Vapor Pressure of Thiodiglycol - Journal of Chemical & Engineering

Jan 28, 2014 - Experimental vapor pressure data have been reported previously for TDG at T = (283.15 to 353.15) K, in addition to reduced-pressure boi...
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Vapor Pressure of Thiodiglycol Ann Brozena and David E. Tevault* Edgewood Chemical Biological Center, U.S. Army Research, Development and Engineering Command, Aberdeen Proving Ground, Maryland 21010, United States

Katherine Irwin SRI International, Menlo Park, California 94025-3493, United States ABSTRACT: Thiodiglycol (TDG), also known as bis(2-hydroxyethyl) sulfide and thiodiethanol, is a precursor of the blister agent sulfur mustard (bis(2-chloroethyl) sulfide). It is also a hydrolytic degradation product, and as such, it is of considerable forensic interest to the chemical defense community. Experimental vapor pressure data have been reported previously for TDG at T = (283.15 to 353.15) K, in addition to reduced-pressure boiling points at T = (353.15 to 454.65) K. New data have been measured using two complementary American Society for Testing and Materials International methods at T = (417.75 to 538.58) K by differential scanning calorimetry and in the ambient temperature range using saturator methodology at T = (298.15 to 313.15) K. The new TDG vapor pressure data are in sharp contrast to widely accepted values from previous literature. entropy of vaporization, ΔvapHom = (26.84) kJ·mol−1 at T = (298.15) K and ΔvapSm = (32.6) J·mol−1·K−1 at T = (848) K, respectively. The previous correlations12,17 produce very similar results, and both appear to disregard the lower half of Bauer and Burschkies’ data, whose inclusion would make these calculated values even more unorthodox. The most recent compilation16 includes three points, two that appear to be based on Bauer and Burschkies’ data and a third near atmospheric pressure. Together these points produce an unlikely vapor pressure plot with positive curvature. All of these observations cast significant doubt as to the validity of Bauer and Burschkies’ data and correlations derived from them. A number of manufacturers’ data sheets available on the Internet list distillation ranges or reduced-pressure boiling points, the majority of which differ from the data of Bauer and Burschkies, but none of those provides information related to the origin of the values given; this oversight renders the accuracy of such values difficult to assess. The NIST Webbook lists a similarly undocumented normal boiling point from a 1968 Union Carbide product bulletin as well as a reduced boiling point from another handbook.18 Historical data plotted in Figure 1 illustrate the contrast between recent manufacturers’ values, reduced boiling point data, and the values published by Bauer and Burschkies; the disparity in the data is conspicuous and unresolved and suggests

1. INTRODUCTION Over the past decade, we have reported the vapor pressure of a number of chemical warfare (CW) agents and their surrogates.1−3 As a precursor and degradation product of the vesicant agent sulfur mustard, thiodiglycol, CAS No. 111-48-8, identified in this report as TDG, has been a compound of interest to the chemical defense community for many years. A literature search for TDG vapor pressure yielded information from a variety of sources. The earliest primary source reference containing experimental vapor pressure data and a detailed discussion of the method used was published by Bauer and Burschkies in 1935;4 that article contains 15 data points at T = (283.15 to 353.15) K and P = (93 to 400) Pa. Several subsequent reports of the normal and reduced pressure boiling points of TDG can be found in the literature.5−11 All of the subsequent data conflict with the data of Bauer and Burschkies; however, none of them cites or discusses the earlier data. The lower value provided by Bronnert and Saunders8 and Szabo and Stiller’s value at P = (4533) Pa11 conflicts with all of the data in the literature, and the reasons for those differences are unclear. Numerous secondary sources, including compilations and handbooks, contain either an Antoine equation,12 tables of smoothed values,13−16 or both.17 All of these references cite Bauer and Burschkies’ paper, either directly or indirectly, as their data source. Some of these values are inappropriately extrapolated well beyond Bauer and Burschkies’ experimental range. Dykyj’s correlation extrapolates to an unreasonably high value for the normal boiling point, T = (848) K, as well as extraordinarily low values for the calculated enthalpy and This article not subject to U.S. Copyright. Published 2014 by the American Chemical Society

Received: August 2, 2013 Accepted: January 16, 2014 Published: January 28, 2014 307

