POLAROGRAPHIC MEASURING ELECTRODE DEVICE

Abstract: An electrochemical measuring electrode device (10) for polarographically measuring the partial pressure of oxygen in an electrolytic medium comprises a cathode (14) which is capable of reducing oxygen and defines an exposed oxygen-reducing cathode surface (20), an anode (18), which defines an exposed anode surface which is arranged rela­ tive to the exposed oxygen-reducing cathode surface so as to communicate electrolytically therewith through the elec­ trolytic medium, and a membrane (24), which covers the exposed oxygen-reducing cathode surface and further defi­ nes an electrolyte chamber (22) in which the electrolytic medium is confined. In order to effectively promote the decomposition H₂O₂ generated in the electrolytic medium as an oxygen reduction intermediate and thereby reduce the response time of the electrode, a stable, non-biological catalytic means is provided catalytically communicating with the H₂O₂. The stable, non-biological catalytic means is preferably constituted by platinum black particles which may be received by the membrane at a central region (42) thereof or alternatively be received by a support structure which may further constitute a covering of at least part of the membrane (24). The platinum black particles may be pressed into a foil material constituting a base material of the membrane or alternatively be applied in a suspension to the membrane whereupon the suspension is solidifyed.

Description: The present invention relates to an electrochemical measuring electrode device for polarographically measuring the partial pressure of oxygen. It is well-known to measure the partial pressure of oxygen by means of a polarographic measuring electrode device. A particular relevant type of polarographic electrode device is the electrode device of the CLARK type. The expression "the electrochemical measuring electrode device of the CLARK type" covers a well-known concept within the measuring electrode technology. This electrode device was first described by Clark at an annual meeting held by the American Society for Artificial Internal Organs in 1956, and it is distinguishable from the electrochemical measuring electrode devices for polarographically measuring the partial pressure of oxygen known up till then in that the electrode device of the CLARK type comprises an oxygen-permeable membrane separating an anode and a cathode of the electrode device from the measuring medium, and in that an electrolyte solution is arranged in an electrolyte chamber defined by the membrane and communicates electrolytically and chemically with the anode and the cathode. In accordance with the principles of the electrode device of the CLARK type, the partial pressure of oxygen of the electrolyte solution constitutes a measure of the partial pressure of oxygen of the medium as oxygen diffuses from the medium into the electrolyte solution. Provided that the supply of oxygen from the medium is sufficiently high and further provided that the rate of diffusion of oxygen through the membrane is sufficiently high, the partial pressure of oxygen of the electrolyte solution is a true measure of the partial pressure of oxygen of the medium. Normally, the cathode is made of a metal which is capable of catalysing cathodic reduction of oxygen, and the anode is made of an oxidizable metal. Suitable cathode metals are noble metals such as platinum and gold and a suitable anode metal is silver. Conventionally, the electrolyte solution is a pH-stabilized chloride ion-containing aqueous solution, but the solvent may also be an organic solvent such as glycol. The membrane is of a foil-shaped plastic material with appropriate oxygen diffusion properties and is normally made of e.g. polyethylene, polypropylene, polytetrafluoroethylene or suitable combinations thereof. Between the cathode and the anode, the potential difference of approximately -0.6V is maintained in order to permit the reduction of oxygen at the cathode. As a result of the reduction of oxygen at the cathode, a current is generated, and the current which is called the electrode response consequently constitutes a measure of the concentration or partial pressure of oxygen of the electrolyte solution and in the electrode device of the CLARK type further a measure of the partial pressure of oxygen of the medium separated from the electrolyte solution by the membrane. As the direct contact between the oxygen-reducing cathode surface and the sample is avoided due to the membrane, the oxygen electrode device of the CLARK type is especially applicable for measuring the partial pressure of oxygen of physiological media or liquids. Thus errors of measurement caused by the presence of other substances such as proteins, which otherwise might give rise to the so-called "poisoning" of the cathode, are avoided. However, when using an electrode device of the CLARK type, other measuring-technical problems occur. After using the electrode for some time, the electrode response, when shifting between samples with different oxygen levels, will become slow (the time constant of the electrode device becomes high). As a result of the slow response, the electrode device will exhibit hysteresis at some applications such as in automatic oxygen analysers in which the electrode response after a relatively short time or the order of 1-2 minutes is used for calculating the oxygen level of an unknown sample. Due to the hysteresis, the measuring result of a given sample will depend on the oxygen content of the previous samples. The slow response is therefore particularly disadvantageous when the electrode device is applied in automatic analysing equipment in which a very high analysing frequency or rate is aimed at. In a notice in Proceedings of