rapidly after the addition of the first electron to yield a n intermediate break. This is in agreement with the polarographic data of Given and Peover ( 5 ) . The final reduction product is presumably the carbinol anion as shown by Wawzonek and Gundersen ( I S ) for the polarographic reduction of benzophenone in DMF. Azobenzene also gave good quantitative results (see Table I). The titration curve (Figure 2 ) exhibited two oneelectron breaks with minor variations in the position of the first break. The one-electron reduction product is a dark red-brown monoanion which gave an excellent electron spin resonance spectrum. The completely reduced product is yellow and can be air oxidized back to the monoanion and then to azobenzene. This is the only compound titrated which appeared to reduce simply to a relatively stable monoanion, and then either to a dianion or to some species which oxidizes to the monoanion. Benzanil was titrated and it was reduced in one two-electron step. The end point break is sharp, but not reproducible. The first titrations were low (94%) in dry DMF, and high (105%) in DMF containing 1% HzO. I n each case, subsequent titrations in the same solution were lower. This might result from an increase in apparent p H as the reaction proceeds (protons are presum-
ably used up in the reaction). However, reduction of the sample size and generating current by a factor of two produced no improvement. These results indicate that because of complicating side reactions, coulometrically generated biphenyl radical anions cannot be used indiscriminately for the determination of any reducible compound. However, it is a new approach t o organic coulometry which has been shown to be useful for the titration of a variety of compounds. It is also an excellent technique for studying the solution reduction of organic compounds and the reactions of anion radicals. Since much longer observation times are involved, coulometry yields information difficult to obtain via a conventional polarographic technique. I n addition, coulometry usually involves the reduction of a t least a milligram of material so that the product can be readily examined by other techniques such as ultraviolet, visible, or electron spin resonance spectrometry. ACKNOWLEDGMENT
The author thanks A. K. Hoffman, R. H. Jura, and C. A. Streuli for many helpful discussions; It7. G. Hodgson for obtaining the electron spin resonance data; J. Koren and R. G. Schmitt for obtaining the ultraviolet data; and
Miss I. Piscopo, who ran some of the titrations. LITERATURE CITED
(1) Adam, F. C., Weissman, S. I., J . Am. Chem. SOC.80, 1518 (1958). (2) Aten, A. C., Buthker, C., Hoijtink, G. J., Trans. Faraday SOC. 55, 324
(1959). (3) Benton, F. L., Hamill, W. B., Ind. Eng. Chem., Anal. Ed. 14,449 (1942). (4) Geske, D. H., Maki, A. H., J . Am. Chem. SOC.82, 2671 (1960). (5) Given, P. H., Peover, M. E., J . Chem. SOC.1960, p. 385. (6) Hoffman, A. K., Hodgson, W. G., Jura, W. H., J . Am. Chem. Soc. 83,4675 (1961). (7) H?ijtink, G. J., deBoer, E., van der Meij, P. H., Weijland, W.P., Rec. Trav. Chim. 75,478 (1956). (8) Hoijtink, G. J., van Schooten, J., deBoer, E., Aalbersberg, W. Y., Ibid., 73,355 (1954). (9) Llggett, L. hf., AXAL.CHEM. 26, 748 (1956). (10) Maricle, D. L., Jura, U'. H.,. Hoffman, A. K., Hodgson, W. G., Division of Analytical Chemistry, 144th National ACS Meeting, Los Angeles, Calif., April (1963). (11) Piette, L. H., Ludwig, P., Adams, R. N., ANAL.CHEM.34, '$16 (1962). (12) Piette, L. H., Ludwig, P., Adams, R. N., J . Am. Chem. SOC.83, 3909 (1961). (13) Wawzonek, S., Gundersen, A., J . Electrochem. SOC.107, 537 (1960). RECEIVED for review October 17, 1962. Accepted February 25, 1963. Presented at the Metropolitan Regional Meeting, Xewark, N. J., January 1963.
Polarographic Determination of Nitrate as 4-Nitro-2,6-XyIe no1 A. M. HARTLEY and D. J. CURRANI Department of Chemistry and Chemical Engineering, University of Illinois, Urbana, Ill. Phenols react with nitronium ion under mild conditions to produce the corresponding nitrophenols. In 6 : 3:1 (volume ratio) sulfuric acid-wateracetic acid, 2,6-xylenol reacts with nitrate to produce 4-nitr0-2~6-xyIenol. The reaction is reproducible, rapid, and nearly quantitative. The product is polarographically reducible with a half-wave potential of - 0 . 2 7 volt vs. the mercury-mercurous sulfate electrode. The diffusion-controlled limiting current is linearly proportional to either nitroxylenol or nitrate concentration over the range 2 X to lO+M. Side reactions, at least one of which produces the 4-nitroso2,6-xylenol, limit the yield to approximately 8870. The nitrosoxylenol is reducible but does not interfere with the polarographic determination. Oxygen is a minor interference which
686
ANALYTICAL CHEMISTRY
can be tolerated in high nitrate samples. Chloride, a serious interference, is removed by a solid-solid metathesis and ion exchange treatment. The over-all reproducibility of the method i s similar to other conventional polarographic determinations relative (standard deviation,
3.0%).
