Simultaneous HPLC Determination of Peroxyacetic Acid and

Sep 1, 1997 - External calibration with the solid standards of MTSO and TPPO leads to a very accurate and reliable method. Samples are stable and ... ...
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Anal. Chem. 1997, 69, 3623-3627

Simultaneous HPLC Determination of Peroxyacetic Acid and Hydrogen Peroxide Ulrich Pinkernell, Stefan Effkemann, and Uwe Karst*

Abteilung Lehrstuhl fu¨ r Analytische Chemie, Anorganisch-Chemisches Institut, Westfa¨ lische Wilhelms-Universita¨ t Mu¨ nster, Wilhelm-Klemm-Strasse 8, D-48149 Mu¨ nster, Germany

The first instrumental method for simultaneous determination of peroxyacetic acid (PAA) and hydrogen peroxide has been developed. The successive quantitative reaction of PAA with methyl p-tolyl sulfide (MTS) and hydrogen peroxide with triphenylphosphine (TPP) yields the corresponding sulfoxide MTSO and phosphine oxide TPPO. The reagents and their oxides are separated by HPLC on reversed-phase columns with acetonitrile/water gradient elution within 5 min. External calibration with the solid standards of MTSO and TPPO leads to a very accurate and reliable method. Samples are stable and can be stored after derivatization for several days prior to analysis. Real samples from brewery disinfection were analyzed in comparison to titration with excellent correlation.

immediately after sampling to avoid decomposition of the peroxides during storage or transportation of the sample. Further methods for the determination of PAA include photometry,3-9 electrochemical sensing,10-13 gas chromatography,14,15 and direct liquid chromatography.16,17 None of these methods, however, is suited to determine both PAA and hydrogen peroxide selectively and simultaneously. Recently, we have described a method for liquid chromatographic determination of PAA using the oxidation of methyl p-tolyl sulfide (MTS) to the corresponding sulfoxide (MTSO):18,19

Peroxides such as peroxyacetic acid (PAA) and hydrogen peroxide find increasing use in industry for disinfection and bleaching purposes due to their ecologically beneficial properties. The technical synthesis of PAA comprises the reaction of acetic acid with hydrogen peroxide, often in the presence of a strong acid as catalyst. Therefore, technical PAA solutions contain significant amounts of hydrogen peroxide as a residue or equilibrium concentration. The determination of these peroxides requires a reliable distinction between the two substances, which are commonly analyzed using their redox properties. Both peroxides are strong oxidizers, and their technical application is mainly based on oxidation chemistry. Hydrogen peroxide, however, may also serve as a reducing agent toward other strong oxidizers in the presence of strong acids. Different titration techniques use this difference in redox properties to determine hydrogen peroxide by permanganate1 or cerium (IV)2 oxidation in the first step. In a second step, iodide is added and converted to iodine by PAA.1,2 Titration with thiosulfate is used to determine the concentration of iodine and, indirectly, the concentration of PAA. On the one hand, this titration is a very convenient way to determine the concentration of both analytes quasi-simultaneously with no need for calibration using the instable peroxide solutions themselves. On the other hand, it is known that Mn(II) ions as a reaction product of the first step may catalyze the decomposition of PAA.1 This results in a severe time dependence of the two-step titration. Furthermore, the titration methods can be applied only for relatively high concentrations and are, therefore, not suitable for the monitoring of peroxide residues. Additionally, titration has to be performed

Di Furia et al. have introduced this reaction with successive extraction of the product into chloroform to determine the PAA concentration by gas chromatography.15 Although both the liquid and the gas chromatographic approaches perform well for the analysis of most real samples, problems may occur in strongly acidic solutions with very high concentrations of hydrogen peroxide.19 This results in a limited storability of the samples. Analysis has to be done within a short time after derivatization to avoid a steadily increasing MTSO peak due to oxidation of the MTS by excess hydrogen peroxide. These problems have been overcome by the addition of manganese dioxide to the sample after derivatization to ensure stability and storability.19 However, more effort has to be put into sample