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taken at the intersection of tangents to the endotherm at the onset of boiling. Experimental pressure, measured using a mercury manometer, is the pressure in the cell as the specimen boils. This process is repeated with new specimens at several different experimental pressures to produce a series of data points to correlate as a function of temperature. The DSC method is recommended for use at P = (5 to 2000) kPa with pinholes ≤ 125 μm diameter. At pressures below 5 kPa, boiling endotherms become increasingly broad reflecting loss of equilibrium boiling conditions and selfpressurization inside the pan as the vaporization rate exceeds the rate at which the vaporized molecules can escape through the pinhole. Accurate determination of boiling onset temperatures is complicated with broadened endotherms. Use of larger pinholes below 5 kPa has been shown to restore boiling endotherm sharpness. Recently published results from our laboratory have demonstrated that accurate DSC data can be obtained down to pressures approaching P = (0.2) kPa by using pinhole diameters up to 350 μm;21 however, the DSC data reported herein were measured prior to optimization of this method modification. As a result, several low pressure DSC data points diverge significantly from the overall trend and were excluded from this report and correlation of the data set, consistent with previous treatment of similar data.3 Boiling endotherm broadening can also occur at higher pressures due to specimen decomposition; this behavior was observed for the DSC data point at atmospheric pressure. The value was inconsistent with the rest of the DSC data and has been excluded from the correlation. Measurement of vapor pressure in the ambient temperature range by vapor saturation was performed by placing 1 mm glass beads inside a 1 cm U tube, plugged at either end with glass wool after the beads had been coated with TDG. After deposition of the TDG, the assembly was submerged in a constant temperature bath (Forma Scientific, model 2095) and purged using UHP nitrogen (Matheson, Inc.) for a minimum of 1 h to establish thermal equilibrium and remove any residual volatile impurities. At that point, the carrier mass flow rate through the U tube was accurately measured and then diverted through a tenax (SKC, Inc.) adsorbent. The TDG contained in the effluent was collected on the tenax, extracted using methanol, and quantified using gas chromatography (HP5890) equipped with a DB-1 column, 5 m × 0.53 mm ID, 2.65 μm film. The vapor pressure at each bath temperature was calculated using the ideal gas equation as described in a previous report.3 Each saturator point was measured a minimum of three times, and the mean is reported for each temperature. Schematic representations of the experimental apparatus used to measure DSC and saturator data are available in previous literature.2,19

Figure 1. Literature vapor pressure data and Antoine equation for thiodiglycol. − − −, ref 17 Antoine equation; ▲, ref 4; △, compilations; □, manufacturers’ data sheets; +, ref 5; ○, ref 6; ☆ ref 7; ⊗ ref 8; *, ref 9; ⧫, ref 10; ●, ref 11. The lower value found in ref 8, equivalent to P = (6.6) Pa, appears to be in error. The value plotted here is P = (66) Pa. The value found in ref 11, equivalent to T = (353.15 to 362.15) K, appears to be in error. The value plotted here is the mean of T = (453.15 to 462.15) K.

that new measurements of the vapor pressure of TDG are required. The new data for TDG reported here were measured using two ASTM International methods. At high temperatures, the differential scanning calorimetry (DSC) pinhole technique19 has been used with modifications to extend the measurable range to lower pressures through the use of larger pinholes. In the ambient temperature range, ASTM vapor saturation methodology has been employed.20 Both new data sets are self-consistent and complement each other over the wide experimental temperature range covered. The new high temperature DSC data are in good agreement with literature reduced boiling points and values from manufacturers’ data sheets. The new ambient temperature saturator data conflict markedly with the only other ambient temperature data available published by Bauer and Burschkies.

2. EXPERIMENTAL SECTION Thiodiglycol was purchased from the Aldrich Chemical Co. and used without purification. Sample details are provided in Table 1. Vapor pressure measurements at high temperatures were Table 1. Sample Information

a

chemical name

source

mole fraction purity

purification method

analysis method

TDGa

Aldrich

0.99

none

NMR

TDG = bis(2-hydroxyethyl) sulfide; CAS 111-48-8.

3. RESULTS Four data points have been measured at T = (298.15 to 313.15) K using the saturator method, and an additional eight points have been measured by DSC at T = (417.75 to 538.58) K. The data sets shown in Figure 2 were combined and fitted to an Antoine equation by minimizing the sum of the squares of the differences between the natural logarithms of the measured and calculated values. The DSC data point measured at atmospheric pressure that was excluded from the Antoine fit is shown in Figure 2 by the open circle. Table 2 lists the experimental data, the calculated values using the resulting Antoine equation, the percent differences

completed using a TA Instruments (New Castle, DE) 910 differential scanning calorimeter (DSC) with a 2200 controller in accordance with an ASTM method for determination of vapor pressure by thermal analysis. With this method, a small specimen (∼ 4 μL) of the test material is sealed in a hermetictype sealable pan with a small orifice (“pinhole”) in the lid. The specimen and an empty reference pan are heated in the DSC cell at a controlled rate, dT/dt = (5) K·min−1, while the pressure in the cell is held constant. At the boiling point, the vaporizing specimen escapes from the pan through the pinhole producing a sharp endotherm reflecting the energy associated with the transition from liquid to vapor. The boiling point is 308

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Table 3. Antoine Equation and Coefficients for TDG ln(P/Pa) = a − b/(c + T/K) a b c

