Sulfonephthaleins

This approximate relationship between the titration exponent and pAin is applicable to phenolphthalein and to many of the sulfonephthalein indicators introduced by Clark and Lubs (sec Table LXIV, page 364). If the darker color is obtained in acid solution, as is the case with methyl orange and methyl red, then it is approximately true that... 

Bromocresol green Tetrabromo-m-cresol- sulfonephthalein Yellow Blue 3.8-S.4 4.7... 

J Sulfonephthaleins (for example, bromophenol blue, bromocresol green,... 

Tetrabromphenol blue Tetrabromophenoltetrabromo- sulfonephthalein 3.0-4.6 yellow - blue Harden and Drake... 

The two transformation regions of the sulfonephthaleins and of the phthaleins. The sulfonephthaleins are not colorless in weakly acid solutions, but are present in the form of a yellow quinoid compound. The color alteration results from the following structural changes ... 

The sulfonephthaleins are much more stable towards alkali than the corresponding phthaleins. Accordingly, their decolori-zation in strongly alkaline solutions proceeds slowly. This was... 

In the following table are recorded the colors of the various sulfonephthaleins in strongly acid solution as observed by Kolt-HOFF.i The change in color of phenol red (P.R.) and of its dichloro- derivative should be observed in reflected light. 

The phthaleins too assume a color in strong acid solutions. In aqueous solution, these compounds exist almost entirely as lactones, although here too we must assume that a small fraction is present in the quinoid form. The acid form, however, does not become visible until the acidity attained is much greater than that required by the corresponding sulfonephthalein. We find for example that thymolphthalein is still colorless in 6 N hydrochloric acid, but pale rose in 9 N and dark violet in concentrated hydrochloric or sulfuric acid. Phenolphthalein is weakly rose-colored in 9 N and 12 N hydrochloric acid, but orange-brown in concentrated sulfuric acid, a-naphtholphthalein also is colorless... 

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. 

The sulfonephthaleins are very valuable indicators because their colors change very distinctly from yellow to red, blue, or 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. 

Dr. Klotz has found furthermore that the unbrominated sulfonephthaleins such as phenol red, cresol purple, thymol blue, etc., possess a marked tendency to form molecular compounds with their corresponding phenols. Apparently the stability of the indicator solutions depends upon this phenol content. 

The properties of the most important of the sulfonephthaleins will be discussed in greater detail in the following pages. 

Aside from the sulfonephthaleins listed in the table on page 108, several others have been reported. Although they offer no practical advantage over the compounds described in detail, their properties will be summarized for the sake of completeness. 

C. B. Wood reports that the indicator is a dark red, amorphous substance. The intensity of its aqueous solutions is only 1/10 that of other sulfonephthaleins. The same investigator finds a satisfactory color change in the pH range 0 to 1.5. Its use in alkaline solutions is not recommended because the color is unstable. 

W. C. Boyd and A. W. Rowe have described the properties of a new series of halogenated sulfonephthaleins. 

These new indicators as well as those of Clark and Lxjbs and of B. Cohen are halogenated sulfonephthaleins. Except for tetrabromo-phenoltetrabromosulfonephthalein, these compounds offer no special advantages over the indicators originally recommended. The properties of these new substances are summarized in the following table. 

It has already been stated that tetrabromophenoltetrabromo-sulfonephthalein (tetrabromphenol blue) is a good substitute for bromphenol blue because it changes from yellow to pure blue and exhibits no dichromatism. 

Tetrabromophenoltetrabromosulfone-phthalein CigHcOsBrsS M = 986 Tetrabromophenoltetrachlorosulfone-phthalein Ci9H606Cl4Br4S M = 808 Dibromo-o-cresoltetrabromophenol-sulfonephthalein C2iHi206Br6S... 

The behavior of thsonolbenzein, dibromothymolbenzein, and a-naphtholbenzein is somewhat abnormal because of their very-slight solubility in water. In alcoholic solution they resemble the corresponding sulfonephthaleins. 

A. Cohen has made use of mixtures of sulfonephthaleins. For example, a mixture of bromcresol purple and bromthymol blue is green-yellow at pH 6.0 and blue at 6.8. He recommends also solutions of bromcresol purple with bromphenol blue and of bromphenol blue and cresol red. Other combinations have also been proposed. I. M. Kolthofp has investigated a large... 

In the last column of the following table, A represents the difference between the indicator constant in alcohol and in water (pAoiss. Aio. — pAdus. Water). We See from the table that, in changing from the yellow to the acid color, the dissociation constants of the sulfonephthaleins in alcohol are 10 -10 times smaller than in water. This is of the same order of magnitude as the difference exhibited by benzoic acid, salicylic acid, and phenol. 

The semi-quantitative derivations may be tested experimentally. In 0.01 N acetic acid, methyl orange has a red-orange color which, upon addition of 40% alcohol, changes to a pure yellow (alkaline). The color of tetrabromophenoltetrabromo-sulfonephthalein, on the other hand, changes slightly towards the acid side under the same conditions. 

The sulfonephthaleins. Although the weak acid form of most phthaleins is colorless, that of the sulfonephthaleins is colored... 

According to Lund (l.c.), it is necessary to assume that in the yellow solution the sulfonephthalein has the following structure ... 

I. M. Kolthofp expresses doubt as to whether the sultone forms (la, Ib, and Ic) actually are present in solution. In any case, they need not be considered in quantitative studies of the color change of sulfonephthaleins. On the other hand, A. Thiel has shown that the sulfonephthaleins, although much more stable than the corresponding phthaleins, are also decolorized by an excess of alkali. The ions are derived from the colorless carbinol modification. 

These forms correspond to those of phenolphthalein. Their concentration in the case of sulfonephthaleins is so small, however, that it may be neglected in the quantitative treatment of the color change. The conversion of the weak acid (yellow) form to the alkaline form is therefore governed by the expression ... 

The color transformation of the sulfonephthaleins in strongly acid media is attributable to the formation of a cation involving the quinoid group see, however, Lund (l.c.)j... 

The sulfonephthaleins are characterized by pronounced color changes and by the stability of both forms. Bromphenol blue and bromcresol purple are less satisfactory for pH determinations since they show a marked dichromatism during transformation. Fortunately these indicators may be replaced by tetrabromo-phenoltetrabromosulfonephthalein, bromcresol green, and chlor-phenol red. 

G. PicHARD and R. Chaminade have studied the behavior of methyl red and various sulfonephthaleins when shaken with organic solvents. They added ten drops of a 0.1% methyl red solution or of a 0.04% solution of a sulfonephthalein to 10 c.c. of aqueous solutions at different pH s, and noted the color of isobutyl alcohol extracts. They made the following observations with isobutyl alcohol as solvent ... 

This conclusion can be tested theoretically. It is of general interest to know how the pH of an indicator mixture or of a buffer mixture changes when subjected to extreme dilution. At infinite dilution, of course, the pH of the solution is that of the pure solvent. Let us consider an indicator which consists of a mixture of a monovalent (acid form) anion and a divalent (alkaline form) sulfonephthalein anion. The equilibrium involved is ... 

The first dissociation constant of the sulfonephthalein is very large so that the hydrolysis of the HI" ions may be neglected. If we add to pure water an indicator solution for which the ratio [HI-]/[I 3 and the total amount of indicator are known, we should find that the ratio has changed due to the variation of pH with dilution. The [HI ] value diminishes with dilution and []I 3 increases if the indicator mixture has an acid reaction. 

---