In the chemical industry, ammonia is an important chemical raw material, which can be used to make urea, soda ash, ammonium nitrogen fertilizer and nitric acid. At the same time, ammonia is also an easy to constitute a hazardous gas, due to the ammonia itself is a flammable and explosive gas, in the event of ammonia leakage, not only will cause harm to the human body, but also prone to explosive accidents, so in the production of the chemical industry, the ammonia monitoring is very important.
Due to the characteristics of ammonia itself, if there are places where ammonia leaks easily in the workplace or living area, fixed ammonia alarms or portable gas detectors should be installed to detect ammonia leakage and prevent dangerous accidents from occurring when the leakage exceeds the standard.
At present, for industrial ammonia monitoring sensors have three broad categories: optical ammonia sensors, metal oxide sensors, conductive polymer ammonia sensors.
First, optical ammonia sensor
The main types of optical sensors are optical interference sensors, ultraviolet absorption sensors, infrared absorption sensors and fiber optic sensors. For ammonia detection of the two main optical principle one is based on the color of the reagent of the ammonia reaction or triggered by the color change of the indicator; the other mechanism is to detect the absorption of the gas on the light to complete the sensing to determine the gas concentration.
The gas to be measured can be analyzed spectrophotometrically after the reaction coloring. As ammonia gas is alkaline gas under a certain concentration, can make the pH test paper color change, so as to analyze whether the atmosphere contains ammonia, but this test needs to ensure that the ammonia concentration is high and for the color change of the paper can not be sensitive to determine the larger error.
Optical sensors can be used to detect the amount of ammonia in the environment, is a kind of gas sensor with high sensitivity and good selectivity. A laser and a spectrograph are the main components of an optical absorption ammonia detection system. The laser emits light through the air, the light arrives at the detector will be different because of the gas components in the air and the characteristics of each component butt the spectrum to produce a certain effect, complete the detection of ammonia content in the gas environment, in terms of sensitivity and selectivity has obvious superiority.
Second, the metal oxide sensor
Metal oxide gas sensors have become one of the more concerned gas sensors in the composition of the gas sensor gas-sensitive materials. It has been found that tin oxide, molybdenum trioxide, and titanium oxide are metal oxides that can be used to detect ammonia. Metal oxide sensors have the advantages of being robust, inexpensive and easy to operate, making them a very promising gas sensor.
The mechanism of the metal oxide sensor is mainly through chemical adsorption of ammonia molecules adsorbed to the metal oxide sensing layer, causing a change in the conductance of the metal oxide sensor to determine the concentration of ammonia.
Third, conductive polymer ammonia sensor
The use of conductive polymers can be realized on the monitoring of ammonia, such as: polypyrrole, polyaniline and polythiophene, etc., as opposed to metal and metal oxides, conductive polymers as conductive sensors can work at room temperature. The sensing mechanism of the conductive polymers for ammonia is mainly dependent on the redox reaction between ammonia and the conductive polymers, and due to the irreversible nature of this reaction, the sensitivity of the conductive polymer sensors decreases when exposed to ammonia for a long period of time.
The three working mechanisms of ammonia sensors are optical class monitoring, metal oxide monitoring, and conductive polymer monitoring in three ways, which are summarized and summarized for ammonia monitoring. Due to the increasing awareness of environmental protection, the future miniaturization of ammonia monitoring sensors is still a trend of gas detection, in order to ensure the miniaturization of the device at the same time, it is also necessary to ensure that the sensor has a low cost, easy to operate, high sensitivity and selectivity.