Determination of Elemental Sulfur in Phosphorus Pentasulfide. 1. A

Article pubs.acs.org/IECR

Determination of Elemental Sulfur in Phosphorus Pentasulfide. 1. A New Approach Using Reverse-Phase UHPLC and HPLC Andre Grange, Jamal Kassir, and Paul Adams* Lubrizol Corp., 29400 Lakeland Boulevard, Wickliffe, Ohio 44092, United States S Supporting Information *

ABSTRACT: There are no direct analysis methods to accurately determine the amount of unbonded sulfur present in phosphorus pentasulfide (PS). Efficient processing and the performance of some industrial PS derivatives can be impacted by the level of residual free sulfur. In this study, an improved analysis of free sulfur was developed that uses a PS/methanol reaction followed by reverse-phase liquid chromatography as a key step. Significant improvements in speed and accuracy have been realized compared to a current industry method, whereby sulfur is crystallized from a methanol solution. The new method worked on two different instruments and columns for samples with free sulfur in the concentration range of 40−15000 ppm. Repeatability and spiking experiments validated the method, even though the solutions likely change with time because a byproduct slowly reacted with elemental sulfur. Another challenge with repeat and variability studies for the new method is the probability that unbonded elemental sulfur is not uniformly distributed throughout a given batch of industrial PS.

1. INTRODUCTION Phosphorus pentasulfide (PS) is an important raw material used for the manufacture of several large-volume commercial chemicals. Examples include lubricant antiwear additives,1,2 agricultural insecticides,3,4 and, to a lesser extent, ore flotation agents for mining.5−7 It is also a component of some amorphous solid electrolytes (e.g., Li2S−P2S5) for certain types of lithium batteries.8,9 Global production of PS is estimated to be in excess of 150000 tons per year. PS is produced from the reaction of molten phosphorus and sulfur at elevated temperature. In addition to major structures P4S10 and P4S9, a small amount of free sulfur remains and is inversely related to the final phosphorus content. During the course of research related to zinc dialkyldithiophosphate additive processing, the need arose to accurately determine levels of free elemental sulfur present in PS. Manufacturers today run a lengthy and labor-intensive free sulfur analysis that may result in significant error. A faster and more accurate analytical method would be a valuable tool for research, as well as help worldwide suppliers ensure consistent quality of their production. Any analysis method faces significant challenges because of the high reactivity, hygroscopic nature, and strong hydrogen sulfide odor of this chemical. Other complications include the low solubility of PS in many common solvents or its reactivity with others such as water, alcohols, and carbonyl compounds. Traditional analytical methods such as chromatography, extraction, titration, and spectroscopy offer no help. Inductively coupled plasma spectrometry measures the total amount of sulfur in a sample but cannot quantify only the unbound elemental sulfur. Therefore, an accurate quantification will likely require some kind of PS reaction, followed by the measurement of sulfur as a single analyte. Liquid chromatography is well suited for this problem because it is capable of separating and detecting elemental sulfur from the other sulfurized species present in the reaction matrix. © 2014 American Chemical Society

Currently, industrial free sulfur analysis of PS starts by reacting it with excess methanol (MeOH) to afford O,Odimethyldithiophosphoric acid and several organophosphorus side products (Figure 1). Cooling the reaction solution to 0 °C precipitates the elemental sulfur, which must be filtered, dried, and weighed. This procedure takes a significant amount of time per sample and suffers from inaccuracy at both low and high free sulfur levels. Errors are partly due to the solubility of some sulfur in cold MeOH and an upper limit as to how much will stay dissolved in the reaction solution. In our hands, the method can be highly variable, and it biases low for PS free sulfur levels in the range of 0−10000 ppm. A recent paper describes a temperature-modulated differential scanning calorimetry approach to the current problem.10 This method seeks to exploit the approximate 185 °C difference in the melting points of PS (mp ∼280 °C) and elemental sulfur. Unfortunately, the analysis is not accurate enough over the entire concentration range of interest. We are unaware of any previous reports that have used chromatography as a key step to analyze free sulfur in PS. HPLC methods have been successfully applied for the determination of elemental sulfur in sulfide minerals,11−13 coal,14 water bottom sediments,15 and foodstuffs.16 A derivatization approach followed by gas chromatorgraphy has been used for the determination of sulfur in soil17 and gasoline.18 If the sulfur present in a MeOH/PS reaction solution could be analyzed by a chromatographic separation, a new rapid and more accurate analysis would be possible. One major advantage would be minimization of sulfur in MeOH solubility errors over the entire concentration range. Received: Revised: Accepted: Published: 4429

