DE102018133297A1 - Reference electrode and potentiometric sensor - Google Patents

Abstract

A reference electrode as a reference for the measurement of relative potentials of other electrodes, in particular for a potentiometric sensor (10), comprising a shaft-shaped housing (12) with a longitudinal axis (a), which delimits an interior (13) at least in some areas, and within the interior (13) an electrolyte (5) and an electrical discharge line (4) are arranged and wherein the shaft-shaped housing (12) has a first liquid transfer (6), the first liquid transfer (6) comprising at least one opening (15), which has a having a first conductive transition (2), the conductive transition (2) extending from a surface which is designed for contact with the measurement medium over the edge of the opening (15) into the interior (13), the conductive transition (2) extends from the edge of the opening (15) over a distance of at least 1 cm parallel to the longitudinal axis (A) along the inside of the shaft-shaped housing (12), and a pot entiometric sensor (10).

Description

The present invention relates to a reference electrode and a potentiometric sensor, in particular in the form of a pH electrode.

As the most important instrument for the electrochemical measurement and control of pH values, a potentiometric sensor, in particular in the form of a so-called glass electrode, is widely used in many areas of chemistry, environmental analysis, medicine, industry and water management. The applicant offers and sells glass electrodes for a wide variety of applications. The glass electrodes used for potentiometric measurements usually have reference half cells, which form highly constant potentials. Usually silver / silver chloride or calomel electrodes are used.

The contact from the reference element to the measurement solution is made via the bridge electrolyte, which can be liquid or solidified and meets certain requirements: On the one hand, it should have little influence on the potential of the reference element; on the other hand, it should form the smallest possible diffusion potential with the measuring medium.

The difference in the electrochemical potential between two electrodes is often used to determine the concentration of different ions. The indicator electrode reacts sensitively to the ion to be determined, while a second reference electrode supplies a defined and constant potential as a reference variable. The indicator electrode is in direct contact with the measuring medium. The electrochemical chain is closed between the indicator electrode / measuring medium and the reference electrode in most applications by means of a liquid transfer from diaphragm and electrolyte. The transition from the electrolyte to the measuring medium has a significant influence on the properties of the measuring chain. The principle of measuring the pH value in a measuring solution using a potentiometric measuring chain is described, for example, in 'Wastewater Measurement and Control Technology', published by Endress + Hauser Holding AG, 1st edition, 2nd edition, p.83 ff. And described in detail in the book: 'pH measurement: fundamentals, methods, applications, devices', Helmut Galster, Weinheim: VCH, 1990, chapter 1.6, p.20 ff.

So far, different forms of liquid transfers have been known, in particular as diaphragms e.g. made of ceramic or Teflon, as a gap diaphragm, as a single-pore diaphragm and / or from platinum fibers.

However, single-pore reference systems can only be used with solid electrolyte, for example, because the pores are too large and the electrolyte would leak. Considerable diffusion potentials generally occur in these solid electrolytes due to diffusion and / or charge separation, for example when changing the measuring medium depending on the medium up to 10 mV.

In Teflon diaphragms, it takes a long time before the old measurement solution is replaced by the new measurement solution by diffusion. This slow exchange causes diffusion potentials. In ceramic diaphragms, the pores have a very rough surface, on which there are additional charges. These two effects cause diffusion potentials when changing the solution depending on the measuring medium up to 10 mV.

Fiber diaphragms with platinum wire have a smooth surface of the transition and due to the conductivity of the platinum wire, the diaphragm resistances are small, but in turn they are susceptible to redox potentials and can therefore only be used in some special applications. These disadvantages of the platinum diaphragms are discussed in DE 10 2011 089 671 A1 extinguished by a transition from, for example, ceramic diaphragms with a conductive layer. This transition is very complex. In the case of transitions with glass fibers, these are generally glued or pressed (e.g. in shrink tubing). This severely limits the pressure and temperature range of the sensors. In addition, glass fibers are not so conductive.

Starting from the aforementioned prior art, it is the object of the present invention to provide a reference electrode with a liquid transfer which is subject to significantly lower measurement errors than in the aforementioned conventional transitions e.g. with ceramic diaphragm.

The present invention achieves this object by providing a reference electrode with the features of claim 1.

A reference electrode according to the invention is known to serve as a reference for the measurement of relative potentials of other electrodes, in particular an indicator electrode, in particular for a potentiometric sensor.

The indicator electrode can preferably have a terminal glass membrane.

The reference electrode comprises a shaft-shaped housing with a longitudinal axis. The shaft-shaped housing delimits an interior, at least in some areas. In particular, it can be the outer wall of the interior, which is in contact with the measuring medium along an outer side stands. An electrolyte, the so-called reference electrolyte, is arranged inside the interior.

