Method of regenerating amperometric sensors

This application claims a right of priority under 35 USC §119 from German patent application 10 2007 061806.0, filed 19 Dec. 2007, the content of which is incorporated by reference as if fully recited herein.

The invention relates to a method and a device for regenerating at least one electrode of an amperometric sensor.

Amperometric sensors are used in the laboratory and in the process field for the determination of a multitude of chemical substances such as the determination of dissolved gases and in particular the determination of oxygen or ozone. Amperometric sensors are known in the form of two-electrode systems or three-electrode systems which have a working electrode, a counter electrode and, optionally, a reference electrode. During operation, the electrodes are immersed in an electrolyte.

During operation of a sensor and in particular during continuous operation in a process system, it is possible that the measurement medium and in particular volatile components dissolved in the measurement medium penetrate and contaminate the electrolyte. In addition, deposits can accumulate over the course of time on the working electrode and/or the reference electrode, which can thereby become passivated. Both of these developments have a negative effect on the measurement result and the functionality of the sensor.

On metal electrodes, for example, deposits of insoluble salts or complexes will accumulate which occur as a result of oxidation or reduction of the electrolyte, the electrolyte contaminants or the electrode material, and/or as a result of a reaction between components of the electrolyte and a subsequent redox reaction.

In order to extend the lifetime and to maintain the functionality of an amperometric sensor, these deposits and/or contaminants should be removed from the electrolyte at certain time intervals. Conventional methods of regenerating amperometric sensors such as oxygen sensors involve for example exposing the electrodes and then manually regenerating them through a chemical and/or physical procedure, as well as exchanging the electrolyte.

It is known, for example, to remove the deposits mechanically by grinding or abrasion. While a removal of the deposits by grinding is certainly effective, it involves the risk, particularly when practiced repeatedly, that the geometry and/or the dimensions, and thus also the performance specifications, of the electrodes are changed, since not only deposits are being removed, but also electrode material. Especially when this removal of deposits is practiced repeatedly, it can thus have a negative effect on the functionality of the sensor and on the quality of the measurement results.

A further known practice is the chemical removal by cleaning the electrodes in highly concentrated acids, for example nitric acid or sulfuric acid. This procedure has on the one hand the disadvantage that the handling of such acids requires extraordinary care and special protective measures. Furthermore, these acids should be properly disposed. On the other hand, electrodes of base metals such as for example zinc or silver can be attacked by acids, so that with this procedure, too, the dimensions of the electrodes are changed and the measurement results are negatively affected.

As an example, US 2005/0236280 A1 discloses a combination of electrical stripping of the electrodes by applying strong electric pulses and using sulfuric acid as electrolytic cleaning solution for the cleaning of the electrode surfaces in an electrochemical cell which is used for the precipitation of copper.

As has been found, the regenerating method with strong electrical pulses has only limited use for electrodes of amperometric sensors, in particular the working electrode and/or the reference electrode, because due to the formation of bubbles the accumulated deposits can be dissolved in the electrolyte only to a limited extent.

Therefore, it is the objective to develop a simple, user-friendly and environmentally compatible method of regenerating an amperometric sensor, wherein even a repeated use of the method has an insignificant effect or preferably no effect on the performance specifications of the sensor.

This objective is met by the method for regenerating an amperometric sensor and by a regenerating device for carrying out the method, as will be described hereinafter.

The amperometric sensor includes a working electrode, a counter electrode and, optionally, a reference electrode which in their operative state are immersed in an electrolyte. The method for regenerating the sensor comprises several steps. First, the electrodes are brought into contact with a chemically active regenerating solution and are connected into an adjustable electric circuit, so that an electrochemical cell is formed. To cause an oxidation and/or reduction of deposits resulting in at least one deposit product, a negative and/or positive voltage pulse is applied to the electrochemical cell. The deposit product which results from reduction or oxidation of the deposits reacts with the regenerating solution, particularly with a complexing agent of the solution, and the reaction product is dissolved in the regenerating solution. The electrodes are removed from the regenerating solution and from the electric circuit and are subsequently rinsed with a preferably neutral rinsing solution and then thoroughly dried. For checking their functionality the electrodes are subsequently brought into contact with the electrolyte or with a new electrolyte and tested.

The regenerating solution used for this procedure is in addition distinguished by the fact that it is less harmful to the environment than the concentrated acids used in prior-art methods and that it can be used without major safety measures even by semiskilled workers.

The method can further include a step of removing a membrane body from the sensor before the electrodes are brought into contact with the regenerating solution. Prior to the function check, the membrane body is reinstalled. These steps are performed primarily with membrane-covered amperometric sensors.

The voltage pulse which is applied for the oxidation and/or reduction of the deposits lasts from about 15 sec to about 300 sec and its magnitude is about −3 V to about +3 V.

In a preferred embodiment the method can further include the polarizing and/or the recalibrating of the electrodes. The term “polarizing” in the context of oxygen- or ozone sensors means the time it takes for a quasi-stationary reaction equilibrium to establish itself. The polarizing can occur either in operation through the transmitter or also before putting the sensor into operation.

To verify that the regenerated electrodes are functioning properly, one or more sensor-specific parameters can be examined by measuring them and comparing them to a specified limit value. Such parameters include for example the slope, a reference potential, the response behavior, or the stability of the measurement signal of the electrochemical cell.

It is particularly advantageous to use an aqueous sodium thiosulfate solution as regenerating solution, as the thiosulfate ion is a good complexing agent, and most thiosulfate compounds show good or even very good solubility in aqueous solutions. In addition, sodium thiosulfate is easy to use and does not require any further safety measures or safety precautions. Methods with this kind of regenerating solution are therefore also suitable for use with instruments for mobile applications in the field, i.e. with instruments that are used outside of a controlled laboratory environment.