Amperometric apparatus, cell and method for determination of different gaseous species.

0001This invention relates to an amperometric method for quanti­tative determination of a first electroactive and normally gaseous species (termed "EAGS"), such as elemental oxygen, and of a second electroactive and also normally gaseous species (the second "EAGS"), such as elemental hydrogen, in a normally fluid medium that may be gaseous or liquid.

0002The terms "normally gaseous" and "normally fluid" are used herein to indicate that the substance or phase is gaseous or fluid when at normal temperatures and pressures of typically 20°C and one bar. The term "elemental" is used to include the said elements in a molecular or atomic state but the invent­ion is not restricted to elemental EAGS since a number of normally gaseous electroactive compounds, such as nitrous oxides, may be of interest as one of the EAGS in the invent­ive method.

0003The normally fluid medium may be a mixture of gases, such as air, or a process gas containing the first and the second EAGS, and may be at reduced, normal or elevated pressures and at temperatures within a typical range of from about -10°C to about 95°C; alternatively, the fluid medium may be a liquid, such as water that contains both the first and the second EAGS in a dissolved state.

0004Reliable measuring or monitoring methods for determining the concentration of elemental hydrogen in the presence of oxygen are of importance wherever hydrogen is generated or used, e.g. in connection with electrolytical processes or when producing or reacting hydrogen in petrochemical or other operations be­cause a gas mixture in which the fractions of oxygen and hydr­ogen exceed certain threshold limits of typically about 4 % by volume may become explosive and may be dangerously unstable; this applies to mixture of hydrogen and chlorine as well.

0005Determination of nitrous oxide in the presence of oxygen is another important problem within the ambit of the present in­vention.

0006Another problem that can be at least as critical as preventing formation of explosive gas mixtures is corrosiveness of water that is subjected to water-decomposing (i.e. oxygenolytic and hydrogenolytic) conditions such as, typically, exposure to radiolytic, e.g. neutronic radiation. For example, the water used in water-moderated nuclear reactors is subject to radio­lytic generation of oxygen and hydrogen but since the latter is relatively more "volatile" in such systems the oxygen con­centration tends to increase and this, in turn, can be a cause for the water becoming corrosive to metallic components in con­tact therewith. One of the most effective methods to prevent such corrosiveness is the addition of elemental hydrogen in well-controlled quantities so that the hydrogen will act as a scavenger for the elemental oxygen.

0007Of course, amperometric methods for determining such EAGS indi­vidually, i.e. not in the same fluid medium or only after chem­ically or otherwise separating one from the other, are con­ventional, and membrane-enclosed amperometric cells (also term­ed "MEACs" for short) have been used for detection of either oxygen and hydrogen, notably since a simple and reliable hydr­ogen sensing method and apparatus have been disclosed by Applicant (cf. EP - A - 124 818).

0008However, MEACs have not been suitable for determining such EAGS as typically hydrogen in the presence of oxygen, or hydro­gen in the presence of chlorine, nitrous oxides in the pres­ence of oxygen etc. because of the amperometric and other inter­ference phenomena between the species of interest. On the other hand, amperometric methods using MEACs have been developed to the point where they provide one of the safest and most prec­ise means for quantitative analyses, particularly in prolong­ed or continuous operation and under severe ambient conditions as is the case in many industrial MEAC applications of prime importance. While it is possible, with prior art methods, to determine oxygen in the presence of hydrogen (while not meas­uring the latter), it has not been possible so far to reliably and simply determine hydrogen in the presence of oxygen by amperometric or other electrochemical methods.

0009Accordingly, it is a main object of the invention to provide for an amperometric method capable of reliable yet simple quantitative determination of a first EAGS, such as elemental oxygen, and of a second EAGS, such as hydrogen, when both are present in one and the same fluid medium as explained above, while the medium contains the first species in a first con­centration and the second species in a second concentration.

0010The method according to the invention is characterized by contacting the medium with a first and selective working el­ectrode that is sensitive but to the first electroactive spe­cies to obtain a first amperometric signal that is indicative of the first concentration; contacting the medium with a sec­ond and non-selective working electrode that is sensitive both to the first electroactive species as well as to the second electroactive species for obtaining a second amperometric signal that is indicative of a sum of the first concentration and the second concentration; and determining the second con­centration of the second electroactive species from the differ­ence between the first amperometric signal and the second am­perometric signal.

