Process and device for continuously detecting at least one substance in a gaseous or liquid mixture by means of a sensor electrode

Processes and devices are disclosed for continuously detecting at least one substance in a gaseous or liquid mixture by means of a sensor electrode to which is applied a variable potential. In an alternative, the substances are concentrated at the surface of the sensor electrode, their concentration is determined by measuring the electrode capacity, the thus obtained measurement value is correlated with the substance and the substance. concentrated at the surface of the sensor electrode is then removed. In another alternative, one or several detection cycles are carried out. In each detection cycle, at least one substance is concentrated at the surface of the sensor electrode, the potential is brought to at least one potential characteristic of the electrochemical reaction of at least one substance, the resulting current is measured and the thus obtained measurement values are correlated with the substances.

FIELD OF THE INVENTION
 The invention is concerned with a method and with a device for continuously
 detecting at least one substance in the gaseous or liquid mixture by means
 of a sensor electrode to which a variable potential is applied.
 BACKGROUND OF THE INVENTION
 The demand for a problem-free, rapid and cost-effective detection method
 for substances, especially for harmful substances, has increased recently,
 one of the reasons being the increased demands for environmental
 protection. In order to comply with harmful-substance limiting values (for
 example, in controlling and reducing emissions) it is necessary to be able
 to detect harmful substance concentrations in gaseous or liquid media
 reliably and continuously, possibly by the use of electrochemical sensors.
 In process monitoring, too, the control of the concentration of products,
 starting materials and impurities may be necessary for optimum performance
 of the process. However, rapid detection is made difficult by the fact
 that the materials or substances are frequently slow to react.
 Frequently, electrochemical detection is based on the amperometric
 principle and is aimed at the quantitative detection of a material
 component.
 For this purpose, substances (Cl.sub.2, HCl, SO.sub.2, NO.sub.x, H.sub.2
 CO, etc.) are reacted electrochemically, that is, by an oxidation or
 reduction reaction on a sensor electrode to which a constant potential is
 applied.
 The current flowing can be set in relation to the concentration of the
 substance to be detected. The selectivity of a sensor electrode based on
 this working principle is limited by the electrode material used and by
 the potential that can be applied to the electrode. The potential that can
 be applied is limited to a range of values at which the oxygen of the air
 is not reduced and/or the electrolyte for the substance to be detected is
 not decomposed. Namely, the currents produced by these perturbing effects
 would overlap the actual measured signal almost completely. Moreover, some
 substances are not sufficiently reacted in the available potential range
 or poison the sensor electrode by adsorption, so that they cannot be
 detected by this method. These substances include many unsaturated
 compounds, halogenated hydrocarbons and aromatics.
 Qualitative and quantitative electrochemical detection can be achieved by
 voltametric techniques. Here, the substance to be detected is not reacted
 at the electrode at a fixed potential. Rather, oxidation or reduction of
 the substance is catalyzed successively while a continuously varying
 potential is applied. The recorded relationship between the amount of
 charge passing through or current and applied potential can be correlated
 with the quantity and also with the nature of the substance to be
 detected.
 In another electrochemical detection method, called the alternating current
 method, an alternating voltage is superimposed on the voltage applied to
 an electrode. The alternating current flowing through is measured. The
 measured alternating current is shifted in phase with respect to the
 applied alternating potential, namely, because of the electrode
 capacitance, which is changed by the adsorption of the substance to be
 detected and also because of oxidation and reduction processes. Therefore,
 a complex resistance is defined, which is called impedance below, which
 describes the processes on the electrode surface appropriately. Its
 frequency-dependent and potential-dependent real and imaginary parts give
 information about the concentration of the substance to be detected.
 A detection method, which is similar to the alternating-current method is
 tensametry, known from the analysis for solutions (see for example
 Nurnberg et al., in Methodicum Chimicum, Volume 1/1, Stuttgart 1973).
 However, in such methods, the sample with the substance to be detected
 must always "be prepared" manually to some extent, that is, interfering
 impurities must be removed and the oxygen of the air must be excluded.
 Thus, continuous detection cannot be performed with these two known
 methods--alternating-current method and tensametry.
 In this connection, the determination of concentration of blood glucose is
 also known (Kasapbasioglu et. al., Sensors and Actuators B, 13-14 (1993),
 p. 749). Here, glucose is oxidized directly electrochemically to gluconic
 acid on a membrane-covered electrode made of a noble metal. The electrode
 functions as an electrocatalyst, to which a potential program that
 decreases and increases stepwise is applied. At each step, an alternating
 potential with a high frequency and one with a low frequency are
 superimposed onto the potential. The glucose concentration in the blood is
 determined from the resulting real and imaginary part of the impedance at
 certain potential steps.
 Furthermore, it is known that the selectivity and sensitivity of an
 electrode can be increased in an electrochemical detection method by
 utilizing the adsorption or absorption of the substance to be detected on
 the electrode surface. The adsorption or absorption can be supported,
 weakened or eliminated by the applied potential or potential program. The
 substance to be detected is adsorbed at a potential at which the substance
 is not electro-chemically active. The amount adsorbed as a function of
 time is then correlated with the concentration of the substance to be
 detected.
 A method is known from the technical journal "Sensors and Actuators B", Ege
 et al., 4 (1991), p. 519, with which the reactive carbon monoxide CO in a
 CO/H.sub.2 mixture can be detected quantitatively based on the
 amperometric principle. For the detection, first the carbon monoxide
 component is adsorbed on a platinum electrode and then reacted
 electro-chemically. The carbon monoxide is adsorbed specifically at a
 potential at which it is not electrochemically active or is not reacted.
 After adsorption of the carbon monoxide to the saturation value, the
 potential is increased to a value at which the carbon monoxide is
 oxidized. The amount of charge flowing during oxidation is measured and is
 integrated over the oxidation time. The measured signal thus obtained is
 correlated with the concentration of the carbon monoxide. However, the
 amount of charge flowing is additionally superimposed by amounts of charge
 stemming from the electrochemical reaction of additionally adsorbed
 substances, such as oxygen. This additional amount of flowing current is
 determined in another reference cycle, in order to correct the measured
 signal. In this reference cycle, a potential is applied over a very short
 period of time to adsorb the additionally adsorbed substances. The time
 period is made so short that the carbon monoxide is not adsorbed on the
 electrode surface of the sensor. Then the potential is brought to a
 suitable value for the electrochemical reaction of these additionally
 adsorbed substances. The amount of charge flowing during this
 electrochemical reaction is used as correction value, because it is
 influenced only by the additionally adsorbed substances. Minimum CO
 concentrations up to 0.05% CO can be detected in a CO/H.sub.2 mixture.
