Balasubramanian, V.

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~'D~"') l l I Adaptability of Nitrate Specific Ion Electrode for Nitrate Analysis in Tropical Soils V. Balasubramanian and Y. Kanehiro I . •· Hawaii Agricultural Experiment Station • College of Tropical Agriculture • University of Hawaii • Departmental Paper 19 ACKNOWLEDGMENTS The technical help rendered by Mr. P. S. C. Rao, Graduate Research Assistant, University of Hawaii, during the course of this study is gratefully acknowledged. •.,, 2 HAWAII AGRICULTURAL EXPERIMENT STATION COLLEGE OF TROPICAL AGRICULTURE UNIVERSITY OF HAWAII Honolulu, Hawaii ADAPTABILITY OF NITRATE SPECIFIC ION ELECTRODE FOR NITRATE ANALYSIS IN TROPICAL SOILS V. Balasubramanianll and Y. Kanehirob' INTRODUCTION The literature on soil nitrate analysis reveals numerous colorimetric and distillation methods for N03determination in soil and plant extracts. Many such methods are time consuming and complicated, and they require special equipment and chemicals. In the past decade, several selective ion electrodes have been intro­duced as a quick means of determining specific ions in solutions. The nitrate electrode is one such introduction that has gained wide usage in recent years. Many workers claim that this electrode procedure for nitrate analysis is simple, easy, and rapid with no appreciable loss in accuracy CI-§.). The question of anion interference on N03electrode is very important for the successful employment of this method for nitrate analysis in tropical 1/Graduate Research Assistant, Department of Agronomy and Soil Science, University of Hawaii, Honolulu. 2/Professor of Soil Science, Department of Agronomy and Soil Science, University of Hawaii, Honolulu. 3 and arid zone soils which may contain large amounts of soluble salts. Another source of these interfering anions in such soils is the irrigation water which is often high in Cl- and HC03ions (~_). Hence this work was taken up to investigate the following aspects in detail: 1. Anion interference in nitrate electrode. 2. Nitrate standard curve vs. time. 3. Storage of nitrate electrode. 4. Extraction of N03for electrode process. MATERIALS AND METHODS General - The apparatus used consisted of the Orion Nitrate Specific Ion Electrode, Model 92-07, a calomel reference electrode, and a Beckman Expandomatic pH meter. Interference - The interference work was concentrated on anions, such as Cl-, HC03, N02, and so42-, which are frequently encountered in soils and irrigation waters or added to soils through fertilizers and pesticides. The interfering anions at concentrations from l to 1000 ppm were added to pure nitrate solutions of known composition, and then the nitrate concentration was measured by the electrode. Electrode Calibration - The electrode was calibrated using the calibra­tion-curve technique with standard nitrate solutions which were preserved with 0.02 ppm phenyl mercuric acetate. The calibration procedure was repeated daily to find out the effect of time on standard curves. Extraction of Nitrate for Electrode Method - Different extracting agents ' (distilled water, 0.02 Ii cuso4, 0.02 Ii Na2HP04, and 2 Ii KCl) were used to extract nitrate from various surface soils. Fifty ml of the extractant were added to 10 g soil (oven-dry basis) and shaken for 1 hour, and the clear 4 supernatant solution was used for nitrate measurement. The soil extract with KCl was used only for the steam distillation method (2) because the Cl concentration in that solution was too high to be employed for the electrode procedure. In all other extractants nitrate was estimated by the electrode. The N03values obtained with the electrode using various extractants were compared with those obtained from 2 !!