Furthermore, an electrical lead, the so-called external lead, is arranged in the interior. wherein the shaft-shaped housing has a first liquid transfer.

The first liquid transfer has at least one opening which has a first conductive transition.

The conductive transition can extend from a surface which is designed for contact with the measurement medium over the edge of the opening into the interior of the reference electrode.

The conductive transition extends from the edge of the opening over a distance of at least 1 cm essentially parallel to the longitudinal axis of the shaft-shaped housing. The conductive transition can be, for example, a coating that is arranged on the inside of the housing or a wire that runs parallel to the longitudinal axis of the housing. Essentially parallel does not rule out that the wire can have smaller bends.

The conductive transition prevents the build-up of diffusion potentials in the area of the liquid transition, which prevents unstable measured values at low conductivities. It has been shown that the transition to the interior should extend at least to a part of 1 cm parallel to the longitudinal axis in order to achieve stable measurement over a longer period, especially with measuring media with low conductivities of less than 50 µS / cm.

Advantageous embodiments of the invention are the subject of the dependent claims.

For the shortest possible distance between the derivation and the conductive transition, they should lie at least on one plane perpendicular to the longitudinal axis or face each other.

Even better results are achieved if the conductive transition extends from the edge of the opening over a distance of at least 2 cm parallel to the longitudinal axis along the inside of the shaft-shaped housing. It is ideal if the conductive transition starts from the edge of the opening up to a second liquid transfer, e.g. with a double junction, or extends up to a sensor head along the inside of the shaft-shaped housing.

The surface which is designed for contact with the measurement medium can advantageously be part of an outer surface of the shaft-shaped housing which is in contact with the medium and on which the conductive transition is fixed.

The distance between the derivative and the conductive transition should advantageously be less than 8 mm, preferably less than 5 mm, in order to achieve the most advantageous measuring effect with the conductive transition.

The conductive transition can in particular be an electron- and / or ion-conductive material.

As already mentioned above, the conductive transition is preferably designed as a coating and / or as a wire, a coating being advantageous for forming a defined distance from the conductor, but the wire is easier to assemble. The aforementioned variant also includes coated wire.

1. a) aus einem Metall, vorzugsweise einem Edelmetall, insbesondere aus Gold, Platin und/oder Silber,
2. b) aus einen Halbleiter, vorzugsweise aus Indium-Zinnoxid
3. c) aus einem leitfähigem Polymer, vorzugsweise aus einem PEDOT-PSS (Poly(3,4-ethylendioxythiophen)polystyrensulfonat)
4. d) aus leitfähigen Fasern, vorzugsweise leitfähigen Kohlenstoffasern, insbesondere CNT-Faser (CNT= carbon nanotubes), wie N/CNT und/oder SW-CNT, ggf. eingebunden in einer Kunststoffmatrix
5. e) aus einer redoxaktiven fixierbaren Komponente, vorzugsweise einem Polymer mit Ferroceneinheiten
6. f) aus einem Anionen oder Kationenleiter, insbesondere einem Protonen- oder Hydroxid- Ionenleiter
7. g) und/oder Kombination der vorgenannten Materialien.

The conductive transition can optionally have the following materials as wire, coating or coated wire or consist of the following materials:

1. a) from a metal, preferably a noble metal, in particular from gold, platinum and / or silver,
2. b) from a semiconductor, preferably from indium tin oxide
3. c) from a conductive polymer, preferably from a PEDOT-PSS (poly (3,4-ethylenedioxythiophene) polystyrene sulfonate)
4. d) made of conductive fibers, preferably conductive carbon fibers, in particular CNT fiber (CNT = carbon nanotubes), such as N / CNT and / or SW-CNT, optionally integrated in a plastic matrix
5. e) from a redox-active fixable component, preferably a polymer with ferrocene units
6. f) from an anion or cation conductor, in particular a proton or hydroxide ion conductor
7. g) and / or combination of the aforementioned materials.

The opening of the liquid transfer can advantageously have a diameter of 0.1-2 mm, preferably 0.3-1 mm.

The electrolyte can be an ammonium and / or alkali salt solution and preferably an electrolyte as a solution Potassium nitrate, potassium chloride, lithium acetate and / or ammonium chloride.

The electrolyte can be a solution with a mass fraction of an organic solvent of more than 50% by weight and a water fraction of less than 30% by weight.

The salt concentration of the electrolyte can advantageously be between 0.01M and 10M.

