LUBS AND CLARK: NEW HYDROGEN-ION INDICATORS 611 



as an acid and found the value, K = 1.05 X 10~^ at 18°. Palitzsch'' 

 states that the region in which methyl red can be used for colorimetric 

 determination of hydrogen-ion concentration lies between Ph 4.2 and 

 Ph 6.3, and that in this region a difference of 0.1 in the Ph^ value produces 

 a marked difference in the tint of the indicator. He also determined its 

 so-called protein and salt errors. The range of color extends from yel- 

 low on the alkaline side to red on the acid side. The brilliant and 

 sharp changes of methyl red and its utility in the presence of protein 

 and protein cleavage products makes it an excellent indicator for the 

 estimation of the hydrogen-ion concentration of bacteriological media. 



It was with the hope of finding an indicator showing color changes 

 at hydrogen-ion concentrations just below those which can be deter- 

 mined by methyl red that we began to synthesize other indicators of 

 this series. In our bacteriological work we found that it was necessary 

 to have an indicator which would give a fairly sharp differentiation be- 

 tween Ph 6 and Ph 6.5. Para nitrophenol is an excellent indicator for 

 this range in a colorless solution, but unfortunately its color cliange is 

 of such a nature that it is obscured by the natural color of most bac- 

 teriological media. 



We found by varying the substituent alkyls in the aniline residue of 

 methyl red that the resulting indicators show but little difference in 

 properties. Their colors vary only in intensity, and the hydrogen-ion 

 concentrations at which the changes occur differ but little. As would 

 be expected, the indicators with two substituent alkyls had a much 

 deeper color than the corresponding monoalkyl derivatives. 



INDICATORS OF THE METHYL RED TYPE INVESTIGATED 



Monomethyl Red, o-Carboxybenzene-azomonomethylaniline. — A speci- 

 men of this material was kindly sent to us by Dr. L. W. Jones. This 

 was recrystallized from alcohol. The nature of the color change was 

 practically the same as that of methyl red but far less intense. Changes 

 occur over the range Ph 4.25 to Ph 6.00. 



Methyl Red, o-Carboxybenzene-azodimethylaniline. — Previously de- 

 scribed. 



Monoethyl Red, o-Carboxybenzene-azomonoethylaniline. — This indi- 

 cator was prepared from anthranilic acid and monoethylaniline by the 



" C. R. d. trav. d. Lab. Carls. 10: 163. 1911. 



* Ph = log 7T-' For a discussion of the significance and convenience of this 

 symbol see Sorensen, loc. cit. 



