Anal. Chem. 1985, 57, 1335-1338
special purposes, the possibility of synthesizing special groups on neutral polymers or chemically modifying existing groups could be considered (32). In some cases, direct use of functionalized polymers as stabilizers might be feasible. During early stages of this work we considered reducing parts of the carboxylic acid groups of Amberlite IRC-84 to aldehyde, thus eliminating the need for a separate stabilization reagent. The idea was never tested, as we found a simpler solution to the problem in the sorbent presented here. ACKNOWLEDGMENT We thank Anders Cedergren for valuable help and Michael Sharp for linguistic revision of the manuscript. Registry No. SOz, 7446-09-5;Amberlite XAD-2, 9060-05-3; Amberlite XAD-7, 37380-43-1; Amberlite IRC-84, 11098-83-2; benzaldehyde, 100-52-7;3-phenylpropenal,104-55-2;formaldehyde, 50-00-0;acetaldehyde, 75-07-0;acetone, 67-64-1;glyceraldehyde, 367-47-5;dihydroxyacetone,96-26-4; glycerol, 56-81-5; ethanedial, 107-22-2. LITERATURE C I T E D (1) von Nieding, G.; Wagner, H. M. Atemw.-Lungenkrkh. 1982, 8 ,
190- 193. (2) von Nieding, G. EHP, Envlron. Health Perspecf. 1978, 2 2 , 91-92. Beckman, G.; Beckman, L.; Rosenhall, L.; Stjernberg, (3) Nordenson, I.; N. Hereditas 1980, 9 3 , 161-164. (4) Anderson, C. C.; Gunderson, E. C.; Coulson, D. M. I n "Chemical Hazards in the Workplace"; Choudhary, G., Ed.; American Chemical Society: Washington, DC, 1981; ACS Symp. Ser. 149, pp 3-19. (5) Huygen, C. Anal. Chlm. Acta 1963, 2 8 , 349-360. (6) Eller, P. M.; Kraus, M. "NIOSH Manual of Analytical Methods"; NIOSH: Cincinatti, OH; Vol. 7, Method no. P&CAM 268. (7) Takamine, K.; Tanaka, S.;Hashimoto, Y. Bunsekl Kagaku 1982, 3 1 , 692-96. (8) Klockow, D.; Teckentrup, A. I n t . J . Envlron. Anal. Chem. 1980, 8 , 137- 148.
(9) Chriswell, C. D.; Gjerde, D. T. Anal. Chem. 1982, 5 4 , 1911-1913.
(IO) Vinjamoori, D. V.; Ling, C.4. Anal. Chem. 1981, 5 3 , 1689-1691. (11) Smith, D. L.; Kim, W.S.;Kupel, R. E. Am. Ind. Hyg. Assoc. J . 1980, 4 1 485-488. (12) Black, M. S.;Herbst, R. P.; Hitchcock, D. R. Anal. Chem. 1978, 5 0 , 848-851. (13) Reiszner, K. D.; West, P. W. Env. Sci. Technol 1973, 7,526-532. (14) Dasgupta, P. K.; DeCesare, K.; Ullrey, J. C. Anal. Chem. 1980, 5 2 , 1912-1922. (15) Dasgupta, P. K.; DeCesare, K. B. Afmos. Environ. 1982, 16, 2927-2934. (16) Lindgren, M.; Cedergren, A.; Lindberg, J. Anal. Chlm. Acta 1982, 14 1 , 279-286. (17) West, P. W.; Gaeke, G. C. Anal. Chem. 1956, 28, 1816-1819. (18) Scaringelli, F. P.; Saltzman, B. E.; Frey, S.A. Anal. Chem. 1967, 3 9 , 1709-1 7 19. (19) O'Keeffe, A. E.; Ortman, G. C. Anal. Chem. 1966, 3 8 , 760-763. (20) Cedergren, A.; Wikby, A.; Bergner, K. Anal. Chem. 1975, 47, 100-1 06. (21) Irgum, K. Anal. Chem. 1983, 55, 1186-1187. (22) Irgum, K. Anal. Chem. 1985, 5 7 , 1335-1338. (23) Irgum, K. Anal. Chem. 1985, 57, 1496-1498. (24) Nishijima, A.; Hagiwara, H.; Kurita, M.; Ueno, A,; Sato, T.; Kiyozumi, Y.; Todo, N. Bull. Chem. Soc Jpn. 1982, 55, 2618-2621. (25) Neuberg, C.; Kobel, M. Biochem. 2. 1932, 256, 475-484. Chem. Abstr. 1933, 2 7 , 1618. (26) Neuberg, C.; Kobel, M. Naturwissenschaften 1932, 16, 953-954. (27) Fieser, L. F.; Fieser, M. "Advanced Organic Chemistry"; Van Nostrand: New York, 1961; pp 443-444. (28) Whipple, E. B. J . Am. Chem. SOC. 1970, 9 2 , 7183-7186. (29) "1982 Catalog and Guide to Air Sampling Standards"; SKC Corp.: Eighty-Four, PA, 1962; p 13. (30) Stevens, T. S.; Davis, J. C ; Small, H. Anal. Chem. 1981, 5 3 , 1488-1 492. (31) Helfferich, F. "Ion Exchange"; McGraw-Hill: New York, 1962; pp 38-39. (32) Frechet, J. M. J.; Farrall, M. J. I n "Chemistry and Properties of Crosslinked Polymers"; Labana, S. s., Ed.; Academic: New York, 1977; pp 59-83. I
