560 J. HOWARD BROWN 



the symbols BI(pH 8 - 5) is the sum of the reserve acidity plus 

 the reserve alkalinity, each value being expressed in terms of 

 per cent normal acid or alkali, i.e., the number of cubic centi- 

 meters of N/1 acid or alkali required to change the hydrogen 

 ion concentration of 100 cc. of medium from one stated hydrogen 

 ion concentration to the other. In figure 1 the reserve acidity 

 is represented on the abscissa by the distance A to B, the reserve 

 alkalinity by A to C, and the buffer index by B to C.^ 



The values referred to may be determined by at least three 

 methods which are as follows. 1. The reserve acidity may 

 be titrated with alkali from pH n to pH 8.0 and then using the 

 same sample the buffer index may be titrated with acid from 

 pH 8.0 to pH 5.0. The reserve alkahnity is calculated by sub- 

 tracting the former from the latter. 2. The reserve alkalinity 

 may be titrated with acid from pH n to pH 5.0 and then using 

 the same sample the buffer index may be titrated with alkaH 

 from pH to 8.0. The reserve acidity is calculated by subtracting 

 the former from the latter. 3. The reserve acidity may be 

 titrated with alkali from pH n to pH 8.0 in one sample and the 

 reserve alkalinity titrated with acid from pH n to pH 5.0 in 

 another sample, the buffer index then being calculated by addi- 

 tion of the other two values. Identical results may be obtained 

 by all three methods if the dilution of the color of the medium 

 and of the indicator is carefully controlled. Alany of the results 

 here reported were obtained by the first method. However, 

 the third method is the simplest and is described in detail in 

 the appendix to this paper. It is hardly necessary to point out 

 that the titrations may be controlled electrometrically quite as 

 well as by the colorimetric method, the potentiometer merely 

 taking the place of the color indicators. 



Samples of bouillon from five different laboratories, indicated 

 by the letters A, B, C, D, and E, have been titrated and the 



2 Whereas the reserve acidity and reserve alkalinity change with each change 

 in the reaction of the medium, the buffer index may remain constant. 



The prevalent method of titrating media against sodium hydrate with phe- 

 nolphthalein as an indicator is actually a titration of the reserve acidity to an 

 end point of about pH 8.5. 



