Sulfonephthalein dyes. П.* Protolytic equilibria of methylxylenol blue

Methylxylenol blue or S^'-bis-Z^jiV'-diicarboxymethylJaminomethyl-^-xylenolsulfonephthalein belongs to the group of metallochromic indicators prepared by Körbt and Přibil [1] via condensation of common sulfonephthalein dyes with formaldehyde and iminodiacetic acid. Recently it has been newly prepared by Cherkesov and co-workers [2] and named ^-xylenolphthalexon S. Two indicators from this group, xylenol orange and methyl thymol blue, have found a wide application very soon in the volumetric analysis and as reagents for spectrophotometric determinations of the trace concentrations of metal ions. Lately, methylxylenol blue has also been often applied, especially in Japan labora­ tories. The formation of coloured complex compounds of methylxylenol blue has been utilized in the methods worked up for the spectrophotometric determination of iron [3], bismuth [4, 5, 27], mercury [6], calcium [7], gallium [8], zirconium [9 — 11], thorium [5, 12], cerium [13], lanthanoides [14], scandium and yttrium [15]. Methylxylenol blue, together with xylenol orange and methylthymol blue, belongs to the most sensitive reagents for the fluoride determination in waters [16] by means of the colour chelates of zirconium(IV) ions. The simultaneous spectrophotometric determination of aluminium and beryllium in mixtures was also studied [28]. Necessary assumption of a more thorough study of the complex compounds of methyl­ xylenol blue is to know its protolytic equilibria. They are studied in this communica­ tion.