REPLACEABLE MEASURING ELECTRODE SYSTEM

Abstract: A measuring electrode system having a measuring body (22) replaceably provided on a rotatable shaft (21). The rotating shaft is provided with a spring loaded contact pin (25, 26). When the measuring body is attached to the shaft the electrodes (16, 18) running through the measuring body will be brought in electrical connection with the contact pins.

Description: TITLE Replaceable measuring electrode system TECHNICAL FIELD OF THE INVENTION The present invention relates to a measuring electrode system for use with a measuring device for electrochemical determination of the content of one or several chemical substances in a solution, and more specifically to a replaceable measuring body on a rotatable shaft adapted therefor. TECHNICAL BACKGROUND A conventional amperometric determination of chemical components (hereinafter referred to as "analytes") in a solution is performed by immersing a number of electrodes, typically three, in the analyte solution. During electrochemical measurements an electrolyte, such as an ion forming salt or any other ion forming compound which when dissolved increases the conductivity of the solution, is usually added to the analyte solution. The three electrodes are referred to as working electrode (WE), reference electrode (RE) and counter electrode (CE). At least one working electrode is required, but there is no restriction to the use of any number of additional working electrodes, if this is advantageous for a specific desired type of measurement. The reference electrode is designed to maintain a constant and well-defined electrical potential. With respect to this potential a fixed electrical potential, negative or positive, is applied on the working electrode by means of an external electronic device, a potentiostat. If analytes in the electrolyte solution being exposed to this potential undergoes reaction at the working electrode, an electrical current will flow between the working electrode and the counter electrode. This current may be measured, and reflects the concentration of the analyte(s) in the solution, while the direction of the measured current flow depends on whether an oxidation or a reduction process is present at the working electrode. Ideally, no current flows through the reference electrode of a triple electrode system as described above. In the simplest case, the current measured between the working electrode and the counter electrode is directly 2 proportional to the analyte concentration. The chemical reactions occurring at the counter electrode are usually of no interest from a measuring point of view. Therefore, the counter electrode is often separated from the analyte solution and is placed in a separate receptacle in a liquid being electrically connected with the analyte solution via a porous sintered glass filter. In a similar way, instead of a potentiometer a control equipment delivering a constant current can be used, whereby a resulting potential is measured across the working electrode. Under certain conditions the reference electrode and the counter electrode may be combined. This is possible when the processes occurring at the counter electrode are reversible with regard to a component in the electrolyte, so that the rate of the electrochemical processes at the combined electrode is not current limiting for measuring an analyte at the working electrode. A difficulty experienced during amperometric measurements is to achieve well-defined conditions in the solution at the boundary layer adjacent to the working electrode. Such well-defined conditions are of importance for obtaining reliable measurement results from which valid conclusions, such as determination of concentration, may be drawn. The rate at which processes occur at the working electrodes is regulated by the rate of electron transfer across the boundary layer between the analyte in the solution and the electrode, and/or by the rate of analyte transport through the solution to this boundary layer. In order to achieve a well-defined transport of analyte towards the electrode the working electrode is rotated. Therefore, this is usually formed as a cylindrical body provided with one or several electrodes, usually working electrodes. In a typical embodiment, a central disk-shaped disk electrode and outside this an annular ring electrode, are positioned on the flat end surface whereby both these electrodes are working electrodes. 