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The standard molar enthalpy of the base catalysed hydrolysis of methyl paraben revisited
J. Chem. Thermodynamics 103 (2016) 176–180
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J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct
The standard molar enthalpy of the base catalysed hydrolysis of methyl paraben revisited Rafael N. Bento a, Miguel A. Rendas a, Valdir A.R. Semedo a, Carlos E.S. Bernardes a, M. Soledade C.S. Santos a, Hermínio P. Diogo b, Fernando Antunes a, Manuel E. Minas da Piedade a,⇑ a b
Centro de Química e Bioquímica e Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal
a r t i c l e
i n f o
Article history: Received 9 June 2016 Received in revised form 12 July 2016 Accepted 26 July 2016 Available online 27 July 2016 Keywords: Enthalpy of reaction Calorimetry Isothermal microcalorimetry Thermal volume Thermochemistry Living cells
a b s t r a c t The standard molar enthalpy of the base catalysed hydrolysis of methyl paraben, Dr Hom , was obtained at 298.02 K, 303.04 K and 310.01 K. The determination relied on isothermal microcalorimetry measurements and on a thermochemical scheme that, in contrast with previous strategies, does not require kinetic information. The results obtained were Dr Hom = (50.99 ± 0.61) kJmol1 at 298.02 K, Dr Hom = (50.60 ± 0.82) kJmol1 at 303.04 K, and Dr Hom = (49.64 ± 0.90) kJmol1 at 310.01 K. The Dr Hom value at 298 K is in very good agreement with the previously recommended benchmark at this temperature. The current procedure allowed, however, a 7-fold improvement in precision. It was also found that the Dr Hom values show a linear variation with temperature. The fact that this variation is small lends support to the previous assumption of a negligible temperature dependency of Dr Hom around 298 K. The present work resolves some important discrepancies in the reported standard molar enthalpies of the base catalysed hydrolysis of methyl paraben. Such inconsistencies can have a considerable impact on the determination of the effective volume of flow-through calorimetric apparatus. This parameter is essential to obtain kinetic and thermodynamic information on the growth, metabolism, and adaptation of living cells from flow calorimetry experiments. Ó 2016 Elsevier Ltd.
1. Introduction Flow microcalorimetry has been widely used to obtain kinetic and thermodynamic information on the growth, metabolism, and adaptation of living cells [1,2]. The experiments are typically carried out in the flow-through mode, where the cell culture is continuously circulated at a known flow rate between an external vessel and the calorimetric cell. The acquired signal represents the power dissipated by the cell population inside the calorimetric cell at a given time. In order to assign that power to a specific cell number or biomass it is necessary to know the thermal or effective volume (V eff ) of the calorimetric cell, which differs from its physical volume since the flow of cell culture results in transport of heat away from the detection area [3,4]. The base catalysed hydrolysis of methyl paraben (MP, Fig. 1) has been proposed as a reference system to determine V eff and also to assess the accuracy of flow-through apparatus for the determination of kinetic and thermodynamic data [3]. In these experiments ⇑ Corresponding author at: Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal. E-mail address: [email protected] (M.E. Minas da Piedade). http://dx.doi.org/10.1016/j.jct.2016.07.042 0021-9614/Ó 2016 Elsevier Ltd.
the reaction is initiated by adding solid MP to a 0.5 moldm3 NaOH aqueous solution outside the calorimetric cell. A 10-fold NaOH molar excess is used so that pseudo-first order conditions are ensured. The initial NaOH concentration corresponds to pH = 12.7. At this pH, >99.99% (mole percent) of MP will consist of MP, as can be estimated from a simple speciation analysis (see Supplementary Data) based on reported pK a data for the methyl paraben –OH group ionization in the temperature range (278–313) K (pK a = 8.42–8.03 [5] and 8.87 [6]). Thus, MP will rapidly deprotonate upon dissolution (step 1 in Fig. 1) and since the reaction mixture will typically reach the calorimetric cell in a few minutes (e.g. 2 min–15 min, depending on the flow rate) the determination of V eff will actually rely on the reaction corresponding to step 2 in Fig. 1. To obtain V eff from a flow-calorimetry run it is necessary to know the initial MP concentration (C o ), the pseudo first-order rate constant (k) for the hydrolysis reaction and the corresponding enthalpy change (Dr Hom ) [3]. The C o value is fixed by the selected initial concentration of MP. The rate constant, k, is accessible from an analysis of the calorimetrically obtained power (P) vs. time (t) pattern. The value of Dr Hom needs, however, to be determined in a separate experiment using a batch calorimeter, except
R.N. Bento et al. / J. Chem. Thermodynamics 103 (2016) 176–180 O
O OCH3
(aq)
OCH3
NaOH(aq) (1)
HO
+
Na- O (2)
Methyl Paraben (MP)
(aq) + H 2O(aq) NaOH(aq)
O
+
Na-O
O- Na + (aq) + CH 3OH(aq)
Fig. 1. Base catalysed hydrolysis of methyl paraben.
