WO1981002831A1

WO1981002831A1 - Apparatus and method for measuring the partial pressure of oxygen and of a gas which in aqueous solution generates an acid or a base

Info

Publication number: WO1981002831A1
Application number: PCT/DK1981/000035
Authority: WO
Inventors: J Severinghaus
Original assignee: Radiometer As; J Severinghaus
Priority date: 1980-04-11
Filing date: 1981-04-13
Publication date: 1981-10-15
Also published as: JPS57500454A; EP0049264A1

Abstract

A measuring electrode device (1) for measuring the partial pressure of oxygen and carbon dioxide (or of other gas which in aqueous solution generates an acid or a base) comprises a cathode (62) and an anode for polarographic measurement of oxygen and a pH-electrode (16) with associated reference electrode for pH-measurement, an electrolyte in contact with these electrodes and a membrane (12) which is permeable to oxygen and carbon dioxide. The anode and reference electrode functions are suitably performed by one electrode (80). To compensate for production of hydroxyl ions at the cathode, the device comprises a compensation electrode (66) which is in contact with the electrolyte and connected to compensation means, typically an operational amplifier, which force current through the compensation electrode to make the compensation electrode electrochemically consume hydroxyl ions in an amount stoichiometrically equivalent to hydroxyl ions generated at the cathode. Compensation electrode may consist of e. g. platinum or aluminum. When used for transcutaneous determination of the partial pressure of oxygen and carbon dioxide in blood, electrode device thermostated to a temperature slightly above skin temperature is applied to a skin area of a patient.

Description

Apparatus and Method for Measuring the Partial Pressure of Oxygen and of a Gas which in Aqueous Solution Generates an Acid or a Base

BACKGROUND OF THE INVENTION

This invention relates to an apparatus and a method for measuring the partial pressure of oxygen and of a gas which in aqueous solution generates an acid or a base .

Conventionally, the partial pressure of oxygen is measured by means of a polarographic apparatus such as disclosed in U. S . Patent No . 2,913,386. The apparatus comprises a pair of electrodes , a cathode and an anode, immersed in an electrolyte and encased at least in part in a membrane which is permeable to oxygen . Typically, the cathode is formed of platinum and is located closely adjacent the membrane; the anode is formed of silver/silver chloride; and the electrolyte is an aqueous alkali halide solution. In operation , oxygen outside of the membrane permeates the latter and is reduced according to the equation :

O₂ + 2H⁺ + 4e- → 2OH-

As appears from the equation, H⁺ is consumed at the cathode and hydroxyl ions are generated as the electrode operates .

The partial pressure, in a liquid or in a gas mixture, of a gas , such as carbon dioxide, whi h in aqueous solution generates an acid or a base is normally measured by electrode device comprising a pH-sensitive electrode (glass electrode) and a referønce electrode, an electrolyte in contact with the electrodes , and a membrane which, in conjunction with the body of the electrode device; defines a chamber in which the electrolyte is present, the said membrane being permeable to the gas . The pH electrode responds to pH changes which are produced by the presence of the gas in the solution and which are indicative of the partial pressure to be measured. Thus , in opefation of such an electrode device adapted for the measurement of the partial pressure of carbon dioxide, carbon dioxide is dissolved in the electrolyte in accordance with the equations :

Recent developments in electrode devices for the clinical measurement of gas partial pressures in blood involve devices for the so-called transcutaneous blood gas measurements performed by applying the device on the skin of the patient, usually with ther mostating of the device at a temperature above normal skin temperature to obtain local hyperemisation of the skin . Such transcutaneous electrode devices , like other gas measurement devices for clinical use, such as catheter devices , are ideally relatively small and compact. In order to facilitate the measurement of the partial pressures of more than one blood gas , typically the partial pres sures of oxygen and carbon dioxide, it is highly desirable to establish combined electrode devices containing the necessary com bination of electrodes for the combined measurement. Thus , e.g. UK Patent Application No . 2 005 418 A discloses an electrode device for simultaneous measurement of Pco₂ and Po₂ comprising an electrode chamber having within a first electrode or pH responsive electrode, a second electrode capable of electrochemically reducing oxygen , a reference electrode for each of, or common to, the pH responsive electrode and the oxygen reducing electrode, an elec trolyte in Contact with the electrodes , and a membrane permeable to oxygen and carbon dioxide.

