CLARK: REDUCTION POTENTIALS 257 



unprecipitated even in markedly alkaline solution. Citric acid 

 is one of the hydroxy-acids which form such complexes, and it also 

 makes an excellent material with which to buffer the hydrogen- 

 ion concentration throughout a considerable range of Ph- Re- 

 duced titanium in the form of titanium trichlorid was therefore 

 added to citrate and the solution was buffered either with citrate 

 mixtures alone or with citrate in addition to a preponderance 

 of other buffer mixtures. The buffer salts were kept about tenth 

 molecular while the titantium was present at about 0.003 ^ 

 concentration. The dye solution containing about the same 

 normal concentration of the dye was buffered by the salt solution. 

 Thus a constant pn was guaranteed throughout the titration. 

 Hydrogen-electrode measurement of these mixtures was imprac- 

 ticable because of the oxidizing action of the dye in the one case 

 and of the unreduced titanium in the other. Therefore depend- 

 ence was placed on the hydrogen-electrode measurements of the 

 Ph of the buffer solutions made up without the oxidizing and 

 reducing agents. The addition of these reagents in concentra- 

 tions of only 0.003 ^ should not have seriously affected the Ph 

 of o.i molecular buffer mixtures. 



The procedure was as follows : 



The indicator solution was placed in an electrode vessel similar 

 to that described by Hostetter and Roberts"^ for the electrometric 

 titration of iron, but provided with gas-tight connections. The 

 vessel was then flushed with nitrogen from a tank. This tank 

 nitrogen contained a little oxygen and was therefore run through 

 a heated tube containing copper wire previously reduced with 

 hydrogen. The solution was then boiled in a vigorous current of 

 nitrogen and while steam was still escaping the tip of the burette 

 containing the reducing agent was forced into a tight-fitting hole 

 on the head of the apparatus. There were then passing into the 

 flask through the "head": The burette tip which delivered the 

 reducing agent above the surface of the solution; the entering 

 tube for the nitrogen which delivered the gas above the middle 

 of the solution surface; the exit for the gas, which was flush with 



* Joum. Amer. Chem. Soc. 41: 1337. 1919. 