October 11, 2013 February 10, 2014 February 14, 2014 February 14, 2014 dx.doi.org/10.1021/ie4034279 | Ind. Eng. Chem. Res. 2014, 53, 4429−4433

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Figure 1. Reaction of PS with MeOH.

industrial procedure. A total of 125 g of MeOH was added to a 250 mL flask equipped with an electric-motor-driven glass stirrer, a subsurface nitrogen line, a temperature probe, and a reflux condenser vented to an aqueous sodium hydroxide trap. A 12−20 g portion of PS weighed to the nearest 0.001 g was added to the stirred MeOH solution over a 1 h period, while the temperature was kept in the range of 35−45 °C. The temperature was raised to reflux at approximately 60 °C and held for 1 h, after which time all of the PS had reacted. After filtration of the warm reaction solution through a Buchner funnel fitted with filter paper, the filtrate was cooled in a filter flask to 0 °C and held for 3 h with periodic swirling to induce crystallization of elemental sulfur. A tared Millipore FS filter was used to collect the sulfur, which was dried to a constant weight in a vacuum oven. Dividing the weight of isolated sulfur by the weight of starting PS affords the percentage of free sulfur in the sample. Liquid Chromatography Instrumentation and Chromatographic Conditions. Most of the free sulfur PS analysis was run on a Waters UPLC system purchased from Waters Corp. (Milford, MA) and equipped with an autosampler, a solvent degasser, a binary pump, a column heater, and a photodiode array detector. An Aquity UHPLC BEH C18 (2.1 × 50 mm, 1.7 μm particle size) column from Waters Corp. was used with the heater set to 40 °C and a detection wavelength of 225 nm. The solvent flow rate was 0.6 mL/min with a mobilephase isocratic gradient of 25% water (solvent A) and 75% acetonitrile (solvent B). The total run time was 4 min, and 2 μL injections were utilized. Chromatography was also performed on an Agilent 1200 series HPLC system purchased from Agilent Technologies and equipped with an autosampler, a solvent degasser, a binary pump, a column heater, and a photodiode array detector. This system used a Phenomenex C18(2) HST (2.0 × 50 mm, 2.5 μm particle size) column purchased from Phenomenex (Torrance, CA) with the heater set to 40 °C and a detection wavelength of 225 nm. The solvent flow rate was 0.5 mL/min with a mobile-phase isocratic gradient of 25% water (solvent A) and 75% acetonitrile (solvent B). The total run time was 5 min, and 5 μL injections were utilized. Preparation of Calibration Solutions. A stock standard solution was prepared by dissolving approximately 100 mg of sulfur in 30 mL of toluene and the concentration recorded in units of mg/kg. The sulfur and toluene weights added were recorded to the nearest 0.1 mg. Five calibration standards were made from the stock solution by adding 40, 80, 120, 160, and

Our primary objective is the development of a more accurate and faster method for the determination of PS free sulfur concentrations of less than 5000 ppm.