 However, this known method cannot provide continuous detection either.
 Moreover, there is no suitable sensor for the commercial utilization of
 this method of detection.
 Similarly, the reactive carbon dioxide, CO.sub.2, can be detected
 quantitatively in air at concentrations from 5% to 0.3% CO.sub.2 (Kuver et
 al.; J. Electroanal. Chem., 353 (1993), p. 255).
 It is also known that several substances can be detected quantitatively
 simultaneously with the aid of a chain of electrodes which mostly consist
 of different electrode materials. Different potentials are applied to the
 individual electrodes and one substance reacts electrochemically at each
 of these potentials. The measured signals obtained at the individual
 electrodes are correlated with the individual substance concentrations
 using pattern recognition technology.
 The electrochemical detection methods mentioned above are not suitable for
 rapid, continuous, both qualitative as well as quantitative detection, or
 are very expensive. Moreover, substances with low reactivity cannot be
 detected with these known detection methods or can only be detected at
 high concentrations.
 In general, known electrochemical detection methods are characterized by
 the fact that the selectivity is frequently too low. Similarly, most of
 the sensors based on these detection methods do not satisfy the general
 criteria of a sensor: The detection should occur rapidly and continuously
 without preparation of the sample "on location" with a time constant of
 the order of one or at most a few minutes. In addition, the sensor should
 operate in the ambient atmosphere, that is, generally in the presence of
 the oxygen of air and should also be cost-effective.
 The goal of the invention is to provide further methods and devices for
 continuous and quantitative as well as qualitative detection of substances
 in gaseous or liquid mixtures.
 SUMMARY OF THE DISCLOSURE
 In accordance with another aspect of the invention, a method is provided
 for the continuous detection of a substance in a gaseous or liquid
 mixture, with the aid of a sensor electrode to which a variable potential
 is applied, in which method the substance is enriched at the surface of
 the sensor electrode, the enrichment is determined with the aid of
 measurement of the electrode capacitance of the sensor electrode, the
 measured value thus obtained is correlated with the substance and then the
 substance enriched on the surface of the sensor electrode is removed. In
 accordance with another aspect of the invention, a device suitable for
 carrying out the above-described method is provided with means for
 carrying out the individual process steps listed above.
 The detection method described above is designated below as a modified
 alternating-current method. Here, the applied potential is preferably
 chosen in such a way that the substance is enriched on the electrode
 surface of the sensor without an electrochemical reaction. The ions of the
 electrolyte form a double layer at the sensor electrode. Together with the
 electrode surface of the sensor, this double layer acts as a type of plate
 capacitor. The degree of enrichment of the substance changes the
 capacitance of this plate capacitor or sensor electrode by the adsorbate
 blocking a part of the electrode surface. The capacitance of the sensor
 electrode can be followed with the aid of a suitable electronic
 measurement method and correlated with the concentration of the substance.
 This modified alternating current method is characterized by high
 sensitivity to changes in the structure of the double layer. Thus, it is
 especially suitable for simple qualitative and quantitative detection of
 small concentrations of surface-active substances, especially halogenated
 hydrocarbons and highly volatile organic solvents.
 First of all, it could be shown that the substance to be detected is
 adsorbed in spite of simultaneous reduction of the oxygen of the air (even
 when the concentration of the oxygen of the air is greater by a factor of
 10.sup.5 to 10.sup.6 or even more than the concentration of the substance
 to be detected) and, thus, the electrode capacitance--and hence also the
 concentration of the substance to be detected--can be detected with the
 reduction of oxygen occurring simultaneously.
 The enrichment of one or several substances present in a mixture can be
 influenced among others by the adsorption time and the applied adsorption
 potential. The adsorption time of a substance depends on different
 thermodynamic and kinetic properties of the various substances in the
 mixture. By suitable selection of the applied adsorption potential and
 other parameters (see below), the substance can be detected selectively in
 the mixture.
 With the modified detection method, the disadvantages of the known
 detection methods described above are avoided. The method is universally
 applicable, especially to substances with low reactivity.
 In a preferred embodiment, the electrode capacitance of the sensor
 electrode is determined by an impedance measurement. Preferably, a dc
 voltage with a superimposed low-frequency alternating voltage is applied
 to the sensor electrode for this purpose. The impedance of the sensor
 electrode or of the double layer is then determined based on the phase
 shift and the change of amplitude of the alternating current flowing as a
 result of the applied low-frequency alternating voltage.
 Preferably, the frequency of the low-frequency alternating voltage is
 optimized with reference to the impedance measurements.
 In a preferred detection method, the concentration of the substance is
 determined based on the difference of the electrode capacitance of the
 sensor electrodes, with and without enrichment of the substance on the
 electrode surface. Here, preferably in a first step, a dc voltage with a
 superimposed alternating voltage is applied to the sensor electrode as
 potential. The potential is chosen so that the substance is reacted
 electrochemically and therefore is not enriched. Thus, in this first step,
 the electrode surface is activated. Then the electrode capacitance is
 measured with the electrode surface being activated or free. In a second
 step, the potential is preferably brought to a value at which the
 substance to be detected is enriched at the electrode surface of the
 sensor and remains there essentially unreacted. Then the electrode
 capacitance is measured with the electrode surface being occupied. The
 concentration value of the substance to be detected is calculated from the
 difference of the electrode capacitances measured in the two steps.
 In an especially preferred detection method, the concentration of the
 substance is determined based on the change of the capacitance of the
 sensor electrode as a function of time. The change of the electrode
 capacitance as a function of time can be measured quasi-differentially--at
 successive points in time during the enrichment process--or as an
 average--measuring it at the beginning and at the end of the enrichment
 phase. It is proportional to the time change of the imaginary part of the
 alternating current caused by the applied low-frequency alternating
 voltage. This flowing alternating current can be measured by a simple
 technology. Thus, the time change of the electrode capacitance and the
 concomitant enrichment of the substance at the electrode surface can be
 followed advantageously almost continuously. This time change is then
 correlated with the substance concentration using known relationships, for
 example, calibration, which is carried out once during manufacture or at
 large time intervals.
 The parameters measured in this detection method--for example, enrichment
 time, enrichment potential and frequency of the applied alternating
 voltage--provide sufficient possible combinations for accurate, simple,
 qualitative and quantitative detection of substances in a gaseous or
 liquid mixture, especially at low concentrations.