(1) D’Ans, J.; Frey, W. Chem. Ber. 1912, 45, 1845. (2) Greenspan, F. P.; McKellar, D. G. Anal. Chem. 1948, 20, 1061-1063. S0003-2700(97)00175-3 CCC: $14.00

© 1997 American Chemical Society

(3) Frew, J. E.; Jones, P.; Scholes, G. Anal. Chim. Acta 1983, 155, 139-150. (4) Davies, D. M.; Deary, M. E. Analyst 1983, 113, 1477-1479. (5) Christner, J. E.; Lucchese, L. J. (Serim Research Corp.). WO 92/22806, 1992. (6) Kru ¨ ssmann, H.; Bohnen, J. Tenside Surf. Deterg. 1994, 31 (4), 229-232. (7) Mallard de la Varende, J.; Crisinel, P. (L’Air Liquide S.A.). US 5 438 002, 1995. (8) Williams, J. (Interox Chemicals Ltd.). EP 0 150 123, 1985. (9) Fischer, W.; Arlt, E.; Braba¨nder, B. (Merck Patent GmbH). US 4 900 682, 1988. (10) Birch, B. J.; Marshman, C. E. (Unilever NV). EP 0 333 246, 1989. (11) Pinkowski, A. (ProMinent Dosiertechnik GmbH). US 5 395 493, 1995. (12) Teske, G. (Dr. Thiedig & Co.). US 5 503 720, 1996. (13) Kaden, H.; Herrmann, S. (Forschungsinstitut “Kurt Schwabe” Meinsberg). DE 43 19 002, 1995. (14) Cairns, G. T.; Ruiz Diaz, R.; Selby, K.; Waddington, D. J. J. Chromatogr. 1975, 103, 381-384. (15) Di Furia, F.; Prato, M.; Quintly, U.; Salvagno, S.; Scorrano, G. Analyst 1984, 109, 985-987. (16) Kirk, O.; Damhus, T.; Christensen, M. W. J. Chromatogr. 1992, 606, 4953. (17) Baj, S. Fresenius J. Anal. Chem. 1994, 350, 159-161. (18) Pinkernell, U.; Karst, U.; Cammann, K. Anal. Chem. 1994, 66, 2599-2602. (19) Pinkernell, U.; Effkemann, S.; Nitzsche, F.; Karst, U. J. Chromatogr. A 1996, 730, 203-208.

