I
PG
IN LARD
1
Figure 4. Addition of PG to lard sample A. 3.60 g lard per 100 ml solution. Solvent: 0.12M sulfuric acid in 1 : 1 ethanol-benzene. B, C , D, and E. Solution A plus 1.0 X lO-jM, 3.0 X lO-jM, 5.0 X 10-jM and 7.0 X lO+M PG, respectively. Scan rate: 0.040 Vlsec BHT (E, = 1.03 V) may also be determined. In fact, preliminary experiments indicate that mixtures of PG, BHA, and BHT in lard may be determined from a single voltammogram. The principal advantage of the proposed method is the rapidity of analysis. The entire operation from sample weighing to the acquisition of the voltammogram requires only about ten minutes. The speed of analysis is aided by
the fact that no prior treatment or separation procedures need be applied to the sample. Furthermore, as dissolved oxygen does not interfere with the voltammetry, no solution deaeration is required. A disadvantage is that the electrode pretreatment procedure must be applied before every voltammogram in order to ensure reproducibility. In addition, the method is of only moderate sensitivity. Typical concentrations of naturally occurring tocopherols or synthetic antioxidants are readily determined but traces (sub ppm) cannot be detected. It may be possible to improve the sensitivity by utilizing a pulse voltammetric method. No attempt was made in this work to minimize sample size. For example, the tocopherol analyses were performed with 10 grams of oil made up to 100 ml. Sample solution volumes may readily be reduced 100-fold by redesigning the electrochemical cell for smaller volumes. The method should find application whenever a rapid analysis of tocopherols or synthetic antioxidants is required. For example, it could be used to monitor tocopherol content at various stages of the refining process. A survey of oil samples for antioxidant content could readily be accomplished or the method could be applied to monitor changes in antioxidant content during manufacture or storage. With modification in sample preparation, the method may be applicable to the determination of antioxidants in other food products such as cereals or processed meat products. RECEIVED for review July 20, 1972. Accepted October 13, 1972. This research was supported by the National Science Foundation through Grant No. GP-19579.
Determination of Nitrogen Oxides in Ambient Air Using a Coated-Wire Nitrate Ion Selective Electrode Barbara M. Kneebone and Henry Freiser University of Arizona, Tucson, Ariz. 85721 A method has been developed for the determination of NO, in ambient air utilizing a coated-wire nitrate ion selective electrode. Air is collected in gas-washing bottles containing 2% H202,the solution is treated with M n 0 2to destroy the remaining peroxide, and the nitrate concentration is measured potentiometrically. The optimum conditions for preparation and use of the electrode were determined. The effect of interfering anions was studied. The method can be used in the presence of at least a 40-fold excess of SO2 or SOs. The results obtained with this method compared well (1-4% relative error) with accepted spectrophotometric methods.
DETERMINATION OF NO, in the atmosphere by a simple potentiometric method would be a highly advantageous alternative to the presently available spectrophotometric methods. The phenoldisulfonic acid ( I ) (PDS) and the xylenol (2) methods are both based on the formation of nitric acid and
its subsequent reaction with the reagent to form a colored product. These methods involve time-consuming sample handling with the attendant dangers of contamination and loss. The PDS procedure takes about three hours to complete, while the xylenol method takes about an hour. Recently the Orion Research nitrate electrode was applied to NO, determination by DiMartini (3). The analytical technique he employed involves gas phase oxidation of NO or NOs by ozone, followed by absorption and hydrolysis of N205. Quantitative determination of the nitrogen oxides as nitrate ion is then performed with the electrode. According to the data reported, a serious problem exists with the efficiency for 5.7 to 0.062 ppm NO,. Driscoll et al. ( 4 ) also used the Orion electrode to determine NO, in combustion effluents by modifying the phenoldisulfonic acid method absorbing solution and measuring the nitrate produced. This method was shown to work well in the case of NO, in combustion effluents where concentra-
(1) American Society for Testing and Materials, Philadelphia, Pa.,
Method D 1608-58T. (2) M. B. Jacobs, “The Chemical Analysis of Air Pollutants,” Interscience, New York, N. Y . , 1960.