24.3482 6224.0 −67.9546 T = (298.15 to 521.15) K

Table 4. Calculated Vapor Pressure (Psat), Volatility (Csat), and Enthalpy of Vaporization (ΔvapHm) for TDG at Selected Temperatures T/K a

263.15 273.15a 283.15a 293.15a 298.15 303.15 313.15 323.15 333.15 343.15 353.15 373.15 393.15 413.15 433.15 453.15 473.15 493.15 513.15 533.15a 553.37a,b

Figure 2. New vapor pressure data and Antoine equation for thiodiglycol. ●, DSC data; ○, DSC data not used to derive Antoine correlation; ⧫, vapor saturation data; , Antoine equation.

between the experimental and calculated values, and the experimental uncertainty of each measurement. The Antoine equation that best describes these experimental data is shown in Table 3. Table 4 lists calculated values for the vapor pressure, volatility or saturation concentration, Csat, and enthalpy of vaporization for TDG at selected temperatures between the melting point, T = (263.15) K,18 and extrapolated normal boiling point, T = (553.37) K, based on the Antoine equation in Table 3.

4. DISCUSSION Figure 3 shows a plot of the Antoine equation calculated from the new DSC and saturator data superimposed on Bauer and Burschkies’ experimental data, several reduced-pressure boiling points reported in the literature,5−11 and the reported manufacturers’ reduced boiling points. Figure 3 demonstrates the drastic contrast between experimental data presented in this report and the original data of Bauer and Burschkies. The new DSC and saturator data are in good agreement with each other, with most of the manufacturers’ values and all but two of the published values based on reduced-pressure distillation data. We suspect that the lower value of Bronnert and Saunders8 is a transcription error and should be listed as P

a

volatility (Csat)/mg·m−3

Psat/Pa −4

−2

5.326·10 2.519·10−3 1.031·10−2 3.725·10−2 6.790·10−2 1.206·10−1 3.550·10−1 9.599·10−1 2.408·10° 5.649·10° 1.249·101 5.218·101 1.829·102 5.544·102 1.488·103 3.605·103 8.004·103 1.648·104 3.182·104 5.803·104 1.013·105

2.974·10 1.355·10−1 5.351·10−1 1.867·10° 3.347·10° 5.846·10° 1.666·101 4.365·101 1.062·102 2.419·102 5.197·102 2.055·103 6.837·103 1.972·104 5.048·104 1.169·105 2.486·105 4.914·105 9.113·105 1.600·106 2.691·106

ΔvapHm/kJ·mol−1 94.05 91.70 89.59 87.69 86.81 85.97 84.41 82.98 81.67 80.46 79.35 77.36 75.64 74.13 72.80 71.62 70.56 69.61 68.75 67.97 67.25

Extrapolated. bDecomposes.

= (0.5) mm Hg, (66) Pa, instead of P = (0.05) mm Hg as published, which would make both of these values consistent with the values predicted by our Antoine equation. Similarly, the value reported at P = (4533) Pa by Szabo and Stiller11 appears to be a typographical error that should have been T = (453.15 to 462.15) K, which is bisected by our Antoine equation, instead of T = (353.15 to 362.15) K. The normal

Table 2. Experimental TDG Vapor Pressure, Calculated Values, Percent Differences between Experimental and Correlated Values, and Experimental Uncertainty

a

T/K

uncertainty/K

298.15 303.15 308.15 313.15

0.1 0.1 0.1 0.1

417.75 429.85 446.15 462.35 476.85 498.15 521.15 538.58a

2.70 2.06 1.48 1.09 0.85 0.60 0.44 0.30

experimental Psat/Pa

uncertainty/Pa

Vapor Saturation 0.0653 0.0033 0.128 0.006 0.200 0.010 0.367 0.018 Differential Scanning Calorimetry 670 92 1310 118 2650 163 5310 221 9330 276 19880 398 39890 519 100440 N/A

calculated Psat/Pa

percent differenceb

0.06790 0.1206 0.2093 0.3550

−3.83 6.14 −4.44 3.38

702.7 1274 2673 5256 9198 19540 40730 67730

−4.65 2.83 −0.86 1.03 1.44 1.74 −2.06 N/A

Value not used to calculate Antoine correlation equation due to incipient decomposition. b100·(Pexpt − Pcalc)Pcalc−1. 309