The electrolyte can also contain a salt dispenser, which gradually releases salt in the electrolyte solution such that the salt concentration is essentially constant.

The reference electrode can also have a second liquid transfer, a so-called double junction.

The reference electrode can have a second conductive transition, this transition extending from a surface which is designed for contact with the measurement medium, over the edge of the opening of the first liquid transfer into the interior, the conductive transition starting from the edge of the opening extends parallel to the longitudinal axis along the inside of the shaft-shaped housing.

The resistance of the first conductive transition can advantageously be less than 5 kOhm

A potentiometric sensor is also according to the invention, in particular in the form of a pH electrode and particularly preferably in the form of a glass electrode.

This comprises an indicator electrode with an inner lead and an inner buffer, the indicator electrode being delimited by a first shaft-shaped housing. Furthermore, the potentiometric sensor comprises a reference electrode according to the invention, the housing of the reference electrode being arranged as a second shaft-shaped housing coaxial to the first shaft-shaped housing, forming the interior.

• 1a-1d schematisch-vereinfachte Darstellung von mehreren Ausführungsvarianten eines erfindungsgemäßen potentiometrischen Sensors mit entsprechender erfindungsgemäßer Referenzhalbzelle
• 2 a, b Diagramm zur Darstellung der Potentialdifferenzen zwischen zwei Halbzellen umfassend eine erfindungsgemäße Referenzhalbzelle CPS91 _Ref bei pH 4 (2a) und pH 7 (2b) gegen eine andere Referenzhalbzelle (CPS43);
• 3a - 3d Balkendiagramm der Potentialdifferenzänderungen bei pH-Wert 4 oder 7 im gerührten oder ungerührten Zustand;
• 4 Tabelle der Potentialdifferenz (CPS91 vs. Urreferenz) in 3M KCl nach Alterung in deionisiertem Wasser (3 mon., 80°C).

Further advantages, features and details of the invention are described in the following description, in which an embodiment of the invention is explained in more detail with reference to the accompanying drawings. The person skilled in the art will expediently also consider the features disclosed in combination in the drawings, the description and the claims in combination and combine them into useful further combinations. Show it:

• 1a-1d schematic-simplified representation of several design variants of a potentiometric sensor according to the invention with a corresponding reference half-cell according to the invention
• 2a, b Diagram showing the potential differences between two half cells comprising a reference half cell CPS91 _Ref according to the invention at pH 4 ( 2a) and pH 7 ( 2 B) against another reference half cell (CPS43);
• 3a - 3d Bar chart of changes in potential difference at pH 4 or 7 in the stirred or unstirred state;
• 4th Table of the potential difference (CPS91 vs. original reference) in 3M KCl after aging in deionized water ( 3rd mon., 80 ° C).

1 schematically shows the structure of a potentiometric sensor using four variants from left to right 10 in the form of a pH sensor. The same components are provided with the same reference numerals. 1a is the left representation of the 1 and 1d the right representation of the 1

The sensors 10 in 1a - 1d each have a sensor head 1 on. The sensor head 1 can be designed as a transmitter or have an electrical interface to a transmitter.

Furthermore, the sensor 10 an inner derivative 3rd on. The inner derivative 3rd can preferably be designed as a conductive wire, which is often often designed as an Ag / AgCI derivative. The inner derivative 3rd is inside an indicator electrode 8th arranged, which by a first shaft-shaped housing 9 with an ion-sensitive membrane in contact with the measuring medium 7 is limited. The ion-sensitive membrane in contact with the measuring medium 7 can preferably be designed as a glass membrane. The indicator electrode is with an inner buffer 11 filled.

Around the first shaft-shaped housing 9 , and preferably coaxial to this, is a second shaft-shaped housing 12th arranged. The second shaft-shaped housing 12th extends parallel to the longitudinal axis of the first shaft-shaped housing 9 on the outside to this over a partial area, so that the terminal area with the measuring medium contacting ion-selective membrane 7 is exposed and in the intended use with the measuring medium by immersing the sensor 10 can come into contact with the measuring medium.

The first shaft-shaped housing 9 and the second shaft-shaped housing 12th delimit an interior 13 with annular cross section. In this interior 13 is an outside derivative 4th arranged, which is preferably also formed as a conductive wire, for example as a silver wire.

The interior 13 is with an outer electrolyte 5 , for example a gel. In the wall of the second shaft-shaped housing 12th is a liquid transfer 6 arranged. This liquid transfer has the function of transferring electrolytes from or into the measuring medium to ensure a salt bridge. For this purpose, the liquid transfers are also in contact with the measuring medium when used as intended. Liquid junctions are known per se. In principle, any known liquid transfer from the field of pH electrodes can be used in the context of the present invention.

