How does the pH scale work?
While highly alkaline waters can corrode the "white metals," the most common reason for water treatment is to prevent or eliminate corrosion of metal surfaces caused by low ph water. The aggressive tendency in many low-solids surface waters is partially caused by low pH values of these waters. The acidity of water is measured by the relative concentration of positive hydrogen ions and the negative hydroxyl ions. pH value contains equal concentrations of hydrogen ions and hydroxyl ions; when the pH is below 7.0, there are fewer hydroxyl than hydrogen ions. A water with 5.0 pH value is 10 times more acidic than one of a pH of 6.0, and a solution of pH 4.0 is 100 times as acidic as one of 6.0 pH value. This vast increment difference occurs because the pH scale is logarithmic rather than arithmetical. Again, a 5.0 pH value is 100 times more acid than the neutral 7.0 pH.