The HI4105 carbon dioxide ion-selective electrode is a complete potentiometric cell that contains both a silver/silver chloride (Ag/AgCl) reference and a pH measurement element. These elements are housed within a plastic body with a chloride ion-containing electrolyte, and are isolated from the sample by a gas permeable membrane made of polytetrafluoroethylene (PTFE).
In solution, carbon dioxide reacts with water to form carbonic acid. Carbonic acid dissociates into bicarbonate and carbonate species. The concentrations of carbon dioxide, carbonic acid, bicarbonate, and carbonate species are pH dependent. As the pH of a solution becomes more alkaline, the equilibrium shifts to favor carbonate as the predominant species. A more acidic pH favors carbon dioxide as the predominant species.
For the HI4105 to measure carbon dioxide accurately, it is important that the pH of the calibration standards and sample be lowered with the carbon dioxide ionic strength adjuster (ISA) solution. The carbon dioxide ISA ensures the pH of solutions is kept between pH 4.2 and 5.2. In this pH range, the carbonate and bicarbonate in solution are converted to carbon dioxide for an accurate measurement. The HI4105 carbon dioxide gas sensing ISE can measure from 400µM (17.6 mg/L) to 0.02M (880 mg/L).
Conversion Table for CO2
moles/L (M) to ppm (mg/L)
4.4 x 104
ppm (mg/L) to M (moles/L)
2.273 x 10-3
Replaceable PTFE Membranes – Gas sensing ISEs are combination electrodes that detect dissolved gases in a solution. The sensing element is separated from the sample solution by a gas permeable, PTFE membrane that is easy to replace.
Durable Delrin® Body – The probe body is composed of durable Delrin plastic.
BNC Connection – The HI4105 has a universal BNC connector for easy connection to any benchtop meter with a female BNC probe input.
Theory of Operation
A gas sensor works due to the partial pressure of the measured gas in solution. The dissolved gas in the sample diffuses into the membrane and changes the pH in a thin film of unbuffered electrolyte on the surface of the internal pH sensor. Diffusion continues until the partial pressure of the sample and the thin film of electrolyte are the same. The pH change is proportional to the concentration of dissolved gas in the sample solution.