If salt depletion does not affect the measurement properties, ideally a liquid transition such as a "single pore" (cross-linked polymer), a ground joint or a platinum diaphragm, glass fiber diaphragm, wood fiber diaphragm or teflon diaphragm can ideally be used. In this way, diffusion potentials on the diaphragm can be excluded. These liquid transitions are in addition to the conductive transition described above, e.g. in the form of conductive materials.

1b shows a second liquid transfer 14 which are above the first liquid transfer 6 towards the sensor head 1 is arranged. This embodiment with a second liquid transfer is known to the person skilled in the art as a so-called “double junction”. There is a second liquid transfer in the housing wall of the housing 12th provided, this second liquid transfer also one in the 1b Has not shown conductive transition.

The respective liquid transfer 6 or 14 includes at least one opening 15 and a first conductive junction 2nd , the conductive transition 2nd in the opening 15 is arranged. The conductive transition 2nd is in contact with the measuring medium and extends into the interior 13 of the second shaft-shaped housing 12th continues.

The second shaft-shaped housing can preferably be formed from glass, but a ceramic and / or a polymer plastic or combinations of the aforementioned materials, ie glass, ceramic, plastic, are also suitable.

The conductive transition extends 2nd at least in regions in the direction of the sensor head along the longitudinal axis of the second shaft-shaped housing 12th .

1. a) einem Metall, idealerweise einem Edelmetall (Gold, Platin, Silber,)
2. b) einen Halbleiter wie zum Beispiel Indium-Zinnoxid
3. c) einem leitfähigen Polymer, z.B. PEDOT-PPS
4. d) aus leitfähigen Kohlestofffasern, wie z.B. CNT-Faser, N/CNT, SW-CNT ggf. eingebunden in einer Kunststoffmatrix
5. e) einer redoxaktiven fixierbaren Komponente, wie z.B. einem Polymer mit Ferroceneinheiten
6. f) einem Anionen und/oder Kationenleiter, insbesondere einen Protonen- und/oder Hydroxid-Ionenleiter
7. g) oder Kombination der zuvor genannten Bespiele.

At the conductive transition 2nd it is preferably an electron and / or ion conductive material, such as a wire or a coating

1. a) a metal, ideally a precious metal (gold, platinum, silver,)
2. b) a semiconductor such as indium tin oxide
3. c) a conductive polymer, for example PEDOT-PPS
4. d) made of conductive carbon fibers, such as CNT fiber, N / CNT, SW-CNT, optionally integrated in a plastic matrix
5. e) a redox-active fixable component, such as a polymer with ferrocene units
6. f) an anion and / or cation conductor, in particular a proton and / or hydroxide ion conductor
7. g) or a combination of the aforementioned examples.

The opening 15 may preferably have a diameter of 0.1 to 2mm, more preferably 0.3-1mm.

With the outer electrolyte 5 , also referred to below as reference electrolyte, can be an electrolyte with alkali and / or ammonium salts, in particular potassium nitrate, potassium chloride, lithium acetate and / or ammonium chloride. The reference electrolyte can be an electrolyte with a mass fraction of an organic solvent greater than 50% by weight and a water fraction less than 30% by weight. The preferred salt concentration of the external electrolyte can be between 0.01 M and 10 M.

The outer electrolyte can also 5 have a salt dispenser which gradually releases salt and thus keeps the salt concentration essentially constant.

The sensor 10 can, however, also have a dry reference, for example a histidine polymer, which is functional when water is absorbed. A dry reference is an anhydrous electrolyte with a water-absorbing material. As a result, the measuring chain or sensor can be stored longer in the dry state. The use of reference half cells with solidified electrolyte has many advantages, for example the electrodes are low-maintenance, compared to the pressurized process there is no need to apply pressure, the refilling of electrolyte is not necessary.

All major electrode manufacturers are currently trying to use these advantages. When using the conventional diaphragms it happens Significantly delayed setting times of the reference elements and under unfavorable process conditions can lead to an interruption of the contact due to electrolyte tear. These can also lead to a considerable measuring error, under unfavorable circumstances up to 0.5pH or if the contact is broken, the electrode may fail.

The sensor 10 can also have a second conductive junction. This can be analogous to the first conductive transition 2nd be formed and preferably along the outer surface, ie the surface in contact with the measuring medium, of the second shaft-shaped housing 12th extend.

The conductive transition is defined along this surface. In the case of the configuration of the second housing 12th as a glass element, the transition along the glass surface can be firmly attached as a coating or as a wire.

