Sulfonephthaleins preparation

Fortunately we are able to replace bromphenol blue and brom-cresol purple with other sulfonephthaleins which do not show dichromatism. W. C. Harden and N. L. Drake have prepared tetrabromophenoltetrabromosulfonephthalein (tetrabrom-phenol blue) which may be used in place of bromphenol blue. Both indicators have the same transformation range (pH 3.0-4.6) with a color change from pure yellow to pure blue. Chlor-phenol red, with a color change from yellow to red, is a good substitute for bromcresol purple. 

Preparation, purity, and properties of the sulfonephthaleins. The preparation of the various sulfonephthaleins from o-sulfobenzoic acid, molten zinc chloride, and a phenol (which may be halogen substituted) has been described by Clark and Lubs. A. Cohen has reported his method of obtaining xylenolsulfone-phthalein, while a number of other valuable sulfonephthaleins have become available through the work of Barnett Cohen. ... 

The common sulfonephthaleins may be procured from a number of manufacturers, although their purity leaves muck to be desired. Several authors have observed that preparations from various sources differ greatly in the color intensities of their solutions. Although the degree of purity of an indicator is of secondary importance in many colorimetric pH measurements,... 

I. M. Kolthoff and T. Kameda have preferred to examine the purity by titrating the indicator solution conductometrically. The sulfonic acid group in sulfonephthaleins behaves as a strong acid and in a titration with alkali, the conductivity diminishes just as in the case of a strong acid. A break occurs in the titration curve when the sulfonic acid group is neutralized, and thereafter the conductivity increases until the phenol group is neutralized. A second break occurs at this point and from there on the conductivity increase is due to the free alkali added. Thus two breaks are obtained and for pure preparations the quantity of alkali required to reach the first point should equal the quantity used up between the first and second. Relatively few of the samples investigated by the author fulfilled this requirement. 

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