PYROLYTIC
SULFATE
PERlMlDYL AMMONIUM BROMIDE (PDA)
these instruments, a higher degree of sophistication in the flow system, would be required. The appeal of this method lies in its simplicity and flexibility. Although the chemistry involved is somewhat sophisticated, the equipment is not. A Variac and two Lindberg heating cylinders would work quite well as a simple constant temperature oven. By using the West-Gaeke procedure to determine the sulfur dioxide evolved in the pyrolysis, the only instrument required was a typical laboratory spectrophotometer. The use of the thermal degradation of (PDA)zS04 generally simplifies the sulfuric acid determination problem. Figure 6 depicts the chemical basis for the precipitation and thermal degradation of sulfate.
METHOD
(PDA),
SO4
0
Lo-N H ;
NH+ 6 I 3 -I/
I
SO2
Figure 6.
+
ORGANIC D E B R I S
ACKNOWLEDGMENT
Precipitation and thermal degradation of sulfate
the expected sulfur dioxide as (PDA12S04. In all cases from 1 to 50 pg, the sulfate, as (PDA)zS04, was converted to sulfur dioxide with 100% efficiency. Both the Bendix and the Meloy Total Sulfur Analyzers were used in an attempt to provide real time analysis and increase the sensitivity. The Meloy SA-120 was more flexible than the Bendix 8300, since the range of the detector could be chosen by selecting the proper amplifier, so the output of the instruments was linear over the entire range of 0.01 to 1 pg of sulfur dioxide. Figure 4 shows the flow system for pyrolysis when a Total Sulfur Analyzer was used as the detector. Surprisingly, the precision varied from f 6 to f 1 0 % at 0.5 gg of sulfate. The organic debris which formed tended to clog the Teflon filter and vary the flow into the hydrogen flame. The nitrogen and oxygen rotameters were not able to maintain the flow rate any better than 43%. As a result of these fluctuations, the precision of Total Sulfur Analyzers was lower than that of the WestGaeke procedure. To take advantage of the flexibility of
The suggestions and interest of Sham Sachdev of KemTech Laboratories, Baton Rouge, La., are greatly acknowledged.
LITERATURE CITED (1) J . Heslinga, Chem. Weekbl., 22, 98 (1925). (2) W. Grote and H. Krekeler, Angew. Chem., 216; 106 (1938). (3) 0. E. Sundberg and G. L. Royer, lnd. Eng. Chem., Anal. Ed., 18, 719 (1946). (4) E. D. Peters, G. C. Rounds, and E . J. Agazzl. Anal. Chem., 24, 710 (1952). (5) W. Kristen, Anal. Chem., 25, 74 (1953). (6) D. B. Hagerman and R. A. Faust, Anal. Chem., 27, 1970 (1955). (7) R. P. Larsen, L. E. Ross, and N . M. Ingber, Anal. Chem., 31, 1596 (1959). (8) P. W. West and G. C. Gaeke, Anal. Chem., 28, 1816 (1956). (9) F. P.Scaringelli and K. A. Rehme, Anal. Chem., 41, 707 (1969). (10) W. I . Stephen, Anal. Chim. Acta, 50, 413 (1970). (1 1) G. L. McClure, Anal. Chim. Acta, 64, 289 (1973).
RECEIVEDfor review July 31, 1974. Accepted October 23, 1974. This research was supported by the National Science Foundation Grant No. GP-18081 and RANN (NSF) Grant No. GI-35114x1.