2. EXPERIMENTAL SECTION Materials and Standards. HPLC-grade toluene and liquid chromatography−mass spectrometry (LCMS)-grade MeOH and acetonitrile were obtained from J. T. Baker (Phillipsburg, NJ) and used as received. Milled PS was obtained from commercial worldwide suppliers as well as purchased from Sigma-Aldrich (St. Louis, MO) and Acros Organics (part of Thermo Fisher Scientific, Bridgewater, NJ). Alfa Aesar (Ward Hill, MA) sublimed elemental sulfur powder was 99.5% pure and 100-mesh particle size. Water was prepared using a Barnstead EASYpure II Water Purification System (Barnstead International, Dubuque, IA). Autosampler vials were purchased from Agilent Technologies (Santa Clara, CA) and the poly(tetrafluoroethylene) (PTFE) filters from VWR Corp. (Radnor, PA). Reaction of PS with MeOH. All PS/MeOH reactions followed this general procedure. A total of 125 g of MeOH and 25 g of toluene were added to a 500 mL four-neck flask equipped with an electric-motor-driven glass stirrer, a subsurface nitrogen line, a temperature probe, and a reflux condenser vented to an aqueous sodium hydroxide trap. Toluene is added to prevent precipitation of sulfur when the reaction solution is cooled to room temperature. A 12−20 g portion of PS weighed to the nearest 0.001 g was added to the stirred MeOH/toluene solution over a 1 h period, while the temperature was kept in the range of 35−45 °C. The temperature was raised to reflux at approximately 60 °C and held for 1 h, after which time all of the PS had reacted. After cooling to room temperature, the entire reaction solution was transferred to a tared polyethylene bottle and accurately weighed just prior to HPLC or UHPLC analysis. It is important to know the exact weight of the reaction solution prior to chromatography because of MeOH volatility losses during the reaction and subsequent handling. Caution! Alcohols and PS react to produce toxic hydrogen sulfide gas. It is very important to ensure that the aqueous trap used for hydrogen sulf ide neutralization has enough equivalents of strong base present. At least 10 equiv of sodium hydroxide per 1 mol of hydrogen sulfide is suf f icient to neutralize. Aqueous sodium sulf ide should never be contacted with strong acids, which can regenerate hydrogen sulfide gas. Determination of PS Free Sulfur by MeOH Recrystallization. Analysis of PS free sulfur was run using the following 4430

dx.doi.org/10.1021/ie4034279 | Ind. Eng. Chem. Res. 2014, 53, 4429−4433

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Figure 2. UHPLC and HPLC sulfur calibration plots.

200 μL to 30 mL of toluene to give concentrations of a, b, c, d, and e. The stock solution and toluene weights added were recorded to the nearest 0.1 mg. Calibration solutions were filtered with 0.45 μm PTFE filters into 2.0 mL autosampler vials. Preparation of Unknown Solutions. Solutions for liquid chromatography were prepared by dissolving 1 mL of the MeOH/toluene reaction in 10 mL of toluene. The sulfur and toluene weights added were recorded to the nearest 0.1 mg. Unknown solutions were filtered with 0.45 μm PTFE filters into 2.0 mL autosampler vials.

(y = mx + c) was performed on the peak areas for the calibration points versus the concentration. S (ppm) =

where S = concentration of sulfur in the sample [ppm], U(p) = peak area of sulfur in the sample [area], c = axis intercept of the calibration line of sulfur [area], m = slope of the calibration line of sulfur [area/mg/kg], and U = sample concentration [mg/ kg]. With the free sulfur concentration of the reaction solution calculated, the corresponding starting PS free sulfur follows from

3. RESULTS AND DISCUSSION Preparation of Sulfur Calibration Plots. The sulfur peak areas from the UHPLC and HPLC standard chromatograms were plotted against the concentrations of the calculated calibration solutions. Both chromatographic methods showed linear plots with coefficients of determination (R2) greater than 0.999 (Figure 2). Sulfur calibration concentrations were calculated from the following equations by the instrumentation software:

SPS (ppm) =

mass sulfur (g) × 1000 mg × 1 g mass sulfur (g) + mass toluene (g) × 1 g × 0.001 kg

S(X )cal (mg/kg) =

S (ppm)/106 × US (g) mass PS (g)

where SPS = PS free sulfur [ppm], US = weight of the MeOH/ toluene reaction solution just before UHPLC or HPLC, and mass PS = weight of the starting PS sample reacted with MeOH. Our chromatography results have been reported for 14 different PS samples (Table S1 in the Supporting Information, SI). Examples include both low (i.e., ≤1000 ppm) and high (4000−15000 ppm) free sulfur PS, which demonstrates the broad application range of the new method. Typical UHPLC chromatograms for sulfur in toluene and PS/MeOH reactions with both high and low free sulfur levels are displayed (Figure 3). For the standard solution and both PS runs, sulfur eluted at 2.7 min and was well separated from any other peaks in the reaction chromatograms. Several HPLC chromatograms in Figure 4 for a different PS sample set also show sulfur (2.2 min) isolated from the early-eluting polar reaction products. It is not surprising that organophosphorus reaction products separate well and elute very early on both columns. The MeOH/PS reaction products shown in Figure 2 are all much more polar than elemental sulfur. One pair of PS samples in Table S1 in the SI (nos. 5 and 6) was analyzed by both UHPLC and HPLC methods. Free sulfur found for each of the samples was within 5% agreement by the two procedures. These results suggest that the new analysis will transfer successfully to a wide variety of different chromatograph instruments equipped with the appropriate reverse-phase column. In the last column of Table S1 in the SI, the corresponding free sulfur levels are reported from the MeOH recrystallization method, where available. Clearly, the new chromatography data