(3) R. DiMartini, ANAL.CHEM., 42, 1102 (1970). (4) J. N. Driscoll et al., J . Air Pollut. Confr. Ass., 22, 119 (1972). ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
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Table I. Effect of Interfering Anions on Aliquat-N03Electrode ResponsebPotentials Reference Coated soh- Test wire, Concn, Ionic tiow, soh, (Av) Orion, Anion M strength mV mV logK log K c10.04 0.045 144 142 0.09 0.095 136 -1.4 -2.2 0.12 0.125 131 NO*0.04 0.045 139 122 0.09 0.095 110 -0.8 -1.2 0.12 0.125 105 C1030.04 0.045 124 58 0.09 0.095 49 0.26 0.3 0.12 0.125 44 Sod*0.04 0.125 134 139 0.09 0.275 140 -3.1 -3.2 0.12 0.365 141 Reference solution, 5 X 10-3M KN03. * Electrode response in series of pure test solutions-54 mV/10fold change in concn. Table 11. Results of Xylenol Method, Average of Ten Determinations Absorbance [NOa-] X lo6 0.0 0.036 0.044 3.2 0.056 6.4 16.0 0.088 0.142 32.0 0.250 64.0 Slope 0.011+0.000 Intercept 0.034 rt 0.000 Correlation coefficient 1 .Ooo
tions are in the range of hundreds ppm. The absorber would have to be modified further for measurement of NO, in the part per hundred million range due to interference from sulfate. This study deals with the application of the coated-wire nitrate selective electrode to NO, determination in ambient air (5). This electrode was developed on the basis of previous studies in the Laboratory on this type of electrode (6-8). The advantages of using this electrode instead of the Orion liquid membrane electrode are ease of construction, increased portability, elimination of an internal reference solution, sturdiness, and economy. EXPERIMENTAL
Apparatus and Measurements. All potentiometric measurements were made with an Orion research model 701/ digital pH meter using a Beckman Fiber Junction calomel electrode as reference electrode. The gas dilution system used was an AID calibration system Model 309 comprising a constant temperature control oven and a diluent and chamber flow controller. AID (Analytical Instrument Development Co., Inc.) Permeation Tubes 3209 and 3393 containing NOz ( 5 ) B. Kneebone, M.S. Thesis, The University of Arizona, 1972. (6) R . W. Cattrall and H. Freiser, ANAL.CHEM., 43, 1905 (1971). (7) G. Carmack, R. Cattrall, H. Freiser, H. James, and B. Kneebone, U. S . Patent applied for January 19, 1972. (8) H. James, G. Carmack, and H. Freiser, ANAL.CHEM., 44, 856
(1972). 450
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were used in the system. A Gilford 2400 spectrophotometer was used for all absorbance measurements. Materials. ACS Reagent grade chemicals were used except as noted. Aliquat 336s (methyl tricaprylyl ammonium chloride) was obtained from the Chemical Division of General Mills. Poly(methy1 methacrylate) was from Monsanto Co. Decyl alcohol, melting point 5.5-6.5 OC, was from Eastman Kodak. Methyl acetate was Baker grade. Hydrogen peroxide (30%) was obtained from Mallinckrodt. 2,4Xylen-1-01 was from K & K Laboratories. Toluene was spectrophotometric grade from Matheson, Coleman and Bell. Aliquat-N03- Solution. A 15-1-111 sample of Aliquat 336s was dissolved in approximately L ml of decanol and this solution was shaken with six 10-ml aliquots of 1.OM KNO, to effect the exchange of NO3- for Cl-. Shaking time on a wrist-action shaker was approximately 10 minutes for each aliquot. After each shaking, the aqueous phase was separated and discarded. The final aqueous phase was tested for the presence of C1- with AgN0,. The absence of C1indicated complete exchange. The organic phase was centrifuged to remove traces of water. Construction of Coated-Wire Aliquat-Nitrate Electrode. The nitrate electrode used in this work was a coated-wire adaptation of the Aliquat-NO,- liquid membrane system investigated by Coetzee (9) and Matsui (10). A solution of poly(methy1 methacrylate) was prepared by dissolving about 0.5 gram of Plexiglas (Rohm & Haas) shavings in a minimum amount of methyl acetate. Four milliliters of this plastic solution was mixed with 1 ml of the Aliquat-NOs- solution. A platinum wire approximately 1 mm in diameter whose tip had been melted in an oxygen-gas flame to form a spherical button was soldered to a length of RG-58 coaxial cable. The wire was then dipped in the Aliquat-NO,--plastic mixture several times to coat it uniformly and allowed to dry for approximately 30 minutes. The sensitive electrode tip was immersed in Aliquat-N03solution and allowed to stand in it overnight. Prior to use, the exposed portion of the wire was wrapped tightly with Parafilm (American Can Co.). Testing of the Electrodes. A series of solutions of K N 0 3 in the range 10-l to 10-jM