T
of the determination of nitrate in air, sewage, soil, water, and other materials is well known. As a result, a large number of methods for nitrate determination have appeared in the literature. Colorimetric methods have been surveyed in monographs by Boltz ( 5 ) and Jacobs ( I S ) and in a review article by Macdonald (20). Polarographic methods for nitrate determination based on the reduction of nitrate ion have been reHE IMPORTANCE
ported and are surveyed in a second review article by Macdonald ($1). I n general, the waves obtained are characterized by a catalytic reaction, and calibration curves are either nonlinear or linear over a narrow range of nitrate concentration. The methods of Rand and Heukelekian ( 2 4 and Hamm and Withrow (9) were exceptions and calibration curves were linear, but for many samples the procedure in the former case required the destruction of nitrate by ferrous ion solution to avoid residual current difficulties. Recently, Asai and Hartley (1, 11) developed a colorimetric method based on the nitration of 2,6-xyleno1(2,6dimethylphenol) in sulfuric acid-waterPresent address, Department of Chemistry, Seton Hall University, South Orange, N. J.
acetic acid medium which appeared to be one of the most sensitive methods yet devised. A solvent composition of 6:3: 1 (v./v.) suliuric acid-wateracetic acid was optirium for the production of the nitrating agent, nitronium ion, according to the reaction: "0,
+ H2SO4 % K02+
+ EESOI- + H2O
(1)
and to control oxidizing reactions of nitrate. The equation for the nitration reaction may then be written:
-.
NO'
The purpose of thi; work was to investigate the polarographic determination of nitrate by the 4',6-xylenol method t o achieve lower limits of detection. EXPERIMENTAL
Apparatus. X Leeds & Korthrup Electro-Chemograph Type E Polarograph was used for all polarographic measurements excep ; for the chloride interference studies where t h e Sargent Model XV Polarograph was used. The cell consisted of a 100-ml. rlectrolysis beaker fittejrl with a rubber stopper which had ztppropriate holes drilled for the dropping mercury electrode (DME), reference electrode, nitrogen dispersion tube, nitrogen outlet, and thermometep. Provision was also made for passing nitrogen over the solution during a polarographic run. Purified (oil-pump, dry) nitrogen presaturated with the i d v e n t was used wj thout further purification. The DLIE n-as of conventional dc sign with a 20-em. capillary of Marine Barometer tubing (Corning Glass Worl.bj whose characteridics in the iolmnt were: m = 1.774 mg. pcr second and t = 4.347 seconds, based on open circuit measurement a t an uncorrected mercury height of 50.0 em. The wference electrode was essentially a mercury-mercurous sulfate (Jlwtrode; since the electrolyte used in the electrode was the sulfuric acid-water-acetic acid solvent saturated nith Hg&04, the cell was without transference. The erperimentally determined potential of the cell: Pt, H1 (1 atm ) H&Od-H,O-HOAc (f!:3:1), HgJSO4, Hg was 0.4079 volt a t 31.5" C. and 760 mm. pres3ure. The electrode design, which allon-ed provision for renewal of the salt bridge electrolyte, was that of Lingane (19). All potentials cited are referred to this electrode. For those experiments requiring constant temperature, the cell was placed in a close-fitting water jacket. Thermal contact was obtained through a small volume of water in the annular space between the jacket and the cell. Water was circulated by means of a constant
temperature bath (Precision Scientific Co., Catalogue KO. 66600). Temgerature control was 25 =k 0.2' C. A 0.25-ml. capacity microburet (California Laboratory Equipment co.)was used in experiments on determinate solutions of 4-nitro-2,6-xylenol. Sulfuric acid-mater mixtures were delivered by means of a buret equipped with a Teflon stopcock (Fischer & Porter Co.). For the chloride separation, an apparatus was developed to incorporate a filtration and a batch process ion exchange procedure. A three-way "Y" form Teflon stopcock (Fischer R- Porter Co.) was joined to the stem of a 60-11 fritted glass Buchner funnel. One stem of the "Y" was connected to the vacuum system trap and used for collecting washings. The other was joined with wction tubing to a short piece of glasq tubing which was inserted through the stopper of a borosilicate glass micro bell jar where the sample was collected in a 50-ml. Erlenmeyer flask when suction was applied. The filtration was carried out in a 60" long stem funnel using K h a t man KO. 42 filter paper. Reagents. .ill chemicals were of reagent grade unless otherwise stated. Primary standard potassium nitrate (Fisher Scientific Co.) which had been dried for 1 hour a t 100" C. was used for nitrate stock solutions. Concentrated sulfuric acid and glacial acetic acid were C.P. grade. -411 further references to these acids imply the use of the concentrated and glacial acids, respectively. Sulfamic acid was Eastman White Label (No. 4659) used without further purification. The 2,6-xylenol used was either Eastman White Label (No. 1772) or Eastman Practical Grade (P-1772) purified by steam distillation and recrystallization from aqueous ethyl alcohol to meet literature melting point data. 4-Sitro-2,6-xylenol was prepared by a modification of the method of Von Auwers and Markovits ( 2 ) . The modifications were: The reaction solvent containing the xylenol was a H2S04HOAc solution of such composition that the acidity after subsequent dilution by addition of aqueous HOAc potassium nitrate solution would remain 30% or greater in sulfuric acid: the rate of addition of the nitrate solution was increased to 0.1 mole in 1.5 hour; no more than a 10 mole yo excess was used; and diphenoquinone was removed from the product by washing with ethyl alcohol in ivhich the former is insoluble. After two recrystallizations from aqueous ethyl alcohol, an average 7Oy0yield of 4-nitro-2,6-xylenol, pale yellow-white plates melting a t 169.5-170' C. (uncorrected) was obtained. 4-?;itroso-2,6-xylenol was prepared by the method of Sabato ($6). By decreasing the rate of acid addition and maintaining ice water temperature, an 85% yield of tan plates melting a t 169170' C. may be obtained. If allowed to proceed out of control, this and the nitration procedure can produce a nearly
