Nomographic determination of free carbon dioxide, Standard Method4500-CO2 B.Thismethodisbaseduponequationsrelating the ionization equilibria of the carbonates and water. The water temperature, total dissolved solids content, pH, and total alkalinity are needed to determine the free CO2 concentration. Alkalinity is measured using HACH method 82030. This method is not recommended for waters of high total dissolved solids, such as those encountered in seawater. Method used in Test Series 3.