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preparation, as the manganese dioxide has to be removed by centrifugation prior to HPLC analysis. To overcome these problems and to obtain information about both peroxide concentrations over a wide concentration range, we have developed a method for quasi-simultaneous HPLC determination of PAA and hydrogen peroxide based on a two-step oxidation which requires no additional sample pretreatment. EXPERIMENTAL SECTION Safety Note: PAA and hydrogen peroxide are strong oxidizers, and their concentrated solutions should neither be mixed with reducing agents nor with organic substances including solvents. Concentrated peroxide solutions should, therefore, be diluted with water prior to the derivatization reaction. The procedures described here have only been tested with peroxide concentrations up to 2000 mg/L each. Chemicals. All chemicals were purchased from Aldrich Chemie (Steinheim, Germany) in the highest quality available. Organic solvents for HPLC were Merck (Darmstadt, Germany) gradient grade. Peroxide solution I was obtained from Aldrich with a concentration of 37% PAA and 5% hydrogen peroxide, while peroxide solution II was P3 Oxonia Aktiv (Henkel-Ecolab, Du¨sseldorf, Germany) with a concentration of 4.5% PAA and 25% hydrogen peroxide. Both solutions were diluted down to the typical industrial disinfection application concentration of 400 mg/L PAA. Derivatization Procedure. The following general procedure is applicable for PAA concentrations from 2.5 × 10-5 to 1.0 × 10-2 mol/L and for hydrogen peroxide concentrations from 7.5 × 10-5 to 3 × 10-3 mol/L in samples: 100 µL of a 20 mM solution of MTS in acetonitrile and 300 µL of deionized water are added to 100 µL of the sample solution. After a reaction time of 10 min, 400 µL of acetonitrile and 100 µL of a 10 mM solution of TPP in acetonitrile are added to start the second derivatization step. The solution is stored in the dark (see below), left to react for 30 min, and analyzed by HPLC subsequently. For sample concentrations exceeding the range stated above, the samples should be diluted, as higher concentrations of the reagents may lead to their precipitation after addition of water. For lower concentrations of analytes in the sample, the concentrations of the reagents may be reduced to at least a molar excess to meet the expected concentrations. It should be noted that reagent concentrations below 10-4 mol/L may result in significantly reduced reaction rates. Reagent excess should not be too large for both MTS and TPP to keep the blanks of the reaction products as low as possible (see below). In general, the reagent concentration should be at least twice as high as the expected peroxide concentration in the sample. This procedure has been applied for all samples described in this work, with dilutions of the samples and reductions of the reagent concentrations whenever required. Photometer. The HP 8453 diode array spectrophotometer (Hewlett-Packard, Waldbronn, Germany) with HP Chem Station 845x biochemical UV/VIS system software was used. UV Absorption Measurements. To determine the optimum detection wavelength for UV detection in HPLC, the spectra of MTSO (10-5 M in acetonitrile/water 40/60) and TPPO (5 × 10-6 M in acetonitrile/water 40/60) were recorded in the wavelength range from 200 to 350 nm. 3624

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HPLC Instrumentation. The high-performance liquid chromatograph consisted of the following components: two LC-10AS pumps (Shimadzu, Duisburg, Germany), a SPD-10AV detector (Shimadzu), an SIL-10A autosampler (Shimadzu), Class LC 10 Version 1.4 software (Shimadzu), and a CBM-10A controller unit (Shimadzu). The injection volume was 10 µL. The column consisted of Nucleosil C8 reversed-phase material (MachereyNagel, Du¨ren, Germany) in ChromCart cartridges (MachereyNagel): particle size, 5 µm; pore size, 100 Å; column dimensions, 70 mm × 3 mm. HPLC Analysis. The injection volume for all measurements was 10 µL, and the UV detection wavelength was 225 nm. The following gradient of acetonitrile and water was selected to achieve separation of MTS, MTSO, TPP, and TPPO: time (min:sec)

cacetonitrile (%)

0:00 3:00 3:10 4:00 4:10 6:00 (stop)

40 40 100 100 40 40

The flow rate of the mobile phase was 1.0 mL/min. Note that, as MTSO and TPPO elute early under these conditions, the gradient ascends steeply at 3 min to rinse the far less polar TPP off the column and, therefore, reduce analysis time. RESULTS AND DISCUSSION It is known from literature that hydrogen peroxide serves as a popular reagent for the preparative-scale oxidation of phosphines to the corresponding phosphine oxides.20 We have, therefore, investigated the suitability of triphenylphosphine (TPP) as a reagent for hydrogen peroxide determination using HPLC with UV/visible detection:

This reaction was intended to be used as the second reaction step after the reaction of PAA with MTS (see above) to obtain a method for simultaneous HPLC determination of PAA and hydrogen peroxide. No oxidation of TPP by MTSO is observed, although it may occur under extreme conditions (T > 100 °C, in concentrated acetic acid21 ) or in the presence of stoichiometric amounts of iodine/iodide.22 UV/visible spectra of MTSO and triphenylphosphine oxide (TPPO) were recorded to investigate the UV/visible detection conditions for HPLC analysis. The spectra are presented in Figure 1. The absorption spectra of both substances exhibit maxima at 223 (TPPO) and 227 nm (MTSO). Therefore, λ ) 225 nm is selected as the detection wavelength for single-wavelength detec(20) Svara, J.; Weferling, N.; Hofmann, T. In Ullmann’s encyclopedia of industrial chemistry, 5th rev. ed.; Elvers, B., Ed.; VCH: Weinheim, 1991; Vol. A 19, pp 545-572. (21) Szmant, H. H.; Cox, O. J. Org. Chem. 1996, 31, 1595-1598. (22) Olah, G. A.; Gupta, B. G. B.; Narang, S. C. Synth. Commun. 1978, 137138.

