Specific-Ion Electrode Determination of Nitrate in Some Freshwaters and Sewage Effluents Donald Langmuir and Roger L. Jacobson Mineral Conservation Section, Department of Geochemistry and Mineralogy, Pennsylvania State University, University Park, Pa. 16802 lated to u N 0 3 through the empirical equation Thirty-seven waters with 1.5 to 122 p.p.m. NO3were analyzed for NOs- with a nitrate ion selective electrode and by the brucine method. Most samples were of the Ca(HCO&type with specific conductances 60 to 558 pmho., HC03- from 17 t o 260 p.p.m., and CI- from 0.8 to 47 p.p.m. Two electrode methods were evaluated. The K-y method involves making corrections for HC03- and C1(the chief interfering anions in most waters) and for the effect of ionic strength on yNOa.C1- has a selectivity ratio of about 3 X thus five times more serious an interference than previously reported. y N 0 3 is evaluated graphically from the specific conductance and the sample’s prevalent chemical character. Agreement between the K-y and brucine methods was generally within + 1 p.p.m. at all NOa- concentrations. An “approximate” electrode method, which ignores interferences and ?NOa corrections, gave results similar t o brucine values for samples above 50 p.p.m. of NO-a, but were systematically about 1.8 p.p.m. greater than brucine values for samples below 50 p.p.m. of NOo-.
ecent publications have described the theory and operation of the nitrate ion selective membrane electrode (Orion Research, Inc.) in pure laboratory systems (Potterton and Shults, 1967) and have shown that the electrode gives results comparable t o established methods of nitrate analysis in soils (Bremner, Bundy, et al., 1968; Mahendrappa, 1969). However, possible use of the electrode for the routine determination of nitrate in water has not been evaluated. This report compares use of the electrode with the brucine method (Amer. P. H. A., 1965) for analysis of the nitrate content of some surface- and groundwaters and sewage effluents. Electrode Response
The nitrate ion selective electrode may be used in combination with a calomel or silver-silver chloride reference electrode and any good quality line or battery operated millivolt p H meter. The electrode measures nitrate ion activity ( a N 0 3 ) related to its molal concentration (mN03) through the expression mNOs
=
aNOa/yNOo
(1)
where y N 0 3 is the activity coefficient of nitrate ion. The potential, E, in millivolts measured with the electrode is re834 Environmental Science & Technology
Kic/J i
(2)
(Srinivasan and Rechnitz, 1969) where E‘ is a constant, T is the temperature in K., and Ki is the selectivity ratio of interfering anion i present with a n activity of ai. The symbol denotes summation of &ai for all interfering anions. T i s the Nernst factor and equals The quantity 1.984 X 59.16 mV at 25 C. Equations 1 and 2 may be formulated in terms of parts per million as NO3-. Thus Equation 1 becomes O
(3) where m ’ N 0 3 is the nitrate concentration in parts per million, and a ’ N 0 3 is the activity of nitrate measured by the electrode in parts per million. Equation 2 similarly becomes E
=
E’
- 1.984 X
T log [@’Nos) X (1.61 X lo-’)
R
+
E = E’ - 1.981 X lo-‘ T l o g (UNO3
+
K~(ai’/Wi)l (4) i
where ai’ is the activity of interfering anion i in parts per million, and W iis its gram formula weight times lo3. In the absence of interfering anions at 25 C., Equation 4 simplifies to
E
=
E”
- 59.16 log (U’NO3)
(5)
where E” is a constant. Equation 5 shows that a tenfold increase in a‘NOocorresponds t o a decrease of 59.16 mV in E. According t o the manufacturer, the electrode exhibits Nernst in pure nitrate solutions. response up to 6000 p.p.m. of Nernst response is obtained down t o about 6 p.p.m. molal) NOa- in K N 0 3 solutions. At lower concentrations, response is less than Nernst, although the electrode is still useful for measurements as low as 1 p.p.m. To determine nitrate concentrations by electrode measurements with Equation 4, it is necessary to convert activities to concentrations through a knowledge of ionic strength, and to correct for the effects of interfering anions. The alternate approach of adding a concentrated salt solution to both standards and samples so that all measurements are made at constant ionic strength has important limitations due to anion interferences, as will be discussed below.
