with analyses of radioactive cadmium ( 5 ) .Results showed that the flameless technique was in better agreement with the radioactive analyses of cadmium than the chelation/ solvent extraction technique, whose results were consistently lower. Thus, the accuracy of the results was better with the flameless method. Summary
Flameless atomic absorption spectroscopy has been shown to be a precise, rapid, and accurate method for analysis of low concentrations of trace metals. When confronted with analyses for this range of concentrations, the’ flameless method proves to be advantageous over the chelation/solvent extraction method. No major interferences are noted and the decrease in sample handling reduces the possibility of sample contamination, while the detection limit is reduced to a lower value.
Literature C i t e d (1) Ediger, R. D., A t . Absorpt. Newsl.,12,151 (1973). (2) Walsh, A., Anal. Chern., 46,698A (1974). (3) Culver, B. R., Surles, T., ibid., 47,920 (1975). (4) Barnard, W. M., Fishman, M. J., At. Absorpt. Newsl., 12, 118 (1973). (5) Kjellstrom, T., Lind, B., Linnman, L., Nordberg, G., Enuiron. Res., 8,92 (1974). (6) Environmental Protection Agency, Cincinnati, Ohio, “Methods for Chemical Analysis of Water and Wastes,” 1971. (7) Perkin-Elmer Corp., Norwalk, Conn., “Analytical Methods for
Atomic Absorption Spectroscopy Using the HGA Graphite Furnace,” 1973. (8) Everson, R. J., Parker, H. E., Anal. Chern., 46,2040 (1974). (9) Varian Associates, Palo Alto, Calif., “Optimum Parameters for Spectrophotometry,” 1973. (10) Guillaumin, J. C., At. Absorpt. Newsl., 13,135 (1974).
Received for review February 3, 1975. Accepted July 10, 1975.
Determination of Nitrate in Water Samples Using a Portable Polarographic Instrument Richard L. Young, J. Everett Spell, Henry M. Slu, and Robert H. Philp’.’ Department of Chemistry, University of South Carolina, Columbia, S.C. 29208
Edwin R. Jones Department of Physics, University of South Carolina, Columbia, S.C. 29208
w A simple portable polarographic analyzer is described. Use of this system for the determination of nitrate in natural water samples employing the polarographic waves developed in the presence of Zr(1V) and U(V1) is reported. The use of these catalytic systems allows direct compensation of background currents due to interferences. Results for both methods are in good agreement with those from spectrophotometric analyses, provided analyses are done a t the same time. While polarographic methods for the determination of a large number of substances of interest in the environment have been reported, they are seldom used routinely. The relative complexity of polarographic instrumentation and the inconvenience of cumbersome dropping mercury electrode (DME) assemblies no doubt account for the lack of popularity of polarography vis-a-vis spectrophotometric methods. With the increased availability of low cost, reliable and small solid state operational amplifiers (OA’s), it would seem that these objections could be largely overcome and that the ease of sample preparation, accuracy, and reliability of polarographic methods would warrant their reexamination. During the course of this work, a portable polarographic analyzer has been described ( I ) that takes advantage of these advances in technology. The possibility of routine polarographic determination of nitrate is particularly attractive. Because nitrate reduction is observed at the DME only in the presence of metal ion catalysts such as La(II1) (2, 3), Zr(IV) ( 4 ) , and U(V1) ( 5 ) ,the possibility exists to compensate directly for interferences that are reduced a t the same potential by a simple differential measurement. On sabbatical at the University of Georgia, Athens, Ga.
