DETERMINA TION OF THE A CIDITY. 5 7 7 



addition of extraneous alkali. The reaction usually betokens that the 

 monohydrogen salts exist in larger proportion than the dihydrogen, but it 

 prevails through a considerable range of variations in this proportion ; 

 its exact limits depending in part upon the delicacy of the litmus paper 

 used. Throughout the range of the amphoteric reaction a solution of 

 litmus, actually mixed with the fluid, retains a violet or neutral colour 

 practically unchanged. 



Heintz attempted to explain this amphoteric reaction as follows. The 

 red colouring matter of litmus acts as a dibasic acid, forming with bases, 

 either unsaturated salts which are violet, or saturated salts which are blue. 

 From the saturated salt the dihydrogen phosphates may extract half the base, 

 leaving the unsaturated violet salt. The monohydrogen phosphates, on the 

 other hand, may yield to the red acid substance sufficient base to form also 

 the violet compound. In an amphoteric mixture the affinities are so balanced 

 that this violet compound can alone exist. When, however, the acid phos- 

 phate is present in sufficient excess, it removes all the base and leaves the red 

 free acid ; with a large excess of the more basic phosphate, on the other hand, 

 the litmus acid obtains its full complement of base and forms its blue salt. 



With other indicators we can obtain a colour change at a more 

 definite point during the process of alkalisation of an acid urine, and to 

 the use of these we shall shortly return. But it should be made clear 

 that only in the interaction between a " strong " acid and a " strong " 

 base is the colour change, with an indicator, synchronous (or approxi- 

 mately synchronous) with the final replacement of all the acidic hydrogen 

 atoms by the base. From this special case we have come to attach a 

 definite value to the expression "degree of acidity," which is not 

 found when we are dealing with such a substance as phosphoric acid. 

 The " acidity " is here a quantity varying with the indicator used. The 

 coloured indicator is itself an unstable compound which, in the play of 

 acid and basic affinities, suffers a definite change when a certain point 

 of equilibrium is reached. This point will depend upon the relative 

 stability of the indicator and of the phosphates with which it is in con- 

 tact, and may or may not occur simultaneously with the removal of 

 all replaceable hydrogen from the latter. 



The " degree of acidity " of a certain quantity of acid phosphate, in 

 solution by itself, will be greater than that of an equal quantity mixed 

 with a proportion of the more basic phosphates ; and this is true, no 

 matter what the indicator used. During the process of neutralisation 

 by the standard alkali, the proportion of the more basic phosphates is 

 gradually increased until the tendency of these to affect the indicator 

 in one direction eventually balances the action of the acid phosphate 

 in the opposite direction. This "neutral" point will evidently be 

 reached the sooner, if some basic salt was originally present before 

 titration was commenced. 



Such considerations as these have led to a proposal to estimate the 

 acidity of urine, not by simple titration, but by actually determining the 

 proportion between the acid phosphates and the more basic phosphates 

 present. For this purpose, Lieblein, 1 after a careful study of the matter, 

 has recommended the process of Freund, which is an application of the 

 barium precipitation method referred to above. The total phosphoric acid 



1 Ztschr. /. physiol. Chem., Strassburg, 1895, Bd. xx. S. 52-88. In this paper a criti- 

 cism of other methods will be found. 

 VOL. I. 37 