the Physiological Society, December 1969, 29P-30P, a polarographic oxygen electrode device of the CLARK type is described in which these effects are explained by H₂O₂ accumulation in the electrolyte solution. To eliminate the effects it is suggested to dissolve the H₂O₂-decomposing enzyme catalase in the electrolyte solution. The accumulation of H₂O₂ in the electrolyte solution may be explained by the fact that the cathodic reduction of O₂ to H₂O occurs in a two-step reaction in which H₂O₂ constitutes an intermediate. The second step of the reaction, in which the intermediate H₂O₂ is decomposed to H₂O, is delayed in an aged oxygen electrode device. The two reaction steps are: O₂ + 2H⁺ + 2e- → H₂O₂ (1) H₂O₂ + 2H⁺ + 2e- → 2H₂O (2) The overall reaction is: O₂ + 4H⁺ + 4e- → 2H₂O (3) Since the rate constant of the O₂ decomposition according to step (1) is large, the rate of step (1) is high. The rate of step (2) is determined by the rate constant of the H₂O₂ decomposition, and this rate constant appears to be much lower in an aged electrode device than in a non-aged electrode device. For the aged electrode device, step (2) will determine the rate of the overall reaction. In a non-aged electrode device, in which step (2) is not delayed, the conditions of diffusion of oxygen from the sample to the cathode determine the rate of the overall reaction. Due to the large rate constant of step (2), no accumulation of H₂O₂ is observed in such a non-aged electrode device. The slow response of step (2) in an aged electrode device has the effect that in such an aged electrode device, which is in a steady-state condition at a constant oxygen level, the part of the electrolyte solution adjacent to the cathode will contain a specific amount of H₂O₂ (the equilibrium concentration of H₂O₂). The equilibrium concentration of H₂O₂ stabilizes on a level so that the rate of formation of H₂O₂ according to step (1) and the sum of the rate of decomposition of H₂O₂ according to step (2) and the rate at which H₂O₂ diffuses away from the electrolyte solution part adjacent to the cathode and to the adjoining electrolyte solution are equivalent. As can be seen, the response of the oxygen electrode device will be a current consisting of two components: One component which is attributable to the oxygen reduction according to step (1), and one component which is attributable to the H₂O₂ reduction according to step (2). Consequently, it is evident that in case the surrounding oxygen level changes or varies, e.g. by bringing a sample of a different oxygen level into contact with the electrode device, the electrode device will not give a stable response (equilibrium current) until a new H₂O₂ equilibrium concentration in the electrolyte solution has been reached. Furthermore, it is clear that the calculation of the concentration of oxygen on the basis of measuring results provided by means of an electrode device, when the electrode device is not in a state of equilibrium, i.e. when the electrode response or electrode current is different from the equilibrium current, will provide results which are different from the results provided by means of an electrode device which is in its equilibrium state. By modifying the electrode device so that the H₂O₂ concentration is kept at or approximately at zero, H₂O₂ being spontaneously decomposed to H₂O + 1/2 O₂, the partial reaction will be: O₂ + 2H⁺ + 2e- → H₂O₂ (4) H₂O₂ → H₂O + 1/2 O₂ (5) The overall reaction is as shown above: O₂ + 4H⁺ + 4e- → 2H₂O (6) As step (5) proceeds rapidly, H₂O₂ will not accumulate in the electrolyte solution and the diffusion conditions of oxygen from the sample to the cathode will determine the rate of the overall reaction. Consequently, such a modified electrode device will generate a stable electrode response soon other a change of the oxygen partial pressure. The above-mentioned reduction of the H₂O₂ level in the electrolyte solution by the addition of catalase is not suitable for improving an electrode device for polarographically measuring the partial pressure of oxygen in routine clinical analyses, e.g. in the so-called blood gas analyzers installed in hospitals in connection with operating sections or in central laboratories. To have a blood gas analyser accepted by the users, it is of the greatest importance that maintenance work is moderate and substantially based on ready-to-use products, which on the one hand reduces the time spent on maintenance work and on the other hand reduces the risk of incorrect analyses caused by errors made by an operator in connection with maintenance work. For routine maintenance of an electrode device of the CLARK type in a blood gas analyser, the membrane and the electrolyte solution are to be replaced approximately once a month. However, an electrolyte solution containing catalase cannot be prepared as a ready-to-use product, as the decline in the activity of the catalase during the normal storage periods would be excessive. Furthermore, to add catalase to the fresh electrolyte solution at the site of use immediately before the replacement of the electrolyte solution is to be considered a measure which cannot be accepted, as hospital personnel themselves are not willing to carry out preparation work such as preparing reagents, electrolyte solutions or other liquid mixtures. Therefore, the object of the present invention is to provide an electrochemical measuring electrode device of the CLARK type for polarographically measuring the partial pressure of oxygen, which electrode device of the CLARK type is capable of generating reproducible and fast responses which are substantially independent of the age and the previous life of the electrode device end which electrode device of the CLARK type is at the same time as easy to maintain as the known