LUBS AND CLARK: NEW HYDROGEN-ION INDICATORS 613 



diphenylamine compound occur between Ph 4.25 and Ph 5.50. A cold 

 saturated alcoholic solution of these two indicators was used. In all 

 other cases a solution of 0.1 gram of indicator in 300 cc. alcohol and 

 200 cc. distilled water was used. 



INDICATORS OF THE SULPHONE-PHTHALEIN TYPE 



Phenolsulphonephthalein was first prepared by Remsen and Sohon^" 

 by the condensation of phenol and the anhydride of o-sulphobenzoic 

 acid. An improved method which has been used in this laboratory is 

 based upon the action of phenol upon the chloride of o-sulphpbenzoic 

 acid in the presence of anhydrous zinc chloride. By the old method 

 a large amount of alkali insoluble material is obtained, while by the 

 new method practically none is formed. An additional advantage of 

 the new method lies in the greater ease with which the starting mate- 

 tials can be prepared. 



PHENOLSULPHONE-PHTH ALE IN 



Preparation. — The starting material was pure saccharin. From this 

 the ammonium salt of o-sulphobenzoic acid was prepared by substan- 

 tially the same method as described by Remsen and Holmes. ^^ The 

 chloride was prepared from the ammonium salt as described by Remsen 

 and Dohme.^2 Ten grams of the acid chloride, 15 grams of phenol 

 and 10 grams of freshly fused and pulverized zinc chloride were mixed 

 in a porcelain jar. As soon as the zinc chloride was added a vigorous 

 evolution of hydrochloric acid gas occurred and the temperature of the 

 reacting mass rose spontaneously to about 55° C, developing a deep red 

 color. The temperature was raised to 140° C. and held there for six 

 hours. During the heating the mass was frequently stirred with the 

 thermometer used to indicate the temperature. The reaction product 

 gradually became more viscous. When the heating was finished the 

 melt was disintegrated with hot water, boiled for a few minutes and 

 filtered with suction. After a thorough washing a bright red powder 

 was obtained. This was then dissolved in strong alkali, the solution 

 filtered and slowly poured into hot dilute hydrochloric acid, with con- 

 stant stirring. After boiling for a few minutes the flask containing the 

 sulphonephthalein was allowed to stand for several hours, and then 

 the indicator was filtered off with suction, washed and air dried. Com- 



10 Amer. Chem. Journ. 20: 257. 1898. 



11 Amer. Chem. Journ. 25: 205. 1901. 



12 Amer. Chem. Journ. 11: 332. 1889. 



