RU2755639C1 - Amperometric method for measuring the content of carbon monoxide in inert gases - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention relates to measuring equipment and can be used to measure the concentration of carbon monoxide in inert gases and other oxygen-free gaseous media. The amperometric method for measuring the concentration of carbon monoxide in inert gases is that an electrochemical cell with a cavity formed by two gas-tightly interconnected disks of oxygen-conducting solid electrolyte, between which there is a capillary, is placed into the flow of the analyzed gas mixture, on electrodes located on opposite surfaces of one from the disks, DC voltage is supplied within 0.5-1 V, with the connection of the positive pole to the outer electrode, whereby electrolysis of water vapor in the analyzed gas is carried out, and the oxygen obtained as a result of electrolysis is pumped from the flow of the analyzed gas into the sensor cavity along the electrochemical circuit: outer electrode - solid electrolyte - inner electrode, in the process of reaching a stationary state, when the diffusion flux of the interaction products of oxygen pumped into the cavity and carbon monoxide in the cavity becomes equal to the amount of carbon monoxide in the analyzed gas entering the cavity, the limiting current flowing through the sensor is measured and the value of the limiting current corresponding to the oxygen content spent on interaction with carbon monoxide, determine the concentration of carbon monoxide in the analyzed gas.

EFFECT: ability to measure the content of carbon monoxide in inert gases at temperatures of 500÷650°C and above, as well as improving the accuracy and reproducibility of measurements.

1 cl, 3 dwg

Description

The invention relates to analytical technology and can be used to measure the concentration of carbon monoxide in inert gases and other oxygen-free gaseous media.

A method for measuring carbon monoxide in gas mixtures using a sensitive element made of solid oxide electrolyte, which is an oxygen-ionic conductor, with polarizable and non-polarizable electrodes deposited on its surface is known from the prior art. In this case, the electrolyte is made in the form of a plate, the non-polarizable reference electrode is applied to the electrolyte surface in the form of a silver oxide paste, and the polarizable measuring electrode is applied to the electrolyte surface in the form of a finely dispersed platinum or palladium powder with the addition of 10% electrolyte powder, and both electrodes are baked, and the polarizable measuring electrode activated [1].

The sensitive element was placed in a glass tube, while silver current leads were pressed to the electrodes, the ends of which were brought out and connected to a voltmeter. A four-component gas mixture consisting of nitrogen, oxygen, carbon dioxide, and carbon monoxide with a given concentration was blown through a glass tube at a constant controlled rate. The concentration of the latter was changed due to the content of nitrogen or carbon dioxide, since their concentrations practically have no effect on the electrode potentials of the cell.

The operating temperature range of this sensor is 300-550 ° C, while the minimum temperature for reaching the equilibrium potential at the platinum electrode in an environment containing hydrogen and oxygen is 640 ° C. At lower temperatures, the equilibrium potential is not established, and the sensor readings are unstable.

Manufacturing the reference electrode from silver, and the working electrode from platinum or palladium with the addition of 10% electrolyte powder complicates the manufacturing technology of the known sensitive element, because requires separate baking of electrodes to solid electrolyte. In addition, the use of precious metals (Pt, Pd, Ag) for the manufacture of electrodes increases the cost of the product, and the addition of CeO ₂ to the platinum electrode reduces the response. But the main disadvantage of sensors based on the use of electrodes with different polarization of the electrodes is that the polarization of the electrodes changes over time and, accordingly, the characteristics of the sensor change. First of all, reproducibility and calibration.

The "Sensitive element of the electrochemical sensor of carbon monoxide in gas mixtures" [2] is aimed at improving the accuracy of measuring carbon monoxide, increasing the stability of readings, as well as simplifying the manufacturing technology of the sensitive element. This element is made in the form of a solid oxide electrolyte tablet, a reference electrode is baked on one of the tablet surfaces, and a measuring electrode on the opposite surface. The solid oxide electrolyte is based on cerium oxide with the composition Ce _0.8 (Sm _0.8 Ca _0.2 ) _0.2 O ₂ , the reference electrode is made of lanthanum-strontium manganite with the composition La _0.6 Sr _0.4 MnO ₃ , and the measuring electrode is made of zinc oxide ZnO. In this case, the reference electrode and the measuring electrode are baked to the surfaces of the solid oxide electrolyte tablet at the same time.