0011Preferably, the medium is in "simultaneous", i.e. spatially close contact with the first and the second working electrode but this is not critical; according to a first embodiment, the inventive method can be worked with the two electrodes as the sensing electrodes of two different MEACs arranged at differ­ent positions in a conduit through which the medium is passed such that no appreciable differences of essential conditions, notably the concentrations of the EAGS, will be encountered at the differing positions of the MEACs; according to a second embodiment, the inventive method is worked with a new integrat­ed type of MEAC which comprises both working electrodes and at least one counter electrode.

0012It has been found according to the invention that a satisfact­ory selectivity of the first working electrode but to the first species can be obtained if that working electrode (e.g. the cathode when oxygen is the first species) is made of an "amperometrically selective" metal as explained in more detail below, e.g. gold, silver, copper, or an alloy of these metals or even stainless steel and similar metals when oxygen is the first EAGS. Obviously, it is but the active or sensing sur­face of the working electrodes that need consist of such a selective metal. The second working electrode, or active sur­face thereof, is made of an "amperometrically unselective" me­tal as will be explained in more detail below, typically plat­inum, a platinum-group metal (Os, Ir, Pd, Rh, Ru) when the sec­ond EAGS is hydrogen.

0013According to a preferred embodiment the inventive method is used for monitoring hydrogen concentrations in a gaseous me­dium so as to indicate, and preferably prevent, formation of a mixture known to be explosive.

0014According to another preferred embodiment the inventive me­thod is applied for monitoring corrosion parameters, notably a relative deficiency of elemental hydrogen in the presence of elemental oxygen, in an aqueous medium that is exposed to oxygenolytic and, specifically, radiolytic conditions, such as typically an aqueous moderator or processing liquid expos­ed to radiolytic effects, such as typically in the core of a nuclear reactor.

0015The invention further provides an apparatus for quantitative determination of a first EAGS and a second EAGS in a gaseous or liquid medium simultaneously containing both the first and the second EAGS; the apparatus according to the invention comprises a first membrane-enclosed amperometric cell (MEAC) having a working electrode that is selectively sensitive but to the first EAGS and a sensing face arranged within a con­tainment means, e.g. conduit or container, for holding or pass­ing the medium; the first MEAC is capable of producing an amp­erometric signal that is indicative of the first concentr­ation; a second MEAC having a working electrode that is sens­itive to both the first and the second EAGS; the second MEAC has its sensing face arranged within the containment means as well and is capable of producing an amperometric signal that is indicative of the sum of the first and the second concentr­ation; further, the inventive apparatus includes a means for receiving the first selective amperometric signal and the second summary amperometric signal and for generating an out­put signal that is indicative, at least, of the second con­centration.

0016The invention also comprises an integrated cell for quantitat­ive determination of a first and normally gaseous electroact­ive species and of a second electroactive and normally gaseous species in a fluid medium containing said first species in a first concentration and said second species in a second con­centration, said cell comprising a first working electrode that is selectively sensitive but to said first electroactive species and is capable of producing an amperometric signal that is indicative of said first concentration; a second working el­ectrode that is sensitive to both said first and said second electroactive species and is capable of producing an ampero­metric signal that is indicative of a sum of said first and said second concentration; at least one counter electrode for cooperation with at least one of said working electrodes, said working electrodes and said counter electrode being arr­anged within an electrolyte space for electrolytically conn­ecting said working electrodes and said at least one counter electrode by means of an aqueous electrolyte; and a semi-perme­able membrane covering said electrolyte space and permitting passage of said first and said second gaseous species but sub­stantially preventing passage of said aqueous electrolyte.

0017Again, a preferred cell has a first sensing electrode that is selective, i.e. sensitive but to the first EAGS, such as oxy­gen in the presence of hydrogen and then is made of Au, Ag, Cu, an alloy thereof, or stainless steel, and a second sensing electr­ode that is non-selective and sensitive both to the first and the second EAGS, e.g. oxygen and hydrogen, and for these EAGS is made of a platinum-group metal (including platinum) or an alloy thereof.

