Environ. Sci. Technd 1986, 20, 515-517
Continuous Flow Method for Simultaneous Determination of Nitrate and Ammonia in Water Toyoaki Aoki, * Satoshi Uemura, and Makoto Munemori Laboratory of Environmental Chemistry, College of Engineering, University of Osaka Prefecture, Mozu-Umemachi, Sakai 59 1, Japan
rn A new continuous flow procedure is proposed for the simultaneous determination of nitrate and ammonia in water. Ammonia is fluorometrically determined with ophthalaldehyde reagent, and nitrate is determined after reducing it to ammonia in alkaline medium by titanium(111)chloride. The ammonia generated in alkaline sample solution is separated with a tubular microporous PTFE membrane. Nitrite stoichiometrically interferes with the present method but can be masked by sulfanilic acid. Detection limits (S/N = 3) for nitrate and ammonia were 1.8 X lo-' and 1.8 X lo-* M, respectively. Nitrate and ammonia in rivers were simultaneously determined with satisfactory results.
Introduction The nitrogen cycle is an important component of aquatic ecosystems. The prevalent species of nitrogen in the aquatic environments are nitrate, nitrite, ammonia, and organic nitrogen (1,2). Nitrite is not as significant in terms of concentration. In general, these species are determined separately by their respective standard methods (3). However, the simultaneous determination of some of these species will save time, and the methods have been studied for nitrate and nitrite (4-7), but scarcely for nitrate and ammonia. This paper presents a flow fluorometric method for simultaneous determination of ammonia and nitrate based on the reduction of nitrate to ammonia by titanium(II1) (8)and the determination of the ammonia after separating it with a tubular microporous poly(tetrafluoroethy1ene)(PTFE) membrane (9). Experimental Section Reagents. A stock solution of titanium(II1) chloride was prepared by dissolving titanium metal (99%) in concentrated hydrochloric acid (analytical reagent grade) and diluted with redistilled water immediately before use. A commercially available titanium(II1) chloride solution gave higher reagent blank. o-Phthalaldehyde and 2mercaptoethanol were of biochemical grade (Wako Pure Chemical Co.). Other reagents were of analytical reagent grade. OPA reagent was prepared with o-phthalaldehyde and 2-mercaptoethanol according to the procedure reported by Lindroth and Mopper (IO). A sulfanilic acid solution was prepared by dissolving 3 g of sulfanilic acid in 160 mL of concentrated hydrochloric acid and diluting it to 1L with redistilled water. Redistilled water was used in the preparation of all solutions, the second distillation being carried out from alkaline permanganate in an allPyrex still. Apparatus and Procedure. A flow diagram of the apparatus is shown in Figure 1. Sample and reagent solutions were propelled by a peristaltic pump (P) through Tygon tubing (Ll-L8). Reaction coils (Rl-R4) were PTFE tubing. The manifold, including a separation unit S1, was used for the determination of the sum of nitrate and ammonia, and the manifold, including a separation unit S2, was for ammonia alone. The construction of the separation units was the same as that described in a previous paper 00 13-936X/86/0920-05 15$01.5010
Table I. Masking of Nitrite by Sulfanilic Acid nitrate 50 50 50 50 50 50
concentration, X106 M ammonia nitrite 0 0 0 50 50 50
RFI"
0 25 50
100 100 99
0
26
100 102
50
99
Relative fluorescence intensity: fluorescence intensity of 5.0 X M nitrate solution containing nitrite and/or ammonia/fluorM nitrate solution in percent. escence intensity of 5.0 X
(9). The outer tube (3.5 mm 0.d. and 2.5 mm i.d.) was made of PTFE and the inner tube (1.8 mm 0.d. and 1.0 mm i.d.) was made of microporous PTFE. Fluorescence intensity was measured by a fluorometer (Japan Spectroscopic) equipped with a flow cell of quartz tubing (2.4 mm 0.d. and 1.0 mm i.d.). A sample (Ll) was allowed to react in the reaction coil R1 with 0.3% sulfanilic acid solution (L2) to mask nitrite which might be present in the sample. Then 7 M sodium hydroxide solution (L3) and titanium(II1) chloride solution (L4) were added successively. Nitrate was reduced to ammonia in the reaction coil R3 and the ammonia permeated through a microporous PTFE membrane in the separation unit S1 into a buffered OPA reagent stream (L5) in the inner tube. The reaction product between ammonia and OPA reagent was fed to the flow cell, and the fluorescence intensity was measured at 486 nm with excitation at 370 nm. The fluorescence signal thus obtained corresponds to the sum of nitrate and ammonia in the sample. Next, a four-way stopcock V was turned 90°, so that the sample was pumped through L6 and mixed with 2 M sodium hydroxide solution (L7). The ammonia produced was determined in the same way as described above after separating it with the separation unit S2. The signal obtained corresponds to the ammonia originally present in the sample. The nitrate content of the sample was obtained by subtracting the two signals.