 Preferably, in order to remove the substance enriched at the electrode
 surface, the potential is brought to a potential characteristic for its
 electrochemical reaction and/or desorption. For the purpose of continuous
 detection, it is necessary that the electrode surface of the sensor used
 be freed completely of the enriched substance from time to time. This can
 be done with the aid of the so-called oxidation cycles. Here, the
 electrode surface is cleaned and activated by appropriate selection of the
 applied potential by first electrochemically reacting and/or desorbing the
 enriched substance or by displacing it with adsorbed oxygen. Finally, the
 reacted substance or the adsorbed oxygen is desorbed completely, for
 example, by reducing the potential or by other methods.
 Furthermore, the enrichment is preferably ended when the measured electrode
 capacitance or the time change of the electrode capacitance of the sensor
 electrode reaches a predetermined value. Hereby, advantageously, the
 occupation of the electrode surface with the substance to be enriched is
 followed. Using a predetermined value, the relationship of the response
 time to the sensitivity of the sensor can be varied. The response time
 depends automatically on the time during which the substance to be
 detected is enriched on the electrode surface to an amount which is
 sufficient for the measured signal to be evaluated. With the predetermined
 value, thus the sensor performs the detection at maximum sensor
 sensitivity and minimum response time. The reactivity and sensitivity of
 the sensor can thus be optimized advantageously and especially simply.
 Preferably, the measured value thus obtained is optionally normalized using
 a measured value obtained through at least one other measurement of the
 electrode capacitance of the sensor electrode, in which step essentially
 not the substance to be detected, but only oxygen or hydrogen is enriched
 at the sensor electrode. Thus, for this normalization, the additional
 measured value can be a measured value that is obtained either in a
 separate measurement or--when measuring the time change of the electrode
 capacitance as an average--it can be the value measured at the beginning
 of the enrichment phase. With this normalization, the alterations, aging
 or wear phenomena of the electrode surface are advantageously taken into
 consideration and the measured signal is appropriately corrected. The
 normalization is based on the fact that adsorption of pure oxygen or
 hydrogen on the sensor electrode is influenced adversely by the quality of
 the sensor electrode surface to the same extent as the enrichment of the
 substance to be detected. Preferably, the measured value can also be
 normalized with the aid of the electrode capacitance.
 According to another aspect of the invention, a method is provided for the
 continuous detection of a substance in a liquid mixture with the aid of a
 sensor electrode to which a variable potential is applied, by enriching
 the substance on the surface of the sensor electrode, then bringing the
 potential to a potential value which is characteristic for the
 electrochemical reaction of the substance, measuring the current thus
 produced and correlating the obtained measured value with the substance.
 In accordance with another aspect of the invention, a device is provided
 for carrying out the method described above, with means which carry out
 the individual process steps listed above.
 The detection method described in the preceding paragraph will be called
 below the liquid-phase potential method. With this liquid phase potential
 method at least one substance is detected in a liquid phase. Similarly to
 the modified alternating-current method, it also uses the enrichment of
 the substance to be detected on an electrode surface to which an
 appropriate potential is applied. After enrichment, preferably, the
 enriched substance is oxidized or reduced at a potential characteristic
 for the electrochemical reaction. The current resulting from the
 electrochemical reaction is then correlated with the enriched substance
 and finally with the substance concentration in the liquid phase or
 solution to be investigated. With the aid of the enrichment, the local
 substance concentration near the sensor electrode is highly increased, so
 that the current flowing during the subsequent electrochemical reaction
 provides a larger measured signal. A sensor to be used in this
 liquid-phase potential method thus advantageously also detects substances
 with low reactivity, because this sensor is overall more sensitive than
 the known sensors. Furthermore, this sensor can be optimized through the
 parameters of enrichment potential and enrichment time.
 According to still another aspect of the invention, a method is provided
 for the continuous detection of at least two substances in a gaseous or
 liquid mixture with the aid of a sensor electrode to which a variable
 potential is applied, in which method at least two detection cycles are
 performed and at least one substance is enriched at the surface of the
 sensor electrode per detection cycle, after which the potential is brought
 to a potential characteristic for the electrochemical reaction of at least
 one substance and the current produced thereby is measured, and,
 subsequently, the measured values thus obtained are correlated with the
 substance. In accordance with another aspect of the invention, a device is
 provided for carrying out the method described above, with means which
 carry out the individual process steps listed above.
 The detection method described in the preceding paragraph will be referred
 to below as the general potential method. It serves for the detection of
 at least two substances in a gaseous or liquid mixture. For this purpose,
 the potential applied to the sensor electrode goes through a given
 potential program in each detection cycle.
 The potential program can be the following: first a potential is applied at
 which at least one of the substances is enriched at the electrode surface,
 whereby the time available for the enrichment is preferably variable. Then
 the potential is changed to a value at which at least one of the
 substances enriched on the sensor electrode is reacted electrochemically.
 The current flowing during this process is measured. The potential is
 changed, preferably by a jump, to a value at which again at least one
 substance is enriched at the electrode surface of the sensor. For the
 detection of several substances in a mixture, in this special potential
 program, as many enrichment steps must be carried out as many different
 substances are to be detected. At least one substance is reacted
 electrochemically in each of these enrichment steps.
 For example, in the detection of two substances in a two-substance mixture,
 in the second enrichment step, additionally, the substance which is not
 enriched in the first enrichment step is additionally enriched. When both
 substances were enriched in the first enrichment step at the electrode
 surface, then, in the second enrichment step, a potential is applied at
 which only one of the two substances is enriched. After the particular
 enrichment steps, a potential is applied at which the enriched substance
 or substances are reacted electrochemically. The current flowing during
 this time is determined.
 In case of three substances to be determined, for example, the potential
 program can be the following: when, in a first enrichment step, substances
 1, 2 and 3 are enriched, and in a second step substances 1 and 2 are
 enriched, then, in this special potential program, in a third step either
 only substance 1 or substance 2 may be enriched. Enrichment of substance 3
 does not lead to an appropriate independent detection of all three
 substances.
 However, alternatively, the potential program can also be the following:
 first a potential is applied at which only one or several substances
 is/are enriched. Then, the individual substances are reacted or oxidized
 at characteristic potentials (when more than two substances to be detected
 are present, several enrichment steps can also be performed; however, for
 this purpose, the number of enrichment steps does not have to be as many
 as the number of substances to be detected contained in the sample).
 The currents flowing at the different electrochemical reactions are used to
 determine the concentrations of the particular substances.
 With this potential program, the cross-sensitivity of a sensor can be
 advantageously highly minimized. In addition, the detection of several
 substances can be done in the presence of the others advantageously, using
 only one sensor cell, instead of one sensor cell for each substance to be
 detected, as done in the prior art. The enrichment with continuous
 detection also provides the advantage that the sensor selectivity can be
 optimized via the enrichment potential and the enrichment time.