RECEIVED for review July 17, 1984. Resubmitted January 22, 1985. Accepted January 22,1985. Grants from the Stiftelsen Bengt Lundqvists Minne are gratefully acknowledged.
Pararosaniline Colorimetric Determination of Sulfur Dioxide Stabilized with Ethanedial Knut Irgum
Department of Analytical Chemistry, University of U m e i , S-901 87 U m e i , Sweden
A hydroxide-consuming intra-Cannirzaro reaction of ethanedial desorbed from the solid sorbent occurs during the hydrolysis step preceding the color reaction. A higher initial OHconcentration is therefore needed In the hydrolysis, as compared to methods without ethanedlal in the sample. Hydrolysis time should be kept between 10 s and 5 min for maximum sensitivity. Optlmum pH and formaldehyde concentration during color development were 1.02 and 5 mM, respectively. The alkaline formaldehyde solution containing 0.5 M NaOH and 20 mM formaldehyde was stable for 1 day at room temperature.
A solid sorbent for sampling sulfur dioxide in occupational hygiene has recently been developed in our laboratory (1). The gas is stabilized as an addition compound with ethanedial a t p H 5 after initial hydrolysis. HSO8- + OHCCHO + OHCCH(OH)SO,(1) Dasgupta e t al. (2) have shown that a modified pararosaniline colorimetric method can be used for determination of S02(g)sampled with buffered formaldehyde solution if the
sample is subjected to an alkaline hydrolysis prior to the color reaction. When this method was applied to samples taken with the solid sorbent, some complications were seen. A discussion of these is presented here and the reaction is optimized for use with ethanedial-containing samples. EXPERIMENTAL S E C T I O N Reagents and Solutions. Apart from the purified (3) pararosaniline ("standard grade"; Fluka, Buchs, Switzerland) and the 6 M aqueous ethanedial (synth.;Merck-Schuchardt,Hohenbrunn, FRG), all chemicals were reagent grade. The formaldehyde was used without removing the 10% methanol that was added as polymerization inhibitor. Acetate-buffered hydroxymethanesulfonate (HMS) ( 4 ) was diluted to 25 pM in 50 mM ethanedial and used as test sample. The alkaline formaldehyde solution was composed of 20 mM formaldehyde in 0.5 M NaOH and the pararosaniline color reagent contained 0.8 mM pararosaniline hydrochloride in 1.00 M HCl. These are the optimized reagents, which were used if not otherwise noted. General Procedure. A 1.5-mL sample was hydrolyzed with 0.75 mL of alkaline formaldehyde for 15 s. The hydrolysate was thereafter added to 0.75 mL of the color reagent. Gilson adjustable air displacement pipets were used to ensure instant mixing of the solutions. After 10 min the absorbance at 580 nm was measured
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
against water. Blanks were prepared from 50 mM ethanedial and run through the same hydrolysis procedure. The temperature of all solutions was 20 k 1 "C before mixing. Hydrolysis pH and Time. Test samples were treated by the general procedure. NaOH concentration in the alkaline formaldehyde solution was varied between 0.1 and 1 M and the hydrolysis time ranged from 10 s to 60 min. HC1 concentration in the color reagent was matched with the NaOH concentration of the alkaline formaldehyde, so that a constant HC1 excess of 0.125 M was maintained in the final color development solution. A test was also included, where an alkaline formaldehyde solution with 0.1 M NaOH, was made 0.4 M in NaC1. In another set of experiments, 10 mM formaldehyde replaced