face areas (1)-are als2 important and work is continuing to evaluate the effect of these variables on gas chromatographic separations. LITERATURE CITED
R. H., Purnell, J. H., J. Chem. SOC. 1960, b44. . (3) Craig, B. M., “Gas Chromatography,” pp. 37-56, N. Brenner, J. E. Callen, and M. D. Weiss, e&., Academic Preas, New York, 1962. (4) Fry, A., Eberhardt, M., Ookuni, I., J . Org. Chem. 25, 1252 (1960). (5) Jones. W. L.. Kieselbach., R... ANAL. CHEM.30,1590(1958). ( 6 ) Martin, R. L., Ibid., 33,347 (1961). (7) Ottenstein, D. M., Seventh Detroit Anachem. Conference, Detroit, Michigan, October, 1959. .
(1) Baker, W. J., Lee, E. W., Wall, R. F., "Gas Chromatography,” pp. 21-31, H. Noebels, R. F. Wall,,and N. Brenner, e&., Academic Press, .New York, 1961. (2) Bohemen, H., Langw, S. H., Perrett,
I
(8) Sawyer, D. T., Barr, J. K., ANAL. CHEM.34, 1052 (1962). ( 9 ) Ibid., p. 1518. (10) Young, J. R., Chem. Znd. (London) 1958,594. RECEIVEDfor review March 25, 1963. Acce ted May 27, 1963. Presented at the Eouthwest Regional Meeting, ACS, Dallas, Texaa, December, 1962. The authors ex reas their appreciation to The Crossett, Ark. (now the Crossett Crossett Division of the Georgia-Pacific Corp.), for providing the Research Grant under which this study wria conducted.
&.,
Spectrophotometric Determination of Nitrate with 2,6-XyIenoI Reagent A. M. HARTLEY and I?. 1. ASAll Deporfment o f Chemistry, University o f Illincis, Urbana, 111.
b Nitrate in a solution of sulfuric acid-water-acetic acid reacts with 2,6-xylenol (2,6-dimethylphenol) to produce the corresponding 4-nitro2,6-xylenol. The readion mixture so produced has an absorption maximum at 320-4 m,u with a molar absorptivity of 7900 which has bee1 identified with the 4-nitro-2,6-xylenol b y extraction and comparison with the authentic compound. Under the optimum conditions of: complete removal of chloride and nitrate, solvent composition of 6 : 3 : 1 v./v. sulfuric acid-wateracetic acid (or 4 : 4 : 1 : 1 sulfuric acidphosphoric acid-water-acetic acid) and room temperature, the absorbance at 320-4 mp is a linear function of nitrate added in the range of 2 i o 30 p.p.m. with an over-all relative standard deviaiion of 0.1 to 1.2%. A procedure has been developed from these conditions which requires 7 to 10 minutes per sample determination. The time, precision, and accuracy compare favorably with existing methods.
T
HE determination of nitrate or oxides of nitrogen in trace concentrations is of interest in connection with problems of air and water pollution, sanitation, and allied areas relating to public health (15). Eoltz has summarized the existing methods for the determination of nitrate and/or nitrite in a monograph (4). ldacdonald had discussed the spectrophotometric and electrometric methods in two review articles (17, 18). For the most part these procedures suffer from the disadvantages attendant cn nonstoichiometric reactions, nonlinear working
Present address, Department of Chemistry, University of Nevada, Reno, S ev.