The liquid flyover 6 , especially the first conductive transition 2nd , is designed such that the resistance of the conductive liquid transfer 6 is less than 5 kOhm.

1a shows a first embodiment of the sensor according to the invention, wherein the first conductive transition 2nd as a coating on the medium-contacting surface of the second housing 12th through the opening 15 the liquid flyover along the interior 13 facing inner surface of the second shaft-shaped housing 12th to the sensor head 1 extends. The conductive transition extends only linearly with a width of preferably less than 2 mm along the inner surface of the second shaft-shaped housing 12th .

In 1b is a too 1a Analog structure shown, but with a second liquid transfer 14 as an arrangement of a double junction

1c assigns a sensor analogously 1b but with a full-surface inside and possibly also outside coated second shaft-shaped housing 12th . The coating represents the conductive transition.

1d shows a further variant in which the coating as in the longitudinal direction of the sensor 10 successively arranged circumferential rings is arranged along the inner surface and / or along the outer surface in contact with the measuring medium. The rings are connected to each other via a conductive connection.

Further variants, not shown, for arranging the coating or a wire arrangement are lattice structures, lines running in the longitudinal direction or the like.

Due to the liquid transfer described above 6 with the conductive transition 2nd are subject to measurements in measurement media with low conductivities of less than 50 µS / cm less fluctuations and measurement errors.

In the present invention, preferably to provide a defined opening z. B. also have perforated diaphragms, which on the pore surface with a conductive transition, e.g. Conductive silver varnish [e.g. B. conductive silver 200 N from Wolbring GmbH] is coated. This conductive silver varnish (conductive transition) is pulled up on the inside of the shaft.

The walls of the perforated diaphragm are given a smooth surface, which reduces diffusion potentials. This smooth surface is conductive so that the diaphragm resistance is kept small and no large transfer voltage is to be expected. Therefore, overpasses with such a transition are also particularly suitable for flowing low-conductivity media, such as Ultrapure water.

If the silver conductive lacquer is also pulled upwards on the inside of the shaft tube, there is no break in the contact in the event of pressure surges or overhead installation with the electrolyte tearing off.

The transition described above does not affect the measured value or only to a small extent. Ease of maintenance can therefore be combined with quick setting behavior of the electrodes and extremely high potential stability.

The sensor according to the invention can thus enable precise pH measurements even with flow in measurement media with low conductivity.

In addition, the electrode is less sensitive to pressure fluctuations and certain installation positions, since in particular when using conductive silver paint on the inside of the second shaft-shaped housing 12th , contact interruption on the electrolyte is prevented by unfavorable process conditions.

1. a) Viskose Medien welche engporige Flüssigübergänge verstopfen würden [> 1000mPas ~ Glycerin]
2. b) Überkopfanwendungen (schräg oder wirklich auf dem Kopf)
3. c) Wasserarme Medien (<30%, noch mehr bevorzugt <10% Wasser, Rest organisches Lösungsmittel)
4. d) Niedrig leitfähige Medien z.B. Kesselspeisewasser, Leitfähigkeit < 50µS/cm << 20µS/cm

The following areas of application are particularly preferred:

1. a) Viscous media which would clog narrow-pore liquid transitions [> 1000mPas ~ glycerin]
2. b) overhead applications (oblique or really upside down)
3. c) Low-water media (<30%, more preferably <10% water, remainder organic solvent)
4. d) Low conductive media eg boiler feed water, conductivity <50µS / cm << 20µS / cm

A thickened electrolyte can be used as the preferred external electrolyte, particularly for applications in media which tend to clog conventional liquid transitions. The electrolyte can be thickened using common polymers such as acrylamides and their derivatives. This also prevents premature salt depletion, especially in applications with a higher flow rate.

As already mentioned, a so-called "double junction" can also be used. In this case, the metallization can once a) on the first liquid transfer, along the inside of the second shaft-shaped housing 12th , until the second liquid transfer 14 be applied.

Alternatively or additionally, in the second liquid transfer 14 also a metallic transition into the second cell. The transition can also consist of a ceramic diaphragm (reduction of the loss of electrolyte) or another material common to the person skilled in the art, since blocking of the second transition is generally prevented by the blocking action of a thick gel.

In some applications, it may be advantageous if the electrolyte concentration has a low ion concentration (e.g. 0.1 M potassium chloride) in order not to contaminate a measurement solution, for example, or to achieve low ion transfer with low ion-poor water. In this case, it is particularly important that the potentials of the reference half-cell do not become too high.

The sensor according to the invention 10 thus enables measurements that may otherwise only be possible with interference signals and are not suitable for permanent measurement.