Figure 1. UV/visible spectra of MTSO and TPPO.

Figure 2. Chromatogram of a derivatized sample of the diluted peroxide solution I. Final concentrations in chromatography: 0.53 mM MTSO, 0.18 mM TPPO, 0.47 mM MTS, 0.32 mM TPP.

tion. If a dual-wavelength detector or a photodiode array detector is available, both substances can be analyzed at their absorption maxima. Chromatographic separation of the reagents and the products was developed on reversed-phase C8 columns (conditions are given above). A chromatogram of a peroxide solution I sample after dilution and derivatization according to the general procedure as stated above is presented in Figure 2. As can be expected from the polarity of the reagents and their reaction products, MTSO elutes first, followed by TPPO, MTS, and finally TPP. The baseline increase at t ) 4 min is due to a very rapid ascent in acetonitrile concentration, which was chosen to rinse the nonpolar TPP rapidly from the column to achieve short analysis times. As only MTSO and TPPO are quantified to obtain information on PAA and hydrogen peroxide concentrations, this shift in baseline does not have to be considered during evaluation of the chromatogram.

Figure 3. Dependence of the oxidation of TPP by hydrogen peroxide on pH and reaction time.

Reaction conditions for PAA determination with MTS have been described in ref 18. A reaction time of 10 min is sufficient to achieve quantitative reaction of PAA in the presence of at least a 2-fold excess of the reagent. Conditions for the reaction of TPP with hydrogen peroxide were investigated as follows. Chromatographic analysis of the commercially available TPP proved that the reagent already contained approximately 1% of TPPO, thus producing a significant blank which worsens the limit of detection considerably. Recrystallization of TPP from ethanol was investigated as a means for reducing the blank and resulted in a reduction of TPPO in the reagent to 0.5%. However, as the blank is rather constant in the absence of light and strong acids, and as the concentration range for hydrogen peroxide is well known for most technical applications, it is easier to adapt the reaction conditions by adding only a double molar excess of reagent where possible. This reduces the blank to a tolerable value, and subtraction of the blank yields reliable data on the hydrogen peroxide concentration. The effects of pH and reaction time have also been investigated. In Figure 3, the concentration data obtained for hydrogen peroxide using TPP as reagent are plotted depending on pH conditions and on two different reaction times. For comparison, the blanks for both reaction times and the complete pH range are given. It is obvious that the reaction occurs quantitatively and without problems for a reaction time of 30 min within the pH range from 2 to 10. On average, the TPPO concentrations determined are only slightly lower when applying a reaction time of 15 min in the same concentration range. Further investigations up to a reaction time of 120 min do not result in significant differences to 30 min. At lower pH, more TPPO is detected due to the increased oxidation of TPP by oxygen from ambient air. This effect depends on the reaction time and is observed for the samples as well as for the blanks. Above pH 10, less TPPO is detected due to either an increased decomposition rate of hydrogen peroxide in alkaline media or a nonquantitative reaction of TPP with hydrogen peroxide under the selected conditions. For the analysis of disinfectant samples, pH was between 2 and 4 due to the presence of acetic acid and sulfuric acid in the samples. Early experiments resulted in a significant increase of the TPPO blank in TPP solutions with time. As this effect was observed exclusively for reagent solutions stored in the light, a Analytical Chemistry, Vol. 69, No. 17, September 1, 1997

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Figure 4. Storability of derivatized samples depending on light during storage. Times indicated in the figure are times after addition of TPP to the sample.

