ACIDITY 305 



hydrochloric acid is 1.0; of one-tenth normal acid, 0.1; of pure 

 water 0.0000001; and of normal sodium hydroxide, 0.000000- 

 00000001; these values expressed in terms of their logarithms 

 become 0, -1, -7, and -14 (10°, lO-i, 10"^ 10-^^). In nature 

 and in usual physiological work, the hydrogen-ion concentration 

 is always less than one (that of a normal and fully dissociated 

 acid); consequently, the exponent, or logarithm, is a negative 

 one, yet it is more convenient to express acidity as a positive 

 value. This is done by using the logarithm of the reciprocal 

 of the hydrogen-ion concentration; this logarithm is known as 

 the pH of the solution. H stands for hydrogen, and p for 

 "Potenz" (Ger., power), i.e., the exponent, or logarithm. The 

 pH of a solution is, therefore, the exponent, or logarithm, of the 

 reciprocal of the hydrogen-ion concentration. 



pH = log|jj+j 



If, now, hydrogen-ion values ranging from 1.0 to 10~^^ are 

 expressed on the same scale mentioned above, but this time in 

 terms of pH, a sheet of paper only 14 in. long is needed, and very 

 cumbersome mathematics is avoided, though some inaccuracy 

 is thereby introduced. 



Decreasing acidity (10~^, 10~^, 10"'^, etc.) means increasing 

 pH (5, 6, 7, etc.). This is unfortunate and confusing, though 

 more troublesome to the beginner than to the advanced worker, 

 who soon gets into the habit of thinking in terms of pH. Wherry 

 has tried to simplify the matter for the nonchemical investigator 

 by calling the neutral point of pure water (pH 7) unity ; then the 

 "specific acidity" of a solution having a pH of 6 is 10, which is 

 the actual relative concentration of the hydrogen ions, i.e., ten 

 times as great as in pure water. Lemon juice, with a pH of 

 about 2 (the pH of iV/100 HCl is 2) has, on this basis, a "specific 

 acidity" of 100,000 (times that of water); and sea water, with 

 a pH of 8.2, has a "specific alkalinity" of slightly over 10. 



But there is a greater difficulty which often leads to a 

 faulty conception of the magnitude of a change in acidity. The 

 difference between pH 4 and 5, 5 and 6, and 6 and 7 (the loga- 

 rithms of the hydrogen-ion concentrations) is in each case 1, 

 but the difference in the actual concentrations of hydrogen ions 

 is in each case ten times that of the preceding value, which 



