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Nitrate-sensitive corroding metal electrode
Sensors and Actuators
B 24-25 (1995) 291-295
Nitrate-sensitive corroding metal electrode Shukuji Asakura, Shinji Okazaki, Hidemoto Nakagawa, Kenzo Fukuda Depormtent of Material Engimwing, Faculty of Engineering, Yokohama National University, 155 Tokiwadai Hodoguyo-ku, Yokohama, Kanogawa
240, Japan
Abstract The electrode potential of Devarda’s alloy in phosphate solutions buffered at pH = 12 containing different amounts of nitrate ions has been measured. A proportional relation behveen the electrode potential and the concentration of nitrale ions [NO,-] is observed when [NO,-] is less than 40 mg I-‘. When [NO,-] is larger than 200 mg I-‘, log[NO,-] is linearly related to the electrode potential, with a slope of about 120 mV decade-’ up to [NOa-.I=10000 mg I-‘. The presence of chloride, sulfate and perchlorate ions has almost no effect on the above relations. The sensitivity and selectivity of the above metal electrode exceed those of nitrate-ion-selective electrodes. Since the electrical impedance of the corroding metal/solution interface is much lower than that of other methods, the potential measurement should be easier. Kqwordrt
Corroding
metal electrode;
Devarda’s alloy; Nitrate sensor
1. Introduction The presence of nitrate ion in nature is an index of environmental pollution [1,2]. In recent years, serious and complicated pollution of aquatic environments by nitrates has been brought about by industrial and domestic waste waters or fertilizers [3,4]. Therefore, the development of inexpensive and convcnicnt methods of detecting nitrate ions in water is required. Calorimetric analysis of nitrate ions had been investigated extensively [5-71. However, the experimental procedures of these methods are troublesome and do not give swift detection. On the contrary, potentiometric determination of nitrate ions with an ISE [8,9], ISFET [lo] or ion-selective photodiode (ISPD) [ll] has been studied intensively as a convenient analytical technique. These methods, which show Nernstian and rapid response to nitrate ions, would be appropriate for environmental and industrial measurement except for their expense. Numerous papers have dealt with developing quantitative analytical methods for the determination of nitrate ions in aqueous solutions mentioned above. However, potentiometric determination of nitrate ions with metal electrodes has not received attention. This is because of the chemically and electrochemically inactive nature of nitrate ions in neutral and alkaline solutions [12]. 09254005A5/$0YSO
0
1995
SSDI 0925.4005(94)01361-K
Elsevier Science S.A. All rights reserved
It is well known that corroding metal powders reduce nitrate ions easily [13-151. Devarda’s alloy (Al: 45 wt.%, Cu: 50 wt.%, Zn: 5 wt.%) is reactive to nitrates in alkaline solution [16], where an electrochemically mixed reaction is presumed. This suggests that the electrode potential of this alloy is affected by nitrate ions. In the present study, it is shown that the concentration of nitrate ions can be determined from the electrode potential of Devarda’s alloy.
2. Experimental
Devarda’s alloy, the composition of which followed an official standard [17], was purchased from Kansai Syokubai Kougyou Co., Ltd. The alloy was coated with epoxy resin to give an clcctrode area of about 1.2 cm’. This electrode was polished to obtain a fresh electrode surface before experiments. The electrode potential was measured with an electrometer of about lo9 R input impedance. The pH value was measured with conventional glass electrodes. The solutions were prepared from superpure reagents and distilled water. The solutions used in all experiments were buffered with phosphate (disodium hydrogenphosphate and t&odium phosphate) at different pH
292
S. Asohm
et 01. I Stx.wrs and Actuators B 24-25 (1995) 291-295
values. Sulfuric acid and sodium hydroxide were used for pH adjustment. An Ag/AgCl electrode was employed as a reference electrode. All potentials are expressed with respect to the standard hydrogen electrode, SHE, in this paper. The experiments were carried out at T=298 K under agitation with a magnetic stirrer.
