Differential Pulse Polarographic Determination of Nitrate and Nitrite Steven W. Boese and Vernon S. Archer” Department of Chemistry, The University of Wyoming, Laramie, Wyo. 82071
Jerome W. O’Laughlin Department of Chemistry, University of Missouri, Columbia; Columbia, Mo. 6520 1
A differential pulse polarographic method for the determination of nitrate and nitrite was based upon the enhancement of the ytterbium peak current by both nitrate and nitrlte. A detection limit of 14 ppb nitrate or nitrite nitrogen was achleved under ideal conditions. The method was adapted to the analysis of soils and natural waters following interference removal. A relative precisionof less than 4 % was routinely achleved for analysls of soil samples containing 10 ppm or more nitrate nitrogen. Simultaneous determination of both nitrate and nitrite was possible under certain conditions. Results obtained by this method were compared to results obtained by the nitrate ion-selective electrode method and the phenoldisulfonic acid spectrophotometricmethod.
The determination of nitrate ion is an important factor in the analysis of soils and natural waters. Nitrate is intimately involved in the overall nitrogen cycle in soils and is a principal source of nitrogen for both microorganisms and higher plants ( 1 ) .A limit of 45 ppm nitrate has been imposed on drinking water since excessive amounts may lead to methemoglobinemia in infants (2). In addition, ruminant animals are particularly susceptible to nitrate poisoning (3).Nitrite, although not normally present in the environment a t levels as high as nitrate, is toxic a t lower levels and is an interference in many methods for determination of nitrate. A comprehensive review of methods for the determination of nitrate would not be appropriate in this paper; however, several of the most commonly used methods in addition to other electrochemical methods will be briefly discussed. One commonly used method for nitrate determination is the phenoldisulfonic acid method which is subject to severe chloride interference. This interference may be eliminated by determining the chloride concentration in a sample and quantitatively removing it by the addition of an equivalent amount of standard silver sulfate solution ( 4 ) . Nitrite and colored materials also interfere with this method. One microgram of nitrate nitrogen can be detected, which corresponds to 10 ppb in a 100-ml water sample, which must be evaporated to dryness prior to undergoing color development. The brucine colorimetric method ( 5 ) of nitrate determination is subject to interference by chloride, nitrite, and iron. The color intensity is also variable from run to run. This method can be used for nitrate nitrogen levels between 0.1 and 2 PPmSeveral methods for nitrate determination involve prior reduction to nitrite. Any nitrite initially present interferes and either must be destroyed or a correction factor must be applied. The quantitative reduction of nitrate to nitrite with no further reduction is difficult to achieve. Prior reduction with zinc (6),copper-cadmium (7), and immobilized enzyme (8) systems with detection limits as low as 2 ppb nitrate have been reported. Direct potentiometric methods using nitrate ion-selective
electrodes have been reported for soils (9),natural waters, and wastewaters (10). Chloride and bicarbonate are commonly occurring interferences in “real” samples; however, perchlorate, although not normally present in natural samples, gives the most severe interference. The detection limit of the ionselective method is about 0.1 ppm nitrate nitrogen in water. A method for the analysis of both nitrate and nitrite using a cation exchange column to remove cation interferences, an anion exchange column to separate nitrate and nitrite, and a tubular cadmium electrochemical detector has been described (11). The detection limit of this method is 0.1 ppm nitrate or nitrite nitrogen. Polarographic currents due to nitrate have been observed in the presence of lanthanum(II1) and cerium(II1) (12, 13), neodymium(II1) (14),zirconium(1V) (15,16),molybdenum(V) (17),and most notably uranium(V1) (18).Analytical methods based on nitrate polarographic currents in the presence of the above metallic ions are characterized, in general, by nonlinear current vs. nitrate concentration response, complex reaction mechanisms, and marked p H dependence. Common interferences include sulfate, phosphate, nitrite, oxalate, and substances which are reduced a t potentials more positive than nitrate, as dc polarography has normally been used. A differential pulse polarographic study of the ytterbiumnitrate system has been reported (19).An enhancement of the current due to reduction of ytterbium(II1) to ytterbium(I1) was observed in the presence of small amounts of nitrate in an ammonium chloride supporting electrolyte. Application of the current enhancement was made to the determination of trace amounts of ytterbium(II1) using an ammonium nitrate supporting electrolyte. It was also suggested that the current enhancement might be used for nitrate determination. The subject of this report is the application of the ytterbium system to the differential pulse polarographic determination of both nitrate and nitrite. Specific application is made to the analysis of nitrate in soils and natural waters with evaluation and minimization of the common attendant interferences.