The most serious problems-involved m combining electrodes for measuring oxygen and carbon dioxide is the generation of OH-ions at the oxygen reducing electϊodev From the equations given above, it is evident that such generation of OH-ions will influence the pH measured, and, frehce, will influence the carbon dioxide partial pressure determined by means of the electrode device . In an attempt to avoid this influence, the combined electrode device disdosed in the above-mentioned patent application further in eludes means for holding the electrodes in spaced apart, insulated relationship in order to reduce the influence. However, in view of the small physical size of such electrode device, the improvement obtainable by holding the electrodes in spaced apart relationship must be believed to be rather limited.

The present invention provides an electrode device which permits combined measurement of the partial pressure of oxygen and of a gas which in aqueous solution generates an acid or a base, and which minimizes the adverse influence of the Po₂ measurement on the measurement of the partial pressure of the acid or base-generating gas .

SUMMARY OF THE INVENTION .

There is provided an electrode device for measuring the partial pressure of oxygen and a gas which in aqueous solution generates an acid or a base, said device comprising a body, a first electrode (cathode) capable of electrochemically reducing O₂ to generate hydroxyl ions , a second electrode (reference electrode or - when considered in relation to the said cathode - anode) communicating with the cathode through means for biasing the said first electrode relative to said second electrode, a third electrode (pH electrode) responsive to pH changes produced by the presence, of the gas which in .solution generates an acid or a base, a fourth electrode (reference electrode) cooperating with the pH electrode, an electrolyte in contact with the above-mentioned electrodes , a membrane defining, in conjunction with the said body an electrolyte chamber in which said electrolyte is present and is in contact with the said electrodes , said membrane being gas-permeable and liquid-impermeable, a fifth electrode (compensation electrode) Joeing in contactwith said electrolyte in said electrolyte chamber, and compensation means electrically connected to said second electrode and to said compensation electrode for compensating for a production of hydroxyl ions at said cathode by forcing current through said compensation electrode to make the compensation ejectrode electrochemically consume hy droxyl ions in an amount stoichiometrically equal to the amount of hydroxyl ions generated at said cathode . In practical embodiments of the invention, the said second electrode and said fourth electrode .will preferably be constituted by a single electrode (reference electrode) as there seems to be no advantage associated with providing separate reference electrodes for the Po₂ and Pco₂ measuring systems .

The compensation means forcing current through the compensation electrode preferably comprise electronic amplifying means connected to the reference electrode, the compensation electrode, and the cathode. Said electronic amplifying means are preferably constituted by an operational amplifier, an inverting input of the said operational amplifier being connected to the reference electrode, an output of said operational amplifier being connected to the compensation electrode, and the non-inverting input of said operational amplifier being connected to the cathode through the above-mentioned biasing means . One important aspect of this circuit layout is that there will be no current flowing in the reference electrode since the operational amplifier forces the current generated at the cathode through the compensation electrode. By this, the hydroxyl ions generated at the cathode will be consumed by the compensation electrode so that pH of the electrolyte does not change and thereby does not influence the Pco₂ measurement. Further, polarization of the reference electrode is avoided, thus eliminating drift with time.

Furthermore, it is preferred that the biasing means comprise a second operational amplifier and a voltage source, the cathode being connected to an inverting input of the operational amplifier, and a terminal of the voltage source being connected to a non-inverting input of the operational amplifier. This second operational amplifier maintains the cathode at the (negative) potential of the said terminal of the voltage source and provides a current-voltage converter which generates a voltage proportional to the O₂ reduction for the measuring instrument used for converting the electrode signals to Po₂ readout; it is wellknown that the use of a voltmeter as the measuring instrument instead of an ammeter is always preferable due to the avoidance of possible impedance mismatch and of loading of the Po₂ measuring system. The reference electrode of the electrode device of the invention is preferably a silver/silver chloride electrode . In one preferred embodiment of the invention the reference electrode is arranged spaced from the electrolyte chamber and communicating electrolytically with the electrolyte in the electrolyte chamber by means of a liquid junction . Without such a barrier, silver ions may diffuse into the electrolyte creating a coating on the cathode resulting in drift of the electrode potential. The liquid junction may , e . g . , be selected from the group consisting of gelled electrolytes and electrolytes absorbed on an absorbent such as cotton .