The method for measuring carbon monoxide in gas mixtures using this sensor is as follows. The sensor is located in a uniform temperature field, which is created by the analyzed gas medium or heater. The analyzed gas washes the surface of the solid electrolyte and the reference and measuring electrodes applied to its opposite surfaces. An oxygen potential is generated at the reference electrode, because lanthanum-strontium manganite, from which the reference electrode is made, is reversible with respect to oxygen and is practically inert to carbon monoxide. Thus, an oxygen potential is generated on the reference electrode 2:

ϕ (e.m.) = RT / 4F⋅ln p ^* O ₂ , (1)

where:

ϕ (e.m.) is the oxygen potential of the reference electrode;

R - gas constant (1.9873 cal / deg * mol);

T is temperature, K;

p ^* O ₂ - partial pressure of oxygen at the reference electrode, Pa;

F is the Faraday number.

A mixed potential is generated at the measuring electrode 3, which is determined by the partial pressures of oxygen and carbon monoxide in the analyzed gas:

ϕ (e.m.) = RT / 4F · ln p ^** (O ₂ + CO) (2)

where:

ϕ (e.m.) is the mixed potential of the measuring electrode;

R - gas constant (1.9873 cal / deg * mol);

T is temperature, K;

p ** (О ₂ + СО) is the total pressure of oxygen and carbon monoxide at the measuring electrode.

EMF of the sensitive element will be determined as:

E = RT / nF⋅ln [p ^* O ₂ / p ^** (O ₂ + CO)] (3)

R - gas constant (1.9873 cal / deg * mol);

n is the charge of oxygen ions;

T is temperature, K;

p ^* O ₂ - partial pressure of oxygen at the reference electrode, Pa;

p ^** (О ₂ + СО) is the total pressure of oxygen and carbon monoxide at the measuring electrode.

From the measured value E (mV) at a known operating temperature (T), it is possible to unambiguously determine the content of carbon monoxide in the analyzed gas.

A sensing element with a solid electrolyte based on cerium dioxide of composition Ce _0.8 (Sm _0.8 Ca _0.2 ) _0.2 O ₂ gives a response to the CO content 1.5-2.5 times higher than a sensing element with a solid electrolyte based on zirconium dioxide of composition 0, 91% ZrO ₂ + 0.09% Y ₂ O ₃ .

The method for measuring carbon monoxide in gas mixtures using this sensor, based on measuring the mixed potential, has low reproducibility and has a nonlinear dependence of the EMF on the concentration of the measured component, which requires initial and periodic calibration of each sensor and complicates its operation.

The objective of the present invention is to expand the field of practical application of solid electrolytes with oxygen-ionic conductivity.

For this, an amperometric method for measuring the concentration of carbon monoxide in inert gases is proposed, which consists in placing an electrochemical cell with a cavity formed by two gas-tightly interconnected disks of oxygen-conducting solid electrolyte, between which there is a capillary, on the electrodes, located on opposite surfaces of one of the disks, supply a DC voltage in the range of 0.5 - 1 V, with the connection of the positive pole to the external electrode, whereby electrolysis of water vapor in the analyzed gas is carried out, and the oxygen obtained as a result of electrolysis is pumped from the stream of the analyzed gas into the sensor cavity along the electrochemical circuit: outer electrode - solid electrolyte - inner electrode, in the process of reaching a stationary state, when the diffusion flow of the interaction products of oxygen pumped into the cavity and carbon monoxide in the cavity type, will become equal to the amount of carbon monoxide in the analyzed gas entering the cavity, the limiting current flowing through the sensor is measured, and the value of the limiting current corresponding to the oxygen content spent on interaction with carbon monoxide determines the concentration of carbon monoxide in the analyzed gas.

Applying a DC voltage to the electrodes in the range of 1 ÷ 1.5 V with a positive pole applied to the inner electrode of the sensor, pumping oxygen obtained as a result of decomposition of the moisture present in the gas mixture from the analyzed gas flow into the cell cavity. In the cavity of the cell, the pumped oxygen interacts with carbon monoxide supplied there as part of the analyzed gas mixture. In this case, on the surface of the inner electrode of the sensor, the process of interaction of oxygen with carbon monoxide will intensively proceed in accordance with the reaction:

О ₂ + 2СО = 2СО ₂ (4)

When the voltage supplied to the cell electrodes is reached, the current through the solid electrolyte disk increases, and when the voltage reaches 1–1.5 V, the current stabilizes and stops growing with increasing voltage. The resulting current is the limiting current, and its value is due to the gas exchange between the analyzed medium and the gas in the sensor cavity. The value of the limiting current of the sensor is limited by the diffusion barrier - the capillary of the sensor and is related to the concentration of carbon monoxide (Ivanov-Shits, I. Murin., Ionika solids, volume 2, St. Petersburg (2010) SS. 964-965) by equation (2 ):

; ( 5)I _{L (CO)} =

; ( 5)

where: F - Faraday number, 96485 C / mol;

D _(CO) - diffusion coefficient of carbon monoxide in nitrogen, cm ² / sec;

X _(CO) - mole fraction of carbon monoxide in nitrogen;

S is the cross-sectional area of the capillary, mm ² ;

P is the total pressure of the gas mixture, atm .;

R is the gas constant, 8.314 * 10 ⁷ erg / mol K;

l - capillary length (mm).

