RU2849140C1 - Electrochemical primary transducer for measuring the concentration of hydrazine vapours and its derivatives - Google Patents

Abstract

FIELD: continuous measurement of the concentration of hydrazine, hydrazine hydrate or asymmetric dimethylhydrazine (heptyl) vapours in the air.

SUBSTANCE: electrochemical primary transducer (PT) according to the invention consists of a housing containing a reaction chamber, an electrode system, a buffer reservoir with electrolyte, a diffusion capillary, channels for supplying and removing the sample, equipped with compression fittings, wherein the electrode system is designed as a two-electrode sensing element (SE), which is a structural element (surface) of the reaction chamber and represents a porous hydrophilic polymer plate with electrolyte bound in the plate, and the electrodes are located on the porous hydrophilic polymer plate on its two opposite sides in such a way that the sensitive layer of the working electrode is located in the reaction chamber and is in direct contact with the gas sample, while the reference electrode is isolated from contact with it, and the end surface of the hydrophilic polymer plate of the SE is connected to a buffer reservoir filled with electrolyte by means of a diffusion capillary, the inlet of which has no direct contact with the electrolyte from the buffer reservoir in any spatial PT, which excludes damage to the PE by liquid, ensuring the operability of the PT in a wide range of relative humidity of the analysed sample.

EFFECT: thus, the invention provides the ability to continuously measure low concentrations of hydrazine vapours and its derivatives without preliminary sample preparation, combined with a rapid response to sample delivery and resistance to overload on the measured component.

1 cl, 4 dwg

Description

Description of the invention

a) The field of technology to which the invention relates

The invention relates to the field of analytical instrumentation, namely to devices for detecting and measuring the concentration of hydrazine vapors and its derivatives in an air sample (electrochemical primary converters) and can be used in medicine, biology, ecology and various industries.

The electrochemical primary transducer (EPT) is designed for continuous measurement of hydrazine, hydrazine hydrate, or unsymmetrical dimethylhydrazine (heptyl) vapor concentrations in ambient air or in the work area of various facilities. To improve sensitivity to the measured component, reduce sample response time, and reduce recovery time after target component overload, the EPT utilizes a two-electrode system based on a newly developed sensing element (SE), a distinctive feature of which is direct contact between the catalytic layer of the working (sensing) electrode and the analyzed sample.

b) State of the art

A known electrical sensor (RU 2 522 735 C9) for measuring hydrazine vapor comprises a dielectric substrate containing electrodes and a sensitive layer whose photoconductivity changes as a result of hydrazine vapor adsorption. The sensitive layer consists of a graphene-semiconductor quantum dot structure, the conductivity of which decreases upon adsorption of hydrazine molecules on the surface of the quantum dots in proportion to the concentration of hydrazine vapor. In the presence of hydrazine vapor in an air sample, hydrazine molecules are adsorbed onto the surface of the quantum dots, reducing their luminescence intensity. As a result, the conductivity of the graphene decreases in proportion to the concentration of hydrazine vapor in the analyzed sample. This sensor has several disadvantages that reduce its performance:

- the need to use a source of coherent optical radiation to excite quantum dots of the sensitive layer;

- the need to perform a procedure of forced desorption of hydrazine molecules from the surface of quantum dots using a vacuum before each subsequent measurement;

- the complexity of implementing the sensor structure due to high requirements for the purity of the source materials and chemical compounds.

A colorimetric sensor (US 4,789,638) for measuring hydrazine vapor is known. Its hydrazine detection principle is based on changes in the absorption parameters of a colorimetric indicator in the presence of iodine, which is released during the interaction of iodate or iodite with hydrazine vapor. Common disadvantages of colorimetric sensors include low sensitivity, the inability to quantitatively assess hydrazine vapor concentration remotely, and a short service life due to the consumption of reagents in the sensitive layer.

A luminescent sensor (US 5,719,061) is known for measuring hydrazine vapor. Its operating principle is based on recording changes in the spectral-luminescent characteristics (in this case, the change in the position of the luminescence band maximum) of the reactant in the sensitive layer upon contact with hydrazine vapor as a result of their chemical interaction. Compared to colorimetric sensors, luminescent sensors offer higher sensitivity and enable quantitative measurement of hydrazine vapor concentration. Common disadvantages of this type of sensor include a limited service life due to reagent consumption and photobleaching of the sensitive layer.

