JPH0579137B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for measuring total nitrogen. More specifically, the present invention relates to a method for accurately measuring the total amount of nitrogen in seawater while eliminating the influence of bromides. It is also useful for measuring seawater pollution, especially eutrophication, predicting the occurrence of red tide, and managing aquaculture in marine areas. Prior Art In the prior art, when measuring total nitrogen in seawater, the only way to eliminate the influence of bromide and obtain correct total nitrogen measurements is to perform complex chemical operations to separate bromide and nitrogen. A method was used in which the amount of nitrogen was measured after separating the two.
Therefore, measurement takes time, requires special techniques, and has the problem that it cannot be automated because the operation is particularly complicated. Although automatic total nitrogen measuring devices have been developed in recent years, these automatic total nitrogen measuring devices were not equipped with a function to separate bromide. Therefore, although this automatic total nitrogen measuring device could be used for lake water and river water samples that do not contain bromide, it had a major drawback in that it could not be used for seawater. Problems to be Solved by the Invention As described above, in the prior art, the amount of nitrogen was measured after separating bromide and nitrogen by performing complicated chemical operations, which took time and required only an experienced person. There were problems in that it could not be operated or automated equipment could not be created. In the conventional method for measuring total nitrogen, sample water is placed in a high-pressure steam sterilizer or a similar container made of pressure-resistant, heat-resistant, chemical-resistant material, and an oxidizing agent such as potassium peroxodisulfate and, if necessary, sodium hydroxide are added to the water sample. Add an alkaline agent such as, seal the container, and heat. Through this operation, all nitrogen compounds are oxidized and decomposed into nitrate nitrogen.
After adjusting the pH of the decomposition solution obtained in this way, the amount of nitrogen is determined by measuring light absorption in the ultraviolet region, for example, absorbance at 220 nm. In this case, if bromide coexists in the sample water, some of the bromide may become a bromate during the oxidative decomposition process. Both bromide and its oxidation product, bromate, absorb in the ultraviolet region, e.g. 220 nm, so when measuring nitrogen, either bromide or bromate will introduce a positive error. become. At this time, the rate at which bromate is generated from bromide is not constant and depends on the salt concentration in the sample water, and the absorption strength of bromide and bromate,
Another problem is that the molar extinction coefficients are different. Means for Solving the Problems In view of the problems of the prior art described above, the present inventor has devised a bromide solution that is easy to operate, does not require skill, and allows for easy creation of an automated device. We have been conducting extensive research to find a method for measuring total nitrogen that eliminates this influence. As a result, the inventors discovered that the object can be achieved by the method described below, and have now completed the present invention. That is, in the case of measuring total nitrogen in a sample containing bromide by a light absorption method, the present invention dilutes a sample with a salt concentration exceeding 2% to bring the salt concentration to 2% or less, then adds an oxide, After converting the nitrogen compound to nitrate nitrogen by heating under pressure, measure the absorbance at two wavelengths in the ultraviolet region where both nitrate nitrogen and bromide absorb, and use the obtained absorbance to calculate the total nitrogen amount. This invention relates to a method for measuring total nitrogen that excludes the influence of bromides. In the method of the present invention, in the case of a seawater sample, the sample water is first diluted with pure water. For example, dilute it twice with pure water to bring the salt concentration below 2.0. Next, this sample water is placed in a high-pressure steam sterilizer or a similar pressure-resistant, heat-resistant, and chemical-resistant container, an oxidizing agent and, if necessary, an alkali agent are added, and the container is sealed and heated. The heating conditions may be the same as those in the conventional method for measuring total nitrogen. The time for this thermal decomposition is usually 30 minutes. Next, take out the decomposition liquid,
If necessary, add acid to adjust the pH to, for example, 2 to 3. The absorbance of this solution is measured, and the measurement wavelengths are two wavelengths in the ultraviolet region where both nitrate nitrogen and bromide have absorption, for example, 220 nm and 210 nm.
Alternatively, select two wavelengths such as 220nm and 215nm. Finally, the amount of nitrogen is calculated from the absorbance at these two wavelengths. The principle of the present invention is as follows. In other words, in the case of a seawater sample with a high salinity concentration, first dilute the sample water with pure water to bring the salinity concentration to 2.0% or less, or use a sample water with a salinity concentration of 2.0% or less, such as a sample that is a mixture of seawater and river water. In the following cases, decompose the undiluted sample water at high temperature and pressure in the presence of an oxidizing agent or alkaline agent to convert nitrogen compounds to nitrate nitrogen. When operated in this manner, the coexisting bromide does not change to bromate because the salt concentration is relatively low, and remains as bromide. The absorbance of the solution after the decomposition operation is measured at two wavelengths in the ultraviolet region in which both nitrate nitrogen and bromide absorb. However, because their molar extinction coefficients are different, for example, if 1 ppm of bromide is present,
0.0021 at 220nm, 0.0074 at 215nm, 210nm
shows an absorbance of 0.0225. On the other hand, nitrate nitrogen, which is a change from total nitrogen to be measured, also exhibits different absorbance at each wavelength. for example,
1ppm nitrate nitrogen is 0.2134 at 220nm, and 0.2134 at 215nm.
0.3486, with an absorbance of 0.4166 at 210 nm. Therefore, if 1 ppm of nitrate nitrogen and 1 ppm of bromide coexist, at each wavelength,
The absorbance corresponds to the sum of the respective absorbances. In reality, this sum of absorbance is further supplemented by salt, other components in seawater, and the absorption of the absorption cell itself used for measurement. As mentioned above, since both nitrate nitrogen and bromide exhibit different absorbance at each wavelength, we can calculate the absorbance per unit concentration in advance using seawater with a known concentration at any two wavelengths in the ultraviolet region where both nitrate nitrogen and bromide absorb. If you calculate this, you can measure the absorbance of an actual seawater sample at these two wavelengths and use calculations to determine the correct amount of nitrogen. Examples The present invention will be explained in more detail by examples. Example 1 Artificial seawater was prepared, and nitrate nitrogen and bromide of known concentrations were added thereto. That is, nitrate nitrogen is 0, 0.50, 1.0, 1.5 ppm each, bromide is 10, 20,
40, 60, and each ppm were added. Next, this artificial seawater was diluted twice with pure water to reduce the salinity to 2.0% or less. Nitrate nitrogen in this diluted seawater is 0, 0.25, 0.50, and 0.75 ppm, and bromide is 5, 10, 20, and 30 ppm. This solution was placed in a high-pressure steam sterilizer, potassium peroxodisulfate and sodium hydroxide were added, and the solution was heated and decomposed for 30 minutes in a closed state. The solution was taken out and acid was added to adjust the pH to 2-3. And 220n
Absorbance was measured at three wavelengths: m, 215 nm, and 210 nm. The dilution due to the addition of various reagents was calculated, and the average absorbance per ppm of nitrogen and bromide was calculated using an absorption cell with an optical path length of 10 mm. The results are shown in Table 1.

