Patent GB-1473250-A (GB)
Title: Method and device for the detection and measurement of no2 gases
IPC: G01N27/416
Filed: 1975-02-10
Abstract: 1473250 Electro-chemical detectors of NO & NO 2  ENERGETICS SCIENCE Inc 10 Feb 1975 [7 May 1974] 5617/75 Heading G1N A device for detecting NO and/or NO 2  in a gas comprises an electro-chemical cell through which gas from an intake is drawn at a controlled rate and means for reading out the amount of gas detected. The electro-chemical cell comprises a working electrode, a counter electrode and a reference electrode at which substantially no current flows all in the same electrolyte. The working electrode comprises a hydrophobic material and a catalyst for either electro-oxidation of NO or electro-reduction of NO 2 . It is maintained at 0À9 to 1À9 V positive relative to a reversible hydrogen electrode potential in the electrolyte for determining NO and at 0À7 to 1À1 V negative relative to a reversible hydrogen electrode potential for determination of NO 2 . As shown, a system for measuring NO and NO 2 . simultaneously comprises two inlets 11 and flowmeters 13 through which air is moved by pumps 15. The air passes through H 2 S removing filters 17 and one stream passes direct to NO 2  sensor 21' which is controlled by a potentostat 24' which monitors the working electrode voltage and a voltmeter which reads the current generated by NO 2  reduction. The other gas stream passes through NO 2  removing filter 19 to NO sensor 21. Both streams are exhausted via passages 23.  The sensor Fig. 2 comprises working electrode 27, counter electrode 25 and reference electrode 29. It is filled with electrolyte 33. Both electrodes 25 and 27 comprise a hydrophobic plastics substrate 25À1, 27À1 and catalytic layers 25À2, 27À2 which preferably comprise a mixture of platinum powder with PTFE particles and gold powder with PTFE particles respectively, the gold preferably being at a concentration of 5-50 mg/cm<SP>2</SP> and 3- 10 times the weight of PTFE. The reference electrode is porous platinum catalyzed PTFE. The gas flow through the cell from inlet 35À1 to outlet 35À2. Electrodes. The working electrode catalyst is preferably gold but may be any noble metal. The substrate may be carbon, a metal or any one of PTFE, polychlorotrifluoroethylene, polyacrylonitrile, polyvinylchloride, polyvinylalcohol or carboxymethyl cellulose; the polymers also serving as binders. The counter electrode construction is not critical but must be a material at which electroreduction occurs (for NO cell) or electro-oxidation occurs (for NO2 cell). The preferred material contains gold. The reference electrode is preferably an Au catalyzed air electrode and may be positioned between or behind the other electrodes or on the same plane or substrate as one of them. The electrolyte may be aqueous acid or aqueous alkaline and may be free or fixed in a matrix of asbestos, Kraft paper, polyvinyl alcohol or PVC treated to make it hydrophilic. The H 2 S filter may be lead acetate or mercuric chloride and the NO 2  filter carbon, alumina or Mallcosorb; the latter three also removing H 2 S. In alternative embodiments when simultaneous measurement of NO and NO 2  is not required, a single intake and pump may be used with, for sample serially connected NO 2  and NO cells.