CH661595A5 - REFERENCE ELECTRODE ARRANGEMENT WITH INTERCHANGEABLE REFERENCE TRANSITION. - Google Patents

Description

The invention relates to a reference electrode arrangement with exchangeable reference transitions. The invention further relates to a liquid-containing transportable or dispatchable container for the interchangeable reference transitions. Reference electrodes generate a constant reference voltage, as is required for electroanalytical processes, for example for ion-selective electrode measurements, in the coulometry operating with control voltages, polarography, and the like.

The most common field of application is a reference electrode with a separately or jointly arranged ion-selective electrode for measuring the activity (as

Function of concentration or proportion) of certain ions used. The further explanation is based on this main, but by no means exclusive, application.

The two electrodes, namely the reference electrode and the ion-selective electrode, are immersed in the measuring liquid and mostly to a measuring device for measuring the voltage difference between the two electrodes, e.g. connected to an electrometer. The reference electrode provides a constant electromotive force or voltage with which the voltage of the ion-selective electrode is compared. The voltage of the latter consists of a constant component from the electrochemical half-cell of the ion-selective electrode and a variable component, the voltage of the measuring membrane dependent on the activity (concentration) of the ions to be measured. This variable component can easily be related in a known manner to the ion activity (concentration). In the interest of accurate measurement results, the voltage of the reference electrode should be independent of the composition of the sample to be measured.

The reference electrode is designed so that it is as independent as possible of changes in the conditions of the ions to be measured. It consists of at least three parts:

1. a half-cell electrode (a mixture of silver and silver chloride is typical),

2. a half-cell electrolyte (a solution of 4 M potassium chloride saturated with silver ions is typical) and

3. a reference connection or a reference transition.

Half-cell electrode and electrolyte form an electrochemical half-cell with a known, constant, constant voltage. The reference transition establishes the direct physical and thus electrical contact of the semi-cell electrolyte with the measuring liquid; this connection usually consists of a porous, ceramic plug, metal or asbestos fiber bundle, sintered plastic or the like, suitable for producing a mechanical leak connection material.

“Half-cell electrode” is understood here to mean the solid-phase, electron-conducting contact with the half-cell electrolyte at which the oxidation-reduction of the half-cell takes place, which in turn produces the constant voltage between the half-cell electrolyte and the contact.

Since the ionic strength and the ion transition of the transition electrolyte and the measurement sample are different, a «liquid transition potential» is regularly created at the reference transition. The differences of this transition potential from sample to sample represent a source of error in the electrode measurement. The aim in reference electrode technology is therefore to make the transition potential as small, stable and reproducible as possible. However, the reference transition can become inoperable for a number of reasons, most often due to constipation. The blockage of the pores of the transition with foreign bodies interrupts the direct physical contact that is required to establish a constant, repeatable tension at the liquid transition between the inner solution and the measurement solution. Blockage also creates a fixed ion charge at the junction, which causes an abnormal increase in the junction voltage when measuring low ionic strengths. It is also important that in some electrode designs the inner filling solution flows from the reference electrode into the measurement solution. It was found that this outflow enables faster and more accurate measurement because it sweeps the previously measured solution away from the transition more quickly and also increases the ionic strength at the transition surface and the abnormal consequences of the fixed2

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stationary space charge at the transition. However, a blockage blocks this favorable outflow of the transition electrolyte and results in slower and less precise measurements. Finally, a blockage also increases the electrical resistance at the transition and thus electrical disturbances of the measurement to a corresponding extent. Typical signs of a blocked transition are slow, erratic, disturbed and often incorrect measurement results.

The transition blockage can arise from various, internal and external causes. The proteins and lipids contained in many measurement samples tend to bind and penetrate many transition substances due to electrostatic and hydrophobic forces. Furthermore, certain components of the filling solution can be precipitated if they come into contact with the measuring solution at the transition; this applies e.g. for AgCl and Ag2Sam transition of the Ag / AgCl reference electrodes immersed in diluted chlorides and sulfides.

In known devices, failure of the reference junction means the need to replace the entire reference electrode. This is expensive and complex because the reference transition is often the cheapest part of the reference electrode. Attempts to have the reference transition removed and replaced by laboratory personnel in laboratory operation regularly failed because in most high-quality electrodes, the transition is permanently melted or cemented onto the electrode body. Even if the transition should only be attached by friction or pressure, it breaks when removed because it is very sensitive. To pull it out, the porous transition would have to protrude above the electrode body. As it turned out, this would introduce a spherical diffusion component instead of the much cheaper flat component and contribute to slow and inaccurate measurements. A flat transition is therefore preferred, but this prevents the porous body from protruding.

