Patent GB-559708-A (GB)
Title: Improvements in or relating to the electrical determination or control of the amount of oxidizing or reducing substances in liquids
IPC: G01N27/38
Filed: 1942-01-26
Abstract: 559,708. Determining physical qualities of materials ; liquid resistances. WALLACE & TIERNAN PRODUCTS, Inc. Jan. 26, 1942, Nos. 1089, 1090 and 1091. Convention dates, Dec. 28, 1940, April 24, 1941 and Sept. 25, 1941. [Class 37] The content or change in content in a liquid of an oxidizing or reducing agent is determined by exposing to the liquid an electrode adapted to be depolarized by the agent, maintaining another electrode separate from the liquid but in electrical communication therewith through an electrolyte, applying a polarizing voltage between the electrodes, and determining the depolarization or change in depolarization produced by the liquid. A system is shown for indicating and recording the concentration of an oxidizing agent such as chlorine in an aqueous liquid flowing through a cell 60 from an inlet 62 to an outlet 63. Suspended in the stream of liquid is a metal plate electrode 61, for example a thin sheet of gold one inch square. The other electrode comprises a coil of silver wire 64 suspended in a solution of potassium chloride in a chamber 65, which has a porous wall or diaphragm 66 exposed to the liquid in the cell 60. A drip cell 70, or other voltage source, is connected in series with a rheostat 71, a dropping resistor 72, and a resistance 73 having a movable contact 74. The voltage across the resistance 72 and the tapped-off part of the resistance 73 is applied to the terminals of the cell 60 through a dropping resistor 75. A milliammeter 76 facilitates adjustment of the voltage applied to the cell. A cell 80 in series with a rheostat 81 and resistor 82 provides a biasing voltage across the resistor 82, which is connected in series with the resistor 75 to the input of a recording potentiometer 85. As the chlorine concentration of the liquid changes in the cell 60 the extent of depolarization of the cathode 61 similarly changes and a corresponding change occurs in the current through the resistor 75, the resulting change of voltage across which is measured by the potentiometer 85. The movable contact 74 is driven by the potentiometer recorder 85 in such a manner as to maintain the voltage across the electrodes of the cell 60 constant in spite of variation of voltage drop across the resistor 75. The cell 60 may be replaced by the cell described in Specification 556,305, [Group XL]. In order to compensate for temperature changes a galvanic cell may be included in the circuit of the potentiometer, the cell being enclosed in a chamber through which the liquid under test passes on its way to the depolarization cell. A suitable galvanic cell may comprise mercury and silver electrodes immersed in a normal hydrochloric acid solution saturated with mercurous chloride and silver chloride. In another arrangement two depolarization cells are fed respectively through constant-level boxes with untreated and treated water and connected differentially so that the potentiometer measures the difference between the currents flowing through them. The potentiometer may have contacts controlling a reversible motor which varies the rate of chlorine supply so as to maintain a desired concentration in the water after treatment. Both cells may be fed with the same liquid under test. e.g. chlorinated water, and may be designed so that the current-chlorine curves of the two cells are different. To effect this the cells may have different values of impressed voltage, or different anodes, or different cathodes. In one such arrangement two depolarization cells are employed which have different impressed voltages and also different cathodes, e.g. of copper and gold. Two potentiometric recorders are used, one being connected to read the difference between the currents through the two cells and the other being connected to measure the current through the goldcathode cell alone. The first of these potentiometers gives a measure of residual chlorine independently of small amounts of substances such as ammonia, whilst the second measures the actual free chlorine as distinguished from the total available residual chlorine including chloramines. Specifications 548,173 and 548,174 and U.S.A. Specifications 1,762,706 and 1,777,987 also are referred to.