CA1115630A

CA1115630A - Analytical method and apparatus for the determination of nitrogen derived from nitrites or nitrates in aqueous systems

Info

Publication number: CA1115630A
Application number: CA320,395A
Authority: CA
Inventors: Tadamasa Itoh
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 1978-01-27
Filing date: 1979-01-29
Publication date: 1982-01-05
Also published as: DE2902876A1; GB2013339B; NL7900699A; GB2013339A; BE873732A; FR2422953A1; FR2422953B1

Abstract

ANALYTICAL METHOD AND APPARATUS FOR THE DETERMINATION OF
NITROGEN DERIVED FROM NITRITES OR NITRATES IN AQUEOUS SYSTEMS

Abstract of the Disclosure The specification discloses an analytical method for the determination of nitrogen derived from nitrites or nitrates contained in aqueous systems, which comprises react-ing an aqueous solution containing nitrites or nitrates with an aqueous solution of sulfamic acid in a reactor which is provided in a carrier gas stream having substantially no nitrogen gas. In the case of a nitrates-containing aqueous solution, the solution is first subjected to a reduction of the nitrates into nitrites, then the nitrogen gas produced by the reaction is removed with the carrier gas, and the nitrogen gas is detected with a nitrogen gas detector. The specification also discloses apparatus for carrying out the method. In this way, nitrites or nitrates can be detected in extremely small quantities in water. The method is simple, inexpensive and very easy to carry out.

Description

1~1563~

The present invention relates to an analytical method for the determination of nitrogen derived from nitrites ' or nitrates which are contained in slight amounts in aqueous systems, such as natural bodies of water (e.g. sea water, river water, lake or marsh water) and various waste waters, and an apparatus therefor.
In view of the problems presented by environmental pollution in large bodies of water, such as nutritional en-richment, various studies have been carried out for the treat-ment of waste water from industrial and sanitation installa-tions. Furthermore, when testing the potability of water, attention is paid to the presence of nitrogen compounds con-tained in the water as an index for the safety of drinking water. There has therefore been a need to develop a rapid, accurate and inexpensive analytical method and apparatus for the determination of nitrogen derived from nitrites or nitrates contained in trace amounts in water.
A conventional analytical method is the Griess method which comprises diazotizing nitrites contained in water with sulfanilic acid, adding a-naphthylamine thereto, and measuring the absorbance of the resulting diazo compound of color. However, the sensitivity of this method is reduced by the presence of oxidizing agents such as residual chlorine, permanganates or hydrochlorates, and also reducing agents such as sulfites, ferric compounds or sulfides, and further by ammonium ions, urea, aliphatic primary amines or the like, when they are present in large amounts in the system. Other interfering substances are metal ions (e.g. silver ions, bismuth ions, lead ions) which produce precipitates in the presence of hydrochloric acid; and iron, gold and metavanadate which produce precipitates in the presence of a-naphthylamine;

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and further ions having a color (e.g. cobalt ions, nickel ions, chromium ions, chromate ions, dichromate ions). In order to obtain accurate analytical values, careful attention must be paid to the influence of these co-existing substances and the reaction used in the analysis, and further, persons employing the analytical method must have a high level of knowledge and skill. A semi-automatized form of the Griess method is used in instrumental analysis, but this method is also affected by co-existing substances as much as the analysis which is carried out by hand.
It is well known that sulfamic acid is quantatively reacted with nitrous acid to produce nitrogen gas and sulfuric acid even at low temperatures, as shown by the fol-lowing equation:

02NH2 + HN02 = H2504 + H20 + N2 Hence, a method for the determination of nitrous acid using sulfamic acid as a standard is employed in Japanese Industrial Standard (JIS~ and Japanese Pharmacopeia. However, according to this method, it is impossible to determine nitrogen de-rived from slight amounts of nitrites contained in wastewaters or natural bodies of water. No analytical method and apparatus was therefore known for the simple, rapid and practical determination of nitrogen derived from small amounts of nitrites in which the above reaction is utilized.
In order to analyze nitrate ions in water, the nitration of phenolic compounds is employed, e.g. methods using phenoldisulfonic acid or sodium salicylate, which are employed in the examination of service water. According to these methods, nitrate ions are determined by measuring the concentration of the colored products which are produced by the nitration of phenol derivatives. However, these methods ~1563{) are also affected by the presence of chlorine ions and nitrite ions, and further, the water to be analyzed must be evaporated to dryness on a water bath for nitration. Thus, these methods are not necessarily easy to carry out, but they can be used for determining nitrogen derived from nitrates in concentra-tions of about 0.01 ppm, in the case of aqueous systems ¢on-taining few interfering substances, such as tap water.
A brucine method is also used in the examination of service water and also in JIS K-0102, which involves the oxi-dation of organic compounds. This method comprises oxidizingbrucine with nitrate ions in the presence of conc. sulfuric acid and measuring the concentration of the colored product, in which the oxidized product shows a red color and thereafter changes to a yellow color. However, this method is also affected by chlorine ions and nitrite ions in just the same way aR the methods using phenoldisulfonic acid or sodium salicylate. Furthermore, when the aqueous systems to be analyzed contain oxidative or reductive substances, they must first be treated with a reducing or oxidizing agent, respectively. Moreover, when the aqueous systems are turbid or colored, they must first be treated with an aqueous sus-pension of aluminum hydroxide and active carbon.
Under these circumstances, the present inventors have investigated new analytical methods for the determination of nitrogen derived from nitrites or nitrates contained in aqueous systems, and have found that nitrogen derived from nitrites can be determined rapidly and accurately by reacting the nitrites contained in the aqueous system to be analyzed with an aqueous solution of sulfamic acid in a reactor, removing the produced nitrogen gas from the aqueous system with a carrier gas and measuring the nitrogen gas with a 1~1563~

nitrogen gas detector. Furthermore, they have found that nitrogen derived from nitrates contained in an aqueous system can also be determined by first converting the nitrates to nitrites by reduction and then following the above procedure.
Thus, according to the invention there is provided a method for the determination of nitrogen derived from nitrites con-tained in an aqueous system, which comprises reacting an aqueous solution containing nitrites to be determined with an aqueous solution of sulfamic acid in a reactor which is pro-vided in a carrier gas stream removing the nitrogen gas thusproduced with the carrier gas from the aqueous system, and measuring the nitrogen gas with a nitrogen gas detector.
An advantage of the present invention, at least in preferred forms, is that it can provide an improved analytical method for the determination of nitrogen derived from nitrites which are contained in small amounts in aqueous systems.
Another advantage of the invention, at least in preferred forms, is that it can provide an improved analytical method from the determination of nitrogen derived from nitrates whi~h are con-tained in a slight amount in aqueous systems.
When the nitrogen is contained in the aqueous systemin the form of nitrates, the nitrates can first be reduced with a reducing agent to nitrites and thereafter the aqueous ~olution containing the resulting nitrites can be subjected to the above procedure.
The aqueous solution of sulfamic acid used in the present invention is usually an aqueous solution containing 0.1 to 15% by weight of sulfamic acid. The aqueous solution should usually contain sulfamic acid in a larger amount than that necessary for converting all nitrites contained in the a~ueous system into nitrogen. The aqueous solution may 1~1563a) contain sulfamic acid alone, but it is preferable that it also contain a salt such as sodium chloride, by which the nitrogen gas produced by the reaction is more rapidly turned out from the aqueous system with the carrier gas.
The salt may be any water-soluble salt which is not reacted with sulfamic acid, for example, chlorides, sulfates, nitrates or phosphates of alkali metals (e~.g. sodium, potas-sium), alkaline earth metals (e.g. magnesium, calcium), or ammonium. Among them, chlorides, sulfates, nitrates or phosphates of alkali metals or ammonium are preferred. The concentration of the salt is not critical, but is usually in the range of 0.1 to 30% by weight, preferably 0.5 to 10%
by weight. The salt may be added to the aqueous solution con-taining nitrites or nitrates instead of being added to the aqueous solution of sulfamic acid, or may be added to both solutions.
The order of addition of the aqueous solution to be analyzed and the aqueous solution of sulfamic acid into the reactor is optional, but from the practical viewpoint, it is preferable first to add a large amount of the aqueous solution o sulfamic acid and thereafter to add the aqueous sGlution to be analyzed, because in this manner a large number of aqueous solutions can be analyzed without adding a new solution of sulfamic acid.
Nitrates contained in the aqueous solution to be analyzed are first reduced to nitrites by known methods as described in JIS K-0102 or as employed in the examination of drainage. Preferably, the nitrates are reduced by treating them with zinc powder in a neutral or weakly alkaline range in the presence of an ammonium compound. Suitable examples of the ammonium compound are ammonium carbonate, ammonium acetate, - 1~1563~