electrode devices of the CLARK type. This object is fulfilled by an electrochemical measuring electrode device of the above-defined type, i.e. for polarographically measuring the partial pressure of oxygen in an electrolytic medium and comprising: a cathode, the cathode being capable of reducing oxygen and defining an exposed oxygen-reducing cathode surface, an anode, the anode defining an exposed anode surface, an oxygen-permeable membrane; an electrolyte chamber defined at least in part by the oxygen-permeable membrane, and containing therein the exposed oxygen-reducing cathode surface and the exposed anode surface; an electrolyte solution confined in the electrolyte chamber and providing an electrolyte medium through which the anode surface and cathode surface communicate with one another and; catalytic noble metal and/or platinum group metal particles contained in the electrolyte chamber arranged so as to be in communication with the electrolyte solution and adjacent to the exposed oxygen-reducing cathode surface. EP-A-0 0 047 898 and US-A-4 051 239 describe an electrochemical sensor designed to measure carbonmonoxide, hydrazine or hydrogen in air. The current-producing electrochemical reactions takes place on platinum black particles which have part of their surface embedded in the electrolyte but are otherwise exposed to the ambient air and are in electrical contact with a current-abducting matrix. US-A-4 051 006 discloses a platinum black electrode for measuring carbon monoxide. The platinum black is enclosed in an electrochemically enclosure with openings covered by membranes of high porosity. The construction makes the electrode portable. In the present specification the term "stable" as used about the catalytic means, is intended to indicate that the catalytic means will substantially retain its efficiency for promoting the decomposition of H₂O₂ under the conditions to which it is exposed in the electrode device; e.g. when the catalytic means is incorporated in the membrane or in a supporting structure, as explained below, the catalytic means should substantially retain its efficiency for promoting the decomposition of H₂O₂ during the normal period between replacements of the electrolyte solution and the membrane. As will appear from the figures and the appertaining description, the good response conditions aimed at according to the present invention are obtained with the electrode device according to the invention. Furthermore, it is extremely easy to prepare preprocessed electrode devices according to the invention or preprocessed components for use in the electrode devices according to the invention and no durability problems have been ascertained in connection with the preprocessed electrode devices or components. It should be noted that certain inorganic catalysts otherwise known to be effective for promoting the decomposition of H₂O₂, have been found not to show the essential stability under the chemical and electrochemical conditions to which the catalyst is exposed in the electrode device. Thus, colloidal silver and manganese dioxide have been found to be dissolved in the electrolyte of CLARK electrodes and thereby apparently to be converted into components which do not act as catalysts for the decomposition of H₂O₂. Activated carbon has been found to absorb oxygen and thereby to interfere with the current generation in the electrode. In the preferred embodiment of the electrode device according to the invention, the catalytic particles are constituted by platinum black particles, and the platinum black particles may be received by the membrane, or alternatively, the electrode device further, preferably, comprises a support structure, the support structure being permeable to H₂O₂ and water, and the platinum black particles being received by the support structure. The support structure means may constitute a covering of at least part of the membrane. It is preferred that the cross-sectional area of the individual platinum black particles is substantially smaller than the exposed oxygen-reducing cathode surface area. In particular, it is preferred that the particulate platinum black is substantially constituted by particles, the maximum diameter of which is less than 1 µm. The platinum black particles received by the support structure or received by the membrane may preferably be located at the side of the membrane facing the electrolyte chamber. Although the catalytic means constituted by the platinum black particles are preferably received by a support structure preferably constituting a covering of at least part of the membrane, the platinum black particles may, alternatively, be constituted by a separate catalytic particles component which is received in the electrode device according to the invention as defined above, however e.g. protruding into the electrolyte chamber or located adjacent to or in front of the oxygen-reducing cathode surface. By providing the catalytic particles of the electrode device as platinum black particles received by the membrane or received by the support structure preferably constituting a covering of at least part of the membrane, it is ensured that the effective promotion of, decomposition of H₂O₂ takes place at the appropriate place in relation to, i.e. adjacent to or in front of the exposed oxygen-reducing cathode surface. To ensure effective H₂O₂ reduction in the entire section of the electrolyte chamber communicating with the cathode surface, the platinum black particles are preferably in accordance with the present invention provided in such an amount and arranged relative to the exposed oxygen-reducing cathode surface in such a manner that the concentration of H₂O₂ is maintained substantially at zero in any volume of the electrolyte in the electrolyte chamber electrolytically communicating with the oxygen-reducing cathode surface, e.g. in a part of the electrolyte chamber having the oxygen-reducing cathode surface as a centre and being within a distance from the cathode surface of max. 100 times the cathode surface diameter. In connection with an oxygen electrode device according to the invention comprising a 20 µm diameter platinum cathode and a 29 µm polypropylene membrane, arranged at a distance of approx. 