Spectrophotometric Determination of Atmospheric Sulfur Dioxide F. P. Scaringelli, B. E. Saltzrnan, and S. A. Frey National Center for Air Pollution Control, Bureau of Disease Prevention and Environmental Control, Public Health Service, U.S. Department of Health, Education, and Welfare, Cincinnati, Ohio 45226 Two improved pararosaniline methods developed for the spectrophotometric determination of sulfur dioxide in ambient air yield greater sensitivity, greater reproducibility, and adherence to Beer’s law throughout a greater working range than the West and Gaeke method. These improvements resulted from optimization of the important parameters. The pararosaniline dye was specially purified and standardized to reduce variability problems. Phosphoric acid was used in the final color development to control pH, to liberate sulfur dioxide from its mercury complex, and to complex heavy metals. The pararosaniline methyl sulfonic acid produced in the reaction exhibited a hypsochromic spectral shift and behaved as a twocolor pH indicator, the color changing from magenta (peak X 575 mp) to red (peak X 548 mp) as the acidity IS decreased. Interferences from oxides of nitrogen, ozone, and heavy metals were minimized. Results were reproducible to within 4.9% (at 95% confidence level) if the analyses were done carefully, with close attention to temperature, pH, purity of the dye and water, and standardization of the sulfite solution. The methods have been tested on air samples from major cities in the United States and have been used routinely for almost a year.
IN1942, STEIGMANN (1) modified the Schiff reaction to produce a sensitive, highly specific test for bisulfite. Acid-bleached fuchsin was reacted with formaldehyde and bisulfite ion to form an intense violet color. This reaction was applied to analysis of air for sulfur dioxide by Urone et al. (2). Early erratic results with this method were attributed to varying impurities of different lots of the fuchsin dye, which was a mixture of pararosaniline and rosaniline. The substitution of pararosaniline, as proposed by West and Gaeke (3), increased the sensitivity but did not eliminate the irregularities. The concentration and purity of the dye still vary in different batches and in batches obtained from different suppliers. For the determination of trace quantities of sulfur dioxide in air, West and Gaeke suggested the use of an absorbing solution of 0.1M sodium tetrachloromercurate, which stabilized the sulfur dioxide from air oxidation by the formation of dichlorosulfitomercurate. The stability of this compound to air oxidation was reported by Ephraims (4). Although the West and Gaeke method has been widely used to determine atmospheric concentrations of sulfur dioxide, the method has been criticized for its lack of reproducibility and of reliability. Interferences associated with the present methods for the determination of sulfur dioxide have been reviewed by Thomas (5) and by Mueller et al. (6). Several (1) A. Steigmann, J . SOC.Chem. Ind., 61, 18 (1942); ANAL.CHEM., 22,492 (1950). (2) P. Urone, W. E. Boggs, and C. M. Noyes, ANAL.CHEM., 23, 1517 (1951). (3) P. W. West and G. C. Gaeke, Ibid., 28, 1916 (1956). (4) F. Ephraims, “Inorganic Chemistry,” 5th ed., P. C. L. Thorne and E. R. Roberts, Eds., 5th ed., Interscience, New York, 1948, p. 562. ( 5 ) M. D. Thomas, J. Air Pollution Control Assoc., 14, 517 (1964). (6) P. K. Mueller, F. P. Terraglio, and Y . Tokiwa, “Chemical Interferences in Continuous Air Analyses,” 7th Conference on Methods in Air Pollution Studies, Union Oil Center, Los Angeles, Calif., January 1965.
investigators have suggested modifications to eliminate interferences or increase reliability (7-12). It was decided that an investigation of important parameters and of possible modifications would be of value. Therefore, an intensive analytical study was undertaken to discover and precisely control any critical factors contributing to the variability of results. Our approach conformed, in essence, to the classical procedure of developing spectrophotometric methods. An excellent critical review of the usual procedure was recently given by Kirkbright (13). The results obtained of studies were used to develop two improved methods for the determination of sulfur dioxide, The pararosaniline dye is specially purified and standardized so that the variability problems associated with this reagent are eliminated. Phosphoric acid is used to control final pH, to aid in liberating sulfur dioxide from its mercury complex, and to eliminate interferences from heavy metals. Larger volumes of reagents at lower concentrations than used in the West and Gaeke procedure are delivered to a 25-ml volumetric flask to increase accuracy. The interference of nitrogen dioxide is eliminated by the addition of sulfamic acid before the addition of the chromogenic reagents as suggested by Pate et al. (14). These and other modifications result in greater sensitivity, greater reproducibility, more complete reaction, and adherence to Beer’s law throughout a greater working range (0 to 35 pg SOz). In Method A, final color is developed at near optimal pH 1.6 f 0.1 and read at 548 mp. Alternative Method B is also proposed for those who prefer to work with low blanks; however, this method yields lower sensitivity. In Method B, color is developed at pH 1.2 i 0.1 and read at 575 mp. These wavelengths are the true maxima; they differ because the reaction product exhibits a hypsochromic spectral shift and behaves as a typical two-color pH indicator; the respective colors are magenta (Method B, pH 1.2) and rose-red (Method A, pH 1.6). The improved methods have been checked with samples collected from 25 major cities in the United States. Other laboratories currently using the methods also have reported favorable results. Standard deviation at the 95 % confidence level is 4.9 % for both methods. EXPERIMENTAL
Apparatus.