curves, limitations of usable concentration range, and overlong time per determination. The most common spectrophotometric methods involve either nitration of a suitable reagent to form the corresponding nitro compound or oxidation by nitrate to yield, usually, quinones. Kitration of 2,4-xylenol has received considerable attention (1, 5, 19, 26, 29). I n this method the 6-nitro-2,4-xylenol is formed, extracted or steam-distilled, and subsequently measured as the yellow nitro-phenolate in aqueous alkali. Phenoldisulfonic acid, presumably the 2,4-disulfonic acid derivative produced by gently warming a solution of phenol in concentrated sulfuric acid, has been utilized in similar fashion (7, 9, 23-26). Brucine, when treated with a sulfuric acid solution containing nitrate, produces a yellow color which may be made a measure of nitrate (8, 10, 16,20, d2,27, 28). The reaction is exceedingly empirical; color development is dependent on reaction time, nitrate level, and source of the alkaloid. Each of these methods suffers from a common list of difficulties. Kone will distinguish between nitrate and nitrite. Halides are a serious interference although less so for the brucine method. Time per determination is a minimum of 30 minutes in each case. The absorbance-nitrate working curves are, for the most part, nonlinear. I n 1949, Holler and Huch published a method based on the nitration of a xylenol (3,4-dimethylphenol) in sulfuric solution followed by steam distillation of the nitro-xylenol and subsequent visible colorimetric estimation of the yellow nitro-phenolate in aqueous alkali (14). This method depends upon the predominant formation of the ortho-nitroxylenols since these are readily
codistilled with water. For this reason Holler and Huch limited their investigation to the five isomeric xylenols which have at least one open ortho position; the remaining isomer, 2,6dimethylphenol, is obviously unsuitable since both ortho positions are blocked. Recently we have developed a series of methods for the determination of nitrate, nitrite, or both based on the nitration of this isomer ( 2 ) . A brief report containing only a recommended procedure for the determination of nitrate in water has been published elsewhere (12). -4polarographic modification of this method has been reported (13). I n the latter two articles the optimum conditions were offered without comment. Since the effects of temperature, acidity, and the like, can be considerable, a more detailed exposition is offered herein. EXPERIMENTAL
Apparatus. All spectral data were recorded using a Cary Model 14 recording spectrophotometer and matched quartz cells. Unless otherwise indicated, the latter were of 1-cm. path length. Reagents. All reagents were of highest quality obtainable. Sulfuric acid, acetic acid, and phosphoric acid were the concentrated c.P., glacial, and 85% reagent grades, respectively. Eastman White Label (No. 1772) 2,6xylenol was used without further purification for the most part; in the later stages of this investigation oxidative deterioration of the xylenol required recrystallization from aqueous ethyl alcohol until the melting point improved t o the literature value. The method of von Auwers and Markovits (3) was used to prepare 4nitro-2,6-xylenol. The compound was obtained as pale-yellow plates melting VOL. 35, NO. 9, AUGUST 1963
e
1207
Table
1.
Wavelengths of Maximum Absorption (mp) and Molar Absorptivities of 2,6-Xylenol and 4-Nitro-2,6-xylenol in Various Solvents
Solvent composition by volume'
2,GXylenol amx
AUUX
Ether Glacial acetic acid 2+i:5 Absolute ethanol (16) 272.5 4: 5: 1 H&OrH~O-HOAc 268.0 5: 4: 1 H~SOI-H~O-HOAC 267.5 267.0 6 :3: 1 H&3OrHsO-HOAc 4: 4:1: 1 H ~ ~ O ~ - H S O ~ H ~ O - H O A C267.0 a Ratios of mixtures refer to volumes taken.
...
1410 1800 960 751 603 631
4-Nitro-2,6xylenol L U X amx 310.0 10,600 315.0 9 ,340 322.7 11,000
33+:5 338.0 338 5 a
...
8,410 8,710
a t 169.5' to 170" C. from aqueous ethyl alcohol; elemental analysis agreed with the calculated values for C, N, and
linear curve is not influenced by prior treatment to remove either nitrite or chloride.
The Hammett indicator (2,4-dinitroaniline) was Eastman White Label (No. 1843) recrystallized twice from 95% ethyl alcohol and vacuum dried; m.p. 178.5' to 179' C., lit,: 180' to 181.5'
RESULTS
H.