0-5 µm from the exposed oxygen-reducing cathode surface, it is preferred that the platinum black particles received by said support structure constituting a covering of at least part of the membrane are present in a surface density of 1-6000 µg/cm², preferably 29-600 µg/cm². The platinum black particles may be arranged in direct contact with the electrolyte solution. Alternatively, the plating black particles may be incorporated in a material which is permeable to H₂O₂ and water to avoid the electron-conducting contact between the platinum black particles and the cathode. Suitable materials are polyurethane, cellulose acetate, polyvinyl acetate and cellophane. The electrode device according to the present invention nay, e.g., be an electrode device of the CLARK type constituting a component of, e.g., a blood gas analyser for analysing samples of whole blood, or it may, e.g., be a transcutaneous electrode device. The transcutaneous electrode device will preferably further comprise means for thermostatically heating the electrode device to a predetermined temperature, said means comprising a temperature sensor means and a heating means. The present invention also relates to a membrane for an electrochemical measuring electrode device for polarographically measuring the partial pressure of oxygen in a medium, the membrane being a foil-shaped plastic material permeable to oxygen and being provided with a coating on at least part of the surface of the membrane, the coating comprising a particulate noble metal and/or platinum group metal catalytic means effective for promoting the decomposition of H₂O₂. Preferred embodiments of the membrane according to the invention are discussed above in connection with the general description of the invention. The present invention further relates to a method of preparing a membrane for an electrochemical measuring electrode device for polarographically measuring the partial pressure of oxygen, the membrane being of a plastic material permeable to oxygen. This method according to the present invention comprises the following steps: providing a foil of the plastic material, providing a suspension of platinum black particles in an organic liquid, applying the suspension to a side surface of the foil of the plastic material, evaporating the organic liquid, heating the plastic material to a temperature in excess of its softening temperature, and forcing the platinum black particles into the plastic material foil being softened by applying mechanical pressure to the platinum black particles. In accordance with an alternative method the membrane may be produced by the following steps: providing a foil of said plastic material, providing a suspension of platinum black particles in a plastic material in liquid state, said material being permeable to oxygen, H₂O₂ and water, applying said suspension to a side surface of said foil of said plastic material so as to provide a homogeneous liquid covering of said side surface of said foil, and solidifying said plastic material so as to establish a solidified covering of said side surface of said foil. In the above-described first method according to the present invention, the organic liquid may e.g. be benzene or ethanol. In the alternative method, the plastic materials may, e.g., be dissolved in a solvent, and/or it may be in a monomeric or oligomeric state which is polymerized to form the solidified covering. The platinum black particles may alternatively be applied to the membrane according to the invention by other techniques, such as printing techniques, rolling techniques or, quite generally, techniques involving gluing the platinum black particles to the plastic material of the membrane. The invention will now be further described with reference to the drawings, in which FIG. 1 is a schematical view of a first, presently preferred embodiment of an electrochemical measuring electrode device of the CLARK type and according to the invention for measuring the partial pressure of oxygen, FIG. 2 is a perspective and partly broken away view of a front part of the electrochemical measuring electrode device shown in FIG. 1 also illustrating a first embodiment of a membrane according to the invention, FIG. 3 is a perspective and partly broken away view corresponding to the view of FIG. 2 also illustrating a second and presently preferred embodiment of the membrane according to the invention, FIG. 4 is a perspective and partly broken away view of a second embodiment of the electrochemical measuring electrode device according to the invention, the electrode device being a transcutaneous, polarographic and potentiometric electrode device, FIG. 5 is an elevational view of the first embodiment of the membrane according to the invention shown in FIG. 2, FIG. 6 is an elevational and partly broken away view of the second and presently preferred embodiment of the membrane according to the invention shown in FIG. 3, FIGS. 7, 8 and 9 are schematical views illustrating separate steps of a first method according to the invention of preparing the first embodiment of the membrane according to the invention, FIGS. 10 and 11 are schematical views of two alternative embodiments of the method of preparing the second and presently preferred embodiment of the membrane according to the invention, FIG. 12 is a