402 



LOUIS J. GILLESPIE 



could be determined with far greater ease and precision than is 

 possible with a one-colored indicator in the usual colorimeter, since 

 the quality changes very rapidly with the adjustment. Plane 

 polished surfaces are desirable in the optical system, but were not 

 used. 



The glass vessels A and C are fixed in position, and B can be 

 moved up or down, the motion being measured by a pointer 

 (not shown) fixed to B and moving upon a scale divided into 100 

 parts. The instrument is so made that the pointer moves from 

 to 100 when B moves from contact with C to contact with A. 

 The acidified indicator solution of suitable strength may be placed 

 in B and an alkaline indicator solution of the same strength 



'A 



B 

 C 



E 



Fig. 1. Colorimeter for Two-colored Indicators 



placed in C. A is left empty.*' Then, if the scale reads 70, the 

 path of light along the left-hand dotted line passes through the 

 alkaline form during 70 per cent of its path in the indicator, and 

 through the acid form during 30 per cent. The light along the 

 right-hand dotted line traverses an indicator solution in tube E, 

 again of the same strength, and over a path equal in length to the 



° For use in the determination of pH, a tube containing unknown solution 

 without indicator can be slipped into tube A in order to compensate for color or 

 turbidity without lengthening the apparatus unduly. In this case, water would 

 be introduced into D to equal height in order to equalize absorption and the 

 meniscus effects. 



