E D I T O R S ' C O L U M N
I H E SEARCH for inexpensive, simple, accurate sensors for measurem e n t purposes, especially for medical and pollutant applications, continues. A recent development was reported by B e r n a r d C. LaRoy, A. C. Lilly, and C. 0 . Tiller of t h e Philip Morris, Inc., Research Center of Richmond, Va., at t h e Electrochemical Society N a t i o n a l Meeting held in Houston, Tex., in M a y . T h e y h a v e developed a solid-state thin-film sensor to detect and monitor reducible gases, which is based on t h e unique properties of rare earth fluorides. M o s t of the work was done with l a n t h a n u m fluoride, although other rare earth fluorides gave similar results. T h e special properties of thin-film L a F 3 as a good anion conductor with low electronic conductivity led to its investigation as a material for the sensor. T h e device, only a fraction of a square millimeter in area, consists of a gas permeable membrane, such as Teflon, t o protect t h e device from contaminating liquids; a noble metal cathode layer in grid form to permit passage of gas; an electrolyte, as thin as 1000 A of L a F 3 or other fluorides of lanthanide rare e a r t h elements; and an anode of a metal such as b i s m u t h which reacts with fluorine and t h e gas being measured to form a sink for t h e charge-carrying ions. T h e sensor is essentially a solid-state analog to a liquid polarographic cell. T h e sensor detects and measures t h e partial pressures of oxygen, carbon dioxide, sulfur dioxide, nitric oxide, a n d / o r nitrogen dioxide in either liquid or gaseous media. Advantages of the sensor over existing instruments are: low cost; can measure several gases sequentially; is easy to use ; is usable over a wide t e m p e r a t u r e range; can be miniaturized; has long shelf life and is stable over long periods; and concentrations can be determined for individual constituents of a gas mixt u r e . These advantages exist with
no loss of sensitivity, response time, wear-out rate, specificity, or linearity of response compared to existing liquid electrolyte systems. P a t e n t applications h a v e been filed for this sensor. H a r d w a r e for specific applications beyond t h a t for laboratory testing h a v e not yet been developed. Suggested applications include the use of such a probe to measure SO2 and N O z sequentially; also, the economical determination of oxygen in water; t h e monitoring of 0 2 and C 0 2 in blood in situ; in space flights for t h e measurement of 0 2 and C 0 2 in cabin atmosphere; and in chemical and petroleum processing, «where t h e continuous monitoring of several reducible gases is desirable. Other applications in industries, such as food and beverage processing, water and sewage t r e a t m e n t processes, and m a n y others, come readily to mind. A secondary development of this research, also reported at the Society meeting, is a solid-state thin-film b a t t e r y with energy densities comparable to those presently commercially available. T h e Coordinating Research Council (CRC) which is involved in research studies under t h e technical direction of E P A , t h e American Petroleum I n s t i t u t e , and t h e Automobile Manufacturers Association has reported studies b y scientists at Argonne N a t i o n a l Laboratory in Illinois. Studies show t h a t a t least 3 billion tons of CO come from t h e oxidation of m e t h a n e emitted b y decaying organic m a t t e r , 100 million tons from chlorophyll in green plants, a n d a n o t h e r 400 million tons from oceans and u n d e t e r m i n e d sources. I n our efforts to clean u p the atmosphere, we are apt to forget t h a t mother n a t u r e is a huge factory too. Indeed, although mother n a t u r e produces carbon monoxide, she also has methods to remove carbon monoxide from t h e atmosphere. A study from Stanford Research I n s t i t u t e shows t h a t h a r m less fungi living in U.S. soils have t h e capability to consume about 550 million tons of carbon monoxide a year, more t h a n twice t h e o u t p u t of automobiles and factories worldwide. J. P.
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972 • 55 A