ethanedial in the samples. These were run through the same procedure as the samples above, except that NaOH solutions with their concentrations matched with the color reagent as above were used for hydrolysis. Optimization of Formaldehyde Concentration. Formaldehyde concentration in the alkaline formaldehyde solution was varied between 5 and 50 mM. The general procedure was followed. Stage of Formaldehyde Addition. NaOH solutions with concentrations ranging from 0.1 to 1 M were used instead of alkaline formaldehyde to hydrolyze the samples. A 0.25-mL portion of 60 mM formaldehyde was added in a separate step to 0.5 mL of 1.2 mM pararosaniline color reagent, whose HC1 concentration was matched as above with the concentration of the NaOH used to hydrolyze the sample. Formaldehyde was added to the color reagent either 1 min or 1 h before adding the hydrolyzed sample or immediately after this addition. Optimization of Color Development pH. Color reagent HC1 concentration was varied so that HCl excess in the color development solution ranged from 0.06 to 0.2 M. The general procedure was used. Influence of Ethanedial Concentration on Color Intensity. The test sample contained 50 pM formate-buffered HMS in various concentrations of ethanedial ranging from 0.1 mM to 1 M. The blank contained ethanedial at the same concentration. The general procedure was followed. Stability of the Alkaline Formaldehyde Solution. Alkaline formaldehyde was stored in a polyethylene bottle in the laboratory and used in parallel with freshly prepared base for test sample hydrolysis using the recommended procedure. RESULTS AND DISCUSSION Optimized Procedure. The general procedure given in the Experimental Section can be followed. Choice of Concentrations i n the Test Sample. When used for sampling of 3 dm3 air a t the current Swedish TLV, the 5-mL sample solution will contain approximately 50 pM S(1V) and 40 mM ethanedial. A test sample containing 25 pM S(1V) and 50 mM ethanedial has been used in most of the experiments, as it gives a signal within the linear working range of the colorimetric method (0-50 pM for a 10-mm cell). Samples with high S(1V) concentrations are diluted with water before determination and the described effects from ethanedial will thus be less pronounced. Hydrolysis pH and Time. The most important effect of ethanedial on the color reaction is the influence of hydrolysis time on final color intensity (Figure 1). Dasgupta e t al. (2) reported briefly that hydrolysis time has no effect on the final color intensity when formaldehyde is used as stabilizer. This was verified (Figure 2) after observing this time dependence with ethanedial-containing samples. Apart from the time dependence, higher p H was needed to achieve complete hydrolysis of ethanedial-stabilized samples, as compared to samples stabilized with formaldehyde. This is explained by a rapid intra-Cannizzaro reaction (eq 2), which converts ethanedial to hydroxyacetate even in moderately alkaline solutions ( 5 ) .
The reaction is first order with respect to ethanedial, while a second-order relationship exists for the hydroxide ion (5).
5' Hydrolysis time
Figure 1. Color intensity as function of hydrolysis pH and time when hydrolyzing a test sample containing 25 p M S(1V) in 50 mM ethanedial with mixed base consisting of 20 mM formaldehyde in (0)0.1 M, (0) 0.2 M, (A) 0.5 M, and (V)1.0 M NaOH. 0.5
5' Hydrolysis time
Figure 2. Color intensity as function of hydrolysis pH and time when hydrolyzing a test sample containing 25 p M S(1V) in 10 mM formaldehyde with sodium hydroxide (symbols as in Figure 1).