quantitative yield of the 3,3',5,5'tetramethyldiphenoquinone. Mercurous sulfate for use in reference electrodes was prepared by the method of Harned and Hamer (10). Fisher Certified Reagent Grade Mercurous Sulfate (M-192) was used in the chloride separations. The latter was found to contain a n appreciable nitrate impurity. Digestion and washing in dilute H1S04 failed to reduce the level of nitrate below polarographic detectability. A more vigorous treatment in which a suspension of the compound in concentrated sulfuric acid was evaporated to near dryness, cooled, decanted, and washed with 1% H2S04 gave a product of requisite purity. Blank corrections for this material were the same within experimental error as those obtained with water with no mercurous sulfate treatment, but which had been passed through the ion exchange resin. The ion exchange resin used was Don.ex 50W-X8 in the sodium form. Demineralized water \Tas used to prepare all aqueous solutions. Preparation of Solutions. Solutions of 6 :3: 1 H2S04-H20-HOAc mere prepared on a volume basis as measured by graduated cylinders. S o correction for nonideality of the solutions was made and the numerical ratios cited refer to original volumes of the respective components. For brevity, further use of the phrase "sulfuric acid-water-acetic acid" will be omitted; the numerical ratios in all cases mill refer to the volume relationship of the three components in that order. Solutions of 3 : 1 Hk304-H20 were prepared in a similar manner. dqueous stock solutions of potassium nitrate were prepared by dissolving 1.0112 and 1.4156 grams of the salt in appropriate volumetric flasks and diluting t o the mark to give 0.02000 and 0.700111 solutions, respectively. Aliquots of these solutions were taken for further dilution. Similarly, a 0.010031 aqueous solution of sodium nitrite was prepared by dissolving 0.1727 gram of the salt in a 250-ml. volumetric flask and diluting to the mark. Aqueous solutions of mixtures of nitrate and nitrite were prepared by the aliquot method. If it was desired t o destroy the nitrite, excess solid sulfamic acid was added to the volumetric flask prior to the dilution to the mark. A 0.1000J1 glacial acetic acid stock solution of 2,6-xylenol was prepared by dissolving 1.2208 grams of the compound in a 100-mi. volumetric flask and diluting to the mark. ;\lore dilute solutions were prepared by volumetric dilution of an appropriate aliquot. A 0.20031 glacial HOAc solution of 4nitro-2,6-xylenol iyas prepared in a similar fashion in a 25-ml. volumetric flask. Solutions for the polarographic investigation of the known compound, 4-nitro-2,6-xylenol, were prepared by delivering from the microburet known volume increments of the nitroxylenol stock solution into 50.00 ml. of 6 . 3 . 1 solvent. Solutions used for the study of the influence of 4-nitroso-2,6-xylenol on the VOL. 35, NO. 6, MAY 1963
a
687
polarographic measurement of the nitration mixture were prepared by appropriate volumetric addition of a determinate nitrosoxylenol solution in 6:3:1 to the completed nitration reaction mixture. Chloride Removal. Chloride was a n interference in the spectrophotometric method and the same was true for the polarographic method. Polarographic solutions prepared to be 10 p.p.m. in nitrate and 0, 1, 2, 5, 10, 50, 100, and 500 p.p.m. in chloride showed a decreasing nitration yield with increasing chloride concentration. Based on the solution containing no chloride, the per cent yield decreased gradually to 90% up to a chloride concentration of 10 p.p.m., but then fell off rapidly until at 100 p.p.m. the yield mas 27%. At 500 p.p.m. the
Table I. Polarographic Characteristics of 4-Nitro-2,6-Xylenol
lSulfuric acid-water-acetic acid solvent, 6:3:1 (v./v.)I Concn., mM
id,
0,02000 0.04000 0.1199 n 2397 0.3594 0.539 0.678 0.836 0.995
0.128 0.213 0.713 1 3in
2.i3s 3.220 4.165 5.138 6.138
EL/*, voltb
-0.276 -0.278 -0.277 -0.277 -0.275 -0.275 -0.274 -0.274 -0.273
IC
3.36 2.79 3.12 3.09 3.12 3.14 3.22 3.22 3.24
Diffusion current measured from maximum excursions of recorder pen and corrected for residual current a t E,,, = -0.600 volt. Versus mercury-mercurous sulfate reference electrode and corrected for ilz drop of the solution. I = id/Cm.2/3t"6.
Table II. Variation of Limiting Current with Height of Mercury Column [ Back pressure correction ( h b a o k ) 1.4 em.]
=
A . Solution of 3.594 X lO-'Af
4-nitro2,6-xylenol in 6 :3 : 1 solvent 44.0 42.6 2.01 0.308 50.1 48.7 2.14 0.307 59.9 58.5 2.35 0.307 69.9 68.5 2.62 0.316 76.0 73.6 2.75 0,318 B. Sitration mixtureb 42.6 3.63 0.557 48.6 3.93 0.565 58.6 4.19 0.547 70.0 68.6 4.54 0.548 80.0 78.6 4.86 0.549
44.0 50.0 60.0
a
Measured a t -0.600
volt us. Hg/
Hgki'O4.
* Nitroxylenol concentration of 7.00 X lO-*M based on nitrate added.
