GB841548A - Apparatus for electrically determining the concentration of hydrogen cyanide or hydrogen sulphide in air - Google Patents

Abstract

841,548. Electric tests. MINE SAFETY APPLIANCES CO. June 25,1958, No. 20348/58 Class 37. Apparatus for determining the concentration of hydrogen cyanide or hydrogen sulphide in air comprises an absorption chamber within which a solution of the compound is formed by passage of the air to be tested, the resultant solution being passed over a silver electrode at high relative velocity, and the e.m.f. produced between the silver electrode and a reference electrode being measured. As shown, Fig. 4, the air to be tested is drawn through a glass-bead absorber 1 by a waterjet aspirator 4, the rate of flow being measured by a "Rotameter" (Registered Trade Mark) 8 and controlled by a needle valve 7, and the absorber 1 is supplied via a capillary tube 27 from a constant head tower 12 fed from a water line 9 or, if tap-water is insufficiently alkaline, by distilled or deionised water, corrected to a pH value of about 10À5, through an inlet 26. The solution from absorber 1 passes to a half-cell 18 comprising a silver electrode 19 which is rotated at 1725 r.p.m. or faster by a motor 23, electrical connection to the electrode being via a slip-ring 24a on the motor shaft. A calomel reference electrode 20 is connected to the solution by a salt-bridge 21, and the output from the electrodes is fed to a meter, preferably a valve voltmeter or a recording potentiometer, and may actuate a visual or audible warning device. Fig. 6 shows portable apparatus in which a measured sample of air is drawn through a bulb absorber 30 containing absorbing solution by a syringe pump 38, the solution then being pumped through a capillary tube 32 containing a silver wire electrode 31 by an air pump 33. Silver electrodes require ageing by operation, or the forming thereon of a black silver sulphide layer, before the apparatus is calibrated. A petticoat bubbler is said to be the preferred form of absorber.