_ KCl by the steam distillation method. RESULTS AND DISCUSSION Interference - The measured potential for a nitrate solution containing interfering ions is given by the equation E = Constant -where AN03 is the nitrate ion activity, Kx is the selectivity constant of interfering anion at a concentration ex with a charge n. The percentage interference may be calculated as follows: l 00 KC l/n Percentage interference= X X The common interfering anions occurring in soils and having significant selectivity constants are N02, HCOj~ Cl-, and so42-. The results in Tables l and 2 indicate that the above anions seriously interfered with nitrate measurement by the electrode. For a particular concentration of interfering anion, the magnitude of interference increased with decreasing contents of nitrate in the solution; solutions having less than 10 ppm N03-N were affected seriously. The interference from Cl was higher than that from HC03, both enhancing the measured N03values over and above the expected values. This confirms the observation of Myers and Paul (.§_), but the HC03 interference con-5 Table 1. The effect of various amounts of interfering anions on the electrode measurement of nitrate* Interfering c1- as NaCl HC03 as NaHC03anions added, ppm 1 ppm 10 ppm 100 ppm l ppm 10 ppm 100 ppm 1 ppm 10 ppm 100 ppm 0 1.00 10. 0 100. 0 1. 00 l 0. 0 100.0 1.00 10. 0 100.0 l 1.00 l 0. 0 100.0 0.95 10.0 99.5 0.96 9.7 100.0 10 1.35 9.7 102.0 0.97 l 0. 2 100.0 0.71 0.5 100.0 100 3.20 11.0 112.0 1.18 l 0.4 100.0 0.61 8.6 92.0 500 5.20 12. 2 114. 0 1. 95 l 0. 6 l 00.0 0. 61 7.3 93.0 1000 8.00 13. 1 114. 0 2.20 10. 5 99.0 0.61 6.7 89.0 *N03-N measured by the electrode at levels of 1 , 10, and 100 ppm N0 3-N. Table 2. Interference of nitrate in nitrate measurement by the electrode in a 10 ppm N03-N standard solution Nitrite (N02) added, ppm 0 l 10 100 500 1000 N03-N measured, ppm 10 9.8 10.8 19. 5 40 62 6 tradicts the results of Oien and Selmer-Olsen (.ZJ. The interference from so42-was of a different type in that it depressed the acti~ity of N03. This may be attributed to the ionic strength effect as discussed below. The data in Table 2 show the interference of N02on the nitrate electrode. When large amounts of N02were present (100 ppm and greater), the magnitude of interference became large. At the greatest level of N02(1000 ppm), the original 10 ppm N03-N was found to erroneously read 62 ppm. Therefore, care should be taken to see that nitrite is not present in solutions used for this method. The valency effect of the cations accompanying the interfering anions on sensitivity of nitrate electrode is shown in Table 3. As the valency of cations accompanying the interfering anions increased, the magnitude of anion interference on nitrate electrode decreased. This may be caused by the depressing effect of increased ionic strength (due to increased cation val ency). Since so4 2- possesses the lowest selectivity constant for interference, it was selected to study the ionic strength effect on electrode sensitivity. It will be noticed in Table 4 that the measured N03concentrations of a standard 10 ppm N03-N solution decreased with increasing ionic strength; in the case of Cuso4 this decrease was stabilized at the ionic strength of 207.14 X 10-4. According to Adams (l), ion pairing with so42- is slight with univalent cations but extensive with multivalent cations. Thus it is probable that the reduction in the measured ionic strength due to ion pairing with so4 2- would be considerably higher in the Cuso4 system than in the Na2so4 solution. This might be the reason for the continued depression of N03activity with increasing so42- concentration in the Na2so4 system as compared to the Cuso4 solution. 7 • Table 3. The effect of ion valency on nitrate determination by the electrode method* Interfering Increasing cation valenct anions added, Cl- Cl- Cl-ppm as NaCl as BaC1 2 as A1Cl 3 0 10.0 l l 0.0 10 9.7 l 00 11.0 500 12. 2 1000 13. l lo. 0 9.5 9.3 10. 0 10. 6 12:0 l 0. 0 9.7 9.5 l 0.4 l 0. 9 11.5 *NOrN measured by the electrode at l O ppm N03-N level . 