EXPERIMENTAL Apparatus. All experiments were performed using a Princeton Applied Research Model 174 Polarographic Analyzer, equipped with a PAR Model 174/70 mechanical drop timer. The current/potential curves were recorded on a MFE Model 815 x-y recorder. A threeelectrode system was used. The reference electrode was a saturated calomel electrode (SCE) with a ceramic liquid junction; the counter electrode was a platinum foil; and the working electrode was a dropping mercury electrode (DME) constructed using a Sargent 2-5 second capillary with a column height of 50 cm. The cell was constructed using a Metrohm EA874 cell top and either a Metrohm EA875-20 or EA875-5 cell bottom. All p H or specific ion measurements were made using an Orion Model 701 p H meter and either a combination p H electrode or an electrode pair consisting of a nitrate specific ion electrode and an Orion single junction reference electrode with 0.1 M NaZS04 saturated with AgCl as the filling solution. A Manostat microburet was used to add base in the pH adjustment ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
479
\1
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16 3.4
Ytterbium
60,000~
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730,000-
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$ 4 0,000
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;
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l
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o
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Figure 2. Plot of slope/intercept ratio from Figure 1 vs. log ytterbium(ll1) concentration for determination of optimum ytterbium concentration
0.6 00
0.2 0
1
2 3 4 5 6 7 8 Y l t r a t e Corcentration
9
1 0 1 1 12
iM ~ 1 0 ~ )
Figure 1. Plots of peak current vs. nitrate concentration in 0.1 M NH4CI, pH 7.00 f 0.05 for different ytterbium concentrations. Molar ytterbium(lll) concentrations are indicated near the respective curves
step. A Hamilton 100 A microliter syringe was used to make standard additions in both the specific ion and polarographic procedures. M YbC13) was prepared Reagents. A stock solution (1.00 X by placing 0.4926 g of Yb203 (99.99%purity, Research Chemicals, Box 14588, Phoenix, Ariz. 85031), which had been heated in a furnace at 900 "C for several hours and stored in a desiccator, into a 100-ml beaker, adding 5 ml of concentrated HC1 and heating on a hot plate until dissolution was complete. Heating was continued until the solution was near dryness. About 50 ml of distilled water and 2.0 ml of concentrated HC1 were added and the solid residue was dissolved. The resulting solution was quantitatively transferred to a 250-ml volumetric flask and made to volume with distilled water. The solution was stored in a polyethylene bottle. More dilute solutions were prepared by quantitative dilution of this solution. The mercury used in the DME was triple distilled and further purified by exhaustive electrolysis at t . 3 V vs. SCE in 0.10 M HC104 to remove dissolved base metals, followed by washing with 0.1 M HC104, rinsing with distilled water, and pinholing; the washing and pinholing steps were repeated several times. The purified mercury was stored under argon at all times. All other reagents were prepared using compounds of analytical reagent grade purity and distilled water. Procedure. All current/potential curves were determined on solutions which had been purged with argon gas for 15 min before each scan and blanketted with argon during each scan. All scans were made from -1.1 to -1.6 volts vs. SCE at 2 mV/s scan rate, 25-mV modulation amplitude, and 1-s drop time. The DME flow rate was measured each day and the capillary cleaned whenever the flow rate decreased by more than 1% from 1.27 mg/s. For the determination of the optimum ytterbium concentration, scans were run on 50.0 ml of 0.1 M NHlCl (pH 7.0) before and after M YbClj (made up from 1.00 the addition of an aliquot of 1.0 X X M YbC13 stock solution and adjusted to pH 7.0), followed by scans made after the addition of various amounts of 1.00 X 10+ M NHdN0.7 (also pH 7.0). In no case was the total volume of the additions more than 5% of the initial solution volume. All work with interferences was done using a combined stock solution containing 1.4 X 10-5 M YbCli and 0.1 M NH4Cl adjusted to pH 7.0. Scans were run on 50.0 ml of this stock solution, followed by a scan run after the addition of 0.040 ml of 2.00 X M KNO4, followed by scans made after addition of various amounts of 0.10 M solutions of interfering anions (prepared from the ammonium salts and adjusted to pH 7.0). The procedure was repeated for each interference without the addition of the KN0.7. The effect of high electrolyte concentrations was investigated by making additions of appropriate amounts of 4 M CaC12,5 M MgC12, 2.5 M KCl, or 5 M NH&1 to the cell containing 0.100 ml of 1.00 X M KNOs and 5.0 ml of a solution prepared in the following way: Ten ml of distilled water was placed in a 20-ml polyethylene beaker with a stir bar. With stirring, 1.00 ml of a stock solution, which was 2.5 M NH4Cl and 5.0 X M YbC13, was added; 0.1 N NaOH was slowly 480
