Reference electrode with improved compressive strength, diaphragm for a reference electrode and method for manufacturing a reference electrode

Abstract

Reference electrode for electrochemical, preferably potentiometric measurements in a measuring fluid, the reference electrode comprising: (a) a housing (H) with a reference electrode compartment (H2; H2, H3) and a housing wall (2) delimiting the reference electrode compartment, (b) a reference electrolyte (12) contained in the reference electrode compartment (H2; H2, H3), (c) a derivative for the potential of the reference electrode and (d) a diaphragm (6; 7; 8, 9) with an inner surface facing the reference electrode space (H2; H2, H3) that is wettable by the reference electrolyte (12) and an outer surface that is wettable by the measuring fluid or that is facing an optional further reference electrode space (H3), (e) wherein the diaphragm (6; 7; 8, 9) comprises a support structure (6'; 7') having at least predominantly open porosity and a hydrogel that fills at least a predominant part of the pore volume of the support structure.

Description

[0001] The invention relates to a reference electrode with improved compressive strength for electrochemical, preferably potentiometric, measurements in a measuring fluid and a method for producing such a reference electrode.

[0002] Potentiometric methods are used to measure the activities of ions in aqueous solutions. At low concentrations, the ion activity is approximately equal to the ion concentration. At higher concentrations, activity and concentration diverge because the ions inhibit each other. This is accounted for by introducing an activity coefficient. The relationship between ion activity and electrode potential is described by the Nernst equation. Potentiometric methods are used, for example, to measure pH or the activity of various ions. The measuring electrodes are accordingly also called ion-selective electrodes.

[0003] Potentiometric measurements use a measuring electrode and a reference electrode. The reference electrode is necessary because the potential of the measuring electrode can only be measured as a voltage relative to the reference electrode. The potential of the measuring electrode depends on the ion or substance being measured. The potential of the reference electrode, on the other hand, is independent of the composition of the measuring fluid. In single-rod measuring chains, the measuring electrode and the reference electrode are integrated into a single sensor. The voltage between the measuring and reference electrodes is measured with high resistance, without current flow, and therefore without any substance transfer at the electrodes. The circuit between the measuring and reference electrodes is closed on the fluid side by an ion-conducting junction. This junction can be a simple, small passage, such as a small hole or gap, or a porous component.Accordingly, one speaks of a hole diaphragm, gap diaphragm, or porous diaphragm. The transition, whether formed as a simple passage or by means of a porous component, can be provided in a wall delimiting the reference electrode space, for example, in a circumferential wall of a housing.

[0004] The diaphragm of an electrode performs inherently contradictory tasks. On the one hand, it must enable electrolytic contact between the measuring fluid and the reference electrolyte; on the other hand, it must also prevent the reference electrolyte and the measuring fluid from mixing too rapidly. Low-maintenance and pressure-resistant electrodes are generally filled with solidified reference electrolytes.

[0005] Chemical processes often occur under pressure and at varying temperatures. Pressure and temperature changes in the process, particularly of the measuring fluid, can cause the fluid to be forced into the reference electrode chamber. If this happens frequently, the reference electrode's capacity will eventually be exhausted because the internal pressure increases with the amount of fluid absorbed. With such an elevated internal pressure, even a relatively small temperature increase can cause the thermal expansion of the reference electrolyte to cause the electrode housing to burst, resulting in a further pressure increase that can occur. The housing is often made of glass.The introduction of measuring fluid into the reference electrolyte also causes the potential of the reference electrode to drift, because the composition of the reference electrolyte changes due to dilution with measuring fluid and / or the absorption of foreign substances from the measuring fluid. Electrodes used in hygienic applications in the pharmaceutical, biotechnology, food technology, and brewing industries must also be pressure-resistant, as these electrodes are sterilized in autoclaves or sanitized during the process.

[0006] For electrodes used in sterilization applications and for higher pressures, the reference electrode chamber can be pressurized during manufacturing. This ensures, at least initially, that only reference electrolyte escapes, but no measuring fluid enters the reference electrode chamber. However, this only applies as long as the pressure in the reference electrode chamber is higher than the pressure of the measuring fluid. With increasing usage, electrolyte is lost in such electrodes, causing the pressure in the reference electrode chamber to gradually decrease. Furthermore, pressurizing the reference electrolyte is complex and poses safety concerns, especially if the housing is a pressurized glass body. If the glass breaks, for example, due to a fall or accidental scratching, glass fragments can fly uncontrollably through the air.Methods for pressurizing the reference electrode chamber are known from EP 1 544 608 A1 and DE 37 02 501 A1.

[0007] Another solution, known from DE 102 07 624 A1, consists of using a compliant or flexible housing or a housing with at least one compliant or flexible wall section. However, the production of such electrodes is complex and involves comparatively high costs.

[0008] The invention relates to a reference electrode for electrochemical measurements in a measuring fluid, comprising a housing with a reference electrode compartment for a reference electrolyte and a housing wall delimiting the reference electrode compartment, a lead for the reference electrode potential, and a diaphragm wetted by the reference electrolyte and wettable by the measuring fluid. The housing can be, for example, a plastic housing or, in particular, a glass housing. The housing can be assembled from several separately manufactured housing components, for example, using one or more sealing elements, or it can be manufactured in one piece. Preferably, the electrode compartment contains the reference electrolyte. However, the invention also relates to a reference electrode whose electrode compartment does not yet contain the reference electrolyte, i.e., it must first be filled with the reference electrolyte.The diaphragm comprises a support structure with at least predominantly open porosity. Such a reference electrode is known from DE 103 54 100 A1.

[0009] One objective of the invention is to create a reference electrode that remains pressure-resistant even over a longer period of use and can be manufactured easily.

[0010] According to the invention, the reference electrode comprises an electrolytically conductive hydrogel that fills at least a predominant part of the pore volume of the pore network formed by the interconnected pores of the support structure. The hydrogel is a water-containing gel based on one or more hydrophilic, but water-insoluble polymers arranged as a three-dimensional network. The degree of filling of the interconnected pores is so high that, under the conditions to which the reference electrode is exposed during intended use, no measuring fluid can penetrate the electrode chamber via the diaphragm. Even at high pressure of the measuring fluid or strong pressure and / or temperature fluctuations, the hydrogel contained in the pores prevents the passage of measuring fluid into the reference electrode chamber.