3. Results and discussion Fig. 1 shows the change of open-circuit potentials with time in a nitrate-free solution buffered at pH= 12 and deaerated with argon. The potential decreased sharply after the immersion. It seemed to result from the destruction of a surface aluminum oxide film. It took about 10 min to reach a steady state. The potential drift was about 1.0 mV min-’ after 30 min and reached 0.15 mV min-’ after 3 h. It was suggested that the selective dissolution of aluminum gradually changed the ratio of anodic to cathodic areas. Fig. 2 represents the typical response of the potential to the addition of potassium nitrate. The addition was made after immersion for 30 min. The response became constant within about 1 min in these cases. When the
solution was replaced with the phosphate buffer free from nitrate ions again, the potential returned to the initial baseline in a few minutes. The sensitivity for nitrate ions was defined by the value of the potential change, dE, responding to a unit concentration change of nitrate ions. Fig. 3 shows the relation between the sensitivity and the immersion time. The sensitivity was independent of the immersion time over a few hours. A plot of open-circuit potentials against the logarithm of the concentration is given in Fig. 4. The potential did not depend on the concentration of nitrate ions in the aerated solution. The dissolved oxygen governed the cathodic process, so that nitrate ions were not reduced at the Devarda’s alloy surface. However, a remarkable dependence of the potential on the concentration of nitrate ions was observed in the solution deaerated with argon. When the concentration of nitrate ions was larger than 200 mg l-‘, the relation between the potential and the logarithm of the concentration was linear with a slope of 120 mV decade-‘. In this range, the hydrogen evolution reaction did not occur.
0
M
loo
L5cl
200
Time / min Fig. 3. Change of sensitivity with time for very low concentrations of nitrate ions.
Time I min Fig. 1. Open-circuit
potentials
vs.
time at pH 12.0.
1.0~ I [ NOs- I
I
i
E
.Omgl
I
=Omgl
I
Fig. 2. Response of open-circuit potentials to the addition of nitrate
Id IO’ NOj concentration / mg
Icp
.I ’
Fig. 4. Dependcnce of open-circuit potentials on the concentration of nitrate ions in solutions aerated (a) and deaerated with argon (‘9.
S. Asa!um et al. / Sen.wrs and Actuators B 24-25 (1995) 291-29.5
This slope was about twice that of ion-selective membrane electrodes. dE was proportional to the concentration of nitrate ions in a very low concentration range, as shown in Fig. 5. A slope of about 1.6 mV (NO,- mg l-1)-1 was obtained. The value of the slope obtained in a low concentration range of nitrate ions was adopted as the sensitivity in the following. In order to avoid experimental complications arising from the inert gas bubbling, the authors tried to remove dissolved oxygen by adding a reducing agent such as sodium sulfite. The electrode potential before and after the addition of sodium sulfite to an aerated solution is shown in Fig. 6. The electrode potential decreased on the addition of a chemically equivalent amount of sulfite ions to remove the saturated dissolved oxygen. The potential was not affected by the addition of an excess amount of sulfite ions. This potential was almost the same as that in the solution deaerated with argon. Fig. I shows the dependence of dE on the concentration of nitrate ions in the presence of 0.5 g 1-l of
293
i
E
0
15 / mg .
5 10 NOj concenlration
Fig. 7. Relation of potential change to a low concentration in the presence of sulfite ions (0.5 g I-‘).
0
5
IO
Fig. 8. Effect of sulfite-ion concentrations concentrations of nitrate ions.
0
IO NO; concentration
Fig. 5. Relation of potential change ions.
20
Time
20 30 / mg. 1.’
to a lowconcentration
60
‘lo
i
of nitrate
80
min
Fig. 6. Open-circuit potentials vs. time in a solution containing sulfite ions (a) and deaerated with argon (b).