Results and Discussion Masking of Nitrite. Nitrite, if present, interferes with the present method, since it is reduced to ammonia by titanium(II1). The interference from nitrite, however, could be eliminated by the addition of sulfanilic acid, sulfanilamide, or sodium azide. Among these reagents, sulfanilic acid was the best. The other two reagents gave higher blanks. Sulfanilic acid reacts with nitrite to give a diazonium salt which is not reduced to ammonia by titanium(II1). The effect of sulfanilic acid on masking of nitrite is shown in Table I. The sulfanilic acid concentration more than 0.05% gave satisfactory results for masking nitrite up to 5 X M. Reduction of Nitrate by Titanium(II1). Nitrate is reduced to ammonia by titanium(II1) in a strongly alkaline medium (8). The reduction depends on the concentration of alkaline solution. In the present method, the fluores-
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 5, 1986 515
P
Ll
Lis L2 L3
14
L5 L6 L7 LB
Figure 1. Schematic diagram of continuous flow system. P, peristaltic pump; L1, sample at 3.5 mL/min; L2, 0.3% sulfaniilc acid solution at 0.7 mL/min; L3, 7 M NaOH solution at 0.6 mL/min; L4, 5 % titanium(111) chloride solution at 0.7 mL/min; L5, OPA reagent solution at 0.29 mL/min; L6, sample at 2.1 mL/min; L7; 2 M NaOH solution at 0.38 mL/min; L8, OPA reagent solution at 0.23 mL/min; S1 and S2,separation units 0.50 m (inner-microporous PTFE tubing 1.0 i.d., 1.8 mm 0.d.; outer-PTFE tubing 2.5 i.d., 3.5 mm 0.d.); R1, 15 m (0.7 i.d., 1.3 mm 0.d.); R2, 5 m (0.9 i.d.); R3, 5 m (2.0 i.d., 3.0 mm 0.d.); R4, 5 m (0.7 i.d., 1.3 mm 0.d.); V, four-way stopcock; D, fluorometer; W, waste.
Table 11. Interference from Various Compounds with Determination of Nitrate compounds
RFI"
compounds
RFIa
glycine aspartic acid phenylalanine acetic acid citric acid tartaric acid humic acid (10 ppm)
100 100 102 103 103 97 104
sodium chloride sodium chloride (0.2 M) sodium sulfate magnesium chloride glucose urea
98 104 100 98 100 101
Relative fluorescence intensity: fluorescence intensity of 2 X lo" M nitrate solution containing 2 X M foreign compound/ fluorescence intensity of 2 X M nitrate solution in percent. Table 111. Results of Simultaneous Determination of Nitrate and Ammonia in Rivers
-01
sampleb Yamato River Kim River Uji River Katsura River Yodo River
NO,, x 105 M present IPC" 18.6 10.6 1.4 7.1 5.8
18.8 10.3 ND' 7.3 6.1
N H ~ x, 105 M present Nessler 6.6 2.2 1.8 9.6 5.2
6.7 2.1 2.1 10.4 5.4
" IPC: indirect photometric chromatography. *Samples were taken on Nov 12, 1985. ND: not detected (detection limit S / N = 3; 2.8 x 10-5 M).
01 0
5 10 Concn. of NaOH, M
15
I
Figure 2. Effects of sodium bye-oxide concentration on fluorescence intensity. (0)Concentration of nitrate, 5 X M; (0)redistilled water.