Irradiation chambers-large
metal boxes, 30
X 46 X 48 inches equipped with air conditioners to control
temperatures at 20°C. Chamber A-WestiGghouse Sun Lamps FS 40, E maximum 3130 A, cut-off 2950 A . Shamber B - G . E. black lights, F42T6BL, E maximum 3500 A, cut-off
(7) L. Acs and S. Barabas, ANAL.CHEM., 36, 1825 (1964). (8) S. Barabas and J. Kaminski, Ibid.,35, 1702 (1963). (9) K. E. Burke and C. M. Davis, Ibid.,34,1747 (1962). (10) H. A. Huitt and J. P. Lodge, Jr., Ibid., 36, 1305 (1964). (11) J. B. Pate, J. P. Lodge, Jr., and A. F. Wartburg, Ibid., 34, 1660 (1962). (12) F. P. Terraglio and R. M. Manganelli, Ibid., 34, 675 (1962). (13) G. F. Kirkbright, Talanta, 13, l(1966). (14) J. B. Pate, B. E. Ammons, G. A. Swanson, and J. P. Lodge, Jr., ANAL.CHEM., 37,942 (1965). VOL. 39, NO. 14, DECEMBER 1967 e
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Table I. Comparative Collections of Various Strengths of Tetrachloromercurate Total SOI collected. ue Polluted air, Laboratory downtown Cincinnati Concentration mixture of TCM, M Expt. 1 Expt. 3 Expt. 2 5.20 8.55 0.1 18.2 4.91 8.77 0.1 17.1 0.04 17.3 4.30 8.40 4.38 0.04 0.01 17.1 4.13 9.75 0.01 10.34 17.5
Table 11. Losses of Sulfite-Tetrachloromercurate Complex by Aeration (0.19 lpm for 24 hours) Molarity Absorbances of TCM Aerated Nonaerated Loss, SODIUMTETRACHLOROMERCURATE 0.00 0.342 0.707 53.0 0.591 0.589 0.01 0.0 0.605 0.607 0.02 0.4 0.597 0.608 1.9 0.04 0.587 0.591 0.10 0.7 POTASSIUM TETRACHLOROMERCURATE 0.00 0.132 0.214 58.0 0.01 0.658 0.669 1.6 0.665 0.668 0.02 0.5 0.659 0.668 0.04 1.5 0.639 0.639 0.0 0.10
--I
0.4
0
002
004
006 TCM, m o l a r i t y
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01
Figure 1. Absorbance a t various concentrations of tetrachloromercurate for three concentrations of SOz (,4ml) -0- 0.338 --A0.672 -00.767
Simulated sample solutions containing sulfur dioxide with varied amounts of TCM (0 to 0.1M) were prepared. Ten milliliters of each sample was transferred to a series of midget impingers in a National Air Sampling Network (NASN) gas sampler apparatus. Air purified with activated carbon was passed through each impinger at the rate of 0.19 liters/min for 24 hours. Results are shown in Table IT. No significant 3150 A. Other apparatus described in Recommended losses were detected in samples containing as little as 0.01.44 Procedures. TCM, whereas the sample without TCM lost 53% of the Reagents. Described in Recommended Procedures. original SOz. This experiment was repeated with potassium Procedure. The procedure as outlined by West-Gaeke substituted for sodium. N o losses were detected in the sam(3) was used with the following modifications: all determinaples that contained TCM, whereas the sample without TCM tions were made in a 25-ml volumetric flask to allow the adlost 58 % of the sulfur dioxide. This experiment was then redition of varied amounts of reagents and to maintain conpeated with samples containing as little as 0.001M TCM. No stant volume; the quantities of reagents were doubled so that concentrations approximated those specified in the appreciable loss of SOz was detected. Apparently the conmethod. Beginning with this method, the new procedure centration of TCM can be reduced 10 to 100 times without was evolved as the parameters were optimized. any significant effect, However, increasing ionization of the TCM in dilute solution reduces the pH and hence, reduces colRESULTS AND DISCUSSION lection efficiency. STABILITY OF THE TETRACHLOROMERCURATE COMPLEXOF Studies of Sampling Reagent. TCM CONCENTRATION. SULFUR DIOXIDE TO LIGHT. Solutions of various concentraConcentrations of TCM higher than 0.04M suppress color tions of snlfur dioxide (as sodium metabisulfite) in 0.1M formation. This was demonstrated by four experiments persodium tetrachloromercurate were prepared and duplicate formed with different concentrations of SOz and varying portions placed in all-glass midget impingers. One set was amounts of TCM. The results are plotted in Figure 1. kept in a dark cupboard, while the other was exposed to artiThe possibility of reducing the commonly used 0.1M conficial light in ChamberoA (Westinghouse Sun Lamps, max. centration of TCM was investigated because reduction would 3130 A, cut-off 2950 A). Light intensity in Chamber A is not only increase sensitivity, but also reduce both the toxic approximately ten times the intensity of sunlight; however, hazard and the cost per sample. Collection efficiency and exposure to sunlight can easily exceed 30 minutes during stability of the tetrachloromercurate-sulfur dioxide complex sampling and handling. After 30 minutes, aliquots of each were examined. solution were analyzed for sulfur dioxide by the West-Gaeke Parallel experiments on collection efficiency were performed method. Concentrations of sulfur dioxide in the exposed in the laboratory and in the field (Cincinnati) with 10-ml solutions were 30 to 4 0 z lower than in the unexposed soluportions of 0.01, 0.04, and 0.1M potassium tetrachloromertions, The experiment was repeated under several types of curate. The data, shown in Table I, indicated no appreciable light sources: after 30 minutes' exposure in Chamber A at differences in collection efficiencies of the various strengths 20" C, losses were 50 to 75%; after 30 minutes' exposure to of solution. Sodium or potassium chloride can be used. light more representative of sunlight (Chamber B, max. 3500 The potassium salt is usually obtained much purer than the A, cut-off 3150 A), losses were 2 to 8%; after 30 minutes' sodium salt; hence, less interference occurs.