diagrammatical view illustrating the responses of an electrode device of the CLARK type according to the invention and the response of a conventional electrode device of the CLARK type, the electrode devices being identical to one another except for the catalytic means characteristic of the present invention, and FIG. 13 is a diagrammatical view illustrating the responses of the electrode devices mentioned above with reference to FIG. 12 in a different test set-up. In FIG. 1, a first embodiment of an electrochemical measuring electrode device of the CLARK type according to the invention for measuring the partial pressure of oxygen in a blood sample is shown. The electrode device is designated the reference numeral 10 in its entirety and comprises a plastic housing 12, a platinum cathode 14 which is constituted by a platinum wire of a diameter of 20 µm, which is sealed into a lead glass tube 16, and an anode 18. The platinum cathode 14 defines an active, exposed oxygen-reducing cathode surface 20, which is in contact with an electrolyte solution, which is confined in an electrolyte chamber 22 defined by an oxygen-permeable plastic membrane 24. As is evident from FIG. 1, the plastic housing 12 encircles the lead glass tube 16 so that an annular chamber 26 is defined between the plastic housing 12 and the lead glass tube 16. The annular chamber 26 communicates with the electrolyte chamber 22 through an electrolyte solution passage 28. The anode 18 is constituted by a silver/silver chloride coating of the lead glass tube 16 and communicates chemically with the electrolyte solution of the annular chamber 26 and further with the electrolyte solution of the electrolyte chamber 22 through the electrolyte solution passage 28. On the membrane 24, platinum black particles of a particle size of less than 1 µm are applied, as shown in FIG. 2 or, alternatively as shown in FIG. 3. The membrane 7 is maintained in position in front of the end surface of the electrode device 10 by means of an O-ring 30 and covers the exposed oxygen-reducing surface 20 of the Pt-wire cathode 14. The electrolyte solution is an aqueous solution of the composition: 191 mmole KH₂PO₄, 298 mmole Na₂HPO₄ · 2H₂O, 139 mmole KCl, 0.26 mmole AgCl and thymol as a germicide. In use, a polarization voltage of -630 mV is supplied to the electrode device 10 from an external measuring apparatus 32 through a cable 34. The electrode device is fitted in a measuring cell 36 of the type described in US patent No. 4,160,714. In the measuring apparatus 32, the current generated by the electrode device by the reduction of oxygen is amplified and converted from analogue to digital form in an A/D converter for calculating the partial pressure of oxygen, which is displayed on a first display 38 of the measuring apparatus 32. The measuring apparatus 32, further comprises a second display 40 for displaying the current generated by the electrode device 10 in digital representation. In FIG. 2, the end part of the electrode device 10 according to the present invention is shown in greater detail also disclosing the structure of the membrane 24 which comprises a catalytic means constituted by platinum black particles effective for promoting the decomposition of H₂O₂ generated in the electrolyte solution by the reduction of oxygen. Although the catalytic means is preferably constituted by a covering of at least part of the membrane of the electrode device, the catalytic means may, alternatively, e.g., be a layer in an annular configuration or an annular groove arranged at the front end of the lead glass tube 16, e.g., encircling the cathode. The membrane 24 is a circular piece of biaxially orientated polypropylene foil of a thickness of 20 µm. The diameter of the membrane 24 is 13 mm and the central part of the membrane 24 contacting a sample is approximately 5 mm. In a central region 42 of the membrane 24, platinum black particles are pressed into the foil of the membrane. The central region 42 measures approximately 7 mm². In FIG. 3, which is a partly broken away view of the end portion of the electrode device 10 basically corresponding to the view of FIG. 2, an alternative and presently preferred embodiment of the membrane 24 according to the invention is shown. The membrane 24 shown in FIG. 3 is a two-layer membrane a first or outer layer of which is designated the reference numeral 44 and constituted by the above-described biaxially oriented polypropylene foil. An inner layer 46 of the membrane 24 is constituted by a solidified plastic material including platinum black particles constituting a stable catalytic means effective for promoting the decomposition of H₂O₂ generated in the reduction of oxygen. The preparation of the first embodiment of the membrane 24 shown in FIG. 2 and the preparation of the second embodiment of the membrane 24 shown in FIG. 3 will be described in greater detail below with reference to FIGS. 7-9 and 10-11, respectively. In FIGS. 5 and 6, the first and the second embodiment, respectively, of the membrane 24 are shown in elevational views and as far as FIG. 6 is concerned also in partly broken away view. In FIG. 4, a second or alternative embodiment of the electrode device 10 according to the invention is shown. The electrode device shown in FIG. 4 is a transcutaneous and combined polarographic and potentiometric electrode device of the type described in the applicant's published international patent application PCT/DK81/00035, publication No. WO 81/02831 to which reference is made. In FIG. 4, components similar to the components described above with reference to FIGS. 1-3 are designated reference numerals identical to the reference numerals of FIGS. 1-3. In the transcutaneous, polarographic and potentiometric electrode device 10 shown in FIG. 4, the anode 