RU2583162C1 - Amperometric method of measurement of concentration of ammonia in nitrogen - Google Patents

Abstract

FIELD: measurement technology.

SUBSTANCE: invention relates to gas analysis and can be used to solve process and environmental monitoring. Concentration of ammonia in analysed gas is defined by relationship changing value of one of electrical characteristics of electrochemical cell of amount of ammonia, oxidised on surface of inner electrodes of electrochemical cells made from electrode material. For this purpose, analysed gas stream is placed electrochemical cell with cavity formed by two discs from solid oxygen conducting electrolyte, on opposite surfaces of discs are arranged along pair of electrodes, to electrodes of discs is DC voltage in range of 400-500 mV with supply of positive pole to inner electrodes by means of which is electrolysis of water vapour contained in analysed gas, and pumping of produced by electrolysis of oxygen from analysed gas flow into the cavity of cell by electrochemical circuit: external electrodes-solid electrolytes-inner electrodes in process of achieving steady state when diffusion flow of products of oxidation of ammonia from cavity of cell is equal to supplied flow of analysed gas supplied to it, measuring flowing through cell current limit and limit current value corresponding to content of oxygen spent on oxidation of ammonia, concentration of ammonia nitrogen.

EFFECT: invention enables simple and reliable measurement of content of ammonia nitrogen.

2 cl, 3 dwg

Description

The invention relates to the field of gas analysis, in particular to detecting devices used for recording and measuring the ammonia content in nitrogen. The invention can be used to solve technological problems and tasks of environmental control.

A known method for determining the concentration of ammonia (RU 2068560, publ. 27.10.1996) [1]. The method consists in transferring ammonia to an aerosol by passing analyzed air over a reagent - crystalline nitrate of iron nitrate for no more than 85 hours. Before passing the analyzed air, the reagent is kept to a constant weight over a solution of nitric acid at a concentration of 55-65 wt. % The resulting aerosol is sent to an electro-induction dust meter and the aerosol concentration is recorded in units of ammonia concentration. The method is characterized by complexity, the need to use consumable reagents, the duration of the analysis.

The most common methods for measuring the ammonia content are methods using dielectric coatings that adsorb ammonia from a carrier gas. So, the known sensor for determining ammonia (RU 2478942, publ. 05/10/2012) [2]. As a result of adsorption, a change in the resistance of this coating occurs and the magnitude of the current change in the circuit is used to judge the concentration of ammonia in the analyzed gas. This measurement method is characterized by poor reproducibility, because applying coatings with the same characteristics in size, thickness, coating composition is almost impossible. In addition, passivation of the coating will occur over time, which will change the characteristics of the sensor.

A known method of measuring ammonia, in which a polycrystalline film of zinc selenide doped with cadmium selenide is applied to a substrate that is an electrode pad of a piezoelectric crystal. This method is implemented in a gas sensor (RU 2464552, publ. 04/22/2011) [3] and a semiconductor gas analyzer (RU 2464553, publ. 10.05.2012) [4]. Depending on the ammonia content in the gas washing the film, a change in the oscillation frequency of the piezoelectric crystal is observed. This method of measuring ammonia has the same disadvantages as the above methods.

Closest to the claimed invention is a method for determining the concentration of ammonia and its derivatives in a gaseous medium (RU 2473893, publ. 01.27.2013) [5]. To implement the method, a sensor is used that is highly sensitive to nitrogen dioxide and ammonia, as well as a converter, which can be used, for example, a heated tube filled with a catalyst or a dielectric plate coated with a heating element with a catalyst layer capable of converting it ammonia to nitrogen oxides, that is, catalytically oxidize ammonia or its derivatives:

The converter can operate in a stationary mode, that is, continuously at a temperature providing a catalytic conversion, or in a non-stationary mode at which the temperature of the converter providing a catalytic conversion is replaced by a temperature at which the catalytic conversion does not proceed.

The sensor and the converter are placed in a chamber having an opening for the studied gas medium to enter the chamber. Nitrogen oxides, unlike ammonia, do not reduce, but increase the resistance of an n-type semiconductor as a result of chemisorption (acceptor signal). After ammonia and its derivatives are converted into nitrogen oxides using a converter, the electrical resistance of the semiconductor sensor is measured and the concentration of ammonia in the gas medium is determined by the value of this resistance. The method provides high selectivity and stability of measurements.

Using this method, it is possible to measure the content of ammonia and its derivatives in air or other oxidizing medium. In an inert gas such as nitrogen, where there is no free oxidizing agent, it is not possible to measure the concentration of ammonia in the described manner.

The objective of the present invention is to provide a method that allows a fairly simple and reliable measurement of the ammonia content in nitrogen.

To solve this problem, an amperometric method for measuring the concentration of ammonia in nitrogen is proposed, which consists in the fact that the concentration of ammonia in the analyzed gas is determined by the dependence of the change in the value of one of the electrical characteristics of the electrochemical cell on the amount of ammonia oxidized on the surface of the internal electrodes of the electrochemical cell made of electrode material while an electrochemical cell with a cavity formed by two disks of ki is placed in the flow of the analyzed gas solid conductive electrolyte, on opposite surfaces of the disks are arranged along a pair of electrodes, a DC voltage of 400-500 mV is applied to the electrodes of the disks with a positive pole supplied to the internal electrodes, through which electrolysis of water vapor in the analyzed gas is carried out, and the result of oxygen electrolysis from the flow of the analyzed gas into the cell cavity along the electrochemical circuit: external electrodes - solid electrolytes - internal electrodes, in the process f attaining the stationary state, when the diffusion flow of ammonia oxidation products from the cell cavity becomes equal to the incoming flow of the analyzed gas entering it, measure the limiting current flowing through the cell and determine the ammonia concentration in the limit current corresponding to the oxygen content spent on the oxidation of ammonia nitrogen. As the catalytic material, platinum is used.

