Determination of Nitrate or Nitrate Plus Nitrite with 1-Aminopyrene Application to Air Pollution EUGENE SAWICKI, HENRY JOHNSON, and T. W. STANLEY Division o f Air Pollution, Robert A. Tuft Sanitary Engineering Center, Cincinnati 26, Ohio
b A new procedure i s introduced for the determination of nitrate or nitrate plus nitrite. The two ions can be determined together since they produce similar spectra and concentration-absorbance curves. These ions can be determined in the presence of up to 50 equivalents of bisulfite. The new nitrate procedure, which utilizes 1-aminopyrene, i s compared with the 2,4-xylenol, 2,6-xylenol, phenol-2,4disulfonic acid, brucine, and chromotropic acid procedures. With the use of sdfamic acid, nitrate can be determined in the presence of 6 pg. of nitrite ion. The advantages of the 1 -aminopyrene procedure are that it i s simple and direct and i s more sensitive than any method in the literature. Amounts ranging from 0.1 4 (A = 0.1) to 1.6 pg. of nitrate nitrogen per milliliter of aqueous solution can be determined. The l-arninopyrene procedure has been applied to the determination of soluble nitrates in airborne particulates. Nitrogen dioxide could be determined as nitrate plus nitrite with the new method.
S
widely used reagents for the determination of nitrate are phen01-2~4disulfonic acid (8,9, I S ) , 2,4xylenol ( I , 11), 2,6-xylenol (4, 6, IO), brucine (2, S), and chromotropic acid (la,14). In the determination of nitrate with phenol-2,4-disulfonic acid or 2,4-xylenol, nitration takes place a t the &position. The resultant o-nitrophenolic chromogen has a low molar absorptivity-e.g., 6-nitro-2,4xylenol in aqueous alkali has a molar absorptivity of 4500 a t a wavelength maximum of 446 mp (7). Consequently. direct analysis by these methods is not possible with mixtures containing low concentrations of nitrate. Another disadvantage of the phenol-2,4disulfonic acid method is that the test solution is evaporated and treatment of the resulting residue with strong sulfuric acid can cause charring of the organic material (I). In addition, chloride and nitrite (I), and ammonium ion ( 8 ) interfere in the determination. A disadvantage of the 2,4xylenol OME OF THE MORE
1934
ANALYTICAL CHEMISTRY
method is that the o-nitrophenolic chromogen must be steam-distilled or extracted with organic solvents to be concentrated and separated from interferences (1). The interferences in this method are chloride and nitrite ions and oxidizing agents. A direct determination is possible with the 2,6-xylenol method since the p nitrophenolic chromogen has a higher molar absorptivity than the o-nitrophenolic type. Interferences in the procedure are nitrite and chloride (4, 6) and periodate, persulfate, hydrogen peroxide, and ferrous ion (IO). I n the Montgomery and Dymock procedure, excess chloride is used to increase the molar absorptivity obtained in the analysis (IO). The 2,6-xylenol method has been slightly modified and used also in the determination of nitrate and nitrite, separately or together, by analysis a t wavelengths 324 and 307 mp using linear simultaneous equations to determine each component (5). The brucine methods have been reported as giving extremely erratic results, although with rigorously controlled conditions and a simultaneous determination of standards reasonably good results are obtained (8, 3). Nitrite reacts about &s well as nitrate. All strong oxidizing and reducing agents are interferences (2, 8). Chromotropic acid in sulfuric acid has been used for the direct determination of nitrate (IC, 14). The main reported interferences are bromate, bromide, chlorate, iron(III), molybdate, titanium(III), titanium(IV), thiocyanate, and tetraborate ions. Kitrite reacts like nitrate but very much more weakly. The procedure introduced for the determination of nitrate with l-aminopyrene will be compared with these literature methods. EXPERIMENTAL
Reagents and Apparatus. I-Amino-
pyrene (K and K Laboratories, Inc., Jamaica, X. Y.)was crystallized several times from ethylcyclohexane t o a constant melting point of 121122O C. (corrected). 1-hrninopyrene reagent solution contained 0.01 gram of 1-aminopyrene in
100 ml. of concentrated sulfuric acid (sp. gr. 1.84). The reagent was made up fresh each dav; " , it was stable for a t least 24 hours. A Carv Model 14 recording suectrophotometer (cells of 1-em. p a b iength, 3-ml. volume) vas used in the spectrophotometric work. Procedure. One milliliter of aqueous test solution is added slowly to 3 ml. of the cold reagent solution while i t is shaken in an ice water bath. The mixture is heated for 15 minutes a t 100' C. It is cooled under the tap, and a reading is taken a t 456 mp against the blank. The color is stable for a t least 1 hour. Nitrate and nitrite react to about the same extent. For the determina,tion of nitrate in the presence of a t most 6 pg. of nitrite per milliliter of test solution, the test solution should be pretreated by the addition of enough sulfamic acid to make a 0.025% solution and allowed to stand for 30 minutes. This procedure will destroy the nitrite without affecting the analyses. DISCUSSION
