Introduction. Carbon dioxide CO2, as a product of functioning of industrial enterprises and motor transport and a product of combustion and expiration of living organisms, is a natural component of the atmosphere. The content of СО2 in the composition of air gas mixture is commonly recognized as one of its most important characteristics whose level permanently increases and, therefore, the necessity of monitoring of the level of СО2 with the help of various instruments is beyond any doubt.

According to GOST R EN 13779-2007, “Ventilation in Nonresidential Buildings. Technical Specifications for Ventilation and Conditioning Systems,” the quality of air in the rooms, where the presence of people is allowed, is classified, according to the СО2 content, into five groups: from 380 ppm (“perfect for well-being”) up to 2000 ppm (“sharp decrease in the productivity of labor”). For the purposes of monitoring of the content of carbon dioxide in air, it is customary to use gas analyzers (monitors) containing, as a rule, СО2 sensors of two types: infrared and electrochemical.

The infrared gas analyzers mainly include the so-called NDIR (nondispersive infrared) sensors based on the phenomenon of nondispersive absorption of infrared radiation, i.e., absorption of the optical radiation with a wavelength of 4.26 μm by СО2 molecules [1, 2]. Air is supplied into the space between a light emitter and a photodetector, and the content of carbon dioxide in the gas mixture is estimated according to the degree of absorption of the optical radiation. NDIR monitors intended for industrial applications have modifications both with diffusion air ingress and with forced pumping of air through the flow chamber, e.g., PKU-4-N carbon-dioxide gas analyzer (Ecological Sensors and Systems, Russia). In general, infrared gas analyzers have acceptable operating characteristics but the contents of dust and moisture in the analyzed samples affect the results of measurements. Among the disadvantages of these instruments, we can also mention their low stability under mechanical actions and relatively high cost explained by the complexity of manufacturing of the equipment with the use of special electronic components.

At the same time, electrochemical СО2 gas analyzers are based on electrochemical cells each of which contains a gas-permeable membrane, special electrolyte in a dielectric body, measuring electrodes, and a temperature sensor [3, 4]. Carbon dioxide penetrates through the membrane and participates in chemical reactions with electrolytes as a result of which the characteristics of its electric conductivity change. The СО2 content of air is found by analyzing the degree of changes in these parameters. The electrochemical gas analyzers are simpler for manufacturing and maintenance and less costly than the monitors based on NDIR sensors. However, they also have serious disadvantages: short service life depending on the content of the measured gas; low resistance to the variations of temperature and relative moisture, and low selectivity to the measured gas, which forces producers to install additional filters in the electrochemical cells.

The aim of the present paper is to study the possibility of creation of СО2 sensors based on conductometric cells with distilled water.

Equipment and the Procedure of Measurements. The possibility of creation of СО2 sensors is based on the well-known reaction of dissolution of carbon dioxide in water accompanied by the formation of hydrogen ions [5,6,7]:

$$ {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_2{\mathrm{CO}}_3\leftrightarrow {\mathrm{H}}^{+}+{\mathrm{H}\mathrm{CO}}_3^{-}. $$
(1)

Reaction (1) shows that the concentration of protons in water, which mainly determines its electric conductivity [5], is in equilibrium with the СО2 concentration over the water surface. The process of leveling of these concentrations is relatively slow (and runs for 10–30 min). However, for the long-term monitoring that lasts for many days, the indicated reaction rate can be regarded as admissible. Thus, it is possible to monitor the CO2 content of ambient air according to the electric conductivity of water in open-type conductometric cells. The processes of dissolution of nitrogen, oxygen, and other atmospheric gases do not affect the electric conductivity of water [8], which guarantees high selectivity of the sensor of conductometric cell to the monitored gas (СО2). Note that the proposed СО2 sensor is actually a modification of electrochemical cell.

Experimental investigations were carried out with the use of measuring complexes. The structural diagram of one of these complexes is shown in Fig. 1 (we present the connection of only one measuring cell). The principle of construction of this measuring complex is, to a large extent, similar to the principles described in [8, 9]. The complex contains an open-type polyethylene measuring conductometric cell CC with two electrodes made of stainless steel SE and aimed at measuring the electric conductivity of water, a thermistor TR mounted outside under the bottom of the cell, and a thermal insulator TI insulated from the ambient medium. The thermistor was used to measure the temperature of water in order to take into account the influence of temperature variations on the electric conductivity. In our experiments, we used distilled water with a specific conductivity of about 1 μS/cm. To measure the electric conductivity of water, we applied sinusoidal voltage with an amplitude of 0.2 V and a frequency of 100 Hz to the electrodes of the cell. Moreover, in order to measure the temperature of water, the frequency of sinusoidal voltage applied to the thermistor was increased up to 200 Hz. These reference signals were formed with the help of a special program in a personal computer PC. From the outputs of the sound card, these signals were applied to the conductometric cell through the matching device MD. The matching device was used to match the levels of signals and resistances between the outputs of the conductometric cells and the inputs of the sound card. The procedures of processing of signals obtained as a result of measurements, evaluation of corresponding parameters, and construction of the plots were performed by using a special computer program. The proposed complex provides a relative error of measurements of the electric conductivity of water by the sensors of at most ±0.1% and a relative error of measurement of the temperature of water of at most ±0.05%. As a result of our experimental investigations, we detected noticeable variations of the conductivity of water with a clear 24-h period [10].