was prepared. The electrode tip was rinsed with deionized water, then immersed in one of the standard solutions. When equilibrium was established, as evidenced by a stable reading (i0.2 mV), the potential of the test cell was measured. Electrodes were evaluated by measuring response as a logarithmic function of the nitrate ion activity and comparing this with ideal, i.e., Nernstian behavior. In most cases with the coated wire electrode there was a 55-mV change for every decade change in activity, which is reasonably close to the 59.2-mV change expected for ideal behavior. The relationship was linear to 10-4M NOs-, although the useful range could be extended to 10-jM. In this region the slope was approximately 25-30. The correlation coefficient of the line obtained in each case varied between 0.995 and 0.999. The response time of the electrodes was rapid. Between 30 and 60 seconds was required for the system to come to equilibrium. The readings could be reproduced during a test run to better than i1 mV, but the absolute potential readings varied from day to day anywhere from 5 to 15 mV. For reliable measurements with these electrodes, it is necessary to restandardize them for each test run and desirable to bracket the sample solution with standard reference samples. The thickness of the coating did not affect the performance of the electrode. RESULTS AND DISCUSSION Interference Studies. The effect of foreign ions on the response of the electrodes was studied by making potenti-
(9) C. J. Coetzee and H. Freiser, ANAL.CHEM., 41, 1128 (1969). (10) M. Matsui and H. Freiser, Anal. Lett., 3, 161 (1970).
ometric measurements on solutions which were 5.00 x 10-3M in NO3- plus 0.04, 0.09, and 0.12M in the interfering anion of interest. The electrode was first immersed and tested in an interference-free 5.00 X 10-3M NO3- solution, then rinsed and immersed in the test solution, then rinsed and again tested in the reference solution. The electrode was found to reach the same potential in the reference solution after each test. This procedure was followed for all ions tested. Selectivity coefficients were calculated from the Eisenman equation : AE
=
(slope) log
Table 111. Comparison of Spectrophotometric (Xylenol) and Electrode Methods [NOs-] found, X Method Solution Xylenol Electrode Re1 dev, 3.38 2.3 A 3.30 2.87 1.9 B 2.82 C 2.20 2.24 2.0 1.42 1.5 D 1.40 2.85 1.9 E 2.80 1.03 2.6 F 1 .oo
1
where al and z are the activity and the charge of the interfering anion, respectively. The change in ionic strength caused by the addition of the foreign ion changed the activity of the nitrate ion, which was suitably accounted for using nitrate ion activity coefficient values. Selectivity coefficients are given in Table I. A log K value more negative than -2.5 indicates that there is virtually no interference from the foreign ion. Sulfate did not interfere even at a concentration of 24 times that of nitrate. This is important in air pollution analysis since SOz interferes with many conventional analyses. At higher levels, dilTerences in response between the reference and the test solutions were obtained because of a change in activity. The study also revealed that C1- and NOZ- were moderate interferences, having log K values of -1.4 and -0.8, respectively. Chlorate anion was a strong interference whose log K value was 0.26. pH Profile. The response of the electrode as a function of pH was studied. The p H of a 10-*M KNO, solution was adjusted with either concentrated H2S04 or dilute ",OH added dropwise. After each addition, the solution was stirred, the p H was measured, and the potential recorded. The electrodes can be used in the p H range 3-8.5. Below p H 3 , there is strong interference from H+ and above p H 8.5, OH- interferes. Determination of Nitrate by the Xylenol Method. Nitrate determinations were carried out on a series of standard K N 0 3 solutions using the xylenol method exactly as outlined by Jacobs (2). The xylenol method was simpler to perform than the phenoldisulfonic acid method because it did not involve time-consuming evaporations. The method was very reliable; the results of ten determinations in the range of 3 to 64 X 10-6M NO;- had a correlation coefficient of 1.000 (Table 11). This method was, therefore, subsequently used in testing the electrodes for their usefulness in air pollution analysis. Testing of the Electrode in H 2 0 2 Absorbing Solutions. Electrode response was measured in test solutions of K N 0 3 made up in 3 %, 2 x , and 1 % H L O ~ .The results indicated that peroxide was a strong interference, both as a foreign, extractable species and as a readily decomposed substance which produced oxygen bubbles which adhered to the coated surface of the electrode. Various reagents were tested for their efficacy in destroying the peroxide. Finely divided MnOn was the most effective. It was determined that 0.01 gram of MnOa was the optimum amount to be used for a 20-ml solution of 3% H L O ~ .It was necessary to wait about 15-30 minutes for the complete decomposition of the H202. As a preferable alternative, the peroxide can be eliminated more quickly by adding from 4-6 mg of MnOz to a 10-ml aliquot, heating in a water bath at 80°C for five minutes, then quickly cooling to room temperature.