20
15
SO~~concentration
“V
20
1’
/ g
of NO,-
25
.I 1
on sensitivity for very low
sulfite ions. The proportional relation between E and concentration still held in these media. The effect of the concentration of sulfite ions on the sensitivity is shown in Fig. 8. Stable sensitivity was obtained independent of the concentration of sulfite ions. Therefore, sulfite ion was proved to be applicable as a dioxidation agent. Fig. 9 represents the dependence of the sensitivity on pH. The sensitivity decreased with increasing pH. However, it was found that the sensitivity was constant in the pH range between 11 and 12. Reliable data could not be obtained below pH 11 because of the appreciable drifts of the baseline. This demonstrated that the adjustment of pH by buffer solutions was necessary to obtain the stable sensitivity. Furthermore, the results shown in Fig. 10 indicated that the concentration of phosphate buffer had almost no effect on the sensitivity. If the electrode surface was continuously scratched to renew the alloy surface, very reproducible data were expected. However, the sensitivity of each electrode was slightly different with respect to that of other electrodes.
294
S. Asakura et of. / Sensors and Actuators B 24-25 (1995) 291-295
300 -
200 # $I 3 0
-
s,o:-
0 cl-
0
_;;
1 In
-
co:- so:-clod
1.
NO;
IO;
-100
I20
II 5
pH
I25
13.0
Fig. 12. Interferences I-’ of nitrate ions.
I -
Fig. 9. Effect of pH on sensitivity for very low concentrations
of conventional
ions in the presence of 30 mg
of
12. However, thiosulfate, nitrite and iodate ions, which are reducing or oxidizing agents, have a considerable effect on the determination. Nevertheless, relatively speaking, the selectivity of the present method is higher than that of presently existing methods. Furthermore, the electrochemical impedance of the Devarda’s alloy electrode is about 20 R cm-‘, which is a few orders of magnitude lower than that of existing ion-selective electrodes. This indicates that industrial instrumentation would be extremely simplified if this electrode was employed.
4. Conclusions Concentration
of phosphate huffer / M
Fig. 10. Effect of the concentration of phosphate buffer on sensitivity for very low concentrations of nitrate ions.
Potential
vs. SHb I V
Fig. 11. Relationship of sensitivity to potential in a phosphate buffer solution free from oxidant after immersion for about 30 min.
Fig. 11 shows the relation of the sensitivity to the potential after the electrodes were immersed in pH 12 buffer solution for 30 min. The sensitivity of the different electrodes was a function of the potential after a fixed immersion time. The interference of conventional ions such as chloride, carbonate and sulfate, was slight, as illustrated in Fig.
The present work showed that the electrode potential of Devarda’s alloy responded quantitatively to the concentration of nitrate ions. In the absence of some oxidative or reducing agents, nitrate ion was determined with high selectivity. Furthermore, the addition of sodium sulfite eliminated the interference of oxygen. Stable and sensitive responses were guaranteed by adjusting the pH between 11 and 12. The sensitivity obtained for an individual electrode was a function of the electrode potential in the phosphate buffer solution after a fixed immersion time. However, surface renewal by scratching could provide reproducible data with no hysterisis. This could simplify the maintenance procedures if applied to the industrial field. The corroding metal/solution potential measurement should be easy because its impedance is very low. The above features, as well as the remarkable inexpensiveness of the present method, should be very advantageous over the presently existing methods.
Acknowledgement K. Fukuda and H. Nakagawa appreciate the financial support from New Cosmos Electric Co., Ltd.
