OPTICAL DETERMINATION OF GASEOUS AMMONIA CONCENTRATION USING THE DIFFUSE REFLECTANCE SPECTROSCOPY

ABSTRACT

In this work optical sensor for the determination of gaseous ammonia based on the diffuse reflectance spectroscopy was prepared. Influence of other gases on the characteristics of the optical material BCP@SiO₂ prepared by sol-gel technology was studied. A linear relationship between the ammonia concentration and the Kubelka-Munk function was obtained and sensors shown stable reflectance signal for a long time.

АННОТАЦИЯ

В ходе выполнения исследования, проведенного авторами, был изготовлен оптический датчик для определения газообразного аммиака на основе спектроскопии диффузного отражения. Изучено влияние других газов на характеристики оптического материала БКП@SiO2, полученного золь-гель технологией. Была получена линейная зависимость между концентрацией аммиака и функцией Кубелки-Мунка, и датчики в течение длительного времени показывали стабильный сигнал отражения.

Keywords: sol-gel, bromcresol purple, ammonia gas, Kubelka-Munk function, diffuse reflectance spectroscopy.

Ключевые слова: золь-гель, бромкрезоловый пурпурный, газообразный аммиак, функция Кубелки-Мунка, спектроскопия диффузного отражения.

Introduction

Due to the development of various branches of industry and national economy in the world, great attention is paid to the use of modern technologies in enterprises. One of the most important problems is monitoring the impact of toxic gases released from production enterprises on the environment. Among such toxic gases, ammonia takes one of the main place. Constant monitoring of its concentration in the air remains one of the important tasks of analytical chemistry today [1].

Therefore, in the past twenty years, a lot of attention is paid to the field of sensors. Since they can be used to continuously monitor the environment. It is common that in order to carry out monitoring with the help of chemical sensors, they should be cheap and able to meet modern requirements. This, in turn, requires the use of new materials and optimization of their properties. At the same time, various toxic gases are analyzed mainly by electrochemical and optical (or infrared) spectroscopy. These methods require the use of expensive and complex equipment. Methods based on UV-vis spectroscopy are attracting attention in recent years due to their simplicity, low cost, rapidness, as well as the possibility of miniaturization. The manufacture of such sensors is much simpler and made of disposable materials. Moreover, sol-gel technology is widely used in the preparation of optical sensors [2]. With the help of this technology, it is possible to control the physicochemical properties of the final materials, and they have a number of advantages over their organic polymer analogues.

Various sensing methods are available for the determination of NH₃. However, the most common detection methods can be divided into three main categories, which are widely used solid-state detection methods, optical methods, and electrochemical analysis methods [3-5]. Metal oxide-based gas sensors are found attracting in the field of gas detection due to their advantages such as simplicity in production, low cost and flexibility. SnO₂, ZnO, WO₃, TiO₂ and MoO₃ are the most widely used metal oxides for ammonia determination [6-8].

Gas sensors using optical absorption have attracted a lot of attention. They are able to overcome the measurement error due to the effect of long-term memory, which is a common drawback of other gas sensors using contact sensing methods [9].

In this work optical sensor for ammonia gas sensing was prepared and the properties were studied. Kubelka-Munk function shown direct linear relationship to the ammonia concentration in a gas phase. 

Materials and methods

Sol-gel solutions were prepared to mix 4 ml of TEOS along appropriate amounts of i-C₃H₇OH for 30 minutes. The alkoxide to solvent ratio was maintained 1:4 in all experiments. 3.0 ml 0,01M HCl aqueous solution was added in order to start hydrolysis and condensation reactions. pH of the final solution was adjusted to below 2. The resulting solution was mixed for 4 hours at room temperature. Then 100 μl of 0.1M BCP in ethanol solution was added and another 30 minutes was mixed.

Gels were dried overnight at 70ºC and grinded until to get a fine powder. Barium sulfate powder was used a background materials and thin layer of BCP@SiO₂ was attached on the surface.

Measurements were also carried out in a gas phase using prepared optical sensors. For this, mixtures of ammonia with different carrier gases with different concentrations (% by volume) were prepared using a gas syringe. The composition of the obtained mixtures and the signal of the sensor in them are presented in Table 1 below. The diffuse reflectance spectrum of the prepared optical material is presented without ammonia gas and after exposure to standard gas mixtures with different concentrations of ammonia.

Table 1.

Standard gas mixtures prepared for determination of ammonia concentration in a gas phase

Results and discussion

The BCP@SiO₂ sensor shown sensitivity to very low concentrations of ammonia. No changes were observed in the sensor diffuse reflectance spectrum compared to the carrier gases Ar, N₂ and air. During all measurements, the sensor showed a stable signal. Each measurement was performed at least five times.

As shown in Fig. 2, the prepared optical material exhibited linear coupling over a wide range of ammonia concentrations. The relationship between ammonia concentration and Kubelka-Munk function (K/S) is expressed by the following equation:

where F(R_∞) -  Kubelka-Munk function;

R_∞ - ideal (absolute) reflectance;

K – absorption coefficint

S – back-scattering coefficient.

Also, diffuse reflectance can ideally depend on absorbance. For this, the response of the optical layer prepared according to the above table to ammonia gas was also calculated in relation to the optical density, and as a result, a straight line relationship was observed.

The effect of temperature on the sensor signal was also studied. For this purpose, ammonia gas was constantly injected from a specially prepared rubber tube and the optical material was heated at different temperatures. It was observed that with the increase in temperature, the reaction of BCP with ammonia takes place slowly.

It can be seen from the spectrum that increasing the temperature up to 50°C does not strongly affect the properties of the indicator. The decrease in the maximum of the signal can be explained by the decrease in the solubility of ammonia gas in the TEOS-based layer as the temperature increases.

Conclusion

Optical sensor for the determination of ammonia in a gas phase was prepared and its characteristics were studie.  Using the diffuse reflectance spectrum, the Kubelka-Munk function (K/S) was determined to be related to NH₃ concentration. Sensors shown good stability and long-term signal in the presence of ammonia gas.

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