The electrode was tested in these solutions, and it was found that the MnOa adhered to the surface of the electrode causing it to drift very badly. The treated solutions were consequently filtered through Whatman No. 1 filter paper and electrode measurements were compared to that of a peroxide-free K N 0 3solution. Testing of the Electrode in a Dynamic System. The AID constant temperature control oven was assembled with the sealed glass chamber containing an NOz permeation tube which leaked at the rate of 1010 ng/min. The temperature was set at 30°C and the system was allowed to equilibrate undisturbed for two days. Dry nitrogen was used as the diluent gas. The system was connected in series with three 250-ml gas washing bottles (Corning No. 1760), each containing 3% HzOz absorbing solution (and 10-4M KN03). The pressure regulator on the tank was set at 40 p.s.i. and the pressure regulator on the oven was set at 30 p.s.i. Collection time was 50 minutes for each run. Chamber flow was 1 l./min and diluent flow was 5 l./min. The solutions were allowed to stand undisturbed for two hours after the gas was bubbled through them. Three 20-ml aliquots were then taken from the first bottle and approximately 0.01 gram of M n 0 2 was added to each. After 30 minutes, the solutions were filtered and the nitrate concentration of each was measured by the xylenol method. A set of standards was run simultaneously. The results of the dynamic system tests revealed that the electrode method compared favorably with the xylenol method. The results for the two agreed within 1.5-2.6% relative deviation, which is within the limits of experimental error (Table 111). In using NOZpermeation tubes, it is important to carefully and completely flush out the permeation tube chamber system between runs in order to avoid a buildup of NO2, which can result in higher nitrate concentration in the absorbing solutions than calculated. Effect of Concentration of Absorbing Solution. Either a 2 or 3% H202 solution could be used for absorbing the NOn. A 1 % solution would also be adequate except in the presence of large amounts of SOn which might reduce the oxidant concentration to a level inadequate for complete NO, oxidation. Because SOz is converted to sulfate by H202,the effect of SOZ absorption was tested by measuring a 10-4M KNO3 solution in the presence of 5 x 10-3M and 10-*M KzS04. The solutions were made up in 2 x H202,then treated with MnOpas previously described. A S042- concentration approximately 30 times that of the nitrate in an absorbing solution would result in a 5x error in the calculated NOs- concentration. Testing of the Electrode on Actual Air Samples. Two 24hour air samples were collected at a roof station on the University of Arizona campus. Air was pumped through a
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
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series of three solutions of 2 % HzOz (and lO-‘M KNOs) at a rate of 2 l./min. After the solutions were allowed to stand overnight, 10-ml aliquots were taken from each and treated with Mn02, filtered, and the nitrate concentration was measured by both methods as described. The air samples were taken and tested to see whether there were any strong interferents present that had not been anticipated in the method development. Because the results of the two methods in each case agreed within 3% of each other, it may be inferred that no such interferences were present. The difference in the two concentrations of atmospheric NO, found-119 and 216 pg/m3-can be attributed to observed differences in traffic flow in the area since the samples were taken on two separate days. The Arizona State Department of Health in its Air Pollution Control Implementation Plan (11) used the Federal primary standard of 100 pg/m3 (annual average) as its ambient air quality standard for NOz. The values obtained for NO, (11) Arizona State Dept. of Health, “State of Arizona Air Pollution Control Implementation Plan,” January, 1972, pp 1-16.