STOCK std (mg/kg) =

U (p) − c × 106 ppm m×U

mass STOCK std(X ) (g) × STOCK std (mg/kg) mass STOCK std(X ) (g) + mass acetonitrile(X ) (g)

X denotes the calibration standard, i.e., a, b, c, d, or e. Sample Analysis and Calculations. An external standard method was used to quantitate the amount of unbonded sulfur in the unknown solutions. Samples for UHPLC or HPLC were prepared immediately after the MeOH/PS reaction. A total of 5 mL of acetonitrile is weighed to the nearest milligram and added to a 10 mL vial. To this vial is added 0.5 mL of the unknown solution weighed to the nearest milligram. The concentration of the unknown solution is calculated as U where U (mg/kg) = mass sample (g ) × 1000 mg × 1 g [mass sample (g) + mass acetonitrile (g)] × 1 g × 0.001 kg

ppm Sulfur Determination. For the percent sulfur determination of the unknown solutions, a linear regression 4431

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Reproducibility can also be influenced by how uniform the sulfur concentration is within a given PS sample. It is likely because the free sulfur is not distributed evenly throughout, although confirmation would require much further study. Two different sulfur blank runs in MeOH/toluene were subjected to the same procedure as that used for the PS/ MeOH reactions and analyzed by UHPLC. Excellent agreement with the expected theoretical amount of sulfur was found for both runs (Table S3 in the SI). Recovery of Free Sulfur Spiked into PS/MeOH Solutions. In order to test the new method’s accuracy after adding a known quantity of free sulfur to the PS/MeOH reaction solutions, we ran a series of three spiked experiments. No. 1 of Table S4 in the SI is a PS sample that analyzed for 7595 ppm of free sulfur by the UHPLC method (Table S1 in the SI, no. 14). This same PS was used to add three different levels of free sulfur during the MeOH reactions, as shown in examples 2−4. In each run, the extra sulfur was added just after the PS addition until the MeOH/toluene solution was complete. All three experiments showed good agreement with the expected theoretical total sulfur levels. Comparison of Chromatographic and Crystallization Free Sulfur Analysis Methods. The preparation of sulfur calibration plots can take as little as 30 min on a chromatograph. After the PS/MeOH reaction is complete, a typical chromatography run including sample dilution is finished in less than 10 min. By contrast, the crystallization free sulfur analysis takes 4 h after the MeOH reaction while using 1 h of analyst time. For a laboratory running a significant number of free sulfur determinations, the time savings should be substantial. This would be true even after the preparation of a new daily calibration plot.

Figure 3. UHPLC chromatograms of sulfur in toluene: MeOH reactions of PS with low free sulfur (Table S1 in the SI, no. 11) and high free sulfur (Table S1 in the SI, no. 10). Sulfur elutes at 2.7 min.

show that this method usually analyzes too low. One likely reason is the solubility of sulfur in cold MeOH, which is reported to be 0.015 wt % at 0 °C.19 Figure S1 in the SI compares PS free sulfur data of 15 different PS samples by the two methods. When the recrystallization procedure usually diverges low, we attribute this to the solubility of sulfur in MeOH and the difficulty of getting all of the sulfur to precipitate from solution. A repeat study on three different days was carried out for two PS samples purchased from laboratory supply vendors (Table S2 in the SI). UHPLC analysis of a new MeOH reaction was run on each of the days. Good reproducibility was observed with divergences of 1.9−6.5% and 3.6−8.7% from the average free sulfur found for both samples. Some of the variability observed may be due to a phosphite byproduct reacting with free sulfur present in solution. This reaction has been previously reported for dialkylmonothio(hydrogen)phosphites.20,21 In fact, 31P NMR spectra of our PS/MeOH solutions do show increased levels over time of the expected phosphite/sulfur reaction product O,O-dimethyldithiophosphoric acid.

