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pH: Log relationship

The pH scale is used to represent hydrogen ion concentration in solution. The range of possible hydrogen ion activity spans many orders of magnitudes from roughly 10 M to 10-15 M. The pH of a solution can be calculated using the equation:

pH = -log[H+]

In order to easily represent this wide range of activity, a logarithmic scale, which uses orders of ten, is used. As an example, a solution with a pH of 4 has a hydrogen ion activity that is 10 times greater than a solution at a pH of 5. On the pH scale, a value of 7 represents a neutral solution while a pH above 7 is basic, and below 7 is acidic.