if the evolution of MP concentration inside the flow cell could be followed in real time, which is normally not the case. The data reported up to now [3,7–10] are all based on isothermal calorimetry measurements in which the reaction is also started outside the calorimeter and Dr Hom is usually obtained from a fit of the equation
lnP ¼ lnðC o kDr Hom Þ kt
ð1Þ
to the recorded P vs. t data. Because the k value derived from the slope of Eq. (1) is also needed to derive Dr Hom from the ordinate, the two parameters are highly correlated. This may perhaps explain the discrepancies found in the published Dr Hom data. Indeed, albeit Dr Hom = (50.5 ± 4.3) kJmol1 has been recommended as a benchmark for measurements carried out at 298 K [3], a literature survey (see Supplementary Data) showed that values ranging from 49.8 kJmol1 to 59.2 kJmol1 have been reported at this temperature throughout the years [3,7–10]. Different selections of Dr Hom within this range may result in considerable variations of V eff . For example, a 20% discrepancy is found when the computation of V eff is either based on Dr Hom = 49.8 kJmol1 or on Dr Hom = 59.2 kJmol1. It is also generally assumed that the enthalpy of MP hydrolysis is temperature independent in the range 298–310 K [3,4,8], even if cell studies are typically carried out at 303–310 K. This led us to revisit the determination of Dr Hom with the following main objectives: (i) to assess the accuracy of the benchmark value mentioned above and, if possible, reduce the corresponding uncertainty; (ii) to test the temperature independency hypothesis. Also worth mention is the fact that, unlike previous methodologies, the determination of Dr Hom was based on a thermodynamic cycle that avoids the need of kinetic information. 2. Materials and methods 2.1. General Elemental analysis (C, H, N) was made on a Fisons Instruments EA1108 apparatus with an uncertainty 0.3% both for the mass percentages in C and H. GC–MS analysis was performed on a TRACEÓ GC gas chromatograph coupled to a TRACEÓ MS mass detector. A DB624 capillary column from Teknokroma (5% diphenyl/95% dimethylpolysiloxane; 30 m 0.32 mm I.D., 1.80 lm df) was used. The carrier gas was helium maintained at a constant flow of 1.0 cm3min1. A vaporization injector operating in the split less mode (1 min), at 483 K, was employed, and the oven temperature was programmed as follows: ramp from 323 K to 363 K at 8 Kmin1, isothermal step (4 min), ramp at 6 Kmin1 to 443 K, isothermal step (1 min), ramp at 4.5 Kmin1 to 513 K, isothermal step (11 min). The transfer line, ion source, and quadrupole analyser were maintained at 493 K, 503 K, and 433 K, respectively, without a solvent delay. In the full-scan mode, electron ionization mass spectra in the range 50–461 Da were recorded at 70 eV electron energy and with an ionization current of 6.8 lA. Data recording
177
and instrument control were performed by the XcaliburÓ v1.2 software. The identity of the compound analysed compound was assigned by comparison of the mass-spectrometric results with the data in Wiley’s reference spectral databank (Mainlib) and its purity was calculated from the normalized peak areas, without using correction factors to establish abundances. The X-ray powder diffractograms were recorded on a Philips X’Pert PRO diffractometer with automatic data acquisition (X’Pert Data Collector v2.0b), operating in the h–2h mode. The apparatus had a vertical PW 3050/60 goniometer and a X’Celerator detector. A monochromatized Cu Ka radiation source set to 30 mA and 40 kV was used. The diffractograms were recorded at 293 K, in the range 5° < 2h < 35° using the continuous scanning mode with a step size of 0.017° (2h) and a scan step interval of 20 s. The samples were mounted on an aluminium sample holder. The indexation