While the Zr02+ and U02*+ methods’have appeared as tentative methods in a standard reference (6), they do not appear to be widely employed. Although numerous studies have been made, the course of the electrode reaction is not completely understood in the case of any of the catalysts. In the La3+ and U0p2+ systems, the reactions appear to be reduction of nitrate to ammonia and/or hydroxylamine, catalyzed by intermediate oxidation states of the metal. The wave observed in the presence of ZrOCl2 has been reported to be due to distinct Zr-NO3 complexes (7). In general nitrite, if present, gives a similar wave under conditions where the nitrate wave is observed and methods have been reported (8) for differentiating between nitrate and nitrite in the polarographic analysis. The presence of nitrite in samples studied was not indicated and no differentiating techniques were employed. Experimental
The Circuit. The circuit employed is shown in Figure 1. The potential is applied through the follower (OAl), and
50 K
50K
IK
Figure 1. Circuit of
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current measurement is carried out in the conventional manner through OA2 and OA3. The diode-follower-50-kf capacitor combination serves as a peak reader. The background current in the absence of the catalyst is bucked out at the final stage through the follower OA6. Spraque ULN 2157A dual OA’s were employed. The circuit was powered by two 12-V Burgess CD27 batteries:The system was normally used in the laboratory in conjunction with a charger. Output voltage was read using a Heath SM-4440 digital voltmeter. The estimated total cost of all components excluding the meter and charger is $65. Cells and Electrodes. A small DME for portable applications was designed using a Pyrex screw cap test tube sealed to a No. 2 stopcock and a standard capillary of 6-cm length. The total height of the assembly was 22 cm. The capillary gave a reproducible drop provided a near constant mercury level was maintained. Capillary characteristics were rn = 1.0 mg/sec and t = 6.7 sec in 0.1M KC1 at -1.0 V vs. SCE. The reference electrode was a minature commercial calomel (Fisher #13-639-79). The cell was purged for oxygen removal by Freon-12 contained in a 15-02 can. The entire system except the charger was contained in a wooden microscope case, with the cell and electrode assembly mounted on the inside of the door. Additional Equipment. Preliminary polarographic investigations were made using a Sargent XV polarograph with conventional cell and electrode. Potentiometric nitrate determinations were carried out with the Orion 92-07-04 nitrate electrode system. Spectrophotometric determinations, using the brucine sulfate method, were done with a Bausch and Lomb Spectronic 20 spectrophotometer.
,
LI
I
0.0 -10 POTENTIAL, VOLTS vs S C E Flgure 2.
Polarogram of a sample containing 5 pprn NOs- (1.1 ppm
N) before and after addition of ZrOClp
Preliminary Chemical Investigations Three catalyst systems were re-examined. While the potential sensitivity of the La3+ system is great (about 0.35 WAlppm NO3- with typical capillaries), findings were in agreement with previous work ( 5 ) indicating that it is not satisfactory for routine analysis. Both the Zr02+ and the U0z2+ systems proved satisfactory for routine analysis, using the portable instrument, and both were employed in studies reported here. Analysis using the enhancement of U0z2+ wave employ the very well-developed plateau of the wave that occurs at about -0.9 V vs. SCE which shows good linearity with nitrate concentration. This method suffers from the disadvantage that the current due to nitrate is measured from the top of a current of about 2 MAa t typical catalyst concentrations, requiring that the final concentration of U0z2+ be accurately known. The Zr02+ method has been difficult to apply because the wave is poorly defined and in some cases not well developed before background. Wharton (7) has reported that improved plateaus can be obtained by using ZrOClz solutions that have been allowed to age; these findings were verified. Even better enhancement was obtained from use of ZrOClz solutions heated a t approximately 70°C for 48 hr prior to use. All solutions employed were treated in this manner. Very satisfactory polarograms, having a well-developed plateau between -1.1 V and -1.3 V vs. SCE, were obtained using natural water samples with nitrate concentrations less than 10 ppm N03- and a final Zr02+ concentration of 0.01M. Typical polarograms of a solution containing nitrate before and after addition of ZrOClz are shown in Figure 2. 1076
Environmental Science & Technology
1
I
I
I
IO
20
30
I
40
I
50
NITRATE CONCENTRATION (pg/,l)
Calibration curve for portable instrument using ZrOC12 method. The instrument response has been normalized to the same gain setting Figure 3.
Analytical Procedure Most of the results reported here were obtained using the Zr02+method. The response to this system is about 0.4 kA/ppm Nos- indicating a lower limit for accurate determination of about 1 ppm NO3- with the present design. The following procedure was employed in the analysis: 1. The gain was set so as to be as high as possible without exceeding the output capability of the system (about 10 V on battery operation) on the most concentrated samples expected. 2. A 5.0-ml sample of test solution and 1.0 ml of a 0.62M KC1 solution were added to the cell followed by 5 min of Freon-12 purging. 3. The DME was inserted and the cell potential set a t -1.25 V VS. SCE. 4. With S-2 shorted to ground the output, voltage (proportional to background current) was allowed to stabilize for 1 min. This voltage is then fed in to buck out the background by using the 500-KQ potentiometer while holding S-1 closed. In typical real samples the background current resulted in an output voltage of about 0.6 V, equivalent to about 0.2 kamp.
