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.
Below are the most common products used for measuring carbon dioxide.
Benchtop pH/ISE meters
Benchtop pH/ISE meters include a two channel option in which a pH probe can be connected to one channel and the carbon dioxide ISE to the other. The benchtop meters have the ion charge and formula weight pre-programmed for use with the carbon dioxide ISE. The benchtop meters also have incremental methods pre-programmed including the known addition, known subtraction, analyte addition and analyte subtraction methods
Portable pH/ISE Meters
Portable pH/ISE allow the flexibility to perform an ISE measurement with a portable even though the ISE measurement method is more suitable to a benchtop environment due to the addition of the ISA to lower the pH and the use of a magnetic stirrer to keep the sample homogenous. The portable pH/ISE meter has the ion charge and formula weight pre-programmed for concentration measurements.
Ion Selective Electrode
The carbon dioxide ISE is a gas sensing electrode. An ISA must be used to lower the pH of the sample in order to change any carbonates/bicarbonates to carbonic acid in which will dissociate into water and carbon dioxide as a gas. The ISE is a convenient way to measure carbon dioxide.
Solutions for carbon dioxide measurement include ISE standards and ISA for calibration and measurement.
Chemical Test Kits
Several chemical test kits are available for measuring carbon dioxide from single parameter test kits to multiparameter test kit for water quality.
Reagents include the reagents used with chemical test kits and photometers.
Accessories include magnetic stirrers to ensure adequate mixing when using an ISE and the spare parts for the carbon dioxide ISE.