0018The invention will now be explained in more detail with refer­ence to the enclosed drawings in which: <ul id="ul0001" list-style="none"><li>Fig. 1 is a diagrammatic illustration of an apparatus for carrying out the novel method;</li><li>Fig. 2 is a diagrammatic illustration of a novel cell for use in the inventive method; and</li><li>Fig. 3 is a diagram of a hydrogen/oxygen determination run according to a preferred example set forth in detail below and showing the hydrogen concentration in per­cent by volume on the ordinate and the oxygen con­centration, again in percent by volume, on the abs­cissa for a series of testing fluids.</li></ul>

0019The diagram of Fig. 1 shows a first MEAC 1 of a generally con­ventional structure as disclosed, for example, in US - A - 4 096 047 having a sensing face 10 exposed to a fluid medium provided within enclosure 3, a working electrode (e.g. cath­ode) 11 being made of Au, Ag, Cu or an alloy of these metals, or having its active surface 110 made of these metals. A coun­ter electrode (e.g. anode) 15, e.g. of the conventional Ag/Ag­Cl-type is arranged within the electrolyte space 112 contain­ing a conventional electrolyte (not shown) while a constant DC-voltage is maintained between the counter electrode and the working electrode in a manner known per se. Additional electr­odes, e.g. guard electrodes for the sensing or working electr­ode may be used but are not shown. The sensing face 10 of MEAC 1 is provided by the surface of a conventional semi-permeable membrane, i.e. one that is substantially impermeable to the el­ectrolyte but is permeable to the first EAGS, but a masked sen­sor as disclosed in EP - A - 205 399 is suitable as well and may be preferred if no guard electrode is to be used.

0020The second MEAC 2 has a structure similar to that of MEAC 1 except that the working electrode 21, or its active surface 210, is made of platinum, a platinum metal or an alloy of platinum group metals (platinum being considered to belong to the platinum group). The sensing face 20 of MEAC 2 again is a semi-permeable membrane of the type known (e.g. as dis­closed in EP - A - 124 818) and the sensing face 20 is expos­ed to the medium within enclosure 3. As shown, enclosure 3 may be provided with an inlet 31 and an outlet 32 both of which could also be the "ends" of a tubular enclosure 3 if the latter is a conduit or the like fluid-passing means. In other words, no particular diameters are critical for enclos­ure 3 and/or its feeding ends.

0021Leads 28 are provided between working electrode 21 and counter electrode 25 to carry the amperometric signal generated at the working electrode 21 by reaction of both the first and the sec­ond EAGS to control instrument 8. The resulting signals may then be evaluated by an operator or fed, via conduits 79 and 89, to an evaluating device that converts the signals from in­struments 7, 8 into a readout, monitoring signal, printer or the like output indicated schematically as output 91. A result obtained by a human operator from the readings of instruments 7 and 8 would also be considered as an "output 91".

0022As diagrammatically shown in Fig. 2, the two MEACs 1, 2 of Fig. 1 can be integrated to form a new cell structure 4 com­prising a first selective (e.g. for oxygen in the presence of hydrogen) working electrode 41 being made, or having an EAGS-­exposed surface 411 made, for example of Au, Ag, Cu, alloys thereof, or stainless steel, and a second non-selective work­ing electrode 42 being made, or having an EAGS-exposed surfa­ce 421 made, for example, of Pt, another metal of the platin­um group, or an alloy thereof.

0023At least one counter electrode 45 made, for example, of Ag/AgCl or another counter electrode material disclos­ed in the above mentioned US - A - 4 096 047, is provided with­in jacket 46 that encompasses an electrolyte space 44 covered by semi-permeable membrane 40 that permits passage of both EAGS contained in medium 43 but substantially prevents pass­age of the aqueous electrolyte that is contained in space 44.

0024A single counter electrode 45 may be used in which case it will serve as the counter electrode for, and cooperate with both, sensing electrodes 41, 42 to produce first and second ampero­metric signals 491, 492 as outputs from control instruments 47, 48 of a generally conventional type for use with MEACs.

0025Instrument 47 is connected with the first sensing electrode 41 and the counter electrode 45 via leads 451, 471, and the ampero­metric signal 471 produced by the first sensing electrode 41 with counter electrode 45 will be indicative but of the con­centration of the first EAGS contained in the fluid medium 43 which, in turn, is optionally included by a container, conduit or the like containment means 431.