I
Table I . Comparison of Methods for Nitrate Ppm NO,-found Sample
Synthetic Synthetic Synthetic River
Ppm NO,added
5.0 5.0 5.0
-
(S,P
Polarographic,
UO,CI,
5.0 (2.8%) 4.9 4.6
2.0 (2.2%)
Potentiometric
4.9 10.9 5.0 4.7
(2.4%) (4.5%) (3.2%) (7.8%)
Colorimetric, brucine sulfate
4.9 5.0 5.2 2.0
Other components
(2.4%) (2.8%) (2.3%) (2.0%)
30 ppm SO,250 ppm CI5 ppm F-
a Relative standard deviation.
5. A 0.1-ml sample of a 0.7M stock solution of ZrOCl2 (previously heated) was added, and after a short additional purge the output was read. The output normally requires about 15-20 sec to stabilize. Repetitive readings made by allowing the output voltage to stabilize after shorting s-1to ground normally agree to within 1%relative. Analysis was done by means of calibration curves. A typical calibration curve is shown in Figure 3.
Table I I . Comparison of. Polarographic (ZrOCI,) and Spectrophotometric (Cd-Column Diazotization) Methods for Effluent Samples Ppm NO,-found Sample
G-704 G-682 G-692 A-459
Results
Comparison with Other Methods. Table I shows analytical results of the U0z2+ polarographic method compared to other methods for real and synthetic samples. The high results in the potentiometric measurement are not unexpected in view of the known chloride interference in this case. A survey was carried out of water treatment effluent samples that had been previously analyzed for nitrate using the cadmium column-diazotization method. In all cases, the polarographic analysis was carried out after the colorimetric analysis a t times varying from a few hours to a few days. Since no preservative was added and there was apparent loss of nitrate, a valid comparison cannot be made. For a total of 16 samples analyzed by both methods, the median result for Zr02+ polarographic measurements was 23% low. Excellent agreement between the two polarographic methods (Zr02+ and U0z2+) were obtained in all except one of the samples. A more valid comparison between the polarographic (ZrOZ+)and the cadmium reduction method is shown in Table 11. In this case the polarographic data were obtained within 4 hr after the spectrophotometric determinations.
Polarographica
Spectrophotometrica
3.1
2.9 2.6b 2.8
4.0 2.4 2.5 2.9
apolarographic determination done 4 h r after spectrophotom e t r i c . b Relative s t a n d a r d d e v i a t i o n 6.1%, s i x d e t e r m i n a t i o n s .
Acknowledgment
The assistance of Carol Jeffcoat in providing analyzed water samples is gratefully acknowledged. Literature Cited
Coleman, D. M., Van Atta, R. E., Klatt, L. N., Enuiron. Sci. Technol., 6,452 (1972). (2) Tokuoka, M., Collect. Czech. Chem. Commun., 4,444 (1932). (3) Tokuoka, M., Ruzicka, J., ibid., 6,339 (1932). (4) Rand, M. C., Heukelekian H., Anal. Chem., 25,878 (1953). ( 5 ) Kolthoff, I. M., Harris, W. E., Matsuyama G . , J. Am. Chem. SOC.,66,1784 (1944).
(1)
(6) American Public Health Association, New York, N.Y., “Standard Methods for the Examination of Water and Wastewater.” 12th ed., pp 202 and 398. (7) Wharton, H. W., J.Electroanal. Chem., 9, 134 (1965). (8) Brezina, M., Zuman, P., “Polarography in Medicine, Biochemistry and Pharmacy,” p 114 Interscience, New York, N.Y., 1958.
Received for review November 6,1974. Accepted July 25,1975.
Identification of Soil Denitrification Peak as N20 P. Zimmerman’ and R. Rasmussen Air Pollution Research Section, College of Engineering, Washington State University, Pullman, Wash. 99 163
Researchers working with soil samples in closed systems should be aware of the possible production of high levels of N20 by some soils. The measurement of this compound is possible during FID gas chromatographic analysis, although high COz levels may interfere with quantitation.
Many researchers in the soil science and air pollution fields have become increasingly aware of the important role soils play in emitting and scavenging hydrocarbons in the
atmosphere. Among these hydrocarbons, ethylene is unique. It is the major component of the light hydrocarbons in auto exhaust. It is also the most actively removed light hydrocarbon from auto exhaust by soils. It has been recognized to be an important plant hormone (1). Recent evidence indicates that it may also serve as a microbial population regulator, by maintaining aerobic conditions in soil (2). Therefore, the observation of a peak just prior to and/or interfering with the elution of ethylene attracted considerable attention in our laboratory during studies quantifying the rates of emission and scavenging of light hydrocarbons in the soil airspace. Volume 9, Number 12, November 1975
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