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exposure to bright sunlight, losses were 4 to 5 %. The potassium tetrachloromercurate indicated a slightly better stability to light, again because of fewer impurities. Although exposure for a few hours to usual indoor lighting does not cause significant losses, collected samples should be shielded from direct sunlight. A dark box is convenient for sampling out-of-doors. STABILITY OF DICHLOROSULFITOMERCURATE WITH TIME. Liter quantities of tetrachloromercurate solutions (0.01, 0.04, and 0.1M) containing sulfur dioxide (as Na2SOs) at three concentration levels were prepared. One half of each solution was placed in a refrigerator (5' C), and the remaining portion was placed in a constant-temperature bath (25' C). Periodically, over 30 days, 10-ml aliquots were withdrawn and analyzed, Freshly prepared standards in 0.1, 0.04, and 0.01M TCM were used as controls for each run. Typical results (Figure 2) indicated that the rates of sulfur dioxide losses were inversely proportional to the concentrations of TCM and varied from 1.0% to 3.2% per day. The radical deviation from the straight line that occurred in the analyses on the second day was traced to the poor quality of the distilled water. Double-distilled water was used in all subsequent analyses. Stabilities were slightly lower for higher concentrations of sulfur dioxide. Different pH values were obtained for each TCM solution (O.lM, pH 5.70; 0.04M, pH 5.40; 0.01M,pH 5.10). These differences in pH values indicated greater ionization of the more dilute TCM solutions, and these lower pH conditions may account for the greater losses of sulfur dioxide observed. The ionization of TCM can be suppressed by increasing the molar concentration of the chloride ion. Preparation of Dye Reagent. IMPURITIES IN PARAROSANILINE. Because concentration and purity of pararosaniline vary in different batches, Pate et al. (11) recommended a method of spectral assay of the dye before use. All lots of dye that did not exhibit maximum absorbances at 540 mp
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Figure 2. Stability of dichlorosulfitomercurate solutions with time -A-0-
-c--
0JM 0.04M O.OIM
were to be rejected. This method of assay, however, was not sufficiently sensitive to an undesirable violet impurity detectable by paper chromatography. By use of a band technique with paper chromatography, we separated quantities of both components. The spectral
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Figure 3. Absorption spectrum of violet impurity in 1-butanol, impurity separated from commercial pararosaniline VOL. 39, NO. 14, DECEMBER 1967
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Table 111. Distribution of Impurities in Organic Phases by Countercurrent Extraction Procedure Absorbance at 560 mu _____ Separatory Separatory Separatory Separatory Dye source funnel 1 funnel 2 funnel 3 funnel 4 Pararosaniline Fisher P389 0.333 0.086 0.059 0.059 Hartman-Leddon 224 5.48 0.538 0.209 0.191 CoIeman & Bell B378 0.114 0.148 0.598 0.935 Fuchsine Hartman-Leddon 163 0.241 0.086 0.074 0.074 Matheson-Coleman Bell B300 0.719 0.667 0.517 0.493 Coleman & Bell B135 6.68 0.975 0.609 0.609 Diflo Lab DF-17 4.09 0.521 0.325 0.299
Table IV. Blank Absorbances. with Pararosaniline Reagents Prepared by Three Methods Purified by extraction Solution in waterb % of nominal % of nominal Absorbance concentration Absorbance concentration Dye source Pararosanilined Fisher P 389 0.039 95 0.025 63 87 0.094 56 Hartman-Leddon 224 0.044 75 0.087 62 0.042 Coleman & Bell B378 Fuchsinee Hartman-Leddon 163 0.043 99 0.033 63 Matheson-Coleman Bell B300 0.053 65 0.101 64 CoIeman & Bell B135 0.057 66 0.109 68 Diflo Lab DF-17 0.039 64 0.088 70 a At 560 mp, pH 1.2 f 0.1, nominal dye reagent concentration 0.04%. 6 West-Gaeke (3). Concentration was 100% of nominal. d C. I. Basic Red 9. a C. I. Basic Violet 14 (mixture).
Solution in methano1,c absorbance 0.053 0.076 0.177 0.045 0.159 0.156 0.119
0
curves (ultraviolet and visible) for pararosaniline were similar to those reported by Nauman et al. (15). The visible spectrum of the violet impurity is shown in Figure 3, which clearly indicates that the impurity contributes significantly to the blank values in the West-Gaeke procedure. A simple (Cstage) countercurrent technique using separatory funnels was applied to study dye impurities. Required amounts (250 ml) of 1 N HCl and 1-butanol were equilibrated before use. Each of the four separatory funnels was charged with 50 ml of the 1-butanol. A 50-mg quantity of dye was weighed in a small beaker, dissolved in 50 ml of the acid phase, and transferred to the first separatory funnel. After thorough shaking of the dye solution with the 50 ml of 1-butanol in separatory funnel 1, the acid phase was successively transferred and extracted with the 1-butanol in the three remaining funnels. Three additional 50-ml portions of 1N HC1 were carried in turn through the extraction procedure. Hence, each of the organic and aqueous phases was extracted four times with the opposite phase. This technique separates mixtures into a Gaussian type of distribution among the various separatory funnels. However, many more stages are required to separate mixtures with closely similar properties. The absorbances of the organic phases containing the impurities were determined, diluting with 1-butanol, when necessary, to obtain on-scale readings, This procedure was repeated with dyes obtained from various sources, Examination of the absorbances for each extract (Table 111) showed (15) R. V. Nauman, P. W. 'West, F. Trow, and G. C. Gaeke, ANAL.CHEM., 32, 1307 (1960). 1712
ANALYTICAL CHEMISTRY
differing concentrations of the impurities in dye batches from various sources. COMPARISON OF METHODS OF PREPARATION OF PRA REAGENT, The absorbances (Table IV) of the reagent blanks were determined with solutions of the dye prepared by three methods: by the extraction procedure, by dissolving in methanol (14), and by dissolving in water (3). With solution in absolute methanol, all of the dye was dissolved but no separation of the violet impurity was accomplished. Some separation was accomplished by initially dissolving the dye in water, but photometric assay showed that the concentration of PRA Alwas only 56 to 70% of the nominal value of 0.04%.' though some of these weaker solutions showed lower blanks, insufficiency of the dye reduced the sensitivity and reproducibility of the method. Furthermore, weaker solutions required a correspondingly longer reaction time for full color development. Preparation by the extraction procedure followed by adjustment to the exact concentration required was best. SPECIFIC ABSORBANCE OF PARAROSANILINE. The amount of PRA in the reagent solution produced by the three methods was determined spectrophotometrically by measuring the absorbance at 540 mp in sodium acetate-acetic acid buffer (pH 4.69). A buffer solution is necessary because pH of the final solution affects color intensity (Figure 4). Since color develops slowly, buffered solutions were allowed to be equilibrated for 1 hour before absorbance values were read. An approximate specific absorbance (absorbance per pg) was determined by starting with the Fisher dye. The dye was dissolved in 1N HC1 and extracted, as described below, with
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Figure 4. Absorbance of pararosaniline at 540 mp as a function of pH 1-butanol, The extraction process was continued until no further violet-colored impurity was visible. A slight red color remained in the butanol extracts because of the solubility of a trace of pararosaniline in equilibrium with the pararosaniline hydrochloride. The pararosaniline base was precipitated by neutralizing the acid solution with a solution of sodium hydroxide. The precipitate was filtered and washed with cold distilled water. After three recrystallizations from boiling water, the precipitate was filtered from the supernatant solution on a coarse-fritted glass crucible. The precipitate was removed from the crucible by dissolving in 0.1N HC1. The resulting solution was taken to dryness in a glass evaporating dish on a steam bath. During the process of evaporation the color of the solution changed, indicating the volatilization of excess HCI. The residue in the evaporating dish was dried to constant weight in an oven set at 110°C. Analysis of a portion of the pararosaniline indicated one chloride ion per mole. (Found 10.9% C1, theoretical 10.92%). This purified pararosaniline was used as the standard for preparation of a calibration curve for the spectrophotometric determination of the PRA stock solution. Because the curve conforms with Beer's law throughout the working range, a calibration factor usually suffices for the analysis of PRA. Study of Color-Producing Reaction and Products. DEVELOPMENT TIME AND TEMPERATURE. Initial studies at controlled temperatures were made for the West-Gaeke procedure, modified so that all determinations were made in a 50-ml volumetric flask with a corresponding increase in quantities of all reagents. The flasks and all reagents were equilibrated in a thermostatically controlled water bath before mixing. After mixing, small portions were removed periodically for color measurements. The results of experiments a t 25" C, 35" C, and 40" C are shown in Figure 5.