[0011] Sterilizable electrodes can be exposed to temperatures of 140 °C and pressures of 6 bar during use. For general applications in water as the measuring fluid, fluid temperatures and pressures of 50 °C and 10 bar or 80 °C and 8 bar are not uncommon. Hydrogel-filled diaphragms have proven their suitability even under such conditions in testing. In each case, a single-rod measuring chain with a reference electrode according to the invention was arranged in a water-filled container, and the container was sealed airtight. The water temperature was then increased from 15 °C to 140 °C and subsequently reduced back to 15 °C. The diaphragms and measuring chains tested in this way withstood more than 50 such temperature and associated pressure cycles without any detectable impairment.

[0012] Despite its impermeability to the measuring fluid, electrolytic contact with the fluid is ensured via ion diffusion through the conductive hydrogel in the pore network of the diaphragm's support structure. By forming the diaphragm as a porous support structure whose internal pore network is coated with the hydrogel, a reference electrode with improved compressive strength can be provided simply and cost-effectively. This electrode is also insensitive to pressure and temperature fluctuations, as the hydrogel permanently prevents the measuring medium from entering the reference electrode chamber. The hydrogel provides an electrolytically conductive, yet convection-preventing seal at the junction created by the diaphragm, while the porous support structure ensures increased mechanical stability of the junction compared to a pure hydrogel plug.

[0013] The invention also relates to a diaphragm of the type described above, suitable for use as the diaphragm of the reference electrode. The diaphragm suitable for use in a reference electrode can, in particular, be a diaphragm for a housing assembled from several housing parts, wherein, in preferred uses, the diaphragm serves as a sealing element for sealing a gap between different components of the assembled housing. Alternatively, the diaphragm can also be a plug-shaped diaphragm that can be inserted into a passage through the housing wall delimiting the reference electrode space and firmly joined to the housing wall.

[0014] The invention further relates to a single-rod measuring chain with a measuring electrode and the reference electrode.

[0015] Finally, the invention also relates to a method for producing a reference electrode for electrochemical, preferably potentiometric, measurements. In this method, a liquid mixture containing one or more monomers polymerizable to form a hydrogel and water, and a housing comprising a reference electrode compartment for a reference electrolyte, a housing wall bounding the electrode compartment, and an open-pore support structure arranged in or on the housing wall with at least predominantly open porosity are provided. The liquid mixture contains precursor components of the hydrogel, thus forming a kind of precursor for the hydrogel. The mixture can be contained within the reference electrode compartment of the housing. Alternatively, the housing can be immersed in the mixture. In yet another embodiment, a portion of the liquid mixture can be contained within the reference electrode compartment, and the housing can be immersed in another portion of the liquid mixture.The mixture wets the support structure, in the first variant only on the inside of the support structure facing the electrode space, in the second variant only on the outside of the support structure facing away from the electrode space, and in the third variant both on the outside and on the inside of the porous support structure.

[0016] To fill the internal pore network of the support structure with the mixture, capillary action can be utilized and / or the pressure within the pore network of the support structure can be reduced. To reduce the pressure, the support structure, wetted on one or both sides with the mixture, can be subjected to a vacuum in a low-pressure chamber, with the vacuum being measured relative to the external environment of the chamber. A hydrogel is formed from the mixture through polymerization of one or more monomers, which fills at least a predominant portion of the pore volume of the support structure formed by the interconnected pores.

[0017] In preferred embodiments, the process conditions, in particular the temperature at which the process takes place, and / or the chemical composition of the liquid mixture, and / or the vacuum in the vacuum chamber, are selected such that the mixture, in its liquid state, penetrates the support structure at least to the extent that it fills the pore network at least predominantly, preferably completely, before the polymerization has progressed to such an extent that the flowability of the mixture is reduced to such a degree that sufficient filling of the pore network can no longer be guaranteed. Advantageously, the process conditions are selected such that the polymerization only begins when the pore network of the support structure is filled with the liquid mixture at least to such an extent that no pathway for convection of the measuring fluid from the outside to the inside of the filled support structure remains.

[0018] The mixture can contain one or more adhesion promoters, each with at least one silane and at least one C-C double bond unit in a single molecule. Advantageous adhesion promoters include vinyltrialkoxysilanes, allyltrialkoxysilanes, allyltrimethoxysilanes, and 3-(trimethoxysilyl)propyl methacrylate. The mixture can contain several different adhesion promoters, particularly of the type mentioned. The one or more adhesion promoters improve the adhesion of the hydrogel to the inner walls of the support structure and thereby anchor the hydrogel in the pore network. Improved adhesion is achieved primarily with ceramic support structures, especially those made of open-pore metal oxide ceramics, such as zirconium dioxide (ZrO2). The adhesion promoter(s) also improve adhesion within the pore network of an open-pore glass support structure.

[0019] The hydrogel is held within the network by the geometry of the pore network itself, sealing it against convective flow. The adhesion promoter, or any of several different adhesion promoters, improves the hydrogel's adhesion to the inner walls of the open-pore support structure through covalent bonding, thereby enhancing the hydrogel's sealing effect. Additionally, the hydrogel's adhesion to glass is improved, for example, when the diaphragm is positioned in or against a glass housing wall. This, too, occurs through covalent bonding.

[0020] The adhesion on the polymer side is based on the fact that the C-C double bond unit of the adhesion promoter is incorporated into the polymer network of the hydrogel via covalent and / or atomic bonding. On the silane side of the adhesion promoter, (glass-Si-O-Si adhesion promoter) bonds form with glass and / or (metal-O-Si) bonds with a metal oxide ceramic, for example, (Zr-O-Si) bonds with zirconium dioxide. Accordingly, a portion of the silicon in the adhesion promoter is present in the polymer network of the hydrogel, while another portion is chemically bonded to the material of the support structure. Yet another portion of the silicon in the adhesion promoter may be chemically bonded to glass in a glass housing wall adjacent to the diaphragm.

[0021] Advantageous features are also described in the dependent claims. The features disclosed in the dependent claims and their combinations may supplement the embodiments described above and / or the aspects described below.

[0022] The following aspects describe features of the invention. These aspects are formulated in the manner of claims and can replace them. Features disclosed in the aspects can further supplement and / or qualify the claims; that is, they can show alternatives to individual claim features and / or supplement or extend claim features. Reference numerals in parentheses refer to embodiments of the invention illustrated in the figures below. They do not limit the features described in the aspects to their literal meaning as such, but rather indicate preferred ways of implementing the respective feature. The features disclosed in the aspects can also further develop the embodiments described above and, conversely, can also be further developed by features described above.