of the powder patterns was performed using the program Celref v3 [11]. Differential scanning calorimetry experiments were performed on a DSC 204 F1-Phoenix from Netzsch at a heating rate of 5 Kmin1. The temperature and heat flow scales of the instrument were calibrated at the same heating rate with the following standards from Netzsch: adamantane (mass fraction > 0.99, Tfus = 208.65 K, Dfus h = 22.0 Jg1), indium (mass fraction = 0.99999, Tfus = 429.75 K, Dfus h = 28.6 Jg1), tin (mass fraction = 0.99999, Dfus h = 60.5 Jg1), bismuth (mass fracTfus = 505.05 K, tion = 0.99999, Tfus = 544.55 K, Dfus h = 53.1 Jg1), zinc (mass fraction = 0.99999, Tfus = 692.65 K, Dfus h = 107.5 Jg1) and cesium chloride (mass fraction = 0.99999, Tfus = 749.15 K, Dfus h = 17.2 Jg1). The MP samples with a mass of 6–9 mg were sealed in aluminium crucibles and weighed with a precision of ±0.1 lg on a Mettler XP2U ultra-micro balance. Nitrogen (Air Liquide N45) was used as both the purge and protective gas, at flow rates of 20 cm3min1 and 50 cm3min1, respectively. The instrument control and data treatment procedures were carried out with the Netzsch Proteus Software v6.1.0.
2.2. Materials Methyl paraben (methyl 4-hydroxybenzoate, Aldrich mass fraction >0.99; CAS number 99-76-3) was purified by sublimation at (338 ± 1) K. Elemental analysis for C8H8O3 (mass percentage): expected C 63.15, H 5.30; found C (63.43 ± 0.08), H, (5.35 ± 0.09) where the assigned uncertainty is twice the mean deviation of two independent determinations. GC–MS analysis gave a mass fraction >0.999 for the purified material. Both PXRD and DSC analysis indicated that the sample corresponds to polymorph I of methyl paraben. The powder pattern, recorded at (296 ± 2) K, was indexed as monoclinic, space group Cc, Z = 12, a = (1350.8 ± 4.9) pm, b = (1691.0 ± 2.0) pm, c = (1243.5 ± 4.4) pm, b = (130.05)±0.18°, crystal density d = (1394 ± 32) kgm3. These values are in good agreement with the following single crystal Xray diffraction data (SCXRD) for form I methyl paraben, at 293 K: space group Cc, Z = 12, a = (1356.8 ± 0.5) pm, b = (1695.9 ± 0.7) pm, c = (1245.8 ± 0.6) pm, b = (130.1 ± 0.3)°, crystal density d = (1383 ± 53) kgm3 [12,13]. Note that the uncertainties quoted for both the PXRD and SCXRD data standard represent standard deviations (u). No phase transitions other than fusion were observed by DSC in the range 283–423 K. The onset, maximum and end temperatures of the fusion peak were Ton = (398.6 ± 0.2) K, Tmax = (402.5 ± 0.4) K, and Tend = (404.1 ± 0.4) K. The corresponding enthalpy of fusion was Dfus Hom = (30.2 ± 0.2) kJmol1. The uncertainties quoted for Ton, Tmax, Tend and Dfus Hom correspond to twice the standard error of the mean of five independent determinations. The Ton value mentioned above is in agreement with previously reported fusion
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R.N. Bento et al. / J. Chem. Thermodynamics 103 (2016) 176–180
temperatures for polymorph I of methyl paraben (399.2 ± 0.4) K [14], 398.6 K [15], (399.2 ± 0.5) K [16]. The corresponding enthalpy of fusion is, however, 4–5 kJmol1 higher than the published results (25.3 ± 0.7) kJmol1 [14], (26.3 ± 0.1) kJmol1 [15]. The sodium hydroxide solutions used in all calorimetric experiments were prepared by dilution of a certified concentrate, Fisher Chemical, molality b = (0.998 ± 0.001 molkg1) using deionized water (conductivity, <0.1 lScm1) from a Millipore system. The KCl sample used to assess the accuracy of the calorimetric system (Fluka, mass fraction 0.999995) through enthalpy of solution measurements in distilled and deionized water was not further purified. Table 1 summarizes relevant information on the provenance and mass fraction purity of the methyl paraben and KCl samples used in this work.
