GB2218525A - Oxygen sensor for use in liquid or gaseous media - Google Patents

Description

I" Device for polarographic measurement of oxygen concentration in liquid

or gaseous media The invention relates to a device for the polarographic measurement of oxygen concentration in liquid or gaseous media.

It is known to carry out the continuous polarogra- phic measurement of oxygen in gaseous and, more especi ally, in liquid media, such as in sewage plants, by means of a so-called Clark cell (Trans. Amc. Soc.

Artit. Intern. Organs, 2, 41 (1956). This method utili zes an oxygen-permeable inert membrane, preferably made of polytetrafluoroethylene, a gold or platinum cathode, a silver anode, and an alkaline electrolyte constituted by an aqueous solution of potassium hydroxide and potas sium chloride, in which between the cathode and the mem brane a small space is provided (Ullman, Encyklopaedie der technischen CheM4 Le,4th Edition, Vol. 5, page 930 to 931). This space is filled out with a fleece or a fabric made of a fibrous material, such as polyamide or paper, said material being impregnated with elecrolyte. The smaller the distance between the membrane and the gold or platinum cathode, the shorter the time required for a stable measuring equilibrium to be established This measuring current 1 1) is given by the formula:

4 F r A dV m p 0 L v r', 21 B 525 Z in which A represents the cathode surface (cm.), Or' is (Ncm cm S mbar- d, is the the permeability coefficient 1 -1 -1 thickness of the membrane (cm) and V m is the molar volume (cm2), F = Faraday Number = 96500 (Cb) and PC is the partial pressure of oxygen (mbar) in the environment.

This means a relatively large number of possible perturbation sources, such as the temperature-dependency of17', which requires the use of a thermistor, the pressure-dependency of oxygen loading as well as the necessity of calibration and the relatively frequent replacement of membrane, cathode and anode material, since no continuously renewed surfaces are available and silver is being consumed, as well as of electrolyte material. The replacement of the membrane is fraught with particular difficulties, since the socalled activity-grid located beneath it must also be changed. In consequence, such measuring cells in measuring probes are relatively maintenance-intensive.

It has now been found that the maintenance of such cells can be reduced to a minimum and moreover a substantial reduction of temperature- and pressuredependency can be achieved if the construction of the auxiliary cell is modified, relative to the known construction, in the manner whereby the membrane is given a gold or platinum coating of a thickness of 0.2 to 2 mm, and that a grid-shaped auxiliary cathode made of silver is provided, which is separated from the gold or V 1 v platinum coating by a thin ion-conducting separation layer.

In a preferred form of embodiment, the oxygenmeasuring cell according to the invention has a silver anode, which is arranged as a thin plate'parallel to the stack formed by the membrane, the gold- or platinum coating, separation layer and auxiliary cathode within the electrolyte space.

In a preferred form of embodiment, the electrolyte consists of silica gel, into which the potassium hydroxide-potassium chloride solution is embedded.

The multilayer stack, with the silver anode and the electrolyte-filled electrolyte-space and the electrical connections is preferably assembled to form an encapsulated, interchangeable component, so that a disposable or throwaway electrode is obtained, which needs no maintenance. Material expenditure, with regard to platinum and gold in particular, is relatively low, owing to the extreme thinness of the layers.

The measuring cell according to the invention preferably has an evacuated and therefore oxygen-free elec-. trolyte space, which shortens the adjustment period until the measuring current is reached. The cell is stable up to a pressure of 8 bar, and surprisingly unaffected by temperature. Thus, measurements can be carried out without any difficulty in media up to 60 0 C.

Although measurements are not possible at higher temperatures, the measuring cell functions normally again when the temperature is reduced from e.g. 135 0 c to 60 0 C, that is to say, the temperature is independent of temperature cycles. By way of gold coating there may be employed goldleaf or vapour-deposited gold. A vapour-deposition of platinum on the membrane is also possible. An additional coating of the rear face with an adhesive, e.g. silica gel, promotes the adhesion of the membrane which, in the conventional manner, is made of polytetrafluoroethylene.

As a separation layer there may be employed a sheet of paper, for which purpose coated paper, e.g. a material treated with a tensid, can also be used. However, another ion-conducting material, such as a plastic fleece or the like are also suitable.

The device according to the invention for polarographic measurement of the oxygen concentration in liquid or gaseous media will now be explained in more detail with reference to the accompanying drawings, in which:

Figure 1 shows a State-of-Art polarographic oxygenmeasuring cell, and Figure 2 shows a polarographic oxygen-measuring cell according to the invention.

t, 1 i- As shown in Figure 1, the measuring unit has a ring-shaped electrolyte chamber 6, which is filled with the electrolyte (KOH + KCl) and is covered by the polytetrafluoroethylene membrane 1. Between the membrane 1 and the gold cathode 3 is located an activity grid 2 made of a fabric or fleece of plastic, paper or the like, which is impregnated with electrolyte.

Electrical connections 5 link the cathode 3 and the silver anode 4 to a current source, e.g. a set of batteries.

As can be seen in Figure 2, the difference relative to the State of the Art is distinguished in particular by the membrane 7 coated with gold or platinum on its rear face, the insulating layer 8, which in particular consists of ion-permeable paper or fabric and the silver auxiliary cathode 9, whilst the multilayer stack 7, 8, 9 is separated from the silver anode by the electrolyte chamber 6. in this arrangement as well the cathode 7, auxiliary cathode 9 and anode 4 are conected by electrical conductors 5 with a current source (not shown).

The combination of multilayer stack 7, 8, 9, electrolyte chamber 6 and anode 4 can be sealed together to form a solid, interchangeable block.

14 - 6

Claims (5)

Claims

1. A device for polarographic measurement of oxygen concentration in liquid or gaseous media comprising a measuring cell with an inert, oxygenpermeable membrane, a gold or platinum cathode, a silver anode, and an aqueous, alkaline electrolyte solution of potassium hydroxide and potassium chloride, characterised in that the membrane has a gold or platinum coating of 0.2 to 2,Pm thickness and that a grid-shaped auxiliary cathode made of silver is provided, which is separated from the gold or platinum coating by a thin, ion-conducting separation layer.

2. A device as claimed in Claim 1 wherein the electrolyte consists of a silica gel, into which the KOH + KC1 solution is impregnated.

3. A device as claimed in Claim 1 or 2, wherein the silver anode is arranged in the electrolyte chamber as a thin plate parallel to a multilayer stack made up of the membrane and its gold or platinum coating, the separation layer, and the auxiliary cathode.

4. A device as claimed in Claim 3 wherein the multilayer stack is firmly integrated with the silver anode, the electrolytefilled electrolyte chamber, and the electrical connections, to form a sealed, "F, 7:

ili .e v interchangeable component.

5. A device for polarographic measurement of oxygen concentration in liquid or gaseous media substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawing.

Qt.+,T-Tniimp RW71 High Holborn, London WO 1R 4TP. Further copies maybe obtained from The Patent Office.