614 LUBS AND CLARK: NEW HYDROGEN-ION INDICATORS 



plete drying can be secured at 120° C. The indicator obtained by this 

 procedure is quite pure. 



Properties. — Levy, Rowntree and Marriot^^ were the first to recog- 

 nize the excellence of phenolsulphone-phthalein as an indicator for the 

 colorimetric determination of hydrogen-ion concentration. A com- 

 plete discussion of the theory of color changes of this compound will 

 shortly be published by White and Acree. The range of color changes 

 is from yellow to reddish-violet and extends from Ph 6.50 to Ph 8.50. 

 Between Ph 7 and Ph 7.50 extremely sharp differentiations can be se- 

 cured but the indicator can be used with entire satisfaction over the 

 wider range. A solution of the sodium salt in water is used and such 

 a solution can now be purchased from any of the dealers in chemical 

 supplies. A 0.01 per cent solution is satisfactory for colorimetric pur- 

 poses. Use 4-6 drops. 



O-CRESOLSULPHONE-PHTHALEIN 



Preparation.- — ^This indicator was also prepared by Remsen and So- 

 hon." The method of preparation used in this laboratorj^ was, in gen- 

 eral, the same as that used for the analogous phenol compound. Ten 

 grams of o-cresol, 15 grams of the acid chloride and 10 grams of freshly 

 fused and pulverized zinc chloride were heated together for 6 hours at 

 165°-170°. The melt was then treated as previously described. The 

 filtration is much more difficult than in the case of the phenol com- 

 pound. Yield about 10 grams. o-Cresolsulphone-phthalein can be re- 

 ciystallized from glacial acetic acid. It is quite soluble in alcohol. 



Properties. — This indicator covers practically the same range as 

 phenolsulphone-phthalein. The color changes are very similar. On 

 the whole the phenol compound is superior to the cresol compound. 

 The phenol compound can be prepared more easily and shows slightly 

 sharper color changes. The indicator solution was prepared by dis- 

 solving 0.1 gram of indicator in 200 cc. alcohol. Use 2-4 drops. 



THYMOLSULPHONE-PHTHALEIN 



Preparation. — Ten grams of the acid chloride, 10 grams of freshly 

 fused and dehydrated zinc chloride and 15 grams of thymol were heated 

 for four hours at 140° C. with frequent stirring. The reaction mixture 

 must be stirred constantly during the first half hour of the reaction in 

 order to prevent a loss of material from frothing. The melt was boiled 



" Archiv. Int. Med. 16: 38. 1915. (See also Trans, of the Assoc, of Physi- 

 cians, 1915.) 



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