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
added until the pH was 4.5 f 0.1 as measured with a pH meter. The resulting solution was transferred quantitatively to a 25-ml volumetric flask and made to volume with distilled water. Investigation of the effect of NH4C104 was made in the same way except the solution contained no NHdCl. Soil Analysis Procedure. Appropriate amounts of various soil samples (0.50, 1.00, or 2.50 g), which had been air dried a t room temperature and ground in a mortar and pestle, were weighed into 100-ml polycarbonate centrifuge tubes. A 20.0-ml aliquot of 1 M CaClz and 0.100 ml of 1 N NaOH were added. The tubes were stoppered and shaken for 10 min. The solution was then filtered through Whatman No. 42 filter paper. A 10.0-ml aliquot of the filtrate (soil extract) was pipetted into a 30-ml beaker containing a stir bar. The beaker and contents were placed on a top loading balance and set to read 30.00 g with the tare. The beaker was then placed on a magnetic stirrer and 5% HC1 was added until the pH indicated by the pH meter was less than 4.0. A 1.00-ml aliquot of a stock solution, which was 2.5 M NH4C1 M YbCl3, was added. Sodium hydroxide (0.1 N) was andb.0 X slowly added until the pH was 4.5 f 0.1. The electrode was rinsed off into the beaker with distilled water; the beaker and contents were placed back on the top loading balance; and distilled water was added until the indicated weight was 40.00 g. The solution was mixed, a portion placed in the 5-ml cell, and the polarogram run after purging with argon. The standard curve was generated by pipetting a 5.0-ml aliquot of a blank prepared by the above procedure into the 5-ml cell and making known additions of 1.00 X M KNO?. Water Analysis Procedure. Water samples were analyzed by pipetting a suitable sized aliquot not greater than 15.0 ml into a 100-ml centrifuge tube. Distilled water was pipetted into the tube if necessary to make the total volume 15.0 ml. A 5.0 ml aliquot of 1M BaC12 and 0.100 ml of 1N NaOH were added. The tube was shaken for several minutes and the solution was filtered through Whatman No. 42 filter paper. A 10.0-ml aliquot of the filtrate was treated in the same manner as the soil extract in the above procedure. The standard curve was generated by pipetting a 5.0-ml aliquot of a blank prepared by the same procedure into the 5-ml cell and making known additions of 1.00 X lo-* M KNOB. The dilutions in the above procedures were made by weight rather than by volume because this was more convenient for routine analyses, not requiring a quantitative transfer of solution to a volumetric flask. All work with nitrite was done using the same procedures as with nitrate except the standard NaNOz solutions were prepared immediately before use. Ion-Selectiue Electrode Procedure. Ion-selective electrode determination of nitrate in soil was done by extracting a 0.50, 1.00, or 2.50 g sample of ground air-dried soil with 25.0 ml of distilled water for 10 min. The mixture was centrifuged and a 20.0-ml aliquot of the supernatant solution was pipetted into a 50-ml beaker. While stirring, 0.006 M HzS04 was added until the pH was 4.0 5 0.1. A 1-ml aliquot of 1 M Na2S04 and 1 ml of saturated AgZS04 in distilled water were added. The ion-selective electrode pair was then immersed in the solution. At least five standard additions of 0.100 M KNO 1 were made totaling at least 0.070 ml. The potential of the specific ion electrode (hO.1mV) was read 1 min after each addition while the solution was being stirred. Water samples were treated in the same manner as the soil extracts. Blanks were carried through the entire procedure. The function 10-E/S, where E was the potential of the nitrate electrode vs. the reference and S was the change in potential of the
~
1
0
1 2 3 Anion C o n c e n t r a t i o n
4
(MX 1 0 3 )
Figure 4. Effect of fluoride, carbonate, and sulfate on ytterbium peak currents with and without nitrate present 2 4 6 N i t r i t e Concentrar 3 n
8
[ M~
1 0 ~ )
Figure 3. Plot of peak current vs. nitrite concentration in solutions 0.1 M ytterbium(lll), pH 7.00 f 0.05 M “&I, 1.4 X
nitrate electrode observed for a 10-fold change in nitrate concentration near the nitrate concentrations observed in the analyses, was plotted vs. the volume of 0.100 M KN03 added. The resultant straight line was extrapolated until it intersected the volume axis. The volume difference between the intercepts for the sample and for the blank was taken as the measure of the nitrate content of the aliquot taken for analysis.