In addition, with a sensor according to the invention 10 potentiometric measurements can be carried out in a low-water measuring medium with a water concentration of less than 30% by weight or even less than or equal to 10% by weight. In this special case, certain internal electrolytes, such as lithium acetate in ethanol or the like, which have a low conductivity, can preferably be used. The device according to the invention also enables a more stable measurement, which is no longer stable with a conventional pH electrode.

The reference electrode of the sensor according to the invention 10 indicates the conductive transition 2nd on, which at least covers the interior to the measuring medium and further into the inside of the shaft of the housing 12th protrudes to bridge any electrolyte interruptions that may occur and to guarantee a high electron and / or ion conductivity.

In a preferred embodiment variant, the conductive transition 2nd be created by a simple line drawn with a special marker, which is the liquid transfer, i.e. the edge of the opening 15 , covered and then along the inside to the top of the case 12th on which the sensor head 1 connects, is pulled. In the simplest case, the marking can be carried out with a conductive silver varnish, which is applied and then optionally also baked. Other conductive lacquers can also be used advantageously.

Alternatively or additionally, pure platinum can be deposited on the inside of the housing by electrochemical deposition of a platinum complex 12th and along the edge of the opening 15 be deposited.

The transition described above 2nd can also be overlaid with a ring made of metal, which is at the transition 2nd and therefore preferably inseparable from the housing 12th is pressed.

Alternatively, the aforementioned metal ring can also be chemically covalently attached to the housing 12th be bound eg by soldering, melting etc.

In a further embodiment, the lacquer line can be branched. If possible, wetting is also given in the event of electrolyte loss, bubbles in gel-like electrolytes or when the electrolyte infiltrates, for example in the case of viscous reference gels. A large area covering the inside of the interior 13 is therefore advantageous, but not mandatory.

Ideally, a framework structure made of a conductive material as shown in Example 1c or 1d can be inserted into the interior of the shaft and pressed through the liquid transfer. It is particularly important to make initial contact with the liquid transfer 6 to the measuring medium. This contact should be stable. However, it is not mandatory that the easy transition be soldered, riveted, coated electronically, burned in or pressed. External pressure loads of up to 12bar and flow velocities of up to 3 m / s should connect the conductive transition to the Housing surface, especially in the area of the opening 15 , preferably withstand.

2a and b as well 3a-d show experimental results for the measurement of a potential difference between a sensor according to the invention against a CPS 43 reference sensor.

As a sensor according to the invention, a CPS91 was provided with an additional silver wire inside the interior of the reference electrode.

For the measurement at low conductivities, dilution series in the ratios of 1: 5, 1:25, 1: 125 and 1: 625 were each prepared with a pH4 buffer from Endress + Hauser and a pH7 buffer from Merck and measured the potential difference of the converted CPS91 against a CPS43_ESA reference (3MKCI). Both sensors are products from Endress + Hauser.

The CPS91 is a pH electrode with single pore liquid transfer with a diameter of approx. 1mm.

To simulate a long-term measurement in liquids with low conductivity, six CPS91 were stirred for 3 months in 80 ° C hot deionized water to induce a reference depletion.

A pH4 buffer solution from Endress + Hauser and a pH7 buffer from Merck were diluted 1: 5, 1:25, 1: 125 and 1: 625, preferably with deionized water.

1. a) Org. Puffer (gerührt 10min, ungerührt 10min),
2. b) 1:5 Verdünnung (gerührt 10min, ungerührt 10min),
3. c) 1:25 Verdünnung (gerührt 10min, ungerührt 10min),
4. d) 1:125 Verdünnung (gerührt 10min, ungerührt 10min),
5. e) 1:625 Verdünnung (gerührt 10min, ungerührt 10min),
6. f) (teilweise deion. Wasser)
7. g) Org. Puffer (gerührt 10min, ungerührt 10min).

The lowest dilution of 1: 625 results in a conductivity of 15-30 µS / cm and is therefore in the range of the lowest conductivities at which a pH measurement still makes sense. The references of the CPS91 (ESA) sensors were then measured with a Lawson device against a CPS43 (reference half cell, 3M KCI) in differential mode against a platinum electrode in the buffers in the following order at 25 ° C:

1. a) Org. buffer (stirred 10min, unstirred 10min),
2. b) 1: 5 dilution (stirred 10min, unstirred 10min),
3. c) 1:25 dilution (stirred 10min, unstirred 10min),
4. d) 1: 125 dilution (stirred 10min, unstirred 10min),
5. e) 1: 625 dilution (stirred 10min, unstirred 10min),
6. f) (partially deionized water)
7. g) Org. buffer (stirred 10min, unstirred 10min).