ammonium chloride, ammonium sulfate, ammonium citrate, or the llke, which are used at a concentration of 0.1 to 0.5~ by weight (as ammonium ions) based on the weight of nitrate;;ion.
Zinc powder is preferably used in an amount of 0.2 to 10~
by weight based on the weight of the nitrate ions. The re-duction of nitrates with zinc powder is carried out by adding an ammonium compound and zinc powder to the aqueous solution containing nitrates and shaking the mixture for about 20 seconds or longer, preferably for about 20 to 180 seconds.
After reducing nitrates contained in the aqueous solution to be analyzed, the resulting aqueous solution con-taining nitrites thus produced is subjected to the determina-tion of nitrogen of the present invention, by which the ni-trogen derived from the nitrates can be determined.
Aqueous solutions to be analyzed usually contain both nitrites andnitrates, and hence, when the aqueous solu-tions are analyzed after reducing the nitrates as mentioned above, the total nitrogen derived from the nitrites and nitrates can be determined. On the other hand, when the aqueous solutions are analyzed by reacting them with an aqueous solution of sulfamic acid without first subjecting them to the above reduction treatment, only the nitrogen derlved from the nitrites is determined. Thus, the nitrogen derived from the nitrates can be calculated by subtracting the amount of nitrogen derived from the nitrites from the total amount of nitrogen derived nitrites and nitrates.
The detection of the nitrogen gas can be carried out by a thermal conductivity method, by mass spectrometry or by discharge spectrometry.
When the nitrogen gas is detected by a gas chromato-graph equipped with a thermal conductivity detector, a carrier t 1~1563iD