[0023] Aspect 1: Reference electrode for electrochemical, preferably potentiometric measurements in a measuring fluid, the reference electrode comprising: (a) a housing (H) with a reference electrode compartment (H2; H2, H3) and a housing wall (2) delimiting the reference electrode compartment, (b) a reference electrolyte (12) contained in the reference electrode compartment (H2; H2, H3), (c) a derivative for the potential of the reference electrode and (d) a diaphragm (6; 7; 8, 9) with an inner surface facing the reference electrode space (H2; H2, H3) that is wettable by the reference electrolyte (12) and an outer surface that is wettable by the measuring fluid or that is facing an optional further reference electrode space (H3), (e) wherein the diaphragm (6; 7; 8, 9) comprises a support structure (6'; 7') having at least predominantly open porosity and a hydrogel that fills at least a predominant part of the pore volume of the support structure.

[0024] Aspect 2: Reference electrode according to the previous aspect, wherein the reference electrolyte (12) is a hydrogel.

[0025] Aspect 3: Reference electrode according to one of the preceding aspects, wherein the hydrogel incorporated in the support structure (6'; 7') and a hydrogel forming the reference electrolyte (12) are identical.

[0026] Aspect 4: Reference electrode according to one of the preceding aspects, where the porosity of the support structure (6'; 7') is at least 20%.

[0027] Aspect 5: Reference electrode according to the previous aspect, wherein the porosity of the support structure (6'; 7') is at least 25%.

[0028] Aspect 6: Reference electrode according to one of the preceding aspects, where the porosity of the support structure (6'; 7') is at most 60%.

[0029] Aspect 7: Reference electrode according to the previous aspect, wherein the porosity of the support structure (6'; 7') is at most 50%.

[0030] Aspect 8: Reference electrode according to one of the preceding aspects, wherein the hydrogel has a water content of at least 40% by weight.

[0031] Aspect 9: Reference electrode according to the previous aspect, wherein the hydrogel has a water content of at least 50 wt%.

[0032] Aspect 10: Reference electrode according to one of the preceding aspects, wherein the hydrogel has a water content of at most 98% by weight.

[0033] Aspect 11: Reference electrode according to the previous aspect, wherein the hydrogel has a water content of at most 95 wt%.

[0034] Aspect 12: Reference electrode according to one of the preceding aspects, wherein the hydrogel contains a polyacrylate and / or a polymethacrylate and / or a copolymer of a polyacrylate and / or a polymethacrylate.

[0035] Aspect 13: Reference electrode according to one of the preceding aspects, wherein the polymeric portion of the hydrogel consists of polyacrylate and / or polymethacrylate and / or one or more copolymers of a polyacrylate or a polymethacrylate.

[0036] Aspect 14: Reference electrode according to one of the preceding aspects, the hydrogel contains a methacrylic acid ester.

[0037] Aspect 15: Reference electrode according to one of the preceding aspects, wherein the hydrogel is formed from one or more monomers and the one or more monomers are selected from the group consisting of acrylic acid, one or more derivatives of acrylic acid, methacrylic acid and one or more derivatives of methacrylic acid.

[0038] Aspect 16: Reference electrode according to one of the two immediately preceding aspects, wherein one or more derivatives are selected from the group consisting of acrylic acid salts, acrylic acid esters, acrylic acid amides, methacrylic acid salts, methacrylic acid esters and methacrylic acid amides.

[0039] Aspect 17: Reference electrode according to one of the preceding aspects, in which the hydrogel contains one or more fillers, such as glass particles and / or silica gels, which may each have different particle sizes.

[0040] Aspect 18: Reference electrode according to one of the preceding aspects, where the hydrogel contains Si.

[0041] Aspect 19: Reference electrode according to the previous aspect, wherein Si is chemically bonded to the support structure (6'; 7') and / or to the housing wall (2) if it is made of glass and / or is present in the polymer network of the hydrogel.

[0042] Aspect 20: Reference electrode according to one of the preceding aspects, wherein the hydrogel has at least one adhesion promoter comprising chemically bonded Si for adhesion to glass or a metal of a metal oxide, such as ZrO2 in particular, and a CC double bond unit incorporated into the polymer network of the hydrogel, wherein the adhesion promoter may, for example, be formed from a vinyltrialkoxysilane and / or allyltrialkoxysilane and / or allyltrimethoxysilane and / or 3-(trimethoxysilyl)propyl methacrylate.

[0043] Aspect 21: Reference electrode according to one of the preceding aspects, in which the hydrogel contains one or more humectants, for example glycerin and / or ethylene glycol and / or polyethylene glycols, which may have different chain lengths.

[0044] Aspect 22: Reference electrode according to one of the preceding aspects, in which the hydrogel contains one or more equitransferent salts, for example potassium chloride and / or lithium chloride.

[0045] Aspect 23: Reference electrode according to one of the preceding aspects, wherein the support structure (6'; 18') is an open-porous ceramic structure, in particular an open-porous metal oxide ceramic structure, such as a zirconium dioxide structure, or an open-porous PTFE structure or an open-porous glass structure.

[0046] Aspect 24: Reference electrode according to one of the preceding aspects, wherein the diaphragm (6; 7, 17; 18) is plug- or ring-shaped.

[0047] Aspect 25: Reference electrode according to one of the preceding aspects, wherein the diaphragm (6; 7; 8, 9) is located in the housing wall (2) or is a sealing element closing the reference electrode space (H2; H3) at one end end to prevent fluid exchange.

[0048] Aspect 26: Reference electrode according to one of the preceding aspects, wherein the reference electrode and a measuring electrode for an electrochemical, preferably potentiometric, measurement are connected to form a measuring chain.

[0049] Aspect 27: Reference electrode according to one of the preceding aspects, wherein the reference electrode and a measuring electrode are jointly designed as a single-rod measuring chain for an electrochemical, preferably potentiometric, measurement.

[0050] Aspect 28: Diaphragm for a reference electrode for an electrochemical measurement, comprising the diaphragm (6; 7; 8, 9): (a) a support structure (6'; 7') with at least predominantly open porosity (H) (b) and a hydrogel that fills at least a predominant part of the pore volume of the support structure (6'; 7').

[0051] Aspect 29: Diaphragm according to the preceding aspect, characterized by its use as the diaphragm (6; 7; 8, 9) of the reference electrode according to one of the preceding aspects.