O
0
60
Figure 5. Plot of absorbance lis. time at specified temperatures (West-Gaeke method) Curves are for blanks (--O-) and for concentrations of SOz in and 0.33 -U-. Distilled water was used as a reference
pg/ml: 0.16-A-
At 25" C the variation in color with time was similar to that at uncontrolled laboratory temperatures. At 25" C, 35" C, and 40" C, the maximum color was reached in 30, 15, and 10 minutes, respectively. With increasing temperatures, however, fading was more rapid and the period of maximum absorbance was shorter. The degree of fading was greater at the higher concentrations of sulfur dioxide and with higher acidity. STABILITY OF FINAL DEVELOPED COLOR TO LIGHT.Duplicate sets of solutions were prepared according to the WestGaeke method with varying amounts of sulfur dioxide. One set was exposed for 30 minutes directly under a fluorescent desk lamp, while the other was kept in the dark. The results obtained for each pair in the duplicate sets were identical. The experiment was then repeated with one set of solutions exposed for 30 minutes in irra(iation Chamber A to an ultraviolet intensity (X max. 3130 A) about 10 times that of sunlight. Color intensity in the exposed samples was 44 to 48% lower than in the control samples. Thus, if the final color is protected from direct sunlight, no appreciable fading should occur. EFFECTS OF PH ON FINAL COLOR. Optimum acidity for maximum final color development was studied in detail using dilute hydrochloric acid to adjust pH of the solution. The PRA dye from two different manufacturers was specially purified by extraction as previously described. The specific absorbances (A/pg/ml) were computed after correcting for the corresponding reagent blank. The pH of the final solutions VOL. 39, NO. 14, DECEMBER 1967
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Figure 6. Absorbance per microgram of SOzper ml as a function of pH Dashed (E) line indicates region of imminent precipitation shows upper limit at 560 mr (National Aniline PRA) Curve 1 (-A+ shows lower limit at 560 mfi (Fisher PRA) Curve 2 (-*) Curve 3 (-U-) shows measurements at 548 m.u
was determined with a Beckman Model G meter, The results, plotted in Figure 6, indicated that pH was a critical factor in the determination. The specific absorbance at 560 m,u varied from almost zero at pH 0 to a high of 0.70 at pH 1.9 (Curve 1, Hartman dye; Curve 2, Fisher dye). Sensitivity was maximum along a plateau at pH values from 1.5 to 1.9. The increase in sensitivity reported by Helwig (16)was probably due to operation at pH 1.5. Closer examination of the color obtained in the reaction with specially purified dye indicated a spectral shift of the peak from 575 mp as pH values changed from 1.0 to 1.7. Curve 3 gives the specific absorbance of the solution at various pH values measured at 548 mp. This curve indicates that maximum sensitivity is in the pH region of 1.6 i 0.1. EFFECTSOF FORMALDEHYDE CONCENTRATION. The formaldehyde concentration in the procedure was varied by the addition of 0 to 3 ml of a 0.2% or a 2% solution. Results indicated that the concentration of formaldehyde was not critical for optimum color development ; however, final concentrations of formaldehyde much higher than the nominal 0.17 reduced sensitivity and increased the absorbance blank. EFFECTSOF SULFAMIC ACID. Sulfamic acid usually supresses color formation. This phenomenon appears to be due mostly to pH effects and exposure of the sulfamic acid solution to air. ABSORPTION SPECTRA. Figure 7 shows the qualitative absorption spectra of the reaction products at selected pH values. These spectra are similar to those of compounds that show a hypsochromic or bathochromic shift with pH. Hence, the (16) H.L.HelwigandC. L. Gordon, ANAL.CHEM., 30,1810 (1958).
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ANALYTICAL CHEMISTRY
reaction product appears to be a single compound with two forms and can be described as a typical two-color pH indicator. Regardless of the pH conditions under which the reaction is carried out, the spectra can be interchanged by the careful addition of acid or base. The sharp peak with an absorbance maximum at 548 mp (rose-red) at the higher pH value (1.68) shifts to 575 m,u (magenta) at a lower pH value
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Figure 8. Continuous variation plot for pararosaniline-SOz complex Straight line (-A-)
after 30 minutes; dashed line (- -0- -) after 75 minutes
(1.02). In the intermediate pH range, the spectrum consists of the additive effect of the two species of the same compound. The observed absorbance maximum is a function of the relative molar extinction and the relative concentrations of the two species. The relative concentrations of the two species depend on the pH of the solution and the pK, of the pararosaniline methyl sulfonic acid, which can be determined from the data in Figure 6. Stoichiometry. Several approaches are commonly used for optimization of reagents and for evaluation of the stoichiometry of the reactions: the methods of continuous variations of Job (17), the mole-ratio method of Yoe and Jones (18), and the slope-ratio method of Harvey and Manning
(19). The mole-ratio method indicated that a 3 :1 molar excess of PRA reagent was required to reach the plateau of the curve at pH 1.6 ==I 0.1. A significantly greater excess of reagent was required at lower pH values because of instability of the products formed under these conditions. The plateau is the portion of the curve that is considered the optimum concentration of the reagent. In this region, the absorbance of the solution is directly proportional to the concentration of SO2 and independent of the concentration of PRA. The stoichiometry of the reaction was not well defined by the above technique, Therefore, the reaction was investigated by the continuous variation technique of Job (17) at three selected pH values. The three pH values gave almost identical diagrams, even though measurements were made at (17) P. Job, Ann. Chim. 9 [lo], 113(1928); 6 [lll, 97(1936). (18) J. H. Yoe and A. L. Jones, IND. ENO. CHEM., ANAL.ED., 16,111 (1944). (19) A. E. Harvey and D. L. Manning, J . Am. Chem. Soc., 72, 4488 (1950).