8 Table 4. The effect of ionic strength on sensitivity of N03 electrode Concentration CuS04 Na2so4 of sulfate in the solution, Measured Measured li I* X 10-4 N03-N, ppm I* X 10-4 N03-N, ppm 0.000 7 .14 10. 0 7 .14 10.0 0.001 27 .14 8.5 22.14 9.4 0.005 107. 14 8.0 82.14 8.6 0.010 207. 14 7.7 157. 14 8.5 0.020 407. 14 7.8 307. 14 8.0 0.030 607 .14 7.8 457. 14 7.8 0.040 807 .14 7.7 607 .14 7.7 0.050 H007 .14 7.8 757. 14 7.4 *I = Calculated ionic strength assuming 100% ionization and no ion pairing. 9 500 -E -Q. Q. 100 z Feb. 19I I fC') 0 50z LL 0 z -0 ~ Feb. 25a:: 10 z ~ w (.) z 50 (.) 'o 120 14020 MEASURED POTENTIAL (mv) Figure l. The effect of time on nitrate standard curve. 10 , __  ., ____________ _ Effect of Time on Nitrate Standard Curve - The electrode should be calibrated with standard nitrate solutions; for each extractant a separate standard curve should be prepared. If the readings are plotted on a semi­log paper with N03concentrations on the log scale and measured millivolts on the normal scale, a straight line relationship is obtained. However, in the presence of interference, the curve deviates from linearity. For instance, when an irrigation water containing 250 ppm of Cl was used to prepare standard solutions, the deviation of the curve from linearity was very much pronounced in the lower concentration range of nitrates (Figure 1). It was also noticed that there was a shift in the nitrate standard curve with time (Figure 1). The curves constructed on two different dates showed a lateral displacement. Therefore, the electrode should be calibrated for each set of readings taken at different times or days. Storage of Nitrate Electrode - When not in use, the nitrate electrode was separated from the reference electrode and stored in a very dilute nitrate solution (0.2 to 0.3 ppm N03-N) containing 0.02 ppm phenyl mercuric acetate as a preservative (.2_). It was found that there was no detectable increase in the N03concentrations of the solution (due to electrode leak­age) in which the electrode was stored for several weeks. This is contrary to the observation of Mack and Saunderson(!} who reported excessive electrode leakage during storage. With the kind of storage as mentioned above, the perfonnance of the nitrate electrode was observed to be good for longer periods of time (4 to 6 weeks). This suggests that the electrode need not be stored in a very concentrated N03solution as speculated by Mack and Saunderson (!). Extraction of N03for Electrode Method - The electrode compared very well with the steam distillation method for nitrate analysis. When 11 11 Table 5. Determination of N03-N (ppm) in various soil extracts by two methods Di sti 11 ed water Electrode method 0.02N CuS04 0.02N Na2HP04 Steam distillation method 2N KCl Soil series extract extract extract extract Molokai silty clay 8.4 6.8 4.8 5.2 Waimea fine sandy loam 24.8 25.0 18.0 25.6 Wahiawa silty clay 1. 5 1.8 0.7 2.8 Akaka silty clay 45.5 47.0 60.0 50.7 Hilo silty clay loam 34.5 42.0 41.0 41.4 Honokaa silty clay loam 22.9 22.3 22.4 21. 5 Maile silty clay loam 80.0 80.0 83.4 78.3 Kawaihae stony loam 2.4 1.6 1.7 1.6 Waikaloa fine sandy loam 46.2 45.0 45.8 45.'9 Lualualei clay 12 .1 12. 3 1o. 2 12. 1 Pahala fine sandy loam 79. 5 78.0 73.8 74.2 Correlation with the steam 0.984 0.996 0.984 distillation method (r2) C. V. for absolute differ­ 7. 19 3.95 9.95 ences between the electrode and the steam distillation methods (%). 12 ·-- -·-·····~ - -- . -----different surface soils were measured for nitrate by the electrode in various soil extracts and compared with the steam distillation method (KCl extract}, very high correlations (r2 = 0.984 to 0.996) were obtained (Table 5). Of the three extractants tried, 0.02 ! Cuso4 gave the highest correlation (r2 = 0.996) and thus was shown to be an effective nitrate extracting solution for the electrode method in the soils studied. This high correlation of the electrode method (CuS04 extractant) with the steam distillation method (KCl extractant) is shown in Figure 2. Moreover the CuS04 