The pH Scale Is Used to Describe Acidity

The pH scale is a numeric scale used to express the acidity of a solution. Mathematically, pH is equal to the negative of the base-10 logarithm of the hydronium ion concentration  

Note again that brackets are used to represent molar concentrations, meaning [H30+] is read the molar concentration of hydronium ions. For understanding the logarithm function, see the Calculation Corner on page 344. 

Consider a neutral solution that has a hydronium ion concentration of 1.0 X 10 7 M. To find the pH of this solution, we first take the logarithm of this value, which is —7 (see the Calculation Corner on logarithms). The pH is, by definition, the negative of this value, which means —(—7) = +7. Hence, in a neutral solution, where the hydronium ion concentration equals 1.0 X 10-7 M, the pH is 7. 

Acidic solutions have pH values less than 7. For an acidic solution in which the hydronium ion concentration is 1.0 X 10-4 M, for example, pH = -log(l. 0x10 4) = 4. The more acidic a solution is, the greater its hydronium ion concentration and the lower its pH. 

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