2i8 wherry: determining soil acidity 



2X5 cm., made of heavy glass, to prevent undue breakage; 

 a container for water, which may conveniently be a screw-capped 

 jar holding 200 cc. or more, or an aluminum canteen : and a pi- 

 pette, most simply constructed of two pieces of glass tubing 

 a few cm. in length, connected by a rubber tube. 



The six indicators which have proved most satisfactory in 

 work with soils are: bromphenol blue, bromcresol purple, brom- 

 thymol blue, phenol red, methyl red, and o-cresolphthalein or 

 phenolphthalein. The first three are used, as recommended by 

 Clark and Lubs^, in about a i per cent solution in water, 

 titrated with dilute sodium hydroxide to their intermediate 

 colors; and the phenol red in a 0.5 per cent solution similarly 

 titrated. The methyl red and phenolphthalein are used as 

 0.02 per cent solutions in 50 per cent alcohol. It should be 

 noted here that litmus paper, which is often recommended for 

 testing soil reaction, is much less sensitive than the above indi- 

 cators, and may give misleading results.^ 



Most of these indicators are dichroic, showing different colors 

 as viewed by reflected and by transmitted light, and in the 

 writer's paper, above referred to, several of their colors were 

 rather inaptly characterized. In the new table here the former 

 descriptions have been improved upon, the colors given being 

 those produced by adding a drop of each indicator solution to 

 a few cc. each of buffer solutions with different reactions, as seen 

 through a i cm. layer against a white background. It has also 

 seemed desirable to add the numbers assigned to the various 

 colors in Ridgway's Color Standards; although in two cases, 

 bromophenol blue in Hquid of specific acidity 1000, and brom- 

 cresol purple in that of specific acidity 10, the colors are non- 

 descript and cannot be accurately placed. 



The special terms used in this table to describe the reactions 

 have recently been defined by the writer.' By way of summary 

 it may be stated here that the specific acidity is the amount of 

 acid present in a given solution, as measured by hydrogen ion, 



^ Journ. Bacteriology 2: 135. 1917. 



^ Gillespie and Wise. Journ. Amer. Chem. Soc. 40: 796. 1918. 



' This Journal 9: 305. 1919. 