688
ANALYTICAL CHEMISTRY
oxidation of mercury occurred. It was obvious from the polarograms that the amount of nitrosation increased at the expense of nitration. The use of silver sulfate as a precipitant for chloride was unsatisfactory because the solubility product of this compound is too high. The solubility of mercurous sulfate appeared to be more favorable. A large excess of mercurous sulfate and considerable time were required for complete chloride precipitation and for particle size growth. Although the removal of chloride by mercurous sulfate was complete, mercurous ion was polarographically active. This difficulty was removed by passing the filtrate over a cation exchange resin. Two milliliters of wet resin was sufficient to remove t.he mercurous ion present after precipitation. With this treatment, no polarographic evidence for the presence of mercurous ion mas obtained. Blanks run on synthetic chloride samples mere identical within experimental error to those obtained on water alone passed through the ion exchange resin. Homever, the latter value was sufficiently different from residual current curves obtained with untreated Ivater that residual current corrections were made using data on Ivater passed through the ion exchange resin for those samples which were treated in a similar fashion. By means of the apparatus and procedure described above it was possible to perform the precipitation, filtration, and ion exchange on the sample without dilution. Procedure. Prepare a n aqueous solution of the sample such t h a t the nitrate concentration falls within the limits of 6 X to 3 X 10-3M. Deliver 40.00 ml. of 3 : l H2S04-H20 solution into a dry polarographic vessel suitably equipped for stirring and deaeration with nitrogen. To this solution add, in order, with stirring, 5.00 ml. of aqueous nitrate sample, 5.00 ml. of a solution of 2,6-xylenol in glacial HOAc whose concentration is about ten times that of the nitrate. Any arrangement of volumes can be used which will produce a final solvent composition of 6:3:1. Place the cell in a water bath, stir, and allow 5 to 10 minutes for completion of the reaction and cooling of the solution to room temperature. (It is convenient for routine determinations to utilize the equipment described previously.) If the final nitroxylenol concentration is such that less than 10 pa. limiting current is expected, deaerate the solution for 7 to 10 minutes using a coarse porosity gas dispersion tube. Measure the current (maximum pen deflection with fast recorders) a t -0.150 and -0.600 volt us. the Hg/Hg2SOa reference electrode. Correct the diffusion current obtained a t -0.600 volt by adding algebraically the current obtained at -0.150 volt to that obtained at -0.150 volt with pure solvent and add this difference to the residual current at -0.600 volt; subtraction of the corrected residual current from the limiting current yields the true diffusion current. For routine purposes the nitrate concentration may be obtained nith equal
ease from a working curve produced from known concentrations of potassium nitrate stock solutions or by use of the diffusion current constant, I , and the capillary characteristics, wz and t. To remove chloride, place 100 ml. of sample in a 400-ml. beaker and acidify with concentrated H2SOI to p H 1-2. For highly basic samples, it may be necessary to use larger samples such that the dilution error viill remain negligible. Add 0.50 gram of nitrate free mercurous sulfate with stirring. Allow to stand for 1 hour and 15 minutes with occasional stirring. Place 2 ml. of thoroughly m-ashed ion exchange resin in the Buchner funnel and remove the water with suction. Filter the sample through a dry funnel using dry Whatman KO.42 filter paper. -4fter the first 10 ml. is collected, stir, and remove by suction. Repeat Kith two more 10-ml. portions. Collect t h e remaining sample in the Buchner funnel n-ith occasional stirring. After filtration is completed, stir for 3 minutes and collect the sample by suction. Stopper the flark and store for use as needed. Sufficient sample is collected for triplicate polarographic determinations. RESULTS
Polarographic Characteristics of 4Nitro-2,6-xylenol. I n this highly acidic solvent, 4-nitr0-2,6-uylenol produced one well defined polarographic wave prior t o hydrogen discharge. The average value of the half-wave potential over the concentration range studied was -0.2i5 =t 0.005 volt. All half-wave potentials cited were estimated by the geometric construction method (22) and are corrected for iR drop ( R = 2800 ohms) Since these data, shown in Table I, were recorded on the 0- to 2-volt span of the Electrochemograph, the precision of half-wave potentials was estimated to be i 5 mv. The height dependence of limiting current was determined and found to be proportional to d h z r (Table 11). The back-pressure correction for the height of the mercury column shown in Table I1 was calculated according to the equation given by Kolthoff and Lingane ( 1 7 ) . To justify the use of this equation for this solvent, the drop times at the electrocapillary maximum in 6:3:1 solvent and 0.1N KC1 were measured and found to be 5.434 and 5.442 seconds, respectively, for a mercury height of 50.0 cm. Thus the ratio fg l / t ~ ~ had e the value 0.998, which was insignificantly different from unity in this case. The diffusion current was directly proportional to concentration over the range 2 x 10-5 to 1 x 10-~Jf4-nitro2,6-xylenol as indicated in Table I. If the behavior of this compound does not differ in kind from that of other nitroaromatics in similar solvents ( 3 ,
Table 111. Effect of Temperature on the Polarographic Characteristics of the Reaction Mixture in Sulfuric AcidWater-Acetic Acid Solvent 6 : 3 : 1 (V./V.,
T;mp.,
C.
Eli?, voltb
id,
3.04 3.38 3.70 4.05 35.0 4.44 40.0 4.80 a Measured a t E,,, 15 0 20 1 25 0 29 8
Hg/”gzSOa.
* Corrected
0
-E
Figure 1 .
1 O-4M, 4.mitroso-2,6-xylenol Residual current of 6 : 3 : 1 H2SOd:HtO: HOAc electrolyte The curi.ent axis for curve b has been shifted by 0.75 pa. ‘Concentration based on nitrate taken
a’.
l e ) , we may write the over-all electrochemical reaction as :
5H-
+
AH~OH4e-
=
2800
Polarograms
7.00 X 1O-5M, 4-nitr0-2,6-xylenoI,~ b. 7.00 X 1 O-5M, 4-nitro-2,6-xylenola in air-saturated solution c. 5.83 X 1 O-5M, 4-nitro-2,6-xylenol‘ in the presence of 1.609 X
rj0,
iR droo ( R
ohms). VS Hg/Hg2S04
a.
-
for
=
-0.281 -0.280 -0.277 -0.275 -0.271 -0.268 -0.600 volt us.