18 is constituted by a solid silver body. Centrally within the solid silver body, a pH-electrode 48 is arranged. At the lower side surface of the silver body 18, recesses 50 are provided which are filled by the electrolyte solution and constitute electrolyte reservoirs. In two further recesses of the silver body 18, a Zener diode 52 and an NTC resistor 54 are arranged. The Zener diode 52 and the NTC resistor constitute a heating element and a temperature detector element, respectively, of the transcutaneous polarographic and potentiometric electrode device, as is well-known in the art. In front of the pH-electrode 48 and further in front of the cathode of the electrode device, not shown in FIG. 4, a spacer structure 56 is arranged. The spacer structure is constituted by a water and H₂O₂ and carbon dioxide permeable plastic element further comprising platinum black particles constituting a catalytic means effective for promoting the decomposition of H₂O₂ formed by the reduction of O₂. As is evident from FIG. 4, the spacer structure 56 is of a basically annular configuration. The platinum black particles of the membrane 24 or of the spacer structure 56 are prepared by grinding commercially available platinum black of the type FLUKA No. 81110. The commercially available platinum black has a variable grain size of approx. 5 µm. When preparing the ground platinum black particles, 1 g is weighed out and suspended in benzene in a concentration of approx. 20 mg/ml. The suspension is transferred to a beaker filled with small glass beads, and stirred on a magnetic stirrer of a conventional laboratory type for 24 hours. Thus a particularly finely distributed platinum black (dust) is obtained with a comparatively uniform particle size distribution and with an average particle diameter < 0.5 µm. The platinum black dust is easily suspended in the suspending agent. In accordance with a first method of preparing a membrane according to the invention, 10 µl of a benzene or ethanol suspension of the above-described platinum particles is, as shown in FIG. 7, taken out by means of a Carlsberg pipette 58 and transferred to a circular polypropylene toil 60. As is evident from FIG.7, the foil 60 may be supported on a support structure 64 which further includes a heating element 66. In FIG. 7, the 10 µl suspension including the platinum black particles is designated the reference numeral 62. The suspending agent is evaporated by standing for about 5 minutes. Alternatively, the evaporation could be performed by applying heat to the support structure 64 from the heating element 66. Thereafter, as is evident from FIG. 8, a TEFLON® foil 68 the melting point of which exceeds 175°C is arranged on top of the foil 60 with the layer of platinum black particles 62 from which the dispersing agent has been evaporated. A pressing tool 70 which also includes a heating element 72 is brought into contact with the upper side surface of the TEFLON® foil 68 as indicated by an arrow 73 and pressed against the foil 60 by a pressure of approximately 300 kg/cm² (29.4 x 10⁶ N/m²), while heat is applied to the pressing assembly from the heating elements 66 and 72 in order to heat the foils 60 and 68 to a temperature of approximately 130°C. The above pressure is maintained for a period of time of approximately 10 min., whereupon the platinum black particles have been pressed into the foil material 60. After the above 10 min. period of time, the heating elements 66 and 72 are disconnected from their supply. When the system has cooled to room temperature, the pressing tool is elevated and, as is evident from FIG. 9, the TEFLON® foil 68 is removed. The platinum black particles adhere completely to the polypropylene foil 60. From an electrode response time point of view, the above-described first method of preparing a membrane according to the invention may be the most advantageous as the membrane prepared in accordance with the above-described method basically has characteristics as to strength and permeability to oxygen which are identical to the characteristics of a polypropylene membrane which does not include platinum black particles. However, it has been found that this method may sometimes result in a membrane which may give rise to a quiescent or zero current of the electrode device when the membrane is arranged on the electrode device and maintained in position by means of its O-ring as shown in FIGS. 1-3. This is believed to be due to the fact that some of the platinum black particles which should have been pressed into the membrane material protrude from the inner side surface of the membrane and are consequently brought into contact with the exposed cathode surface so that a galvanic current is generated by the contact between the exposed cathode surface and the platinum black particles. Thus, in this method, it seems important to ensure that substantially all of the platinum black particles are pressed into the membrane material. In accordance with a second and presently preferred embodiment of preparing the membrane according to the invention, the platinum black particles are suspended in a plastic material which is in a liquid state, e.g., in solution, whereupon the suspension is applied to the polypropylene foil and solidified, e.g., by evaporation. Two alternatives of applying the suspension to the foil in accordance with the above-described presently preferred embodiment of the method according to the invention are shown in FIGS. 10 and 11. In FIG. 10, the suspension of the platinum black particles in the liquid plastic material is poured to an outer or upper side surface of the polypropylene foil 60 which is arranged in an oblique position, whereupon the suspension which is designated the reference numeral 74 in FIG. 10 flows down the oblique