gas such as helium, argon or hydrogen is used, and the sep-arating column is packed with a filler of the type usually used in the gas chromatography of inorganic gases, such;as silica gel, active carbon, porous polymer beads, or molecular sieves. In the case of mass spectrometry, hel~um, argon or hydrogen is used as a carrier gas, and the nitrogen gas pro-duced in the reaction zone is preferably,measured by mass fragmentgraphy at m/e = 28. In case of a discharge spectro-metry, argon is used as a carrier gas, and the nitrogen gas produced in the reaction zone is passed through a detecting cell and is discharged therein by applying a high electrical voltage to the electrodes provided at both sides of the cell, and a wavelength of 3371 A is isolated from the emission spectrum with an optical filter and is detected with a photo-multiplier. By this method, the nitrogen can be determined with high sensitivity.
One embodiment of the analysis of the present in-vention using a gas chromatograph equipped with a thermal con-ductivity detector is illustrated with reference to the accompanying drawings, in which:
Figures 1, 2 and 3 are schematic diagrams of embodi-ments of the apparatus used in the present invention; and Figures 4, 5, 6 and 7 are the calibration curves of the solutions referred to in the Examples disclosed herein-after, which show the relation between the peak height of the nitrogen in the gas chromatogram and the amount of the nitrogen derived from nitrites or nitrates.
As is shown in Figures 1, 2 and 3, a carrier gas (e.g. helium or argon) from a device for supplying such a gas (e.g. helium cannister or argon cannister) is divided into two streams, and one of them is sent to a reference side of a ` ~lS63~) gas chromatograph via a pressure controller 4 and the other one is sent to a reaction tube 9 via a pressure controller 3 and optionally a switching cock 7. The carrier gas flows at a rate of 20 to 100 ml/minute. The reaction tube 9 is made of a hard glass and the upper part above a glass filter 10 has an inner diameter of 8 to 15 ~n and an inner volume of 5 to 20 cm3. In the upper part, a capil~ary tube 12 and a cock 13 are provided and are used for removing the solution to be reacted (i.e. an aqueous solution of sulfamic acid), the solution to be analyzed, the reaction mixture and the washing liquid. The glass filter 10 preferably has a grade of 2G or 3G (JIS R-3503-1958) so that the reaction mixture does not pass through when the carrier gas is passing upwardly therethrough and has a thickness of 2 to 5 mm. At the top of the reaction tube 9, an inlet 14 is provided for introducing the aqueous solution of sulfamic acid and the solution to be analyzed. These sol-utions are introduced into the reaction tube 9 from an inlet 14 by means of, for example, a microsyringe or an automatic injector 15. The nitrogen gas produced in the reaction mix-ture 11 is removed from the aqueous sys tm with the carriergas and passes through a splash removing tube 17 and is optionally led to an oxidation-reduction tube 19. The splash removing tube 17 is packed with a water-absorptive material (e.g. gauze) which is impregnated with an indicator, and when splashes of the reaction mixture are carried on the ca~rier gas, the splash removing tube becomes colored.
When the aqueous solution to be analyzed contains volatile organic substances, the organic substances vaporize, and hence it is preferable to oxidize them in the oxidation-reduction tube 19, as is shown in Figures 2 and 3, and thenthe resulting carbon dioxide is removed in an acidic gas-1~1563~) removing tube 25. The oxidation-reduction tube 19 may be made of quartz and is packed with an oxidizing agent (e.g. copper oxide or cobalt oxide) and a reducing agent (e.g. reduced copper or reduced nickel) and has an inner diameter of 8 to 15 mm and a length of 15 to 30 cm. The oxidation-reduction tube 19 is preferably heated at 300 to 700C. with an electric furnace 20. The gas passed through the oxidation-reduction tube 19 is optionally further passed through a moisture-removing tube 22 and a switching cock 7 and is led into the acidic gas-removing tube 25, as is shown in Figure 3. The moisture-removing tube 22 is provided in order to remove the moisture which is produced by vaporization of the aqueous solutions contained in the reaction system. The moisture-removing tube 22 may be made of a glass and is packed with a dehydrating agent such as magnesium perchlorate, hydroscopic ion exchange resin, calcium chloride or silica gel. The acidic gas-removing tube 25 may be made of a glass and packed with soda asbestos or soda lime.
The gas ~assed through the acidic gas-removing tube 25 is introduced into a gas chromatograph equipped with a thermal conductivity detector 30. The gas chromatograph may be any double column flow or single column flow type. Sep-arating columns 28 and 29 are packed with a filler which is usually used for gas chromatography of an inorganic gas, such a6 silica gel, active carbon, polar polymer beads or molecular sieves. The signals obtained in the thermal con-ductivity detector 30 are sent to a recorder 32 through a signal line 31 and are recorded therein.
In Figures 1, 2 and 3,the numerals 2, 5, 6, 8, 16, 30 18, 21, 23, 24, 26, 27 and 33 identify pipes for connecting each of the various devices.

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As is shown in Figure 3, the carrier gas divided from the pipe 2 may optionally be led into a dissolved air-removing tube 37 via a needle valve 34 and a capillary tube 35. Usually, water contains dissolved air which contains about 15 ppm of nitrogen gas, and hence it is preferable to remove the dissolved air from the aqueous solutions to be subjected to the reaction by feeding the aqueous solution to the dissolved air-removing tube and bubbling the solutions with a carrier gas. Furthermore, in order to prevent further dissolving of air, a septum 36 having fine pores for collect-ing the reaction mixture is provided at the upper part of the dissolved air-removing tube 37. The dissolved air may also be removed by other conventional methods before introducing the aqueous solutions into the reaction system of the present invention, or may also be removed by bubbling the solutions in the reaction tube with a carrier gas by blowing the carrier gas into the reaction tube from the lower end thereof.
The present invention is illustrated by the following Examples but is not limited thereto.
Example 1 Nitrogen derived from nitrites was determined by using an apparatus as shown in Figure 1. The reaction tube had an inner diameter of 13 mm and an inner volume of 12 ml, and contained a 3G glass filter having a thickness of 2 mm. An aqueous solution (3 ml) containing 5 ~ by weight of sulfamic acid and 5 % by weight of sodium chloride was added to the reaction tube. The moisture-removing tube had an inner diameter of 8 mm and a length of 150 mm and was packed with 10 - 20 mesh magnesium perchlorate, and the acidic gas-removing tube had an inner diameter of 8 mm and a length of 70 mm and - ~lS630 was packed with 20 - 40 mesh soda asbestos. A stainless steel column having an inner diameter of 3 mm and a length of 50 cm was used as the separating column of the gas chromatograph, which was packed with 60 - 80 mesh active carbon. Under the conditions of a column temperature of 50C, a temperature of the thermal conductivity detector of 60C and an electric current of the bridge of 180 mA, a carrier gas (helium) was flowed at a rate of 60 ml/minute. An aqueous solution of sodium nitrite having an appropriate concentration was prepared and the dissolved air was turned out with helium gas, and the aqueous solution (100 ~1~ thus obtained was introduced into the reaction tube by means of a microsyringe, and then a calibration curve was drawn. The chromatogram was obtained in about 2 minutes.
A relation between the peak height Oc the spectrum ~nitrogen) and the concentration of nitrogen derived from nitrites contained in the test solution showed a good straight line as shown in Figure 4.

