ACIDITY 169 



Concentration of hydrogen ions may be expressed directly in gram-ions 

 per liter as above (comparable to moles per liter when considering a 

 given reagent). Usually, however, when dealing with biological ma- 

 terials this necessitates the use of relatively small decimal fractions 

 with attendant possibility of an error in writing. A more convenient 

 method is that of pH, by which one merely expresses the logarithm of 

 the reciprocal of the hydrogen-ion concentration. Thus the reciprocal 

 of 0.0000001, the hydrogen-ion concentration of water, is 10,000,000 or 

 10^, and the logarithm of this number is 7. The pH of pure water there- 

 fore is 7, and all solutions of such pH are said to be neutral. 



That acidic solutions have pH values less than 7 is apparent in view 

 of the fact that any concentration greater than 0.0000001 will have a 

 correspondingly smaller reciprocal and consequently a smaller logarithm 

 (pH value). Conversely, all basic solutions have pH values greater than 

 7. To those unfamiliar with this system of expressing active acidity, 

 it may seem that small differences in pH correspond to unbelievably great 

 differences in actual hydrogen-ion concentration. This can best be 

 realized by a comparison of several hydrogen-ion concentrations and 

 corresponding pH values simultaneously: 



Thus far only a tenfold (or some power thereof) increase or decrease 

 in hydrogen-ion concentration has been considered, but it is obvious 

 that between any two such values, e.g., 0.1 and 0.01 gram-ions per liter, 

 are countless possible concentrations with corresponding pH values. To 

 make interpretation of pH values easy Table 7-2 has been included. 

 Column 1 merely gives the approximate logarithms of some appropriate 

 numbers between 1 and 10 (listed in column 2) . 



Table 7-2 

 Comparison of pH values 



Approximate equivalent 

 Change in pH • change in acidity 



0.1 1.25 times 



0.2 1.6 time.s 



0.3 2.0 times 



0.6 4.0 times 



0.9 8.0 times 



1.0 10.0 times 