In the preferred embodiment of the invention, the cathode is a noble metal such as platinum or gold and the pH-sensitive electrode is a pH glass electrode . The cathode is preferably formed as an about 25 μm platinum wire in a 4 mm glass tubing . The glass electrode is preferably substantially filled with bodies of a metal showing high heat conductivity in order to increase the heat conductivity of the pH glass electrode so that the thermal response time of the electrode system is increased which may be of importance especially when the electrode device is used for transcutaneous measurements . The bodies may be constituted by a structure defining channels containing the interior electrolyte such as sintered silver.

The compensation electrode may be made of a conductive material selected from the group consisting of carbon and noble metals , such as platinum or a platinum metal. In this case, the hydroxyl ions will be consumed by formation of oxygen at the compensation electrode . However, this surplus concentration of oxygen will tend to diffuse out through the membrane, and with proper construction of the electrode, has been found not disturb the oxygen measurement at the cathode . In the electrode constructions herein illustrated, a suitable distance between a noble metal cathode and a carbon or noble metal compensation electrode would be more than 1 mm, preferably more than 2 mm, typically 3 mm or above . In one aspect of the invention, the compensation electrode is made of a metal which in operation of the electrode device consumes hydroxyl ions by precipitation of a hydroxide or oxide. A typical and sui table example of such metals is aluminum (optionally anodized) , but also tantalum, titanium, iron, chromium , tin, zirconium , or cadmium may, in suitable modifications or alloys , be useful. When the compensation electrode is made of optionally anodized aluminium, OH--ions which are generated at the cathode by reduction of O₂ are consumed at the compensation electrode in accordance with the following equations :

Al + 3OH- → Al(OH)₃

2Al(OH)₃ → Al₂O₃ + 3H₂O.

The fact that the Al₂O₃ will be precipitated on the aluminum compensation electrode will not to any substantial degree detract from the long term reliability of the electrode; thus , e. g. , at a current of 10 ^-9 amperes, with a 1 cm diameter aluminum compensation electrode, it would take about 70 years to build up a 1 μm deposit of Al₂O₃.

The electrolyte in the electrolyte chamber may be a hydrogen carbonate solution of a concentration which is preferably in the range 5 - 50 mM, and may include 0.1M KC1, 0.5M KNO₃ (or other soluble potassium salt inhibiting the growth of microorganisms) , 0.02M NaHCO₃ in mixtures containing ethylene glycol and/or propylene glycol and optionally a small amount of a wetting agent such as 0.01% Trition X.

In a preferred embodiment of the invention, the membrane is a membrane permeable to oxygen and carbon dioxide and is typically made of polytetrafluoroethylene of a thickness of about 6 - 25 μm.

The body may be made of an electrically-insulating material such as ceramic material or glass . In one preferred embodiment, the the body may constitute the compensation electrode in which the reference electrode, the pH glass . electrode, and the cathode are embedded. For transcutaneous measurement of Po₂ og Pco₂ , the electrode device according to the invention comprises means for controlled heating of the body, e. g. a zener diode for heating the body and a thermistor for controlling the heating of the body .

The invention further relates to a method for determining the partial pressure, in a liquid or a gas mixture of oxygen and of a gas which in aqueous solution generates an acid or a base, comprising determining the partial pressure of oxygen by determination of the current consumed in reducing oxygen at a cathode contacting an electrolyte solution which, through a gas-permeable membrane, is in communication with the said liquid or the said gas mixture, determining the partial pressure of the gas which in aqueous solution generates an acid or a base, typically carbon dioxide, by determining pH in the said electrolyte solution and compensating for any influence on the pH of the said electrolyte solution caused by the generation of hydroxyl ions at the said cathode being compensated for by substantially stoichiometrically consuming hydroxyl ions from the said electrolyte solution by electrochemical oxidation at a compensation electrode, the current to which is controlled in dependency of the generation of hydroxyl ions at the said cathode . According to one aspect of the invention, this method is performed without generation of O₂ at the compensation electrode, such as by consuming the hydroxyl ions by precipitation of a hydroxide or oxide as a result of the electrochemical oxidation .

