252 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



vary slightly but to lie within the limits P H , 10 ~ 7 ' 68 and P H , 10 " 824 . 

 The latter value lies well within the phenol-phthalein range. 



It is to the Carlsberg school of workers, and notably to 

 Sorensen and Palitzsch 2 ' 3 ' 4 , that we owe, first, extensive and 

 careful determinations in various seas of the hydrogen-ion 

 concentration of sea-water, secondly, a painstaking and 

 elaborate investigation of the so-called " Salt Error " introduced 

 into colorimetric observations by the action of the neutral 

 salts of sea- water and chiefly the sodium chloride upon the 

 coloured indicators. 



Thanks to the labours of these observers, the deviation 

 caused by the salt is now accurately known : it has been 

 established that under given conditions this deflection in the 

 colorimetric results is constant, and hence can be allowed for 

 by deductions placed on record in the Sorensen-Palitzsch 

 tables. 



Accordingly, a rapid and accurate method is placed in 

 the hands of observers for estimating the hydrogen-ion 

 concentration of sea-water. 



This colorimetric method of Sorensen has therefore been 

 mainly used by us, but we have on certain occasions controlled 

 it, by using the electrometric method with the electrodes. We 

 have found concordance good when allowance is made for the 

 " Salt Error " by use of the tables. 



The Sorensen method has now become so well known that 

 it need not be described in detail,* so its principle only will be 

 pointed out. Mixtures in varying proportions of two solutions, 

 one with a higher the other with a lower hydrogen-ion 

 concentration, are prepared in a series of test-tubes. The 

 hydrogen-ion concentrations of these mixtures have been 

 directly determined once for all by Sorensen and tabulated so 

 that they can be referred to, and have also been plotted in 



* See Walpole, Bio-Chemical Journal, Vol. V, 1911, p. 207, and 

 Vol. VIII, 1915, p. 628. 