in this study, expressed in terms of NOz, are somewhat high reflecting the presence of abnormally heavy truck traffic in the area (construction projects) during the test periods. CONCLUSIONS
A new potentiometric method for NO, determination has been developed. The importance of this to air pollution work is that this method is much simpler and faster than the spectrophotometric methods currently in use, and just as reliable. Additional advantages include the facts that the electrodes are small, portable, inexpensive, sturdy, and easily constructed, conditioned, and stored. Use of the coated-wire nitrate electrode with the absorption system described is more efficient than the flowing gas technique of DiMartini, and the system is certainly much less complex. Air sampling rates will have to be adjusted to accommodate individual situations ; the method could thus be an advantageous alternative to the existing ones. RECEIVED for review July 17, 1972. Accepted October 19, 1972. This work was carried out in part in the Atmospheric Analysis Laboratory supported by a grant from the Arizona Mining Association.
Theoretical and Experimental Evaluation of Staircase Volta mmetry J. J. Zipper and S. P . Perone Department of Chemistry, Purdue Unicersity, Lafayette, Ind. 47907 The electroanalytical technique of staircase voltammetry (SCV) was investigated as a computer-compatible approach to stationary electrode polarography (SEP). Theoretical relationships presented here show a marked dependence on the data sampling characteristics. Moreover, the severe restrictions upon correlating SCV and SEP data were established. Experimental studies were conducted to verify theoretical predictions for SCV. In addition, the analytical capabilities were assessed experimentally. A linear concentration dependence to 5 x lO-7M was observed, and optimum experimental parameters were defined. .A projected limit of detection of about 5 x 10-*M i s suggested.
THEELECTROANALYTICAL TECHNIQUE of staircase voltammetry was suggested by Barker ( I ) , and has been explored epperimentally by Mann ( 2 , 3) and by Nigmatullin and Vyaselev ( 4 ) . However, these earlier works were carried out prior to the development of an adequate theoretical description, presented later by Christie and Lingane (5). Moreover, the recent emergence of digital electronics and computerized instrumentation, with which the technique is inherently com(1) G. C. Barker, Adcan. Polarog., 1,144(1960). ( 2 ) C. K. Mann, ANAL.CHEM., 33,1484 (1961). (3) Ibid., 37,326 (1965). (4) R. S . Nigmatullin and M. R. Vyaselev, Zh. A n d . Khim., 19, 545 (1964). (5) J. H. Christie and P. J. Lingane, J. Electroanal. Chem., 10, 176 (1965).
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patible, allows a much more definitive theoretical and experimental evaluation of the method than possible previously. In our laboratory, we are particularly interested in the generation of a large computer-compatible data base for practical electroanalytical applications based on the measurement technique of stationary electrode polarography (SEP) (6). Because staircase voltammetry (SCV) is a computer-compatible approach which produces SEP-like results, it is the logical experimental choice for modern computerized electroanalytical systems ( 7 ) . Thus, an experimental evaluation of this technique under computer-controlled conditions appeared desirable, and this work is reported here. EXPERIMENTAL
Instrumentation. The potentiostat used in this work is shown in Figure 1. It uses a common three-electrode design. Current measurements are made across a load resistor, RL, placed in the controlling loop. A diode (H.P. 5082-2811) was placed parallel to RL and limited the voltage drop across RL to about 0.4 V. Thus, during large charging current pulses, nearly the total output voltage of the controlling amplifier can be applied to the cell, and a minimal charging time is required. Moreover, a booster amplifier can be added to provide additional current capacity if needed. (6) R. S . Nicholson and I. Shain, ANAL.CHEM., 36,706 (1964). (7) G. Lauer and R. A. Osteryoung, ibid., 40 (lo), 30A (1968).
