256 ClyARK: REDUCTION POTENTIALS 



tion of various biochemical phenomena will be evident, but the 

 significance of such data is of broader scope, because the efforts 

 that have previously been made to bring organic compounds 

 within the range of potential measiu-ements have yielded few 

 data of value. 



In a subsequent paper the writer hopes to discuss in detail some 

 of the theoretical aspects concerned in the measurement of reduc- 

 tion potentials, together with certain biochemical applications. 

 But for the present only the data obtained with indigo and 

 methylene blue will be presented. 



The original sample of the indigo used in the following experi- 

 ments was labeled "Indigo sodium sulfonate dye." Mr. Zoller 

 made a sulfur determination upon the purified material which 

 indicated that it was Ci6H9N202S03Na. 



Mr. Zoller prepared from the original material some purer 

 material by precipitating the dye from its aqueous solution with 

 pure alcohol. This was dried in a current of air and then in an 

 air oven at 105° C. It was this material that was used in the 

 following experiments. 



The methylene blue was prepared by Mr. Zoller from Schu- 

 maker and Busch's "medicinal methylene blue." For the puri- 

 fication the method of Bemthsen- was followed in the main. The 

 crystals were dried in vacuo over stick KOH and concentrated 

 sulfuric acid. It thus attained constant weight. 



The most convenient form of these two substances is their 

 oxidized state. Therefore it is required to find a reducing agent 

 whose own oxidation-reduction equilibrium does not seriously 

 overlap the range of potential covered by the dyes in question. 

 Titanium trichlorid has been used by Knecht and Hibbert^ for 

 the quantitative estimation of methylene blue, indigo, and a 

 variety of other dyes. However it is customarily used in very 

 acid solutions and in a range of Ph which would have little sig- 

 nificance for biochemical studies. Titanium, however, like iron, 

 enters into complexes with hydroxy-acids and then remains 



^ Ann. 230: 139. 1885. • 



3 New reduction methods in volumetric analysis. 



