Apparatus for simultaneous determination of different gas components.

p0001The invention proceeds from a device for the simultaneous detection of different gas components having a plurality of electrochemical Dreielektrodenmeßzellen (working electrode counter electrode-reference electrode) with a common electrolyte and a potentiostatic evaluation circuit for setting and regulation of different potentials at the working electrodes and for measuring the electrochemically generated, with the individual gas concentrations correlated electric signals.

p0002Potentiostatic three-electrode sensors are widely used for measuring gases in trace amounts to tests of pure gases. These gas sensors are reproducible, sensitive and feasible for a variety of different gases in the rule. The selectivity can be influenced by the choice of the catalyst at the measuring electrode, the electrolyte and the potential on the measuring electrode (working electrode). However, not all cross sensitivities off simultaneously. Rather, in practice, from application to application, a compromise between sensitivity and suppression of cross-sensitivity to disturbing other gases must be found.

p0003Alternatively makes sense to combine several sensors to a sensor array and to use the different sensitivities for the desired measured component and the interfering cross components in the measured value. By considering all sensor signals then the composition of the gas can be determined (pattern recognition). Such sensor arrays have been realized on the basis of conductivity solid state gas sensors.

p0004An electrochemical multi-electrode sensor in which the gas passes through successively arranged electrochemical measuring cells is described in DE 24 35 813th The measuring cells are via an associate with the electrode resistance network so interlinked that only a certain pollutant associated measurement voltage is formed in each cell. Requirement for this is that in each stage a complete reaction takes place, so that longer residence times, and thus also longer dead times have to be taken in the measurement into account. The underlying measurement principle is further limited in that all the electrodes are brought out separately for each measuring cell. These properties are the claim contrary to a simply constructed, compact electrochemical multi-electrode sensor.

p0005In addition, temperature differences and different incoming flows must be considered in the individual measuring cells.

p0006Furthermore, 64 337 potentiostatic Four electrode sensors for measuring specific gas systems are known from US 43 15 753 and EP OD. However, there is no way to select the potentials of the working electrodes independently. This eliminates a substantial degree of freedom for the individual optimization of selectivities. That being said, in this prior art, in part of an elaborate gas guide is required.

p0007The invention is based on the object of several gas components to be measured simultaneously and independently with the aid of an electrochemical multi-electrode sensor, with a view to optimizing the selectivity for the detection of individual gas components complete freedom in determining the electrode potential, the selection of the electrode material including catalytic additives and the use of appropriate gas filter and gas diffusion barrier prevails.

p0008This object is achieved with an electrochemical sensor having a plurality of Dreielektrodenmeßzellen inventively, <ul><li>a) that the Dreielektrodenmeßzellen be formed by a plurality of working electrodes with a common counter-electrode and a common reference electrode, which are connected with the same electrolyte,</li><li>b) that a potentiostatic evaluation circuit contains control loops that the potentials of the working electrode relative to the reference electrode, individually and independently maintain constant</li><li>c) and that said evaluation circuit means for detecting the working electrode in the flowing, correlated with the gas concentrations electrical current signals.</li></ul>

p0009Preferably, there is a loop in the potentiostatic evaluation circuit each consist of two cascade-connected operational amplifiers, wherein the first amplifier, the potential difference between a working electrode and the common reference electrode high impedance taps and the second amplifier, this potential difference at the output of the first amplifier with a preset reference value U_n compares and the current flowing from the output of the second amplifier to the working electrode current I_n readjusts that the deviation from the nominal value U_n is minimized. The currents I_nWhich are established in the scheme to the different working electrodes, the measured values ​​for the gas concentrations of the incident at the working electrodes gas components.

p0010Preferably, a liquid electrolyte is used. The working electrodes are advantageously on the accommodated in the form of spatially separated measurement fields one surface of the electrolyte, while the common counter electrode and the common reference electrode are disposed on the opposite side of the electrolyte. In this way, a compact electrochemical multi-electrode sensor can be realized particularly well.

p0011The sensitivity of a working electrode for a specific gas, and thus the sensitivity of the measurement field for a specific gas component is dependent in a known manner on the electrode material and the electrode potential and can thus be adjusted via the electrode potential Furthermore, the selectivity of a measuring field by catalytic activation of the working electrode and be improved by prescreening of gas-specific filters. Furthermore, the sensitivity of a measuring cell by diaphragms or diffusion membranes can be set differently. A multi-electrode sensor having different sensitivities of the individual measuring areas in principle permits a pattern recognition and thus the identification of certain gas mixtures.

