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The dihydrochloride crystallises from water or aqueous EtOH and is washed with acetone, then diethyl ether. Alternatively convert it to the histidine di-(3,4-dichlorobenzenesulfonate) salt by dissolving 3,4-dichlorobenzenesulfonic acid (1.5g/10mL) in the aqueous histidine solution with warming, and then the solution is cooled in ice. The resulting crystals (m 280o dec) can be recrystallised from 5% aqueous 3,4-dichlorobenzenesulfonic acid, then dried over CaCl2 under vacuum, and washed with diethyl ether to remove Purification of Biochemicals — Amino Acids and Peptides excess reagent. The dihydrochloride can be regenerated by passing the solution through a Dowex-1 (Cl-form) ion-exchange column. The solid is obtained by evaporation of the solution on a steam bath or better in a vacuum. [Greenstein & Winitz, The Amino Acids Vol 3 p 1976 1961, Beilstein 25/16 V 366.]