foil whereby a homogeneous suspension layer is provided and is solidified, e.g., by evaporation. The suspension 74 may, as is shown in FIG. 11 alternatively be applied to the outer or upper side surface of the foil 60 in a screening process by employing a squeegee or spatula 76. The liquid plastic material may, e.g., be prepared as a solution of 0.35 g PUR (polyurethane) in 5 ml DMF (dimethylformamide), and 20 ml THF (tetrahydrofuran) which may be prepared within a few hours. In 3 ml of this solution, 5 mg platinum black particles are suspended and ground by stirring with a magnetic stirrer for about two hours in a beaker of diameter 20 mm filled with about 20 glass beads of a diameter of 3 mm. As will be understood, the response time of the electrode device comprising the above-described second and presently preferred embodiment of the membrane according to the invention may be slightly slower than the response time of the electrode device comprising the above-described first embodiment of the membrane according to the invention if the membrane thickness is not reduced to compensate for the added layer. However, the increase in the response time by applying the inner layer 46 to the outer layer 44 of the membrane 24 shown in FIG. 3 only amounts to approximately 50%, and the response time of the electrode device shown in FIG. 3 may obviously be reduced by simply decreasing the thickness of the outer membrane layer 44. The membrane 24 shown in FIG. 3 and 6 has the advantage that the platinum black particles are all embedded in the layer 46 and thus cannot give rise to a quiescent current or zero current. In FIG. 12, a diagram is shown illustrating the response A of an aged oxygen electrode device as shown in FIGS. 1 and 2 according to the invention comprising a membrane of the type described above with reference to FIGS. 2 and 5 and prepared as described above with reference to FIGS. 7-9 and the response B of the same electrode device comprising a polypropylene membrane of a conventional type, i.e. not including platinum black particles. The responses were recorded in an experiment in which the electrode devices were mounted in a set-up of the type shown in FIG. 1 for measuring the oxygen partial pressure on gas samples. Atmospheric air saturated with aqueous vapour, i.e. air of an oxygen partial pressure of 145 mm Hg (19.3 kPa) was introduced into the measuring cell 36 every 6 minutes and conducted past the electrode device. After the oxygen electrode device had been mounted in the measuring cell for approx. 3 days, a sample of the composition of 95% argon, 5% CO₂, i.e. of an oxygen partial pressure of zero, was introduced into the measuring cell. The digitized response of the electrode device was recorded every 2 seconds and converted to mm Hg on the basis of calibration data at 145 mm Hg. The two response curves A and B shown in FIG. 3 were plotted on the basis of the response data thus recorded. It is seen that after 90 seconds the electrode current and/or the response B of the conventional electrode device reaches a value corresponding to an oxygen partial pressure of approximately 9 mm Hg, whereas the electrode current of the response A of the electrode device according to the invention already after 45 seconds reaches a stable value corresponding to an oxygen partial pressure of 2 mm Hg. Thus it can be seen that the response of the electrode device of the present invention reaches its stable level considerably faster than the conventional electrode device. Based on the theoretical reflections above, it is evident that the response curve B of the conventional aged electrode device will be positioned above the response curve A of the electrode device according to the invention, when the oxygen level in the sample is lover than the oxygen level at which the electrode device has stabilized. This is due to the fact that the H₂O₂ level in the electrolyte solution while the response curve is recorded is higher than the level of H₂O₂ originating from the present oxygen level and thus provides a higher current than the current to be expected from the present oxygen level. The rate of the response is usually expressed by the time constant of the electrode device, which is approximately 5 seconds for a non-aged conventional oxygen electrode device. The same value is ascertained for the electrode devices according to the invention. Aged conventional oxygen electrode devices have a small time constant during the first part of the response and a large time constant during the last part of the response. In connection with measurements on samples with an oxygen level lower than the oxygen level at which the electrode device has stabilized (145 mm Hg), the time constant during the last part of the response for aged conventional oxygen electrode devices was experimentally found to be more than 15 seconds. On the basis of the response curves, it is estimated that the analysing time may be reduced by more than 50% in an analysing apparatus in which the electrode device according to the invention replaces the conventional device. In FIG. 13, the response C of an aged electrode device according to the invention and of the type described above with reference to FIG. 12 and the response D of the conventional electrode device also described above with reference to FIG. 12 are shown. With the one exception that a sample of a high oxygen level (80% O₂, 5% CO₂, 15 % N₂) was used instead of a sample of an oxygen level of zero, the experimental work which led to the plotting of FIG. 4 was carried out in the same manner as described in connection with FIG 3. In this case a lower oxygen content was recorded (as expected) when employing the conventional electrode device than when employing the electrode device according to the invention.