[0052] Aspect 30: Diaphragm according to one of the two immediately preceding aspects, wherein the diaphragm has one or more features that are described in one of the preceding aspects for the diaphragm as such.

[0053] Aspect 31: Method for producing a reference electrode for electrochemical measurements, wherein (a) a liquid mixture (12') containing one or more monomers polymerizable to form a hydrogel and water, and (b) a housing (H) of the reference electrode comprising a reference electrode space (H2) for a reference electrolyte, a housing wall (2) delimiting the reference electrode space (H2) and a porous support structure (6'; 7') arranged in or on the housing wall (2) with an internal pore network of at least predominantly open porosity, shall be provided, (c) such that the mixture (12') is contained in the reference electrode space (H2) and / or the housing (H) is immersed in the mixture (12') and the mixture (12') wets the support structure (6'; 7'), (d) the mixture (12') penetrates the pore network of the support structure (6'; 7') by capillary action and / or the pressure in the pore network of the support structure (6'; 7') is reduced and the mixture (12') penetrates the pore network of the support structure (6'; 7') due to the reduced pressure in the pore network. (e) and from the mixture (12') a hydrogel is formed by polymerization of one or more monomers, which fills at least a predominant part of the pore volume of the support structure (6'; 7').

[0054] Aspect 32: Method according to the preceding aspect, wherein the pressure acting on the mixture (12') outside the support structure (6'; 7') is reduced to reduce the pressure prevailing in the pore network of the support structure (6'; 7').

[0055] Aspect 33: Method according to one of the preceding aspects, wherein the pressure acting on the mixture (12') outside the support structure (6'; 7') is reduced and then increased again to draw the mixture (12') into the pore network of the support structure (6'; 7').

[0056] Aspect 34: Method according to one of the preceding aspects, wherein the liquid mixture (12') is introduced into the reference electrode space (H2) so that the mixture (12') wets the support structure (6'; 7') in the reference electrode space (H2), and the reference electrode space (H2) which is at least partially filled with the liquid mixture (12') and / or a vacuum chamber (25) surrounding the housing (H) and the support structure (6'; 7') is / are subjected to vacuum and the mixture (12') is drawn into the pore network of the support structure (6'; 7') by the vacuum generated in the pore network.

[0057] Aspect 35: Method according to one of the preceding aspects, wherein the housing (H) with the support structure (6'; 7') is immersed in the mixture (6'; 7') located in a vacuum chamber (25), such that the mixture (6'; 7') wets the support structure (6'; 7') in the vacuum chamber (25), and the vacuum chamber (25) and / or the reference electrode space (H2) is / are subjected to vacuum and the mixture (7') is drawn into the pore network of the support structure (6'; 7') by the vacuum generated in the pore network.

[0058] Aspect 36: Method according to one of the two immediately preceding aspects, wherein the pressure in the reference electrode chamber (H2) and / or in the vacuum chamber (25) is increased again, so that an overpressure is created in the reference electrode chamber (H2) and / or in the vacuum chamber (25) compared to the pressure prevailing in the pore network of the support structure (6'; 7') and the mixture is drawn into the pore network of the support structure (6'; 7').

[0059] Aspect 37: Method according to one of the preceding aspects, wherein the support structure (6'; 7') is sealed externally, on an outer side facing away from the reference electrode space (H2), with a seal (28; 29), for example with a shrink tube shrunk onto the housing wall (2) covering the support structure (6'; 7'), and the reference electrode space (H2) is subjected to negative pressure with the support structure (6'; 7') sealed externally.

[0060] Aspect 38: Process according to any of the preceding aspects, wherein the mixture (12') contains one or more monomers from the group consisting of acrylic acid, one or more derivatives of acrylic acid, methacrylic acid and one or more derivatives of methacrylic acid.

[0061] Aspect 39: Method according to the preceding aspect, wherein one or more derivatives are selected from the group consisting of acrylic acid salts, acrylic acid esters, acrylic acid amides, methacrylic acid salts, methacrylic acid esters and methacrylic acid amides.

[0062] Aspect 40: Method according to one of the two immediately preceding aspects, wherein one or more organic, preferably hydrophilic, substituents are attached to the one or more monomers.

[0063] Aspect 41: Process according to any of the preceding aspects, wherein the mixture (12') contains one or more difunctional or multifunctional crosslinking agents, preferably one or more di(meth)acrylates, e.g. ethylene glycol diacrylate and / or ethylene glycol dimethacrylate, and / or one or more bisacrylamides (e.g. n,N'-methylenebisacrylamide).

[0064] Aspect 42: Method according to any of the preceding aspects wherein the mixture (12') contains one or more fillers, such as glass particles and / or silica gels, which may each have different particle sizes.

[0065] Aspect 43: Method according to any of the preceding aspects, wherein the mixture (12') contains one or more adhesion promoters, each having at least one silane and CC double bond unit in one molecule, for example vinyltrialkoxysilanes and / or allyltrialkoxysilanes and / or allyltrimethoxysilanes and / or 3-(trimethoxysilyl)propyl methacrylate.

[0066] Aspect 44: Method according to any of the preceding aspects wherein the mixture (12') contains one or more humectants, for example glycerin and / or ethylene glycol and / or polyethylene glycols, which may have different chain lengths.

[0067] Aspect 45: Process according to any of the preceding aspects wherein the mixture (12') contains one or more equitransferent salts, for example potassium chloride and / or lithium chloride.

[0068] Aspect 46: Process according to any of the preceding aspects wherein the mixture (12') contains one or more monomers, water and one or more water-soluble radical initiators, for example potassium peroxodisulfate and / or azoisobutyronitrile (AIBN), for radical polymerization of the one or more monomers.

[0069] Aspect 47: Method according to one of the preceding aspects, wherein a first solution containing water and one or more monomers and a second solution containing water and one or more water-soluble radical initiators, for example potassium peroxodisulfate and / or azoisobutyronitrile (AIBN), are mixed to form the mixture (7') and the mixture (12') is filled in liquid form into the reference electrode space (H2) and / or the housing (H) is immersed in the liquid mixture (12') so that the liquid mixture (12') wets the support structure (6'; 7') inside and / or outside.