respectively, where Ac is the area of the calibration curve; Vi and Ii are the ith voltage and current readings by the power supply and the digital multimeter, respectively; Dt i is the time difference between two consecutive data acquisitions during calibration; A represents the area of the curve obtained in the main experiment; and m and M are the mass and molar mass of the sample, respectively. The obtained calibration constants were: e= (6.97566 ± 0.02640) lWmV1 at 298.02 K, e = (6.90676 ± 0.00879) lWmV1 at 303.04 K, and e = (6.83374 ± 0.00773) lWmV1 at 310.01 K. The indicated uncertainties are the standard errors of the mean of 6, 12, and 7 determinations, respectively. It is interesting to note that the e values show an approximate linear dependency with temperature. Indeed, a least squares fit to these values led to:
e=ðlW mV1 Þ ¼ ð1:1762 0:1802Þ 102 ðT=KÞ þ 10:477 0:547
2.3. Isothermal microcalorimetry
ð4Þ 2
Isothermal microcalorimetry experiments were carried out at 298.02 K, 303.04 K, and 310.01 K using a LKB 2277 Thermal Activity Monitor (TAM), with a 15.0 cm3 stirred stainless steel cell containing in-house designed systems for electrical calibration and solid sample dissolution. In a typical experiment, a MP sample with mass m 30 mg was dropped into 12 g of a (0.496 ± 0.001) molkg1 NaOH aqueous solution (NaOH:112H2O) and the output of the thermopiles was monitored with an Agilent 34420A nanovoltmeter until the calorimetric signal returned to the baseline. Completion of the hydrolysis reaction under the concentration and temperature conditions of the calorimetric experiments had been previously reported [8]. This was nevertheless confirmed in the present work by performing spectrophotometric runs where the absorbance of the solution was followed at 272 nm. This wavelength corresponds to the formation of the final MP-hydrolysis product, identified in preliminary scans of the reaction mixture over time. Electrical calibrations were performed before initiation or after completion of the calorimetric process. The electrical calibration circuit consisted of a glass encased 22 X manganin resistance connected in a four-wire configuration to an Agilent 34401A multimeter and an Agilent 6611C DC power supply. The calorimeter was kept in an air-conditioned room whose temperature was regulated to (295 ± 1) K. Instrument control and data acquisition were performed with the CBCAL 3.0 program [17]. The energy equivalent, e, of the calorimeter and the enthalpy of the calorimetric process, Dr Hom , were obtained from:
e¼
P
Dr Hom
i V i I i Dt i Ac
ð2Þ
M ¼ eA m
ð3Þ
for 95% probability, with a regression coefficient R = 0.994. The accuracy of the electrical calibration was tested by using the enthalpy of solution of KCl in water to yield a KCl:1525H2O solution. The obtained value was Dr Hom = (17.39 ± 0.12) kJmol1, where the assigned uncertainty is twice the standard error of the mean of seven independent determinations. This result is in excellent agreement with the corresponding value, Dr Hom = (17.48 ± 0.08) kJmol1, calculated from data in the NBS tables [18] after correction for the molar mass used in this work (M = 74.548 gmol1). 3. Results and discussion All molar quantities were based on the molar mass of methyl paraben M = 152.149 gmol1, calculated from the conventional atomic masses recommended by the IUPAC Commission in 2013 [19]. The enthalpy of hydrolysis of methyl paraben corresponding to step 2 in Fig. 1, which is given by reaction (5), can be derived by combining the enthalpy of reaction (6) obtained in this work by isothermal calorimetry and the enthalpies of processes (7)–(9): O
O OCH3
−
O−
(aq) + OH -(aq)
−
O O
ð5Þ
O OCH 3
O−
(cr) + 2OH− (aq)
−
HO
(aq) + CH3 OH(aq) + H 2O(l)
O
ð6Þ O
O OCH3
OCH 3
(cr) + H 2O(l)
HO
Material
CAS number
Supplier
Mass fraction purity
Methyl 4-hydroxybenzoate (cr I) Potassium chloride (cr)
99-76-3 7447-40-7
Aldrich Fluka
>0.999a 0.999995b
Mass fraction purity from GC–MS analysis after purification carried out in this work by sublimation and identification of the sample as polymorph I of methyl paraben (cr I) from X-ray powder diffraction analysis. The initial mass fraction indicated by the supplier was >0.99. b Given by the supplier.