HzO
Using the assumption that = 4, the diffusion coefficient, D,was calculated to be 1.3 X l o p F sq. cin. per second from the simple Ilkovic equation, i,, = 706 nCD112m2 31 Although the polarographic wave was n-ell defined, it x a s irreversible a3 evidenced by a plot of log ( i d - i ) l i us. E,,,, (applied potential corrected for zR drop) which had 3, slope of 22.5 mu, per decade over it. linear midrange. For a four-electron rwersible step, this slope should have a value of 14.8 niv. per decade a t 25” C. As was expected for an irreversible xocess, the halfwave potential varied with concentration a5 shown in Table I. Throughout the concentration range examined, the wave was well behaved and showed no detectable maximuri or adsorption effect. Polarographic Characteristics of the Reaction Mixtures. Similar studies of the reaction miuture showvcd with a high degree of certainty t h a t 4nitro-2,6-xylenol wa: the polarographically active substance (a typical polarogram is shown in Figure 1, curve a ) . When a polarographic reaction mixture W E I S diluted with 6:3:1 solvent to a concentration desirable for spectral examination, the spectrum was in all respwts identical with
those obtained in the spectrophotometric study (A,, = 324 mp, a = 7900 l/cm.-mole) ( I , 1 1 ) . The average value of the half-wave potential over the concentration range 2 X lop6 to 2 X 10-3X 4-nitro-2,6-xylenoI (based on nitrate taken) Fvas -0.276 volt with a range of values from -0.267 to -0.287 volt, respectively. This is identical within experimental error to that of the known nitro compound. A plot of log (id - i ) / ius. E,,,, for a concentration similar to that of the known compound, gave a slope of 23.7 mv. per decade which compares favorably with 22.4 inv. per decade. The height dependence of the limiting current mas determined and again found to be proportional to d G as shown in Table 11. In addition, the temperature dependence of the diffut-ion current over the range 15-40” C. vas determined. The temperature coefficient as calculated from the data in Table I11 was 1.91% per degree. The temperature coefficient of the half-wave potential was -0.19% per degree.
Table IV.
A plot of i dus. C was again linear but of different slope than that of the known compound. The precision of the method for triplicate determinations a t several concentration levels is shown in Table IV. Interferences. Since the half-wave potential for the first wave of oxygen in this solvent was about -0.54 volt, it was desirable t o investigate the possible interference of this substance. I n air-saturated reaction mixtures, the current a t either the foot or the plateau of the oyygen wave was directly proportional to concentration of the nitroxylenol as shown in Table V. A typical polarogram is shown in Figure 1, curve b. The contribution of oxygen to the total current at the foot of the oxygen wave was a nonlinear function of oxygen concentration (Figure 2). This is not qurpriqing qince in this region of the curve two processes, both of which involve hydrogen ion, are occurring simultaneously. The magnitude of the current due to oxygen in an air-saturated solution, as measured at an applied potential (E,,,) of -0.900 volt was about one microampere. h-itrite was a n interference in the spectrophotometric determination of nitrate due to the reaction of nitrite with 2,6-xylenol ( 1 , 11). Hartley and
Precision of the Polarographic Determination of Nitrate as 4-Nitro-2,6Xylenol (Triplicate Determinations)
Relative Concn., m X a 2.000
h v . i,, pa. 10.41
id
Calcd., pa.b 12.28 4.298 0.430
UI
7c”
0.85
Relative error, yod -15.23 -14.70 -8.14
3.666 1.52 0.0700 0.395 2.99 a Based on nitrate added. * Based on the diffusion current of a prepared solution of 4-nitro-2,G-xylenol of identical concentration assuming lOOql, 5-ield. Standard deviation calculated from the range R; u,, = k , ( E ) where h: = 0.591 for n = 3 (7). Based on i d (calculated) as the “true” value. 0.700
VOL. 35, NO. 6, M A Y 1 9 6 3
689
"--I
I
trons. The protonation of the primary product, hydroxylamine, is regarded by Pearson as sufficiently stabilizing as to prevent immediate further reduction to the corresponding anilinium ion in acidic solutions (25). The fact that the limiting current is directly proportional to the square root of the corrected mercury height and a plot of id us. extrapolates to zero intercept, along with the comparatively normal temperature coefficient found for this current, demonstrates that diffusion was the rate-controlling process in the plateau region of the currentvoltage curve. Kolthoff and Lingane (18) state that the temperature variation of a diffusion controlled process should be primarily due to a change in solution viscosity and should be estimable from the temperature coefficient of conductivity which for simple ions is about 3 to 5%. The present solvent system is sufficiently nonaqueous to preclude direct comparison and yet is not similar enough to the pure sulfuric or acetic acid systems used by Bergman and James (S), Conant ( 6 ) , and Hall (8) to be comparable. Our best estimate is that this solvent lies a t or near the maxima of compositionphysical property functions such as viscosity, conductivity, and freezing point. Thus the temperature coefficient, while somewhat low, is taken as evidence in the negative sense that other rate-controlling processes should have produced a much larger value. Curve d, Figure 1, shows the residual current curve for the 6:3:1 solvent. Comparison of this curve with a polarogram of a reaction mixture (curve a) shows a rise in current in the region of the foot of the wave, which cannot be accounted for on the basis of residual current alone, This rise in current was found to be a nonlinear function of nitrate concentration and its magnitude also depended on the ratio of 2,6xylenol to nitrate in the reaction mixture, for the same iiitrate concentration a xylenol-nitrate ratio of 100 produced a current of 0.08 pa. while a tenfold excess produced a 0.31 pa. current in this region. The procedure for measuring the diffusion current of the reaction product, 4-nitro-2,6-xylenol, in view of these facts was to record the entire polarogram, measure the rise in current over that expected from the residual current a t E,,, = -0.150 volt and extrapolate linearly the residual current to E,,, = -0.600 volt. Curve c, Figure 1, shows a polarogram of 4-nitro-2,6-xylenol in the presence of 4-nitroso-2,6-xylenol. Plots of current us. 4-nitroso-2,6-xylenol concentration at constant nitroxylenol concentration as measured a t the foot (E,,, = -0.200 volt) and the top (EQPP= -0.600 volt) of the nitroxylenol wave were linear and parallel, indicating no
cr
100
L 20
40
80
60
OXYGEN ( % A I R SATURATED
100
I
Figure 2. Variation of 4-nitro-2,6xylenol diffusion current with oxygen concentration in 6:3:1 HzS04:HzO:HOAc electrolyte Current measured a t E,, = -0.400 volt for a 2.00 X 1 O - W , 4-nitro-2,6-xylenol solution based on nitrate taken
