Carbon dioxide easily diffuses into water forming carbonic acid (H2CO3). In water carbonic acid quickly dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3–). These ions increase the conductivity and decrease the resistivity and pH of the water.
Lakes and rivers contain carbon dioxide naturally at concentrations of less than 10 mg/L. However, stagnant or polluted water can generate large amounts of carbon dioxide due to organic or mineral decomposition. Higher amounts of carbon dioxide can make the water corrosive and toxic to aquatic organisms. Monitoring carbon dioxide levels is also critical in the man-made environment. Carbon dioxide is added to drinking water during the final stages of the purification process. In water softening systems, a delicate balance of carbon dioxide must be maintained to prevent corrosion or encrustation of pipes and storage tanks.
Hanna Instruments offers a variety of methods to measure carbon dioxide in water, soft drinks, and wine. Products include portable and benchtop pH/ISE meters that can use a gas membrane carbon dioxide Ion-Selective Electrode (ISE) for direct measurement. In addition, carbon dioxide levels can be measured with a chemical test kit that is based on a phenolphthalein titration. The phenolphthalein is first added to the sample and then titrated with a sodium hydroxide standard solution until the sample changes color at the endpoint pH of 8.3.