 With these two potential methods according to the invention (liquid phase
 and general), the disadvantages of the known detection methods of the
 prior art can be avoided. They are universally applicable, especially to
 substances with low reactivity.
 The variable potential can be altered cyclically in all three detection
 methods according to the invention. After completion of a potential
 program with enrichments and subsequent electrochemical reactions, the
 potential is brought again to the initial value of the same potential
 program and is thus available for the next detection cycle. Accordingly,
 in this way, the concentration of at least one substance to be detected
 can be followed quasi-continuously.
 In order to maintain the sensitivity of the sensor electrode through a
 continuous detection operation, the electrode surface must be freed, in an
 appropriate desorption step, both from the oxidized and reduced enriched
 layer of the substance to be detected as well as from the additionally
 adsorbed oxygen layer (or also hydrogen layer). For this purpose, in a
 first step, preferably the enrichment layer is oxidized and an oxygen
 layer is enriched. In the next desorption step, preferably, a low
 potential is applied to the sensor electrode for a short time during which
 the enriched oxygen layer is reduced (optionally, the entire enrichment
 layer is desorbed). Then the potential is again brought to the potential
 that is characteristic for the electrochemical reaction. Repeated passage
 through these desorption steps ensures that the electrode surface is
 actually unoccupied. Additionally, the sensor electrode can be activated
 by adsorbing oxygen in a first step and desorbing it again in a second
 step.
 In another practical example of a continuous detection method, the
 potential which is characteristic for the electrochemical reaction is
 changed linearly in time. For this purpose, after the enrichment step, the
 potential is brought to a potential value, preferably in a jump, and then
 varied linearly as a function of time. Thus, the potential goes through a
 range at a given potential change rate. Due to the sudden change of the
 potential values, one obtains an enrichment step which is accurately
 defined in time and also the time for this enrichment step and thus for
 the entire detection cycle is shortened. This value of the potential
 change rate has an upper limit by the fact that the electrochemically
 reacted substances should find sufficient time to desorb. The potential
 change rate should not be chosen too slow, because otherwise the response
 time of the sensor would be increased. Owing to the linear increase of the
 potential from an initial value to an end value, all substances that are
 reacted between these two electrochemical values are advantageously
 desorbed from the electrode surface.
 Furthermore, the concentration of the substance is determined preferably
 through the current produced by the electrochemical reaction at a specific
 potential. Preferably, for this purpose, the maximum current flowing
 during the electrochemical reaction is determined instead of the current
 integrated in time over the entire electrochemical reaction and is thus
 correlated with the enriched substance concentration. The measured current
 can be correlated with the substance concentration using a previously
 performed calibration.
 Preferably, the measured value(s) obtained with the above detection method
 is/are correlated with a measured value, called oxygen value below, in
 which the oxygen value is obtained through at least one other detection
 cycle--called reference cycle below--in which the substance is not
 enriched in a first step on the sensor electrode. If said potential values
 are in ranges in which other substances which perturb the detection are
 reacted electrochemically or enriched simultaneously, then the
 additionally flowing currents must be taken into consideration in
 reference cycles in the determination of the substance concentrations.
 These interfering substances are present in the electrolyte initially,
 such as bound oxygen or water. During the reference cycle, no substance is
 enriched, for example, only oxygen is adsorbed. Then the oxygen is
 oxidized or reduced at the same potential at which the substance to be
 detected was already reacted electrochemically and the current flowing
 during this process is determined. For the purpose of measurement value
 correction, the current measured in the reference cycle is subtracted from
 the current measured in a normal detection cycle.
 In this method, the measured value thus obtained is preferably normalized
 to the oxygen value determined in the reference cycle. With this
 normalization, changes, alterations or wear phenomena of the electrode
 surface are taken into consideration advantageously and the measured
 signal corrected correspondingly. The normalization is based on the fact
 that pure oxygen or hydrogen adsorption is influenced adversely to the
 same extent by the quality of the sensor electrode surface as the
 enrichment of the substance to be enriched on the sensor electrode.
 Preferably, the measured values are also normalized with the aid of the
 electrode capacitance.
 Especially preferably, during the potential method according to the
 invention (liquid phase and general), the electrode capacitance is
 measured and the enrichment ended when the electrode capacitance or the
 time change of the electrode capacitance reaches a predetermined value. In
 this detection method--called combined detection method below--in
 principle, the potential method (liquid phase and general) according to
 the invention, is combined with the modified alternating current method
 according to the invention. The enrichment is followed as in the modified
 alternating current method. The electrochemical reaction is started only
 when a sufficient amount of substance has become enriched on the electrode
 surface. The resulting oxidation current thus yields a sufficiently large
 measurement signal and thus concomitantly a reliable concentration result
 for the substance to be detected. Preferably the concentrations of the
 individual substances to be detected are determined only through the
 enrichment, and the determination of the other substances is carried out
 either in combination, through a previous enrichment and subsequent
 electrochemical reaction, or directly through an electrochemical reaction.
 In another preferred variation of the modified alternating current method
 or potential method (liquid phase, general or combined), the substance to
 be detected is first reacted electrochemically at the sensor electrode or
 at another electrode at an applied potential and at least one product thus
 obtained is then detected by the sensor electrode. If a substance to be
 detected is characterized by a small tendency to become enriched at the
 sensor electrode used, then this substance can advantageously be reacted
 electrochemically at an electrode at a given potential to form an
 intermediate product. The intermediate product formed should then be able
 to be enriched at the electrode surface of the sensor electrode at another
 potential value. The final detection of this intermediate product is then
 carried out either through the modified alternating current method
 presented above or by one of the potential methods. Preferably, the
 electrode necessary for the production of the intermediate product can be
 separated in space from the sensor electrode, so that the substance that
 has a low tendency to become enriched can be detected continuously. The
 spatial separation must not be too large, so that the intermediate product
 can diffuse from the separate electrode to the sensor electrode within a
 short time--preferably in fractions of a minute.
 Preferably, the detection can be optimized through the parameters of
 electrode material, electrolyte composition, enrichment, enrichment
 potential, potential for electrochemical reaction and/or time change of
 the potential for the electrochemical reaction. The detection methods
 according to the invention provide a multiplicity of optimization
 parameters for a highly sensitive and selective sensor electrode. The
 optimization parameters for the enrichment--enrichment time, enrichment
 potential, electrode material and electrolyte composition--as well as for
 the electrochemical reaction--electrolyte composition, electrode material,
 characteristic potential and its time change--can vary.
