GB858677A - Improvements in and relating to methods of and apparatus for measuring the concentration of oxygen dissolved in liquid - Google Patents

Abstract

<PICT:0858677/III/1> <PICT:0858677/III/2> In an apparatus for estimating the amount of oxygen in a liquid comprising a scrubbing vessel through which the liquid can be passed and wherein the liquid can be contacted with hydrogen, an electrochemical measuring cell through which the resultant oxygen-containing hydrogen is passed, means to circulate the hydrogen between said scrubbing vessel and said cell, a hydrogen-generating electrolytic cell and means to conduct the hydrogen generated into the circulating stream of hydrogen, there is provided for the purpose of calibrating the apparatus an oxygen-generating electrolytic cell, together with means to conduct the oxygen generated by said cell into the circulating stream of hydrogen. In the apparatus illustrated in Fig. 1, the liquid to be tested, e.g. boiler feed water, is fed from a constant head apparatus 2 through constriction 3 into the scrubber 1 and thence discharged. Hydrogen is bubbled through the scrubber 1, entrains oxygen and is passed through conduit 4 into the upper part of the measuring cell 5 containing a buffer solution, which thereby is able to pass a current proportion to the amount of oxygen in the buffer solution. The hydrogen is thereafter passed by means of pump 9 back to the scrubber so that an equilibrium reading may be obtained. A hydrogen-generating cell 11 supplies hydrogen via line 27 and a furnace 14 containing palladised asbestos which eliminates any oxygen by reaction with the hydrogen to form water. The cell comprises a vessel containing a suitable electrolyte such as potassium hydroxide, a bell jar 26 enclosing a cathode 25, and an anode 23 outside the bell jar. An anode 24 is also supplied so that oxygen can also be generated in the bell jar: the furnace can then be tested by exciting anode 24 and noting any change indicated by the measuring cell. Hydrogen is passed into the circulating gases to make up losses and maintain pressure; by opening valve 15 the pressure in the system is maintained constant at a value determined by the depth of immersion of the tube in the bubbler 16. Circulating gases may alternatively be vented through valve 15. In the oxygen-generating cell 10 the anode terminates above the level of the electrolyte, e.g. potassium hydroxide, but maintains contact by means of a fibrous thread: oxygen is generated where the wet thread meets the anode. In order to calibrate the apparatus, the flow of water through the scrubbing tower is stopped, or the circulating gases are made to by-pass the scrubber by means of a passage between ducts 4 and 8, and the circulating gases vented via line 17 and valve 15. When the apparatus is filled with hydrogen, a current is passed through oxygen-generating cell 10. When equilibrium is reached, the ratio of oxygen to hydrogen in the apparatus is equal to half the ratio of the currents through the respective generating cells, thus enabling the apparatus to be calibrated. The oxygen is normally less than 1% by volume of the gas stream. A suitable calibration circuit is shown in Fig. 2: by adjusting resistor 35 until no current flows through galvanometer 36, the ratio of the currents through the cells is made equal to the ratios of resistors 33 and 34 alternatively a ratiometer may be used. The measuring cell 5 consists of a triangular vessel containing a solution, e.g. a borate buffer solution of suitable pH, e.g. pH 5-12, and two electrodes of which the anode consists of a substantially chemically inert material such as platinum wire coated with platinum black. The Provisional Specification is not restricted to the use of electrolytically generated oxygen, and refers to apparatus in p which only a part of the circulating gas stream passes through the measuring cell. It also refers to the use as the wet thread in the oxygen-generating cell of a glass silk thread. Specification 818,732 is referred to.