0
0
1
2 3 T I ( I P ) , '10
4
5
Flgure 3. Effects of titanium(II1) concentration on reduction of nitrate M. to ammonia. Concentration of nitrate, 5 X
cence signal increased with the increase in the concentration of sodium hydroxide (Figure 2). At the same time, however, the blank also increased. As a compromise, 7 M sodium hydroxide solution was selected. The reduction also depends on the amount of titanium(II1). As shown in Figure 3, the signal increased as the concentration of titanium(II1) was increased. When 5% titanium(II1) solution was used, the percent of conversion of nitrate to ammonia was 100 f 1% . This conversion efficiency was maintained for at least 75 days after preparation of titanium(II1) chloride solution. The precipitation of titanium(II1) did not interfere with the fluorometric measurement because of the membrane separation. Response Time. The response time of the system depends on a path length of the sample and its reaction product flows and the flow rates. In the present systems, by selection of the path lengths and the flow rates as shown in Figure 1,the sum of nitrate and ammonia and the am516
Environ. Scl. Technoi., Vol. 20, No. 5, 1986
monia alone in the same sample could be alternately determined every 5 min. Calibration and Interferences. Linear calibration to 2 x curves were obtained over the range from 5 x M for nitrate and from 2 X to 2 X loT4M for ammonia (9). The detection limits (S/N = 3) for nitrate and 1.8 X lom8M, respecand ammonia were 1.8 X tively. The relative standard deviations (n = 5) for nitrate and ammonia were 3.3% at 2.0 X lo4 M and 4.4% at 1.0 X lo4 M, respectively. Any organic and inorganic species common to aquatic environment did not interfere with the present method for the determination of nitrate as shown in Table 11. There was no interference in this determination from soluble species that generally did not permeate through the microporous PTFE membrane in alkaline medium and did not react with OPA reagent. Amino acids and proteins reacted with OPA reagent (10) but did not permeate through the microporous PTFE membrane. However, primary amines such as methylamine and ethylamine interfered, but the interference could be reduced to a low level by lowering the pH of the outer solution in the separation unit as reported in a previous paper (9). Application to Water Analysis. The proposed method was applied to the simultaneous determination of nitrate and ammonia in several rivers. The results were in good agreement with those obtained by an indirect photometric chromatography (IPC) for nitrate (11) and by the Nessler method for ammonia (12) as shown in Table 111. The present method, being so sensitive to nitrate, gave a satisfactory result even in the case of Uji River where the indirect photometric chromatography failed because of the deficiency of the sensitivity. A correlation coefficient between the present method and the IPC was 0.998 ( n = 10) for nitrate, and that between the present method and the Nessler method was 0.997 ( n = 10) for ammonia. The same samples as listed in Table I11 were analyzed without the addition of sulfanilic acid, and the results are compared in Table IV with the data from which the concentrations of nitrate and ammonia shown in Table I11
Environ. Sci. Technol. 1986, 20,517-520
Table IV. Feasibility of Determination of Nitrite
samplea Yamato River Kizu River Uji River Katsura River Yodo River
IN,* x 105 M without with SAc SA difference 26.1 13.2 3.5 19.0 11.7
25.2 12.8 3.2 16.7 11.0
0.9 0.4
0.3 2.3 0.7
NO;, x lo6 M colorimetricd 0.8 0.3 0.1 2.8 0.4
Samples were taken on Nov 12, 1985. bIN inorganic nitrogen. SA: sulfanilic acid. dSulfamide/N-(1-naphthy1)ethylenediamine method.
were derived. The difference between the two results for each sample was in fairly good agreement with the value obtained by the standard colorimetric method for nitrite (3). This fact indicates that nitrite is stoichiometrically reduced to ammonia by titanium(III),and therefore, the present method can also be applied to the determination of nitrite. Registry No. NO3-, 14797-55-8;NH3, 7664-41-7; H 2 0 , 7732-
L i t e r a t u r e Cited (1) Stanley, W. D.; Hobbie, E. J. Limnol. Oceanogr. 1981,26, 30-42. (2) Knowles, R.; Lean, D. S.; Chan, Y. K. Limnol. Oceanogr. 1981,26,855-866. (3) “Standard Methods for the Examination of Water and Wastewater”, 16th ed.; American Public Health Association: Washington, DC, 1985; pp 391-406. (4) Connors, J. J.; Beland, J. J.-Am. Water Works Assoc. 1976, 68, 55-56. (5) Kamphake, J. L.; Hannah, A. S.; Cohen, M. J. Water Res. 1967,1, 205-216. (6) Anderson, L. Anal. Chim. Acta 1979, 110, 123-128. (7) Tanaka, K. Bunseki Kagaku 1982, 31, T107-Tl12. (8) Cresser, S. M. Analyst (London) 1977, 102, 99-103. (9) Aoki, T.;Uemura, S.; Munemori, M. Anal. Chem. 1983,55, 1620-1622. (10) Lindroth, P.; Mopper, K. Anal. Chem. 1979,51,1667-1674. (11) Small, H.; Miller, T. E. Anal. Chem. 1982, 54, 462-469. (12) “Standard Methods for the Examination of Water and Wastewater”, 16th ed.; American Public Health Association: Washington, DC, 1985; pp 379-382. Received for review J u n e 10, 1985. Accepted J a n u a r y 3,1986.