The closest invention to the claimed invention and accepted as a prototype is "Amperometric Sensor and Method for Detecting Gaseous Analytes Including a Working Electrode Made of End-Faced Pyrolytic Graphite" (US 2010/0147705). Its operating principle is based on the phenomenon of a specific chemical reaction (an electrochemical reaction) occurring in an electrochemical cell, which is a container containing an electrolyte solution and an electrode system consisting of a working electrode and a counter electrode. End-faced pyrolytic graphite is used as a catalyst for the working electrode. The analyzed sample undergoes a chemical reaction with the electrolyte filling the cell. As a result, charged ions are formed in the solution, and an electric current begins to flow between the electrodes, proportional to the concentration of the analyzed component in the sample.

The prototype has the following disadvantages:

1 Long response time: not less than 120 s.

2 Low threshold sensitivity: 0.5 - 1.0 ^ppm .

3 Dependence of the output signal on the temperature and humidity of the analyzed gas sample.

The problem of increasing sensitivity and reducing response time is being solved.

c) Disclosure of the essence of the invention.

The operating principle of the developed PP is based on the voltammetric measurement method, which relies on the electrochemical oxidation reaction of the target substance on the surface of the working electrode and the transfer of the resulting ions into the electrolyte. The reverse reaction occurs on the opposite (reference) electrode. As a result, an electric current begins to flow between the PP electrodes, the magnitude of which is proportional to the concentration of the substance being measured.

The essence of the proposed invention lies in the fact that the electrochemical primary converter (PC), consisting of a housing (3) (see Figs. 1, 2), contains a sensitive element (SE), based on a porous hydrophilic dielectric plate (8) with a working (12) and a reference (7) electrodes applied to it on both sides, based on powder of graphite grade MPG of a certain fractional composition with additives of transition metal oxides. The PE contains an electrolyte retained by the porous system of the dielectric plate, and is installed between the current-collecting terminals (6, 13) inside the reaction chamber (14) of a small volume in such a way that the sample is forcibly pumped through the reaction chamber above the surface of the working electrode, and the reference electrode is insulated from contact with it. The channels for supplying (2) and removing (10) the sample from the reaction chamber are equipped with compression fittings (1, 11), ensuring a hermetically sealed connection of the PC to the gas tract of the device.

The vast majority of commercially available electrochemical PPs designed for gas analysis are equipped with a gas-permeable membrane on the working electrode side, which ensures sample diffusion to the electrode system while simultaneously preventing electrolyte leakage from the PP's interior. The use of a gas-permeable membrane significantly reduces sensitivity and limits the PP's use in analyzing low concentrations.

The design of the SE of the developed PP ensures operation with direct contact of the sample with the sensitive layer of the working electrode, eliminating the need for a gas-permeable membrane. This design solution significantly increases sensitivity to the target component and reduces response time. However, eliminating the gas-permeable membrane increases the evaporation rate of the electrolyte in the sensitive layer of the working electrode, which can lead to a decrease in sensitivity and response time.

To compensate for this effect, the PCB design includes a buffer reservoir with electrolyte (5) for replenishing the SE, and a diffusion capillary (4, 9) connecting the SE to the reservoir. The capillary ensures the transport of electrolyte vapor to the SE and limits the vapor diffusion rate to the required level. As an additional measure to reduce the rate of electrolyte evaporation, a moisture-retaining component is introduced into the SE dielectric plate. The electrolyte in the reservoir can be refilled or replaced if necessary.

The developed catalyst and electrolyte compositions ensure the required sensitivity and high selectivity for the target component. To further enhance selectivity, a specific external potential difference is maintained between the PP electrodes.

The implemented technical solutions ensure the stability of the PP characteristics in a wide range of relative humidity (from 15 to 90% Rh) and elevated (up to +50°C) ambient temperatures.