The pH7 Merck buffer and the pH4 buffer from Endress + Hauser were used due to their approximately comparable conductivity. The results of this are in 2a, b shown, where 2a the measurement results in the pH = 4 buffer and 2 B shows the measurement results in the pH = 7 buffer.

Another measurement was carried out with a kink transmitter after the end of the aforementioned tests a) - g). The reference was measured against the original reference. The results are reflected in 4th again.

The measurement was carried out for leached CPS91 vs. Original reference (product from Endress + Hauser). in 3M KCl after aging in deionized water (3 months, 80 ° C). This was used to check the final state of salt depletion or electrolyte depletion of the CPS91.

2a and 2 B each show measurements with stirring at 500 rpm and subsequent measurement in the unstirred state. The dilution was then increased. In the course of the x-axis, the dilution of the respective solution increases at pH = 4 or pH = 7. The measurements were carried out twice for leached CPS91 sensors without silver wire and with silver wire.

The thick dark gray curves of the 2a show the measurement results or measurements ( 100 , 200 ) of the CPS91 sensors without silver wire and the thin light gray curves show the measurement results or measurements ( 300 , 400 ) of the CPS91 sensors with silver wire. The same applies to 2 B .

It turned out that the silver wires, which reached far into the reference half-cell, showed very stable measured values. The CPS91 half cells without wire sometimes showed very unstable measurement behavior with the highly diluted buffers, which is noticeable by sudden changes in potential. (please refer 2a, b )

It was also found that the arrangement of the silver wire ( 0 , 5 mm diameter, 5 cm length, 3 cm protrude into the reference interior) a more stable measurement was possible.

In addition, very small changes in the potential difference between the diluted buffers and the undiluted original buffer solution were found in the CPS91 with silver wires. (please refer ) In total it can be stated that reference half cells with the silver wire contribute to very small changes in potential difference compared to the undiluted buffer. This enables one stable measurement even with larger flows, which in the present case were simulated by an agitator at a speed of 500 rpm and with low conductivities up to approx. 20 µS / cm and also after leaching of the reference interior or interior 13 of the sensor.

This speaks for a long-term stable measurement with low conductivities. The change in potential of the CPS91 Ref was measured after the end of the test against a 3MKCI reference in 3M potassium chloride in order to control the leaching of the reference. As expected, the single pore electrodes experienced greater salt depletion, which has a rather disadvantageous effect on the measurement stability if there is no conductive transition, such as a silver wire, as in 2a and 2 B pictured, is present. This is in the table of the 4th reproduced.

Furthermore, the potential differences from stirred and unstirred diluted buffers to the original buffer were calculated. In the 3a-3d The bar diagrams shown refer to changes in the potential difference between the diluted buffers and the original buffers or undiluted buffers.

These are in 3a given for pH = 4 at various dilutions in the stirred state at 500 rpm. 3b shows the potential differences at pH = 4 in the unstirred state at different dilutions. 3c shows the potential differences at pH = 7 in the stirred state at various dilutions and 3d shows the potential differences at pH = 7 in the unstirred state at different dilutions.

The white bars are the measured reference ( 500 ) from two CPS43 sensors for error estimation of the measurement. The light gray bars show the two measurements ( 300 , 400 ) of CPS91 sensors with silver wire and the dark gray bars ( 100 , 200 ) show two measurements of two CPS91 sensors without silver wires in the interior of the sensor's reference half-cell.

The invention can be used in all pH electrodes and also in other potentiometric sensors. The reference half-cell can also be spatially separated from the measuring half-cell.

Reference list

1.

Sensor head

2nd

Conductive transition

3rd

Internal drainage

4th

External derivative

5.

electrolyte

6.

Liquid transfer

7.

ion sensitive membrane

8th.

Indicator electrode

9.

Shank-shaped housing

10th

Pot sensor

11.

Inner buffer

12th

Shank-shaped housing

13.

inner space

14.