3 Such a configuration is called a rotating ring-disk electrode. In addition to working electrodes, reference and counter electrodes could be integrated in the rotatable body as well. The US patent 4,889,608 to G. Eickmann describes an electrode system including one electrode, wherein the electrode is disposed in a replaceable electrode fixture, which electrode fixture is mounted via an adapter on an elongated solid metal shaft, the shaft also acting as an electric conductor. When changing an electrode, the electrode fixture is dismounted to allow replacement of the electrode. Replacement of an electrode according to US 4,889,608 requires a special tool. Alternatively, the entire electrode fixture is replaced. However, the electrode fixture includes several components making it comparatively expensive. Furthermore, the electrode system is adapted for the case of a single electrode only. Therefore, there is a need for a simplified replaceable electrode system for an arbitrary number of electrodes. SUMMARY OF THE INVENTION It is the object of the present invention, as it is defined in claim 1, to provide a measuring electrode system having an easily replaceable measuring body and allowing an arbitrary number of electrodes. According to the invention, this object is achieved with a measuring body of a generally cylindrical shape and being impermeably connectable to a rotatable shaft of the measuring system, wherein the rotatable shaft is provided with spring urged contact pins, and wherein each contact pin during the attachement of the measuring body is brought into contact with an end of an electrode, which electrode is provided with a second end on the outside of the measuring body. 4 According to the invention, the measuring body can be manufactured very simply and without moving parts. In addition, it may be adapted to any number of contact pins by being provided with a corresponding number of electrodes. BRIEF DESCRIPTION OF THE DRAWINGS Illustrative embodiments of the invention are described in more detail below, with the embodiments being illustrated in the attached drawings wherein: Fig. 1 is a schematical elevation view of a conventional measuring system; Fig. 2 is a cross sectional view of a first embodiment of a rotatable shaft and a replaceable measuring body according to the present invention; Fig. 3 is a cross sectional view of a second embodiment of a rotatable shaft and a replaceable measuring body according to the present invention; Fig. 4 is a separate cross sectional view of the measuring body shown in fig. 2; Fig. 5 is a top view of the measuring body according to fig 4.; DETAILED DESCRIPTION OF EMBODIMENTS First, a typical conventional measuring system shall be described in general terms as a background, with reference to the schematical illustration of Fig. 1. A typical conventional measuring system includes a measuring body 2 attached to a shaft 1. The shaft is, in its turn, connected to a motor 5 via a coupling device 4, in order to make the measuring body and the shaft rotatable. The motor is mounted onto a stand 7. On the bottom surface of the measuring body 2 is disposed at least one working electrode 6. This working electrode is electrically connected to a potentiostat 10 via a lead (not shown) running through the shaft 1 , a 5 conventional collector connection 8 and a cable 9. A reference electrode 11 and a counter electrode 12 are connected to the potentiostat via the cables 13 and 14, respectively. At use, the electrodes 6, 1 1 and 12 (the working electrode 6 being rotated) are immersed in an electrolyte and analyte solution 19 in a vessel 20. By providing a voltage across the working electrode a current is generated at the working electrode and is transferred to the potentiostat via the collector connection 8. The potentiostat outputs a measuring signal 15 based on the received current signal to a registering means 17. Power feeder cables have been omitted in Fig. 1 for reasons of clarity. The present invention relates to an improved design of a rotatable shaft and measuring body. A first embodiment of the invention is illustrated in Fig. 2, showing a cross sectional view of the lower section of a rotatable shaft 21 having a replaceable measuring body 22. The rotatable shaft 21 , which is formed of any suitable material withstanding the environment in which the measuring equipment is to operate, is hollow to house therein electronic leads 23, 24 to a collector connection (not shown in Fig. 2) for communication with the environment. In the lower section of the rotatable shaft 21 are stiff contact pins 25, 26 disposed that are connected to the electrical leads 23, 24 by, for example, soldering. The number of contact pins corresponds to the number of electrodes 16, 18 for which the measuring equipment is adapted, two being shown in Fig. 2. Each contact pin 25, 26 extend through an elongated bore 30, essentially in parallel with the central axis 21 of the shaft. A shrinking tubing, partitioned into an upper portion 32 and a lower portion 33, is shrunk onto the connecting pin and serves as insulation with respect to the bottom shaft section but also as a stiffening means. In its upper end the lead, thus stiffened, extends with a clearance through a hole in a washer 34 6 provided in a groove 39 in a centrally disposed recess 35 in the bottom of the hollow interior of the rotatable shaft. The washer is made of a stiff, preferably electrically insulating material, such as polyamide, PA. The washer is locked in the groove with a conventional locking ring 36. In a position below the washer 34, but above the bottom of the recess 35, is the upper shrinking tubing 32 separated from the lower shrinking tubing 33 by a sleeve 37. The sleeve has a wider diameter than the shrinking tubing so that, for each contact pin, one end of a compression spring 38 abuts against the upper sleeve end while the other end of the compression spring abuts against the washer 34. Thus, the compression spring will bias each