MEASUREMENT OF H-ION CONCENTRATION 403 



total path on the left. The merit of the instrument consists in 

 the fact that the length of this total path is not affected by the mo- 

 tion of tube B, though the percentages of the path lengths in the 

 two solutions are varied directly thereby. The indicator solu- 

 tion in tube E consists of a buffer mixture (or solution, the pH 

 of which is to be determined) to which the proper amount of indi- 

 cator has been added. If conditions are such that 70 per cent 

 of the molecules encountered along the path on the right are in 

 the alkaline modification, and 30, in the acid, then the eye will 

 perceive identical impressions upon looking through the two sys- 

 tems from above. This will be the case, even if each modifica- 

 tion is not pure, but admixed with the other, ^ or if each modifica- 

 tion absorbs to some extent like the other, or if the indicator 

 exhibits dichromatism. Consequently the apparatus may be used 

 to determine the apparent percentage transformation of the indi- 

 cator at different hydrogen-ion exponents; the relation being 

 studied at different temperatures and subsequently being used 

 to determine unknown hydrogen-ion exponents.^ 



To control the temperature of the buffer solutions or of the 

 unknown solution, water can be circulated in a jacket (not shown 

 in the figure) about the tube E. The temperature should be 

 controlled to about one degree, or possibly better. 



It is evident that titrations can be carried out in the tube E, 

 a proper quantity of strong indicator solution being added for 

 every cubic centimeter, or smaller unit, of added reagent. 



^ It need not be the case if the indicator is grossly contaminated with another 

 indicator of different apparent dissociation constant, or if the indicator behaves 

 like a dibasic or polybasic acid. Wegscheider (1915) has made statements 

 equivalent to those in the text above. 



^ The instrument can of course be used at once and dependence put for the time 

 being on the apparent dissociation constants and tables published (Gillespie, 

 1920). If the indicator used, the temperature, and what information as may be 

 available as to the salt content of the solution, be recorded, the corrections can 

 be applied at any time when better values for the indicators and other data are 

 obtained. Although the writer can not admit that the method previously pub- 

 lished or the use of a double colorimeter is to be classed as approximate because 

 of doubtful optical assumptions, it is of course only approximate until precise 

 calibration of the standards is made. At present the instrument is capable of 

 giving more precision than could be obtained in the calibration made without 

 it, and it may possibly disclose some small deviations from the simple dissociation 

 curve. 