S. Asakura et al. I Sensors and Actuators
References
B
24-25 (199s) 291-29s
295
K.S.V. Santhanam, Ion-Seming Electrodes and Electrochemical World Scientific, Singapore, 1990, p. 118. [91 J. Koqa, Theory and application of ion-selective electrode part 8, Anal. Chim. Acta, 233 (1990) l-30. PI
Instrumentafion,
[ll
PI t31 I41 I51
I61
171
K. Ito and H. Sunahara, Simple determination of ammonium and nitrate ions in activated sluge process waters with plasticmembrane ion selective electrode, Buns& Kagaku, 36 (1987) 123-125. D.D. Beer, Measurement of nitrate gradients with an ionselective microelectrode, Anal. Chim. Acra, 229 (1989) 351-356. J.J. Fried, Nitrates and their control in the EEC aquatic environment, Nafo ASI Ser., G30 (1991) 3-11. R.F. Spalding and M.E. J&w. Nitrate contamination in the contiguous United States, N&o ASI Ser., G30 (1991) 134. A.M. Hartley and R.I. Asai, Spectrophotometric determination of nitrate with 2,6,-xylenoi reagent, Anal Chem., 35 (1963) 1207-1213. P.W. West and T.P. Ramachandran, Spectrophotometric determination of nitrate using chromotropic acid, Anal. Chim. Acta, 35 (1966) 317-324. F.L. Fisher, E.R. Ibert and H.F. Beckman, Inorganic nitrate, nitrite, or nitrate-nitrite rapid calorimetric determination of micro quantities in aqueous solution, Ano!. Chem, 30 (1958) 1972-1974.
WI
S. Wakida, M. Yamane, A. Kawahara, S. Takasuka and K. Higasi, Durable poly(vinyl chloride) matrix ion-selective field effect transistors for nitrate ions, Bunreki Kagaku, 38 (1989) 510-514.
1111 PC. Hauaer, P.M.J. Perisset, S.S.S. Tan and W. Simon, Optode forbulk-responsemembranes,~inal. Chem., 62 (1990) 1919-1923. A.J. Bard, Encyclopedia of Electrochemical Elemenfs, Vol. 8, PI Marcel Deckker, New York, 1978, p. 450. I131 M. Nishimura and K. Matsunaga, Determination of nitrate by reduction to nitrite, Bun&i Kaguku, I8 (1969) 154-158. 1141 APHA AWWA WPCF, Saandard Methods for fire Examination of Water and Wastewaler, New York, 14th edn., 1976, p. 423. I151 A.P. Murphy, Chemical removal of nitrate from water, Nature, 350 (1991) 22>225. WI F. Feigel, Spot tests based on redox reactions with Devarda’s alloy and Raney alloy, Anal. Chem., 33 (1961) 111%1121. I171 Japanese Industrial Standards, JIS K 8653 (1985).
B 24-25 (1995) 291-295
Nitrate-sensitive corroding metal electrode Shukuji Asakura, Shinji Okazaki, Hidemoto Nakagawa, Kenzo Fukuda Depormtent of Material Engimwing, Faculty of Engineering, Yokohama National University, 155 Tokiwadai Hodoguyo-ku, Yokohama, Kanogawa
240, Japan
Abstract The electrode potential of Devarda’s alloy in phosphate solutions buffered at pH = 12 containing different amounts of nitrate ions has been measured. A proportional relation behveen the electrode potential and the concentration of nitrale ions [NO,-] is observed when [NO,-] is less than 40 mg I-‘. When [NO,-] is larger than 200 mg I-‘, log[NO,-] is linearly related to the electrode potential, with a slope of about 120 mV decade-’ up to [NOa-.I=10000 mg I-‘. The presence of chloride, sulfate and perchlorate ions has almost no effect on the above relations. The sensitivity and selectivity of the above metal electrode exceed those of nitrate-ion-selective electrodes. Since the electrical impedance of the corroding metal/solution interface is much lower than that of other methods, the potential measurement should be easier. Kqwordrt
Corroding
metal electrode;
Devarda’s alloy; Nitrate sensor
1. Introduction The presence of nitrate ion in nature is an index of environmental pollution [1,2]. In recent years, serious and complicated pollution of aquatic environments by nitrates has been brought about by industrial and domestic waste waters or fertilizers [3,4]. Therefore, the development of inexpensive and convcnicnt methods of detecting nitrate ions in water is required. Calorimetric analysis of nitrate ions had been investigated extensively [5-71. However, the experimental procedures of these methods are troublesome and do not give swift detection. On the contrary, potentiometric determination of nitrate ions with an ISE [8,9], ISFET [lo] or ion-selective photodiode (ISPD) [ll] has been studied intensively as a convenient analytical technique. These methods, which show Nernstian and rapid response to nitrate ions, would be appropriate for environmental and industrial measurement except for their expense. Numerous papers have dealt with developing quantitative analytical methods for the determination of nitrate ions in aqueous solutions mentioned above. However, potentiometric determination of nitrate ions with metal electrodes has not received attention. This is because of the chemically and electrochemically inactive nature of nitrate ions in neutral and alkaline solutions [12]. 09254005A5/$0YSO
0
1995
SSDI 0925.4005(94)01361-K
Elsevier Science S.A. All rights reserved
It is well known that corroding metal powders reduce nitrate ions easily [13-151. Devarda’s alloy (Al: 45 wt.%, Cu: 50 wt.%, Zn: 5 wt.%) is reactive to nitrates in alkaline solution [16], where an electrochemically mixed reaction is presumed. This suggests that the electrode potential of this alloy is affected by nitrate ions. In the present study, it is shown that the concentration of nitrate ions can be determined from the electrode potential of Devarda’s alloy.
