Alkalinity is a measure of the capacity of water to neutralize acids (see pH description). Alkaline compounds in the water such as bicarbonates (baking soda is one type), carbonates, and hydroxides remove H+ ions and lower the acidity of the water (which means increased pH). They usually do this by combining with the H+ ions to make new compounds. Without this acid-neutralizing capacity, any acid added to a stream would cause an immediate change in the pH. Measuring alkalinity is important in determining a stream’s ability to neutralize acidic pollution from rainfall or wastewater. It’s one of the best measures of the sensitivity of the stream to acid inputs.
Alkalinity in streams is influenced by rocks and soils, salts, certain plant activities, and certain industrial wastewater discharges.
Total alkalinity is measured by measuring the amount of acid (e.g., sulfuric acid) needed to bring the sample to a pH of 4.2. At this pH all the alkaline compounds in the sample are “used up.” The result is reported as milligrams per liter of calcium carbonate (mg/L CaCO3).
Analytical and equipment considerations
For total alkalinity, a double endpoint titration using a pH meter (or pH “pocket pal”) and a digital titrator or buret is recommended. This can be done in the field or in the lab. If you will analyze alkalinity in the field, it is recommended that you use a digital titrator instead of a buret because the buret is fragile and more difficult to set up and use in the field. The alkalinity method described below was developed by the Acid Rain Monitoring Project of the University of Massachusetts Water Resources Research Center.