text: Water molecules dissociate into equal amounts of HO and OH, so their concentrations are almost exactly  at 25 °C and 0.1 MPa. A solution in which the HO and OH concentrations equal each other is considered a neutral solution. In general, the pH of the neutral point is numerically equal to pK.
  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 hydroxide ion. Water samples that are exposed to air will absorb some carbon dioxide to form carbonic acid (HCO) and the concentration of HO will increase due to the reaction HCO + HO = HCO + HO. The concentration of OH will decrease in such a way that the product [HO][OH] remains constant for fixed temperature and pressure. Thus these water samples will be slightly acidic.  If a pH of exactly 7.0 is required, it must be maintained with an appropriate buffer solution.