(aq)
ð7Þ
HO
O
Table 1 Provenance and mass fraction purity of the material used in this work.
a
(aq) + CH3 OH(aq)
O
O OCH 3
(aq)
HO
OCH 3 −O
(aq) + H+(aq)
H2 OðlÞ ! Hþ ðaqÞ þ OH ðaqÞ
ð8Þ
ð9Þ
by using the following equations:
Dr Hom ð5Þ ¼ Dr Hom ð6Þ Dsln Hom ð7Þ Dr Hom ð8Þ þ Dr Hom ð9Þ Dsln Hom ð7Þ=kJ mol
1
¼ 0:26086ðT=KÞ 49:96
ð10Þ ð11Þ
R.N. Bento et al. / J. Chem. Thermodynamics 103 (2016) 176–180
Dr Hom ð8Þ=kJ mol
1
¼ 0:15800ðT=KÞ þ 65:20
ð12Þ
Dr Hom ð9Þ=kJ mol
1
¼ 0:20409ðT=KÞ þ 116:68
ð13Þ
Eq. (11) was derived from a linear least squares regression to Dsln Hom (7) values calculated from critically analysed solubility data [20]. Eqs. (12) and (13) were analogously obtained from literature data for the enthalpies of acid dissociation of MP [5] and autoionization of water [21], respectively. The values of Dr Hom (5) to Dr Hom (9) at the three temperatures studied in this work are summarized in Table 2. The uncertainty assigned to Dr Hom (6) corresponds to twice the standard error of the mean of seven, ten and six determinations, for T = 298.02 K, T = 303.04 K, and T = 310.01 K, respectively. In the case of Dr Hom (7) and Dr Hom (8) a 1% uncertainty was assumed in this work. The uncertainty of Dr Hom (9) is that given by the authors [21]. As shown in Fig. 2, a very good linear relationship was found when the Dr Hom (6) results obtained in this work (Table 2) were plotted as a function of temperature. A linear least squares regression to those values led to (95% probability): 1
Dr Hom ð6Þ=ðkJ mol Þ ¼ ð0:4219 0:0324ÞðT=KÞ 186:75 9:86
ð14Þ
with a regression coefficient R2 = 0.998. From the slope of Eq. (14) the heat capacity change of the reaction in the temperature range studied can be derived as Dr C op;m ± U c = (422 ± 32) JK1mol1. Given the linear variations of Dr Hom (6) to Dr Hom (9) with temperature, a linear relationship of Dr Hom (5) with temperature was necessarily found (95% probability): 1
Dr Hom ð5Þ=ðkJ mol Þ ¼ ð0:1142 0:0334ÞðT=KÞ 85:08 10:16
ð15Þ
the regression coefficient being R2 = 0.98 and Dr C op;m ± U c = (114 ± 33) JK1mol1. It should, nevertheless, be noted that the observed variation of Dr Hom (5) with temperature is covered by the uncertainty of the determinations, as can be concluded from Table 2. The standard molar enthalpy of the base catalysed hydrolysis of methyl paraben here obtained at 298.02 K, Dr Hom (5) = (50.99 ± 0.61) kJmol1, is in good agreement with the previously recommended benchmark value at 298 K temperature, Dr Hom (5) = (50.5 ± 4.3) kJmol1 [3], within their combined uncertainty intervals. An approximately seven fold improvement in precision was, however, achieved in the present work. Finally, the small temperature dependence found for Dr Hom (5) in the range 298.02– 310.01 K also lends support to the assumption that the enthalpy of the base catalysed hydrolysis of methyl paraben is approximately constant around 298 K [3], particularly if the ±4.3 kJmol1 uncertainty of the previously recommended benchmark is considered.