 In the devices according to the invention, the sensor electrode is
 preferably a membrane provided with an electrocatalyst on one side. The
 sensor electrode is wetted mostly on one side with the electrolyte
 solution necessary for the electrochemical detection. The electrolyte
 solution is preferably hygroscopic, so that evaporation of the water is
 largely avoided and that its composition--concentration of the conducting
 salt--remains as constant as possible. Preferably, the membrane is made of
 teflon and/or the electrocatalyst is a thin platinum, rhodium or palladium
 layer applied by sputtering. Platinum is characterized by good enrichment
 properties toward a number of substances, while palladium is especially
 suitable for the detection of saturated halogenated hydrocarbons. However,
 other metals of the platinum group can also be used as electrocatalyst.
 The test liquid itself can replace the electrolyte solution for the liquid
 detection method. Here, the membrane is preferably prepared from a porous,
 hydrophilic or ion-conducting material (for example, Nafion), and the
 electrocatalyst is applied onto the side that faces the test liquid--and
 not the electrolyte liquid. For this purpose, the electrocatalyst is
 applied in such a thin layer that the membrane with the electrocatalyst is
 still porous or ion-conducting. Furthermore, the sample liquid side of the
 membrane can be provided with a thin Nafion or cellulose acetate film, as
 protecting film. With this structure, the sensor electrode can be used
 relatively universally in various (even nonaqueous or poorly conducting)
 liquids.
 For the gaseous detection method, the electrocatalyst is sputtered onto the
 side of the membrane which faces the electrolyte solution. Here, the
 gaseous substances to be detected can diffuse through the membrane and the
 electrocatalyst and dissolve in the electrolyte solution before they reach
 the electrode surface. However, for the gaseous detection method, the
 sensor electrode described above for the liquid detection method may also
 be used.
 The invention complex will be explained in more detail below with the aid
 of practical examples and the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 shows in a schematic representation a cross-section through a sensor
 generally designated 2, which is suitable for carrying out the represented
 detection method of substances in gaseous mixtures.
 The core part of sensor 2 is a sensor electrode 4 consisting of a teflon
 membrane 6, shown schematically in detail in an enlarged form. The teflon
 membrane 6 preferably has a thickness of 75 .mu.m, a pore size of 0.2
 .mu.m and a diameter of 6 mm. It separates the gaseous mixture with the
 substance to be detected from an electrolyte solution necessary for the
 electrochemical detection. The electrolyte solution is selected to be
 strongly hygroscopic (for example, perchloric or sulfuric acid) and thus
 prevents rapid drying of sensor 2, so that the electrolyte concentration
 in the inner space of sensor 2 hardly changes at all. The teflon membrane
 6 is sputtered with an electrocatalyst 8 (for example, platinum) on the
 side facing the electrolyte solution. A thin, noble metal layer with a
 layer thickness of preferably 90 nm is produced. In addition, the
 roughness factor of the noble metal layer is reduced considerably in
 comparison to the roughness factor of known sensor electrodes for
 amperometric detection methods. The teflon membrane 6 thus modified
 functions at the same time as a sensor and as a gas-diffusion electrode.
 It is secured tightly to a sensor housing 12, with the aid of a pressing
 disk 10 through an O-ring 11. The sensor electrode 4 is dimensioned in
 such a way that, on the one hand, edge effects (interfering
 electrochemical processes at the edge or in the electrolyte gaps at the
 seal) become negligible, while, on the other hand, the resistance of the
 metal layer toward the center of the sensor electrode becomes sufficiently
 small. The resistance is measured here from the edge of the sensor
 electrode 4 where the electrical contact to an external electronic is
 provided, to the center of the sensor electrode 4, where the
 electrochemical processes occur mainly--such as enrichment and
 electrochemical reaction, etc.
 The sensor electrode 4 is embedded into the cylindrical sensor housing 12
 in such a way that it and an opposite counter-electrode 14, as components
 of a three-electrode arrangement, close this inner sensor housing 12
 tightly at the open sides. The counter-electrode 14 is pressed tightly
 against sensor housing 12 with the aid of a ring 16, a means to secure
 against turning and an O-ring 17. Preferably, a reference electrode 18,
 for example, a hydrogen electrode, is introduced through a conical bore 20
 into the cylinder wall of sensor housing 12, so that it can be placed in
 the immediate vicinity of the sensor electrode 4. The sensor 2 can be
 filled with electrolyte solution through another conical bore 22. This
 bore 22 is then closed for the practical operation of sensor 2, in order
 to prevent running out of the electrolyte solution. Optionally, the
 gaseous products that are formed in the electrolyte solution or the gases
 that are formed at the counter-electrode 14 can be liberated directly
 through the porous teflon membrane 6, as long as their amount, based on
 the area of the teflon membrane 6, is not too large. Therefore, the area
 of the teflon membrane 6 must be greater than the area of the sensor
 electrode 4 or of the electrocatalyst. Thin wires 24a and 24b provide the
 electrical contact of sensor electrode 4 and counter-electrode 14 toward
 the outside.
 In order to provide good tightness or a high pressing pressure of sensor 2,
 the sensor housing 12 is surrounded by a steel mantle 26, which presses
 the sensor housing 12 together under pressure, with the aid of leaf
 springs 28 and a lock nut 30.
 The gaseous mixture with the substance(s) to be detected enters through an
 opening 31 of the steel mantle 26 and the pressing disk 10 in the
 direction shown by the arrow and impinges onto the outside of the porous
 teflon membrane 6. From there, it goes through the pores of the teflon
 membrane 6 inside sensor housing 12 and dissolves in the electrolyte
 solution located there.
 The sensor 2, together with the corresponding electronics for carrying out
 the individual detection methods according to the invention (potential
 program, automatic ending of the enrichment phase, etc.), can be
 dimensioned in such a way that it is easily transportable. For this
 purpose, the heavy steel mantle 26 can be replaced by another suitable
 housing.
 Overall, the sensor 2 has good contact between the thin wires 24a and 24b
 and the respective electrocatalyst layer of sensor electrode 4 or
 counter-electrode 14, especially with regard to a small distance of the
 reference electrode from the working electrode, has small dimensions; a
 special type of the sensor electrode 4 as well as a small roughness factor
 of the electro-catalyst layer were optimized for the detection method of
 the invention.
 FIG. 2 shows an equivalent circuit for the electrical behavior of sensor
 electrode 4, counter-electrode 14 and reference electrode 18 in the
 modified alternating current method.