548 and 575 mfi. These results further indicated the formation of a single product. A typical curve is shown in Figure 8. With 30-minute reaction time, the extensions of the two tangents to the curve intersect at point indicating a 0.58 to 0.60 mole fraction of sulfur dioxide, one mole of PRA apparently reacting with approximately 1.5 moles of so~. With 75-minute reaction time, the absorbance increases in those solutions in which the concentration of PRA is the limiting factor. The apparent stoichiometry is 1.7 moles of SO2 reacting with 1 mole of PRA, independent of pH. The method of continuous variation, as pointed out by Rossotti and Rossotti (20) applies only under conditions in which one reaction product is formed. As more than one reaction product is formed, this technique does not give unequivocal results. One may reasonably assume, however, that the reaction in which a considerable excess of PRA is present yields essentially a monosubstituted product. At the other extreme, in which SOz is present in considerable excess, the reaction would go essentially to disubstituted or trisubstituted products. Which of these products are actually formed cannot be rigorously defined under these conditions, unless the molar extinction of each species is known. The ratio of the slopes of the two tangents was 1.2, which indicated a 20% increase in molar absorptivity of the reaction products under conditions in which SOz concentration is in considerable excess. As the two extreme portions of the curve are linear, the value obtained is the same as that given by the slope-ratio technique of Harvey and Manning (19). Definite conclusions cannot be drawn without fundamental assumptions concerning which species are present. Under (20) F. J. C. Rossotti, and H. Rossotti, “Determination of Stability Constants,” McGraw-Hill, New York, 1961. VOL. 39, NO. 14, DECEMBER 1967
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the conditions of the reaction involved in the determination of SOa, the monosubstituted product apparently is formed if the concentration of PRA is kept at 0.02% or above and is present in a threefold molar excess. RECOMMENDED PROCEDURES
Apparatus. Spectrophotometer, midget impingers, calibrated rotameters or calibrated critical orifices (hypodermic needles) (21), vacuum pump (capable of maintaining liZ atmosphere). Reagents. All chemicals, except for the dye, are Analytical Reagent grade. A good quality of distilled or double-distilled water, free from oxidants, must be used. I-Butanol. Certain batches of 1-butanol contain oxidants, which create an SO2 demand. Check by shaking 20 ml of 1-butanol with 5 ml of 20% KI. If a yellow color appears in the alcohol phase, redistill the butanol from silver oxide, and collect middle fraction. Buffer Stock Solution (pH 4.69). In a 100-ml volumetric flask, dissolve 13.61 grams of sodium acetate trihydrate in water. Add 5.7 ml of glacial acetic acid and dilute to volume with water. Purified Pararosaniline, 0.2 % (Nominal) Stock Solution. In a large separatory funnel, equilibrate 100 ml each of 1-butanol and 1N HC1. Weigh 100 mg of pararosaniline hydrochloride (PRA), in a small beaker. Add 50 ml of the equilibrated acid and let stand for several minutes. To a 125-ml separatory funnel, add 50 ml of the equilibrated 1-butanol. Transfer the acid solution containing the dye to the funnel, and extract. The violet impurity will transfer to the organic phase. Transfer the lower (aqueous) phase into another separatory funnel and add 20 ml of 1-butanol; extract again, Repeat the extraction with three more 10-ml portions of 1-butanol. This procedure usually removes almost all of the violet impurity that contributes to the blank. After the final extraction, filter the acid phase through a cotton plug into a 50-ml volumetric flask and bring to volume with 1 N HC1. This stock reagent will be yellowish red. The concentration of PRA need be assayed only once for each batch of dye in the following manner: 1 ml of the stock reagent is diluted to mark in a 100-ml volumetric flask with distilled water. A 5-ml aliquot is transferred to a 50-ml volumetric flask. Five milliliters of 1M sodium acetateacetic acid buffer are added, and the mixture is then diluted to 50-ml volume with distilled water. After 1 hour, the absorbance is determined at 540 mp with a spectrophotometer. The per cent of the nominal concentration of PRA is determined by the formula :
%PRA =
Absorbance X K grams of dye taken
For 1-cm cells and 0.04 mm slit width in a Beckman D U spectrophotometer, K = 21.3 Pararosaniline Reagent for Method A. To a 250-ml volumetric flask add 20 ml of stock PRA reagent. Add an additional 0.2 ml of stock for each per cent that the stock reagent assays below 100%. Then add 25 ml of 3M HsPOa and dilute to volume with distilled water. This reagent is stable for at least 9 months. Parasoaniline Reagent for Method B. Prepare as above, but use 200 ml of 3M H3P04 instead of 25 ml. This reagent is stable for at least 9 months. Absorbing Reagent, 0.04M Potassium Tetrachloromercurate (TCM). Dissolve 10.90 grams of mercuric chloride and 5.96 grams of potassium chloride in water and bring to volume in a I-liter volumetric flask. (Caution: highly poisonous. If spilled on skin, flush ~ f with f water imme(21) J. P. Lodge, Jr., J. B. Pate, B. E. Ammons, and G. A. Swanson, J. Air Pollution Control ASSOC.,16, 197 (1966).