At low initial hydroxide ion concentration, this consumption can decrease pH to a level where the hydrolysis equilibrium (eq 3) is less in favor of free aldehyde and sulfite ion with a lower signal as an obvious result, as seen in Figure 1.
+ S03*- + HzO
This was not a matrix effect from the varying amount of sodium chloride in the color development solution, as a 0.1 M NaOH alkaline formaldehyde solution with 0.4 M NaCl added gave the same effect as that without added NaC1. By use of rate constants and equations from the literature (5), the ethanedial concentration (and pH) in the solution could be calculated as a function of initial hydroxide concentration and hydrolysis time when the reaction proceeds. An iterative computer routine was used to solve the integrated forms (ref 5 , eq 2 or 2a) of the third-order rate expression for this reaction with respect to x for various reaction times. Input concentrations in the calculations were chosen to match the experiments, and the concentration was 33 mM for ethanedial, while the initial OH- concentration was assigned values between 33 and 330 mM. The calculations indicate that approximately M ethanedial is remaining after the recommended 15 s hydrolysis time when an alkaline formaldehyde solution containing 0.5 M NaOH is used for hydrolysis (see Figure 3). Removal of stabilizer aldehyde from the sample is fortunate, as this could be a means of circumventing the sensitivity to the way of mixing seen by others ( 2 ) . Formaldehyde Concentration. Formaldehyde is an essential part of the color reaction; 5 mM in the final color development solution was found to be the optimum concentration, which is the same as that seen for samples without ethanedial ( 2 ) . This was not surprising, as practically all
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
Table I. Sensitivity and Precision of the Color Reaction When Formaldehyde Is Added at Different Stages in the Reaction
stage of formaldehyde addition
as mixed base to the color reagentc 1 min before mixing 1h before mixing after mixing
37800 f 180
Hydrolysis time (s) Figure 3. Conversion of ethanedial during hydrolysis when different initial hydroxide concentrations are used. Curves are calculated from literature data (5), using 33 mM as input concentration for ethanedial. ethanedial originally present in the solution has been converted to hydroxyacetate, which will not form an addition compound with sulfite nor react with the other reagents used. Formaldehyde Addition. In contrast with other methods (2, 6), samples from the sorbent allow formaldehyde to be added at different stages in the color reaction: to the desorbed sample solution or to the color reagent. Addition to the sample can be done in a separate step or mixed with the base used for hydrolysis, whereas addition to the color reagent is possible before or after mixing with the hydrolyzed sample. Rapid addition of hydrolyzed sample to the color reagent with vigorous mixing is the only way to get reproducible results with samples containing formaldehyde (2). Hydrochloric acid is in excess all the time, and the equilibrium that existed in the solution during hydrolysis is frozen by the slow kinetics of HMS formation at low pH (7). In the transition pH interval, HMS formation is fast, so it will always take place to some extent, regardless of the way of mixing. If formaldehyde is added to the color reagent instead of to the sample as an alkaline formaldehyde solution, addition compound formation would be eliminated, as practically no aldehyde will be present in an ethanedial-stabilized sample hydrolyzed with sodium hydroxide. A mixture of formaldehyde and acidic color reagent is, however, not stable, so formaldehyde addition must be done either shortly before or immediately after adding the hydrolyzed sample. A 6-8% increase in color intensity was seen when formaldehyde was added to the color reagent in a separate step, as compared to adding it to the sample as an alkaline solution (Table I). The increase is due to decrease in or lack of HMS formation. The efficiency of the HMS hydrolysis reaction (eq 4), calculated by using eq 23 in ref 7, was 94.7% when 16.7 KMHMS,6.7 mM formaldehyde, and a pH of 13.00 were used as inputs. CH,(OH)SO,-
+ S032- + H20 (4)
Input OH- concentration was calculated by subtracting the OH- consumed in 15 s by the intra-Cannizzaro reaction of ethanedial from the concentration a t the start of hydrolysis. The activity coefficient was computed using eq 5 , which is derived by the extended Debye-Huckel theory (by multiple regression analysis on literature data (8), using the OHconcentration and its square root as predicting variables for the logarithm of the activity coefficient). log,, y+ = 0.3445[OH1] - 0.4680[0H-]'/2 (5) The gain in sensitivity obtained with this separate addition of OH- and formaldehyde is, however, marginal, and the alkaline formaldehyde approach is therefore recommended for routine use as it reduces the number of steps and thereby the complexity of the method. If a separate addition of form-
% re1 to mixed baseb
40900 f 370 40600 f 380 40000 f 320
+8.2 f 0.7
+7.4 f 0.7 +5.7 f 0.6
a Calculated as S(1V) concentration in the color development solution. Mean f standard deviation. Six determinations at each way of adding formaldehyde. *Mean f standard error (95% confidence interval). Variances added. e Same amount of formaldehyde as in the mixed base was added directly to the color reagent before or after mixing hydrolyzed sample and color reagent.
