The pCO₂ electrode (E788) is a combined pH and Ag/AgCl reference electrode mounted in a plastic jacket, which is filled with a bicarbonate electrolyte.

The jacket is covered by a 20 µm silicone membrane moulded on a 50 µm nylon net. The net both reinforces the silicone membrane and serves as a spacer in order to trap a layer of the electrolyte between the membrane and the glass tip of the electrode. The electrolyte also contains glycerol to prevent collection of air bubbles in the electrode jacket thus improving electrode stability.

The membrane allows any uncharged molecules of CO₂, O₂, N₂ to pass through it. Charged ions such as H+ will not pass. Consequently, dissolved CO₂ from the sample will diffuse into the thin layer of bicarbonate electrolyte until the equilibrium is reached.