M ytterbium(lll), 1.6 X Upper three curves (1.4 X M NOs-, 0.1 M NH4CI,pH 7.00 f 0.05); lower three curves (1.4 X M ytterbtum(lll), 0.1 M NH4CI,pH 7.00 f 0.05). The two sets of curves are offset by 0.025 fiA. ( W ) sulfate; ( 0 )carbonate; (A)fluoride
0.4
I
RESULTS AND DISCUSSION Choice of Supporting Electrolyte. Preliminary work with the ytterbium-nitrate system had indicated that larger current enhancements were observed using NH&l rather than KCl as the supporting electrolyte, other factors being equal (20). As the current enhancement in NH4C1 solutions was observed to be essentially independent of pH from 3.5 to 7.2, ammonium ion appears to act as a proton donor for the nitrate reduction, thus providing some buffering effect in the reaction layer at the electrode-solution interface. At pH values of 3.5 or less, the peak corresponding to the reduction of H+overlapped with the ytterbium peak at -1.42 V vs. SCE and rapidly became massive as the p H decreased. At pH values greater than 7.2, ytterbium hydroxide precipitated. Ammonium perchlorate was found to be as suitable a supporting electrolyte as ammonium chloride. Ammonium sulfate caused the ytterbium reduction peak to be shifted to potentials sufficiently negative that resolution of this peak from the supporting electrolyte reduction wave was difficult. Acetate buffer systems caused the same negative peak shift and the same problem of resolution. The unbuffered solution of 0.1 M NH4Cl was the preferred supporting electrolyte because more concentrated stock solutions of NH&1 than of NHdC104 could be prepared and because maximum current enhancement was observed in solutions 0.1 M in ammonium ion (see Figure 6). Determination of Optimum Ytterbium Concentration. Peak current vs. nitrate concentration curves for various ytterbium concentrations in 0.1 M NH4C1 at pH 7.00 f 0.05 are shown in Figure 1. Both the peak currents for the ytterbium with no nitrate present (peak current intercepts in Figure 1) and the degree of current enhancement by nitrate (slopes of curves in Figure 1)were observed to increase with increasing ytterbium concentration. Since maximum sensitivity for nitrate would be achieved when the peak current enhancement relative to the peak current due to ytterbium alone was a maximum, a plot of the slope-to-intercept ratio vs. the log of the ytterbium concentration was constructed using data from “least-squares-lines” of the points in Figure 1(see Figure 2).