p0012With the invention the following advantages are achieved:<ul><li>Compared with the previously known electrochemical multi-electrode sensors can be achieved due to the space-saving design, a significant volume reduction, as reference electrode, counter electrode, electrolyte and the entire sensor housing are shared for all working electrodes.</li><li>When using a solid electrolyte results in a further reduction in volume when the sensor is manufactured in hybrid technology.</li><li>All working electrodes are operated at identical temperature, pressure and Anströmungsbedingungen. In this manner, disturbing influences, which are due to different variations of these parameters can be avoided.</li><li>All measuring signals at the working electrodes (working electrodes I_n) Refer to the same reference electrode and the same electrolyte, can be so that potential drifts of the reference electrode detected and optionally compensated.</li><li>The multi-electrode sensor according to the invention can be a cyclic voltammetric diagram (voltammogram) with very high resolution record currently and simultaneously static (according to the number of working electrodes), whereas one in the classic cyclic voltammetry even at very slow measurements (speeds of up to 1 mV / min) receives only dynamic limits.</li><li>The inventive multi-electrode sensor also allows for a targeted suppression of cross-sensitivities to other undesirable gas components.</li><li>Since the individual working electrodes are operated potentiostatic totally independent, there is complete freedom in choosing and setting the electrode potential, the selection of the catalysts to the working electrode, the upstream connection of gas-specific filters, so that the individual sensing elements can be adapted with optimum selectivity of the measurement problem ,</li><li>The gas to be examined is the same on every working electrode, since the gas paths are equal. This may have different response times, which are attributable to different flow paths or diffusion distances for the individual gas components can be avoided.</li><li>Basically, there are no restrictions on the multi-electrode sensor according to the invention as regards the monitoring of certain gas components or a particular mixture. For example, solid, liquid, inorganic or organic electrolytes may be used.</li></ul>

p0013In the following an embodiment of the invention will be explained in more detail with reference to drawings. Show it<dl id="dl0001"><dt>Fig. 1</dt><dd>the basic structure of a multi-electrode sensor with a multiple-potentiostat regulating the working electrodes, potentials and measuring the current flowing in the working electrodes,</dd><dt>FIG. 2</dt><dd>the practical implementation of a multi-electrode sensor,</dd><dt>FIGS. 3a-3c</dt><dd>Embodiments of the multi-electrode sensor with two, three and four Maßfeldern on the gas side of a liquid electrolyte,</dd><dt>FIGS. 4-6</dt><dd>various measurement examples for explaining the influence of the working electrode potential on the selectivity,</dd><dt>Fig. 7</dt><dd>the time curve of the measurement signal (response curve) at two measuring areas in a multi-electrode sensor components manufactured in compliance FIG. 1 with gassing with Cl₂ and HCl and</dd><dt>Fig. 8</dt><dd>the response curves in the same multi-electrode sensor during fumigation with Cl₂ and HCl.</dd></dl>

p0014In Fig. 1, the multi-electrode sensor with the housing 1, the Meßzellenelektrolyt 2 and the electrodes is shown only schematically. The Gagenelaktrode 3, the reference electrode 4 and the working electrode 5_1.. 5_n dive into the Elaktrolyt. 2 It can, for example, up to eight working electrodes (n = 8) are provided. It forms each working electrode 5_n with the common counter-electrode 3 and the common reference electrode 2 has a three-electrode sensor.

p00156, the evaluation circuit consists of n over the common electrodes 3 and 4 are coupled together potentiostatic control circuits 7_1.. 7_n, Each loop consists of two connected in Keskade operational amplifiers 8_n and 9_n, With the aid of the first amplifier 8_n will each high-resistance, the potential difference between a working electrode 5_n and the common reference electrode 4 is measured. This difference is the second amplifier 9_n at the output of the first amplifier 8_n with each setpoint U_n compared, and the current through the working electrode 5_n leading feedback line 10_n automatically readjusted so that the deviation from the nominal value U_n (Deviation) is minimized. The setpoints U_n and therefore the potentials at the working electrodes 5_n can be set individually and independently. The currents I_n by the feedback lines 10_n flow of the working electrodes 5_n through the electrolyte 2 to the common, grounded electrode 3 from. Since the potentials between the working electrode 5_n and the common reference electrode 4 are measured to high impedance and the common counter electrode 3 is grounded, the individual Potentiostatenstufen work 7_n (Potentiostatic control loops) independently. The using the display device 11_1.. 11_n in the feedback lines 10_1.. 10_n measured currents I_1.. I_n are a direct measure of the working electrodes 5_1.. 5_n unreacted gas quantities. Instead of the display device 11_1.. 11_n Also, other means for detecting the in the working electrodes 5_1.. 5_n currents flowing for example, electronic memory, can be used. Here, the gas to be measured all working electrodes is the same 5_1.. 5_n offered.

