A sensor of the kind referred to above is described by T. Ishiji et al in an article entitled "Amperometric Acidic Gas Sensors using Platinum Oxide Reduction and Iodine Reduction", published in the journal "Sensors and Actuators B", 13-14 (1993), pages 583 and 584. The measuring or work electrode is defined by a sputter coating on a gas-permeable membrane made of polytetrafluorethylene (PTFE). The sputter coating is formed by vapor depositing a thin coating of platinum oxide. The counter electrode and the reference electrode are silver wires. A 0.1 M aqueous KCl solution is used as an electrolyte. Such a platinum sensor is used, inter alia, to detect carbon dioxide (CO.sub.2). However, it has been shown that the precious metal oxide, which is available only in limited quantities, is rapidly consumed during operation of the sensor. Thereafter, the measurement signal collapses.
A known method for detecting acidic and alkaline gases is based upon the potentiometric measurement with ion-specific electrodes. Here, it appears to be disadvantageous that the potentials of the measurement electrode and the reference electrode must be held very stable. Even slight deviations significantly affect the precision of the measurement. The gas to be detected accumulates in the electrolyte during long time spans of operation and continuous exposure to the gas. If the gas concentration is then subsequently reduced, an adjustment of the new equilibrium value takes place extremely slowly.
A further known method for detecting acidic gases comprises shifting a redox equilibrium by means of a pH change. Here, the position of a redox equilibrium is changed by the entry of the acidic gas into the electrolyte. In this way, a current is generated for a potentiostatic circuit of the measuring electrode and this current is proportional to the concentration of the gas to be detected. In this method, the equilibrium shift of the redox reaction is utilized for detection and does not only occur because of the change of the pH value, but also because of temperature jumps. Furthermore, the detection sensitivity appears to be satisfactory only for intensely acidic gases.
A sensor disclosed in U.S. Pat. No. 4,552,624 also belongs to the known state of the art. The electrolyte comprises a mixture of bromide and bromate. The dissociated gas dissolved in the electrolyte reacts directly with the electrolyte and free bromine occurs from the comproprotionation of the educts. The bromine is then electrochemically detected on a metallic measuring electrode. It is disadvantageous in this method that an adequate sensitivity is obtained only with intensely acidic gases.
USPTO Backgrounds
USPTO Backgrounds