Figure 5. Effect of phosphate on peak currents of 1.4 X M ytterbium(ll1) in 0.1 M NH4CI, pH 7.00 f 0.05 with and without nitrate present (0)nitrate concentration = 1.6 X
M; ( 0 )nitrate concentration = 0
The optimum ytterbium concentration as determined by this M. The peak current vs. nitrate conmethod was 1.4 X centration curve for this ytterbium concentration is included in Figure 1. Nitrite was found to give a much larger current enhancement than nitrate (see Figure 3) under certain conditions (1.4 X M Yb3+, 0.1 M NH4C1, pH 7.0), although the peaks occurred at essentially the same potential (-1.43 V vs. SCE) and had the same shape. Interference Studies. Figure 4 shows the effect of fluoride, carbonate, and sulfate; and Figure 5 shows the effect of phosphate, both on the ytterbium peak with no nitrate present and on the ytterbium peak with a nitrate concentration of 1.6 X M. The supporting electrolyte was 0.1 M NH&l of pH 7.00 f 0.05 and the ytterbium concentration was 1.4 X 10-5 M in all cases. The concentrations of fluoride, carbonate, sulfate, and phosphate sufficient to decrease the current enM nitrate by 10%were 3.8 hancement produced by 1.6 X ppm, 45 ppm, 170 ppm, and 0.07 ppm, respectively. Thus, the first three anions constitute moderate interferences; however, interference by phosphate is severe. The peak potentials shifted in a negative direction with increasing concentrations of fluoride, carbonate, or sulfate whether nitrate was present or not. However, the peak potentials remained constant at -1.43 V vs. SCE in the presence of increasing amounts of phosphate in spite of the decreasing peak currents and regardless of the presence or absence of nitrate. The negative potential shifts in the presence of fluoANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
481
Table 1. Nitrate Recovery from Water Samples High in Sulfate after Sulfate Removal with Barium Chloride
Sample size, ml
Sample la
0.50
la
la
0.50 0.50
2a
0.50
2a 2a
0.50 0.50 15.0 15.0 15.0 15.0 15.0 15.0
606 606 606 618 618 618
No. of replicates 2 2 2 2 2 2 2 2 2 3 2 1
Mean total ppm NO:j--N found 35.6 63.0 91.5 40.5 67.5
95.4 0.558 0.708
0.911 0.107 0.284 0.860
ppm NO:j--N added
Mean ppm N03--N recovered
0.0
0.0
28.0
27.4
56.0 0.0 28.0 56.0 0.000
55.9 0.0
27.0
0.373 0.000
54.9 0.000 0.150 0.353 0.000
0.177 0.747
0.753
0.177
0.177
Range 1 .o 0.0 0.0 0.0 0.0 0.0 0.008 0.008
0.004 0.015 0.000
....
I
4
2.
--14.0 i
5
5 13.0 m
1
B '2.0
11.0
3 C'11crlde or Perchlorate Concentration
(MI
Figure 6. Effect of electrolyte concentration on peak currents of 2.0 X M ytterbium(lll), 2.0 X M nitrate
lov4
( 0 )"&I;
( 0 )CaCIp; (A)MgClp; (A)KCI; (0) "&I04
ride, carbonate, or sulfate indicated the formation of soluble complexes with ytterbium(II1). The phosphate interference was due to formation of an insoluble ytterbium phosphate precipitate. Because fluoride, carbonate, and sulfate interfered by complexing ytterbium(III), concentrations of ytterbium more than an order of magnitude greater than the optimum (1.4 X 10-5 M) were used in the analysis of samples likely to contain one or more of these interferences in order to minimize their effect. Phosphate was removed from samples having low sulfate concentrations by precipitation from basic solution with CaC12. Both phosphate and sulfate were removed from solutions containing interfering amounts of both ions by precipitation with BaClz in basic media. Studies done on synthetic samples containing large amounts of phosphate indicated that phosphate interference was eliminated by treatment with CaC12 and that no detectable nitrate was lost in the procedure. The results of nitrate recovery studies on real samples high in sulfate, which were analyzed by the polarographic method after treatment with BaC12, are listed in Table I. These results indicate that interferences were effectively removed and that nitrate losses by coprecipitation did not amount to more than a few percent of the total nitrate content in spite of the fact that large amounts of precipitate were formed. Effect of pH. The effect of the pH of the final solution was investigated by generating standard curves by addition of nitrate to blanks as described under the procedure section for soil analysis except that adjustments to various pH values other than 4.5 were made. The results indicated that the peak current was independent of pH in the range 4.02 to 5.52 for 482
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
VOLTS E. SCE
Figure 7. Differential pulse polarograms for solutions (2.5 X M 0.25 M BaCI2, pH 4.5 f O.l), to which a ytterbium(lll), 0.1 M "&I, series of concentration increments of nitrate (A), nitrite (E),or equal increments of nitrate and nitrite (C) have been added