Table 2 Standard (p° = 100 kPa) temperatures. Reaction
(5) (6) (7) (8) (9)
molar
enthalpies
of
reactions
(5)–(9)
at
different
Dr Hom /(kJmol1) T = 298.02 K
T = 303.04 K
T = 310.01 K
(50.99 ± 0.61) (60.96 ± 0.50) 27.78 ± 0.28 18.11 ± 0.18 55.86 ± 0.10
(50.60 ± 0.82) (59.02 ± 0.74) 29.09 ± 0.29 17.32 ± 0.17 54.83 ± 0.10
(49.64 ± 0.90) (55.92 ± 0.82) 30.91 ± 0.31 16.22 ± 0.16 53.41 ± 0.10
179
Fig. 2. Standard molar enthalpy of reaction (6) as a function of temperature.
4. Conclusions Results from an isothermal microcalorimetry study together with literature data allowed the determination of the standard molar enthalpy of the base catalysed hydrolysis of methyl paraben according to reaction (5), Dr Hom (5), at 298.02 K, 303.04 K and 310.01 K. Unlike previous determinations, the adopted strategy avoided the need of kinetic information, an aspect that may be at the root of the significant discrepancies found in the reported MP enthalpy of hydrolysis data [3,7–10]. Furthermore, it also allowed a ca. 7-fold improvement in precision relative to the previously proposed benchmark value of Dr Hom (5) at 298 K [3], which is, nevertheless, in excellent agreement with the present determinations. Finally, the small temperature dependence found for Dr Hom (5) in the range 298.02–310.01 K, also supports the assumption [3] that the enthalpy of the base catalysed hydrolysis of methyl paraben is approximately constant around 298 K. Eq. (15) is, nevertheless, recommended for the calculation of benchmark Dr Hom (5) values used to determine the effective volume of flow-through calorimetric apparatus, within the approximate range 298–310 K. Acknowledgments This work was supported by FCT, Portugal, through Projects PEst-OE/QUI/UI0612/2013 and PEst-OE/QUI/UI0100/2013, and a post-doctoral grant awarded to C.E.S. Bernardes (SFRH/ BPD/101505/2014). Thanks are also due to PARALAB (Portugal) and NETZSCH (Germany) for providing the Netzsch DSC 204 F1Phoenix apparatus. We finally thank Prof. C. Teixeira (CQE-IST, Portugal) for all her help during the set up of the TAM apparatus at CQB-FCUL and Prof. M.C. Oliveira (CQE-IST, Portugal) for the GC– MS analysis. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jct.2016.07.042. References [1] A.M. James, Thermal and Energetic Studies of Cellular Biological Systems, IOP Publishing, Bristol, 1987. [2] A.E. Beezer, Biological Microcalorimetry, Academic Press, London, 1980. [3] M.A.A. O’Neill, A.E. Beezer, C. Labetoulle, L. Nicolaides, J.C. Mitchell, J.A. Orchard, J.A. Connor, R.B. Kemp, D. Olomolaiye, Thermochim. Acta 399 (2003) 63–71. [4] M.A.A. O’Neill, A.E. Beezer, G.J. Vine, R.B. Kemp, D. Olomolaiye, P.L.O. Volpe, D. Oliveira, Thermochim. Acta 413 (2004) 193–199. [5] A. Kroflicˇ, A. Apelblat, M. Bešter-Rogacˇ, J. Phys. Chem. B 116 (2012) 1385–1392.