I _{L (CO)} - limiting current corresponding to the amount of pumped out oxygen formed after the decomposition of nitrogen oxide in the analyzed gas mixture, mA;

T is the analysis temperature, K.

In accordance with equation (5), it is quite easy to calculate the carbon monoxide content from the measured value of the limiting current I _{L (CO).} As can be seen from figure 2, the volt-ampere dependences of the electrochemical cell have pronounced values of the limiting currents for each measured CO concentration. The concentration dependence of the limiting cell currents on the CO concentration in nitrogen for temperatures of 500 and 550 ° C are shown in Fig. 3. As seen in FIG. 3 for a cell, the concentration dependence is linear over the entire concentration range.

The new technical result achieved by the claimed method consists in creating a method that allows for a fairly simple and reliable measurement of the content of carbon monoxide in inert gases, including at temperatures of 500 ÷ 650 ° C and above, as well as to improve the accuracy and reproducibility of measurements.

The invention is illustrated in the drawings, where FIG. 1 shows an electrochemical cell for implementing the method; in fig. 2 shows the dependence of cell currents on voltage for carbon monoxide concentrations in nitrogen from 0.49 to 10% vol .; in fig. 3 shows the concentration dependence of the cell for a temperature of 500 ° C.

To implement the method, an electrochemical cell is used containing disks 1 and 2 made of an oxygen-conducting solid electrolyte having an ion transfer number equal to 1, for example, of the composition 0.9 ZrO ₂ + 0.1Y ₂ O ₃ , interconnected by a gas-tight sealant 3. On the opposite surfaces of the disc 1 there is an outer electrode 4 and an inner electrode 5. On the opposite surfaces of the disc 1, electrodes 3 and 4 are located. The voltage is supplied to electrodes 4 and 5 from a DC voltage source, and a minus is supplied to the inner electrode 6, and a plus is supplied to the electrode 4 located on the outer side of the disk.

The DC voltage is supplied to the electrodes of the disk 1 from DC voltage sources IT and is controlled by the ammeter A. The electrochemical cell of the above-described design is placed in the analyzed gas mixture of carbon monoxide with an inert gas (nitrogen), which washes the cell from the outside and flows through the capillary 7 into the cavity 6 .Under the action of a direct current voltage applied from the IT source to electrodes 4 and 5, oxygen is pumped through disk 1, formed as a result of electrolysis of water vapor in the gas mixture, into cavity 6. In this cavity, oxygen interacts with carbon monoxide supplied to cavity 6 through capillary 7 with the formation of carbon dioxide. The resulting interaction products in accordance with equations (4) are exchanged through the capillary 7 with the CO + N ₂ gas mixture washed by the cell. By measuring the resulting limiting current, equation (5) can be used to calculate the concentration of carbon monoxide in the sample gas.

Thus, the claimed method makes it possible to expand the area of practical application of solid electrolytes, primarily for analytical measurements.

Sources of information:

1. RU 2326375, publ. 06/10/2008 "The sensitive element of the electrochemical sensor of carbon monoxide in gas mixtures."

2. RU 2522815, publ. 07/20/2014, "The sensitive element of the electrochemical sensor of carbon monoxide in gas mixtures."

Claims (1)

An amperometric method for measuring the concentration of carbon monoxide in inert gases, which consists in placing an electrochemical cell with a cavity formed by two gas-tightly interconnected disks of an oxygen-conducting solid electrolyte, between which there is a capillary, on the electrodes located on opposite surfaces one of the disks, a DC voltage is supplied within 0.5-1 V, with the connection of the positive pole to the outer electrode, whereby electrolysis of water vapor in the analyzed gas is carried out, and the oxygen obtained as a result of electrolysis is pumped from the flow of the analyzed gas into the cavity sensor along the electrochemical circuit: outer electrode - solid electrolyte - inner electrode, in the process of reaching a stationary state, when the diffusion flux of the interaction products of oxygen pumped into the cavity and carbon monoxide in the cavity becomes equal in the amount of carbon monoxide entering the cavity in the analyzed gas, the limiting current flowing through the sensor is measured, and the concentration of carbon monoxide in the analyzed gas is determined by the value of the limiting current corresponding to the oxygen content spent on interaction with carbon monoxide.

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