Definition of pH

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Definition of pH - pH is defined mathematically as the negative logarithm (base 10) of the H3O+ (oxonium) concentration.

pH = -log [H3O+]      (Brønsted-Lowry)

H2O    <-->    H3O+   +   OH-

Example: The oxonium ion (H3O+) concentration of a solution with a pH of 1.0 is 100 times greater than a solution with a pH of 3.0.  In pure neutral water the concentration of oxonium ions and hydroxide ions (OH-) are both equal to 10-7 per liter.

When the pH is below 7, the solution is acidic

When the pH is 7, the solution is neutral

When the pH is above 7, the solution is basic (alkaline)

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