0026The amperometric signal 492 produced by the second sensing el­ectrode 42 together with counter electrode 45 is indicative of the sum of the concentrations of both the first and the second EAGS in medium 43.

0027Since only one counter electrode 45 is used in the MEAC struct­ure 4 of Fig. 2, each instrument 47, 48 will have an input lead 451, 452 connected with the one and only counter electrode 45 and an input lead 471, 472 from the first and the second sensing electrode, respectively. However, it is within the scope of the invention that each sensing electrode of an inte­grated cell as exemplified by Fig. 2 has a separate counter electrode (not shown).

0028Further, both in the separate cells 1, 2 of Fig. 1 as well as in the integrated cell 4 of Fig. 2, each sensing electrode and counter electrode could consist of several segments or port­ions, and guard electrodes or physical guards could be used in a manner known per se for all sensing electrodes.

0029Again, in Fig. 2, the outputs 491, 492 from instruments 47, 48 can be evaluated by a human operator by an instrument 49 to produce, in either case, an output or signal 499 that is ind­icative of the difference of the amperometric signals produced by the two sensing electrodes and may, but need not, indicate the concentration of the second EAGS, or a derived parameter, such as a threshold value for indicating that explosive or corrosive conditions as explained above are approached and/or for triggering control or alarm signals to prevent that such conditions prevail.

Example 1

0030For the embodiment of the invention where the EAGS contained in enclosure 3 (or 43) are oxygen (first EAGS) and hydrogen (second EAGS) - typically in mixture with inert gases such as nitrogen - and the selective working electrode surface 110 (or 411) is made of gold while the working electrode surface 210 (or 421) is made of platinum, electrode/EAGS-reactions are as follows:

0031When using an alkaline electrolyte, oxygen reacts at the sel­ective electrode 11 or 41 or its surface 110 or 411 as fol­lows:     O2 + 2H2 O + 4e → 4 OH⁻      (1) At the non-selective platinum electrode 21 or 42 or its surfa­ce 210 or 421 the above reaction occurs as well but hydrogen reacts as     H2 → 2H⁺ + 2e      (2) Accordingly, the non-selective platinum electrode or surface reacts simultaneously as a cathode in the oxygen reaction and as an anode in the hydrogen reaction. Since the electron flow through the platinum/electrolyte-interface is in opposite dir­ections in the two reactions, the partial currents associated with the two reactions will cancel each other so that there will be a difference between the magnitudes of the electric currents measured with the two sensing electrodes. By combin­ing these two currents the partial pressure of O₂ and H₂ in the sample stream can be calculated as explained in detail be­low.

0032For simplicity, the sensors or sensing electrodes are assumed to operate in the manner of conventional MEACs or their resp­ective working or sensing electrodes, that is to say, the magn­itude of the current measured is limited by the rate of diff­usion of the electroactive gases from the sample through the membrane to the electrode surface. In the case of electrode 21 or 42 this means that both reactions specified above must pro­ceed very rapidly at the electrode surface 210 or 421, respect­ively. The conventional relation between current and partial pressure of the EAGS in a MEAC may be written as follows:<maths id="math0001" num=""><img file="EP0293541A1_D0001.tif" /></maths> If the current is measured in microamperes and the partial pressure in kilopascals, the sensitivities will have units of microamperes per kilopascal. The symbol "i" is used for curr­ent, "S" for sensitivity and "p" for partial pressure; then, the relation may be written as follows:     i = S · P      (4) Generally, S will be dependent upon the temperature and the membrane. This relation is applied to the elective gold el­ectrode surface with the following result:     i_g = S_g,O2 · p_O2 Checksum = (x_O2/y_O2) + (x_H2/y_H2) = 100 %      (5) At the non-selective platinum electrode both of the cathodic and anodic react­ions may be described in this way, so that the net current be­comes: x_O2=100·p_O2/b = S_p,O2 · p_O2 - S_p,H2 · p_H2      (6) In order to use these relations for determination of the parti­al pressures of oxygen and hydrogen, the sensitivities S_g,O2, S_p,O2 and S_p,H2 must be determined. This is done by the follow­ing calibration procedure: First, both MEACs 1 and 2 are, or MEAC 4, is, exposed to a gas stream contain­ing a known fraction Y_O2 of oxygen, but no hydrogen. Assuming that the stream passes the sensors at atmospheric pressure B, then the partial pressure P_O2 in this calibration medium by Dalton's law is:     P_O2,1 = y_O2 · B      (7) It follows from equation (5) that     S_g,O2 = i_g,1 / p_O2,1 = i_g,1 / (y_O2 · B)      (8) Furthermore, since p_H2 = 0 in this calibration medium, it fol­lows from equation (6) that     S_p,O2 = i_p,1/p_O2,1 = i_p,1/(y_O2 · B)      (9) Subsequently, MEACs 1 + 2 or 4 are exposed to a mixed gas stream containing known fractions of O₂ and H₂ in order to de­termine S_p,H2. This stream is conveniently prepared by bleeding into the O₂-containing stream used for O₂ calibrating a sec­ond stream containing a known fraction of H₂, y_H2 but no oxygen.