Claims: A polarographic electrode device (10) of the Clark type for polarographically measuring the partial pressure of oxygen, comprising: a cathode (14) capable of reducing oxygen and defining in exposed oxygen-reducing cathode surface (20); an anode (18) defining an exposed anode surface; an oxygen-permeable membrane (24, 44); an electrolyte chamber (22) defined at least in part by the oxygen-permeable membrane (24), and containing therein the exposed oxygen-reducing cathode surface (20) and the exposed anode surface; an electrolyte solution confined in the electrolyte chamber (22) and providing an electrolyte medium through which the anode surface and cathode surface (20) communicate with one another; and catalytic noble metal and/or platinum group metal particles (42, 46) contained in the electrolyte chamber (22) arranged so as to be in communication with the electrolyte solution and adjacent to the exposed oxygen-reducing cathode surface (20). An electrode device according to claim 1, in which the particles comprise platinum black particles. An electrode device according to claim 2, in which the platinum black particles are received by the membrane (24). An electrode device according to claim 2, in which cross-sectional area of the individual platinum black particles is substantially smaller than the exposed oxygen-reducing cathode surface area (20). An electrode device according to claim 4, in which the platinum black particles are substantially constituted by particles having a maximum diameter of less than 1 µm. An electrode device according to claim 2, which further comprises a support structure (56), the support structure being permeable to H₂O₂ and water, and the platinum particles being received by the support structure. An electrode device according to claim 6, in which the support structure constitutes a covering of at least part of a surface of the membrane facing the electrolyte chamber. An electrode device according to claim 2-7, in which the platinum black particles are present in a surface density of 1 - 6000 µg/cm², preferably 20 - 600 µg/cm². An electrode device according to any of the preceding claims, the electrode device further comprising means for thermostatically heating the electrode device to a predetermined temperature and comprising a temperature sensor means and a heating means. A membrane (24) for an polarographic electrode device for polarographically measuring the partial pressure of oxygen in a medium, the membrane being a foil-shaped plastic material permeable to oxygen and being provided with a coating on at least part of the surface of the membrane, the coating comprising a particulate noble metal and/or platinum group metal catalytic means effective for promoting the decomposition of H₂O₂. A membrane (24) according to claim 10, in which the catalytic means comprised a platinum black means. A membrane (24) according to claim 11, in which the platinum block means is constituted by platinum black particles received by the membrane. A membrane (24) according to claim 11, in which the platinum black means is constituted by platinum black particles, the platinum black particles being received by a support structure (56) permeable to H₂O₂ and water, the support structure constituting a covering of at least part of a surface of the membrane. A membrane (24) according to claim 11-13, in which cross-sectional area of the platinum black particles is less than 1 µm. A membrane (24) according to any of claims 10-14, in which the coating comprising the catalytic means is arranged on a first side surface of the membrane, the first side surface being adapted to face the front measuring surface of the polarographic electrode device. A membrane (24) according to any of the claims 10-15, in which the platinum black particles are present in a surface density of 1 - 6000 µg/cm², preferably 20 - 600 µg/cm². A method of preparing a membrane (24) for a polarographic electrode device for polarographically measuring the partial pressure of oxygen, the membrane being of a plastic material being permeable to oxygen, the method comprising the following steps: providing a foil (68) of the plastic material; providing a suspension of platinum black particles in an organic liquid; applying the suspension to a side surface of the foil of the plastic material (60); evaporating the organic liquid; heating the plastic material to a temperature in excess of its softening temperature; and forcing the platinum black particles into the softened plastic material foil (60) by applying mechanical pressure to the platinum black particles.

Inventors: CHRISTIANSEN, TORBEN FALCH, KOKHOLM, FINN
Applicants: RADIOMETER A/S
Application year: 1986
Main classification: G01N27
IPC codes: G01N27