2. Experimental
Devarda’s alloy, the composition of which followed an official standard [17], was purchased from Kansai Syokubai Kougyou Co., Ltd. The alloy was coated with epoxy resin to give an clcctrode area of about 1.2 cm’. This electrode was polished to obtain a fresh electrode surface before experiments. The electrode potential was measured with an electrometer of about lo9 R input impedance. The pH value was measured with conventional glass electrodes. The solutions were prepared from superpure reagents and distilled water. The solutions used in all experiments were buffered with phosphate (disodium hydrogenphosphate and t&odium phosphate) at different pH
292
S. Asohm
et 01. I Stx.wrs and Actuators B 24-25 (1995) 291-295
values. Sulfuric acid and sodium hydroxide were used for pH adjustment. An Ag/AgCl electrode was employed as a reference electrode. All potentials are expressed with respect to the standard hydrogen electrode, SHE, in this paper. The experiments were carried out at T=298 K under agitation with a magnetic stirrer.
3. Results and discussion Fig. 1 shows the change of open-circuit potentials with time in a nitrate-free solution buffered at pH= 12 and deaerated with argon. The potential decreased sharply after the immersion. It seemed to result from the destruction of a surface aluminum oxide film. It took about 10 min to reach a steady state. The potential drift was about 1.0 mV min-’ after 30 min and reached 0.15 mV min-’ after 3 h. It was suggested that the selective dissolution of aluminum gradually changed the ratio of anodic to cathodic areas. Fig. 2 represents the typical response of the potential to the addition of potassium nitrate. The addition was made after immersion for 30 min. The response became constant within about 1 min in these cases. When the
solution was replaced with the phosphate buffer free from nitrate ions again, the potential returned to the initial baseline in a few minutes. The sensitivity for nitrate ions was defined by the value of the potential change, dE, responding to a unit concentration change of nitrate ions. Fig. 3 shows the relation between the sensitivity and the immersion time. The sensitivity was independent of the immersion time over a few hours. A plot of open-circuit potentials against the logarithm of the concentration is given in Fig. 4. The potential did not depend on the concentration of nitrate ions in the aerated solution. The dissolved oxygen governed the cathodic process, so that nitrate ions were not reduced at the Devarda’s alloy surface. However, a remarkable dependence of the potential on the concentration of nitrate ions was observed in the solution deaerated with argon. When the concentration of nitrate ions was larger than 200 mg l-‘, the relation between the potential and the logarithm of the concentration was linear with a slope of 120 mV decade-‘. In this range, the hydrogen evolution reaction did not occur.
0
M
loo
L5cl
200
Time / min Fig. 3. Change of sensitivity with time for very low concentrations of nitrate ions.
Time I min Fig. 1. Open-circuit
potentials
vs.
time at pH 12.0.
1.0~ I [ NOs- I
I
i
E
.Omgl
I
=Omgl
I
Fig. 2. Response of open-circuit potentials to the addition of nitrate
Id IO’ NOj concentration / mg
Icp
.I ’
Fig. 4. Dependcnce of open-circuit potentials on the concentration of nitrate ions in solutions aerated (a) and deaerated with argon (‘9.
