Shen ChangHong Ammonia nitrogen refers to the nitrogen in the form of free ammonia (NH3) and ammonium ion (NH4 +). The nitrogen content of animal organic matter is generally higher than that of plant organic matter. At the same time, the nitrogen-containing organic matter in human and animal feces is very unstable and easy to decompose into ammonia. Therefore, when the ammonia nitrogen content in water increases, it refers to the combined nitrogen in the form of ammonia or ammonium ion. The following problems should be paid attention to during the determination of ammonia nitrogen in water quality:
1、 Laboratory environment
(1) Attention should be paid to avoid dust phenomenon in the laboratory, pollution of water samples and reagents, affecting the level of experimental blank value, so as to affect the final test results;
(2) The ammonia used in the nitrate nitrogen test is highly volatile, and the ammonia after volatilization will exist in the air in the laboratory, while the Nessler reagent is very easy to absorb the ammonia after volatilization, which makes the ammonia nitrogen test result higher,
Therefore, the analysis items such as ammonium compounds should not be carried out at the same time with the nitrate nitrogen test, so as to avoid affecting the sensitivity of the reagent, thus affecting the test results;
(3) Reagents, glassware and other experimental articles used for ammonia nitrogen test shall be stored separately to avoid cross contamination,
Thus, the blank value is affected.
2、 Experimental water
The experimental water for ammonia nitrogen test is ammonia free water. Generally, the middle distillate is selected as the experimental water for ammonia nitrogen test after secondary processing with the fresh distilled water. In order to effectively ensure the precision and accuracy of the experimental blank value, the distilled water can neither contain ammonium salt nor free ammonia.
In order to prevent the ammonia in the air from melting into the water for the experiment, the distilled water used in the experiment should be sealed and the reagents related to the ammonium salt should be isolated and stored.
It is laborious to prepare the secondary distilled water. A large number of experiments show that the blank experimental value can reach about 0.030, which meets the requirements of ammonia nitrogen and ammonia free water.
3、 Pay attention to the preservation and pretreatment of sample water
1.1 preservation of water samples
When the ammonia nitrogen water sample to be measured is preserved, adding sulfuric acid drop can make the water sample acidify to pH less than 2, and prolong the preservation time.
The main principle is that when the pH of ammonia nitrogen water sample is less than 2, it can effectively inhibit the hydrolysis and oxidation-reduction reaction of ammonia nitrogen, so that the composition of the water sample remains stable, thus prolonging the storage time.
1.2 pretreatment of water sample
1.2.1 remove chromaticity, turbidity and some interfering substances
The flocculation sedimentation method can be used for pretreatment of clean water samples. The flocculation reagents used in the flocculation sedimentation method mainly include zinc sulfate and sodium hydroxide. Add a proper amount of zinc sulfate in the water samples, and then add sodium hydroxide to make the pH value of the water samples reach about 10.5, forming zinc hydroxide precipitation, and then filter out the color and turbidity.
1.2.2 deal with the problem of high background value caused by ammonium content in domestic filter paper
(1) The problem of ammonium content in domestic filter paper used in ammonia nitrogen test can be solved by soaking in 3% boric acid or 5% sulfuric acid and cleaning repeatedly. The filter paper can be treated manually to make it neutral and reduce the measurement error, which can well solve the problem of high experimental background value;
(2) It has been proved that the sand core funnel can also be used instead of the domestic filter paper, but before use, the sand core funnel also needs to be cleaned with tap water, soaked in dilute sulfuric acid for 10 minutes, washed with tap water and ammonia free water, dried or dried under control. The sand core funnel after treatment does not need to discard the primary filtrate when it is used, which can shorten the filtering time and improve the experimental efficiency to the greatest extent.
1.3 treatment of water samples with high ammonia nitrogen content
(1) For water samples with high concentration, the method of diluting first and then flocculation sedimentation or distillation can overcome the phenomenon of no color or turbidity in the determination process.
(2) When potassium sodium tartrate solution and Nessler's reagent are added to the water sample, the color will be darker, so that the absorbance value beyond the range can be determined after quantitative dilution with ammonia free water. This method is more suitable for the analysis and test of a large number of samples.