all nitrate concentrations studied. Higher apparent peak currents were observed a t p H 3.50 due to overlap with a hydrogen ion reduction peak. Higher peak current values observed at pH 6.55 may be attributed to baseline changes due to negative shift of the supporting electrolyte reduction wave. Effect of High Electrolyte Concentrations. It was noted that larger current enhancements were observed in solutions containing only 0.1 M NH4C1 as supporting electrolyte in comparison with solutions containing 0.4 M CaC12 in addition to 0.1 M NH4C1, which resulted from the interference removal procedure previously described. The results of an investigation into the effect of high concentrations of NH4c1, CaC12, MgC12, KC1, and NH4C104 on the peak current enhancement observed for solutions containing 2.0 X M ytterbium and 2.0 X M nitrate (as described in the Procedure section) are plotted in Figure 6. The general decrease in peak current with increasing electrolyte concentration demonstrates the necessity to analyze all samples and standards a t the same overall electrolyte concentration. The electrolyte concentrations of most soil and water samples are low enough that this would not be a source of interference. However, high results would be obtained if soil extract aliquots smaller than 10.0 ml (see Soil Analysis Procedure) were taken for analysis and the differences between the volumes taken and 10.0 ml were not made up with 1 M CaCl2 solution. Analysis of Soil Samples. Soil samples were analyzed by both the polarographic and the specific ion methods as outlined in the Procedure section. The results are given in Table 11. Numbered samples were Bayard fine sandy loam soils from
Table 11. Comparison of t h e Polarographic and Ion-Selective Electrode Methods of Nitrate Determination in Soils" Polarographic method
Ion-selective electrode method
Sample size, Sample
Sample size, n
g
2.50 2.50 1.00 1.00
1
3 7 11
22 24 P G
X
0.13 0.02 14.4 53.5 96.4 134
1.00
0.50 0.50
175
16.8
1.00
n
S
g
0.2 0.2 0.4 2.0 3.6 3 (2)
2.50 2.50
0.0 0.0
1.00 1.00 1.00 0.50 1.00 1.00
16.2 54.9 95.4 139
(0.5)
X
S
181 18.0
n = number of replicates; x = mean ppm nitrate-nitrogen; s = standard deviation or (range).
Table 111. Comparison of the Polarographic, Ion-Selective Electrode, and Phenoldisulfonic Acid Methods of Nitrate Determination in Natural Watersa
Sample 1 2
3 4 5 6 606 618 a
Sample Size, ml 0.50 0.50 0.50
0.50 0.50
15.0 15.0 15.0
Polarographic method n
X
39.8 43.5 75.0 66.8 48.0 2.08 0.558 0.107
S
Sample Size, ml
0.8
1.00
1.5 1.5 0.7 1.5 0.03
1.00 1.00
1.00 1.00
20.0
Ion-selective electrode method n 2 2 2
2 2 2
X
S
45.4 50.1 81.9 68.1 50.3 2.00
2.2
0.3 0.0 1.5 0.1
0.17
Phenoldisulfonic acid method X
30.3 37.0 59.6 53.5 34.3 1.94
0.008
0.00
0.015
0.05
n = number of replicates; x = mean ppm nitrate-nitrogen;s = range
Wyoming and lettered samples were Barnes silt loam soils from Minnesota. The results averaged 5% lower for the polarographic method as compared to the specific ion method. Standard deviations of the polarographic method ranged from 0.2 ppm nitrate nitrogen for soil samples containing less than 1 ppm nitrate nitrogen to 3 ppm nitrate nitrogen for soil samples containing more than 100 ppm nitrate nitrogen. The consistent difference between the two methods was probably due to a combination of negative error due to nitrate coprecipitation with phosphate in the polarographic method and positive error due to chloride and/or bicarbonate interference in the specific ion method. Analysis of Water Samples. Natural water samples which had been analyzed for nitrate by the phenoldisulfonic acid method were obtained from another laboratory. Nitrate determinations were performed on these samples by both the polarographic and the specific ion methods as described in the procedure section. The curve defined by the closed circles in Figure 8 is a typical standard curve for the polarographic determination of nitrate in water. I t should be noted that this curve is nonlinear with a definite inflection point. This inflection point was observed in all nitrate calibration curves obtained in this work. The nitrite curve in Figure 3 has a similar inflection point. The results of the analyses are listed in Table 111. The polarographic results averaged 7% lower than the specific ion results. The consistent difference in results was probably due to coprecipitation of nitrate with sulfate (The samples were high in sulfate and large amounts of B a s 0 4 precipitated.), and possibly to chloride and/or bicarbonate interference in the specific ion method. The results of the phenoldisulfonic acid method averaged lower than the po-
a0
N i t r a t e o r N i t r i t e C0ncentra:lon in Final S o l u t i o n X105) 2 0 . 4.0 61) 8.0 10.0 121) 14.3 16.0
