SEA-FISHERIES LABORATORY. 235 



3. The hydrogen-ion concentration was also determined 

 electrometrically on several occasions, although this is a difficult 

 estimation with sea- water on account of nearly all the " Re- 

 activity " or " Buffer " action being due to bi-carbonate of 

 magnesium. This was done to give a control to the colorimetric 

 method, and on the whole the agreement was found to be 

 excellent, when Sorensen's correction for the salt error had been 

 applied. 



A somewhat exclusive importance has been attached in 

 recent years to a determination of the hydrogen-ion concentra- 

 tion of a physiological fluid such as blood-serum or sea- water. 

 Important as this figure may be in indicating the actual 

 reaction of the fluid, it gives no idea of the resistance of that 

 reaction to change when alkali or acid is added, and from the 

 point of view of the protective action of the physiological 

 fluid to any living organisms of which it may form the 

 environment or " external medium." This resistance to 

 change is an all-important factor. 



It is fashionable to-day to look askance at earlier methods of 

 titration to coloured indicators, and to say that these only give 

 the amount of acid or alkali necessary to bring the solution to 

 an arbitrary level of hydrogen-ion concentration at which 

 the indicator employed changes colour, and so titrations can 

 furnish no indication of the hydrogen-ion concentration of the 

 solution in its original condition. While this is true, and 

 while we may now smile at the old-time results with coloured 

 indicators which led to the same solution being written down 

 as both acid and alkaline, and gave rise to such phrases as the 

 " acidity " and " alkalinity " of the blood, it does not follow by 

 any means that titration of physiological solutions is based on 

 error and should be abandoned. In order to appreciate the 

 properties as to alkalinity and acidity of a solution two things 

 are essential, (1) to know the actual hydrogen-ion concentration 

 and (2) the amount of alkali or acid required to be added to 