[0070] Aspect 48: Method according to one of the two immediately preceding aspects, wherein the concentration of one or more radical formers in the liquid mixture (12') is selected such that the mixture (12') remains fluid within a temperature range of 20 °C to 30 °C for a period of 30 ± 20 minutes and / or is no longer fluid after a period of 30 ± 20 minutes.

[0071] Aspect 49: Method according to any of the preceding aspects wherein the mixture (12') is left to cure at room temperature or is cured at a temperature of at least 40°C or at least 50°C.

[0072] Exemplary embodiments of the invention are explained below with reference to the figures. Features that become apparent in the exemplary embodiments, individually and in every combination of features, advantageously further define the subject matter of the claims and aspects and the embodiments explained above. The figures show: Fig. 1 a single-rod measuring chain with a reference electrode of a first embodiment, Fig. 2 a first configuration for generating a hydrogel in a porous support structure of the reference electrode of the first embodiment, Fig. 3 a second configuration for generating a hydrogel in the porous support structure of the reference electrode of the first embodiment, Fig. 4 a third configuration for generating a hydrogel in the porous support structure of the reference electrode of the first embodiment, Fig. 5 a fourth configuration for generating a hydrogel in a porous support structure of a reference electrode of a second embodiment, Fig. 6 a fifth configuration for generating a hydrogel in the porous support structure of the reference electrode of the second embodiment, Fig. 7 a single-rod measuring chain with a reference electrode of a third embodiment. Fig. 8 the function of an adhesion promoter in relation to a metal oxide ceramic support structure, and Fig. 9 the function of the adhesion promoter in relation to glass.

[0073] Fig. Figure 1 shows a longitudinal section of a single-rod measuring chain with a measuring electrode and a reference electrode of a first embodiment. The single-rod measuring chain comprises a housing H with a tubular inner housing wall 1 and a tubular outer housing wall 2. The housing H is designed as a double-walled housing tube, for example, as a double-walled glass tube. The inner housing wall 1 surrounds an inner or first cavity H1 extending longitudinally along the housing H. The outer housing wall 2 surrounds the inner housing wall 1 at a radial distance, so that an outer or second cavity H2 is formed between the housing walls 1 and 2. The second cavity H2 is an annular space extending completely around the longitudinal axis of the housing H. While a completely circumferential annular space is preferred, the second cavity H2 can also extend only over a portion of the circumference of the housing H.

[0074] A measuring membrane 3 is arranged at an axial front end of the housing H, closing the first cavity H1 at the front end. The measuring membrane 3 can be, in particular, a glass membrane, but also, in principle, a plastic membrane. It is bonded to the open front end of the housing tube, preferably by means of a fusion bond. The measuring membrane 3 is ion-selective, for example, pH-sensitive. An inner buffer 4 is contained in the first cavity H1, which contacts the measuring membrane 3 at its rear. The potential of the measuring membrane 3 is derived via a lead 5 extending through the inner cavity H1 and projecting into the inner buffer 4 into a terminal head 15 and can be transmitted via this to measuring equipment, for example, a display device and / or evaluation and / or monitoring device.The connection head 15 serves for the electrical connection and also for mounting the measuring chain at the measuring point and can, for this purpose, be provided with an external thread, as in the exemplary embodiment. The measuring diaphragm 3, inner buffer 4 and inner lead 5 form the measuring electrode of the measuring chain.

[0075] To form the reference electrode, an electrode cartridge 10 is arranged in the second cavity H2 and immersed in a reference electrolyte 12 contained in cavity H2. The reference electrolyte 12 can, in particular, be a potassium chloride solution. The reference electrolyte 12 is provided with a salt reservoir 13. The potential of the reference electrode is derived via a lead connected to the electrode cartridge 10 (not visible in the figure) to the terminal head 15 and can be transmitted via this to the measuring equipment.

[0076] In the front axial section of the housing H, near the measuring membrane 3, an electrolytically conductive diaphragm 6 is arranged as an ion junction. This diaphragm connects the electrode compartment of the reference electrode, filled with the reference electrolyte 12, to the measuring fluid near the measuring membrane 3. The diaphragm 6 comprises an open-porous support structure, i.e., a support structure permeated by a network of interconnected internal pores. This inherently hydrophobic pore network is filled with an electrolytically conductive hydrogel. The hydrogel permeates the pore network and electrolytically connects the measuring environment of the measuring chain, i.e., the measuring fluid, to the reference electrode compartment. Dissolved ions can diffuse almost freely through the pore network in the hydrogel, while the hydrogel prevents convection of the measuring fluid.

[0077] The reference electrolyte 12 is also a hydrogel. The hydrogel that fills the pores of the pore network of the support structure can differ from the hydrogel that forms the reference electrolyte 12 or, as in the exemplary embodiments, be the same hydrogel.

[0078] The hydrogel should have a water content of at least 40% by weight. It is advantageous if the hydrogel consists of at least 50% by weight of water. Conversely, it is advantageous if the water content of the hydrogel is at most 98% by weight, or even better, at most 95% by weight.

[0079] The hydrogel may, in particular, contain a polyacrylate and / or a polymethacrylate and / or a copolymer of a polyacrylate or a polymethacrylate. Advantageously, the polymeric portion of the hydrogel consists of polyacrylate and / or polymethacrylate and / or one or more copolymers of a polyacrylate or a polymethacrylate. Preferably, the hydrogel contains a methacrylic acid ester.

[0080] The hydrogel may contain fillers such as glass particles and / or silica gels.

[0081] To stabilize the water content of the hydrogel, the hydrogel may contain humectants such as glycerin and / or ethylene glycol and / or polyethylene glycols.

[0082] The hydrogel advantageously contains an equitransferent, preferably chloride-containing salt, for example potassium chloride and / or lithium chloride. This is automatically the case if the same hydrogel also forms the reference electrolyte 12.

[0083] In advantageous embodiments, the porosity of the support structure is at least 20%, with a porosity of 25% or more being preferred. On the other hand, the porosity of the support structure should not exceed 60%. It is advantageous if the porosity is 50% or less.

[0084] Regarding the material of the support structure, an open-pore ceramic structure, such as a zirconia structure, or alternatively an open-pore PTFE structure, or as yet another alternative, an open-pore glass structure, are good choices. For the first embodiment, a ceramic structure is selected.