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ANALYTICAL CHEMISTRY
diately.) If excessive heavy metal interferences are expected, add 0.07 gram of ethylenediaminetetraacetic acid disodium salt (EDTA). Sulfamic Acid, 0.6 %. Dissolve 0.60 gram of sulfamic acid in 100 ml of distilled water. This reagent can be kept for a few days if protected from air. Formaldehyde, 0.2%. Dilute 5 ml of 4 0 x formaldehyde to 1 liter with distilled water. Prepare daily. Standard Sulfite Solution. Dissolve 0.400 gram of sodium sulfite in 500 ml of recently boiled and cooled high-quality distilled water. (Double-distilled water that has been deaerated is preferred). Prepare daily. This solution contains 320 to 400 pg per ml as SOz. Determine the concentration in the standard solution by adding excess iodine and back-titrating with sodium thiosulfate which has been standardized against potassium iodate or dichromate (primary standard). Sulfite solution is unstable. Immediately after analysis, dilute with tetrachloromercurate as described in the calibration section. Sampling and Analysis. COLLECTION OF SAMPLE.Draw the air sample at the rate of 0.2 to 2 liters/min through a midget impinger containing 10 ml of the absorbing reagent. For long-term or 24-hour sampling periods, use 20 ml of the reagent. Flow rate is not critical as long as it is low enough to prevent entrainment of the liquid. It is desirable to collect 5 to 30 pg of SOz(2 to 13 pl at 760 mm Hg, 25" C). If the sample must be stored for more than a day before analysis, keep it at 5 O C in a refrigerator, if possible. ANALYSIS.After collection, transfer the sample quantitatively to a 25-ml volumetric flask, filtering if necessary; use about 5 ml of distilled water for rinsing. If ozone is present at concentrations greater than 0.01 ppm, delay analyses for 20 minutes to allow the ozone to decompose. For each set of determinations, prepare a blank with 10 ml of the unexposed absorbing reagent. To each flask add 1 mi of 0.6% sulfamic acid and allow to react for 10 minutes to destroy the nitrite from oxides of nitrogen. Accurately pipet 2 ml of the 0.2% formaldehyde, then 5 ml of pararosaniline reagent (select Method A or B). Start a laboratory timer set for 30 minutes. Bring all flasks to volume with freshly boiled distilled water. After 30 minutes, determine the absorbance at the wavelength of maximum absorbance (548 mp for Method A; 575 mp for Method B), using distilled water in the reference cell. CALIBRATION. Dilute 2 ml of the freshly standardized sulfite solution to 100 ml with 0.04M potassium tetrachloromercurate (TCM) reagent. Accurately pipet graduated amounts of this solution (such as: 0, 1, 2, 3, 4, and 5 ml) into a series of 25-ml volumetric flasks. Add enough 0.04M TCM to each flask to bring the volume of its contents to approximately 10 ml. Then add the remaining reagents as described in the procedure. For greatest precision, a constant-temperature bath is preferred. Calibration temperature should not differ from the analysis temperature by more than a few degrees. The total absorbances of the solutions are plotted (as ordinates) against the micrograms per milliliter of SOZ. A linear relationship is obtained. The intercept with the vertical axis of the line best fitting the points usually is within 0.02 absorbance unit of the blank (zero standard) reading. Under these conditions the plot need be determined only once to evaluate the calibration factor (reciprocal of the slope of the line). This calibration factor can be used for calculating results provided there are no radical changes in temperature or pH. At least one control sample is recommended per series of determinations to ensure the reliability of this factor. Each method requires a different factor. CALCULATION. Compute the concentration of sulfur dioxide in the sample by the following formula : (A
PPm
- A,) 0.382 B V
Table V. Reproducibility of Method A (pH 1.6 f 0.1) Std. dev. of points,a
Date
Set no.
absorbance units
1/18/66
1 2 3 4 5 6 7 8 9 10 11 12 13
0,005 0.015 0.039 0.008 0.026 0.004 0.015 0.013 0.011 0.006 0.004 0.015 0.008
1/31/66 2/3/66 2/11/66 2/ 18/66
Slope of calibration line,b Std. dev. of slope,c absorbance/pg/ml absorbance/pg/ml 0.757 0.762 0.757 0.752 0.726 0.729 0,728 0.732 0.797 0.749 0.744 0.735 0.726
0.004 0.013 0.033 0.008 0,022 0.003 0.012 0.011 0.015 0.007 0.004 0.018 0.009
Absorbance units Computedd Observed 0.105 0.093 0.100 0.125 0.128 0.128 0.134 0.155 0.165 0.147 0.154 0.135 0.137
0.106 0.106 0.104 0.118 0.129 0.127 0.128 0.163 0.170 0.147 0.154 0.143 0.138
Per cent error (single observation) at 1 pg/ml (25 pg SOntotal): 4.6% (at 95% confidence level). ReproducibiIity of slope: 0.746 f 0.030 (at 95 % confidence Ievel). a Deviation from the mean, six observations per set. b Line of best fit by Method of Least Squares. c Deviation from the mean slope, six observations per set. d Intercept of line fitted by Method of Least Squares.
Table VI. Reproducibility of Method B (pH 1.2 i 0.1) Date
Set no.
2/2/66
1 2 3 4 5 6
2/16/66 2/23/66
7 3/1/66
8 9
Std. dev. of points,a absorbance units
Slope of calibration line,b Std. dev. of slope,c absorbance/pg/ml absorbance/pg/ml
0.012 0,008 0.013 0.021 0.017 0.014 0.011 0.015 0.014
0.582 0.585 0.612 0.602 0.574 0.577 0.567 0.558 0.551
0.009 0.007 0.011 0.017 0.013 0.011 0.008 0.012 0.011
Blank, absorbance units Computedd Observed 0.043 0.048 0,052 0,052 0.026 0.031 0.039 0.039 0.050
0,037 0.039 0.038 0.038 0.028 0.028 0.028 0.045 0.046
Per cent error (single determination) at 1 pg/ml (25 pg SO2 total): 4.8% (at 95% confidence level). Reproducibility of slope: 0.579 i: 0.025 (at 95% confidence leveI). a Deviation from the mean, six observations per set. 5 Line of best fit by Method of Least Squares. c Deviation from the mean slope, six observations per set. d Intercept of Iine fitted by Method of Least Squares.