Asai recommended sulfamic acid as a reagent for the removal of nitrite; the same procedure proved to be effective for the polarographic determination. That the procedure for chloride removal was effective for actual samples was shoa7n by the standard addition analysis of a sample of river water containing nitrate and chloride. The plot of diffusion current us. nitrate added was linear and the slope of the line was equal within experimental error to that of the normal working curve. The value of the intercept corresponded to 13.6 p.p.m. nitrate for the sample. The reported analysis was 14.4 p.p.m. nitrate. The analysis without standard addition of a soil water sample known to contain 12.2 p.p.m. nitrate gave a result of 12.3 p.p.m. nitrate. DISCUSSION
Generally, the first over-all reaction in the electrochemical reduction of aromatic nitrocompounds in acidic solutions involves the transfer of four electrons and is followed by a second which is believed to involve two electrons (4). Polarographically, two waves may be found, the first corresponding to reduction to the hydroxylamine and the second to reduction of the hydroxylamine to the amine (IS). I n many instances the second wave is not well defined or its half-wave potential lies well beyond the discharge potential of the supporting electrolyte. The latter situation appears to prevail here as no evidence of a second wave was obtained. On this basis, the electrochemical reaction in the present case is believed to be the reduction of 4nitro-2,6-xylenol to the corresponding hydroxylammonium ion (Equation 3). Work currently in progress in these laboratories substantiates the work of Pearson in that the shift in half-wave potential with p H requires one more proton than elec-
690
ANALYTICAL CHEMISTRY
Table V. Effect of Oxygen on the Polarographic Wave of 4-Nitro-2,6Xylenol in Air-Saturated 6 :3 :1 Sulfuric Acid-Water-Acetic Acid Nitroxvlenol diffusion
4-Nitro-2,6xylenol, mMn 0.02000 0.0700
current, Ma. I* II C 0.269 0.114 0,505 0.359
1.048 1.193 0.2000 Based on nitrate added. * Measured at ESP, = -0.4 volt Hg/HgBOt in air-saturated solution. 6 Measured at ESP, = -0.4 volt Hg/Hg,S04 with oxygen removed. a
us.
us.
polarographic interference of nitrosoxylenol. However, the effect of the presence of nitrite on the nitrate determination was not investigated except that it was possible to destroy nitrite with sulfamic acid prior to reaction with the xylenol. The slope of the working curve for 4nitro-2,6-xylenol was 6.19 X lo3 pa. per molar while that of the reaction mixture was 5.45 x 103 pa. per molar. Interestingly, the rise in current a t the foot of the polarographic waves for the nitroxylenol occurs in the same region that the limiting current for 4nitroso2,g-xylenol is found. This would indicate that some nitrosation takes place during the nitrating reaction. A possible explanation would be that nitrate oxidizes the 2,6-xylenol reagent with the resultant production of some nitrite. Some evidence for this was indicated in the spectrophotometric study in that adjustment of the acidity of the solvent was necessary in order to avoid serious reagent deterioration. Further, the spectral curves for either nitration or nitrosation showed a peak in the visible region which was attributed to an indophenol formed by condensation of 4nitroso-2,6-xylenol with excess 2,6xylenol and which was unstable in the reaction mixture ( 1 , 1 1 ) . That the side reaction producing some nitrosoxylenol does not account for the total difference between nitroxylenol found and that expected is not surprising in view of Asai's observations of the nitrosation reaction under conditions such that only nitrosoxylenol and 3,3',5,5'-tetramethyldiphenoquinone were produced. I n these studies a material balance calculation of nitrosoxylenol and diphenoquinone produced from a known amount of nitrite added showed that approximately 2.5 moles of benzoquinone were produced per mole of nitrite which did not appear as nitrosoxylenol. Therefore, the loss in nitrate is attributed to some nitrosation which takes place but cannot be quantitatively accounted for because of further side reactions of nitrosated xylenol. Despite this, production of nitroxylenol is proportional to nitrate added. This would
also explain why the rise in current at the foot of the wave of the reaction mixture is not linear with nitrate concentration but yet a linear extrapolation of the residual current is a success ful procedure. Althotgh the nature of these side reactions is not yet clear, it is believed that the explanation of the polarographic and spectrophotometric behavior lies in the relative rates and molecularities of Equation 1 and the following reactions: ” 0 3
+ xylenol
-c
+
+
diphenoquinone NO+ ? (4) NO xylenol -c nitrosoxylenol (5) KO2+ j-xylenol + nitroxylenol (6) +
+
Reactions 5 and 6 are pseudofirst order in nitrosonium or nitronium ion ( I ) . Equation 1 and a n allied host of reactions have been shomn to be fast (12). The ultiinate fatc of the nitrate and thus the final polarogram will be dependent upon the relative rates of Equations 4 and 6. Evidently the molecularity with respect to xylenol of these two reactions is such that an increase in xylenol conzentration favors Equation 6, thus leading to the lower “residual current” which is a function of Equations 4 and 5 Nitrite, per se, was not investigated as an interference. The nitrosation product, Cnitroso-2,6xylenol, is clearly not 3, polarographic interference. However, the prior spectrophotometric studies showed the nitrosation reaction to be considerably more complex and less precise than the nitration reaction. Oxygen has been shown to be a partial interference. If the concentration of oxygen is variable, nonlinearity is introduced, but if the Concentration of oxygen is constant, lnearity is maintained. Since the magnitude of the current due to oxygen in air-saturated solvent is about 1 pa., the effect would be troublesome at low nitrate concentrations, but could probEibly be neglected at high nitrate concent mations. Chloride has been shown to be a serious interference. Due to the increase in current in the region of the Cnitroso2,6-xylenol wave for samples containing chloride, it is suspected that the loss in nitrate is due t o the formation of nitrosyl chloride. A quantitative investigation of this matter is difficult since the total current in this region is the m m of contributions from several sources. It should be pointed out that the amount of mercurous sulfate and the amount of resin required should be experimentally determined for a particular type of sample since eflects such as total ionic strength and types of cations present are important. Obviously many cations which would be polarographically active are removed by this procedure as well as anions of other insoluble mercurous salts.