A most important embodiment of this method is when performed for the transcutaneous determination of the partial pressure of oxygen and carbon dioxide in blood, comprising applying, to a skin area, an electrode device, thermostated at a temperature above the skin temperature to generate local hyperemia at the application site, the electrode device comprising an electrolyte solution contained in a reservoir placed behind an oxygen- and carbon dioxide-permeable membrane mounted on the side of the electrode device contacting the skin and a polarographic oxygen-reducing cathode and a pH sensitive potentiometric electrode in electrolytical communication with the said electrolyte, determining the oxygen partial pressure by determination of the current consumption of the polarographic electrode and determining the pH of the electrolyte solution, indicative of the carbon dioxide partial pressure in the blood, by determination of the potential between the pH sensitive electrode and a reference electrode, and compensating for any influence, on the pH measurement, of hydroxyl ions generated at the polarographic oxygen-reducing cathode by forcing a current through a compensation electrode in electrolytic communication with the said electrolyte, the size of the said current being determined in dependency of the current involved in the reduction of oxygen at the polarographic oxygen-reducing cathode to substantially stoichiometrically consume hydroxyl ions generated at the said cathode. The preferred features of this method are as stated above.

According to a special feature of the invention, the membrane is spaced from the body of the electrode device by means of paper of the type of Joseph paper, that is , fluffless porous paper of the type which is also useful e. g. as "lens paper" , e . g. , Joseph paper. The paper is typically of a thickness of about 20 - 50 μm, e.g. about 25 μm. Joseph paper has been found to function well in the combination electrode of the invention . However, good results have also been obtained without any spacer, utilizing a groove in the front area of the electrode device as an electrolyte reservoir.

In the combination electrode of the invention, nylon mesh was also used as a spacer, but resulted in somewhat slower response to

CO₂. Cellophane (cuprophane) was found to be slower and to decrease the sensitivity to CO₂ slightly. With Joseph paper under 25 μm polytetrafluoroethylene, the air liquid ratio, measured in 50% ethylene glycol, was found to be 1.015 at 43ºC , which is closer to unity than has been found for cuprophane . This is presumed to be due to the freer lateral diffusion of O₂ within the paper.

Pressure sensitivity of the O₂ electrode may be minimized by having the cathode and its glass shaft project slightly, e. g. , about 0.5 mm, above the natural lie of the membrane, thus keeping a tight contact. BRIEF DESCRIPTION OF THE DRAWINGS .

The invention will now be further described with reference to the drawing, wherein Fig. 1 is a plane view partially in section of a first embodiment of an electrochemical measuring electrode device according to the invention for transcutaneous combined measurement of the partial pressures of oxygen and carbon dioxide, Fig. 2 is a sectional view along the line II - II in Fig. 1 , Fig. 3 is a sectional view along the line III - III in Fig. l ,

Fig. 4 is a block diagram showing an electrical circuit for use in connection with the electrochemical measuring electrode device according to the invention, Fig. 5 is a graphical representation of measurements of Pco₂ plotted versus time and showing the importance of using a compensation electrode,

