Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-09-24
2003-11-25
Tung, T. (Department: 1253)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S408000, C204S409000, C204S272000, C205S782000
Reexamination Certificate
active
06652721
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to a sensor for measuring O
2
concentrations in liquids, with a working electrode, with a counterelectrode and with a reference electrode, wherein the working electrode and the counterelectrode are in contact with the liquid, wherein the reference electrode is separated from the liquid via a diaphragm and wherein the reference electrode measures a polarization voltage effectively acting at the working electrode and is connected to a potentiostat regulating the potential between the working electrode and the counterelectrode.
BACKGROUND OF THE INVENTION
Oxygen in the molecular form, dissolved in liquids, is undesirable in many chemical and/or food technological processes. For example, very low oxygen concentrations are necessary for reasons of purity and/or corrosion protection in the manufacture of semiconductors or in the water-steam cycle in power plants. Low oxygen contents are desirable in food technology for reasons of good shelf life. For example, only very little oxygen may be present in the end product at most in the area of beer-brewing if satisfactory stability of the taste is to be achieved. The accurate measurement and monitoring of oxygen concentrations is therefore of broad analytical significance.
A sensor of the design mentioned in the introduction has been known from the reference source
Brauindustrie,
3/88. This is a three-electrode sensor operating by amperometric measurement with a working electrode, a counterelectrode as well as a reference electrode. The working electrode and the counterelectrode are arranged coaxially to one another and the liquid flows through the intermediate space between the working electrode and the counterelectrode. The reference electrode is arranged on the side and is connected via a diaphragm passing through the reference electrode.
The basic functions of the sensor known so far and also of a sensor according to the present invention are as follows. An electric current between the cathodically polarized working electrode and the counterelectrode, which results from the reduction of O
2
molecules at the working electrode, is measured. It is obvious that reduction of O
3
molecules can also take place and the sensors known so far, just as a sensor according to the present invention, can be used to determine ozone or total (molecular) oxygen. On the one hand, accurate setting of the potential between the working electrode and the counterelectrode is necessary for the measurement of the current intensity. On the other hand, determination of the rate of flow of the liquid must be performed if the measurement is to be performed in flowing liquids, because the measured values obtained are also determined by the thickness of the boundary layer at the working electrode in which the O
2
transport takes place in a diffusion-controlled manner. The thickness of the boundary layer is in turn a function of the rate of flow.
A method for determining the rate of flow is, e.g., the technique known from practice in which two NTC resistors are used. One NTC resistor is heated and is arranged in the flow of the liquid. The second NTC resistor is likewise arranged in the liquid, but in an area without flow or only low flow. The second NTC resistor is used as a compensating resistor concerning the temperature of the liquid, while the voltage drop over the first NTC resistor (in case of temperature compensation) is an indicator of the “cooling effect” of the flowing liquid and therefore also of the rate of flow. It is obvious that the amount of liquid is so large that no appreciable increase in the temperature of the liquid takes place as a whole due to the heating of the first NTC.
In conjunction with a potentiostat, the reference electrode ensures that the electrochemical conditions are maintained at a constant value between the working electrode and the counterelectrode. A suitable circuit can be found, e.g., in the reference source
Handbuch der industriellen Me&bgr;technik
[Handbook of Industrial Measuring Technique], 6th edition, 1994, R. Oldenbourg Verlag, Munich, Vienna, p. 1102.
One disadvantage of a sensor according to the above-mentioned state of the art is that it has a protruding design. Due to the fact that the working electrode and the counterelectrode, on the one hand, and the reference electrode, on the other hand, are arranged side by side, there also arises a disturbing effort to contact the sensor as a whole. In addition, a compact conductor system is necessary, which leads to a relatively great pressure drop and a considerable risk for microbial contamination. In addition, sterilization is cumbersome.
SUMMARY AND OBJECTS OF THE INVENTION
The basic technical object of the present invention is to provide a sensor of the design mentioned in the introduction, which has a compact design, has a reduced risk for microbial contamination and can be sterilized in a simple manner.
To accomplish this object, the present invention teaches that the reference electrode, the working electrode and the counterelectrode are coaxial to one another, the working electrode and the counterelectrode being arranged around the reference electrode. In other words, the sensor has an essentially rotationally symmetrical design, with the reference electrode forming the middle part.
The liquid can be easily passed through between the working electrode and the counterelectrode or past the working electrode and the counterelectrode, without the need for longer lines of a narrow cross section. The system can be rather designed as an open system, e.g., as a cylindrical system, wherein the liquid can flow through at least one front surface of the cylinder which is open over its full area.
A very compact design is obtained with the present invention due to the coaxial arrangement of all electrodes. Other advantages are a small pressure drop, reduced risk for microbial contamination, simpler sterilizability, simpler manufacturability, and reduced maintenance requirement.
When viewed in the radial direction, the working electrode and the counterelectrode may follow each other, in principle, in any desired arrangement. The counterelectrode is preferably arranged around the working electrode. The working electrode or the counterelectrode may form an outer wall of the reference electrode and thus form a structural unit.
The electrodes may have, in principle, any shape, e.g., a square shape relative to their cross section at right angles to the axis. However, the counterelectrode and/or the working electrode are preferably of a cylinder jacket design, i.e., they have a circular cross section for manufacturing technical reasons.
The reference electrode is, e.g., a class 2 electrode with constant input potential, preferably an Ag/AgCl electrode. The material of the working electrode may be silver or a silver alloy. The material of the counterelectrode may be a stainless steel.
All electrical contacts for the connection of the working electrode, the counterelectrode and the reference electrode may be arranged at one end of the axis of the sensor. As a result, simple contactability and higher electrical reliability are achieved.
A coaxial opening, through which the liquid can flow, preferably a coaxial opening extending over the entire cross section of the sensor, may be arranged at the end of the axis of the sensor located opposite the contacts. In conjunction herewith, a radial opening, through which the liquid can flow, may be arranged in the electrode that is the outermost electrode in the radial direction. For example, liquid can be passed through the radial opening into the interior of the sensor, and the coaxial opening functions as an outflow opening in this case. If the electrical contacts are not arranged in the area of one end of the axis of the sensor (but, e.g., laterally or radially), a coaxial opening of the above-mentioned design each may be arranged at both ends. An extremely low flow resistance is achieved in this case.
A temperature sensor, which is in thermal contact with the liquid, prefe
Biotechnologie Kempe (GmbH)
McGlew and Tuttle , P.C.
Tung T.
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