GB1512002A - Gas monitors - Google Patents

Abstract

1512002 Monitoring concentration of a gas BRITISH STEEL CORP 22 April 1975 [22 Jan 1974] 02841/74 Headings G1A and G1B A method for monitoring the toxic constituent of an ambient gas comprises bringing the gas into contact with a liquid having one component in which the constituent is selectively soluble and an indicator having optical transmission characteristics responsive to the quantity of constituent in solution, passing an electric current representative of the change in transmission characteristics from a datum through the liquid in which a second component is arranged to electrolyze, thereby to neutralize changes in the first component arising from the dissolved constituent and hence restore the transmission characteristics to the datum. A suction pump connected to outlet tube 6 draws air contaminated by sulphur dioxide through inlet tube 10 and a water based liquid containing hydrogen peroxide, methyl orange, and sodium chloride. The SO 2 is oxidized by the H 2 O 2 to sulphuric acid which produces a colour change in the methyl orange and an out-of-balance condition in the light received from lamp 20 in photo-electric cells 26 and 28. An out-of-balance signal is applied to an amplifier 48 which controls a power transistor T supplying current between the silver anode 11 and platinum wire cathode 16 of the cell 2. The NaCl is electrolyzed to sodium hydroxide, which neutralizes the H 2 SO 4 , and chlorine, which is retained in the cylinder 12 by a semi-permeable membrane 14 in the form of an agar plug on a sintered glass disc. The chlorine reacts with the silver of the anode to form an inert chloride. The instantaneous current between anode and cathode is indicative of instantaneous SO 2 concentration in the air and is applied for display to a recorder by means of the resistance network 52. The datum pH of the liquid is selected to lie between 3À9 and 4À0 by the initial addition of hydrochloric acid and the outputs of amplifiers 40, 42 are balanced by potentiometer 27 so that amplifier 44 produces a zero output at the datum condition. For increased sensitivity the methyl orange may be replaced by bromocresol green. For monitoring carbon monoxide the liquid in container 4 comprises dimethylformamide with an indicator of thymolphthalein, potassium iodide, ethanolamine and water. The contaminated air is first passed through absorption tubes containing respectively in sequence; silica gel to remove moisture, soda asbestos to remove carbon dioxide, magnesium perchlorate to remove moisture, molecular sieve to remove hydrocarbons and iodine pentoxide to convert CO to CO 2 for titration in cell 2. This method may be adapted to monitor carbon dioxide concentration by omitting the absorption tube containing soda asbestos.