Fig. 6 is a plane view partially in section of a second embodiment of an electrochemical measuring electrode device according to the invention for transcutaneous combined measurement of the partial pressures of oxygen and carbon dioxide, fig. 7 is a sectional view along the line VII - VII in fig . 6 , and fig. 8 is a sectional view along the line VIII - VIII in fig . 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 shows a sectional view of a first embodiment of an electrochemical measuring electrode device according to the invention for transcutaneous , combined measurement of the partial pressures of oxygen and carbon dioxide . This electrode device is contained in an electrode housing 1 as also shown in Figs . 2 and 3. As best shown in Figs . 2 and 3 the electrode housing 1 comprises two annular parts 2 and 3 of which the part 2 has an outer diameter which is greater than that of the part 3. The annular part 2 has a tube stub 4 formed thereon for fastening an outer insulating jacket of a multicore cable 5. The annular part 3 is furthermore provided with two projecting thread parts 6 and 7 adapted to cooperate with corresponding inner threads for fastening the measuring electrode device during use . A ring-shaped anodized aluminum body 8 is mounted within the electrode housing 1. The body 8 has an upper end portion with a reduced outer diameter for engaging the inner surface of the annular part 3 with a tight fit. A space defined within the electrode housing by an upper cover (not shown) is filled with a solidified material 9, such as epoxy, which has been introduced into the space in a moulden condition . An annular peripheral groove 10 is formed in the outer surface of the aluminum body 8 and is adapted to received an O-ring 11 as shown in Figs . 2 and 3 for securing a membrane 12 which is permeable to oxygen and carbon dioxide, in an extended condition over the end surface of the aluminum body 8. This end surface is slightly concavely curved in order to facilitate mounting and securing of the membrane 12. Between the said end surface and the said membrane, a spacer is arranged, e.g. , a 25 μm Joseph paper spacer. A shallow annular groove 13 is also formed in the end surface of the aluminum body 8, and this groove constitutes a reservoir for an electrolyte which is enclosed between the membrane 12 and the end surface of the aluminum body 8. A pair of bores 14 and 15 extend substantially axially through the annular parts 2 and 3 of the electrode housing 1. The upper ends of these bores open at the upper end surface of the annular part 2, while the lower ends of the bores extend through the aluminum body 8 and open into the annular groove 13 in the end surface of the body 8 as indicated in Fig. 2. The upper ends of the bores 14 and 15 may be connected to outer suction and pressure sources , not shown. Thus , these sources are connected to the space formed between the concavely curved end surface of the aluminum body 8 and the membrane 12. It is understood that it will thereby be possible to replace the electrolyte in the measuring electrode device.

A pH-glass electrode generally designated by 16 and described more in detail below is positioned in the central opening or bore of the aluminum body 8. The body 8 also defines a through-going axially extending, excentric bore opening into the annular groove 13 as shown in Fig. 2. An Ag/AgCl-electrode generally designated by 17 and described more in detail below is positioned in the last-mentioned excentric bore . The Ag/AgCl-electrode 17 constitutes a reference electrode as well in relation to the pH-glass electrode 16 in the Pco₂ -measuring system of the measuring electrode device as in a Po₂-measuring of the electrode device. Furthermore, the Po₂-measuring system of the electrode device comprises a cathode 18 which is shown in Fig. 3 and will be described more in detail below.

A thermistor 20 with terminals 21 and 22 and embedded within a body 19 is positioned in a pocket formed in the aluminum body 8. The body 19 with the thermistor 20 is in turn embedded in a mass 19a having a good thermal conductivity, such as silver epoxy, shown in Figs . 1 and 2.

As shown in Figs . 1 and 3, the aluminum body 8 defines a further pocket containing a zener diode 23 having terminals 24 and 25 which are connected to individual cores of the multicore cable 5 just like the terminals 21 and 22 of the thermistor 20 , so that the zener diode and the thermistor may be connected to an outer circuit (not shown) for thermostatic controlling the heating of the body 8 by means of the zener diode 23 controlled by the thermistor 20. As explained below in connection with Fig. 4, the aluminum body 8 constitutes a compensating electrode for stoichiometrical consumption of the hydroxyl ions which are generated at the cathode in connection with the Po₂-measurement and which alter the pH of the electrolyte of the electrode device and thereby influence the Pco₂ measurement. As apparent from Fig. 1 the aluminum body 8 is connected to a conductor 30 connected to one of the individual cores of the multicore cable 5.

As shown in Figs . 2 and 3, the pH-glass electrode 16 comprises a glass cylinder 33 integrally formed with a pH-sensitive glass membrane 34. A paste 37 containing silver powder, agar, and a pH-buffer, such as an aqueous solution of KCl, is arranged within the inner space of the pH-glass electrode 16 defined by the glass cylinder 33 and the glass membrane 34. A conductor 26 of silver is positioned within the paste 37 so as to establish electrical conductive connection between the said paste and an individual core of the multicore cable 5. The upper end of the conductor 26 is surrounded by an insulation 27. A pair of stoppers 35 and 36 which may, for example, be of neoprene, are arranged in the upper end of the pH-glass electrode 16 and close the inner space thereof . A remaining space defined within the pH-electrode 16 between the lower end surface of the stopper 36 and the upper surface of the paste 37 is filled by oil 38. As already mentioned the entire pH-glass electrode 16 is positioned within the aluminum body 8, and the electrode is embedded in a solidified mass 39, such as silver epoxy.