[0085] The Fig. 2, Fig. 3 and Fig. Figure 4 shows setups for filling the porous support structure. The support structure is labelled 6' in each case, indicating that it is not yet the finished diaphragm 6, but rather a support structure 6' as such, still free of hydrogel. In the process variants to which the Fig. 2, Fig. 3 and Fig. Figure 4 shows a setup; the housing H, including the measuring membrane 3, is provided. The inner buffer 4 can be already filled as indicated or filled later, after the hydrogel has been produced.

[0086] A liquid mixture 12' is poured through the still open rear end of the housing H into the second cavity H2, which will later serve as the reference electrode chamber, so that the support structure 6' is completely wetted with the liquid mixture 12' on its inner surface facing cavity H2. The mixture 12' contains the precursor components of the hydrogel. A polymerization reaction is initiated by combining one or more monomer components and one or more hardener components, resulting in the hydrogel. The components, monomer(s) and hardener, are mixtures of various substances that positively influence the properties of the hydrogel. The ingredients include one or more monomers, one or more crosslinkers, one or more humectants, one or more adhesion promoters, water, and potassium chloride or another equitransferent salt.The hardener component can be, for example, azoisobutyronitrile (AIBN) or, preferably, potassium peroxodisulfate. In advantageous embodiments, the composition of mixture 12' is selected such that polymerization begins in the temperature range between 15 °C and 35 °C and the hydrogel is no longer flowable at temperatures outside this range within a period of at most 10 hours, preferably at most 1 hour.

[0087] The polymerization is a radical polymerization. To initiate the polymerization reaction, a first solution and a second solution are mixed together. The first solution contains the one or more monomers to be crosslinked, but no hardener. The first solution can contain all components except the one or more hardener components. In addition to the one or more monomers, it can contain, for example, one or more adhesion promoters and / or one or more humectants and / or one or more filler particles. The second solution contains the one or more hardener components, whereby, as already mentioned, potassium peroxodisulfate can be used as a hardener component. The mixture 12' can also be formed from more than two solutions.The solution(s) containing the one or more hardener components may also contain other components, such as one or more humectants and / or one or more adhesion promoters, but not the monomer(s) to be crosslinked. The two or more solutions are advantageously mixed together immediately before filling the cavity H2, thereby initiating the polymerization.

[0088] The hardener concentration, for example, potassium peroxodisulfate concentration, is chosen such that the filling of the cavity H2 and the porous support structure 6' occurs in the liquid state of the mixture 12'. Approximately 30 minutes after adding the solution containing the hardener, polymerization has progressed to the point where the gel is no longer flowable. The curing time can be adjusted by changing the hardener concentration. Curing can be carried out, for example, overnight at room temperature or, to shorten the curing time, at an elevated temperature of 40°C or higher, for example, at 50°C or higher.

[0089] Fig. Figure 2 shows the setup for carrying out a first process variant, in which the porous support structure 6' is filled with the liquid mixture 12' by capillary action. Advantageously, the capillary action is sufficient to fill the pore network of the support structure 6' within a period of at most 1 hour to such an extent that the desired effect, the prevention of convection and the provision of a diffusion path for ions through the pore network of the support structure 6', is achieved.

[0090] In Fig. Figure 3 shows a setup for carrying out a second process variant in which the gas pressure prevailing in the pore network of the support structure 6' is reduced for filling with the liquid mixture 12'. The filling of the pore network of the support structure 6' takes place in a vacuum chamber 25. A vessel 26 filled with the liquid mixture 12' is arranged in the vacuum chamber 25. The housing H, with the cavity H2 filled with the liquid mixture 12' as in the first process variant, is immersed in the vessel 26 so that the support structure 6' is completely wetted on its outer surface facing away from the cavity H2 with the mixture 12' in the vessel 26.

[0091] In the submerged state, the pressure in the vacuum chamber 25 is reduced, allowing the gas, typically air, present in the pore network of the support structure 6' to escape. The cavity H2 is open at its rear axial end, allowing the gas to escape inwards into the cavity H2 and outwards. After this evacuation of the support structure 6', the pressure in the vacuum chamber 25 is increased again. This pressure increase can be achieved by simply venting the vacuum chamber 25, i.e., the vacuum chamber 25 can simply be vented to the pressure of the external environment. The pressure increase draws the liquid mixture 12' from the cavity H2 (from the inside) and from the vessel 26 (from the outside) into the pore network of the support structure 6'.

[0092] Fig. Figure 4 shows a setup for carrying out a third process variant, in which the support structure 6' is filled with liquid mixture 12' from the inside, from the cavity H2, as in the first process variant. However, the filling takes place as in the second process variant by reducing the pressure in the pore network of the support structure 6' and subsequently increasing the pressure, whereby the mixture 12' is drawn into the pore network of the support structure 6'.

[0093] To prevent the still liquid mixture 12' from not only penetrating the support structure 6' but also from leaking out at its outer surface, the outer housing wall 2 is sealed at its outer circumference in the region of the support structure 6' by means of a removable seal 28. The seal 28 surrounds the outer housing wall 2 in the axial section where the support structure 6' is inserted. The seal 28 can, in particular, be a heat-shrink tube that is shrunk onto the outer housing wall 2. The vessel 26 serves as a collection vessel for any mixture 12' that may leak out despite the seal 28.

[0094] In Fig. Figure 5 shows a setup for filling an open-porous support structure 7' to produce a reference electrode of a second embodiment. The setup corresponds to that of the Fig. 3. As in the first embodiment, the reference electrode is an integrated component of a single-rod measuring chain. However, unlike the first embodiment, the housing H is assembled from several housing parts. It comprises a tubular inner housing wall 1, which surrounds an inner cavity H1, and an outer housing wall 2 surrounding the inner housing wall 1, which in turn surrounds an outer cavity H2, which is bounded internally by the inner housing wall 1. The annular cavity H2 is closed at its front end, near the measuring diaphragm 3, by the support structure 7'.

[0095] The support structure 7' surrounds the inner housing wall 1 and is enclosed in a rear axial section by the outer housing wall 2. To seal the cavity H2, the support structure 7' tightly encloses the inner housing wall 1 and is itself tightly enclosed by the outer housing wall 2 at its outer circumference. In the contact area between the support structure 7' and the housing wall 2, an elastic sealing element 17, in this embodiment a sealing ring, is arranged to improve the seal. The outer housing wall 2 is positively engaged with the support structure 7' in a manner that ensures the cohesion of the housing wall 2, the support structure 7' (or the diaphragm to be produced from it), and the housing wall 1, including the materially bonded measuring diaphragm 3. The support structure 7' can, in particular, be a PTFE structure.