Investigation of the variables in the recommended procedure disclosed that optimum results were obtained with 0.01 to 0.04% (w./v.) reagent. The higher concentrations of reagent, >0.03%, gave optimum results after 10 hours of standing. For best results the 0.01% reagent should be made and used the same day; it is stable a t least 24 hours. A change in the order of the addition has only a small effect on the absorbance. Fifteen minutes of heating on the m-ater bath was found to give optimum absorbances. The color intensity was stable for at least 1 hour. Although Beer's law was not obeyed (Figure I), there is a linear relationship between absorbance and concentration from 0.3 to 1.6 pg. of nitrate nitrogen. The slope of the calibration curve decreased with a lower concentration of nitrate. Sit,rite gave a closely similar curve. The absorption spectrum obtained in the determination of nitrate or nitrite is very distinctive, Figure 2. hlixtures of nitrate and nitrite can be readily analyzed by the procedure (Table I). A relative standard deviation of 4Q/, w m obtained in the
I
1
pg NITRATE NITROGEN
Figure 1. Concentration-absorbance curve obtained in the determination of nitrate by the recommended procedure determination of nitr3te. The 11 determinations ranged in absorbance from 0.67 to 0.75. With lower concentrations of nitrate the relative standard deviation increased (Table 11). Various other reagents were tried in the procedure-e.g., plienarsazonic acid, acridone, 1-hydroxy-:!-naphthoic acid, Table 1. Analysis of Mixtures of Nitrate and Nitrite for Total Nitrogen Nitrogen added, pg.Nitrate Nitrite Total Founda 0.59 0.040 O.fI30 0.63 f 0.00 0.052 O.fi32 0.64 =t 0.01 0.58 0.56 0.084 O.fi44 0.60 f 0.01 0.31 0.41 0.720 0.68 =t 0.00 0.062 0.039 0.030
0.75 0.78 0.80
0.1112 0 . 7 6 i n.02 .
o.si9 0.82 i 0.01
0.830 0.82 i 0.03 Duplicate determinations.
3-hydroxy-Znaphthoic acid, pyrene, 1hydroxypyrene, 1-anthramine, 2-anthramine, I-naphthol, Ai-trifluoroacetyl 1:trninopyrene, and rtzulene. The last reagent gave bands at 389 and 665 mfi with molar absorptivities of approximately 11,000 and 3300, respectively. The same procedure was used except for 20 minutes of heat and 0.06~' reagent. None of these reagents approached the sensitivity of 1-aminopyrene. When i t is desirable to determine nitrate and nitrite in the presence of suEte, the nitrate and nitrite must be collected or extracted into an alkaline solution-e.g., 0.1N aqueous sodium hydroxide solution. When this procedure is used, the presence of sulfite in proportions ~ 1 high s as 50 to 1 (sulfite to nitrate plus nitrite) will not interfere in the determination of nitrate plus nitrite nitrogen, The main interferences in the determination of nitrate are nitrite and ferric ion. In the analytical procedure 56 fig. of ferric ion gives a molar absorptivity of 4000. Since water extracts only minute traces of this metal from airborne particulates, ferric iron is not a serious interference in the analysis of airborne particulates for nitrate. Comparison of Methods. The various methods for the determination of nitrate are compared in Table 11. Because of their high dilution factors, the 2,6-xylenol methods are the least sensitive. With the concentration effect of extraction procedures, their sensitivities could be considerably increased. The 1-aminopyrene method is the most sensitive. The sensitivity is equivalent to the absorbance mole-' per ml. per cm.-l multiplied by the fraction of test solution volume in the final analyzed volume. The greater the sensitivity the larger the absorbance per unit weight of nitrate ion. The hrucine and 1-aminopyrene procedures had the lowest determination limits in terms of pg. per ml.-l test solution, although the
1.5
I .o
A 0.5
1
I
450
500
I 550
X,mJJ
Figure 2. Visible absorption spectrum obtained in the determination of 1 pg. of nitrate nitrogen 1-aminopyrene procedure could be used for the analysis of much smaller volumes of test solution. The 2,6-xylenol1 brucine, and 1aminopyrene procedures are reasonably precise. The color stabilities obtained in the 2,6-xylenol, chromotropic acid, and 1-aminopyrene procedures appear to be adequate. The 2,4-xylenol and 2,6xylenol procedures obey Beer's law. The brucine procedures have been reported to obey Beer's law over a narrow range (2) and in other cases not to obey it (3). The chromotropic acid and 1aminopyrene procedures do not obey Beer's law. The most complicated procedures for the determination of nitrate are the phenol-2,4-disulfonic acid and 2,4-xylenol procedures. The simplest is the 2,6-xylenol procedure of Hartley and Asai (6). Nitrite reacts relatively weakly in the chromotropic acid pro-
Table II. Comparison of Methods for the Determination of Nitrite Dilution factor
net. limit,b fig. Total M1.-*
Color
stability,
Proc.