Flgure 4. Color intensity and blank as function of ethanedial concentration in a test sample containing 50 pM S(IV). Sample was hydrolyzed with alkaline formaldehyde solution.
aldehyde is preferred for some reason, it should be done to the color reagent. Optimum pH in the Color Development. The color reaction is strongly affected by pH variations during color development ( 2 ) ,and different ways of buffering have been described in the literature. Phosphoric acid (3) will not be ideal, as the buffering action of its second dissociation constant will increase the time the sample spends in the critical neutral pH range. HC1 excess in the color reagent (2) was therefore used to buffer the reaction. An HC1 concentration of 0.125 M in the color development solution was a good compromise between sensitivity and blank. This corresponds to a pH of 1.02, which is close to that reported earlier with formaldehyde as stabilizer (2). Influence of Ethanedial Concentration on Color Intensity. A slight increase in sensitivity took place on adding up to approximately 50 mM ethanedial to the sample, whereafter a rapid signal decrease and an increase in blank absorbance occurred (Figure 4). The signal decrease could be due to incomplete hydrolysis, as the consumption of OHdecreases pH. This effect is strong when the initial ethanedial concentration is higher than that of the hydroxide ion. In this case, the solution will contain excess ethanedial, which can react with the pararosaniline and account for the increase in blank that accompanied the decrease in color intensity. Stability of Alkaline Formaldehyde Solution and Color Reagent. Formaldehyde will also undergo a Cannizzaro reaction (eq 6) in alkaline solutions, so stability of the alkaline formaldehyde was checked. 2HCHO
The Cannizzaro reaction normally requires highly alkaline conditions to proceed at a rate appreciable for utilization in
Anal. Chem. 1985, 57, 1338-1341
organic synthesis (91, but the reaction could be suspected to take place to some extent even in the 0.5 M OH- concentration of the alkaline formaldehyde. Decreases of 2.4 and 6.3% in color intensity were observed when alkaline formaldehyde which had been stored for 12 and 30 days, respectively, was used in parallel with freshly prepared base for analysis of test samples. (Whether this was actually due to a Cannizzaro reaction was not investigated). The alkalide formaldehyde should therefore be freshly prepared every day. No difference was seen in blank magnitude or the color intensity obtained when freshly prepared color reagent and reagent that had been stored 30 days in the laboratory were used in parallel for analysis of a test sample. We are currently working on an adaption of this method for flow injection analysis.
Registry No. Sulfur dioxide, 7446-09-5;pararosaniline, 56961-9; ethandial, 107-22-2; formaldehyde, 50-00-0.
ACKNOWLEDGMENT Thanks are due to Michael Sharp for linguistic revision of the manuscript.
RECEIVED for review July 17, 1984. Resubmitted January 22, 1985. Accepted January 22,1985. Grants from the Stiftelsen Bengt Lundqvists Minne are gratefully acknowledged.
