The present invention relates to a method for generating a constant derivation potential be means of a reference electrode in analytical chemistry, in particular for a pH measuring chain for use in the meat-producing industry or for measurements in semi-solid substances such as cheese, pastes or the like, in which for carrying out the measurement, at least the pH test electrode is introduced into or brought into contact with the medium to be measured by a corresponding movement.
There have been known a large variety of different measuring arrangements for measuring the pH value by potentiometric means, all of them comprising a glass electrode, a reference electrode and a transducer intended for processing the derivation potentials of both electrodes and for indicating, for example, the pH value so measured.
Most of the pH measuring arrangements consist of a combination of glass electrodes and reference electrodes, thus forming a measuring electrode chain which, when the two electrode types are united in one unit, form a single-point test electrode of the type which is widely in use in industry. The principle of determining the pH value consists in measuring the ion concentration and is largely known so that it need not be described here in more detail. In order to enable the H.sup.+ ion concentration to be measured by means of the glass membrane of the measuring electrode and to be converted to a useful pH value, a reference electrode is needed which provides a constant standard or derivation potential for the measuring electrode. The separation at the contact point between the interior electrolyte of the reference electrode and the test solution consists in this case of a capillary connection, the so-called diaphragm.
This design (i.e. a glass membrane in the measuring electrode and a diaphragm in the reference electrode) requires that both the glass membrane and the diaphragm be located in the test fluid during the measuring process. In addition, the diaphragm needs at least one outer contact support.
Problems have been encountered in certain applications, especially in the area of the reference electrode, in connection with measurements carried out, for example, in the meat-processing trade or in semi-solid substances, such as cheese, pastes, or the like. It is of course possible in such cases, too, to introduce the measuring chain into the substance to be measured, to pull it out again upon completion of the measurement and to clean it as required, if the glass membrane of the measuring electrode should have been soiled, but the steps of introducing and withdrawing the measuring chain will necessarily lead to the diaphragm of the reference electrode being soiled or blocked, and this condition will necessarily result in adulterations of the measuring results, maybe even in gradual contamination of the interior electrolyte.
Consequently, it has also been known (DE Utility Patent 87 09 937.3) to equip a pH measuring chain intended for carrying out measurements on such semi-solid substances with a reference system using a gel-like reference electrode in the form of a suitable polymer arranged in an electrolyte vessel of given dimensions. In order to achieve long-term zero-point stability of the reference system, the polymer is enriched with potassium chloride in crystalline form. Further, it is essential that the lower boundary surface of the polymer forming the reference electrode must simultaneously act as a measuring diaphragm which means that the electrolyte vessel may not be closed at this point. During the measuring process, the glass membrane of the pH electrode is then of course introduced into the substance to be measured, but the lower exposed surface of the gel-like reference electrolyte is brought into contact only with the surface of the medium to be measured. The particular physical properties of the polymer provide a self-cleaning effect, and if the design and material of the electrolyte vessel are selected conveniently, there is in addition the possibility to cut off from time to time the worn boundary surface of the diaphragm. If necessary, the reference system and the pH electrode can be detached individually from the handle by which the operator holds the pH measuring chain so formed.
However, it remains a problem of this known pH measuring chain that the measuring diaphragm may deteriorate gradually in the course of the measurement to be carried out, before diaphragm is renewed completely by the radical step of cutting off the lower portion of the reference electrolyte.
Now, it is the object of the present invention to provide a reference electrode, especially for measurements to be carried out on semi-solid substances, which when used in connection with a pH measuring electrode provides zero-point stability and a self-cleaning effect.