Abstract: Nessler's reagent colorimetry has the advantages of simplicity, rapidity and high sensitivity, and is widely used in the determination of ammonia nitrogen in water. This paper discusses several problems that should be paid attention to when determining ammonia nitrogen by Nessler's reagent colorimetry: the choice of pretreatment method; Eliminate interference in water samples; Problems that should be paid attention to in preparing potassium sodium tartrate solution and Nessler's reagent, and the control of color development conditions, etc.

Key words: Nessler's reagent colorimetry, pretreatment, Nessler's reagent, color development conditions

1 Selection of pretreatment methods

The water sample is colored or turbid, and contains other interfering substances, which affects the determination of dim and needs corresponding pretreatment. For clean water samples, flocculation precipitation method [1] can be used, and for seriously polluted water or industrial wastewater, distillation method [1] can be used for pretreatment to eliminate interference. Among them, the former is the preferred method because it is simpler and faster.

1. 1 Flocculation precipitation method and its improvement

1. 1. 1 instrument

100ml plug measuring cylinder or colorimetric tube

1. 1.2 reagent:

(1) 10% sulfuric acid solution

(2)25% sodium hydroxide solution

1. 1.3 step

Take 100ml water sample in a graduated cylinder or colorimetric tube with plug, add 1ml 10% zinc sulfate solution and 2~4 drops of 25% sodium hydroxide solution, adjust the pH value to about 10.5, mix well, and let stand for precipitation. Take a proper amount of supernatant for later use. There is an improvement in this method, that is, the supernatant after standing is taken without filter paper filtration. The standing time shall be based on the fact that floc cannot be taken during sampling.

1. 1.4 discussion: in the fourth edition of monitoring and analysis methods for water and wastewater, the water samples after flocculation and precipitation are filtered with medium-speed filter paper which is not fully washed with ammonia water, and the filtrate after the initial filtrate of 20ml is discarded. Some experiments show that the content of ammonium salt varies greatly between different filter papers or on the same filter paper, and some filter papers with high content can not meet the experimental requirements even after repeated washing. So every batch of filter paper should be sampled and washed several times before use. It is also found that there are about 0.25% soluble substances in filter paper, and the average weight loss rate of filter paper is 0.58%, which will affect the accuracy of analysis results. This disadvantage is avoided by directly taking the supernatant.

2 eliminate all kinds of interference in water samples:

In practical work, due to the diversity of samples and the complexity of interferents, sometimes the color solution of samples after flocculation and precipitation pretreatment is turbid, which seriously affects the light transmittance and causes high results. At this time, distillation pretreatment method should be adopted. Methods See Monitoring and Analysis Methods of Water and Wastewater (4th Edition).

2. 1 Elimination of color (turbidity) interference.

Take 50 ml water sample in a 50 ml colorimetric tube, add 1.00 ml potassium sodium tartrate solution and 1.00 ml potassium hydroxide solution, and measure the absorbance (corrected absorbance). After the water sample is colored with Nessler reagent, the measured absorbance is subtracted from the corrected absorbance.

2.2 Eliminate the interference of metal ions.

In alkaline environment, metal ions are easy to hydrolyze, and potassium and sodium tartrate are generally added for complexing; Adding a small amount of sodium thiosulfate can mask mercury-containing salts; When Mn2+ is contained, the interference of Mn2+ can be masked by using 50% potassium sodium tartrate1.00 ml+2% Na2EDTA1.00 ml instead of pure potassium sodium tartrate [2]; It contains a large number of metal ions such as copper and iron, which are determined after distillation pretreatment.

2.3 Eliminate organic interference.

When the water sample contains organic substances such as glycine, hydrazine and some amines, the pH value of the water sample is adjusted to about 9.5 for distillation. When it contains ketones, aldehydes and other amines, it is removed by boiling at low pH value.

2.4 Countermeasures for turbidity of chromogenic solution

After flocculation and precipitation pretreatment, the supernatant was taken, and after adding potassium sodium tartrate solution and Nessler's reagent, sometimes turbidity appeared, which seriously affected the light transmittance and the error was great. The author often encounters this situation when measuring the effluent water sample of sewage treatment plant. The color solution without potassium and sodium tartrate is not turbid, which indicates that it is the problem of potassium and sodium tartrate. Potassium sodium tartrate solution purified by (3. 1) method was determined after distillation pretreatment.

3 reagent preparation should pay attention to the problem

The purity of drugs and the configuration method of reagents will affect the experimental results.

3. The purity of1potassium sodium tartrate is directly related to the determination result, which leads to the high blank value of the experiment and the turbidity of the actual water sample, which affects the determination and needs to be purified to remove the ammonium salt. In practical work, there are two ways to deal with it.

