec). Similar procedures were used for the dl-isomer. [Kenner J Org Chem 13 28 1948, McKennis & Yard J Org Chem 23 980 1958, Beilstein 4 III 1528, 4 IV 3025.]
