THE PREPARATION OF CULTURE MEDIA 139 



point. Solutions in which the hydrogen ion concentration is less than 

 1X10~ 7 (and the hydroxyl, therefore, more than 1X10~ 7 ) are alkalin, 

 and vice versa. This method of expression offers certain difficulties 

 to the plotting of curves, and it has been found simpler for such purposes 

 to plot curves according to the logarithms of the expressions used 

 above. Sorensen 5 has initiated a method of doing this, by intro- 

 ducing the symbol P H which signifies the logarithm of the reciprocal 

 of the hydrogen ion concentration, expressed as above, thus, P H equals 



tog of -L. 



Thus, if: 



H = 2X10- 4 , 



5> 2X10- 4 

 = log 5000 = 3.7. 



Another way of getting at this is as follows: 

 Supposing that, 



[H] = 2X10- 6 , 



Pj?/ = log. of reciprocal, 

 or, 



Now, 



log. 1 = 0, 



log. 2 = 0.3, 

 log. 10~ 6 =-6. 

 Therefore : 



or, 



P 7/ = 0-[+0.3-6] or 5.7. 



The following table gives the relationship of the old method of 

 expression to the new: 



5 Sorensen, Biochem. Zeit., 1909, 21, 131, 201. 