[Table] From this result, when nitrogen and bromide coexist, the absorbance at each wavelength can be expressed by the following formula when using an absorption cell with an optical path length of 10 mm. In these formulas, the contents of N and Br are expressed in ppm, and A indicates the absorbance at each wavelength. A220=0.2134N+0.0021Br+0.0140...(1) A215=0.3486N+0.0074Br+0.0260...(2) A210=0.4166N+0.0225Br+0.0810...(3) Next, any two of these equations By solving the simultaneous equations using these two equations, we obtain equations (6) and (9) below. For example, when using equations (1) and (3), first multiply both sides of equation (1) by 10.7 to obtain equation (4). 10.7A220=2.283N+0.0225Br+0.150...(4) Subtracting equation (3) from equation (4) yields equation (5). 10.7A220−A210=1.866N+0.0690 ……(5) Transforming equation (5), we obtain equation (6). N=10.7A220−A210/1.866−0.0370……(6) Similarly, when using formulas (1) and (2), first multiply both sides of formula (1) by 3.52 to obtain formula (7). 3.52A220=0.7512N+0.0074Br+0.0493...(7) Subtracting equation (2) from equation (7) yields equation (8). 3.52A220−A215=0.4026N+0.0233 ……(8) Transforming equation (8), we obtain equation (9). N=3.52A220-A215/0.4026-0.0579...(9) That is, by this method, for example, 220nm
Measure the absorbance at two wavelengths: and 210nm, or 220nm and 215nm, and calculate the absorbance at each wavelength.
By putting it into equation (6) or equation (9) and solving each equation, the concentration of N can be determined in ppm value. After that, the nitrogen concentration in the original seawater sample can be determined correctly by diluting the sample with pure water and correcting the volume with various reagents in the first operation. Here, for convenience, 220n
Although m and 210 nm and 220 nm and 215 nm were selected as two arbitrary wavelengths, the present invention is not limited to these wavelengths. Note that according to the present invention, the concentration of bromide ions can also be determined as necessary. Example 2 Coastal seawater was collected from Amagasaki City, Nishinomiya City, and Kobe City in Osaka Bay, and was designated as Seawater 1, 2, and 3. The absorbance of these samples was measured in the same manner as in Example 1. In addition, samples in which a certain amount of nitrogen or bromide was added to these seawaters were similarly treated and measured. At the same time, the nitrogen concentrations in these samples were determined by spectrophotometry using an autoanalyzer. The results are shown in Table 2. Based on the measured value by the autoanalyzer, the error in the measured value by the method of the present invention was calculated for the case of using equation (6) and the case of using equation (9). As a result, it was found that the results agreed with the autoanalyzer measurement results with an error of about 7% or less.

【table】

Claims (1)

[Claims] 1 When measuring total nitrogen in a sample containing bromide by optical absorption method, dilute a sample with a salt concentration of more than 2% to bring the salt concentration to 2% or less, then add oxides and heat under pressure. After turning the nitrogen compound into nitrate nitrogen, the absorbance is measured at two wavelengths in the ultraviolet region where both nitrate nitrogen and bromide have absorption, and the total nitrogen amount is calculated using the obtained absorbance. A total nitrogen measurement method that eliminates the effects of bromides.