0033Indicating by i_g,2 and i_p,2 the currents measured at MEACs 1 + 2 or 4 in this mixed stream: since S_g,O2 is known, the O₂ parti­al pressure in the stream can be calculated from equation (5)     p_O2,2 = i_g,2 / S_g,O2      (10) Hence, the contribution to the total pressure represented by the original O₂ containing stream will be     p_O2,2 / Y_O2 = i_g,2 / (y_O2 · S_g,O2)       (11) and that of the hydrogen containing stream will be:     B - i_g,2 / (y_O2 · S_g,O2)      (12).

0034From this, the hydrogen partial pressure of the mixed stream results as:     p_H2 = y_H2 · {B - i_g,2 / (y_O2 · S_g,O2)}       (13).

0035Returning to equation (6), the unknown term S_p,H2 can now be calculated as follows: S_p,H2 = {S_p,O2·i_g,2 / S_g,O2 - i_p,2} {y_H2·[B - i_g,2/y_O2·S_g,O2]}      (14).

0036This completes the calibration procedure.

Example 2

0037The MEACs 1 and 2 were standard probes manufactured by Orbis­phere Laboratories Geneva, i.e. Model 21152 having a gold cathode, and Model 2230 having a platinum electrode. The elect­rolyte used in both sensors contained 1 mol/lit potassium chloride and 1 mol/lit potassium hydroxide. A membrane TEFZEL (registered trademark DuPont) having a thickness of 25,4 micro­meters was used on the platinum sensor (MEAC 2) and TEFZEL of 12,7 micrometers on MEAC 1. The voltage applied to both sens­ors was -0.6 volts (platinum or gold negative) versus the sil­ver/silver chloride reference electrodes 15, 25.

0038Oxygen calibration was performed in pure O₂ (y_O2 = 1). It was found that     S_g,O2 = 0.625 µA/kPa and S_p,O2 = 0.291 µA/kPa      (8-1) The mixed O₂ and H₂ containing stream was prepared by bleeding pure H₂ (y_H2 = 1) into the oxygen. Very low flow rates of only several ml/min were used. Then it turned out that:     S_p,H2 = 0.494 µA/kPa      (14-1).

0039Once calibration had been completed, the fractions of O₂ and H₂ in the mixed stream were varied and the percentages of the con­stituent gases were calculated by application of equation (5) as:     x_O2 = 100·_p<img file="EP0293541A1_D0002.tif" /> / B = 100 · i_g / B · S_g,O2      (15) and     x_H2 = (100 / S_p,H2 · B) {S_p,O2 · i_g / S _g,O2 - i_p}      (16).

0040The correctness of the measurement was checked in two ways. First, since the mixture is made from two component streams there should be a constant checksum:     Checksum = (x_O2 / y_O2) + (y_H2 / y_H2) = 100 %       (17).

0041This was verified for all mixtures and the result is illustrat­ed in Fig. 3. The rule is obeyed within 1 % for all mixtures (0-35 % H₂, 100-65 % O₂) tested.