Ionic Strength
The normal ionic strength ( p ) of samples and standard solutions may be calculated from the expression
where m,‘ is the concentration of ionic species j in parts per million, z j is its charge, W, is its gram formula weight times l o 3 , and denotes summation of the expression for all ions 3
in soluti3-x At ionic strengths below 0.1 (equivalent t o specific conductances less than 4000 t o 10,000 pmho.), yNOa may be calculated by using the expanded Debye-Huckel equation. At 25’ C. this equation is log yNO:r
=
-(0.5091 p1i2)/(l
+ 0.9858 p”*)
(7)
Because both ionic strength and specific conductance are proportional to the concentration of ionic species in solution, the ionic strength of samples can be estimated from a measurement of specific conductance, if the prevalent chemical character of the water is known. Specific conductance measurements at 25’ C. in pure salt solutions of known ionic strength have been used to construct Figure 1. Interfering Anions
Nitrite, bicarbonate, chloride, and sulfate anions can interfere with electrode response in freshwaters and municipal waste waters. However, only chloride and (or) bicarbonate represent significant interference in most waters. In the presence of interfering ions, the apparent nitrate activity measured with the electrode (a”NOa) equals the bracketed term in Equation 4. Solving for a’NOa,and because usually HCO3- and C1- are the only interferences, we may write u ’ N O : ~= a”NOs
KEC03was calculated from electrode measurements in 14 solutions containing from 10 to 100 p.p.m. of NOa-, and 25 t o 1000 p.p.m. of HC03-. The selectivity ratio ranged and averaged 3 X 10-2. I n general, from 1 X lo-* t o 5 X the lowest values of KHcolwere measured in solutions which contained 50 p.p.m. or less of NO8-. Because most of the 37 samples analyzed in this study contained less than 50 p.p.m. of NOa-, a value of KHCO, = 2 X lo-* was used in subsequent computations. This is the same ratio suggested by the electrode manufacturer. Electrode measurements in 14 different mixed NOd-Cl- solutions containing from 12 t o 100 p.p.m. of NOaand 25 t o 500 p.p.m. of CI- showed that KClranged from 2 X lo-* t o 5 X lo-* and averaged 3 X lo-*, independent of the relative or absolute amounts of NOa- and C1- ions. This average value is five times the value of Kcl = 6 X recommended by the electrode manufacturer. Thus, chloride ion is a much more serious interference than had been previously assumed. The revised selectivity ratio for CI-, and the manufacturer’s ratios for NO?-, HCOs-, and SO4%-are listed in Table I. Calculated from these values and also given in the table are the approximate amounts of each anion which, if present along with 10 p.p.m. of NOa-, produce a + 1 % error in the nitrate determination. O n a parts per million basis, NO2is the chief interfering anion. However, nitrite is rarely present in sufticient amounts relative to nitrate t o effect electrode response. Except in waters that contain several thousand p.p.m. of SO4*-,that ion can also be ignored. and Kcl = 3 X lo-‘>, When we introduce KHCOl= 2 X Equation 9 reduces to m‘N08
=
a”NOs/yNOa (0.02 X m ‘ H C 0 3
- (1.02 KHco~X
+ 0.05 X m’C1)
(12)
+
u ’ H C O ~ 1.75 Kci X u’CI) (8) Dividing by yNOa, but because ?NOa and yC1 are equal, and equal yi-iCOa to within 3 % up t o p = 0.1, Equation 8 becomes
ITI’NO,~ = ~ ” N O d / y N 0 a- (1.02 KHCO, X m’HC03
+ 1.75 KCI X m‘C1)
(9)
In a solution containing only NOs- and HCOa- as anions, Equation 9 may be simplified and solved for K H C Oto~ give K i ~ ( ~ (=0 30.98(ar’N0a - a’NOs)/yNOa X m’HC03
(10)
Similarly, in a solution with only NOa- and C1- as anions, Equation 9 is equivalent to Kci
0 . 5 7 ( ~ ” N 0 3- a’NOs)/yNOs X m‘CI
SPECIFIC CONDUCTANCE AT 25OC I micromhos)
(11)
T o determine K H C Oand ~ Kcl for waters similar in composition to the 37 examined in this study, electrode measurements were made in a variety of mixed NO3--HCOsand NOa--C1- solutions. Values of a ” N O a were read off a plot of E cs. u‘NOa in pure K N 0 3 standards. Values of ?NO.; and a’NOa in the mixed solutions were calculated with Equations 6 and 7, and 3, respectively. Selectivity ratio measurements were limited to solutions in which nitrate was not the major anion. This was because a ” N O a and u’NOa values approach each other in NOa--predominant solutions and thus meaningful K values are not obtainable. The calculations generally involved small differences in ct’NOs and a’NOs so that the uncertainties in selectivity ratios range from about *30-50%.