LITERATURE CITED (1) Irgum, K.; Lindgren, M. Anal. Chem. 1985, 57, 1330-1335. (2) Dasgupta, P. K.; DeCesare, K.; Ullrey, J. C. Anal. Cbem. 1960, 52, 1912-1922. (3) Scaringelli, F. P.; Saltzman, 6.E.; Frey, S.A. Anal. Cbem. 1967, 39, 1709-1719. (4) Irgum, K. Anal. Cbem. 1965, 57, 1496-1498. (5) Saiomaa, P. Acta Cbem. Scand. 1956, 10, 311-319. (6) West, P. W.; Gaeke, G. C. Anal. Cbem. 1956, 28, 1816-1819. (7) Skrabal, A.; Skrabal, R. Monatscb. Cbem. 1936, 69, 11-41. (8) Harned, H. S.;Owen, 6.6. "The Physical Chemistry of Electrolyte Solutions"; Reinhold: New York, 1958; ACS Monograph Series, p 729. (9) March, J. "Advanced Organic Chemistry", 2nd ed.; McGraw-Hill: New York, 1977; pp 1139-1140.
Sulfate Determination in Industrial Wastewater by Liquid Chromatography with Postcolumn Solid-Phase Reaction Detection Kommer Brunt
Analytical Department, Potato Processing Research Institute-TNO, Rouaanstraat 27, 9723 CC Groningen, T h e Netherlands
A sulfate selective postcolumn solid-phase reactor (SPR) detectlon system is described for the sulfate determination at parts per million (ppm) levels in wastewaters of the potato starch Industry. The effect of the chromatographic flow rate on the converslon reaction in the SPR Is dlscussed. The linear dynamic range of the detection system In combination with the used anlon-exchange separation is from about 5 to 400 ppm. The repeatability of the system at the 30 ppm sulfate level Is better than 3.5 %. The SPR system can be used for about 13 h without noticeable depletlon effects. Day-to-day repeatablllty Is good. Application of the described detection system for the sulfate determlnatlon in industrlal wastewaters Is dlscussed. Recoveries in spiked wastewater samples were better than 90 %
Sulfate is widely distributed in nature. It is present in natural waters such as surface water and rain water in concentrations ranging from almost zero to rather high levels of hundreds of parts per million. Industrial wastes often contain considerable amounts of sulfate. It is possible to remove sulfate by anaerobic wastewater treatment, as is done by the Dutch potato starch industry ( I ) . Knowledge of sulfate concentrations in the different steps may be an important parameter for the process control. Moreover these data are also important for a better understanding of the chemical and microbiological processes which occur in the anaerobic purification reactors (1). Different analytical procedures are described for the determination of sulfate (2, 3). It is beyond doubt that gravimetric and turbidimetric methods are cumbersome and
time-consuming, especially when the chemical oxygen demand (COD) of the wastewater is high (10000 mg of 0 2 / L and higher). Colorimetric ( 4 , 5 ) ,continuous flow (6, 7 ) ,and flow injection methods ( 5 , 6) are not satisfactory because too many interfering compounds are present in a lot of industrial wastes. The sulfate determination by modern ion chromatography with silica-based columns and with the two-column system marketed by Dionex Corp. have been described extensively (e.g., ref 8-10). However, the described applications concern mostly analyses in surface, rain, and ground waters, plating baths, and brine solutions. Unfortunately silica-based columns will deteriorate rapidly due to the high COD levels in the wastewater samples if no (expensive) guard columns are used. The more classical polystyrene-divinylbenzene cross-linked ionexchange resins are much less susceptible to deterioration by the organic compounds present in the wastewater. In contrast, with silica-based stationary phases, ion-exchange resins are easy to regenerate in case the column efficiency decreases due to contamination. However, application of these resins is attended by the use of solutions of relatively highly ionic strength as a mobile phase. Hence, it is not possible to detect sulfate in the parts per million range with a conductivity detector. Therefore, we have applied a sulfate-selective postcolumn solid phase reaction detection system. Such heterogeneous derivatization systems are usually applied to enzymatic conversions by immobilized enzymes in flow systems (11). Recently, Ruter and Neidhardt described a solid-phase reactor (SPR) for the detection of manganese species (12). The use of SPR systems for derivatization in liquid chromatography was reviewed in depth by Xie et al. (13). According to these papers, SPR systems do not introduce any additional
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