Online ammonia nitrogen measurement method

　　The main methods for ammonia nitrogen in China are: salicylic acid colorimetry, Nessler's reagent colorimetry and ammonia sensitive electrode method. Simply put, there are two categories: colorimetry and electrode method. Foreign countries basically use ammonia sensitive electrode method.

　　Introduction of Ammonia Sensitive Electrode Method

　　1. Principle

　　When the pH value is greater than 11, the ammonium ion turns into ammonia, and the ammonia transfers through the hydrophobic membrane of the ammonia-sensitive electrode, causing the electromotive force of the ammonia-sensitive electrode to change. The instrument measures the concentration of ammonia nitrogen based on the change in electromotive force.

　　2. Detection steps

　　Rinse the measuring water sample, reagent volume containers and electrode installation tubes with new water sample.

　　The sample is injected using a peristaltic pump. The water sample does not directly contact the peristaltic pump tube, and there is an air buffer. The volume of the injection is controlled by a visual measurement system.

　　As with sample injection, auxiliary reagents are also added via a peristaltic pump and the volume is controlled by a visual measuring system. The sample and reagents are mixed by bubbling. The reaction time is automatically controlled by the measuring system. The residual liquid is discharged by a peristaltic pump. During the user-defined measurement cycle, the analyzer is automatically calibrated and cleaned using the built-in calibration standard solution and cleaning solution.

　　3. How to distinguish the performance of the ammonia sensitive electrode method instrument

　　3.1. Range: The range specifications of ammonia nitrogen by electrode method are divided into: 0-1200; 0-2000; 0-3000; 0-10000. And the range can be switched freely. The larger the range, the stronger the adaptability of the electrode used in the instrument.

　　3.2. Minimum detection limit: The lower the minimum detection limit of the instrument, the better the quality of the electrode, which is generally 0.05 mg/l.

　　4. Comparison between the ammonia sensitive electrode method and the traditional Nessler reagent colorimetric method.

　　Electrode method

　　Response time: fast, continuous testing can be achieved, the fastest is only 3 minutes, and the longest is 10 minutes for low-range fine measurement below 1 mg/L. The test range is wide, from 0.00-10000 mg/l NH4-N, and the full range test can be achieved with only one electrode. The instrument can automatically switch the range and automatically adjust the resolution.

　　Minimum detection limit: 0.05 mg/l

　　Interference: Strong anti-interference ability, not affected by chromaticity and turbidity, no need for additional compensation.

　　Injection requirements: No special requirements.

　　Reagent operation cost: low, the electrode method does not require colorimetric reagents, the electrode has a long service life, the reagent formula is public, and domestic reagents are used, which are convenient and cheap to purchase.

　　Lifespan: The electrodes have a long service life and the cost of replacing them is low.

　　Colorimetry

　　Response time: Slow, can only be tested in batches, need to wait for the color reaction to complete before testing. One measurement takes at least 30 minutes.

　　Test range: small range, or segmented range. When changing the range, a new instrument must be replaced (the range is determined by the colorimetric cell), and the resolution is low.

　　Minimum detection limit: 5.0 mg/l

　　Interference: It is easily affected by the color and turbidity of the sample, and the photometric method is easily affected by changes in ambient temperature, humidity and other conditions.

　　Injection requirements: Strict to avoid contamination of optical components and affecting absorbance.

　　Test reagent cost: High

　　Lifespan: The light source is aging, and the cost of replacing the light source is high. The colorimetric cell should be replaced regularly.