The Estimation. of Acidity. 237' 



tion of hydrogen ion concentration. Its sensitiveness is remarkable, 

 and this will perhaps be better realised when it is pointed out 

 that its use has made possible as an ordinary laboratory procedure 

 the estimation of the change in the hydrogen ion concentration 

 in the blood, which controls the mechanism of respiration. (5) 



The method is free from the objections' inseparable from the 

 use of colorimetric methods, and it may be used to determine 

 with great exactitude the end point in a titration, where a definite 

 cencentration of hydrogen ions is aimed at. Furthermore, since 

 no reagents are introduced, the solution undergoing investigation^ 

 remains the same at the end of the examination as at the be- 

 ginning. 



In comparing the electrometric with the colorimetric method 

 this very important fact must be kept in mind — titration methods 

 exhibit the total amount of acid or alkali present, i.e. dissociated, 

 plus undissociated, since with every addition of standard acid 

 or alkali chemical combination takes place, and a further portion 

 of the dissolved substance becomes dissociated. This process will 

 continue until the whole of the reacting acid or base is neutralised,, 

 and thus, not the actual, but the available amount of reacting salt 

 is shown. 



The routine use of such an exact method has become a necessity 

 to the biochemical worker. To take a case in point, one of the 

 notable advances in both human and comparative physiology 

 has been the recognition of the great importance which attaches, 

 to the maintenance of the blood at an hydrogen ion concentration, 

 which varies only within the narrowest limits. In the blood, an 

 elaborate mechanism, involving the " buffer " action of its con- 

 tained carbonates and phosphates exists for this purpose. As Bay- 

 liss points out, in the case of the frog, " the heart is affected by so 

 small a change in hydrogen ion concentration as that from neu- 

 trality, HxlO -7 - 7 , to one of HxlO -6 - 5 , while a rise in H. concentra- 

 tion corresponding to that effected by adding 0.036 mgm. of HC1 

 to a litre of distilled water, i.e., to HxlO -6 , is fatal." 



To quote Bayliss's words, " If we were dealing with distilled 

 v/ater only, the addition of one-millionth of a gram molecule of 

 HO to a litre of distilled water would raise its hydrogen ion con- 

 centration from HxlO -7 to HxlO -6 - 5 , and such a change as this re- 

 presents would be ten times in excess of that which would be 

 fatal to many protoplasmic processes." ( 1) 



The electrometric method is of service in investigations involv- 

 ing the optimum reaction of enzymes. 



