162 CULTURE MEDIA 



Normal Na2S04 is 71 grams per liter. 



For a substance with three replaceable H atoms or equiv- 

 alent the normal solution will contain the number of grams 

 per liter equal to one-third the molecular weight. Normal 

 Na3P04 will contain one-third of 164.15 or 54.7 grams per 

 liter. 



The strength of a solution is then most commonly expressed 

 in terms of the normal solution, as § normal (N/2), 1/100 

 normal (N/100), etc. Reaction may be expressed in terms 

 of a normal solution of H ions for acid solutions, or of OH 

 ions for alkaline solutions. It is the usual practice to express 

 both in terms of a normal solution of H ions for the sake 

 of uniformity and convenience. 



The bacteriologist deals mainly with solutions whose 

 reaction is near the neutral point. Pure water is neutral in 

 reaction. The concentration of H ions in pure water is 

 1/10,000,000 of the concentration in a normal solution of 

 H ions, or is N/ 10,000,000. The H-ion concentration in 

 a normal NaOH solution is 1/100,000,000,000,000 of that of a 

 normal H ion solution. These fractions are unwieldy. It 

 is an improvement to write 1/10^ or 1/lO^S etc., though these 

 are still awkward. These fractions mean that in pure water 

 the number of H ions per cubic centimeter is the same as 

 in a cubic centimeter of a normal solution of H ions diluted 

 10,000,000 times and in normal NaOH solution it is the same 

 as in 1 cubic centimeter of a normal solution of H ions 

 diluted 100,000,000,000,000 times. 10,000,000 = 10^ = 

 log 7 and 100,000,000,000,000 = 10^^ = log 14. Sorensen 

 suggested the symbol pH for this logarithm of the dilution. 

 This expression has come into quite general use on account 

 of its simplicity. Thus, pure water would have a reaction 

 expressed as 2^117, normal NaOH, 2?H14, normal HCl, 2^H0. 

 With this method of expression the larger the figure the less 

 the acidity, and mce versa. Reactions expressed by whole 

 number and a decimal as 2^H7.2, 2^H 7.4 would really mean 

 that the number of H ions per liter corresponds to the number 

 per liter of a normal solution diluted the number of times 

 corresponding to that number of which 7.2 is the logarithm 

 or 8.4, etc. 7.2 is the logarithm of 12,195,121.9. 



It should be noted that a difference of unity in 2^H means 