Fig. 1. The relationship between yN03 and specific conductance in various pure salt solutions at 25 C.
Table I. Selectivity Ratios ( K , ) for Various Interfering Anions Concentration in p.p.m. for +1% error when Anion K m’NOa = 10 p.p.m.
NO?c1HCOa-
so 24
6 X 10-2 3 x 10-2 2 x 10-2 6 X
1.2 2 5
5000
Volume 4, Number 10, October 1970
835
Table 11. Concentration (m ’NOa)and Activity (a ’NOs) of Nitrate in KNOBSolutions at 25” C. in Parts Per Million n~‘NO8
1 5 10 25 50 100 150
N‘NO,~
m‘NOa
1 .oo 4.95 9.86 24.4 48.4 95.5 142
200 300 400 500 750 1000
CI‘NO~
188 278 367 454 668 876
Equation 12 was used to calculate rn’NOa from electrode measurements in 37 freshwater and sewage effluent samples. The results are described below under the K-y method. Procedures for Analysis
Because the electrode measures nitrate activity in samples and standards, a calibration curve must be constructed showing E measured in standard solutions as a function of a‘NOa. This can be done for K N 0 3 solutions using a’NOs values from Table 11, which are based o n Equations 3 and 7. Figure 2 shows such a plot along with the curve of E cs. m’NO3 in the standards. Taking into account both ionic strength and interfering anions (the K-y method), the procedure for nitrate analysis with the electrode is as follows: Prepare a stock solution of 1000 p . p m of NO3- by dissolving 1.631 g. of anhydrous K N O a in distilled water and dilute to 1 liter. Make up nitrate standards t o cover the range of nitrate concentrations expected in the samples. Measure E for each standard, and make a plot of E against a’NOa on semilog paper using the data in Table 11. Measure the specific conductance corrected t o 25 O C., and the HCOj- and CI- content of the sample. Estimate yNO3 from Figure 1 and calculate the interference term in Equation 10. Measure E in the sample and determine a ” N 0 3 in the sample from the calibration curve of E cs. a‘NOa. (In the standards,a”NOs = a’NOa.) Calculate rn’NOa from Equation 9. The 37 water samples analyzed in this study contained from 1.5 to 122 p.p.m. of NO3-. Because few fresh or waste waters containing more than 50 p.p.m. of NOswere available, samples 1, 3, and 5 to 9 are sewage effluents to which different amounts of K N O , have been added. Sample 2 is an unaltered sewage effluent. Samples 21 and 35 are from streams. The other 27 samples are spring and well waters. All chemical analyses were made in the laboratory a t 2 3 ” C . i 2’. The samples were analyzed for NO1- in duplicate or triplicate using the K-y method. Because the specific conductances of NaNOa and K N O a solutions of equal normality are about the same (Amer. P.H.A., 1965), and samples 1, 3, and 5 to 9 were predominantly KNOs-type waters, values of ?NOa for these samples were based on their specific conductances and the N a N 0 3 curve in Figure 1. Samples 2, 4, and 10 to 37 were predominantly Ca(HCOa)?-type waters so that the Ca(HCO,)? curve in Figure 1 was used to evaluate ?NO3. A rapid, approximate method of electrode NOj- analysis 836
Environmental Science & Technology
was also employed, in which E measured in the sample is used to read m’NOa for the sample directly off the calibration plot of m’N03 cs. E for the standards. The NOa- content of the same samples was also analyzed in duplicate or triplicate by the brucine method. Results of the two electmde and brucine methods are shown in Table 111, along with sample analyses of specific conductance and bicarbonate and chloride concentrations.