C. All solutions of reagents were determinate preparations by standard volumetric procedures. Dilute solutions were, unless otherwise specified, prepared by aliquot dilution. The acid solvents: sulfuric acid-acetic acid and sulfuric acid-phosphoric acid-acetic acid were prepared by appropriate admixture of the respective components measured by graduated cylinder. For the most part these solvents were prepared in liter batches. The composition ratios cited in the text refer to the volume ratios of respective solvents added and do not pertain to fractions existing in the final solution. The solvent compositions used in reaction mixtures resulted from comixing a sulfuric-acetic acid solution with an acetic acid solution of the xylenol and an aqueous solution of nitrate. The sulfuric acid and phosphoric acid solutions were dispensed from burets equipped with Teflon stopcocks (Fischer & Porter Co.); acetic acid, acetic acid solutions of the xylenol, and the aqueous nitrate solution were delivered by pipet. Procedure. Obtain aqueous samples containing nitrate nitrogen in the range such t h a t accurate final dilution to 2-30 p.p.m. is possible. If chloride is present t o excess (see text) remove by the addition of solid silver sulfate as described by Hartley and Curran (13). Nitrite, an interference a t all levels, may be effectively removed by addition of solid sulfamic acid a t this point. T o a cooled 40-ml. portion of 3:1 sulfuric acid-water (v./v.) add, in order, 10 ml. of the prepared sample (filtered if necessary) and 10 ml. of 0.1M 2,6-xylenol in glacial acetic acid. Adjust the temperature to 25" f 0.3' C. and record the absorbance of the reaction mixture after 5 minutes over the interval 320-30 mp. For accurate results compare the absorbance with that obtained for an aliquot of stock potassium nitrate prepared by determinate weighing of the dried salt. For convenience a working curve may be constructed from measurements of stock nitrate solutions; the resulting 1208
e
ANALYTICAL CHEMISTRY
Spectral Characteristics of 2,6Xylenol and 4-Nitro-2,6-xylenol in Various Solvents. Spectral curves for determinate solutions of 2,6xylenol and 4-nitro-2,6-xylenol in several solvents were obtained. Molar absorptivities and wavelengths of maximum absorption are given in Table I. Solutions in the various acid mixtures were prepared by pipetting 1.00 ml. of 2,6-xylenol or 4-nitro2,6-xylenol standard solution in glacial acetic acid into 50-ml. Erlenmeyer flasks containing 9.0 ml. of an appropriate acid mixture to give final compositions as shown in column 1, Table I. The data for absolute ethyl alcohol solvent are from the work of Burawoy and Chamberlain (6). Although the spectral characteristics in acid solvent differ markedly from those in water or alcohol the change with increased acidity is slight. The ultraviolet spectra for 2,6-xylenol, 4-nitro-2,6-xylenol, and a nitration mixture all in the same solvent, 6:3:1 sulfuric acid-water-acetic acid by volume, are reproduced in Figure 1. All solutions are 10.0 ml. in total volume containing 1.00 ml. of 1.001 X 10-zM 2,6-xylenol, 1.00 ml. of 1.00 X 10-SM 4-nitro-2,6-xylenol, and 1.00 ml. of 14.0-p.p.m. nitrate nitrogen plus 1.OO ml. of 1.001 x lO+M 2,6-xylenol, respectively. QuJitative Effects of Acid Composition. The reaction between nitrate and 2,6-xylenol was first investigated in sulfuric acid-glacial acetic acid mixed solvent. This particular solvent was chosen because a strong acid such as sulfuric acid is required to convert nitrate t o the nitronium ion, NOz+, which is believed to be the nitrating species, and glacial acetic acid is a good solvent for 2,6-xylenol reagent. Nitration in 1 :3 sulfuric acidacetic acid required nearly 90 minutes for attainment of maximum absorbance a t 320 mp. I n 3 : 5 sulfuric acid-acetic acid the maximum absorbance was
Figure 1 , Spectra of ( A ) 1.001 X 1 O-3M 2,6-xylend, (B) 1 .OO X 1 O-'M 4-nitro-2,6-xylenol, and (C)nitration mixture (14.0 pg. nitrate N) in 6 : 3 : 1
HJOrHsO-HOAc reached within 15 minutes, still somewhat slower than desired, and the plot of absorbance a t 320 mp us. nitrate concentration although linear did not pass through the origin. In all these nitrations an absorption maximum was observed at 512 mp which wm not dependent upon the nitrate concentration. The plot of absorbance us. nitrate concentration did not pass through the origin because of the deterioration of 2,6-xylenol reagent in this medium to a species having an absorption maximum a t 345-50 mp which contributes to absorbance measurements a t 320 mp. Since all the spectra had been recorded against the mixed solvent containing no 2,6-xylenol reagent, a positive error was introduced in the foregoing measurements, The blank in 1:3 sulfuric acidacetic acid (v./v.) showed no change in absorbance at 345 mp over a 6-hour period, whereas in 3:5 sulfuric acidacetic acid considerable deterioration occurred within minutes as indicated by increases in absorbance a t this mavelength with time. The species responsible for this absorption a t 345 mp is not affected by dilution with glacial acetic acid, but additions of small amounts of water cause the gradual disappearance of this absorption with a corresponding reappearance of the 2,6-xylenol absorption maximum a t 268 mp. Sulfuric acid-acetic acid mixtures as nitration media therefore, are unsatisfactory. If the sulfuric acid to acetic acid ratio is increased such that the time for completion of the nitration reaction is short enough to be practical, the 2,6-xylenol deterioration product absorption a t 345 mp seriously interferes with the measurement of the nitration product absorbance a t 320 mp. If the ratio is decreased such that reagent deterioration is absent, the nitration reaction becomes too slow.