Based upon the calibration curve obtained above, test solutions having various concentrations of nitrites were tested and the concentration of nitrogen derived from nitrites was measured. The results are shown in the following Table 1.

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1~15~3~) Example 2 Nitrogen derived from nitrites was deter~ined by using an apparatus as shown in Figure 2. The reaction tube was made of a hard glass and had an inner diameter of 13 mm and an inner volume of 12 ml and contained a 3G glass filter having a thickness of 2 mm. The moisture-removing tube had an inner diameter of 8 mm and a length of 150 mm and was packed with 10 - 20 mesh magnesium perchlorate, and the acidic gas-removing tube had an inner diameter of 8 mm and a length of 70 mm and was packed with 20 - 40 mesh soda asbestos. The oxidation tube had an inner diameter of 10 mm and a length of 200 mm and was packed with linear copper oxide having a diameter of 0.6 mm and a length of 2 - 4 mm, and at both sides of the oxidation tube, quartz was packed in a length of about 2 cm. A stainless steel column having an inner diameter of 3 mm and a length of 1 m was used as the separating column of the gas chromatograph and was packed with 60 - 80 mesh active carbon. A carrier gas (helium) was flowed at a rate of 60 ml/minute. The determination was carried out under the conditions of' a column temperature of 50C, a temperature of the thermal conductivity detector of 60C, an electric current of the bridge of 180 mA and a full scale of the recorder of 1 mV. The aqueous solution to be reacted was a 5 ~ by weight aqueous solution of sulfamic acid.
Aqueous solutions (3 ml) having various concentrations of sodium nitrite were each collected in a syringe and were introduced into the reaction tube and were allowed to stand for about 4 minutes in order to turn out the _, . .
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dissolved air. An aqueous solution of sulfamic acid (100 ~ rom which dissolved air was turned out with helium gas, was introduced into the reaction tube with a microsyringe. The produced nitrogen gas was recorded with a chromatogram. After the reaction, the test solution was taken out from the system by opéning the needle valve, and the reaction tube was washed by introducing therein distilled water or an aqueous solution to be analyzed (4 ml) with a syringe. The chromatogram was obtained in about 2 minutes after introduction of the aqueous solution of sulfamic acid. A relation between the peak height of the spectrum (nitrogen) and the concentration of nitrogen derived from nitrites showed a good straight line as shown in Figure 5. The test results obtained by using the calibration curve are shown in Table 2.
Table 2 ~mount of NO2 ~ N Found amount f NO2 ~ N (ppm~
~dded to the test solution No. 1 No. 2 No. 3 Average 0.300 0.287 0.303 0.306 0.299 :
0.200 0.207 0.194 0.199 0.200 0.100 0.102 0.097 0.111 0.103 0.050 0.046 0.049 0.052 0.049 O . 010 O . 009 O . 009 O . 010 û . 009 0.005 0.004 0.006 0.004 0.005 Example 3 Nitrogen derived from nitrates was determined by using an apparatus as shown in Figure 3. The reaction tube was made of a hard glass and had an inner diameter of 13 mm and an inner volume of 12 cm3 and contained a 2G glass 111563~