NOTES
Voltammetric Determination of Trace Quantities of Nitrate in an Anion Exchange Membrane Isolated Cell G. L. Lundquist, G. Washinger, and J. A. Cox’ Department of Chemistry and Biochemistry, Southern Illinois University at Carbondale, Carbondale, Ill. 6290 7
Voltammetric methods have been used for the determination of nitrate by reduction at a rotating cadmium electrode ( I ) , by dc polarography in the presence of multivalent cations (see citations 1-13 in Reference 1 ), and by polarographic reduction of 4-nitro-2,6-xylenol formed by reaction of nitrate with 2,6-xylenol ( 2 ) .Only the latter two have presently been applied to trace level determinations. The polarographic methods have significant limitations for field investigations. In addition to the inherent difficulty of field use of a dropping mercury electrode, the direct polarographic methods generally have nonlinear working curves, and the detection limits (0.1-1.0 ppm as nitrate) are insufficient for many studies. The 2,6-xylenol polarographic method yields linear working curves and has a 0.4A u t h o r t o w h o m correspondence should be addressed
ppm (as nitrate) detection limit; however, the necessities of chloride removal from the samples and of separation of polarographically active cations make the method somewhat inconvenient. Spectrophotometric procedures using brucine or phenoldisulfonic acid are generally accepted methods for nitrate analysis (3, 41, but they have limited utility for field studies. The former is highly toxic and may yield nonlinear working curves, whereas the latter is subject to chloride interference and requires extensive sample pretreatment. The present method is based upon the reduction of nitrate by linear potential scan voltammetry at a stationary mercury drop electrode in a small volume electrolysis cell. The cell is isolated from the test solution by an anion exchange membrane. The membrane serves two basic functions. Preconcentration of the nitrate (and other anions) in
A N A L Y T I C A L CHEMISTRY, VOL. 4 7 , NO. 2 , FEBRUARY 1975
319
-1
3
"0
I .I
6
-1
7
-1
9
20
potep,'.ial, v a i s
Figure 1. Reduction of nitrate by linear potential scan voltammetry ( A ) 4.76 X lO-'M Nos-, 0.1M KCI, 0.01M La&; (6)0.10M LaCI,: v, 10 rnV/sec: electrode area, 0.034 cm2
KCI, 0.01M
5G
IC2
:No,] x1c;
Flgur 2. Relative sensitivities of dc polarography and linear potential scan voltammetry for nitrate determination ( A ) linear
the electrolysis chamber is accomplished by the mechanism described by Blaedel and coworkers (5, 6 ) , and the membrane prevents net loss of the supporting electrolyte from the electrolysis chamber. Hence, a dip-type system may be designed for direct voltammetric measurements in low ionic strength media. The membrane will also tend to exclude high molecular weight surfactants ( 7 ) which have been a general problem in the application of voltammetric methods to natural samples.
EXPERIMENTAL The instrumentation consisted of a conventional three-electrode solid state potentiostat, a Wavetek Model 114 function generator, and a Hewlett-Packard 7004B x-y recorder. A Metrohm microburet hanging mercury drop electrode was used for the linear potential scan voltammetry (LSV) experiments. The mercury surface area was 0.034 cm2. All potentials are reported us. a saturated calomel electrode (SCE). All chemicals used were ACS Reagent Grade. The KC1 and Lac13 were twice recrystallized from deionized water. The water was purified with a Millipore Super Q system. The anion exchange membranes were type P-1025 (Teflon pyridinium methyl iodide composition) obtained from RAI Research Corporation, Hauppauge, L.I., New York. The pretreatment procedure was that described by Blaedel and Kissel (6) except that the membranes were stored in a 0.1M KC1,O.OlM Lac13 mixture prior to use. The membrane-isolated cell consisted of an open 1-cm diameter soft glass tube. An anion exchange membrane was attached to the base with Teflon tape and O-rings. Side-arms allowed entry of a Luggin capillary reference probe and a platinum counter electrode. The deaerating tube and indicator electrode entered through a cell cover. The electrolytic cell capacity was 2 ml. In experiments utilizing the above cell, 2 ml of the 0.1M KC1, 0.01M Lac13 supporting electrolyte were pipetted into the assembly. The preconcentration step was initiated by dipping the electrolytic assembly.into the sample which was agitated-by a magnetic stirrer (exact control of the rate of stirring and cell position is not important). After a prescribed time, the electrolytic cell was removed from the sample and deaerated for 10 minutes. The nitrate concentration of the original sample was determined by LSV. The peak current, i,, was correlated to a working curve prepared with standard solutions which had been preyoncentrated for the prescribed time. Unless otherwise noted, the scan rate, u, was 10 mVlsec. The electrolyte p H was 6.0 and did not change during the preconcentration step. Experiments without preconcentration were performed in a Metrohm polarographic cell.