 Ions and solvent molecules with dipole character (that is, water molecules)
 interact with the metallic electrode surface 8 of sensor electrode 4 and
 of counter-electrode 14 and develop an electrolytic double layer there. In
 the simplest case, this electrolytic double layer behaves as a plate
 capacitor 32a or 32b with a certain double-layer capacitance. This
 double-layer capacitance includes in principle all electrostatic
 interactions of the ions (sulfate ions, etc.) and solvent molecules with
 the sensor electrode.
 If a potential is applied between the sensor electrode 4 and the reference
 electrode 18 in order to enrich the substance, then the substance can be
 converted to an adsorbate such as
 C.sub.2 Cl.sub.4 +4e.sup.-.fwdarw.(C.sub.2).sub.ads. +4Cl.sup.- between
 0-0.3 V
 This adsorbate can consist of ions as well as neutral molecules with and
 without dipole character and forms an additional adsorbate layer on the
 particular electrode surface. This adsorbate layer blocks the sensor
 electrode 4 where it is adsorbed. Then at those places, the double-layer
 capacitance is reduced significantly because the distance of the double
 layer from the electrode surface is enlarged as a result of the adsorbate
 located in between.
 In addition to the conversion of the substance to be detected to the
 adsorbate, in case of enrichment of the substance, at the same time, a
 competing electrochemical reaction can also occur, such as
EQU C.sub.2 Cl.sub.4 +6H.sup.+ +10e.sup.-.fwdarw.C.sub.2 H.sub.6 +4Cl.sup.-
 between 0-0.2 V
 Even at an optimum adsorption potential, possibly to a small extent, an
 undesirable competing electrochemical reaction of the substance to be
 detected can occur to a product which is no longer adsorbed. Furthermore,
 at potentials for the adsorption of the substance to be enriched, the
 oxygen of the air is also reduced, which leads to a large additional
 current which is highly superimposed onto the actual measured signal.
 These electrochemical reactions that occur parallel to the adsorption, are
 described with the aid of resistors 40a and 40b.
 In addition to the double layer capacitance, another pseudocapacitance 34a
 and 34b also arises at the particular electrodes 4 and 14, due to the
 following effect: protons from the solution adsorb as hydrogen on the
 electrocatalyst 8--and the following reaction occurs on a platinum layer:
EQU Pt+H.sup.+ +e.fwdarw..rarw.Pt-H (respectively: H.sup.+
 +e.sup.-.fwdarw..rarw.H.sub.ad)
 The current flowing as a result of this behaves exactly as a capacitive
 current, and, therefore, we speak of a pseudocapacitance 34a or 34b. This
 pseudocapacitance 34a or 34b is highly potential-dependent and is an order
 of magnitude larger than the actual double layer capacitance 32a or 32b.
 At the point of the sensor electrode 4, where the substance to be detected
 is adsorbed (irreversibly), no hydrogen can adsorb any longer, as a result
 of which, in addition to the double layer capacitance (see above), the
 pseudocapacitance 34a or 34b is also reduced.
 Resistors 36a and 36b describe the limited rate of hydrogen adsorption.
 However, this rate is extremely high and the resistors 36a and 36b are
 thus correspondingly small, so that the double layer capacitances 32a and
 32b as well as the pseudocapacitances 34a and 34b can hardly be
 distinguished from one another.
 Instead of hydrogen adsorption, the adsorption of metal ions, such as
 copper, can also be utilized according to
EQU Cu.sup.2+ +2e.sup.-.revreaction.Cu.sub.ads.
 Since some of the substances to be detected prevent the adsorption of
 copper, in this way, the selectivity of sensor 2 can be increased. In this
 case, the value of the resistors 36a and 36b must be taken into
 consideration.
 The reference electrode 18 is described by a complex impedance 18a.
 However, this impedance 18a, similarly to the current flowing through it,
 is so low that no potential drop occurs.
 The ohmic resistance of the electrolyte solution before the particular
 electrode 4 and 14 is represented by an electrolyte resistor 38a and 38b.
 The ohmic resistance of the particular electrocatalyst layer is
 represented by the corresponding resistors 42a and 42b. However, it cannot
 be distinguished from the ohmic resistance of the electrolyte solution by
 technical measurements.
 With the aid of the equivalent circuit shown in FIG. 2, the double layer
 capacitance (optionally also the pseudocapacitance) are derived from the
 measured impedance of the sensor electrode 4 through the imaginary part of
 the alternating current and correlated with the enrichment or with the
 time change of the enrichment. Then, from the time-dependent amount of
 enrichment, the concentration and the type of enriched substance can be
 determined.
 FIG. 3a shows a diagram of a potential-time curve for the modified
 alternating current method using the detection of perchloroethylene in
 synthetic air as example. FIG. 3b shows a corresponding time plot of the
 alternating current imaginary part called alternating current transient A'
 below. In FIGS. 3a and b, the ordinate gives the potential in volts and
 the imaginary part of the alternating current in milliamperes and the
 abscissa shows the time in seconds. The electrode surface is a platinum
 layer in this example and the electrolyte solution contains 1 M HClO.sub.4
 as supporting electrolyte.
 In the first detection step Z (FIG. 3a)--also called desorption Z
 below--the electrode surface is freed from any impurities present using an
 oxidation-reduction reaction and is activated. For this purpose, the
 potential applied to the sensor electrode 4 is increased or decreased to
 values at which any substances adhering to the electrode surface are
 reacted electrochemically and desorbed.
 In a second detection step A--also called enrichment A below--a dc
 potential is applied at which, when possible, no electrochemical reaction
 of the substance to be detected is catalyzed in the neighborhood of the
 electrode, but rather, as selectively as possible, the substance to be
 detected is enriched. The accurate value for this potential depends on the
 thermodynamic and kinetic properties of the substance to be enriched. The
 enrichment rate is also dependent on the applied potential. For example,
 perchloroethylene becomes enriched as a potential value of 100 mV with a
 high rate of enrichment.
 In the modified alternating current method, during enrichment A, an
 alternating voltage with an amplitude of 10 mV and a frequency of 10 Hz is
 superimposed onto the dc potential. The alternating current flowing A'
 (FIG. 3b) is recorded by the sensor electronics as a measured signal. For
 the evaluation of the measured signal, the initial drop of the alternating
 current transient A' is taken, is related to the enrichment rate and this
 is correlated with the concentration of the enriched substance. As can be
 seen in FIGS. 3a and b, the beginning of enrichment A and the use of a
 constant alternating current transient A' are shifted in time with respect
 to one another, which is caused by the process of establishment of the
 enrichment potential, by the electrolyte resistance or also by the
 measurement technology. This modified alternating current method is
 characterized by high linearity within the measured signal and substance
 concentration, because the enrichment rate can be measured almost directly
 and can be represented as a simple function of the concentration.