extract gives a clear supernatant solution in about 1 hour after shaking, facilitating the direct de­termination of N03 in the supernatant solution without any necessity for fil­tration. The percentage coefficient of variability for absolute differences between the electrode and the steam distillation methods is minimum for Cuso4 extractant. CONCLUSIONS Anion interference in the nitrate electrode method was found to be sig­nificant. The electrode can be used for N03analysis in soils containing large amounts of soluble salts if proper precautions are taken to avoid or compensate for the anion interference. This method is highly useful when rapid determinations of nitrates are required in a large number of samples or in routine nitrate analysis. In between actual use, the electrode can best be stored in a very dilute nitrate solution to which phenyl mercuric acetate is added as a preservative. SUMMARY The performance of nitrate specific ion electrode in presence of inter­fering anions was evaluated in this study. This was done by adding known 13 0 0 :c I-w ~ w 0 0 0::I-_ (.) w ...J w ..-E 0.. 0..-z I I rt> 0 z r = 0.998 r2 = 0 .996 /1. 1.02 X - 0.42y = 60 40 20 NO;-N (ppm), STEAM DISTILLATION Figure 2. Comparison of methods for nitrate analysis: electrode method vs. steam distillation method. 14 amounts of interfering anions to standard nitrate solutions and then esti­mating the amount of nitrate by the electrode. The following are the anions which significantly interfered with electrode sensitivity and which are commonly found in tropical and arid soils, irrigation waters, fertilizers, and pesticides: ~o;, Cl-, HC03, and so42~ The magnitude of interference from these anions was large in the lower concentration range of nitrates (less than 10 ppm N03-N). There was a slight change in the nitrate standard curve with time, and so the electrode should be calibrated for each set of readings taken at different times or days. In between actual use, the pre­pared electrode can best be stored in a very dilute N03solution (0.2 to 0.3 ppm N03-N) and preserved with 0.02 ppm phenyl mercuric acetate solution. Cuso4 solution (0.02 !!_) was shown to be an effective N03-extracting solution for the electrode method in the soils studied. A high correlation coeffi­cient (fz = 0.996) was obtained when the electrode procedure was compared with the steam distillation method for soil nitrate analysis. Thus in soils with high soluble salts the nitrate electrode can be successfully employed for nitrate analysis if proper precautions are taken to avoid or compensate for the anion interference. 15 BIBLIOGRAPHY 1. Adams, F. 1971. Soil Sci. Soc. Amer. Proc. 35:420. 2. Bremner, J. M. 1965. Inorganic forms of nitrogen. P. 1179 in C. A. Black, ed. Methods of soil analysis, part 2. Agronomy series 9. Amer. Soc. Agron. Madison, Wisconsin. 3. Dahnke, ~J. C. 1971. Soil Sci. and Plant Anal. 2:73. 4. Mack, A. R., and R. B. Saunderson. 1971. Canad. J. Soil Sci. 51 :95. 5. Mahendrappa, M. K. 1969. Soil Sci. 108:132. 6. Myers, R. J. K., and E. A. Paul. 1968. Canad. J. Soil Sci. 48:369. 7. Oien, A.,and A. R. Selmer-Olsen. 1969. Analyst 94:888. 8. Orion Research. 1967. Instruction manua 1. Nitrate ion activity electrode, Model 92-07. Orion Res. Inc., 11 Blackstone St., Cambridge, Mass. 9. Tenorio, P. A., R. H. F. Young, and H. C. Whitehead. 1969. Water Resources Res. Center, Univ. Hawaii. Tech. Rep. 33. Hawaii Agricultural Experiment Station, College of Tropical Agriculture, University of Hawaii C. Peairs Wilson, Dean of the College and Director of the Experiment Station Leslie D. Swindale, Associate Director of the Experiment Station Dep. Paper 19-February 1974 (l.SM) 

Adaptability of nitrate specific ion electrode for nitrate analysis in tropical soils

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Adaptability of nitrate specific ion electrode for nitrate analysis in tropical soils

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