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[16] T. Gelbrich, D.E. Braun, A. Ellern, U.J. Griesser, Cryst. Growth Des. 13 (2013) 1206–1217. [17] C.E.S. Bernardes, CBCAL 3.0 – A Program for Calorimeter Control and Data Acquisition, FCUL, Lisbon, 2016. [18] D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Churney, R.L. Nuttall, The NBS tables of chemical thermodynamics properties, J. Phys Chem. Ref. Data 11 (Suppl. 12) (1982) 11. [19] J. Meija, T.B. Coplen, M. Berglund, W.A. Brand, P. De Bièvre, M. Gröning, N.E. Holden, J. Irrgeher, R.D. Loss, T. Walczyk, T. Prohaska, Pure Appl. Chem. 88 (2016) 265–291. [20] A. Goto, H. Miyamoto, M. Salomon, R. Goto, H. Fukuda, E. Konigsberger, L.C. Konigsberger, J. Phys. Chem. Ref. Data 40 (2011) 013101. [21] B.A. Patterson, E.M. Woolley, J. Chem. Thermodyn. 34 (2002) 535–556.
JCT 16-461
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The standard molar enthalpy of the base catalysed hydrolysis of methyl paraben revisited Rafael N. Bento a, Miguel A. Rendas a, Valdir A.R. Semedo a, Carlos E.S. Bernardes a, M. Soledade C.S. Santos a, Hermínio P. Diogo b, Fernando Antunes a, Manuel E. Minas da Piedade a,⇑ a b
Centro de Química e Bioquímica e Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal
a r t i c l e
i n f o
Article history: Received 9 June 2016 Received in revised form 12 July 2016 Accepted 26 July 2016 Available online 27 July 2016 Keywords: Enthalpy of reaction Calorimetry Isothermal microcalorimetry Thermal volume Thermochemistry Living cells
a b s t r a c t The standard molar enthalpy of the base catalysed hydrolysis of methyl paraben, Dr Hom , was obtained at 298.02 K, 303.04 K and 310.01 K. The determination relied on isothermal microcalorimetry measurements and on a thermochemical scheme that, in contrast with previous strategies, does not require kinetic information. The results obtained were Dr Hom = (50.99 ± 0.61) kJmol1 at 298.02 K, Dr Hom = (50.60 ± 0.82) kJmol1 at 303.04 K, and Dr Hom = (49.64 ± 0.90) kJmol1 at 310.01 K. The Dr Hom value at 298 K is in very good agreement with the previously recommended benchmark at this temperature. The current procedure allowed, however, a 7-fold improvement in precision. It was also found that the Dr Hom values show a linear variation with temperature. The fact that this variation is small lends support to the previous assumption of a negligible temperature dependency of Dr Hom around 298 K. The present work resolves some important discrepancies in the reported standard molar enthalpies of the base catalysed hydrolysis of methyl paraben. Such inconsistencies can have a considerable impact on the determination of the effective volume of flow-through calorimetric apparatus. This parameter is essential to obtain kinetic and thermodynamic information on the growth, metabolism, and adaptation of living cells from flow calorimetry experiments. Ó 2016 Elsevier Ltd.
1. Introduction Flow microcalorimetry has been widely used to obtain kinetic and thermodynamic information on the growth, metabolism, and adaptation of living cells [1,2]. The experiments are typically carried out in the flow-through mode, where the cell culture is continuously circulated at a known flow rate between an external vessel and the calorimetric cell. The acquired signal represents the power dissipated by the cell population inside the calorimetric cell at a given time. In order to assign that power to a specific cell number or biomass it is necessary to know the thermal or effective volume (V eff ) of the calorimetric cell, which differs from its physical volume since the flow of cell culture results in transport of heat away from the detection area [3,4]. The base catalysed hydrolysis of methyl paraben (MP, Fig. 1) has been proposed as a reference system to determine V eff and also to assess the accuracy of flow-through apparatus for the determination of kinetic and thermodynamic data [3]. In these experiments ⇑ Corresponding author at: Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal. E-mail address: [email protected] (M.E. Minas da Piedade). http://dx.doi.org/10.1016/j.jct.2016.07.042 0021-9614/Ó 2016 Elsevier Ltd.
