Relationship With The Neutral Point of Water
Water molecules dissociate into equal amounts of H3O+ and OH−, so their concentrations are equal to 1.00 × 10−7 mol∙dm−3 at 25 °C. A solution in which the H3O+ and OH− concentrations equal each other is considered a neutral solution. In general, the pH of the neutral point is numerically equal to pKw/2.
Pure water is neutral, but most water samples contain impurities. If an impurity is an acid or base this will affect the concentrations of hydronium ion and hydoxide ion. Water samples which are exposed to air will absorb the acid carbon dioxide and the concentration of H3O+ will increase. The concentration of OH− will decrease in such a way that the product remains constant for fixed temperature and pressure.
Read more about this topic: Self-ionization Of Water
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