Discussion of Results The tabulated results show that agreement between the K-y and brucine methods is generally excellent at all concentrations. Of the 37 analyses, eight are equal, and 25 of the 37 agree within i l p . p m The average difference between values measured by the K-y and brucine methods is $0.04 p.p.m. The K-y method assumes constancy for the selectivity ratios of chloride and bicarbonate ions. Errors in m’NO1 due t o possible variations in Kcl and KHco3 might be expected to increase as the size of the interference correction term in Equation 12 increases. For the 37 samples analyzed, this term ranged from 0.4 to 6.0 p . p m and averaged 3.2 p.p.m. of nitrate. The good general agreement between K-y and brucine values in Table 111 regardless of the size of the interference term indicates that nonconstancy of Kcl or KEco3is not a problem for the samples analyzed. It may be worth noting, however, that for samples which contain more than 50 p.p.m. of NOa-, the agreement between K-y and brucine values is improved if KHCo3 = 4 X lo-* instead of 2 X lo-? is used in Equation 12. The apparent accuracy of measurements based on the approximate method above 50 p.p.m. of nitrate is probably because at these concentrations the samples contain appreciable alkali metal nitrate or added K N O and so resemble the nitrate standards used to draw up the calibration curve. Below 50 p.p.m., however, m’NOs values based on the approximate method average rather systematically 1.8 p.p.m. higher than brucine values. This is chiefly because the approximate method does not correct for interfering ions. The systematic nature of the error, however, suggested that this method might be calibrated against the brucine method for a range of sample nitrate concentrations in a given study and so permit subsequent analyses to be made with a minimum of effort. Such an approach works well with the 28 samples in Table 111 below 50 p.u.m. of nitrate, which are
I6Ot
t
i1
\\
\
2
4
6
810 NO3 (ppm)
20
40
60 80 100
Figure 2. Typical calibration plot for the nitrate electrode in KNOa solutions at 25’ C.
Sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
Table 111. Analysis of NO3- in Natural Water Samples the Brucine Method Specific conductance HCOICI(pmho., 25” C.) (p.p.m.) (p.p.m.) 37 558 62 522 47 121 427 29 48 415 17 125 372 42 26 366 24 41 336 22 38 285 19 33 246 27 16 352 11 156 335 122 9.0 297 5.8 150 519 31 187 340 19 107 473 27 146 432 35 168 327 7.8 163 254 126 8.6 366 11 173 407 28 164 218 58 18 475 260 16 439 217 19 284 148 6.8 258 143 9.8 310 157 10 407 194 16 330 168 6.8 325 171 8.9 268 162 2.7 190 116 2.4 163 3.7 81 127 59 1.5 239 154 2.5 198 7.0 85 151 1.2 79 60 0.8 17
predominantly Ca(HCO&-type waters. Thus, when 1.8 p.p.m. of NOI- is subtracted from each value based on the approximate method, the corrected values for seven of the twenty-eight are equal, and for twenty-three of the twentyeight they agree within + 1 p.p.m. In principle, ionic strength corrections can be avoided in electrode measurements by the addition of a relatively concentrated salt solution such that all samples and standards are analyzed at the same ionic strength. Unfortunately, the constant ionic strength approach is of limited use with the nitrate electrode. This is because electrode response is strongly interfered with by all the common anions excepting sulfate that might be considered as additives, including perchlorate and chlorate. In the presence of 500 p.p.m. or more of strongly interfering anions such as C1- and HC03-, and NOs- concentrations less than 50 p.p.m., electrode potential drift can make measurements dificult as interfering anions slowly diffuse into the electrode internal exchange solution during measurement. In samples which contain more than 100 p.p.m. of NO:,-, drift effects are less serious. Nitrate electrode measurements by one of the authors on soil waters beneath cattle
by Two Electrode Methods and NOI- (p.p.m.)
