CH. I.] 



DETERMINATION OF P H . 



21 



or " comparator." Cole and Onslow somewhat improved 

 this by using a comparator containing three pairs of 

 tubes, the arrangement being diagramatically shown in 

 fig. 3. The addition of a ground glass plate fixed to the 

 comparator between the eye and the tubes has been found 

 by the author very much to improve the apparatus, slight 

 differences in colour being readily detected. 



The standard solutions taken are such that the appear- 

 ance seen through Y is either intermediate between that 

 seen through X and Z, or identical with one of them. The 

 colour changes, and the ranges of the most useful indicators 

 are given below in Table I. Certain other interesting data 

 are presented in chart form in Table II. 



Fig 4. Cole and Onslow's Comparator. 



The author can very strongly recommend the new 

 sulphone-phthalein indicators* introduced by Mansfield 

 Clark, Lubs and Acree. For further information on the 

 subject of the colorimetric method of determination of P H 

 the student is referred to an important series of papers by 

 Clark and Lubs, " Journal of Bacteriology," Baltimore, 

 Vol. II., pp. i, 109 and 191 (1917). 



* The Cooper Laboratory for Economic Research, Watford, Herts., 

 has undertaken the manufacture of the sulphone-phthalein indicators. 

 They can be obtained in convenient standardised solutions, or in the solid 

 form, either direct or through Messrs. Baird and Tatlock and other agents. 



