The hydrogen-ion exponent pH

19 THE HYDROGEN-ION EXPONENT (pH) In the practice of chemical analysis one frequently deals with low hydrogen-ion concentrations. To avoid the cumbersome practice of writing out such figures with factors of negative powers of 10, Sorensen (1909) introduced the hydrogen-ion exponent or pH, defined by the relationship  

the quantity pH is equal to the logarithm of the hydrogen-ion concentration with negative sign, or the logarithm of the reciprocal hydrogen-ion concentration. It is very convenient to express the acidity or alkalinity of a solution by its pH. From the considerations of Section 1.18 it follows that the pHs of aqueous solutions will in most cases lie between the values of 0 and 14. In a 1m solution of a strong monobasic acid 

If a solution is neutral, pH = -log 10-7 = 7 From the above definition it follows that 

The term pOH is sometimes used in an analogous way for the hydroxyl-ion exponent, that is 

Example 3 Calculate the pH of a solution of 0 0 1m acetic acid (cf. Example 1 in Section 1.16). 

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For many purposes, especially when dealing with small concentrations, it is cumbersome to express concentrations of hydrogen and hydroxyl ions in terms of moles per litre. A very convenient method was proposed by S. P. L. Sorensen (1909). He introduced the hydrogen ion exponent pH defined by the relationships ... 

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