[0096] The pore network of the support structure 7' is determined according to the Fig. 3 explained second process variant simultaneously filled from the inside and outside with the liquid mixture 12' of the precursor components.

[0097] Fig. Figure 6 shows a setup for filling the support structure 7' of the reference electrode of the second embodiment according to the Fig. 4 explained the third method variant. Accordingly, filling takes place only via the inside of the support structure 7'. Reference numeral 29 denotes a removable seal, for example a shrink sleeve, which seals the free outside of the support structure 7' to prevent the mixture 12' that has penetrated the support structure 7' from escaping into the vacuum chamber 25 on the outside. As a precaution, the housing H, filled with the mixture 12' in the cavity H2, is arranged in a vessel 26, which, however, in the third method variant serves only as a collection vessel for any mixture 12' that may escape.

[0098] Apart from the differences explained above, the reference electrode of the second embodiment and the single-rod measuring chain formed with it correspond to the reference electrode and the single-rod measuring chain of the first embodiment.

[0099] Fig. Figure 7 shows a longitudinal section of a single-rod measuring chain with a measuring electrode and a reference electrode of a third embodiment. In contrast to the first embodiment, no electrolytically conductive diaphragm or other type of electrolytically conductive transition is arranged in the front axial section of the housing H near the measuring membrane 3 to connect the electrode space of the reference electrode near the measuring membrane 3 to the measuring fluid. However, such a diaphragm 8 is provided axially distant from the measuring membrane 3 in a rear axial section of the housing H.Apart from the arrangement of the diaphragm 8 in the rear axial section of the housing H, the housing H, the measuring electrode, and also the reference electrode, insofar as they are the components of the reference electrode housed in the housing H, correspond at least substantially to the first embodiment, so that the reference numerals of the first embodiment are used for these functionally and structurally identical components, and reference is made to the description of the first embodiment. The diaphragm 8 also corresponds as such to the diaphragm 6 of the first embodiment.

[0100] In the third embodiment, a casing structure 20 surrounds the housing H, including the measuring diaphragm 3. The casing structure 20 serves a protective function for the housing H and, in particular, the measuring diaphragm 3, as a front axial section 21 of the casing structure 20 surrounds the measuring diaphragm 3. The casing structure 20 may have open circumferential sections in the axial overlap area with the measuring diaphragm 3. A rear axial section 22 accommodates a connection head 15 for electrical connection and, optionally, for mounting the entire assembly. A special feature is that the casing structure 20 is also an integrated component of the single-rod measuring chain, i.e., the measuring arrangement. Specifically, the casing structure 20 extends the reference electrode space.

[0101] The casing structure 20 surrounds the housing H along the length of the outer housing wall 2. A third cavity H3 is formed radially between the outer housing wall 2 and the casing structure 20. The cavity H3 extends completely around the outer housing wall 2 along its entire length. The cavity H3 is sealed at its axial front end near the measuring diaphragm 3 by a gasket 9. A gasket 23 seals the cavity H3 at its axial rear end furthest from the measuring diaphragm 3, providing a liquid-tight and electrolytically tight seal.

[0102] The seal 23 can, in particular, be a sealing ring, for example, a silicone sealing ring. The front seal, on the other hand, is implemented as an electrolytically conductive outer diaphragm 9. The diaphragm 9 forms the interface to the measuring fluid. While in one modification the outer diaphragm 9 can only form a partial area of ​​the front seal, the implementation of the diaphragm 9 as a circumferential seal made of a uniformly open-porous support structure is preferred. The front seal can, in particular, be formed by means of an open-porous PTFE sealing ring. The front seal at 9 and the rear seal 23 each seal the cavity H3 by direct sealing contact with the housing H on the one hand and the hollow structure 20 on the other. The seal 23 is held in position by means of a pressure washer 24.

[0103] Cavity H3 also contains the reference electrolyte 12. The reference electrode chamber thus comprises an inner electrolyte chamber formed by the second cavity H2 and an outer electrolyte chamber formed by the third cavity H3. Diaphragm 8 accordingly forms an inner ion interface.

[0104] By using a reference electrode cartridge 10, as in the other embodiments, the presence of the metal halide can be limited to the interior of the cartridge. The reference electrolyte 12 remains at least substantially free of the metal halide of the reference electrode.

[0105] In the third embodiment, both diaphragms 8 and 9 are formed according to the invention and therefore each comprise an open-porous support structure whose pore network is loaded with an electrolytically conductive hydrogel. If, preferably, the reference electrolyte 12 is also a hydrogel, the same hydrogel can form both the reference electrolyte 12 and the hydrogel located in the pore network of the respective support structure. The support structure of the inner diaphragm 8 is advantageously an open-porous ceramic or glass structure. The support structure of the outer diaphragm 9 is preferably an open-porous PTFE structure.

[0106] In some modifications, only one of the two diaphragms 8 and 9 is formed according to the invention, while the other can be formed in a conventional manner. For example, the outer diaphragm 9 can be formed as an open-porous support structure with embedded hydrogel, as described above, and the inner diaphragm can be formed as a through-hole, for example as a hole diaphragm or gap diaphragm, or as a conventional porous ceramic diaphragm. In other modifications, the inner diaphragm 8 is formed according to the invention, while the outer diaphragm 9 is a porous sealing element not filled with hydrogel, for example a porous PTFE sealing element.

[0107] In the exemplary embodiments, the diaphragms are loaded with the hydrogel while the respective support structure is joined by material bonding or form-fit and / or friction bonding. In variations, the respective support structure can also be loaded with the hydrogel independently, separate from the housing H of the reference electrode or single-rod measuring chain, and then joined to the housing or housing parts.

[0108] Fig. Figure 8 illustrates the function of an adhesion promoter A for improving the adhesion of the hydrogel to the inner surfaces in the pore network of a support structure 6' made of a metal oxide ceramic. In the exemplary embodiment, the metal oxide ceramic is zirconium dioxide and the adhesion promoter A is (trimethoxysilyl)propyl methacrylate. On the inner surfaces of the pore network, the adhesion promoter A binds to the zirconium hydroxide groups B of the support structure 6' in a condensation reaction K with the elimination of methanol, CH3-OH. During the radical polymerization of the C-C double bonds in the mixture 12', the adhesion promoter A is, on the other hand, polymerized into the polymer chains of acrylates or methacrylates.