RESULTS AND DISCUSSION Preliminary experiments were performed without preconcentration. Figure 1 shows a typical current-voltage curve for the reduction of NO3- by LSV a t a stationary mercury drop electrode. As the scan rate, u, increases from 4.9 X t o 1.0 X 10-1 V/sec, the function ipu-1/2 de320
scan voltammetry;( B )dc polarography: 0.1 M KCI, 0.01M LaC13; v, 10 rnV/sec; electrode area, 0.034 crn2
creases from 54 ,uA(V/sec)-'/2 to a limiting value of 30 ,uA(V/sec)-*/2; hence, the process is not diffusion-controlled. Nevertheless, working curves for the method are linear over two orders-of-magnitude of nitrate concentration and intercept the origin. For example, when u is 10 mV/sec, i, is 0.10 and 10.0 ,uA for solutions containing 1.0 x and 1.0 X lO-*M NO3-, respectively. A comparison of the working curves obtained by LSV and dc polarography (Figure 2) demonstrates that the former method is also more sensitive. A further increase in sensitivity can be attained a t higher u; however, a t NO3concentrations below 10-5M, the signal-to-background ratio is a maximum with u of 10 mV/sec. Studies with the anion exchange membrane isolated electrochemical cell were restricted to the LSV method. For such a cell to be analytically useful, certain basic requirements must be met. The rate of mass transfer of the test anion into the electrolytic chamber must be fast in order to design a reasonably rapid procedure; the rate must be independent of the ionic strength of the sample over a wide range; and, a t a given time, the concentration of the test ion in the electrolytic chamber must be proportional to the original concentration in the sample. The alternative to the latter requirement is to employ total transfer of the test anion from the sample into the electrolytic chamber which would result in a restrictively long analytical procedure. Figure 3 shows the concentration of Nos- in the electrolytic cell, determined by LSV, as a function of contact time with 50- and 250-ml samples of 1.0 X 10-6M NO3-. The cell concentration increases linearly with time until nearly complete removal of Nos- from the test solution. At given preconcentration times up to 2 hours with sample volumes equal to or greater than 50 ml, the electrolytic cell concentration of N03- is independent of sample volume. The plots in Figure 3 reach limiting values corresponding to greater than 98%transfer of NO3- from the sample into the electrolytic chamber after 6 and 9 hours for the 50- and 250-ml samples, respectively. Working curves prepared a t preconcentration times of 15 and 60 min with a 50-ml sample are shown in Figure 4.The 15-min preconcentration resulted in the same sensitivity as direct LSV (compare Figures 4B and 2A). The reported data are corrected for blank currents. For the direct determination and the 15-min preconcentration, typical blank currents are 0.1 f 0.01 PA; with 1- and 5-hr preconcentra-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 2, FEBRUARY 1975
53
go $3 0 L
L
623 10 2
3
4 13
me ilou-s
Figure 3. Effect of time and sample volume on the preconcentration of nitrate into the supporting electrolyte. ( A ) 250-ml voiume; ( B ) 50-ml volume, 0 10M KCi, 0 01 M Lac13 electrolyte; nitrate concentrations determined by LSV
tions, the blank currents are 0.25 f 0.02 and 0.35 f 0.03 respectively. The minimum NO3- concentrations determined were those which resulted in corrected peak currents equal to the mean blank currents. With l- and 5-hr preconcentrations, the corresponding values were 5 X 10-'M and 1 X 10-;M Nos-, respectively. The precision of the method was studied by separately determining the reproducibility of i, with and without preconcentration. The results of the study are shown in Table I. The relative standard deviation is greater in the case of preconcentration but does not exceed 5%. The effect of sample ionic strength on the rate of transfer of NO3- through the membrane was studied by adding various concentrations of KCl to the samples and, after a certain preconcentration time, comparing i ,,to the data in Figure 4. With KCI concentrations below 1 X 10-4M, i, was 4.45 FA after a 60-min preconcentration of a 6.0 X and 2 X 10-6M NOt3- sample; however, with 2 X 10+M KC1, i decreased to 4.00 and 2.21 pA, respectively. Even a t the latter ionic strength, the working curves were linear with a zero intercept. The standard addition method can, therefore, be used for nitrate determinations in moderate ionic strength solutions with the preconcentration cell. With high ionic strength samples, preconcentration would, of course, not occur. As previously reported ( 8 ) , sulfate interferes with the polarographic determination of Nos- in the presence of La(II1). Similarly, sulfate interferes with the LSV method. Without preconcentration, the peak current of a sample containing 2.8 X 10-4M Sod2- and 4.8 X 10-jM N03- is 3.5 FA, a 22% decrease from the expected value. The use of the anion exchange membrane isolated electrolytic cell has little effect on the interference since all anions are preconcentrated. With a 60-min preconcentration of a 6.0 X 10-6M NO:j-, 3.2 X 10-5M S042-solution, a 17% decrease in i, is observed. Likewise, the interference of nitrite, which is reduced at the same potential as nitrate, is not eliminated with the present method. The presence of dissolved oxygen causes a decrease in i , possibly because the OH- produced by the electrochemical reduction increases the pH a t the electrode surface (independent experiments demonstrate that i decreases as the pH increases above 7). Attempts to perform the experiments in citrate and acetate buffer to test the hypothesis failed, as reduction of NO3- prior to breakdown of the supporting electrolyte was not observed in these buffers. The product of the NO3- reduction can act as an interference unless a new mercury drop is employed for each pA,
,
,
3 0
[zc 1 X ' Z "
'So
Figure 4. Effect of preconcentratlon time on the sensitivity of nitrate determination ( A ) 60-min preconcentration; ( s ) 15-min preconcentration; O.lOMKCI, 0.01 M La&; v, 10 mV/sec; electrode area, 0.034 om2
Table I. Reproducibility of Nitrate Determinations by LSV Sample
Tech-
PJ03-1
nique a
ipb
9.1 x 10-6 NP 0.90 i 0.01 6.0 x P 4.45 i- 0.14 1.8 x io-fi NP 0.20 i 0.01 1.0 x 10-R P 0.50 i 0.02 a NP, no preconcentration; P, 60-min preconcentration. * Standard deviations based upon seven trials.