S. Asa!um et al. / Sen.wrs and Actuators B 24-25 (1995) 291-29.5
This slope was about twice that of ion-selective membrane electrodes. dE was proportional to the concentration of nitrate ions in a very low concentration range, as shown in Fig. 5. A slope of about 1.6 mV (NO,- mg l-1)-1 was obtained. The value of the slope obtained in a low concentration range of nitrate ions was adopted as the sensitivity in the following. In order to avoid experimental complications arising from the inert gas bubbling, the authors tried to remove dissolved oxygen by adding a reducing agent such as sodium sulfite. The electrode potential before and after the addition of sodium sulfite to an aerated solution is shown in Fig. 6. The electrode potential decreased on the addition of a chemically equivalent amount of sulfite ions to remove the saturated dissolved oxygen. The potential was not affected by the addition of an excess amount of sulfite ions. This potential was almost the same as that in the solution deaerated with argon. Fig. I shows the dependence of dE on the concentration of nitrate ions in the presence of 0.5 g 1-l of
293
i
E
0
15 / mg .
5 10 NOj concenlration
Fig. 7. Relation of potential change to a low concentration in the presence of sulfite ions (0.5 g I-‘).
0
5
IO
Fig. 8. Effect of sulfite-ion concentrations concentrations of nitrate ions.
0
IO NO; concentration
Fig. 5. Relation of potential change ions.
20
Time
20 30 / mg. 1.’
to a lowconcentration
60
‘lo
i
of nitrate
80
min
Fig. 6. Open-circuit potentials vs. time in a solution containing sulfite ions (a) and deaerated with argon (b).
20
15
SO~~concentration
“V
20
1’
/ g
of NO,-
25
.I 1
on sensitivity for very low
sulfite ions. The proportional relation between E and concentration still held in these media. The effect of the concentration of sulfite ions on the sensitivity is shown in Fig. 8. Stable sensitivity was obtained independent of the concentration of sulfite ions. Therefore, sulfite ion was proved to be applicable as a dioxidation agent. Fig. 9 represents the dependence of the sensitivity on pH. The sensitivity decreased with increasing pH. However, it was found that the sensitivity was constant in the pH range between 11 and 12. Reliable data could not be obtained below pH 11 because of the appreciable drifts of the baseline. This demonstrated that the adjustment of pH by buffer solutions was necessary to obtain the stable sensitivity. Furthermore, the results shown in Fig. 10 indicated that the concentration of phosphate buffer had almost no effect on the sensitivity. If the electrode surface was continuously scratched to renew the alloy surface, very reproducible data were expected. However, the sensitivity of each electrode was slightly different with respect to that of other electrodes.
294
S. Asakura et of. / Sensors and Actuators B 24-25 (1995) 291-295
300 -
200 # $I 3 0
-
s,o:-
0 cl-
0
_;;
1 In
-
co:- so:-clod
1.
NO;
IO;
-100
I20
II 5
pH
I25
13.0
Fig. 12. Interferences I-’ of nitrate ions.
I -
Fig. 9. Effect of pH on sensitivity for very low concentrations
of conventional
ions in the presence of 30 mg
of
12. However, thiosulfate, nitrite and iodate ions, which are reducing or oxidizing agents, have a considerable effect on the determination. Nevertheless, relatively speaking, the selectivity of the present method is higher than that of presently existing methods. Furthermore, the electrochemical impedance of the Devarda’s alloy electrode is about 20 R cm-‘, which is a few orders of magnitude lower than that of existing ion-selective electrodes. This indicates that industrial instrumentation would be extremely simplified if this electrode was employed.
4. Conclusions Concentration
of phosphate huffer / M
Fig. 10. Effect of the concentration of phosphate buffer on sensitivity for very low concentrations of nitrate ions.
Potential
vs. SHb I V
Fig. 11. Relationship of sensitivity to potential in a phosphate buffer solution free from oxidant after immersion for about 30 min.