p0016Fig. 2 shows the practical embodiment of an electrochemical multi-electrode sensor. The electrolyte 12 here consists of an aqueous electrolyte solution (50% H₂SO₄), which is enclosed by the housing thirteenth The lower end is delimited by the counter electrode 14 and reference electrode 15 (for example, a Pt / air electrode). The counter electrode 14 and reference electrode 15 are led out via the terminals 16, 17th

p0017At the upper end are two working electrodes 18₁ and 18₂ arranged, which are connected to the leads 19 and twentieth

p0018The working electrodes are on its outside, ie facing the gas side, provided with a diffusion membrane 26th About the diffusion membrane 26, which consists for example of a PTFE film, there is a gas-permeable spacers 25 are arranged on the terminating and required sensitivity Aperture 24₁ and 24₂. The standing with the electrolyte 12 in connection inner surface of working electrodes 18₁, 18₂ can be catalytically activated. The selection of suitable catalysts in order to affect the electrochemical reaction at the interface of the working electrode / electrolyte to a particular gas component selectively, is prior art. The sensitivity of the multi-electrode sensor can be adjusted by means of an upstream aperture 21 of each measurement problem.

p0019As shown in Fig. 3a, 12 are semi-circular measuring fields 22₁ and 22₂ formed by the working electrodes 18₁ and 18₂ on the surface of the electrolyte, which are separated by a gap 23. Figs. 3b and 3c show embodiments of a sensor surface with three or four sector-shaped measurement fields for different gas components. To improve the selectivity of the measurement fields of different gas-specific filters to be installed. The sensor construction shown in FIGS. 2 and 3 allows a space-saving and compact design of the multi-electrode sensor.

example 1

p0020As an example, the system Au / H₂SO₄ is described, for which in Fig. 4, the potential-dependent sensitivity for the measurement of various gases is shown by means of a classical cyclic voltammetry. It is used a PTFE powder-gold gas diffusion electrode. The electrolyte consists of 0.5 M sulfuric acid. The measurements are performed at room temperature and at a test gas flow of 5 l / h. The diagram shows quasi-stationary current-voltage curves for the gas components NO, SO₂, NO₂, HCl, Cl₂ and H₂S. The so-called wallpaper (thick solid line) is taken in pure air. The potential is measured against a reversible H₂ electrode in the electrolyte. This is measured using a standard three-electrode sensor diagram a discount gas specific electrode potential can be determined in each case. In determining the potential it usually goes a compromise one. Between the desired sensitivity and to suppressing cross-sensitivities The inventive multi-electrode sensor creates the conditions that individual Working electrodes are each operated at the best potentials. For example, the potentials of 1.2 V for NO, 1.1V to SO₂ measurement and 1 V are set to NO₂ measurement. As experience shows that the measurement sensitivity for all measuring components is sufficiently high, the means by reducing the measuring electrode according to the measuring areas 22 (Fig. 3a-3c) caused loss of speed restriction. The measured currents may be used for the direct detection of the individual detected gases. In addition, you can correct the cross-sensitivity of one or more principal components calculated by the parallel measurement of multiple components.

example 2

p0021Fig. Figure 5a shows the dependence of the measurement currents of a predetermined HCl concentration for a multi-electrode sensor with only two working electrodes. Both working electrodes made of gold. The first working electrode is operated at a potential of 150 mV and the second working electrode at 0 mV against a Pt / air electrode in a Schwefelsäurelektrolyt. One can see that the first working electrode sensitive to HCl. Fig. 5b shows the measuring currents of the same multi-electrode sensor to a fixed Cl₂ concentration. Due to the different working electrode potentials here the second working electrode has a higher sensitivity to Cl₂ than the first working electrode.

p0022The selectivity can be increased in a known manner in that catalytically active materials are used as working electrodes. Thus, for example with the aid of a ruthenium-black working electrode, which is operated with an electrode potential of 0.6 V relative to the reversible hydrogen electrode at a platinum working electrode, which is set at a potential of 1.1 V, with sulfuric acid as electrolyte is a sensor for simultaneous and independent measurement of NO₂ and CO can be realized.

example 3

p0023As a final example, the timing and the cross sensitivity of a multi-electrode sensor of Fig. 1 was examined for the gases Cl₂ and HCl. The two working electrodes consist of gold powder-diffusion electrodes. As a reference electrode and a counter electrode platinum black-diffusion electrodes were used. The electrolyte consisted of 50% sulfuric acid.

p0024The potential for the chlorine sensitive working electrode was set at 1000 mV and for the HCl-sensitive working electrode to 1150 mV. In Fig. 7, as a function of time simultaneously registered progressions of the measurement signals to the Cl₂-working electrode and the HCl-working electrode at a gassing of the multi-electrode sensor are presented with 5 ppm Cl₂, wherein the test gas was fed at a flow rate of 5 l / h. The Cl₂ working electrode shows a significant measuring signal, while the HCl working electrode has only a low cross sensitivity.

p0025Fig. 8 shows in a manner analogous to the response curves for the HCl-gassing of the same multi-electrode sensor. Otherwise working with the same conditions as in the test according to FIG. 7. In this case the HCl working electrode shows a significant measuring signal, while conversely the Cl₂-working electrode having a low cross-sensitivity.