Fig. 1.
figure 1

Structural diagram of the measuring complex: CC – conductometric cell; SE – steel electrode; ТM – thermistor; TI – thermal insulator; MD – matching device; PC – personal computer.

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Experimental results. The possibility of application of conductometric cells with distilled water for the purposes of monitoring of the СО2 content of air was confirmed as a result of a series of experiments carried out in 2019. Each of two complexes contains two structurally identical conductometric cells with an internal volume of 12 ml, a volume of distilled water poured into the cell of 1.5 ml, an area of the water–air interface of about 2 cm2, a cross-sectional area of the inlet hole of about 5 mm2, a diameter of the electrode wire of 0.6 mm, and an interelectrode distance of about 10 mm. The first measuring complex was placed in a laboratory room and the second measuring complex was located in the forest, outside the city, at a distance of about 15 km from the first complex. For two days, the temperature of water varied within the range 26.2 ± 0.3°С in the cells of the first complex and within the range 24.4 ± 0.7°С in the cells of the second complex. Moreover, in all cells, we detected a uniform increase in the electric conductivity of water at a rate of 8% per day, which was obviously caused by the dissolution of the material of electrodes.

In Fig. 2, we show the time dependences of the electric conductivity of water in two cells recorded on 10–11.06.2019. In constructing these dependences, we performed the software compensation of the indicated trend in the behavior of electric conductivity, as well as the compensation of changes in the electric conductivity under the action of temperature with regard for the known temperature coefficient of the electric conductivity of water equal to 2.5%/°С. The experimental data were obtained by using two identical measuring complexes. To increase the reliability of measurements, each complex recorded the data of two identical conductometric cells. Since the data obtained for two cells of the same complex are practically identical, in Fig. 2, we present a single dependence for each complex.

Fig. 2.
figure 2

Time dependences of the electric conductivity of water (СО2 content) in the cells placed in the laboratory (1) and in the forest (2).

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As indicated above, the electric conductivity of water Y in the cells caused by the concentration of protons is proportional to the СО2 content in the composition of air mixture. The initial electric conductivity of water in the cell located in the laboratory room is somewhat higher than the conductivity of water in the cell located in the room outside the city, which is explained by the corresponding mean values of the СО2 content (see Fig. 2). In [5], it was shown that the relationship between the СО2 content N and the electric conductivity of water Y is close to linear. To determine the СО2 contents in standard measurement units, we carried out special experiments: in a bounded volume of air, in the process of changes in the concentration of СО2, we measured the electric conductivity and temperature of water in both cells and the СО2 content of air with the help of an MT8057s monitor (Finder Technology, Hong Kong, China). In the process of comparison of the data of measurements, we took into account the time lag of the conductometric cells (of about 20 min) mainly caused by the diffusion of air through the inlet hole of the cell. As a result, we obtained a conversion coefficient K = 820 ppm/μS with the help of which we computed the СО2 content in ppm (right scale in the dependence presented in Fig. 2). Note that the values of the coefficient K are specified by the specific features of the design of conductometric cells and are individual for cells of each type.

Discussion of the results. The time dependences of the electric conductivity of water in both cells had a periodic (daily) character (see Fig. 2). The electric conductivity of water in the first cell increased by about 18% starting from 10 a.m., the maximum of the curve was detected at 5–6 p.m., and a decrease in the electric conductivity down to the initial values occurred in the evening and at night. The periods of increase in the electric conductivity exactly coincided with the time of presence of the employees who controlled the process of experiment and served as a source of increase in the amount of СО2 in the room. The electric conductivity of water in the second cell increased by about 14 % at night and in the morning but became lower during the daytime. This is explained by the variations of the СО2 content of air in the process of “breathing of the vegetation cover” [11], when plants release carbon dioxide in the darkness and absorb it under the action of solar radiation. Thus, the correlation between the time of action of the sources of СО2 and the variations of the electric conductivity of water in the open conductometric cells is obvious, which enables us to use them as carbon-dioxide sensors.

In the performed experiments, the temperature of the ambient medium varied within narrow ranges and, hence, the compensation of variations of the electric conductivity of water under the action of temperature was carried out fairly correctly with regard for the corresponding constant temperature coefficient. In the case where it is necessary to perform monitoring of the СО2 content under the conditions of noticeable changes in the ambient temperature, it is reasonable to use an active thermostat, similar to that described in [8], in order to stabilize the temperature of water in the cell.

Conclusions. The experimental investigations of the developed hardware-software complex based on the use of conductometric cells with distilled water and intended for measuring the СО2 content in air revealed the principal possibility of practical application of the analyzed complex for the long-term monitoring of the level of carbon dioxide in air under different conditions of the ambient medium. The application of conductometric cells with distilled water as СО2 sensors with the indicated relatively high time lag of the process of measurements (several tens of minutes) as compered with the known electrochemical cells demonstrates that they have several advantages, namely, a quite high selectivity of the release of carbon dioxide from the air gas mixture, a simple design of the cell and measuring channel, the absence of costly accessories and expendable materials, and a low power intensity of the process of measurements.