① Purify the potassium sodium tartrate solution (50%) with Nessler's reagent, the dosage of Nessler's reagent is 2% of the volume of potassium sodium tartrate solution, and the blank absorbance is the smallest and basically stable;

(2) Add a small amount of lye to the potassium sodium tartrate solution, boil and evaporate it to about 50mL, and then cool it to constant volume of 100mL. Experiments show that the blank values purified by the above two methods can also meet the requirements of analysis and determination.

3.2 Preparation of Nessler's reagent

Understanding the color principle of Nessler's reagent for determination of ammonia nitrogen is helpful to understand the preparation method of Nessler's reagent. The principle is as follows: 2k2 [hgi4]+3NaOH+NH3 → nh2hgio+3nai+4ki+2h2o.

There are two preparation methods of Nessler's reagent, both of which can produce chromogenic group [HGI 4] 2. In the first method, mercury chloride and potassium iodide are used, and the key point is that the amount of HgCl2 added determines the content of chromogenic groups, which in turn affects the sensitivity of the method. However, this method does not give an accurate dosage of HgCl2, and it needs to be judged according to the phenomena in the process of reagent preparation, which is empirical and difficult to grasp. According to the experience, when the dosage ratio of HgCl2 to KI is 0.44∶ 1 (that is, 8.8gHgCl2 is dissolved in 20gKI solution), the effect is very good. I won't go into details here. In the second method, mercuric iodide and potassium iodide are used: 16g sodium hydroxide is weighed, dissolved in 50ml water, and fully cooled to room temperature. Weigh 7g of potassium iodide and 10g of mercuric iodide and dissolve them in water, slowly inject the solution into sodium hydroxide solution while stirring, and dilute it to 100ml with water. Pay special attention to the ratio of mercury iodide to potassium iodide here. I- should not be excessive, otherwise the reaction will be reversed, the chromogenic group [hgi 4]2- will decrease, and the color of Nessler's reagent will become lighter. The low point of ammonia nitrogen working curve made with this Nessler reagent is insensitive, almost no difference, and the linearity is poor, so the experiment will fail. Mercury iodide is slightly soluble in water, and only when I- exists in the solution can it react to form [hgi 4]2- red precipitate and disappear. In excess, it exists in the form of red mercuric iodide precipitation, which will not reverse the color reaction. Therefore, in practical work, mercury iodide should be slightly excessive, and the prepared Nessler's reagent should be allowed to stand, and then the precipitate should be discarded, carefully poured into polyethylene bottles, plugged and stored at low temperature.

4 control of color reaction conditions

4. 1 reaction temperature and time. The experiment shows that when the reaction temperature is 25℃ and the reaction time is 10 ~ 30 min, the color development is the most complete and the color of the solution is stable. In practical work, the color temperature is controlled at 20℃ ~ 25℃ and the time is controlled at 10min, which makes the monitoring data accurate and reliable.

4.2 pH value of the reaction system. The change of pH value of water sample has a significant effect on color development. The water sample is neutral or alkaline, the relative deviation of the determination results meets the analysis requirements, and the water sample is acidic and incomparable. Experiment [3] found that the measured value of water sample was 0.24mg/L when it was acidic, 1.03 mg/L when it was alkaline, and 0.92 mg/L when it was neutral. The experiment shows that [4]: When the pH of the solution

Conclusion: Nessler's reagent colorimetry is sensitive, easy to operate and popularize. Different pretreatment methods should be selected for different water samples, otherwise it will bring great errors to the results. For relatively clean water samples with less interference, simple and time-saving flocculation precipitation method can be used to take supernatant to avoid ammonia nitrogen pollution introduced by filter paper filtration. For water samples with serious pollution and interference, distillation should be used for pretreatment. According to different interference, corresponding elimination measures should be taken. The preparation of reagents is also very important. The commercial potassium sodium tartrate was purified to eliminate the error caused by high ammonium salt. The preparation of Nessler's reagent should be slightly excessive, and a small amount of red precipitate will not affect the experimental results. On the contrary, excessive potassium iodide will lead to color insensitivity and the experiment will fail. Controlling the color development time and temperature and the pH value of the reaction system are also important conditions for obtaining accurate and reliable results.

References:

[1] Editorial Board of Monitoring and Analysis Methods of Water and Wastewater, State Environmental Protection Administration. Monitoring and analysis methods of water and wastewater. Fourth edition. [M] Beijing: China Environmental Science Press, 2002

Ding Jiansen, Li Ling. Discussion on the influence of manganese in drinking water on the detection of ammonia nitrogen [J] Shanghai Journal of Preventive Medicine,1997,9 (10): 474–475.

[3] Su Aimei, Wang Junrong. Discussion on Some Problems in Determination of Ammonia Nitrogen [J] Drought Environment Monitoring, 2003,17 (2):123-125.

Chen Guoqiang, Lu Mingyu. Determination of ammonia nitrogen in domestic sewage by ion-selective electrode method [J]. Chongqing Environmental Science,1998,20 (3): 58-90

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