Where A is the sample absorbance A . is the blank absorbance 0.382 is the volume (pl) of 1 pg SOzat 25" C, 760 mm Hg B is the calibration factor X 25 ml, pg/A unit Vis the sample volume in liters corrected to 25' C,760 mm Hg (by perfect gas law) CALIBRATION, RANGE,AND SENSITIVITY. Calibrations for both Method A and Method B are linear (conform to Beer's law) and reproducible. Statistical tabulations of the data for 1-cm cells obtained over a month of use of the recommended methods A and B appear in Tables V and VI, respectively. Old or freshly prepared pararosaniline-phosphoric acid reagents were used in the procedure with which over 60 determinations were made. Subsequently, hundreds of determinations over a year gave results within the standard deviation. The slopes of the calibration lines for Method A and Method B were 0.746 f 0.03 and 0.579 i: 0.025 absorbance unit per pg per ml, respectively. Temperature control and a better calibration standard than aqueous sulfite solution would, of course, narrow the dispersion of the calibration
factor. Gaseous calibration with permeation tubes (22, 23) is preferable because of the instability of aqueous solutions of sulfur dioxide. The standard deviation computed from results obtained in our laboratory (several technicians) for Method A and Method B at the 9 5 x confidence level, are below 5 % at the 1 pg/ml level of concentration. The nonlinearity and poorer reproducibility of the previous pararosaniline methods required the use of standards for various points of the calibration curve. The working range was expanded from 0 to 16 pug (WestGaeke method) to 0 to 34 pg (Method A) and 0 to 43 pg (Method B). Some deviations from linearity occur with Method B at concentrations higher than 43 pg of sulfur dioxide. This deviation appears to be due to rapid fading of the color; therefore, development time becomes critical at the high Concentration of sulfur dioxide. All conditions were not fully optimized in Method B; it is primarily useful when low blanks are desirable. (22) A. E. O'Keeffe and G . C. Ortman, ANAL.CHEM.,38, 760 (1966). (23) F. P. Scaringelli, S. A. Frey, and B. E. Saltzman, Amer. Ind. Hyg. Assoc. J., 28, 260 (1967). VOL. 39, NO. 14, DECEMBER 1967 e
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Table VII. Interferences of Metals
so2 14.1 14.1 14.2 14.2 14.4 14.4 14.4 14.4 14.4 13.1 13.1 13.1 13.1 13.1
Metal salts, concentration pg/lO ml. Fe I11
EDTA
Aerated Found, pg SO2 Recovered,
448
13.1 13.8 14.2 14.6 14.0 14.0 14.0 11.1 1.95 13.1 12.3 12.8 10.6 12.3
92.9 97.9 100 102.8 97.2 97.2 97.2 77.1 13.5 100 93.9 97.7 80.9 93.9
14.1 14.4 14.2 14.6 14.4 13.6 11.4 3.48 0.40 13.1 11.8 12.7 10.5 12.3
100 102.1 100 102.8 100 94.4 79.2 24.2 2.77 100 90.1 96.9 80.2 93.9
70
15.05 14.6 13.7 14.3 15.3
98.4 95.4 89.6 93.5 100
15.3 14.9 13.8 12.0 15.3
100 97.4 90.2 83.7 100
70
15.4 14.8 14.7 14.6 15.4
100 96.1 95.5 94.8 100
15.4 15.4 15.2 15.1 16.8
100 100 98.7 98.1 109.1
10 20 30 60 100 200
30 30 60 60
Nonaerated Found, pg SOz Recovered,
224
Mn I1 15.3 15.3 15.3 15.3 15.3
2 5 10 10
Cr I11 15.4 15.4 15.4 15.4 15.4
2 5 10 10
Method A, the more sensitive procedure, yields a mean molar absorptivity ( a ) of 47,700 for sulfur dioxide. Slightly higher molar absorptivity is possible with higher pH values, but precipitation and high blanks are a problem in this region. Method B is somewhat less sensitive ( E = 37,000). The procedures yielding these values include the use of sulfamic acid reagent for the destruction of nitrite. The reported value of e for the West-Gaeke method (3) is approximately 36,700; the sulfamic acid modification (14) reduces this value by at least 15 Z. Interferences. NITROGENDIOXIDE.Known concentrations of sulfite in TCM were exposed to atmospheric levels of nitrogen dioxide in purified air for 24 hours. Criticalorifice hypodermic needles regulated the rates of flow at 0.2 liter/min. The interference produced by adding sulfamic acid to the impingers before collection of sulfur dioxide was verified. The method outlined by Pate et a/. (14) of adding sulfamic acid for the destruction of nitrite after the collection is completed was found satisfactory and was incorporated into the procedure. The suppression of color formation observed with the Pate procedure was primarily due to the effect of the sulfamic acid on the pH value. The pH was controlled closely in the present procedure. OZONE. The interference due to ozone was evaluated by the passage of ozonized air through five all-glass midget impingers in parallel. Two of the impingers contained alkaline iodide reagent (24) for determination of the amount of ozone in the air stream. The remaining three impingers contained known quantities of sodium sulfite in 0.04M TCM. All parts of the apparatus were made of glass or Teflon and were conditioned with several ppm of ozone for 1 hour. This system gave erratic nonstoichiometric results. The experiments were repeated individually with a Mast oxidant analyzer placed downstream to ascertain the amount of ozone, if any, that passed through the solutions. The results indicated that ozone was not soluble in the TCM absorbing solutions and passed through the impinger unchanged. (24) B. E. Saltzman and N. Gilbert, ANAL.CHEM., 31,1914 (1959).
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0
ANALYTICAL CHEMISTRY
A small amount of ozone sufficed to saturate the absorbing medium. If the sulfur dioxide was liberated from its complex by the addition of reagents, interferences were caused by reaction with the residual ozone. Ozone is unstable in aqueous solutions and decays rapidly (25). Therefore, a 20-minute delay time was established to ensure the complete reaction of ozone. The dichlorosulfitomercurate once formed is stable in the presence of ozone and other strong oxidants. However, interferences can occur before the formation or after the liberation of SO2from its complex. HEAVYMETALS. Zurlo and Griffini (26) reported that heavy metals interfered with the West and Gaeke procedure. These investigators recommended the use of EDTA to complex these heavy metals. Salts of heavy metals [Fe(III), Cr(III), Cu(II), V(V), Mn(II)] were added in increasing quantities to solutions of sulfite in TCM, both with and without the presence of EDTA. A 10-ml portion of each solution was aerated in an impinger for 24 hours at 0.2 liter/min with charcoal-purified air. Reference samples were kept in a dark cupboard. These samples were later analyzed for sulfur dioxide. No significant interferences were found (losses were less than 5 %) with 10 pg copper(I1) and 22 pg vanadium(V). The interferences produced by various concentrations of Fe(III), Cr(III), and Mn(I1) are reported in Table VII. Up to 30 pg of iron can be tolerated. Significant losses (greater than 10%) of SO2 were found with higher concentrations of this element. EDTA was found effective, as indicated by Zurlo and Griffini (26), in preventing sulfur dioxide losses. At least 5 pg of Cr(II1) and 2 pg of Mn(I1) can be tolerated in the procedure. Again, EDTA was effective in preventing losses when higher concentrations of these elements were added to the TCM solution.