[0109] Fig.Figure 9 illustrates the function of the same adhesion promoter A in improving adhesion to a glass surface, for which, as an example, the housing wall 2 is assumed to be a glass wall. On this glass wall 2, the adhesion promoter A binds to the silanol groups D of the glass surface in a condensation reaction K, releasing methanol, CH3-OH. As already mentioned, the C-C double bonds of the adhesion promoter A are polymerized into the polymer chains of acrylates or methacrylates during radical polymerization. Reference symbol: 1 inner housing wall 2 outer casing wall 3 measuring membrane 4 inner buffers 5th derivative 6 Diaphragm 7 Diaphragm 8 Diaphragm 9 Diaphragm 10 reference electrode cartridges 11 Derivative 12 Reference electrolyte 13 Salt supply 14 - 15 Connection head 16 - 17 Sealing element 18 Sealing, heat shrink tubing 19 Sealing, heat shrink tubing 20 Envelope structure 21 front axial section 22 rear axial section 23 Seal 24 pressure plate 25 Low-pressure chamber 26 Vessels H Housing H1 first cavity H2 second cavity H3 third cavity

Claims

[1] Reference electrode for electrochemical, preferably potentiometric measurements in a measuring fluid, the reference electrode comprising: (a) a housing (H) with a reference electrode compartment (H2; H2, H3) and a housing wall (2) delimiting the reference electrode compartment, (b) a reference electrolyte (12) contained in the reference electrode compartment (H2; H2, H3), (c) a derivative for the potential of the reference electrode and (d) a diaphragm (6; 7; 8, 9) with an inner surface facing the reference electrode space (H2; H2, H3) that is wettable by the reference electrolyte (12) and an outer surface that is wettable by the measuring fluid or that is facing an optional further reference electrode space (H3), (e) wherein the diaphragm (6; 7; 8, 9) comprises a support structure (6'; 7') having at least predominantly open porosity and a hydrogel that fills at least a predominant part of the pore volume of the support structure. [2] Reference electrode according to the preceding claim, wherein the reference electrolyte (12) is a hydrogel. [3] Reference electrode according to any of the preceding claims, wherein the porosity of the support structure (6'; 7') is at least 20% and / or at most 60%. [4] Reference electrode according to any of the preceding claims, wherein the hydrogel has a water content of at least 40 wt% and / or at most 98 wt%. [5] Reference electrode according to any of the preceding claims, wherein the hydrogel contains a polyacrylate and / or a polymethacrylate and / or a copolymer of a polyacrylate or a polymethacrylate. [6] Reference electrode according to any of the preceding claims, wherein the hydrogel contains one or more fillers, such as glass particles and / or silica gels, which may each have different particle sizes. [7] Reference electrode according to any of the preceding claims, wherein the hydrogel contains Si. [8] Reference electrode according to any of the preceding claims, wherein the hydrogel contains one or more humectants, for example glycerin and / or ethylene glycol and / or polyethylene glycols, which may have different chain lengths. [9] Reference electrode according to any of the preceding claims, wherein the hydrogel contains one or more equitransferent salts, for example potassium chloride and / or lithium chloride. [10] Reference electrode according to one of the preceding claims, wherein the diaphragm (6; 7; 8, 9) is located in the housing wall (2) or is a sealing element closing the reference electrode space (H2; H3) at an end face to prevent fluid exchange. [11] Reference electrode according to one of the preceding claims, wherein the reference electrode and a measuring electrode for an electrochemical, preferably potentiometric, measurement are connected to form a measuring chain. [12] Diaphragm for a reference electrode for an electrochemical measurement, comprising the diaphragm (6; 7; 8, 9): (a) a support structure (6'; 7') with at least predominantly open porosity and (b) a hydrogel that fills at least a predominant part of the pore volume of the support structure (6'; 7'). [13] Method for producing a reference electrode for electrochemical measurements, wherein (a) a liquid mixture (12') containing one or more monomers polymerizable to form a hydrogel and water, and (b) a housing (H) of the reference electrode comprising a reference electrode space (H2) for a reference electrolyte, a housing wall (2) delimiting the reference electrode space (H2) and a porous support structure (6'; 7') arranged in or on the housing wall (2) with an internal pore network of at least predominantly open porosity, shall be provided, (c) such that the mixture (12') is contained in the reference electrode space (H2) and / or the housing (H) is immersed in the mixture (12') and the mixture (12') wets the support structure (6'; 7'), (d) the mixture (12') penetrates the pore network of the support structure (6'; 7') by capillary action and / or the pressure in the pore network of the support structure (6'; 7') is reduced and the mixture (12') penetrates the pore network of the support structure (6'; 7') due to the reduced pressure in the pore network. (e) and from the mixture (12') a hydrogel is formed by polymerization of one or more monomers, which fills at least a predominant part of the pore volume of the support structure (6'; 7'). [14] Method according to the preceding claim, wherein the pressure acting on the mixture (12') outside the support structure (6'; 7') is reduced and then increased again to draw the mixture (12') into the pore network of the support structure (6'; 7'). [15] Method according to any of the preceding claims, wherein the mixture (12') contains one or more monomers from the group consisting of acrylic acid, one or more derivatives of acrylic acid, methacrylic acid and one or more derivatives of methacrylic acid. [16] Method according to any of the preceding claims, wherein the mixture (12') contains one or more difunctional or multifunctional crosslinking agents, preferably one or more di(meth)acrylates, e.g. ethylene glycol diacrylate and / or ethylene glycol dimethacrylate, and / or one or more bisacrylamides (e.g. n,N'-methylenebisacrylamide). [17] Method according to any of the preceding claims, wherein the mixture (12') contains one or more adhesion promoters, each comprising at least one silane and CC double bond unit in a molecule, for example vinyltrialkoxysilanes and / or allyltrialkoxysilanes and / or allyltrimethoxysilanes and / or 3-(trimethoxysilyl)propyl methacrylate. [18] A method according to any of the preceding claims, wherein the mixture (12') contains one or more monomers, water and one or more water-soluble radical initiators, for example potassium peroxodisulfate and / or azoisobutyronitrile (AIBN), for radical polymerization of the one or more monomers. [19] Method according to the preceding claim, wherein the concentration of one or more radical formers in the liquid mixture (12') is selected such that the mixture (12') remains fluid within a temperature range of 20 °C to 30 °C for a period of 30 ± 20 minutes and / or is no longer fluid after a period of 30 ± 20 minutes.

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