time,
Reagent , , ,A E X 10-3 Rel. std. dev. Sema hours nun. Ref. 2,4-Xylenol (1) 446 3.3c 2 1.65 4.2 0.84 90 2,6-Xylenol 324 7.9 0 . 1 t'0'2.17~ 10 0.79 1 .s 1.8 >i'&in. 8 (4' 304 17' 2,6-Xylenol ( 1 06) , ... 25 0.68 2.1 2.1 2 45 Brucine (2) 410 20 2 2 10 2.1 0.14 ... 30 Chromotropic acid (12,14) 357 43f ... 3.33 12.9 -0.74 -0.25 >96 -2.5 1-Aminopyrene 466 948 4h 4 23.5 0.14 0.14 >1 20 x 10-3. Sens. = dil. factor 6 Total micrograms of nitrate nitrogen in test solution (and micrograms of nitrate nitrogen per milliliter of test soliition) giving &I)sorbance of 0.1 in 1-cm. cell. For pure 6-nitro-2,4-xylenol Amax 446, e 4500 in aqueous alkali. Varying with concentration. * W'ith N02--Xaux 304, 6 16,000. 4-Sitroso-2,6-xylenoI has, a, A 304, E 20,400 (10). f With 3 pg. NOa-. 9 With 6.2 pg. NOa-. * Based on 11 determinations with 6.2 pg. of NOa-. With 3.1 and 1.2 p g . of YO3- relative standard deviations of 6 and 10 were ohtained, respectively. ~
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VOL. 35, NO. 12, NOVEMBER 1963
1935
Table 111. Analysis of Aqueous Extracts of Airborne Particulate Samples
Nitrate/ml. ____.Extract 1-Aminopyrene 2,4-Xylenola 4 . 3 i 0.16 3.8 11.0 i 0 . 2 9.4 10.3 i 0.1 9.8 - 8 . 9 0:i 8.3 7.6 f 0.1 8.1 1.2 f 0 . 1 1.5 1 . 8 0.1 2.0 4.3 f 0.1 4.8 2.6 f 0 . 0 2.5 a These results were taken from a series of routine analyses obtained with the help of a Beckman DU spectrophotometer by n group under the direction of Norman Huey of the Air Quality Network at the Robert A. Taft Sanitary Engineering Center. Duplicate determinations. pg. ______.
cedure as compared to its reaction in the brucine, xylenol, and aminopyrene procedures. Formaldehyde is an interference. The main difficulty in the chromotropic acid procedure is associated with the interference of the blank. The chromotropic acid in the blank has strong bands (with decreasing intensity) at 347, 331, and 313 mp, while the band at which analysis takes place is a t 357 mu.
Application. Aqueous extracts of airborne particulates were analyzed by the 1-aminopyrene procedure and the 2,4xylenol method ( I ) , Table 111. The latter method is the one used routinely in the local laboratories. The comparative procedure times of the two methods emphasize the simplicity of the 1-aminopyrene procedure compared to the 2,4xylenol method. The xylenol method involves about 17 steps consisting of a 30-minute heating period, two liquid-liquid extractions, and a filtration, all of which tend t o make the method much more tedious and prone to error in routine analysis than the 1aminopyrene procedure. Consequently, the latter procedure is preferred. Since the aqueous extracts of the airborne particulates did not contain the nitrite ion, the step involving destruction of nitrite was not necessary. Although airborne particulate samples contain a fairly large amount of iron, the aqueous extracts contained only minute traces and therefore ferric salts were not a serious interference in the analyses. The 1-aminopyrene procedure is especially recommended where a simple, sensitive, and direct method is needed for the determination of nitrate in mixtures containing little or no nitrite and ferric salts. It should also be useful in
Determination of Phenolic Substances UItravioIet Difference Spectrometry
the determination of total nitrate and The nitrite nitrogen in mixtures. method does not have the Beer's law relationship or the reproducibility of the 2,6xylenol method (4-6)but does have a much greater sensitivity. LITERATURE CITED
( 1 ) Barnes, H., Analyst 75, 388 (1950). (2) Fisher, F. L., Ibert, E. R., Beckman, H. T., ANAL.CHEM.30, 1972 (1955).