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filter having a thickness of 2 mm. An aqueous solution (3 ml) contair.ing 3 % by weight of sodium chloride-and 10 ~ by weight of sulfamic acid was introduced into the reaction tube. The moisture-removing tube had an inner diameter of 9 mm and a length of 150 mm and was packed with an equal amount of 20 - 40 mesh magnesium perchlorate and 20 - 40 mesh hydroscopic sulfonic acid type ion exchange resin, and the oxidation-reduction tube had an inner diameter of 10 mm and a length of 200 mm and was packed with 80 mm of a linear copper oxide having a diameter of 0.6 mm and a length of 2 - 4 mm, and 80 mm of a linear reduced copper having a diameter of 0.6 mm and a length of 2 - 4 mm, in this order, and at the both sides of the oxidation-reduction tube, quart~
wool was packed in a length of 20 mm and was heated at 500C with an electrical furnace. The glass-made splash removing tube had an inner diameter of 9 mm and a length of 70 mm and was packed with a gauze which was impregnated with a dye, Congo Red, and the acidic gas-removing tube had an inner diameter of 9 mm and a length of 70 mm and was packed with 20 - 40 mesh soda asbestos. A stainless steel column having an inner diameter of 3 mm and a length of 1 m was used as the separating column of a gas chromatograph and was packed with 60 - 80 mesh active carbon. A carrier gas (helium) was f lowed at a rate of 60 ml/minute. The determination was carried out under the conditions of a column temperature of 60C, a temperature of the thermal conductivity detector of 60C and an electric current of a bridge of 160 mA.
Aqueous solutions containing various concentration r 1563~

of sodium nitrate (20 ppm, 10 ppm and 5 ppm as nitrogen derived from the nitrate) were prepared by using g~aranteed sodium nitrate reagent and each 25 ml thexeof was collected in a 50 ml test tube, to which was added an aqueous solution of an ammonium compound (30 ~ by weight ammonium carbonate solution, 0.65 ml; 26 % by weight ammonium chloride solution, 1.0 ml; 50 % by weight ammonium acetate solution, 0.6 ml;
30 ~ by weight ammonium sulfate solution, 1.0 ml; or 30 % by weight ammonium citrate solution, 2.0 ml~, and thereto was further added zinc powder tO.2 - 0.3 g).
The test tube was sealed and shaken for 1 minute.
Immediately, the mixture was filtered with a filter paper (No. 5, C grade) and about 0.6 ml thereof was added to a dissolved air-removing tube having an inner diameter of 15 mm and a length of 7 cm and therein`dissolved air was turned out with helium gas. The solution thus obtained (100 ~1) was introduced into the reaction tube with a microsyringe, and a calibration curve was drawn.
The chromatogram was obtained in about 2 minutes. A
relation between the peak height of the spectrum (nitrogen) and the concentration of nitrogen derived from nitrate was obtained for each ammonium compound. The relation showed a good straight line in all solutions as shown in Figure 6.
By using the data of sodium nitrate as a standard, various test solutions containing nitrate ion, which was previously reduced with ammonium carbonate and zinc powder into nitrite ion, were reacted with a 10 %
by weight aqueous sulfamic acid solution or a 3 to 10 % hy weight aqueous sulfamic acid solution containing .

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various salts, and the concentration of nitrogen derived from nitrates was measured based upon the `
calibration curve. The results are shown in Table 3.

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I O t~ O ¦ ~ ~ _ i 1~1563~) Example 4 Nitrogen derived from nitrates was determined by using an apparatus as shown in Figure 3.
A test solution containing a slight amount of nitrate ion (25 ml) was collected in a test tube and thereto were added a 26 % by weight aqueous ammoniu~
chloride solution (1.0 ml) and zinc powder (0.25 g~ and the mixture was shaken for 1 minute. After the mixture (about 10 ml) was filtered with a filter paper ~o. 5, C grade), the filtrate (about 3 ml~ was immediately introduced into the reaction tube with a 5 ml syringe and was allowed to stand for about 4 minutes in order to turn out the dissolved air. To the reaction tube was added a 10 % by weight aqueous sulfamic acid solution (100 ~1) containing 3 % by weight of sodium chloride, from which dissolved air was previously turned out, by means of a microsyringe. The chromatogram of the produced nitrogen gas was recorded. Other conditions than the above were the same as in Example 3.
After the reaction, the test solution was taken out by opening the cock, and the reaction tube was washed with distilled water or ion exchange water (4 ml) and the washing water was taken out likewise. Thereafter, the next test solution was introduced into the reaction tube and followed by the above procedure. The chromatogram was obtained in about 2 minutes after introduction of the aqueous solution to be reacted. A relation between the peak height of the spectrum (nitrogen) and the concentration of nitrogen derived from nitrates had a good straight line as shown in Figure 7.