The object of the invention is an improved reference electrode arrangement with an improved reference transition; in particular, a reference transition is to be created which can be removed and replaced without risk of damage and which works better.

This object is achieved by a reference electrode arrangement according to claim 1. Another object of the invention is to provide a liquid-containing, transportable or dispatchable container for reference transitions according to the invention. This further object is achieved by a container according to claim 11.

The reference electrode can contain a sealing ring. This is sealed in the electrolyte outlet and prevents the flow of liquid around it. It has at least one opening through which the interchangeable reference transition can be inserted and removed.

The reference transition contains a sleeve body, e.g. a capillary tube, preferably made of glass, which supports the porous body, e.g. encloses a ceramic rod. The ceramic rod is shorter than the capillary tube and is preferably located at the end immersed in the measuring liquid. Virtually no part of the liquid flows around the capillary tube through the sealing ring.

According to a further advantageous embodiment of the invention, one end of the porous body is mounted in the sleeve body or the capillary tube in such a way that it lies in one plane with one end of the sleeve tube, in particular is flat. The length of the porous body is also no more than half the length, and preferably no more than a third of the length of the capillary tube.

In the case of designs in the manner of combination electrodes, a measuring element, e.g. a pH meter or an ion-selective element.

In the drawings considered in the further explanation show:

Figure 1 schematically shows the essential parts of a typical pH measuring system;

FIG. 2 shows a known reference connection of a reference electrode in longitudinal section;

3 shows in longitudinal section a reference connection with a reference transition according to the invention in a reference electrode;

FIG. 4 shows in longitudinal section several interchangeable reference transitions according to the invention in a container ready for dispatch;

5 shows a reference transition according to the invention, in an enlarged longitudinal section;

6 shows a partial view in longitudinal section of a combination electrode with a reference transition according to the invention;

7 shows a further embodiment of the invention.

1, a pH electrode 1 and a reference electrode 3 are partially immersed in a measuring liquid 5 in the container 8. Both electrodes are connected to an electrometer 17 by conductors 13, 15. The voltage applied to the glass membrane 7 changes according to the pH difference between the measuring liquid 5 and a buffer solution 9 enclosed in the membrane. The electrical connection is established between the buffer solution and the line to the electrometer by means of an electronic half cell 10. The voltage of the half cell is regularly determined by the chloride ion concentration in the buffer. Thus the voltage difference between the measuring solution 5 and the positive connection of the electrometer changes with the pH value. The function of the reference electrode is to establish a fixed half-cell voltage between the measuring liquid and the negative connection of the electrometer. If an unknown liquid is to be measured, the half-cell electrode cannot be immersed directly in this liquid, because its voltage depends on the unknown anion component, e.g. the chloride ion activity of the measuring liquid. An indirect reference connection is therefore established by immersing the reference half-cell 11 in a known electrolyte 19 (mostly 4M KCl saturated with AgCl) and establishing the physical and chemical connection between the latter and the measuring liquid through a reference transition 21 arranged in the outlet 23. The reference transition usually consists of a plug made of porous ceramic, asbestos fibers, or the like material of mechanical permeability, which has a flow-inhibiting, filtering effect and defines the shape of the interface between the two liquids. If possible, it should make a contact of low resistance, preferably below 1 OK ohm, but without mixing the liquids together.

FIG. 2 shows a known reference electrode in section. FIG. 3 shows in section the reference electrode of FIG. 2, which, however, has a reference transition according to the invention.

The reference electrode 30 of FIGS. 2 and 3 contains an electronic half cell 32, the electrical conductor 34, the electrolyte solution 36 and the outlet 38, through which the reference transition is connected to the measurement solution (not shown here). In FIG. 2, a ceramic plug 40 is inserted into the outlet 38, while in FIG. 3 a sealing ring 42 is permanently installed. Through this is a

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Capillary tube made of glass 44 which surrounds a porous ceramic stopper 46 extending over a third of the tube. The tube forms a reference transition according to the invention with the stopper.

The shipping container 39 of FIG. 4, consisting of a housing 43 with a screwed-on cap 41, contains a plurality of interchangeable reference transitions 45 which are completely immersed in the liquid 50, preferably the electrolyte, so that no adjustment time is lost for establishing the equilibrium when the reference electrode is inserted.