Figure 6. Analysis of PAA in disinfectant solutions and brewery CIP samples using the simultaneous HPLC determination and the two-step titration.

Figure 5. Logarithmic plot for the calibration of PAA and hydrogen peroxide.

Figure 7. Analysis of hydrogen peroxide in disinfectant solutions and brewery CIP samples using the simultaneous HPLC determination and the two-step titration.

study about the storability of the samples after derivatization over 1 week was conducted. The results are presented in Figure 4 for two different peroxide solutions (see above). In general, MTSO concentrations do not increase significantly during 1 week of storage, neither in the light nor in the dark. TPPO concentrations of those samples stored in the light, however, exhibit a strong increase during the period of investigation. In contrast to this, TPPO increases are very moderate for those samples stored in the dark. Important information can be gained from this experiment. First, the data obtained for PAA are very reliable, even after a storage time of 1 week either in the light or in the dark. Storability of MTSO-containing solutions is even better in this case compared with stabilization by addition of manganese dioxide. Second, storability for TPPO is at least 1 week when the samples are stored in the dark. Although the relative increase of the TPPO concentration with time is observed in a similar way for both peroxide solutions when stored in the light after derivatization, the absolute increase is stronger for peroxide solution 2. In Figure 5, a logarithmic plot for the calibration of both peroxides is presented. Concentrations for the reagents are

reduced by a factor of 10 compared to the general derivatization procedure (see above). Linearity for the calibration curves is achieved from 2.5 × 10-6 to 1 × 10-3 mol/L for PAA (r > 0.999) and from 7.5 × 10-6 to 3 × 10-4 mol/L for hydrogen peroxide (r > 0.999) after subtraction of the blanks. Limits of detection (LOD S/N > 3) are 10-6 mol/L for PAA and 3 × 10-6 mol/L for hydrogen peroxide under these conditions. This problem is more serious for TPP than for MTS, as TPP may contain rather high TPPO concentrations (see above). The RSD for 10-fold reaction and analysis of samples with concentrations in the linear range of the calibration curves is (1% for both analytes. Commercially available disinfectant solutions and real samples from the cleaning-in-place (CIP) process of a brewery have been analyzed using the simultaneous HPLC method in comparison to the two-step titration according to refs 1 and 19. The data for PAA are presented in Figure 6, and those for hydrogen peroxide are presented in Figure 7. For both types of samples, good agreement of the data using both methods has been achieved. This proves the applicability of the simultaneous HPLC determination of PAA and hydrogen peroxide for real sample analysis in the field of disinfection control. Reproducibility problems with

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the titration are described in detail in ref 2 and may account for the differences in the disinfectant solutions 1 and 2. CONCLUSIONS The simultaneous HPLC determination of PAA and hydrogen peroxide is a valuable and reliable tool for peroxide analysis. Limits of detection are very low for both peroxides, and the linear ranges for determination can be adapted easily to varying sampling conditions by dilution of either the sample or the reagents. This HPLC method is the first instrumental method which is able to simultaneously determine PAA and hydrogen peroxide. Results obtained for disinfection samples correlate well with titration data. In comparison to titration, the new method is characterized by considerably lower limits of detection for both

analytes. Samples can be stored or shipped, after derivatization but prior to analysis, for at least 1 week. Only standard HPLC equipment is required to successfully perform this method. ACKNOWLEDGMENT Financial support by the DFG (Deutsche Forschungsgemeinschaft, Bonn, Germany) and the Fonds der Chemischen Industrie (Frankfurt, Germany) is gratefully acknowledged. U.P. thanks the Stiftung Industrieforschung (Ko¨ln, Germany) for a scholarship. Received for review February 11, 1997. Accepted June 6, 1997.X AC9701750 X

Abstract published in Advance ACS Abstracts, July 15, 1997.

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