The reference electrode 17 shown in Fig. 2 contains an

Ag/AgCl-body 40 which by means of a connection 41 is electrically connected to a silver rod 28, the upper part of which is isolated in relation to the adjacent electrode parts by means of an insulating layer 29. As shown in Fig. 2 the reference electrode 17 is mounted recessed in relation to the concavely curved end surface of the aluminum body 8. The Ag/AgCl-body 40 is enclosed in a tubular member 42 also enclosing the insulating layer 29 of the silver rod 28, and the said tubular member defines a passage between the end surface of the aluminum body 8 and the Ag/AgCl-body 40. This passage is filled with a porous material, such as cotton, forming a liquid barrier between the reference electrode and the electrolyte of the electrode device . The tubular member 42, which may be made from heat shrink teflon, is pressfitted in the said excentric bore of the aluminum body 8 and surrounded by a filling mass 43, such as silver epoxy.

The cathode as shown in Fig. 3 comprises a platinum wire 44 constituting the cathode metal in the Po„-measuring system of the measuring electrode device. The platinum wire 44 is connected to a conductor 32 which in turn is connected to a core of the multicore cable 5, and enclosed in a glass tube 45 which is tapered at the end adjacent to the end surface of the aluminum body 8. The glass tube 45 is in turn surrounded by a tubular member 46 which also surrounds the end part of the insulation of the conductor 32. The tubular member 46 may, for example, be made from heat shrink teflon or heat shrink polyvinyl chloride. The tubular member 46 which is positioned within the aluminum body 8, is embedded in a filling mass 47, such as silver epoxy. Fig. 4 shows a block diagram of an electrical circuit for use in connection with the electrochemical measuring electrode device according to the invention . As wellknown for those skilled in the art, this circuitry contains a polarization voltage source 53 for establishing a voltage difference between the electrodes of the

Po₂-measuring system of the electrode device . The negative terminal of this voltage source 53 is connected to a non-inverting input of an operational amplifier 48. The positive terminal of the voltage source 53 is also connected to a non-inverting input of a second operational amplifier 50 and establishing simultaneously the ground of the circuit. The output of the first-mentioned operational amplifier 48 is connected to the inverting input of the operational amplifier 48 through a feed-back resistor 52 and also to an input of a measuring instrument 49. The inverting input of the operational amplifier 48 is also connected to a terminal 54 for establishing connection to the cathode of the measuring device, i. e . the cathode 18 in Fig. 3. The inverting input of the said operational amplifier 50 is also connected to a terminal 55 for establishing connection to the reference electrode of the measuring device, i. e. the Ag/AgCl-electrode 17 in Fig. 2. The output of the same operational amplifier 50 is connected to the compensation electrode or anode, i. e . the aluminum body 8 in Figs . 2 and 3 , via a terminal 56. A fourth terminal 57 is provided for establishing connection between a measuring instrument 51 and the pH-electrode of the measuring device, i. e. the electrode 16 in Figs . 2 and

3.

As will be understood, the measuring instrument 49 is used for measuring the partial pressure of oxygen, while the measuring instrument 51 is used for measuring the partial pressure of carbon dioxide. As is wellknown to a person skilled in the art each of the measuring instruments may be of various types . Thus , they may be provided with various kinds of readout or indicating devices and adapted to transform the measuring signals provided to suitable measuring units .

As apparent from the diagram the operational amplifier 48 maintains the cathode of the measuring device connected to a teminal 54 at the negative potential in relation to ground as determined by the polarizing voltage 53. However, the operational amplifier 50 tends to maintain the potential of the electrolyte at zero by driving the compensation electrode or anode of the electrode device connected to the terminal 56 to whatever potential required to keep the potential of the electrolyte at zero measured by the operational amplifier at the terminal 55.

A very important feature of the layout of the circuit shown in Fig. 4 is that no current is flowing through the reference electrode connected to the terminal 55, because the operational amplifier 50 forces the current generated in the Po₂-measuring system to flow between the anode connected to the terminal 56 and the cathode connected to the terminal 54 and not between the reference electrode connected to the terminal 55 and the said cathode. Hereby is obtained that the hydroxyl ions generated at the cathode are consumed at the anode so that the pH-value of the electrolyte does not change due to the Po₂-measurement and, consequently, does not influence the Pco«-measurement. Furthermore, polarization of the Ag/AgCl-reference electrode is prevented so that the potential of said electrode does not drift with time.

