Water contains both hydrogen ions, H+, and hydroxide ions, OH–. The relative concentrations of these two ions determine the pH value. Water with a pH of 7 has equal concentrations of these two ions and is considered to be a neutral solution. If a solution is acidic, the concentration of H+ ions exceeds that of the OH– ions. In a basic solution, the concentration of OH– ions exceeds that of the H+ ions. On a pH scale of 0 to 14, a value of 0 is the most acidic, and 14 the most basic. A change from pH 7 to pH 8 in a lake or stream represents a ten-fold increase in the OH– ion concentration.
Rainfall generally has a pH value between 5 and 6.5. It is acidic because of dissolved carbon dioxide and air pollutants, such as sulfur dioxide or nitrogen oxides. If the rainwater flows over soil containing hard-water minerals, its pH usually increases. Bicarbonate ions, HCO3–, resulting from limestone deposits react with the water to produce OH– ions, according to the equation:
As a result, streams and lakes are often basic, with pH values between 7 and 8, sometimes as high as 8.5.
In this experiment, you will use a pH Sensor to measure the pH of a body of water.