Brucine 122 101 93 89 78 77 73 65 55 43 40 34 33 28 27 26 24 24 21 21 19 17 16 15 14 14 12 12 12 11 4.5 4.3 3.8 3.5 3.5 3.2 1.5
electrode (K--/)
123 99 95 92 82 81 73 63 56 43 40 34 34 29 28 26 23 20 21 21 20 15 14 16 11 14 10 13 13 12 3.5 3.7 4.8 2.2 2.5 2.9 1.2
Electrode (approx.) 120 99 94 88 81 79 73 62 56 44 41 35 36 30 29 28 25 22 24 25 21 19 17 18 14 17 14 16 16 14 5.6 5.2 6.0 5.0 4.2 4.0 1.5
feed lots (Jacobson, 1968), showed that for nitrate concentrations ranging from 100 to at least 400 p.p.m., the constant ionic strength approach gave results accurate to within a few percent in solutions which contained several 100 p.p,m, ofinterfering anions such as Cl- and HC03-, at ionic strengths up to about 3 X 10-2. Presumably such measurements are also possible at higher ionic strengths as long as NOs- concentrations are in the hundreds of parts per million range. Another possible electrode method for nitrate analysis involves the initial removal of interfering anions. Bicarbonate can be removed with HrSOl and chloride precipitated with AgiS01 solution. Sulfate may be reduced to a noninterference level with Ba(OH)?. Similarly, Mahendrappa (1969) used sulfamic acid to eliminate nitrite interference. Interference removal is a n acceptable approach if only minor species such as nitrite are involved. However, several factors make the approach less satisfactory in general than electrode methods previously described. First, addition of reagents to the sample in excess of what will remove a n interfering anion may alter pH to below 2 or above 12, under which conditions the nitrate electrode does not respond properly. Second, and more Volume 4, Number 10, October 1970 837
important, the addition of excess reagents to eliminate major anions such as C1- or HC03- can increase sample ionic strength such that accurate estimation of yNOa from specific conductance measurements becomes di.6cult. T o avoid problems associated with excess additions, it is necessary t o know interfering anion concentrations prior to their removal. Electrode Techniques
The most accurate and reproducible results are obtained with the electrode if samples and standards are stirred continuously and at a constant rate during measurements with a device such as a magnetic stirrer, if the depth of immersion of the electrode in solutions is always the same, and if the temperature of the electrode and solutions are the same. Electrode response is most rapid if successive samples and standards are of similar composition. Response is also most rapid and reproducible in solutions which are high in nitrate and low in dissolved solids. The number of samples which may be analyzed between electrode calibrations will depend on such factors as the electronic stability of the millivolt meter, changes in the composition and temperature of samples and standards, and the age of the electrode. The authors have found that once the calibration curve of E cs. u'NO, has been drawn up at the beginning of a sequence of analyses, a check of the curve with a single standard after every two or three samples is often sufficient. When the electrode is used in solutions which contain relatively high concentrations of interfering anions, these ions will tend to diffuse through the electrode membrane displacing NO:,- in the internal anion-exchange liquid. This effect can lead to a slowing of response times and a gradual drift in E values. The electrode may be restored t o its original condition by soaking overnight in a pure K N 0 3 solution. Subsequent electrode response will be most rapid if this soaking solution has a nitrate content within the range of the samples to be analyzed. The electrode will normally exhibit Nernst response for one to six months. When response becomes much less than Nernst, the electrode membrane and internal solutions should be replaced. Teinperature Effects