for nitrations (sulfuricacid-phosphoric 7 1
I
, 021-
II
i
Figure 2. Absorbance at 322 mp vs. time for nitration i i sulfuric acidwater-acetic acid mixtures 0-5:4:1,
29: l6:5,
.-27:18:5, @--6:3:
and
8-28:17:5, C1 by volume. Dotted
line represents expected obsorbonce for 100% yield
acid-acetic acid, sulfuric acid-phosphoric acid-water-acetic acid, and sulfuric acid-water-acetic acid) the sulfuric acid concentration was increased at the expense of phosphoric acid in the first two systems, or water in the third system, a point mas reached where the rcagcnt deterioration became a serious interference. Sulfwic acid-phosphoric acid-acetic acid (7 :11:2), sulfuric acidphosphoric acid-water-acetic acid (4:4 : 1 : I), and sulfuric acid-water-acetic acid (6:3:1) mixtures by volume all pro.r-ed to be satisfactory nitrating media. I n these mixtures the nitration reaction was complete within 5 minutes, and the interference from reagent deterioration was either absent or required very slight blank corrections. Since & h i l a r nitration reaction behavior was found for these solvents of widely different composition, the similarities must lie in the intrinqic acidities of the solvents. This may be memired bv the IIaniniett acidity function (U0): (11) log
Sitration in phosphoric acid-acetic acid mixture is very slow if i t occurs at all; no change in the ultraviolet spectrum was observed over a 25minute period. Acid :systems found to be satisfactory as nitrating media were: sulfuric acid-phosphoric acidacetic acid, sulfuric acid-phosphoric acid-water-acetic acil, and sulfuric acid-acetic acid-water mixtures. The latter two are sufficiently similar t h a t results given in Table I1 in the quaternary solvent are identical to those obtained in t'he ternary mixture otherwise iised for simplicitj,. Effect of Acidity. I n t h e systems mentioned i n t h e previous paragraph a s satisfactory nitrating media i t was observed t h a t varyiiig t h e sulfuric acid concentration affected both t h e nitration rate and t h e deterioration of t h e 2,6-xylenol reagent. This effect of acidity was determined b y preparing five nitration niixbures in various sulfuric acid-acetic acid-water mixtures. T h e sulfuric acid t o water ratios were varied at a constant acetic acid concentration of 10% by volume so that final composiiions of sulfuric acid : water : acetic acid were 5:4:1, 27:18:5, 28:17:5, 29: L6:5, and 6 : 3 : 1 Iy volume. The ab-orbances at 322-4 mp, the wai-elengthof maximum absorption for nitration mixtures in these solvents, were recorded as a function of time against the coi~espondingacid mixtures as referencec. The absorbance ZIS. time ['lots shi8wn in Figure 2 are typical C J ~the beiavior observed in the two other acid sy,stems previously r i i t ~ r i t i o r i c ~ l :is s:ttid~:lory iiikttiiig Incdia. If, in the three sj-stcmis satisfactory
CRH+ - CB
Thc acidities of these mixtures wrre measured by the usual spectrophotometric techniques using 2.4-dinitroaniline as the indicator baqe Since onlv relati1 e values of acidity were deqired for the different acid mittures, no attempt was made to control the temperaturc or to correct for the 0 25 ml of concentrated sulfuric acid added as the indicator solvent. I n the sulfuric acid-phosphoric acid-water-acetic acid s-stem the sulfuric acid to phoqphoric acid ratio way varied while maintaining the water and acetic acid concentrations constant a t 10% each by volume. I n the sulfuric acid-water-acetic acid system the sulfuric acid to water ratio mas varied a t a constant acetic arid concentration of 1Oyoby volume. Plot. of -Ifo tis. sulfuric acid concentration b r volume are found in Figure 3. The values of --Ho were calculated using the relation:
n here AX is the experimental absorbance of the indicator solution; A d and .I R I I + are the respective absorbances of identical concentrations of the indicator when the acidities were such as to convert the indicator completely to either the base (13) or conjugate acid form (BH+). This approximation is sati5factory provided the indicator concrntration i+ snfficiently low that the lkrr's law IS obeyrcl aiid appropi iate correction for solvent abborbance IS lll:ld(s. \ :LlLlk' ( J f ]>T