trial or the electrolytic solution is vigorously stirred between scans. Accumulation of the product on the electrode passivates the mercury against further nitrate reduction. The use of the membrane eliminates two types of interference noted in direct polarographic determinations of nitrate. Reducible metal ions such as Cd(II), Pb(II), and Zn(I1) lower the dc polarographic limiting current. For example, 5 X 10-jM Zn(I1) lowers the limiting current for 2.0 X 10-4M NO3- from 5.2 to 4.7 PA. Isolation by the anion exchange membrane excludes the Zn(I1) from the electrolytic chamber and, hence, eliminates the interference. Strong surfactants are a major potential interference for voltammetric methods applied to natural samples. For example, without membrane isolation, the presence of 0.001% gelatin causes i, to be decreased to 2.70 p A from the expected value of a 4.45 p A for a 4.8 X 10+~MNO3- solution. When the membrane-isolated cell was used, the addition of gelatin to the sample did not alter i from expected values after preconcentration. The above results demonstrate that linear scan voltammetry in the anion exchange membrane isolated cell is a suitable method for trace nitrate determinations in samples of low-to-moderate ionic strength. That surfactants, chloride, and cations do not interfere, makes the method potentially attractive for studies of natural samples.
,
LITERATURE C I T E D (1) D. C. Johnson and R. J. Davenport, Anal. Chem., 45, 1979 (1973). (2) A. M. Hartley and D. J. Curran. Anal. Chem., 35, 686 (1963). (3) "Annual Book of ASTM Standards," (Part 23), American Society for Testing and Materials. Philadelphia, Pa., 1973, p 363. (4) "Standard Methods for the Examination of Water and Wastewater." 13th ed., American Public Health Association, Inc., Washington, D.C., 1971, p 234. (5) W. J. Blaedel and T. J. Haupert. Anal. Chem., 38, 1305 (1966).
A N A L Y T I C A L CHEMISTRY, VOL. 47, NO. 2, FEBRUARY 1975
321
W. J. Blaedel and T. R. Kissel, Anal. Chem., 44, 2109 (1972). H. B. Mark, Jr., U. Eisner, J. M. Rottschafer. F. J. Berlandi, and J. s. Mattson, Environ. Sci. Techno/., 3, 165 (1969). I. M. Kolthoff, W. E. Harris, and G. Matsuyama, J. Arner. Chem. Soc.. 66, 1782 (1944).
RECEIVEDfor review July 26, 1974. Accepted September 25, 1974. This research was supported by the Water Resources Center, University of Illinois, Grant S-033-ILL.
Determination of the Sodium Salt of 2-Mercaptopyridine-NOxide by Differential Pulse Cathodic Stripping Voltammetry 0. A. Csejka, S. T. Nakos, and E. W. Ou6ord Chemicals Division, Olin Corporation, New Haven, Conn. 06504
Compared to anodic stripping voltammetry, the technique of cathodic stripping has been used rather sparingly. Its main application has been the analysis of halogens, although a variety of other inorganic anions which form insoluble mercury compounds have also been determined. Most of this work has been summarized in several reviews (1-3). Cathodic stripping is not limited solely to the determination of inorganic compounds. For example, the analysis of several mercaptans have been described ( 4 , 5 ) . By conventional cathodic stripping, limits of detection are quite low, usually in the order of 10-6 to 10-8 M , depending on the solution matrix. However, with the advent of differential pulse stripping voltammetry (6-IO), general increases in sensitivity, shorter deposition times, and better peak resolution have been realized. All of the pulse work to date has been applied to the determination of metal ions by anodic stripping, except for the cathodic stripping of selenium (11). In this work, differential pulse cathodic stripping voltammetry with a hanging mercury drop electrode was used to determine the 2-mercaptopyriM . Potentially, this techdine-N-oxide ion to 8 X nique can also be used to determine the concentrations of other mercaptans several orders of magnitude lower than previously obtained.