 After the enrichment A, desorption steps Z are performed in order to desorb
 the enriched substance as completely as possible from the electrode
 surface via an oxidation or reduction reaction. After the fifth desorption
 step Z, consequently, the sensor electrode is sufficiently purified and,
 at the same time, activated again.
 The amount of enriched substance contributes significantly to the
 sensitivity of sensor 2. In order to ensure that sensor 2 is equally
 sensitive, even at different substance concentrations, the alternating
 current transient A' is followed during the entire enrichment A and
 whether or not a sufficient amount of substance has been enriched is
 derived from this.
 FIG. 4a shows a diagram of a potential-time curve for one of the potential
 methods for the selective quantitative detection of the substance to be
 detected. In the diagram, the ordinate gives the potential in volts and
 the abscissa shows the time in seconds. The potential-time curve is shown
 for the example of a detection of benzene in the liquid phase (as well as
 in the gaseous phase) on a sensor electrode 4 sputtered with platinum.
 In a first detection step, the enrichment step A, a potential of 200 to 300
 mV is applied at sensor electrode 4 for 20 seconds in order to enrich a
 certain amount of the substance to be detected. As it is known from
 heterogeneous catalysis, here the internal bonds of the enriched or
 adsorbed substance are weakened. A subsequent oxidation can then occur at
 lower potentials than that needed for the oxidation of a free, that is,
 not adsorbed substance. During the enrichment or adsorption of the
 substance, oxygen of the air is reduced simultaneously. This leads to a
 large negative current (not shown) at sensor electrode 4, but this has no
 influence on the detection process.
 In a second detection step B--also called potential jump B below--the
 potential is changed suddenly to 900 mV. This potential value is chosen so
 that oxidation of the enriched substance or layer just does not occur.
 In a third detection step C--also called oxidation C below--the potential
 is increased linearly in time with a potential change rate of 300 mV/s.
 Hereby, the enriched layer becomes oxidized and at the same time, largely
 desorbed--for example, benzene becomes oxidized at platinum electrodes
 according to
EQU (C.sub.6 H.sub.6).sub.ads.+12H.sub.2 O.fwdarw.6CO.sub.2 +30e.sup.-
 +30H.sup.+.
 At potentials higher than about 0.7 V, additionally, the oxygen bound in
 the electrolyte liquid begins to adsorb:
EQU H.sub.2 O.fwdarw.O.sub.ads. +2e.sup.+ 2H.sup.+.
 Here, a clearly defined oxide layer is formed as a monolayer. In some
 cases, the already adsorbed substance to be detected (benzene) and now to
 be reacted electrochemically, is to be displaced at sensor electrode 4:
EQU (C.sub.6 H.sub.6).sub.ads. +H.sub.2 O.fwdarw.C.sub.6 H.sub.6 +O.sub.ads.
 +2H.sup.+ +2e.sup.-.
 The potential steps serve to achieve oxidation and desorption of the enrich
 ed substance as completely as possible.
 In a fourth detection step D--also called reduction D below--the potential
 is reduced again to a highly cathodic potential of 50 mV for a fraction of
 a second (for example, 0.5 sec); this potential corresponds approximately
 to the enrichment potential. The time for the fourth detection step D is
 chosen to be so short that the substance to be detected cannot be
 deposited on the electrode surface again and, on the other hand, the
 entire oxide layer will be reduced and desorbed.
 The three detection steps, B, C and D, that is, the potential jump B,
 oxidation C and reduction D form a detection cycle E. This detection cycle
 E is repeated five times in succession. As a result of this, the substance
 to be detected and additionally enriched substances are removed completely
 from the electrode surface until finally only the clearly defined oxide
 covering layer that is formed again in the detection cycle E remains. It
 can be assumed that no enriched substance is present on the electrode
 surface any longer in the fifth detection cycle E.
 FIG. 4b shows a diagram of the current-time curve, which flows as a result
 of the potential program shown in FIG. 4a. In the diagram, the ordinate
 gives the current in milliamperes and the abscissa gives the time in
 seconds.
 The current-time curve shows a current peak B' and a subsequent oxidation
 current C' for each detection cycle E of the potential-time curve.
 The current peak B' occurs during the potential jump B and results from the
 recharging of the produced double layer at the sensor electrode 4.
 The oxidation current C' increases strongly to a maximum, which is at the
 highest potential value of the potential range of oxidation C in the
 practical example shown in FIG. 4b. This maximum can also be reached at a
 different value of the potential of the potential range. The oxidation
 current C' results from a superimposition of two currents, one of which
 flows because of the oxidation C of the substances to be detected,
 enriched and additionally enriched and the other flows because of the
 development of the oxide layer on the electrode surface. The maximum of
 the oxidation current C' decreases steadily from the first to the fifth
 detection cycle E. In the fifth detection cycle E, the magnitude of the
 current that flows due to the oxidation C of the enriched substances is so
 small that only the development of the oxide layer contributes to the
 oxidation current C'. For example, in the example of benzene, the
 oxidation current C' is measured at 1.44 V.
 A measured signal to be correlated to the concentration is obtained, for
 example, by forming the difference between the measured oxidation currents
 C' in the first and in the fifth detection cycles E. The potential at
 which these two oxidation currents C' are measured within a detection
 cycle E can be chosen in such a way that the resulting difference is
 maximum. In this example, the difference of the maximum values of the
 oxidation currents C' is formed.
 The enrichment of a substance at the electrode surface is influenced
 significantly by the properties of this surface. Thus, the obtained
 measured signal should be suitably normalized in order to be able to be
 reproduced well. For this normalization, the difference of the oxidation
 currents C' in the first and in the fifth detection cycles E is formed and
 normalized to the oxidation current C' determined in the fifth detection
 cycle E. The oxidation current C' measured in the fifth detection cycle E
 reflects the real surface conditions that influence the enrichment
 conditions. After this normalization, the measured signal is a
 dimensionless quantity.
 The sensitivity of sensor 2 is greatly improved by enrichment A. The
 oxidation current C', which flows during the electrochemical reaction of
 the enriched substance, which is to be correlated with the concentration,
 is dependent on the amount of substance accumulated during the enrichment
 A. Thus, the measured signal is influenced considerably by the amount of
 time available for the enrichment A. This time can also be optimized
 automatically by measuring the electrode capacitance. For this purpose,
 the enrichment potential is applied only until the electrode capacitance
 reaches a predetermined value. For the detection of benzene,
 concentrations to 1 ppm can be detected reliably. (For example,
 perchloroethylene can be detected to 30 ppm, but with an improved
 evaluation electronics even to 3 ppm.)