4. CONCLUSION A new, faster, and more accurate analysis of the PS free sulfur content has been developed. It can be run with either UHPLC or HPLC instruments and is applicable over a wide concentration range. The new method shortens the analysis time by around 80% compared to crystallization of the PS free sulfur from a MeOH solution. In addition, this new

Figure 4. HPLC chromatograms of MeOH reactions of PS (Table S1 in the SI, nos. 4 and 5) and sulfur in toluene. Sulfur elutes at 2.2 min. 4432

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(10) Adams, T.; Adams, P. Determination of Elemental Sulfur in Phosphorus Pentasulfide by Temperature Modulated Differential Scanning Calorimetry. J. Therm. Anal. Calorim. 2012, 109, 1285. (11) McGuire, M.; Hamers, R. Extraction and Quantitative Analysis of Elemental Sulfur from Sulfide Mineral Surfaces by HighPerformance Liquid Chromatography. Environ. Sci. Technol. 2000, 34, 4651. (12) Mustin, C.; de Donato, P.; Berthelin, J.; Marion, P. Surface Sulphur as Promoting Agent of Pyrite Leaching by Thiobacillus Ferrooxidans. FEMS Microbiol. Rev. 1993, 11, 71. (13) McGuire, M.; Banfield, J. F.; Hamers, R. Quantitative Determination of Elemental Sulfur at the Arsenopyrite Surface After Oxidation by Ferric Ion: Mechanistic Implications. Geochem. Trans. 2001, 2, 25. (14) Buchanan, D. H.; Coombs, K. J.; Murphy, P. M.; Chaven, C. A Convenient Method for the Quantitative Determination of Elemental Sulfur in Coal by HPLC Analysis of Perchloroethylene Extracts. Energy Fuels 1993, 7, 219. (15) Azarova, I. N.; Gorshikov, M. A.; Grachev, M. A.; Korzhova, E. N.; Smagunova, A. N. Determination of Elemental Sulfur in Bottom Sediments Using High-Performance Liquid Chromatography. J. Anal. Chem. 2001, 56 (10), 929. (16) Wenzel, K. The HPLC Determination of Elemental Sulfur in Foods. Eur. Food Res. Technol. 1980, 170 (10), 5. (17) Clark, P. D.; Lesage, K. L. A New Method for the Analysis of Elemental Sulfur in Oils, Soils and Other Materials. Alberta Sulfur Res. Bull. 1987, 24, 1. (18) Pauls, R. E. Determination of Elemental Sulfur in Gasoline by Gas Chromatography with On-Column Injection and Flame Ionization Detection Following Derivatization with Triphenylphosphine. J. Chromatogr. Sci. 2010, 48, 283. (19) The Sulphur Data Book; McGraw Hill Book Company, Inc.: New York, 1954; Vol. 79, p 84. (20) Perlikowska, W.; Gouygou, M.; Mikolajczyk, M.; Daran, J. C. Enantiomerically Pure Disulfides: Key Compounds in the Kinetic Resolution of Chiral PIII-Derivatives with Stereogenic Phosphorus. Tetrahedron: Asymmetry 2004, 15, 3519. (21) Wada, T.; Hata, T. A Facile Conversion of Dialkylphosphonates to Dialkyl Phosphorodithioates. Tetrahedron Lett. 1990, 31, 7461.

chromatography approach confirms that the free sulfur content of commercial PS can be much lower than 2000 ppm. With this relatively simple procedure for a difficult chemical analysis, commercial PS suppliers should be in a better position to monitor the free sulfur content and help to ensure a more consistent and less variable product. Furthermore, implementation of the chromatographic technique described herein should result in cost savings over time for those running many PS analyses.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S4 and Figure S1, which contains a graphical comparison of the HPLC and recrystallization free sulfur data for 15 different PS samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1 440 347 2217. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Lydia Raser and Dr. Paul Byers for their review of this manuscript and helpful suggestions.

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NOMENCLATURE

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REFERENCES

PS = phosphorus pentasulfide ppm = parts per million HPLC = high-performance liquid chromatography UHPLC = ultrahigh-performance liquid chromatography μL = microliter nm = nanometer mL = milliliter mg = milligram

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