The Nernst factor is temperature dependent and so the slope of the E cs. u'NOa calibration plot changes with temperature. The factor equals 57.17 mV at 15" C. and 61.14 mV at 35" C. Temperature change also effects ?NOa through the Debye-Hiickel equation, however, at 15" C. and 35" C., yNOI differs from its value at 25" C. by less than t 0 . 3 and - 0 . 3 z , respectively. Thus ?NO3 values at 25" C. are still useful at these temperatures. The chief problem of sample measurement at other than 25" C. is specific conductance increases about 2 to 3 z per O C. Figure 2 is based on specific conductance measurements at 25 C. Thus conductance measurements should be corrected to this temperature. Appropriate procedures for making such corrections are given in Amer. P.H.A., (1965). Summary and Conclusions
This study was of 37 freshwater and sewage effiuent samples containing from 1.5 to 122 p.p.m. of NOa- with specific conductances up to 558 pmho., and CI- and HC03concentrations LIP to 47 and 260 p.p.m., respectively. For these waters the K-y electrode and brucine methods gave NO3- values which agreed within i l p,p.m. for 25 of the 37 838 Environmental Science & Technology
samples. The average difference between K-y and brucine values was f0.04 p.p.m. Because selectivity ratios for interfering anions are not constant, accurate nitrate analysis by the K-y method may be restricted to samples in which anion interference corrections do not exceed about 10 p.p.m. of NO3- or 10 to 2 0 z of the true nitrate concentration. AIthough the approximate method of reading rn'N03 for the samples directly off the E cs. ni'NOt plot for the standards was only studied up to 122 p.p.m. of NOa-, the method should give accurate results up t o about 6000 p.p.m. of NO:,as long as nitrate is the predominant anion. If nitrite is present as a n important interference, its removal with sulfamic acid prior to electrode analysis may be desirable. Removal of major interfering anions such as C1- and HCOa- by the addition of large amounts of reagent may, however, alter pH out of the range for proper electrode response. Further, if excess reagents are added, ionic strength may be increased such that the accurate estimation of ?NO3 from specific conductance becomes difficult. The constant ionic strength medium approach to nitrate analysis with the electrode was not examined in detail in this study, but should be useful for nitrate concentrations greater than 100 p.p.m. A potassium or sodium sulfate solution is probably the best ionic strength medium to minimize any increase in anion interference effects. Measurements with the electrode by any method are difficult due to drift problems when 500 p.p.m. or more of strongly interfering anions such as CI- or HCO:j- are present and NOa- concentrations are less than about 50 p.p.m. Acknowledging the above limitations, in solutions where electrode measurements are appropriate, there are several important advantages of the electrode over the brucine method : Measurements are generally unaffected by sample color or turbidity. NO1- concentrations of 1 t o about 6000 p.p.m. may be determined without sample dilution or concentration. The method is portable and rapid; one person can often analyze 30 to 40 samples per hour. Analytical procedures are relatively simple, and yet the accuracy and precision of measurement is comparable to that of the brucine method. The reproducibility of measurements with the nitrate electrode is often better than 1 1 %. For a possible = 1 z accuracy of analysis, the millivolt pH meter should have a readability and reproducibility equal to or better than ~ 0 . 0 6 mV (0.01 pH units). A ckn o wledginent
Financial support for this study was provided by the Institute for Research on Land and Water Resources, and the Mineral Conservation Section, both of the Pennsylvania State University, University Park, Pa. Literature Cited American Public Health Association, "Standard Methods for the Examination of Water and Waste Water," 12th ed., pp. 198-200, New York, 1965. Bremner, J. M., Bundy, L. G., Agarwal, A. S., Anal. Lett. 1, 837-844 (1968). Jacobson, R. L., M.A. thesis, University of Missouri, Columbia, Mo., 1968. Mahendrappa, M. K., Soil Sci. 108, 132-136 (1969). Potterton, S. S., Shults, W. D., Anal. Lett. 1, 11-22 (1967). Srinivasan, K., Rechnitz, G. A., Anal. Cl7ern. 41, 1203-1208 (1969). Receiredjor reciew Junuurj. I S , 1970. Accepted Muy 18, 1970.