The DC voltage applied to the electrodes in the range of 400-500 mV with the positive pole applied to the electrodes located inside the cell, pumps the oxygen obtained by dissociation of the moisture present in the gas mixture from the analyzed gas stream into the cell cavity. In the cell cavity, the pumped oxygen interacts with ammonia, which is received therein in a mixture with nitrogen from the analyzed medium. In this case, the interaction of ammonia with oxygen will intensively proceed on the surface of the cell’s internal electrodes made of electrode material in accordance with the reactions (1, 2). When the DC voltage reaches 400-500 mV, the current stabilizes and stops growing with increasing voltage. The resulting current is the limiting current, and its value is due to gas exchange between the analyzed medium and the gas in the cell cavity. The limit current of the sensor is limited by the diffusion barrier — the capillary of the sensor and is associated with the concentration of ammonia (Ivanov-Shits, I. Murin, Solid State Ionika, Volume 2, St. Petersburg (2010). P. 964-965 by equation (3):

Where:

D _{(ammonia-inert gas)} is the diffusion coefficient of ammonia in an inert gas, cm ² / s;

X _(ammonia) - molar fraction of ammonia in an inert gas;

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

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

T is the analysis temperature, K;

L is the length of the capillary, mm

In accordance with equation (3), it is quite easy to calculate the ammonia content from the measured value of the limiting current I _{L (ammonia is an inert gas)} .

A new technical result achieved by the claimed method is to obtain the possibility of measuring ammonia in a mixture with an inert gas and simplifying the measuring device by making it from a simple and well-studied oxygen-conducting solid electrolyte.

The invention is illustrated by drawings, where in FIG. 1 shows an electrochemical cell for implementing the method; in FIG. 2 - current-voltage characteristic in the analysis of ammonia in a mixture of ammonia + nitrogen at 500 ° C; in FIG. 3 - concentration dependence of the limiting current on the concentration of ammonia in a mixture with nitrogen. The electrochemical cell for implementing the method of measuring ammonia consists of two disks 1 made of an oxygen-conducting solid electrolyte of the composition 0.9ZrO ₂ + 0.1Y ₂ O ₃ . On the opposite surfaces of each of the disks 1, two outer electrodes 2 and two inner electrodes 3 are located. The disks 1 are interconnected by a gas tight sealant 4 with the formation of an internal cavity. A capillary 5 is located between the disks. Voltage is supplied to the electrodes 2 and 3 from a DC voltage source (IPT). The current arising in the cell circuit is measured with an ammeter (A). The electrochemical cell is placed in the stream of the analyzed gas, which washes its outer surface and through the capillary 5 enters the inner cavity of the cell. Under the action of a DC voltage applied from a source (IPT) to electrodes 2 and 3, and a plus is applied to the internal electrodes 3, oxygen is pumped from the analyzed gas into the internal cavity of the cell through a solid oxygen-conducting electrolyte. In the cavity, the incoming oxygen interacts on the surface of the catalytic electrodes 3 with ammonia. The resulting interaction products, in accordance with equations (1-2), are exchanged through capillary 5 with the analyzed gas. In this case, capillary 5 is a diffusion barrier that limits this gas exchange flow. This current flow will correspond to the cell current. When the applied voltage is reached within 400-500 mV, the gas exchange between the cell cavity and the analyzed medium is stabilized and the limiting diffusion current is set in the circuit - I _{L (ammonia - nitrogen)} , which is measured using an ammeter (A). Using equation (3), the value of X _(ammonia) can be determined from the value of the measured I _{L (ammonia - nitrogen )} , i.e. the concentration of ammonia in nitrogen.

Thus, the claimed method allows to measure the ammonia content in a mixture with nitrogen or other inert gas by means of an amperometric cell with an oxygen-conducting solid electrolyte.

Claims (2)

1. Amperometric method for measuring the concentration of ammonia in nitrogen, which consists in the fact that the concentration of ammonia in the analyzed gas is determined by the dependence of the change in the value of one of the electrical characteristics of the electrochemical cell on the amount of ammonia oxidized on the surface of the internal electrodes of the electrochemical cell, made of electrode material, characterized in that an electrochemical cell with a cavity formed by two oxygen-conducting solid disks is placed in the stream of the analyzed gas electrolyte, on opposite surfaces of the disks are arranged along a pair of electrodes, a DC voltage of 400-500 mV is applied to the electrodes of the disks with a positive pole supplied to the internal electrodes, through which electrolysis of water vapor in the analyzed gas is carried out, and the resulting electrolysis is pumped oxygen from the flow of the analyzed gas into the cavity of the cell along the electrochemical circuit: external electrodes - solid electrolytes - internal electrodes, in the process of achieving a stationary TATUS when the diffusion flow of ammonia oxidation product from the cavity of the cell becomes equal to the incoming flow of the sample gas flowing into it is measured flowing through the cell limiting current, and limiting current of magnitude corresponding to the oxygen content of spent in the oxidation of ammonia, the concentration of ammonia nitrogen. 2. The method according to p. 1, characterized in that platinum is used as the electrode material.

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