(25) R. P. Selm, “Ozone Chemistry and Technology,” Advan’ Chem. Series, American Chemical Society, Washington, D. C., 1959. (26) N. Zurlo and A. M. Griffini, Med. Lavoro, 53, 330 (1962).
The maximum reported values in the atmosphere for Fe, Mn, Cu, Cr, and V were 22, 9.98, 10, 0.24, and 2.2 yg per cubic meter, respectively. These metal concentrations should not produce significant interference with the methods as outlined; however, addition of EDTA is recommended when low levels of sulfur dioxide are to be measured or when contamination by heavy metals is unusually high. A minimum EDTA concentration of 7 times the anticipated amount of heavy-metal interference should be added. SUMMARY
This study indicates that many critical variables-i.e., pH, temperature, impurities in reagents and in water-must be controlled for reliable results. The pH is critical because of the spectral shift phenomenon and the large difference in intensity of the two colored species. There is an apparent isosbestic point at a wavelength of 564 mp, but the possibility of measuring at this wavelength was not investigated. Also, the lower the pH value, the less stable the final color. Phosphoric acid provides pH control. Temperature affects the rate of color formation and fading of the final color. The reagent blank has a very high temperature coefficient (0.015 absorbance unit per "C) and the temperature effect is reversible. Any change in temperature upward or downward causes a corresponding increase or decrease in absorbance value. Hence, for best results a constant-temperature bath is recommended. Purity of the reagents and the dye, in particular, is extremely important in obtaining reproducible results because of the high sensitivity of the method. Therefore, the dye is specially purified and standardized to eliminate problems associated with this reagent. All other reagents should be ACS grade or the best available. Potassium chloride was
used in the absorbing solution, because it is usually obtained in purer form than the sodium salt. Double-distilled water, free from oxidants, is necessary. Although the dichlorosulfitomercurate is stable in the presence of strong oxidants, oxidation can occur before the complex forms or after the sulfito radical is liberated. The effects of interferences are minimized by adding sulfamic acid to destroy nitrite, by using complexing agents (H3P04,EDTA) to bind heavy metals, and by allowing time for ozone to decay. Solutions of bisulfite or sulfite are unstable and unsatisfactory as calibration standards. Careful standardization and immediate dilution with TCM are required for accurate results. The modified methods give more reliable, more specific, and more reproducible values for sulfur dioxide in ambient air than earlier methods. Although specifically developed for SOz in the atmosphere, the methods can be applied to determination of sulfite or sulfur in other materials (e.g., sulfur in metals or sulfite in agricultural products). ACKNOWLEDGMENT
The authors wish to thank Robert Hurtubise for the analysis of SOzin the presence of ozone and Kenneth Rehme for the analysis of SOzin the study of the reaction.
RECEIVED for review June 2, 1967. Accepted September 1, 1967. Presented in part before the Division of Water, Air, and Waste Chemistry, 149th National Meeting, ACS, Atlantic City, N. J., September 13, 1965,and before the 151st National Meeting, ACS, Pittsburgh, Pa,, March 24, 1966. The mention of a commercial product does not constitute an endorsement by the Public Health Service.
An Improved Spectrophotometric Method for the Determination of Sulfate with Barium Chloranilate as Applied to Coal Ash and Related Materials H. N. S . Schafer Coal Research Laboratory, CSIRO Division of Mineral Chemistry, P . 0.Box 175, Chatswood, N.S. W . , 2067, Australia The indirect method for the determination of sulfate, based on spectrophotometric measurement at 330 or 530 mp of the chloranilate ion formed by the reaction of sulfate with barium chloranilate, has been studied with 80% isopropyl alcohol as the solvent medium. In 80% isopropyl alcohol, the reaction between sulfate and barium chloranilate is almost stoichiometric and the blank value is considerably reduced. An investigation of the spectrum of chloranilic acid revealed an isobestic point, not previously reported, at 310 mp, which proved analytically useful. These new features have been used in developing a semimicro method for the determination of sulfate in coal ash and related materials. Anionic interferences, and the ways in which these may be eliminated or minimized in the present application, are discussed.
estimate of the amount of sulfate even when this is present at very low levels, and because the amount of sample available is often small. In line with the trend toward spectrophotometry and atomic absorption spectroscopy in the analysis of fuel residues ( 1 , 2), a spectrophotometric method for the determination of sulfate is desirable. The method developed by Bertolacini and Barney (3) appeared to be the most suitable for this purpose. It is based on the reaction of sulfate with the barium salt of chloranilic acid (2,5-dichloro-3,6,-dihydroxy-p-benzoquinone) in 50 % ethyl alcohol to produce barium sulfate and chloranilate ion. The chloranilate absorbance is measured at-530 mp or, with improved sensitivity, at 330 my (4).
STUDIESOF THE COMBUSTION chemistry of sulfur require, inter alia, the reliable and rapid determination of sulfur, in flue gases and in solid combustion residues (coal ash, slags, and fly ashes). Further, a method of high sensitivity is required because frequently it is desirable to have a precise
(1) R. A. Durie, H. N. S. Schafer, and D. J. Swaine, CSIRO Diu. Coal Res., Znaest. Rept. 36 (formerly Misc. Rept. 178) (1963). (2) R.A. Durie, H. N. S. Schafer, and D. J. Swaine, CSIRO Diu. Coal Res., Tech. Cornrn. 47 (1965). (3) R.J. Bertolacini and J. E. Barney, ANAL.CHEM., 29,281 (1957). (4) Ibid., 30,202 (1958). VOL. 39, NO. 14, DECEMBER 1967
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