The developed PP for measuring the concentration of hydrazine vapors and its derivatives has the following advantages:

1 Threshold sensitivity at the level of 0.02 ^ppm .

2 Instant response to the supply of a sample containing the target component.

3 High selectivity to the target component.

4. The manufacturing technology for PP is relatively simple. This advantage is realized because the preparation of the catalytic mass of the electrodes does not require complex and expensive technologies, whereas the production of the SE requires applying the electrodes as a layer of a certain thickness to opposite sides of a hydrophilic dielectric plate and fixing them in place by pressing.

d) Implementation of the invention

The refusal to use a gas-permeable membrane in the PP design, in addition to the advantages described, led to the emergence of some undesirable consequences that limit the possibilities of using PP with continuous sample supply, namely:

1 Drift of background current arising due to accumulation of the target component in the electrolyte of the SE with insufficient catalytic activity of the working electrode.

2 A decrease in sensitivity to the target component due to the occurrence of an oxidation reaction of the target component accumulated in the electrolyte of the SE on the reference electrode and the occurrence of a “parasitic” current opposite to the current of the main oxidation-reduction reaction.

The use of a cyclic sample feed mode or a gas modulation mode made it possible to avoid the disadvantages characteristic of the developed PP design.

For this purpose, a pneumatic circuit was developed for the device, providing forced sample delivery to the PP in continuous gas modulation mode. The pneumatic circuit for the device is shown in Fig. 3.

The pneumatic circuit of the device includes the following main components:

1 - flame arresters at the inlet and outlet of the gas tract of the device, ensuring compliance with explosion protection requirements;

2 - a three-way electromagnetic valve that ensures the operation of the device in gas modulation mode and supplies the sample or “zero” gas to the PP;

3 - an absorbing filter with high sorption capacity in relation to the measured substance, designed to create a “zero” gas from the sample;

4 - electrochemical primary converter;

5 - a block of flow, temperature and humidity sensors designed to automatically maintain a set flow rate and compensate for the impact of changes in temperature and humidity of the sample on the output signal of the PP;

6 - flow regulator;

7 - flow stimulator, providing forced sampling.

Using the gas analyzer's pneumatic circuit, the sample to be analyzed is fed to the PP at a predetermined rate, followed by a "zero" gas. The "zero" gas is the sample purified by the gas analyzer's sorption filter to remove the measured component. As a result, the PP generates a signal, an approximate form of which is shown in Fig. 4. The area bounded by the "zero line" and the signal change curve in each "feed-purge" cycle is proportional to the measured concentration value.

Selecting the ratio of sample feed time and purge time with zero gas, as well as the overall duration of the measurement cycle, allows for the required sensitivity of the device to the measured component (range of measured concentrations) and a reduction in the threshold alarm response time.

The physicochemical properties of the measured substances necessitate careful selection of PP construction materials and optimization of its design, as well as increased requirements for the quality of processing and cleaning of the internal surfaces of parts, since hydrazine and its derivatives are highly reactive and can interact with materials, foreign inclusions, oxides, or contamination products.

A series of electrochemical PDs designed for measuring hydrazine, hydrazine hydrate, and unsymmetrical dimethylhydrazine (heptyl) has been developed and put into production. These PDs are based on the operating principle of the claimed invention. A commercially available automatic gas analyzer/sensor has been created using these PDs and is used at nuclear power facilities and ground-based defense industry facilities.

d) a brief description of drawings and other graphic materials

Fig. 1 External appearance of a serially produced electrochemical PP

Fig. 2 Schematic representation of the design of the PP

Fig. 3 Pneumatic diagram of the device

Fig. 4. Cyclogram of the PP in gas modulation mode.

Claims (1)

An electrochemical primary transducer (PT) consisting of a housing in which a reaction chamber, an electrode system, a buffer reservoir with electrolyte, a diffusion capillary, channels for feeding and removing a sample, equipped with compression fittings, are placed, wherein the electrode system is made in the form of a two-electrode sensitive element (SE), which is a structural element - the surface of the reaction chamber and is a porous hydrophilic polymer plate with an electrolyte located in the plate in a bound state, and the electrodes are located on the porous hydrophilic polymer plate on its two opposite sides in such a way that the sensitive layer of the working electrode is located in the reaction chamber and has direct contact with the gas sample, the reference electrode is insulated from contact with it, and the end surface of the hydrophilic polymer plate of the SE is connected to the buffer reservoir filled with electrolyte, by means of a diffusion capillary, the inlet of which does not have direct contact with the electrolyte from the buffer reservoir in any spatial PT, which eliminates damage to the sensitive element by liquid, ensuring the operability of the PP in a wide range of relative humidity of the analyzed sample.