Fig. 11 shows the relation of the sensitivity to the potential after the electrodes were immersed in pH 12 buffer solution for 30 min. The sensitivity of the different electrodes was a function of the potential after a fixed immersion time. The interference of conventional ions such as chloride, carbonate and sulfate, was slight, as illustrated in Fig.
The present work showed that the electrode potential of Devarda’s alloy responded quantitatively to the concentration of nitrate ions. In the absence of some oxidative or reducing agents, nitrate ion was determined with high selectivity. Furthermore, the addition of sodium sulfite eliminated the interference of oxygen. Stable and sensitive responses were guaranteed by adjusting the pH between 11 and 12. The sensitivity obtained for an individual electrode was a function of the electrode potential in the phosphate buffer solution after a fixed immersion time. However, surface renewal by scratching could provide reproducible data with no hysterisis. This could simplify the maintenance procedures if applied to the industrial field. The corroding metal/solution potential measurement should be easy because its impedance is very low. The above features, as well as the remarkable inexpensiveness of the present method, should be very advantageous over the presently existing methods.
Acknowledgement K. Fukuda and H. Nakagawa appreciate the financial support from New Cosmos Electric Co., Ltd.
S. Asakura et al. I Sensors and Actuators
References
B
24-25 (199s) 291-29s
295
K.S.V. Santhanam, Ion-Seming Electrodes and Electrochemical World Scientific, Singapore, 1990, p. 118. [91 J. Koqa, Theory and application of ion-selective electrode part 8, Anal. Chim. Acta, 233 (1990) l-30. PI
Instrumentafion,
[ll
PI t31 I41 I51
I61
171
K. Ito and H. Sunahara, Simple determination of ammonium and nitrate ions in activated sluge process waters with plasticmembrane ion selective electrode, Buns& Kagaku, 36 (1987) 123-125. D.D. Beer, Measurement of nitrate gradients with an ionselective microelectrode, Anal. Chim. Acra, 229 (1989) 351-356. J.J. Fried, Nitrates and their control in the EEC aquatic environment, Nafo ASI Ser., G30 (1991) 3-11. R.F. Spalding and M.E. J&w. Nitrate contamination in the contiguous United States, N&o ASI Ser., G30 (1991) 134. A.M. Hartley and R.I. Asai, Spectrophotometric determination of nitrate with 2,6,-xylenoi reagent, Anal Chem., 35 (1963) 1207-1213. P.W. West and T.P. Ramachandran, Spectrophotometric determination of nitrate using chromotropic acid, Anal. Chim. Acta, 35 (1966) 317-324. F.L. Fisher, E.R. Ibert and H.F. Beckman, Inorganic nitrate, nitrite, or nitrate-nitrite rapid calorimetric determination of micro quantities in aqueous solution, Ano!. Chem, 30 (1958) 1972-1974.
WI
S. Wakida, M. Yamane, A. Kawahara, S. Takasuka and K. Higasi, Durable poly(vinyl chloride) matrix ion-selective field effect transistors for nitrate ions, Bunreki Kagaku, 38 (1989) 510-514.
1111 PC. Hauaer, P.M.J. Perisset, S.S.S. Tan and W. Simon, Optode forbulk-responsemembranes,~inal. Chem., 62 (1990) 1919-1923. A.J. Bard, Encyclopedia of Electrochemical Elemenfs, Vol. 8, PI Marcel Deckker, New York, 1978, p. 450. I131 M. Nishimura and K. Matsunaga, Determination of nitrate by reduction to nitrite, Bun&i Kaguku, I8 (1969) 154-158. 1141 APHA AWWA WPCF, Saandard Methods for fire Examination of Water and Wastewaler, New York, 14th edn., 1976, p. 423. I151 A.P. Murphy, Chemical removal of nitrate from water, Nature, 350 (1991) 22>225. WI F. Feigel, Spot tests based on redox reactions with Devarda’s alloy and Raney alloy, Anal. Chem., 33 (1961) 111%1121. I171 Japanese Industrial Standards, JIS K 8653 (1985).