 The duration of a detection with 5 detection cycles E takes, for example,
 20 seconds for benzene (for example, 36 seconds for perchloroethylene).
 On the other hand, the selectivity of the sensor 2 is dependent on the
 selection of the electrode material and of the electrolyte. Moreover, the
 maximum potential-dependent enrichment rate and also the
 potentialdependent oxidation current C flowing during the electrochemical
 reaction play a significant role, since different substances are oxidized
 or, in the reverse case, reduced, at different potentials. In the method
 presented here, several measurement parameters are available for
 increasing the selectivity of the measured signal. On the side of the
 sensor, this includes the electrode material and the electrode metal, the
 electrolyte, the pH value of the electrolyte solution and the material of
 the film on the solution side (in the case of a sensor for the liquid
 phase). The extent and the rate of adsorption of the substance to be
 detected can be influenced by ions or additives in the electrolyte, which
 themselves adsorb at a certain potential without being reacted. This
 effect depends strongly on the nature of the substance to be detected and
 therefore leads to a higher selectivity of the sensor. On the electronics
 side of the sensor, these measurement parameters include the adsorption
 potential, the oxidation potential (in the case of linear potential
 ranges, this corresponds to the potential at which the oxidation current
 is detected), the time at which--in the case of oxidation at constant
 potential--the oxidation current is detected, and the slope of the
 potential range (different substances are oxidized at different rates).
 The special advantage of the electronically alterable parameters lies in
 the fact that they can be altered automatically or manually very rapidly.
 FIG. 5 shows a diagram of a potential-time curve for the general potential
 method for the detection of two substances. In the diagram, the ordinate
 shows the potential in volts and the abscissa the time in seconds. The
 detection of two substances to be detected will be explained on the
 example of perchloroethylene and toluene in air using a platinum-coated
 electrode surface and a 1M HClO.sub.4 electrolyte solution.
 In a first detection step A1, a low potential of 50 mV is applied to the
 sensor electrode 4 for 20 seconds, during which both the perchloroethylene
 as well as the toluene become enriched at the electrode surface.
 In a second detection step B--also called potential jump B below--the
 potential is increased suddenly to 900 mV. The magnitude of this potential
 value is chosen so that oxidation of the two enriched substances still
 does not occur.
 In a third detection step C--also called oxidation C below--the potential
 is increased linearly in time at a potential increase rate of 300 mV/s.
 During this, the two enriched substances are oxidized.
 In a fourth detection step D--also called reduction D below--the potential
 is decreased suddenly to a low potential at which the adsorbed oxygen is
 reduced and any residues of toluene and perchloroethylene present are
 desorbed.
 The three detection steps B, C and D again form a detection cycle E. This
 detection cycle E can be repeated five times in succession (not shown). As
 a result of this, the toluene and perchloroethylene are removed from the
 electrode surface as completely as possible.
 In a fifth detection step A2, a low potential of 300 mV is applied to the
 sensor electrode 4 at which mainly only toluene is enriched. Then the
 potential jump B, oxidation C and reduction D are repeated, where, during
 oxidation C, only the enriched toluene is oxidized. Perchloroethylene
 would also become oxidized at the applied potential. However, due to the
 exclusive enrichment of toluene in the fifth detection step A2, this
 oxidation of perchloroethylene is eliminated.
 Subsequent detection cycles E follow, consisting of potential jump B,
 oxidation C and reduction D in order to determine the oxygen adsorption on
 the electrode surface and the changes of the electrode surface. During the
 entire detection, the current-time curve is measured and recorded in order
 to correlate the corresponding current values with the substance
 concentrations. However, it is also sufficient to report only the current
 values during the oxidation at the maximum or at a characteristic is
 potential. For this purpose, first, the oxidation currents are measured
 that arise from the oxidation C of the first enriched layer--consisting of
 perchloroethylene and toluene--and from the oxidation C of the second
 enriched layer--consisting largely of toluene. Analogously to the
 detection method shown in FIGS. 4a and 4b, the difference values of the
 oxidation currents measured in the first and fifth detection cycles E are
 determined.
 The difference value obtained in this way, which originates from the
 oxidation C of the second enriched layer--consisting largely of
 toluene--is a measure of the toluene concentration in the investigated
 substance mixture, since the portion of the current to be attributed to
 the adsorption of oxygen is eliminated from the oxidation current C'. The
 difference of the difference values obtained above is again a measure of
 the perchloroethylene concentration, since the part of the oxidation
 current C, attributable to oxygen adsorption as well as to oxidation of
 toluene is eliminated.
 Thus, corresponding to the applied potential program, the sensor 2 can
 distinguish between different substances to be detected. This applies
 especially to substances to be detected in a mixture, where the enrichment
 potentials differ greatly--such as perchloroethylene and toluene, benzene
 or vinyl acetate. These can also be enriched at considerably more anodic
 potentials.
 FIGS. 6a-c show in diagrams a potential-time curve (FIG. 6a), a
 corresponding alternating current transient A' (FIG. 6b) and a
 corresponding dc time curve C' (FIG. 6c) for the combined detection
 method, that is, for the measurement of the electrode capacitance during
 substance enrichment and the measurement of the current during the
 subsequent electrochemical reaction of the substance(s) thus enriched. In
 the diagram of FIG. 6a, the ordinate gives the potential in volts and the
 abscissa the time in seconds. In the diagrams of 6b and 6c, the ordinate
 gives the imaginary part of the alternating current and the direct current
 in mA, respectively, the abscissas show the time in seconds.
 A low frequency alternating voltage is superimposed on the established
 potential value during enrichment A. As in the modified alternating
 current method, thus, by measuring the alternating current transients, the
 enrichment of the substance to be detected on the electrode surface is
 followed. The electrochemical reaction or oxidation C is started only when
 a sufficient amount of substance has been enriched on the electrode
 surface. The resulting oxidation current C' thus yields a sufficiently
 large measured signal and thus a reliable determination of the
 concentration of the enriched substance. This concentration determination
 is related to the enrichment time in order to obtain the actual
 concentration of the substance to be detected in the investigated mixture.
 Thus, in this method presented here, the enrichment time is no longer
 constant, but is adapted automatically to the existing substance
 concentration. A shorter enrichment time is sufficient for a higher
 substance concentration, while a lower substance concentration requires a
 longer enrichment time in order to enrich a sufficient amount of substance
 on the electrode surface. Furthermore, this method offers advantageously a
 continuous function control of the sensor by measuring the current in the
 fifth detection cycle and the capacitance without enrichment.