contact pin, respectively, such that the pin always presses downwards. At the same time, each sleeve 37 is wider than each bore 34, respectively, thereby preventing the contact pins from being urged out of the shaft when the measuring body is removed. The replaceable measuring body 22 adapted to the rotatable shaft 21 is formed from a material having suitable mechanical and chemical properties, such as polyamide, PA. The embodiment of the measuring body 22, which is shown in Fig. 2 and in more detail in Fig. 4 and 5, is formed as a hollow cylinder with a bottom held in its position on the rotatable shaft 21 by the clamping force of an 0-ring 29. The O-ring is disposed in a groove 41, 42 in the rotatable shaft and in the inside of the measuring body, respectively, whereby the shape and the measures of the grooves are selected such that the measuring body can be detached and attached by hand and also such that the O-ring will act as a sealing. Two electrodes 16, 18 are introduced through the bottom of the measuring body 22. One of the electrodes 18 has an annular shape and, thus, the measuring body will correspond to a ring-disk electrode. The electrodes are exposed at each side of the bottom of the measuring body such that one side of each electrode is exposed at the outside bottom surface of the measuring body to be brought into contact with the solution to be analyzed, while the 7 other side of each contact pin is adapted to be brought into contact with its respective contact pin 25, 26 in the shaft. By placing one of the contact pins 25 in the center of the rotatable shaft and a second contact pin 26 at a distance from the shaft center, said distance corresponding to the ring electrode 18 radius, the effect that the measuring body is mounted on the shaft with proper electrical connection regardless of the angular position of the measuring body is obtained. When fastening the measuring body on the shaft, the contact pins are pressed into the shaft by the inner electrode surfaces of the measuring body, thereby providing good electrical contact by means of the reaction force from each compression spring, respectively. A second embodiment of a measuring body 22 for attachment to the shaft 21 with threads is illustrated in Fig. 3. In this embodiment, an O-ring 43 is used as sealing only. Threads 44 are provided on the measuring body as well as on the inside of a recess 45 in the lower section of the shaft, thereby allowing the measuring body to be screwed onto the shaft. Similarly to the previously described embodiment, a ring-disk configuration having contact pins correspondingly situated always provides a proper electrical connection, while other positions of the measuring electrodes may require that the threaded connection between the measuring body and the shaft is formed with such a precision that the electrodes always ends in a position directly below their respective contact pins when the measuring body is screwed onto the shaft. A measuring body according to this second embodiment has the advantage of being of small size and of a simple design, but the screwing operation during electrode replacement could be perceived as inconvenient. A so-called bayonet-mount is also useful for attaching the measuring body to the shaft. The bayonet-mount, which may be designed in any suitable manner known by anyone skilled in the art, has the advantage to allow for high precision positioning of the measuring body with respect to the shaft, but it is typically rather expensive. It should be noted that even in the case that the shaft is provided with numerous contact pins, measuring bodies having a fewer number of electrodes could be used as well, and vice verse, although the possibility to fully utilize all components is then lost. Thus, the present invention provides a low cost measuring body having an arbitrary number of electrodes, and a shaft adapted thereto, for allowing a simple electrode replacement procedure.

Claims: 9 CLAIMS 1. A rotatable shaft (21) in a measuring electrode system, characterized in that the shaft is provided with one end comprising at least one contact pin (25, 26) being moveably provided in a bore (30) that ends at the shaft end and the contact pin furthermore being provided with an obstructing means (37) to prevent the contact pin from entirely passing through the bore, and wherein the shaft comprises a resilient means (38) to press the contact pin in a direction out of the shaft, and the shaft further comprising means (43; 44) for retaining a replaceable measuring body (22). 2. A measuring body (22) for attachment to the rotatable shaft (21) of claim 1, characterized in that at least one electrode (16, 18) is fixedly provided through the measuring body, said electrode being arranged to be brought in contact with a contact pin (25, 26) in the shaft, and comprising means (42, 44) for attachment to the shaft. 3. A measuring equipment for electrochemical measurements including a rotatable shaft according to claim 1 , and a measuring body according to claim 2.

Inventors: NORDLING MAGNUS, NORDLING, MAGNUS
Applicants: CLL CONNECTORS & CABLES AB, NORDLING MAGNUS, NORDLING, MAGNUS
Application year: 1999
Main classification: G01N27
IPC codes: G01N27