18-5.
Measurement of Total Reduced Sulfur Compounds in Ambient Air Neil R. McQuaker,” Glenn E. Rajala, and Davld Pengllly Environmental Laboratory, Ministry of Environment, Vancouver, British Columbia V6S 2L2, Canada
Methods for the determination of total reduced sulfur (TRS) compounds in the ambient air based on coulometric detection (Philips Model PW 9700 analyzer) and thermal oxidation followed by detection using pulsed fluorescence (Teco Model 43 analyzer) have been evaluated. Analytical response factors, relative to HzS,were determined for both the individual TRS compounds and compounds such as terpenes and carbonyl sulfide that may be a potential source of interference. The results for COS and terpenes indicate that in a typical monitoring situation normally encountered concentrations of these compounds are not expected to cause significant measurement bias. The results for the individual TRS compounds indicate that while variations in TRS composition are not a factor in assessing measurement bias for the thermal oxidation/pulsed fluorescence method, they are a factor for the Philips coulometric method; i.e., increasing positive measurement bias may be introduced as the TRS composition shifts toward relatively less H2S. Philips-Teco comparison data collected at a single site in the vicinity of three operating kraft pulp mills are compatible with these expectations. Introduction
Total reduced sulfur or TRS may be defined as [TRS] + [CH3SH] + [(CH3),S] + B[(CH,),S,], and reference methods for its measurement in the ambient air have not been established by Federal Regulatory Agencies in either Canada or the United States. However, in many regional jurisdictions where there are kraft pulp mills there is a requirement, often compliance related, to measure ambient levels of TRS. Often, the method of choice has been coulometric detection using the Philips Model P W 9700 analyzer (I). In this method a scrubber is used to remove SO2 from the original sample; the sample then passes into a titration cell where the response to TRS is referenced to H,S. Details of the titration procedure ap= [HzS]
0013-936X/86/0920-0517$01.50/0
pear elsewhere (I),but the essential element involves titration of the sample with bromine which is electrolytically generated. An alternate method, based upon thermal oxidation of TRS to SO2, has recently become available. A scrubber is used to remove SO2 from the original sample; the sample then passes into a thermal oxidizer where TRS is oxidized to SOz and then detected by using pulsed fluorescence. One of the available instruments satisfying the analytical requirements of this method is Teco Model 43 (2). Current regulatory limits for TRS in the province of British Columbia, Canada, are specified as follows: existing mills, 20 ppb hourly average; new or upgraded mills, 5 ppb hourly average (3). The regulatory limits of 20 and 5 ppb compare with the detection limit of 2 ppb provided by each of the instrumental methods identified above. The Philips Model PW 9700 analyzer is no longer manufactured, and because of its unavailability increased monitoring activities will, more and more, have to depend upon the alternate thermal oxidation/pulsed fluorescence method. This immediately raises the question of compatability between the two methods and is related to the larger question of what measurement biases, if any, are inherent to each method. The answers to these questions have implications in terms of interfacing data provided by the two methods as well as ‘the suitability of the individual methods as possible reference procedures. Data that would assist in answering these questions are evidently not available, and in the present work we have attempted to remedy this by (i) collecting Teco-Philips comparison data, at a single site, in the vicinity of three adjacent kraft pulp mills and (ii) assessing known potentials for measurement bias by determining analytical response factors relative to H2S for both the individual TRS compounds and compounds that have a potential for interference. This latter group included (i) COS which under overload conditions
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 5, 1986
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