Distillation-microcoulometric titration method and instrument for determining ammonia nitrogen in water

Abstract

The invention discloses a novel method and an instrument for measuring ammonia nitrogen in water. The method comprises the following steps: injecting 5ml of a to-be-measured sample or blank secondary water and a small amount of sodium hydroxide solution into a small distillation flask, distilling off water and generated ammonia gas, cooling, feeding into a 5ml volumetric flask filled with 1ml of electrolyte, taking down the volumetric flask when the solution just reaches a 5ml scale line, uniformly shaking, accurately measuring a certain amount of solution each time, the sample is injected into a micro-coulometric titration cell with a photoelectric indication signal in normal operation, the ammonia nitrogen content in the sample is automatically measured, and after a blank value is deducted, the instrument automatically displays the ammonia nitrogen content in the sample. Compared with a classical distillation-neutralization titration method, the method disclosed by the invention has the advantages of simple distillation device, less required sample amount, low energy consumption and no need of calibration solution. The method is especially suitable for measuring ammonia nitrogen in domestic sewage and industrial wastewater treatment plants.

Description

Technical Field

[0001] The invention relates to the technical field of determination of ammonia nitrogen in water, and belongs to a method and an instrument in the field of ecological and environmental analysis. Background Art

[0002] Ammonia nitrogen content in water is a key indicator of wastewater discharge and requires strict monitoring. Seven standard methods exist for its determination, most of which have strict requirements for sample color and particulate sediment, requiring pre-distillation of the sample. Consequently, most wastewater treatment plants prefer to use the HJ537-2009 distillation-neutralization titration method to determine ammonia nitrogen content in discharged wastewater.

[0003] The distillation-neutralization titration method has two disadvantages. First, its sensitivity is too low, requiring a large sample volume, a large distillation apparatus, and high energy consumption. Second, the solution needs to be calibrated daily, necessitating the development of a more advanced method. Summary of the Invention

[0004]

Solution

[0005] The characteristic of the present invention is that it also uses the classic pre-distillation method to convert all ammonia nitrogen compounds in the sample into ammonia gas, and also eliminates related interference factors. The ammonia content in the distillate is determined by microcoulometric titration with photoelectric indicator signal. Because the microcoulometric method is more sensitive than the volumetric method, the sample injection is small each time, and the distillation device can be very small. + It is produced by electrolysis and does not require daily calibration of the titration solution, which solves the problems of the classic distillation-neutralization titration method.

[0006]

Detailed description

[0007] The whole set of instruments consists of a set of 6-way small distillation devices and a microcoulometer with photoelectric indication signal. The small distillation device consists of a 10ml distillation conical flask, a polytetrafluoroethylene transparent tube with an outer diameter of ￠2 and an inner diameter of ￠1, a 5ml distillate receiving volumetric flask, a shared cooling water tank and a heating and temperature control system. The upper end of the tube is inserted into the silicone rubber cover of the conical flask, and the lower end is inserted below the absorption liquid level of the volumetric flask, with a cooling water tank passing through the middle. Before distillation, 1ml of electrolyte should be added to each absorption volumetric flask first, and 5ml of sample or blank water with a pH adjusted to about 7 and 0.2ml of 0.5% sodium hydroxide solution should be added to the distillation conical flask. The heating distillation can be started only after the cover is immediately sealed. The speed should not be too fast. The volumetric flask should be removed in time after the distillate reaches the scale line of the volumetric flask. The microcoulometer used is a general-purpose microcomputer coulometer with photoelectric indication signal, including a host, a computer, a stirrer and a photoelectric titration cell. There is an inlet on the titration cell, and electrolysis produces H +The anode is composed of a platinum sheet, the back is a Pt wire cathode, and there are pulse light emitting diodes and photodetector diodes on the left and right. The photoelectric signal caused by the addition of the sample is input to the host to generate H + It reacts equivalently with the ammonia gas therein, and the computer displays the signal and titration process. It automatically calculates the ammonia nitrogen content in the blank and sample based on the consumed electricity and displays the ammonia nitrogen content in the water sample in a table.

[0008]

Method demonstration 1

[0009] The volume of the titration cell is about 70 ml. Each time, add 60 ml of 4 g / L Na2SO4 aqueous solution, about 5 ml of the auxiliary electrode on the side wall, and 0.2 ml of methyl red alcohol solution. Adjust the maximum current to 1 mA. Use 350 μl of standard concentration Na2CO3 solution 35 mg N / L to generate H + The current efficiency is shown in Table 1. The data show that when the above electrolyte is used, as long as the current does not exceed 1 mA, the current efficiency can reach 100%, and the parallelism is very good.

