Atkins — Factors affecting Hydrogen ton Concentration of Soil. 375 



noL necessarily the most favourable for living cells, which can, in different 

 species, tolerate a large range of acidity and alkalinity, a reaction suitable 

 for one being too acid for another. 



(f) The colorimetric method of determining liydrogen ion concentrations. — 

 In addition to the electrical method, using a potentiometer and hydrogen 

 electrode, or the more recent method with a condenser, which requires a less 

 expensive outfit, the colorimetric method introduced by Friedenthal (1904) 

 and Salm (1904) is available. It is based upon the potentiometer method 

 as a standard, the latter being used to determine the pH values of the buffer 

 solutions, made up to be at convenient intervals on the pH scale. Measured 

 amounts of various indicators are added to these solutions. The electrical 

 method is also used to check sources of error, such as those due to proteins 

 and salts, which cause the indicators to give readings higher or lower than 

 the true values. 



The colorimetric method was improved and extensively tested by Sorensen 

 (1909), who introduced new indicators, and eliminated those liable to 

 mislead. More brilliant water-soluble indicators of the sulphone phthalein 

 series were introduced by Clark and Lubbs (1917), as well as standard buffer 

 solutions, having certain advantages over those of Sorensen. The indicators 

 and standard solutions used in this research are those described by Clark 

 and Lubs. A full account of the method is also given by Clark (1920) 

 and by Cole (1920). Clark gives a coloui'ed chart, which can also be 

 purchased separately, and is a great aid to field work and to approximate 

 work in the laboratory. In most cases the colours are faithful renderings 

 of those in the standard tubes, with the same indicator in the specified 

 amount. 



It may be added that the preparation of standard buffer solutions by 

 each worker is no longer a necessity, as, owing to the work of Meacham, 

 Hoptield, and Acree (1920), a stable series of salts has been found, which is 

 issued on the market in tabloid form. This series covers the range from 

 pH 2'65 to 9*65 by twenty-eight standards, at convenient, but irregular, 

 intervals. Those in the Clark and Lubs series are uniformly pH 0-2, from 

 pH 1'2 to pH lO'O. In work of this nature care should be taken to check 

 and cross-check the solutions and indicators, both when freshly made up and 

 after storing. It is always advisable to use two or more indicators in 

 orienting experiments, and the use of several indicators greatly increases the 

 accuracy of work carried out without standard tubes. 



Methods have aLo been devised for colorimetric determinations without 

 recourse to standard buffer solutions, by employing, in varying number, 