Fig. 5 is a graphical representation of measurement of Pco₂ plotted versus time and showing the importance of using a compensation electrode. Within the first one hour and twenty minutes , the compensation electrode connected to the terminal 56 in Fig. 4 was disconnected, and the inputs of the operational amplifier 50 of Fig. 4 were shorted so that the Ag/AgCl electrode was connected directly to the positive terminal of the polarizing voltage source 53 in Fig. 4. Thus , the Po₂ measuring system of the measuring electrode device functioned as a conventional polarographic measuring device. It is evident from the figure that the hydroxyl ions produced at the cathode of the Po₂ measuring system influence the Pco₂ measurement since the Pco₂ measurements plotted drift down about 6.5% per hour. After one hour and twenty minutes , the circuit shown in Fig. 4 was re-estabhshed, and the electrochemical measuring device started measuring correctly after 20 minutes, as shown in Fig. 5. The drift of the Pco₂ measurement was less than 0.5 mm Hg after 40 hours .

Figs . 6, 7, and 8 show sectional views of a second embodiment of an electrochemical measuring electrode device according to the invention for transcutaneous combined measurement of the partial pressures of oxygen and carbon dioxide . Like the first embodiment of the electrode device, the embodiment shown in figs . 6 - 8 is contained in the electrode housing 1 comprising the two annular parts 2 and 3 shown in figs . 1 and 2. Generally, substantially identical parts in the embodiments shown in figs . 1 , 2, and figs . 6 - 7, respectively, are designated by identical reference numerals .

In contrast to the embodiment shown in figs . 1 and 2 which comprises two projecting thread parts 6 and 7 serving mechanical mounting purposes , the embodiment of the present invention shown in figs . 6 - 8 comprises an O-ring 58 adapted to cooperate with an inner surface of a holder (not shown) and also serving mechanical mounting purposes .

Like the first embodiment of the electrochemical measuring electrode device according to the invention shown in figs . 1 and 2, the second embodiment shown in figs . 6 - 8 comprises a pH-measuring electrode generally designated by 16 and mounted in a central hole of the aluminum body 8.

The embodiment shown in figs . 6 - 8 comprises a silver body 80 which serves as the reference electrode for the pH-measuring electrode 16 and simultaneously as the anode or reference electrode for a platinum cathode 62 for the Po_? measurement. In the embodiment shown in figs . 6 - 8 , the slightly concavely curved end surface of the silver body 80 is provided with an optional coating or layer 59 of a noble metal, preferably gold. An uncoated area 60 provides an exposed area of the silver body 80 contacting the electrolyte solution. The numeral 65 designates the front of a platinum cathode projecting to the active surface of the electrode measuring device . The platinum cathode comprises a platinum wire 62 of a diameter of 20 μm embedded in an insulating tubing 63 and connected to a single core 64 of the multicore cable 5 extending through the tube stub 4 also shown in figs . 1 and 2. The numeral 69 designates the front of a compensation electrode projecting to the active surface of the electrode measuring device . The compensation electrode comprises a platinum wire 66 of a diameter greater than the diameter of the platinum cathode wire 62, e . g. , 200 μm. Like the wire 62 embedded in the insulating tubing 63, the platinum compensation electrode wire 66 is also embedded in an insu-lating tubing 67. Furthermore, the platinum wire 66 is connected to a single core 68 of the multicore cable 5.

A suitable electrolyte for this embodiment of the electrode device is, e.g. 75 - 100% ethylene glycol with 50 mM KHCO₃ and 100 mM KCl, with an 1 mil polytetrafluoroethylene membrane.

