CULTURE MEDIA 89 



acidity by the concentration of the hydrogen ion. Hydrogen ion 

 concentration, therefore, is the physical measure of acidity. 



Pure water is in part a solution of dissociated H+ and OH- 

 in undissociated H2O. Roughly the actual amount of dissociated 

 H+ in a liter of water is 0.000,000, i gram and the dissociated OH- 

 is 0.000,001, 7 gram, in each instance representing a concentration 



°^ 10 000 000 N<^™^^ °^ Normal X lo-^ which is Normal log- 7.0. 

 Sorensen and others after him have employed the symbol Pfj 

 to signify the logarithm of the reciprocal of the hydrogen ion 

 concentration, thus omitting the minus sign, so that in pure water 

 Pa = 7.0. In a watery solution the concentration of hydrogen 

 ions X the concentration of hydroxyl ions = a constant which is 



Normal or Normal X io~'*. As hydrogen ion 



I 



100,000,000,000,000 

 concentration increases, hydroxyl ion concentration diminishes. 

 Thus a Deci-normal NaOH has an H+ concentration of approxi- 

 mately Normal Xio-^^ This is expressed on Sorensen's scale as 

 P^ = 13. Conversely Deci-normal HCl has an H+ concentration 

 of about Normal X 10-^ or Pg — 1. 



In the presence of feebly dissociated compounds, phosphates, 

 salts of organic acids and especially proteins, a large fraction 

 of the hydrogen ions, introduced by the addition of a mineral acid, 

 are quickly combined in the undissociated state. The same fate 

 is met by hydroxyl ions introduced. The relatively lesser degree 

 of dissociation in such solutions therefore requires the addition 

 of greater amounts of acid or of alkali in order to bring about a 

 definite alteration in chemical reaction or measured alteration 

 in hydrogen ion concentration . This property of such solutions is 

 spoken of as a buffer effect. Obviously bacteriological media are 

 such as to exhibit this property in a conspicuous manner and to a 

 different degree, according to their composition. 



Clark and Lubs^ recommend a series of indicators which change 

 color at various hydrogen ion concentrations and their chart has 



^ Clark, W. M. an4 Lubs, H. A.: Journ. Bact., 1917, II, i, 109, 191. 



