It is generally contaminated with starting material (cresol red) and semi-xylenol orange. Purify it by ion-exchange chromatography using DEAE-cellulose, eluting with 0.1M NaCl solution, which will give the sodium salt [3618-43-7] . Cresol Red, semi-xylenol orange and iminodiacetic acid bands elute first. This procedure will give the sodium salt of the dye. To obtain the free acid, dissolve the salt in H2O and acidify it with AcOH. Filter it off, wash it with H2O and dry it first in air and then in a vacuum desiccator over P2O5 in the dark [Sato et al. Anal Chim Acta 94 317 1977]. [Beilstein 19 II 111, 19 III/IV 1135, 19/3 V 461.]