EXPERIMENTAL Apparatus. All experiments were performed using a Princeton Applied Research, Model 174, Polarographic Analyzer with a Metrohm E-410 hanging mercury drop electrode (HMDE) assembly. Voltammograms were recorded with a Houston Instrument Model 2000 X-Y recorder on a sensitivity of 1.0 V/in. for the Y axis and 0.1 V/in. for the X axis. The cell, maintained at 25 f 0.1 "C, contained a saturated calomel reference electrode isolated from the sample solution by a saturated K N 0 3 salt bridge, a platinum wire auxiliary electrode, and a gas purging assembly for deoxygenation and stirring. Reagents. The sodium salt of 2-mercaptopyridine-N-oxide was prepared and purified by the Biocides Group, Chemicals Division, Olin Corporation. It exceeded 99.5% purity as determined by an iodimetric assay procedure. Trace electroactive impurities, as the disulfide, were determined polarographically. Mass Spectrometry aided in the detection of other trace materials. Triply distilled mercury and deionized water were used throughout this study. All other chemicals were reagent grade and used without further purification. Procedure. Several stock solutions were prepared by weighing and diluting the sodium salt of 2-mercaptopyridine-N-oxide to a concentration of about 1 Mg per ml using deionized water. Precautions were taken to avoid exposure to light because of the instability of these solutions. An aliquot of a stock solution was transferred uia a microliter pipet to 10.00 ml of deionized water containing 0.002% Triton X-100, 35 ppm NzH4, and 0.05% disodium ethylenediaminetetraacetate (EDTA). The sample was then deoxygenated with nitrogen (oxygen-free) for 10 minutes a t 100 cmJ/min. 322
ANALYTICAL CHEMISTRY, VOL. 47, N O . 2 , FEBRUARY
The PAR 174 Polarograph was adjusted for 50-mV pulse amplitude, an initial deposition potential of - 0.10 V, a pulse repetition time of 1 sec, and a scan rate of 2 mV/sec. A mercury drop having a surface area of 2.9 mm2 was used throughout the study. The drop was obtained by dialing six divisions of the E-410 HMDE assembly. All glassware and cell parts were soaked in 1:l HN03-HZO prior to analysis. With nitrogen bubbling through the solution a t 100 f 5 cm3/min as the only means of stirring, the deposition was begun. After exactly 2 minutes, the bubbling was stopped and the nitrogen flow was directed over the solution. Precisely 1 minute later the scan was initiated. The peak potential ( E), for the 2-mercaptopyridineN-oxide occurred at -0.24 V us. SCE. After the initial scan, a 10-pl to 50-p1 spike of a 1 pg/ml stock solution was added and the solution was thoroughly mixed by nitrogen bubbling. Another voltammogram was obtained after 2-min stirring and 1-min quiescent deposition. This procedure was repeated for not more than three additions. The peak current, i,, was determined by subtracting the blank current from each scan. Suitable blank scans were obtained as above using solutions which did not contain the mercaptan. These blank scans had currents lower than 0.01 pA in the vicinity of the Z-mercaptopyridineN- oxide peak. Higher currents indicated contaminants which were detrimental to the analyses, and were usually removed by further cleaning of the glassware or cell.
RESULTS AND DISCUSSION Using this technique, a fifty- to one-hundredfold increase in sensitivity is obtained over direct current cathodic stripping voltammetry. This is due to the elimination of capacitance currents by the pulse sampling technique of the polarographic analyzer and to the large modulation amplitude (50 mV) which causes large current sampling differences in the vicinity of the oxidation potential of the compound of interest. The reaction occurring a t the HMDE during the deposition is RS-
+
Hg
-
RSHg
+
e-
(1)
Krivis and Gazda ( 1 2 ) investigated this reaction and found that the one-electron reaction was consistent with a logarithmic analysis of the current-voltage curve obtained from dc polarographic measurements. Further support for the one-electron transfer was obtained from controlled-potential electrolysis experiments. During stripping, the reverse of reaction 1 is true. This was shown by repetitive analyses of various solutions according to the procedure described in the previous section. A solution was analyzed and its peak current measured. The mercury drop was then removed and a new drop formed. The solution was stirred to ensure homogeneity and the analysis was repeated. This procedure was performed as many as four times on the same solution with the resulting peak currents being equal (within 1975
