Separation and acid equilibria of xylenol orange and semi-xylenol orange

A mixture of Xylenol Orange (XO) and Semi-Xylenol Orange (SXO) was chromatographed on a cellulose column and ion-exchanged with Diaion SK-1 resin. XO and SXO in the acidic forms were titrated with sodium hydroxide to determine the basicities and the acid formation constants. The absorption spectra of XO and SXO were measured over a wide pH range, and were used for the calculation of acid formation constants, molar absorptivities, and the effect of ionic strength on the activity of XO. The purity of a commercial XO was measured by absorption spectroscopy, and it was found that the sample contained 36.3% of XO and 17.2% of SXO. Pure XO and SXO form a 1:2 and a 1:1 complex respectively with Zn(II). On the basis of these data the ionic structure of XO and SXO have been compared with those of Cresol Red and iminodiacetic acid, and discussed.