Table VI.
Sensitivities of Eight Months of Nitrate Determination
Method Sensitivitya Reference Remarks 1, Brucine 0.700 absorbance/mM (5) C 2,PXylenol 3.18 absorbance/mM (1) c Phenoldisulfonic acid 6.36 absorbance/& (1) 2,6-Xylenol (spectrophotometric) 7.90 absorbance/mM (1) d Uran 1 ion method 36.1 pa./mM (16) 2 , 6 - 2ylenol (polaro5.45 pa./mM graphic) Sodium molybdate 0.033 pa./mM (ikj f Zirconyl chloride ... (24) * Defined as (1) AA/AC for the spectrophotometric methods and (2) Ai/AC for the polarographic methods. * Extracted from the cited reference. Computed values taken from experimental values obtained by R. I. Asai by repetition of literature procedures. Obtained from data for the working curve. e Estimated from the most linear portion of nonlinear working curve. f No data on the magnitude of the limiting current are available in the literature. d
The precision of the method as shown in Table IV is in accordance with that expected for a polarographic procedure. The relative error as expressed in the last column in Table IV reflects the loss in nitrate as discussed. At the lowest concentration the precision is least good due to accumulated volumetric errors starting with a highly concentrated stock nitrate solution, increased noise and irreproducibility of the polarograph at high sensitivity settings, and increased importance of unknown and adventitious impurities contributing to the residual current. A comparison of the more common methods for determining nitrate is tabulated in Table VI. The sensitivities listed in column 2 are essentially the slopes of concentration-measured variable functions. In several instances, notably the brucine spectrophotometric procedure and the metal-ion catalyzed polarographic procedures, the working curves are markedly nonlinear. In these cases a n attempt was made to cast the most favorable light on the method in question. On the basis of the magnitude of the sensitivity criterion the uranyl ion catalytic direct nitrate would appear most sensitive. This particular method, however, suffers from a severely limited concentration range over which the measurements are valid. The working curve for the sodium molybdate method is so nonlinear over the concentration range investigated that the slope was taken at higher nitrate concentrations where the increase in current is nearly zero. In any event, direct comparison of absorbance units and microamperes is useless, particularly since the former is a logarithm of a ratio. A comparison which is not evident from Table VI concerns the working range for the several methods. With the exception of the 2,g-xylenol spectrophotometric and polarographic meth-
ods, all others in Table VI are characterized by a useful working range extending over not more than a hundredfold concentration change, and in most cases this figure is reached only by use of a nonlinear working curve. The present method has been used over a thousandfold concentration range from about 10-6 to l O + N with linearity within experimental error. The lower limit of detection is estimated t o be 0.4 p.p.m. nitrate in the original sample which amounts to approximately 28 p.p.b. nitrate nitrogen in the final solution taken for analysis. ACKNOWLEDGMENT
The authors thank the Illinois State Water Survey for providing analyzed water samdes. LITERATURE CITED
(1) Asai, R. I., Ph.D. Thesis, University of Illinois. Urbana. Ill.. 1959. (2) Auwers,’K. von, ’Maikovits, T., Ber.
41,2332 (1908). (3) Bergman, I., James, J. C., Trans. Faraduu SOC.48, 956 (1952). (4) 1bid.,‘50, 60 (i954): (51 . . Boltz. D. F.. “Colorimetric Determination of Nonmetals,” Chemical Analysis, Vol. 8, pp. 135-47, Interscience, New York, 1959. (6) Conant, J. B., Hall, N. F., J. Am. Chem. SOC.49, 3062 (1927). (7],Dixon, W. J., Massey, F. J., Jr., Introduction t o Statistical Analysis,” p. 239, McGraw-Hill, New York, 1951. (8) Hall, N. F., Sprengelman, W. F., J. Am. Chem. SOC.62 2487 (1940). (9) Hamm, R. E., d t h r o w , C. D., ANAL. CHEM.27, 1913 (1955). (10) Harned, H. S., Hamer, W. J., J. Am. Chem. SOC.57, 27 (1935). (11) Hartley, A. M., Asai, R. I., J. Am. Water Works Assoc. 52, 255 (1960). (12) .Ingold, C. K., “Structure and Mechanism in Organic Chemistry,” p. 269, Cornell University Press, Ithaca, N. Y., 1953.
(13) Jacobs, M. B., “The Analytical Chemistry of Industrial Poisons, Hasards, and Solvents,” 2nd ed., p. 350, Interscience, New York, 1949. VOL. 35,
NO. 6, MAY 1963
0
691
(14) James, J. C., T r a n s . Faraday Soc. 47, 1240 (1951). (15) Johnson, M. F., Robinson, R. J., ANAL.CHEILI.24, 366 (1952). (16) Kolthoff, I. M.,Harris, IT. E.. Matsuyama, G., J . A m . Chem. SOC.6 6 , 1782 (1912). (17) Kolthoff, I. M., Lingane, J. J., “Polarography,” 2nd ed., Vol. 1, p. 81, Interscience, Sew York, 1952. (18) Zbid.,pp. 90-3. (19) Lingane, J. J., “Electroanalj-tical
Chemistry,” 2nd ed., p. 362, Interscience, Sew York, 1958. (20) Macdonald, A. M. G., Ind. Chemist 31, 515 (1955). (21) Zbid.,p. 568. (22) M e i p , L., “Polarographic Techniques, Interscience, New York, 1955. (23) Pearson, J., T r a n s . Faraday SOC.44, 683 (1948). (21) Rand, 11.C., Heukelekian, H., ANAL. CHEhf. 25, 878 (1953). (25) Sabato, .I.,Hol. Infornz. Petrol.
