84 CARNEGIE INSTITUTION OF WASHINGTON. 



Refinements in the Indicator Method of Hydrogen-ion Determination, 

 hy B. M. Duggar. 



The inherent complexity of the hydrogen electrode, or gas-chain 

 method, of determining hydrogen-ion concentration, and the care 

 requisite for the maintenance of the apparatus in standardized condi- 

 tion, are some of the factors which operate against the general appli- 

 cability of this method in most biological laboratories. Further than 

 this, there appear to be relatively few data bearing upon the possible 

 errors or complications arising from the use of the gas-chain method 

 with complex biological fluids of unknown composition. 



In biological work, moreover, a proximate determination promptly 

 made at any moment with a relatively small volume of fluid may be 

 far more important than a delayed precise determination, accurate to 

 the second decimal of the pH exponent. With the relatively stable 

 and easily prepared thallate and phosphate standard solutions, like- 

 wise with the newer thymol, phenol, cresol, and other indicators, 

 which have recently been developed through the work of Clark and 

 Lubs, the determination of the pH of colorless solutions is readily and 

 accurately carried out. 



The chief difficulty with the indicator method as applied to plant 

 juices, decoctions, etc., has been the factor of color or pigment in the 

 sample. Simple comparators, whereby ''shield" solutions have been 

 employed in test-tubes, have been unsatisfactory in compensating for 

 the color of the sample. It has been shown in an earlier paper that 

 practically all difficulties are removed by the adaptation of certain 

 types of colorimeters in this work, and the development of the method 

 has been pursued during the summer of 1919. 



The microcolorimeter of Duboscq has proved admirable for this 

 work, both because of its form, the solidity of the cups, and the fact 

 that it may be employed, where necessary, when the total volume of 

 solution available does not exceed 2 c.c. 



The usual method of employing a colorimeter is of course that of 

 comparing the sample colored solution in one cup with a standard 

 solution in the other cup, the latter corresponding to a known quantity 

 of the substance for which the test is made. The cups are raised or 

 lowered by the adjustment screws until a match is obtained, and the 

 depths of the columns are then inversely proportional to the concen- 

 tration, so that the value of the sample is determined in terms of the 

 standard. 



In determining the hydrogen-ion concentration of pigmented fluids 

 or juices, the colorimeter has two essential functions: (1) the con- 

 venient use of shield solutions, thus compensating with the least 

 possible optical difficulty for the native color of the sample, and (2) 

 affording an accurate means of determining in which standard solution 

 (of known pH) the color-cliange of the indicator corresponds with 