Second liquid transfer

15.

opening

A

Longitudinal axis (first, second shaft-shaped housing and sensor)

100

Measurement (CPS91 without Ag wire)

200

Measurement (CPS91 without Ag wire)

300

Measurement (CPS91 with Ag wire)

400

Measurement (CPS91 with Ag wire)

500

Reference measurement

QUOTES INCLUDE IN THE DESCRIPTION

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Patent literature cited

• DE 102011089671 A1 [0008]

Claims (19)

Reference electrode as a reference for the measurement of relative potentials of other electrodes, in particular for a potentiometric sensor (10), comprising a shaft-shaped housing (12) with a longitudinal axis (a), which delimits an interior (13) at least in some areas, whereby within the interior ( 13) an electrolyte (5) and an electrical discharge line (4) are arranged and the shaft-shaped housing (12) has a first liquid transfer (6), the first liquid transfer (6) comprising at least one opening (15) which has a first The conductive transition (2) is characterized in that the conductive transition (2) extends from a surface which is designed for contact with the measuring medium over the edge of the opening (15) into the interior (13), the conductive transition ( 2) extending from the edge of the opening (15) over a distance of at least 1 cm parallel to the longitudinal axis (A) of the shaft-shaped housing (12). Reference electrode after Claim 1 , characterized in that the derivative (4) and the transition (2) lie at least on one plane perpendicular to the longitudinal axis (A). Reference electrode after Claim 1 or 2nd , characterized in that the conductive transition (2) extends from the edge of the opening (15) over a distance of at least 2 cm parallel to the longitudinal axis (A) along the inside of the shaft-shaped housing (12). Reference electrode according to one of the preceding claims, characterized in that the conductive transition (2) extends from the edge of the opening (15) to a second liquid transfer (14) or to a sensor head (1) along the inside of the shaft-shaped housing (12 ) extends. Reference electrode according to one of the preceding claims, characterized in that the surface which is designed for contact with the measuring medium is part of a medium-contacting outer surface of the shaft-shaped housing (12), on which the conductive transition is fixed. Reference electrode according to one of the preceding claims, characterized in that the distance between the lead (4) and the conductive transition is less than 8 mm, preferably less than 5 mm. Reference electrode according to one of the preceding claims, characterized in that the conductive transition (2) is an electron and / or ion conductive material. Reference electrode according to one of the preceding claims, characterized in that the conductive transition is designed as a coating and / or as a wire. Reference electrode according to one of the preceding claims, characterized in that the conductive transition a) from a metal, preferably a noble metal, in particular from gold, platinum and / or silver, b) from a semiconductor, preferably from indium tin oxide c) from a conductive Polymer, preferably from a PEDOT-PSS (poly (3,4-ethylenedioxythiophene) polystyrene sulfonate) d) from conductive fibers, preferably conductive carbon fibers, in particular CNT fiber (CNT = carbon nanotubes), such as N / CNT and / or SW-CNT , optionally integrated in a plastic matrix e) from a redox-active fixable component, preferably a polymer with ferrocene units f) from an anion or cation conductor, in particular a proton or hydroxide ion conductor g) and / or combination of the aforementioned materials. Reference electrode according to one of the preceding claims, characterized in that the opening (15) of the liquid transfer (6) has a diameter of 0.1-2 mm, preferably 0.3-1 mm. Reference electrode according to one of the preceding claims, characterized in that the electrolyte (5) is an ammonium and / or alkali salt solution and preferably an electrolyte as a solution of potassium nitrate, potassium chloride, lithium acetate and / or ammonium chloride. Reference electrode according to one of the preceding claims, characterized in that the electrolyte (5) is a solution with a mass fraction of an organic solvent of greater than 50% by weight and a water fraction of less than 30% by weight. Reference electrode according to one of the preceding claims, characterized in that the salt concentration of the electrolyte (5) is between 0.01 M and 10M. Reference electrode according to one of the preceding claims, characterized in that the electrolyte (5) contains a salt dispenser which gradually releases salt, such that the salt concentration is essentially constant. Reference electrode according to one of the preceding claims, characterized in that the reference electrode has a second liquid transfer (14). Reference electrode according to one of the preceding claims, characterized in that the electrolyte (5) is a dry reference, that is to say a water-free electrolyte with a water-absorbing material. Reference electrode according to one of the preceding claims, characterized in that the reference electrode has a second conductive junction, this junction extending from a surface which is designed for contact with the measurement medium over the edge of the opening (15) of the first liquid transfer (6) to in extends the interior (13), the conductive transition (2) extending from the edge of the opening (15) parallel to the longitudinal axis (A) along the inside of the shaft-shaped housing (12). Reference electrode after Claim 1 , characterized in that the resistance of the first conductive transition (2) is less than 5 kOhm. Potentiometric sensor (10), in particular pH electrode, comprising an indicator electrode (8) with an internal lead (3) and an internal buffer (11), the indicator electrode (8) being delimited by a first shaft-shaped housing (9), characterized in that that the potentiometric sensor (10) has a reference electrode according to one of the preceding claims, wherein the housing of the reference electrode is arranged as a second shaft-shaped housing (12) coaxial to the first shaft-shaped housing (9) while forming the interior (13).

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