[0010] Table 1 Current efficiency results (see spectrum Figure 1 )

[0011]

[0012]

Method demonstration 2

[0013] The measurement range and measurement accuracy of the entire method were determined using standard ammonium chloride solutions of different concentrations. The results are listed in Tables 2, 3, 4, 5, and 6.

[0014] Table 2 50mgN / L standard sample measurement results (see spectrum Figure 2 ,3)

[0015]

[0016] Table 3 10mgN / L standard sample measurement results (see spectrum Figure 4 )

[0017]

[0018] Table 4 1.25mgN / L standard sample measurement results (see spectrum Figure 5 )

[0019]

[0020]

[0021] Table 5 0.25mgN / L standard sample measurement results (see spectrum Figure 6 )

[0022]

[0023] The following points can be seen from the data in Tables 2 to 5:

[0024] 1. The detection limit of this method is about 0.25 mgN / L, which is suitable for the determination of ammonia nitrogen in domestic sewage.

[0025] 2. The sample and blank should be measured under the same conditions, especially the current and bias voltage. The higher the sample content, the greater the current. Therefore, the same blank will have a high value at a high content and a low value at a low content.

[0026]

Method demonstration 3

[0027] The pH value of the sample was adjusted to about 7, and the ammonia nitrogen in the wastewater discharged from the sewage treatment plant was measured according to the above-mentioned method for determining the standard sample. The results are listed in Table 6.

[0028] Table 6 Sample measurement results (see spectrum Figure 7 , 8, 9)

[0029]

[0030]

[0031] Note: Sample 3 is diluted 5 times as much as Sample 2.

[0032]

in conclusion

[0033] 1. The method can fully meet the minimum concentration of 1.5 mgN / L of domestic sewage discharge currently stipulated by the state;

[0034] 2. There are still problems in attempting to increase the injection volume by 50-100 times and use the microcoulometric full titration method to expand the method to low ammonia nitrogen samples of surface water and drinking water.

[0035]

Attached drawings

[0036] Figure 1 , current efficiency result spectrum;

[0037] Figure 2 , 50mgN / L standard sample measurement results (blank) spectrum;

[0038] Figure 3 , 50mgN / L standard sample measurement results (standard sample) spectrum;

[0039] Figure 4 , 10mgN / L standard sample measurement result spectrum;

[0040] Figure 5 、 1.25mgN / L standard sample measurement result spectrum;

[0041] Figure 6 , 0.25mgN / L standard sample measurement result spectrum;

[0042] Figure 7 , spectrum of sample measurement results;

[0043] Figure 8 , spectrum of sample measurement results;

[0044] Figure 9 , sample measurement result spectrum.

Claims

1. A method and apparatus for measuring ammonia nitrogen in water, characterized by the following; - The instrument mainly consists of two parts: a small distillation device and a microcoulometer with photoelectric indication signal; - The former is used to convert ammonia nitrogen compounds into ammonia gas and distill it out to eliminate sample color, particulate matter and various interfering substances; - The latter measures the ammonia content by generating hydrogen ions equivalent to ammonia through electrolysis, and automatically converts it into the ammonia nitrogen content in the water and displays the entire titration process and the final measurement results.

2. According to claim 1, the small distillation apparatus consists of a 10 ml glass conical flask, a polytetrafluoroethylene transparent tube with an outer diameter of ￠2 and an inner diameter of ￠1, a 5 ml volumetric flask, a shared cooling water tank and a heating temperature control system, and can distill up to 6 samples each time.

3. According to the requirements of claims 1 and 2, each sample is first adjusted to a pH of about 7, then 5 ml of each sample and 0.2 ml of 0.5% sodium hydroxide solution are accurately measured and added to a distillation flask, 1 ml of electrolyte is added to each absorption volumetric flask, and the distillation flask cap connected to the catheter is immediately covered, and each catheter outlet is inserted below the liquid level of the absorption volumetric flask. Heating and distillation can be started until the liquid level of the absorption volumetric flask reaches the scale line, and the absorption volumetric flask is immediately removed so that the total volume of the absorption liquid and the distillate is completely equal to the sample amount.

4. according to claim 3, must have 1-2 times that double distilled water is as blank sample each time.

5. According to claim 1, the electrolyte used in the microcoulometer is a 4 g / L aqueous solution of NaSO, and the indicator is a 50 mg / 50 ml methyl red alcohol solution. Add 0.2 ml at a time, with 60 ml added to the central cell and approximately 5 ml added to the auxiliary electrode compartments. A sodium carbonate standard solution is used to adjust the current and bias voltage appropriately, and only after a stable baseline is established can the sample be injected for analysis. Each injection should preferably be 0.35-0.5 ml, and the injector must be accurately calibrated.

6. As described in claim 5, the blank and sample should be measured at the same current and bias voltage, because different bias voltages and currents will have different blank values.

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