482 LUBS AND CLARK : HYDROGEN-ION INDICATORS 



better products. These modifications will be discussed under 

 the indicators in question. At the end of the article will be found 

 a list of the sulphone-phthaleins which have been investigated in 

 this laboratory, with their color changes and the r«anges over which 

 the changes occur. An account of our investigations of the 

 more useful indicators will shortly appear in the Journal of 

 Bacteriology. 



O-CRESOLSULPHONE-PHTHALEIN 



Ten gramsof the chloride of o-sulphobenzoic acid, 10 grams of freshly 

 fused zinc chloride, and 15 grams of o-cresol were heated for 6 hours 

 at 110°-120°, instead of at 165°-170° as previously recommended. At 

 the lower temperature a purer product was obtained. 



DIBROM-O-CRESOL-SULPHONE-PHTHALEIN 



This indicator was prepared practically as described by Sohon. 3 Two 

 grams of o-cresolsulphone-phthalein was suspended in 10 cc. of glacial 

 acetic acid and 2 cc. of bromine was added. The flask was allowed to 

 stand over night and the reddish-white crystals were filtered off the 

 next morning. These crystals can be recrystallized by dissolving in 

 boiling toluol and allowing the solution to cool. 



The color changes are from yellow to a brilliant purple and occur over 

 the range Pj 5.2 to P+ 6.8. A 0.04 per cent aqueous solution of 

 the mono-sodium salt is satisfactory for the indicator solution. 



THYMOLSULPHONE-PHTHALEIN 



This compound can be prepared more satisfactorily by heating the 

 mixture of the chloride of o-sulphobenzoic acid, zinc chloride, and 

 thymol at 100°-110°for 6 hours, instead of at 140° as recommended in 

 our previous paper. 



DIBROMTHYMOL-SULPHONE-PHTHALEIN 



In our earlier paper this compound was described simply as brom- 

 thymol-sulphone-phthalein. Subsequent analyses have shown that it 

 is the dibrom compound. 



3 Amer. Chem. Journ., 20: 257. 1898. 