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By using the data of sodium nitrate as a standard, various tes~ solutions containing nitratè
ion were tested and the concentration of nitrogen derived from nitrates was measured based on the calibration curve. The results are shown in Table 4.
Table 4 Components of Amount of Found amount of NO - N
aqueous solution _ (ppm) 3 to be analyzed added to the No. 1 No. 2 Average .
test solution (ppm) ~ ~
NH4NO3 0.500~ 0.491 0.497 0.494 NH4NO3 + 3%NaCl 0.500 0.505 0.502 0.504 KNO3 0.2000.200 0.19 a 0.195 .KNO3 + 1%Na2SO4 0.200 0.203 0.197 0.200 I~NH4NO3 0.0500.046 0.048 0.047 NH4MO3 + 3~NaCl 0.050 0.052 0.049 0.050 ,IKNO3 0.0200.018 0.020 0.019 KNO3 + 1%Na2SO4 0.020 0.021 0.021 0.021 ',KNO3 i 0.0100.008 0.009 0.008 !IKNO3 + 3%NaCl ~ 0.010 ~ 0.011 0.010 0.010 :

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for the determination of nitrogen derived from nitrites contained in an aqueous system, which comprises reacting an aqueous solution containing nitrites to be deter-mined with an aqueous solution of sulfamic acid in a reactor which is provided in a carrier gas stream, removing the nitrogen gas thus produced with the carrier gas from the aqueous system, and measuring the nitrogen gas with a nitrogen gas detector.

2. A method according to claim 1, wherein the aqueous solution of sulfamic acid is first added to the reactor and thereafter the aqueous solution containing nitrites is added thereto.

3. A method according to claim 1, wherein the aqueous solution containing nitrites is first added to the reactor and thereafter the aqueous solution of sulfamic acid is added thereto.

4. A method according to claim 1, wherein the aqueous solution of sulfamic acid has a concentration of 0.1 to 15%
by weight of sulfamic acid.

5. A method according to claim 4, wherein the con-centration of sulfamic acid is in the range of 1 to 10% by weight.

6. A method according to claim 1, wherein the aqueous solution of sulfamic acid contains 0.1 to 30% by weight of a salt.

7. A method according to claim 6, wherein the concen-tration of the salt is in the range of 0.5 to 10% by weight.

8. A method according to claim 7, wherein the salt is a water soluble compound selected from the group consisting of a chloride, sulfate, nitrate or phosphate of an alkali metal, an alkaline earth metal or ammonium.

9. A method according to claim 8, wherein the salt is a member selected from the group consisting of a chloride, sulfate, nitrate or phosphate of an alkali metal or ammonium.

10. A method according to claim 1 wherein the aqueous solution containing nitrites is an aqueous solution pre-pared by subjecting an aqueous solution containing nitrates to reduction.

11. A method according to claim 10, wherein the aqueous solution containing nitrites is prepared by adding an ammonium compound and zinc powder to an aqueous solution containing nitrates and shaking the mixture and thereby reducing the nitrates into nitrites.

12. An apparatus for the determination of nitrogen derived from nitrites or nitrates contained in an aqueous system, comprising a reactor for reacting an aqueous solution con-taining nitrites and an aqueous solution of sulfamic acid, a device for supplying a carrier gas to the reactor, a nitrogen gas detector for detecting nitrogen gas produced in the reactor and removed therefrom by the carrier gas, and pipes connecting these devices.

13. An apparatus according to claim 12, wherein an oxidation-reduction tube is provided between the reactor and the nitrogen gas detector.