n 15 ml W a t e r
Figure 8. Plot of peak current vs. nitrate or nitrite concentration ( 0 )nitrate (from Figure 7A); (A)nitrite (from Figure 78); (0, A ) nitrate, nitrite (from Figure 7C)
larographic method by 34%, probably because of nitrate losses in the phenoldisulfonic acid method due to chloride interference. The relative range for the polarographic method varied from 2% to 10%for samples containing 75 to 0.1 ppm nitrate nitrogen, respectively. Simultaneous Determination of N i t r a t e a n d Nitrite. Differential pulse polarographic curves for various nitrate and/or nitrite concentrations in solutions, which were 2.5 X M ytterbium, 0.1 M "&I, and 0.25 M BaClz of p H 4.5 f 0.1, prepared as described in the procedure section for ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
483
generation of standard curves for water analysis, are shown in Figure 7. Curves in Figures 7A and B, in order of increasing peak height, correspond, respectively, to nitrate and nitrite M from concentrations increasing in increments of 2.0 X zero to 1.6 X M. Curves in Figure 7C, in order of increasing peak height, correspond to equal nitrate and nitrite M from concentrations increasing in increments of 2.0 X M. Peak potentials for nitrite shift from zero to 1.4 X -1.33 to -1.37 V vs. SCE for the nitrite concentration range 2.0 X M to 1.6 X M. The peak potential for nitrate over the same range of concentrations was constant a t -1.43 V vs. SCE. Note the different behavior of nitrite in more concentrated ytterbium solutions as compared to its behavior in 1.4 X 10-5 M ytterbium solutions (see section on Determination of Optimum Ytterbium Concentration), where the peak potentials for nitrite and nitrate were identical. Plots of peak currents vs. nitrate or nitrite concentrations taken from the curves in Figure 7 are shown in Figure 8. The peak currents for nitrate taken from Figures 7A and C were measured from the extrapolated baselines on the positive sides of the peaks. The peak currents for nitrite taken from Figure 7B were measured from the curve corresponding to zero nitrite concentration. The peak currents for nitrite taken from Figure 7C were corrected for overlap with the nitrate peak. The curves in Figure 8 indicated that nitrite interfered very little with the determination of nitrate even when the two were present in equal concentrations. The results further showed that estimates of nitrite content were possible in the range of 0 to 3 ppm nitrite nitrogen, the estimates being low by up to 25% for solutions containing an equivalent concentration of nitrate. Decreasing error with increasing nitritehitrate mole ratio would be expected. Standard addition techniques would be useful for the determination of nitrite from 0 to 5 ppm nitrite nitrogen since the peak current vs. nitrite concentration plot was linear over this range of concentrations and had an intercept at the origin. Standard addition techniques were not useful for polarographic nitrate determination since the standard curves were nonlinear and did not intersect the origin, with the "unenhanced" ytterbium peak current also subject to matrix effects.
polarographic and specific ion methods was observed. The detection limit for nitrate or nitrite by the polarographic method under optimum conditions was 14 ppb nitrate or nitrite nitrogen in water. Simultaneous determination of nitrate and nitrite was possible under certain conditions. Analysis time was approximately 10 min per sample. An investigation into the nature of the reaction leading to the ytterbium peak current enhancement by either nitrate or nitrite is presently being conducted in this laboratory.
ACKNOWLEDGMENT Grateful acknowledgment is made of the receipt of water samples from the Wyoming Department of Agriculture, Division of Laboratories, and the receipt of soil samples from the Plant Science Division of the College of Agriculture of The University of Wyoming, and Leo Boese, Swift County, Minnesota.
LITERATURE CITED H.0. Buckman and N. C. Brady, "The Nature and Properties of Soils", 7th
(14) (15) (16) (17) (18) (19)
CONCLUSION A differential pulse polarographic method for the determination of nitrate has been described which is applicable to the analysis of soils with nitrate nitrogen levels greater than 0.2 ppm and natural water samples with nitrate nitrogen levels greater than 0.1 ppm. Relative standard deviations less than 4% may be routinely achieved. Good agreement between the
484
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RECEIVEDfor review August 9,1976. Accepted November 19, 1976. Some preliminary investigations on the ytterbiumnitrate system were performed a t the Ames Laboratory, USAEC, a t Iowa State University.
