Dissolved carbon dioxide (CO₂) is present naturally as a result of animal respiration, the decay of organic matter, and the decomposition of certain minerals. It is the major source of acidity in unpolluted water samples. Surface waters typically contain less than 10 ppm (mg/L) dissolved CO₂, while ground waters, particularly if deep, may contain several hundred ppm (mg/L). Oil production brines also contain high CO₂ concentrations.

High levels of CO₂ in surface waters can indicate abnormal organic or mineral decomposition. Measurement of CO₂ is a means for monitoring the quality of municipal water treatment systems. Dissolved CO₂ is corrosive to water handling equipment, particularly steam condensate systems. Some CO₂ is desirable as it helps maintain the carbonate equilibrium, thus avoiding the formation of calcite scale on exposed surfaces. Due to the delicate balance between corrosion and scale formation, CO₂ concentrations must be carefully monitored.

The Titrimetric Method. CHEMetrics' carbon dioxide test kits employ a caustic titrant and phenolphthalein indicator. The kits contain a neutralizer solution to correct for sulfide interference.  Results are expressed as ppm (mg/L) CO_2.