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The pH scale represents more than simple numbers, with each number corresponding to a 10-fold increase in acidity or alkalinity over the next number.

What exactly does a pH number mean?

It is a measure of the concentration of hydrogen ions in solution. The higher the concentration of H⁺(aq), the more acidic is the solution and the lower the pH.

If the concentration of hydrogen ions of a solution is known and represents the degree of acidity, why is the pH scale needed?

The concentration of hydrogen ions in solution is indeed a useful way of expressing the degree of acidity or alkalinity of a solution. However, this involves manipulating negative powers of 10. The pH scale was developed (Soren Sørenson, 1909) as a convenient method of working with various concentrations of solutions that varied by factors of ten.

The numbers are more manageable using base 10 logarithms. For example, the log of the concentration 0.001 mol dm^–3 is simply –3. In order to simplify the numbers further, Sørensen applied a negative sign to these numbers to make a positive, so in this example, –3 becomes 3.

In general, it is expressed as pH = –log [H⁺(aq)].

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