Purify the acid by recrystallisation from H2O or aqueous EtOH as needles or plates. [pKa: Evans & Brown J Org Chem 27 3127 1962, IR: Arnett & Chawla J Am Chem Soc 100 214 1978.] UV in 50% aqueous EtOH has at 208 and 262nm. The ammonium salt max has m 243o (from H2O), the sulfonyl chloride has m 133-134o (from pet ether), the amide has m 110-111o (from H2O), the hydrochloride has m >300o(dec), and the N-methyl betaine has m 130o (from H2O). [Gastel & Wibaut Recl Trav Chim Pays-Bas 53 1031 1934, McIlvain & Goese J Am Chem Soc 65 2233 1943, Machek Monatsh Chem 72 77 1938, Beilstein 22 I 616, 22 II 309, 22/7 V 552.]