(3) Greenberg, A. E., Rossum, J. R., Moskowitz, N., Villaruz, P. A , , J . Am. Wufer Works Assoc. 5 0 , 821 (1958). (4) Hartley, A. M., Asai, It. I., ANAL. CHEM.35, 1207 (1963). ( 5 ) Ibid., p. 1214. (6) Hartley, A. M., Asai, R. I., J . Am.
Water Works Assoc. 52, 255 (1960). (7) Holler, A. C., Huggett, C., Rathmann, F. H., J. Am. Chem. SOC.72, 2034 (1950). (8) Hora, F. B., Webber, P. J., Analyst 85, 567 (1960). (9) Johnson, C. M., Ulrich, A., ANAL. CHEM.22, 1526 (1950). (10) Montgomery, H. A. C., Dymoclr, J. F., Analyst 87,374 (1962). (11) Swain, J. C., Chem. Ind. Lmdon 1957, 470. (12) Swinehart, B. A., Brandt, W. W., Proc. Indiana Acad. Sn'. 63, 133 (1953). (13) Taras, M. J., ANAL.CHEM.22, 1020 (1950). (14) West, P. W., Lyles, G. L., Anal. Chim. Acta 23, 227 (1960).
RECEIVEDfor review June 13, 1963. Accepted August 16, 1963.
bY
ARTHUR S. WEXLER Dewey and Almy Chemical Division, W.
b The selective determination of phenols in the presence of nonionizing interfering substances by ultraviolet difference spectroscopy is discussed. The difference spectrum of the alkaline form of a phenolic substance in water or alcohol is recorded directly in an ultraviolet recording spectrophotometer against an identical concentration of the substance in neutral or slightly acidified sofvent. The resulting difference spectrum is a characteristic and useful indication of the concentration and chemical identity of the phenolic substance. Possible interferences due to nonionizing, nonphenolic species are usually canceled out in the difference spectrum. Applications to the polymer field of analysis are discussed.
T
CHANGE in the ultraviolet spectriim of phenolic materials as a function of basicity ha.: been the basis of determinations of the phenolic content of gasolines by Murray ( Y ) , waste water HE
1936
ANALYTICAL CHEMISTRY
R. Grace and Co.,
Cambridge 40, Mass.
by Schmauch and Grubb (8), rubber by Wadelin (IO), surfactants by Smullin and Wetterau (9), and food products by Englis and Wollerman (4). The basis of these determinations is the well known bathochromic shift of the long wavelength maximum of phenols in alkaline solutions due to the formation of phenolates as shown by Coggeshall and Glassner (a) and by Doub and Vandenbelt (3). The differenre spectrum obtained by subtraction of the absorbance of the neutral solution of phenolic type materials from the absorbance of the alkaline solution has been utilized in the study of types of lignin and lignosulfonates by Aulin-Erdtman (1). Goldschmid (5) and Maranville and Goldschmid (6) developed difference spectra methods for the analysis of lignosulfonates and of tannins (polyphenols). I n this paper the identifications and quantitative estimation of some phenols by mwns of their ultrnviolet difference sj)~(+r:i me re1iortmI. While the method
was developed for the rapid and direct estimation of phenolic antioxidants in synthetic rubber latex and in rubber products, it is obvious that the reported procedure is applicable to a wide variety of materials and problems involving the detection and identification of phenolic materials. EXPERIMENTAL
Apparatus. All ultraviolet determinations were made on a Beckman DK-2 recording spectrophotometer. Absorbances and wavelengths were occasionally checked on a Beckman D U spectrophotometer. Both instruments were calibrated for wavelength with a mercury source and a holmium oxide standard. Matched 1-cm. and 0.2-rm. silica cells were used. Materials. Commercial and reagent grade phenols were used without purification. Rragent grade potassium hydrolide and reagent grade methanol were used. Procedure. One per cent stock solutions of phenols ill methnnol were