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result in a vapor pressure error of greater than 1 % at T = (300) K. As stated in an earlier report from our laboratory,3 based on comparison to standard octanol data, the DSC data uncertainty is T = (0.3) K near the normal boiling point, T = (1.0) K at 5000 Pa, the ASTM recommended limit of the method, and T = (3) K at 640 Pa. The resulting vapor pressure uncertainties are 0.8 %, 4.0 %, and 14.4 %, respectively, for TDG. These values have been correlated, and the resulting uncertainty at each of the experimental data points is listed in Table 2. Recent method modifications using larger pinholes have significantly decreased the method uncertainty at low pressures.21 As noted earlier, the impact of the uncertainty in the lower DSC data is greatly diminished by the addition of saturator data. Our assessment is that the combination of data sets gathered using complementary methodologies greatly reduces the uncertainty attached to either data set alone. Clearly, the totality of the data strongly suggests that the new data are accurate. It is noteworthy to the chemical defense community to recognize that the vapor pressure of TDG approximates the vapor pressure of the CW agent VX (O-ethyl-S-[2(diisopropylamino)ethyl] methylphosphonothiolate)1 over the entire experimental range and thus could serve as a less toxic simulant for the toxic nerve agent in selected applications. Specifically, the vapor pressure of TDG is about one-half that of VX in the ambient temperature range, equal to VX at T = (403) K, and about 25% higher than VX near the estimated normal boiling temperature of TDG based on the current data, T = (553.37) K.

Figure 3. Vapor pressure data for thiodiglycol. ◊, this work; ▲, ref 4 ; +, ref 5; ○, ref 6; ☆, ref 7;⊗, ref 8; *, ref 9; ⧫, ref 10; ●, ref 11; □, manufacturers’ data sheets. The lower value found in ref 8, equivalent to P = (6.6) Pa, appears to be in error. The value plotted here is P = (66) Pa. The value found in ref 11, equivalent to T = (353.15 to 362.15) K, appears to be in error. The value plotted here is the mean of T = (453.15 to 462.15) K.

boiling point extrapolated from the new data, T = (553.37) K, is more reasonable than the value extrapolated from Bauer and Burschkies’ data using the earlier correlations.12,17 The calculated heat of vaporization, ΔvapHom, at T = (298.15) K based on the present work, shown in Table 4, is 86.81 kJ·mol−1. This value is within the anticipated tolerance of the value estimated using the methodology of Chickos and co-workers, 91.0 ± 5.0 kJ·mol−1.22 The calculated entropy of vaporization based on the new data is 121.5 J·K−1·mol−1 at T = (553.37) K. Trouton’s Rule predicts a value of 88 J·K−1·mol−1, but higher values can be expected for hydrogen-bonded species such as TDG. The value based on the new data is more reasonable than the 32.6 J·mol−1·K−1 calculated using Dykyj’s correlation of Bauer and Burschkies’ data. Degradation at atmospheric pressure, indicated by the broadening of the DSC curve, is noted. Our DSC data point at ambient pressure has been included in Table 2 and Figure 2 but was not used to derive the correlation between vapor pressure and temperature. The experimental uncertainties of the data measured using the methods described in this report have been addressed in detail in a previous report from our lab.3 In summary, we conclude that the major causes of uncertainty for saturator data are related to flow rate measurements through the saturator and to the collection medium, followed by analyte calibration. Each of the flow rate uncertainties is estimated by the manufacturer to be 1 % of the full scale reading for flow rates, amounting to approximately 2 % for each of the flow measurements. The estimated calibration uncertainty is about 1%. The resulting overall uncertainty is believed to be less than 5 %. We have also measured data using vapor saturation methodology for naphthalene, and the resulting data at T = (263.35 to 303.15) K averaged 3.2 % difference from values predicted by the correlation provided by Marsh.23 The average difference between experimental and correlated data shown in Table 2 for the saturator data is 4.44 %; however, one of the values reported in Table 2 has a difference between experimental and calculated vapor pressure of just over 6 %, suggesting an experimental anomaly in that measurement, possibly due, for example, to measurement of the water bath temperature. A ΔT = (0.1) K temperature excursion would

5. CONCLUSIONS TDG vapor pressure data have been measured over a wide temperature range using standard ASTM DSC and vapor saturation methods. The consistency of the data measured using these two complementary methods supports the validity of both. Data published by Bauer and Burschkies and widely cited in the literature appear to be significantly flawed. The new data reported here agree with more recent published data based on reduced-pressure boiling points and with distillation data reported by TDG manufacturers and extend the range of reliable experimental vapor pressure data for TDG well into the ambient temperature range. The new DSC data fill the T = (100) K gap between the two highest values available in previous literature. The vapor pressure of TDG and VX are similar over a very wide temperature range. TDG is deemed to be an excellent vapor pressure simulant for VX over this entire temperature range.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 410-436-3860. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to express their thanks to Ms. Linda Szafraniec and Mr. Bill Beaudry who performed NMR purity analysis on the thiodiglycol sample. We thank Mr. William Murphy, Dr. Subhash Narang, and Ms. Georgina Hum, SRI International, who facilitated obtaining the saturator data 310

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reported herein. We extend our profound gratitude to Professor James Chickos who calculated the estimated enthalpy of vaporization that appears in this manuscript.

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REFERENCES

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