A Simple Method for Measuring Carbon Dioxide in Soils

Abstract

Polyethylene ('Polythene'), polytetrafluoroethylene ('PTFE', 'Fluon' or 'Teflon') and several other organic polymer plastics are relatively permeable to uncharged molecules including carbon dioxide, oxygen and nitrogen, but impermeable to electrolytes and polar liquids, so that it is possible to use membranes of these substances to make a simple electrode for measuring the partial pressure of carbon dioxide or oxygen (Severinghaus & Bradley 1958; Snell 1960). To measure the percentage of carbon dioxide either in the atmosphere or in solution, a combined glass and calomel reference electrode is immersed in a small volume of sodium bicarbonate solution which is enclosed in a membrane of Polythene or PTFE. The pH of the internal solution is then related only to the partial pressure of carbon dioxide, because neither oxygen nor nitrogen affect the pH of a bicarbonate solution, and electrolytes from the external solution, including hydrogen ions and hydroxyl ions, are excluded. The time for the internal solution to come to equilibrium with the external gas pressure depends partly on the volume of liquid enclosed and partly on the surface area and thickness of the polymer membrane. The device described by Severinghaus & Bradley uses a PTFE membrane 0-001 in. (25 ,u) thick and other methods use thin Polythene membranes. These electrodes are suitable for use in air or in liquids but are too delicate to be inserted into soil without damage. A membrane thick enough not to be damaged by abrasion slows down the diffusion of gas molecules and the time for the electrode to respond is too long to be practical. This paper describes a simple modification of the polymer membrane electrode which has proved to be convenient for measurements in the soil. The system is unfortunately not suitable for following short-term fluctuations in carbon dioxide pressure and has the disadvantage that a considerable volume of carbon dioxide is lost from the soil. However, the tubes can be inserted with very little soil disturbance and once the internal solution has come to equilibrium less than a minute is required to make each