0042Secondly, combining equations (15), (16) and (17), it results that: B·y_H2 · S_p,H2+i_p = (i_g/S_g,O2)· {(y_H2·S_p,H2 / y_O2) + S_p,O2}      (18) In the series of mixtures studied B,y_O2, y_H2, S_p,O2, S_g,H2 and S_p,O2 were all constant. Hence, the equation (18) is that of a straight line     i_p = a + b · i_g.      (19) The correctness of this relation was also tested and found to be valid.

Example 3

0043In equations (5) and (6) different expressions were written for the currents measured at the gold and platinum electrodes 11, 12, respectively, even though assumed to be exposed to the same medium containing both oxygen and hydrogen. This co­mes about because the non-selective platinum electrode of MEAC 2 or 4 permits virtually instantaneous reaction of both hydrogen and oxygen at its surface 210/421, while the select­ive gold electrode 11 or 41 permits reaction but of oxygen. Thus, one can write an equation similar to (6) instead of (5) for the current at the gold cathode sensor but employing a sensitivity term S_g,H2 which is practically zero. In order to quantify the difference between the gold and platinum electr­odes 11 or 41 and 21 or 42, a suitable equation was found as follows:     i = n F A_p,H2 / [(1/k_eS_e) + (z_m/D_mS_m)]      (20) and this supports the viability of the physicochemical basis of the inventive method.

0044The symbols used in equation (20) are defined as follows: n = 2 = number of electrons participating in the oxidation electrochemical reaction of the second EAGs, e.g. hydr­ogen; F = 96,500 Coulombs per mole, the Faraday constant; A = the area of the working electrode; P_H2 = the partial pressure of hydrogen in the medium; k_e (cm/sec) = the rate constant of the hydrogen oxidation reaction; S_e = the solubility of hydrogen dissolved in the electrolyte at the electrode surface; z_m = the thickness of the membrane; D_m = the diffusion coefficient of hydrogen in the membr­ane; and S_m = the solubility of hydrogen in the membrane.

0045The given equation is best understood in terms of an electric­al analog: by Ohm's law, the observed current "i" equals the voltage applied divided by the resistence to current flow en­countered. The numerator of the equation is then analogous to the voltage, and the denominator analogous to the resist­ance. The numerator of equation (20), i.e. the resistance term     R = (1/k_eS_e) + (Z_mD_mS_m)      (21) is of main interest. Its first term on the right hand side can be designated as the "resistance of the electrode reaction" and the second term can be designated as the "resistance of the membrane". It should be noted that this "resistance" is not the "electrical" resistance but rather the resistance to transport of the EAGS, e.g. hydrogen.

0046Hence, the difference between the selective and non-selective sensing electrodes used in the invention may be expressed as follows: The "resistance" of the hydrogen oxidation reaction on electr­ode 21 or 42, e.g. having a platinum surface 210, is negli­gible in comparison with the "resistance" of the membrane 20 or 40, e.g. amounts to one percent of the latter "resist­ance" or less.

0047In contrast, the "resistance" of the electrode reaction on el­ectrode 11 or 41, e.g. having a gold surface 110 or 411, exceeds by far the "resistance of the membrane 10 or 40, e.g. is a hundred times or more greater than the latter "resistance".

0048In practice, the magnitude of the membrane "resistance" can be determined, according to the present invention, because the term (1/k_eS_e) is negligible for the non-selective electrode 21 or 42 (see equation 20) and the current delivered may be written as:     FA D_m S_m p_H2 / z_m      (22) Equations (13) and (14) provide for determination of the term S_p,H2 which, by comparison, is:     S_p,H2 = F A D_m S_m / z_m      (23) Since the area A of the sensing electrode is known or can easily be determ­ined, it is possible to calculate directly the membrane resist­ance term (z_m/D_mS_m) and for Example 2 S_p,H2 was determined as 0.494 µA/kPa since "A" of MEAC 2 was 0.3142 cm². Hence, the resistance term is as follows:     (z_m/D_mS_m) = 1.272 · 10 cm²·kPa·S / mol.

