1919] 



DUGGAR AND DODGE COLORIMETER IN H ION DETERMINATION 67 



PI 



ID 



optical difficulties, and unless the indicator combines with the 

 test solution, the comparison may be perfect. 



In order that equal depths of liquid may be examined it is 

 only necessary to know, or gauge by suitable washers, the 

 distance a to b in the figure; then (if the inner cup is not the 

 exact length of the plunger) after placing the cups on the 

 carriers they are raised until the tip of 

 the plunger barely touches the bottom of 

 the inner cup when the position is read 

 on the scale. The cups are then lowered 

 to a distance equal to the line ab. It is 

 to be noted that the quantity of solution 

 to be placed in the cups is not neces- 

 sarily determinate, so long as there is 

 at least sufficient depth in each to equal 

 the distance ab. We have found that a 

 depth of 10-15 mm. of liquid is not too 

 great with the instrument employed, 

 assuming that the red indicators are 

 utilized. In order that air bubbles may 

 not catch under the lens the inner diam- 



. 



fiL 



Ji 





eter of the inner cup should be 4-5 mm. 



greater than the diameter of the plunger, Fig . L Special co i rimeter 



and similarly the inner diameter of the cu p s - 



outer cup should be correspondingly greater than the outer 



diameter of the inner cup. It is also evident that good optical 



glass should be employed for the bottoms of the cups. 



It is believed that the colorimeter may be employed in this 

 work almost as rapidly as the comparator, and certainly with 

 greater confidence and accuracy. For rapid work it is essen- 

 tial that one should understand the particular indicator in col- 

 ored solution; likewise the effects of the quality of light em- 

 ployed on the color of the field, but these are minor difficulties. 

 The red indicators, with color change red-yellow or yellow- 

 red, have proved particularly satisfactory. In the use of 

 these it soon became obvious that the usual P„ range of each 



might be considerably extended by the use of the colorimeter, 

 but in any case the extent of the useful range is somewhat 



