| Total nitrogen is one of the important indicators to control water quality,and is a routine monitoring project for lakes,reservoirs and rivers.The determination of total nitrogen in water is listed as the basic item of national surface water environmental quality standard.At the same time,this index is also the basic control item in the pollutant discharge standard of urban sewage treatment plant.Therefore,it is necessary to determine the total nitrogen in water accurately.Ultraviolet Spectrophotometry with alkaline potassium persulfate digestion(HJ 636-2012)is a commonly used method for determination of total nitrogen in water quality.Although this method is simple to operate,it is affected by many factors such as reagents in the actual detection process.In this paper,the factors affecting the test results are systematically analyzed,and the experimental error properties are clarified.Some links are improved and optimized.The main results are as follows:1.Water consumption is one of the most direct factors affecting test blanks.Laboratory errors come from hardware and operation errors.For laboratories with pure water preparation conditions,ultra-pure water can be selected as test water;for laboratories without pure water preparation conditions,Watson’s drinking water can be selected as test water.2.Potassium persulfate is one of the important factors affecting the test blank.The laboratory error comes from the hardware error.(1)According to the analysis results of blank absorbance and standard sample,potassium persulfate can be dissolved at 50℃+manual stirring,and the solution is prepared with good results.(2)The recommended storage time of alkaline potassium persulfate solution in thecurrent standard of potassium persulfate digestion ultraviolet method is 1 week.The storage time of alkaline potassium persulfate solution can be extended to 2 weeks(according to 12 days)according to the measured absorbance and relative deviation of ultra-pure water in laboratory.3.Dissolution method is one of the important factors affecting the results of test analysis.Laboratory errors come from hardware and operation errors.(1)Under the same digestion time(30 min),it is relatively appropriate to select a pressure steam sterilizer to keep the digestion mode for 30 min at 130℃.(2)At the same digestion temperature(120℃),it is relatively appropriate to select a pressure steam sterilizer for 45 minutes at 120℃.(3)Under the same digestion time(30 minutes),it is relatively appropriate to choose the digestion mode of electrothermal constant temperature drying box at 140℃ for 30 minutes.(4)At the same digestion temperature(120℃),it is relatively appropriate to select the electrothermal constant temperature drying chamber to keep the digestion for 40 minutes at 120 ℃.(5)It is feasible to use Yiyao WX-8000 microwave digestion instrument with 5 ml sample size and 2.5 ml alkaline potassium persulfate solution to maintain 30 minutes digestion at 124℃ and 5 ATM pressure.4.The containers used in digestion are one of the important factors affecting the results of test analysis.The laboratory errors come from the errors in hardware and operation.(1)It is feasible to use a covered PTFE digestion tank to digest in a pressure steam sterilizer at 130℃ for 30 minutes.(2)It is feasible to use a covered PTFE digestion tank to digest in an electrothermal constant temperature drying box at 140℃ for 30 min.(3)The PTFE digestion tank with cover has the advantages of strong material,no leakage,avoiding pollution or loss of digestion solution,and being able to withstand certain temperature and pressure,compared with the common glass colorimetric tube with plug.However,the heating temperature should be higher than that of the glass container by more than 20 degrees Celsius to obtain a better digestion effect.Based on the difference of method principle,the empirical principle of corrected absorbance formula in alkaline potassium persulfate digestion ultravioletspectrophotometry limits the accuracy of determination.In this paper,the experimental comparison shows that the determination of total nitrogen in water by alkaline potassium persulfate digestion ultraviolet spectrophotometry and gas-phase molecular absorption spectrometry is affected by the complexity of water samples,and there is a certain degree of interference in the determination of different water samples by ultraviolet spectrophotometry,while the interference degree of gas-phase molecular absorption spectrometry is obviously smaller than that by ultraviolet spectrophotometry.In terms of anti-interference ability and accuracy of detection results,gas-phase molecular absorption spectrometry has certain advantages.In view of the shortcomings in the detection process of alkaline potassium persulfate digestion ultraviolet spectrophotometry,this paper considers,proposes and verifies the optimized method of total nitrogen detection and carries out practical detection and application.The main results are as follows:1.Verify the use of hydrogen peroxide or hydrogen peroxide+sulfuric acid as oxidation reagent to detect total nitrogen in water.Under the conditions set up in this experiment,good results have not been achieved,and further research needs to be improved.2.Polytetrafluoroethylene(PTFE)digestion tank was selected to digest the samples with alkaline potassium persulfate solution as oxidant under the condition of constant temperature drying box at 140 C and 30 min.After digestion,total nitrogen was determined by gas phase molecular absorption spectrometer.According to the test data,the detection limit of this method is 0.02mg/L.The relative standard deviation of the actual samples with different kinds and concentrations,such as clean surface water,effluent from sewage treatment plant and treated water samples from chemical industry,is 0.58%-7.1 1%,and the recovery rate is 95%-103%.The total nitrogen standard sample numbered 203251 by the Ministry of Environmental Protection was determined with an average value of 2.55 mg/L in the range of 2.31-2.65 mg/L.3.According to the minimum detection limit,precision,accuracy and comparative test of the method,the optimum method is proved to be a feasible method for the determination of total nitrogen.The optimization method is simple,fast and easy to operate.The minimum detection limit of the method is lower than that of the national standard method,and it has ideal precision and accuracy.Two methods were used to detect and compare different types of water samples.The results showed that there was a good comparability between the two methods,and there was no significant difference. |
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