0049With regard to MEAC 1 exposed to the same partial pressure of the second EAGS, e.g. H₂, tables can be used to determine the solubility thereof in its electrolyte; for hydrogen in pure wa­ter the solubility is approximately 7.5 · 10⁻⁹ mol/(cm³ · kPa) at 25°C. The presence of salt in the electrolyte normally low­ers the solubility in comparison with pure water but the ef­fect is not large with normally used electrolyte compositions. Hence, the threshold magnitude of the electrode reaction rate constant can be calculated according to the invention as follows:     k<maths id="math0002" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>c</mtext></mrow><mrow><mtext>e</mtext></mrow></mfrac></mrow></math><img file="EP0293541A1_D0003.tif" /></maths> = D_m S_m / z_m Se = S_p,H2 / n FAS _e      (24) and this permits classification of electrode metals into those suitable for non-selective electrodes of the non-selective el­ectrode, i.e. sensitive to both the first and the second EAGS.

0050In the one class     k_e » k<maths id="math0003" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>c</mtext></mrow><mrow><mtext>e</mtext></mrow></mfrac></mrow></math><img file="EP0293541A1_D0004.tif" /></maths>      (24-1) while in the other class     k_e « k<maths id="math0004" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>c</mtext></mrow><mrow><mtext>e</mtext></mrow></mfrac></mrow></math><img file="EP0293541A1_D0005.tif" /></maths>      (24-2).

0051Reaction rate constants k_e are not always tabulated while the "exchange current density" for the electrode reaction is given more frequently. The exchange current density i_o is defined as the partial current for oxidation of hydrogen at the equilibri­um potential (where the net current is zero), when the activi­ty of hydrogen is unity (p_H2 = 100 kPa). Thus:     i_o = 2 F k <maths id="math0005" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>o</mtext></mrow><mrow><mtext>e</mtext></mrow></mfrac></mrow></math><img file="EP0293541A1_D0006.tif" /></maths> S_e (p_H2 = 100 kPa)      (25) Where k<maths id="math0006" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>o</mtext></mrow><mrow><mtext>e</mtext></mrow></mfrac></mrow></math><img file="EP0293541A1_D0007.tif" /></maths> would be the value of the rate constant introduced earlier at the equilibrium potential. The rate constant varies with electrode potential in an exponential manner:     k_e = k_e exp (-η/ b)      (26) wherein η = V-V_o is the "overpotential" of the electrode, V is the electrode potential and V_o is the equilibrium potential b call­ed the Tafel constant (J. Tafel, Z. Physik. Chem. <u style="single">50</u>, 641, 1905) which determines the steepness of the variation of rate constant with electrode potential.

0052Combining equations (24) and (28) the criterion for classific­ation of electrode metals in terms of the exchange current can be expressed as:     i_o exp (η / b) = S_p,H2 / A      (27).

0053From the experimentally determined magnitude of S_p,H2/A and the chosen value of the overpotential η applied to the sensor, a critical or threshold value of the exchange current density can be evaluated for various possible values of the Tafel con­stant b. Tables can then be consulted to determine which met­als (having i_o » i<maths id="math0007" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>c</mtext></mrow><mrow><mtext>o</mtext></mrow></mfrac></mrow></math><img file="EP0293541A1_D0008.tif" /></maths>) would be suitable for sensing both the first and the second EAGS.

0054For example, in the case of the preferred determination of hydrogen in the presence of oxygen according to the invention, a recent tabulation can be found in Chapter IX 2-3, "Hydrog­en", by A. J. Appleby, H. Kita, M. Chemla and G. Bronoël, in "Encyclopedia of Electrochemistry of the Elements", edited by A. J. Bard, Volume IXA, Marcel Dekker, New York 1983, incorpor­ated herein by reference.

0055Preferred electrodes for the selective sensing electrode 11 or 41 should have a sufficiently high exchange current for reduction of the first EAGS, e.g. oxygen, such that the rate of this process will be limited by the diffusion rate of the first EAGS through the membrane.

0056Various modifications of the above described specifics, e.g. regarding currents, electrolytes and suitable MEAC materials are apparent to one skilled in the amperometric art. For ex­ample, wherever "hydrogen" or "oxygen" or "chlorine" are men­tioned herein, any isotopes thereof (deuterium, tritium etc.) are encompassed because the electrochemical behaviour depends upon the election shells of the elements in question. Further, "H2" and "O2" stand for "H₂" and "O₂" but so simplify print­ing.