Claims

Claims :

1. An electrode device for measuring the partial pressure of oxy¬gen and of a gas which in aqueous solution generates an acid or a base, comprising

a body,

a first electrode (cathode) capable of electrochemically reducing O₂ to generate hydroxyl ions ,

a second electrode (reference electrode or anode) communicating with the cathode, through means for biasing said first electrode relative to said second electrode,

a third electrode (pH electrode) responsive to pH changes produced by the presence of the gas which in solution generates an acid or a base,

a fourth electrode (reference electrode) cooperating with the pH electrode,

an electrolyte in contact with the above-mentioned electrodes ,

a membrane defining, in conjuntion with the said body, an electrolyte chamber in which said electrolyte is present and is in contact with the said electrodes , said membrane being gas-permeable and hquid-impermeable,

a fifth electrode (compensation electrode) being in contact with said electrolyte in said electrolyte chamber, and

compensation means electrically connected to said second electrode and to said fifth electrode for compensating for a production of hydroxyl ions at said first electrode by forcing current through said fifth electrode to make the fifth electrode electrochemically consume hydroxyl ions in an amount stoichiometrically equal to the amount of hydroxyl ions generated at said first electrode

2. An electrode device as claimed in claim 1, in which said second electrode and the said fourth electrode are constituted by a single electrode (reference electrode) .

3. An electrode device as claimed in claim 1 or 2 in which the compensation means comprise an operational amplifier, an inverting input of the said operational amplifier being connected to the reference electrode, an output of said operational amplifier being connected to the compensation electrode and a non-inverting input of the said operationsal amplifier being connected to the cathode through the said biasing means .

4. An electrode device as claimed in any of the preceding claims in which the means for biasing the cathode relative to the reference electrode comprise an operational amplifier and a voltage source, the cathode being connected to an inverting input of said operational amplifier, and a terminal of said voltage source being connected to a non -inverting input of said operational amplifier.

5. An electrode device as claimed in claim 2 in which the reference electrode is a silver or silver/silver chloride electrode .

6. An electrode device as claimed in any of the preceding claims in which the cathode is of a noble metal such as platinum or gold.

7. An electrode device as claimed in any of the preceding claims in which the pH-sensitive electrode is a pH glass electrode, the interior electrolyte of which is substantially filled with bodies of a metal showing high heat conductivity .

8. An electrode device as claimed in claim 7 in which the bodies of metal are constituted by a structure defining channels containing the interior electrolyte, such as sintered silver.

9. An electrode device as claimed in any of the preceding claims in which the compensation electrode is made of a conductive material selected from the group consisting of e. g. platinum and aluminium which is optionally anodized.

10. A method for determining the partial pressure, in a hquid or a gas mixture, of oxygen and of a gas which in aqueous solution generates an acid or a base, comprising determining the partial pressure of oxygen by determination of the current consumed in reducing oxygen at a cathode contacting an electrolyte solution which, through a gas-permeable membrane, is in communication with the said liquid or the said gas mixture, determining the partial pressure of the gas which in aqueous solution generates an acid or a base, typically carbon dioxide, by determining pH in the said electrolyte solution, and compensating for any influence on the pH of the said electrolyte solution caused by the generation of hydroxyl ions at the said cathode by substantially stoichiometrically consuming hydroxyl ions from the said electrolyte solution by electrochemical oxidation at a compensation electrode, the current to which is controlled in dependency of the generation of hydroxyl ions at the said cathode.

11. A method as claimed in claim 10 for transcuta neously determining the partial pressure of oxygen and carbon dioxide in blood, comprising applying, to a skin area, an electrode device, thermostated at a temperature above the skin temperature to generate local hyperemia at the application site, the electrode device comprising an electrolyte solution contained in a reservoir placed behind an oxygen- and carbon dioxide-permeable membrane mounted on the side of the electrode device contacting the skin and a polarographic oxygen-reducing cathode and a pH sensitive potentiometric electrode in electrolytical communication with the said electrolyte, determining the oxygen partial pressure by determination of the current consumption of the polarographic electrode and determining the pH of the electrolyte solution, indicative of the carbon dioxide partial pressure in the blood, by determination of the potential between the pH sensitive electrode and a reference electrode, and compensating for any influence, on the pH measurement, of hydroxyl ions generated at the polarographic oxygen- reducing cathode by forcing a current through a compensation electrode in electrolytic communication with the said electrolyte, the size of the said current being determined in dependency of the current involved in the reduction of oxygen at the polarographic oxygen-reducing cathode to substantially stoichiometrically consume hydroxyl ions generated at the said cathode .