In a favorable embodiment of the invention, a glass capillary tube 52 is provided. This has e.g. an outer diameter of about 2 mm, an inner diameter of about 1 mm, and is about 12 mm long. A porous ceramic plug 54 is inserted into this sleeve, e.g. Is 1 mm in diameter and 3 mm long. By passing along e.g. At a temperature of 1250 ° C, the glass tube is melted or coated on the ceramic. The tube end 56 is then ground flat with the ceramic and the end 58 is fire polished or chamfered to facilitate its insertion into the sealing ring. If desired, the coating can be left on or relaxed.

In the electrode arrangement 60 of FIG. 6, the glass electrode 62 and the reference transition 70 are guided through the sealing ring 66. The reference half cell 64 extends into the electrolyte 68 located in the electrode housing.

The basic concept of enclosing the porous transition part in a sleeve and thus facilitating insertion and removal can also be achieved in other ways; so instead of attaching the sealing ring to the electrode, a seal can be provided in one piece as part of a removable housing of a sleeve.

In a further embodiment, the sealing ring can be omitted, and instead the replaceable sleeve body itself or the opening of the reference electrode is made of an easily compressible material, e.g. Polypropylene, which creates a tight, leak-free fit between the sleeve body and the electrode body. For example, the interchangeable sleeve body 80 made of polypropylene for the porous ceramic part 82 of FIG. 7 fits tightly into the glass opening 84 formed in the electrode body 86.

Further modifications of these designs of the inventive concept are possible.

Basically, all previously common porous parts are suitable for the connection or the transition in reference electrodes, provided that a corresponding sleeve to be agreed is available. Some porous parts are e.g. cannot be used when the glass tube is heated. It is possible to use a plastic sleeve made of material that flows at lower temperatures or can be sealed with a plasticizing solvent or with epoxy cement, e.g. a porous plug made of polypropylene or polyvinylidene, which is held in the end of a non-porous tube of the same material.

The porous part can protrude from the sleeve, but this will usually be 20. not be so cheap. In contrast, the reference transition preferably extends beyond the seal on both sides. Preferably, the filter end of the reference transition extends far enough beyond the seal that it can be gripped and pulled out with a tool; 25 the other end of the reference transition extends into the electrolyte solution.

The concept of the invention can also be used in arrangements with double reference electrodes. The reference transition of the invention is preferably used here on the outer reference connection between the outer electrolyte liquid and the measuring liquid, although it can also be used as an inner connection between the half-cell electrolyte and the outer electrolyte.

35 The invention can be used for all electrochemical half cells (e.g. silver-silver chloride, calomel etc.) and even for gelled internal electrolytes.

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Claims (11)

661595 PATENT CLAIMS 1. Reference electrode arrangement for electroanalytical measurements with a half-cell electrode (11) arranged in a housing (3) and connectable to an external measuring device (17), a half-cell electrolyte (19) and a reference transition (23) for the electrolyte. 21) to a measurement solution when the housing (3) is immersed in the measurement solution, characterized in that the reference transition is removable and interchangeable and has a removable sleeve body (44) which is guided in the outlet and surrounds a porous body (46) whose length does not exceed half the length of the sleeve body. 2. Reference electrode arrangement according to claim 1, characterized in that the outlet (38) or the removable sleeve body (44) contains a compressible sealing ring (42). 3. Reference electrode arrangement according to claim 2, characterized in that the sealing ring (42) is provided in the outlet (38) and the sleeve body consists of a cylinder tube (44). 4. Reference electrode arrangement according to claim 3, characterized in that the length of the porous body (46) does not exceed a third of the length of the sleeve body (44). 5. Reference electrode arrangement according to one of claims 1-4, characterized in that the entire porous body (46) is enclosed in the sleeve body (44). 6. Reference electrode arrangement according to claim 5, characterized in that one end (56) of the porous body (54) lies in the same plane with a first end of the sleeve body (52). 7. reference electrode arrangement according to claim 6, characterized in that the reference transition in the sealing ring (42) is mounted so that the second open end of the sleeve body (44) projects into the half-cell electrolyte (36). 8. Reference electrode arrangement according to claim 7, characterized in that the first end of the sleeve body protrudes beyond the outer side of the sealing ring and is in contact with the measurement solution. 9. Reference electrode arrangement according to one of claims 2-8, characterized in that the porous body consists of a ceramic stopper (46) to which a glass tube forming the sleeve body (44) is applied. 10. Reference electrode arrangement according to claim 9, characterized in that the length of the porous ceramic stopper is 5 mm and that of the glass tube is 10-20 mm. 11. A liquid (50) -containable or transportable container (39) with immersed therein at least one of the exchangeable reference transitions (45) for attachment into the outlet (23) of a reference electrode arrangement according to claim 1.