(Buenos Aires) 27, 59 (1950); 45, 1333
(1951).
c.
A.
RECEIVED for review August 15, 1960. Resubmitted November 2, 1962. ACcepted February 8, 1963. Taken in part from the Ph.D. thesis of D. J. Curran. Kork supported in part by a Grant from the Sational Institutes of Health. Presented in part before the Division of i\nalytical Chemistry, 138th Meeting, -4CS, Sew Tork, September 1960.
End-Point Detection and Current Efficiencies for Coulometric Titrations Using the Dual Intermediates M a nga nese(ll1) and Iron(II) RICHARD
P. BUCK
Bell & Howell Research Center, Pasadena, Calif.
b Several proposed versions of amperometric and potentiometric endpoint detection systems have been investigated. For routine applications the null potentiometric method has been most satisfactory. Efficiencies were determined by a potentiostatic method for the generation of manganese(ll1) and iron(l1) on paraffin-impregnated carbon, boron carbide, and platinum foil for a wide range of current densities. Efficiencies were confirmed by generating Mn(lll) for a known time and back-titrating with Fe(ll). Carbon i s preferred as generator anode material.
T
of electrolytically generated Xn(II1) for the coulometric titration of Fe(II), As(III), and oxalic acid was introduced by Tutundzic and Mladenovic (7‘). Subsequently, current efficiencies for the generation of Mn(II1) on smooth platinum in sulfuric acid media and disproportionation equilibria of Mn(II), (111), and (IT’) were investigated by Selim and Lingane (6). Conditions for the efficient generation of hln(II1) as a function of Mn(II), and sulfuric acid concentrations and current density on platinum were confirmed and extended to gold anodes by Fenton and Furman ( 3 ) . Both groups agreed that current efficiencies for the generation of Mn(II1) were less than 1 0 0 ~ by o 0.2 to lyOunder optimum circumstances,-Le., Mn(I1) greater than 0.28’; sulfuric acid between 2F and 7 F ; current densities between approximately 1 and 4 ma. per square centimeter. In this paper, results are reported on the current efficiencies for the generation of Mn(II1) and Fe(l1) on platinum foil, paraffinimpregnated carbon, and boron carHE USE
692
rn
ANALYTICAL CHEMISTRY
bide which confirm and extend the earlier LTork. The resp0n.e of several forms of amperometric and potentiometric endpoint detection systems was measured for the dual intermediates in the vicinity of the equivalence point. The amperometric end pointq include use of two platinum foils with defined, impressed potential difference, with cathode held a t a constant potential difference (more negative) with respect to the solution potential, and with the cathode held a t a constant potential with respect to a calomel reference electrode. The potentiometric end points include using direct measurement of solution potential a t a platinum wire us. a calomel cell, null mcasurement of solution potential us. a preset value corresponding to the equivalence point potential, and constant current potentiometric method a t two platinum electrodes. EXPERIMENTAL
Apparatus and Reagents. Currents
constant t o 0.1% from approximately 100 pa. to 250 ma. mere obtained from an electronically controlled constant current supply following the t n o-electrode designs of DeFord (g), and shown schematically in Figure 1. The modular arrangement selected uSes a high gain d.c. amplifier as an adder-inverter and normally provides currents up to 25 ma. For currents between 25 and 250 ma., a second booster stage, indicated by B . is used. Generation times were measured TTith a precision timer Model 8-10 of the Standard Electric Time Co. The titration vesael was a waterjacketed 150-ml. lipless beaker fitted with a plastic cap into which were inserted the electrodes, salt bridge, gas dispersion tube for deaeration with prepurified nitrogen, and isolation tubes
terminating in frittcd glass disks Solution volume of 7 5 nil. nas used. The generator electrode pair was 0.003in. thick platinum foil electrodes sealed into soft glass. The generator anode wm selected from three foils with areas of 2, 3, or 6 sq em. The generator cathode had an area of 2 sq. em. and n-as separated from the bulk of the solution by a tube containing 4.5F H2S04 terminating in a fine fritted glass disk. The carbon generator anode n-as Spex Industries, Inc , spectrographic grade carbon rod, 0.3-cm. diameter, n hich had been imp:egnated a i t h paraffin !\ax Rods were vaxed into glass tubes ‘0 that 0.34 and 1.14 sq. em mere eyposed. The exposed portion n a. poliqhed with carboiundum The cloth to remole surface n a x boron carbide anode was fine porosity solid cylinder, O..km. diameter, made by the Korton Co., and was sealed into glass JTith sealing T T ~ Y so , that 0.91 sq. em. was exposed. Nercury contacts ivere used n i t h all of the electrodes. A liquid, a t u r a t e d K2S04 bridge separated the referrnce calomel cell from the coulometric vessel. Tefloncovered magnetic Etirring bar R as used. The stock qolution uwd in this study was 0.4F 1\InS04, 0 12F Fe2(S0J3, and 4F H2S01. and n as prepared according to the procedure of Fenton and Furman ( 3 ) . Potential drift, indicating a loss of generated X€n(III), n a s eliminated by boiling the stock splution nith a fen- crystals of p o t a s w m permanganate and filtering. The constant impresqed potential amperometric end-point source, current meter, and shunts n ere constructed from the schematic given by Meier, Myers, and Snift ( 6 ) . The potential was impressed across a pair of 2-sq. em. platinum foil of the same construction as the generator pair. For the amperometric end point in which one electrode of the polarized pair was held at a constant potential difference n ith respect to the 3olution potential,
