1922] 



ROSEN — A BACTERIAL DISEASE OF FOXTAIL 



355 



pure water is 0.0000001 normal acid and at the same time 

 0.0000001 normal alkali. Such fractions as 1/10,000,000 (0.000- 

 0001 ) being unwieldy, the logarithmic equivalents are used ; thus 

 1/10,000,000 may be written 10" 7 , and the hydrogen-ion concen- 

 tration of theoretically pure water may then be written 10 7 , or 

 expressed in the form of a negative logarithm as Sorensen does, 

 P H 7, which denotes the pressure of hydrogen ions (written va- 

 riously pH, P H , p^, the last being Sorensen's original method 

 [II, '09, p. 4] ) or, in other words, the hydrogen-ion concentration. 



TABLE I 



H-ION CONCENTRATION OF STANDARD SOLUTIONS OP ACIDS 



HC1 at 18° C. 



Acetic Acid at 18° C. 



Concentration 



H-ion con- 

 centration 



Normal 



(1.0) 



0.1 



0.01 

 0.001 



N 

 N 

 N 



0.0001N 



0.8 

 0.084 

 0.0095 

 9.7x10"* 

 9.8x10"° 



Ph 



Concentration 



H-ion con- 

 centration 



0.10 

 1.071 



2.022 

 3.013 

 4.009 



Normal 



(1.0) 



0.1 N 



0.01 N 

 0.001N 



Ph 



4.3 xlO 

 1.36x10 

 4.3 xlO 

 1.36x10 



-3 



4 

 4 



2.366 

 2.866 

 3.366 

 3.866 



At a given temperature the product of the concentration of H 

 and OH ions is constant and this value is called the "dissociation 



constant." For 



o 



C. this 



Iready been stated, the concentration of H ions and of 



OH ions is each 



As the dissociation 



temperature is always the same, the concentration of either 



+H or 



OH 



be expressed 



terms of the other. Thus 



1/100 N hydrochloric acid which has an hydrogen-ion concen- 

 tration of 10 2 and an hydroxyl-ion concentration of 10~ 12 is said 

 to have a value of Ph 2. Similarly, a 1/100 N sodium 



hydrogc 

 acids h 



hydroxyl-ion concentration of 10" 2 and 



a 



greater hyd 



concentration th 



12. All 

 an pure 



have a value 



(the smaller the logarithmic 



exponent of a number less than 1 the greater the number) ; con- 

 versely all alkaline solutions having a lower hydrogen-ion con- 

 centration than pure water have a value greater than Ph 7. 

 Table II adonted with modifications 'from Medalia 



(II, '20, p. 



3) may help in an understanding of the matter. 

 Good accounts of the principles involved in hydro 



may be had in th 



Sorensen (II. '09). Wash 