the reaction is initiated by adding solid MP to a 0.5 moldm3 NaOH aqueous solution outside the calorimetric cell. A 10-fold NaOH molar excess is used so that pseudo-first order conditions are ensured. The initial NaOH concentration corresponds to pH = 12.7. At this pH, >99.99% (mole percent) of MP will consist of MP, as can be estimated from a simple speciation analysis (see Supplementary Data) based on reported pK a data for the methyl paraben –OH group ionization in the temperature range (278–313) K (pK a = 8.42–8.03 [5] and 8.87 [6]). Thus, MP will rapidly deprotonate upon dissolution (step 1 in Fig. 1) and since the reaction mixture will typically reach the calorimetric cell in a few minutes (e.g. 2 min–15 min, depending on the flow rate) the determination of V eff will actually rely on the reaction corresponding to step 2 in Fig. 1. To obtain V eff from a flow-calorimetry run it is necessary to know the initial MP concentration (C o ), the pseudo first-order rate constant (k) for the hydrolysis reaction and the corresponding enthalpy change (Dr Hom ) [3]. The C o value is fixed by the selected initial concentration of MP. The rate constant, k, is accessible from an analysis of the calorimetrically obtained power (P) vs. time (t) pattern. The value of Dr Hom needs, however, to be determined in a separate experiment using a batch calorimeter, except
R.N. Bento et al. / J. Chem. Thermodynamics 103 (2016) 176–180 O
O OCH3
(aq)
OCH3
NaOH(aq) (1)
HO
+
Na- O (2)
Methyl Paraben (MP)
(aq) + H 2O(aq) NaOH(aq)
O
+
Na-O
O- Na + (aq) + CH 3OH(aq)
Fig. 1. Base catalysed hydrolysis of methyl paraben.
if the evolution of MP concentration inside the flow cell could be followed in real time, which is normally not the case. The data reported up to now [3,7–10] are all based on isothermal calorimetry measurements in which the reaction is also started outside the calorimeter and Dr Hom is usually obtained from a fit of the equation
lnP ¼ lnðC o kDr Hom Þ kt
ð1Þ
to the recorded P vs. t data. Because the k value derived from the slope of Eq. (1) is also needed to derive Dr Hom from the ordinate, the two parameters are highly correlated. This may perhaps explain the discrepancies found in the published Dr Hom data. Indeed, albeit Dr Hom = (50.5 ± 4.3) kJmol1 has been recommended as a benchmark for measurements carried out at 298 K [3], a literature survey (see Supplementary Data) showed that values ranging from 49.8 kJmol1 to 59.2 kJmol1 have been reported at this temperature throughout the years [3,7–10]. Different selections of Dr Hom within this range may result in considerable variations of V eff . For example, a 20% discrepancy is found when the computation of V eff is either based on Dr Hom = 49.8 kJmol1 or on Dr Hom = 59.2 kJmol1. It is also generally assumed that the enthalpy of MP hydrolysis is temperature independent in the range 298–310 K [3,4,8], even if cell studies are typically carried out at 303–310 K. This led us to revisit the determination of Dr Hom with the following main objectives: (i) to assess the accuracy of the benchmark value mentioned above and, if possible, reduce the corresponding uncertainty; (ii) to test the temperature independency hypothesis. Also worth mention is the fact that, unlike previous methodologies, the determination of Dr Hom was based on a thermodynamic cycle that avoids the need of kinetic information. 2. Materials and methods 2.1. General Elemental analysis (C, H, N) was made on a Fisons Instruments EA1108 apparatus with an uncertainty 0.3% both for the mass percentages in C and H. GC–MS analysis was performed on a TRACEÓ GC gas chromatograph coupled to a TRACEÓ MS mass detector. A DB624 capillary column from Teknokroma (5% diphenyl/95% dimethylpolysiloxane; 30 m 0.32 mm I.D., 1.80 lm df) was used. The carrier gas was helium maintained at a constant flow of 1.0 cm3min1. A vaporization injector operating in the split less mode (1 min), at 483 K, was employed, and the oven temperature was programmed as follows: ramp from 323 K to 363 K at 8 Kmin1, isothermal step (4 min), ramp at 6 Kmin1 to 443 K, isothermal step (1 min), ramp at 4.5 Kmin1 to 513 K, isothermal step (11 min). The transfer line, ion source, and quadrupole analyser were maintained at 493 K, 503 K, and 433 K, respectively, without a solvent delay. In the full-scan mode, electron ionization mass spectra in the range 50–461 Da were recorded at 70 eV electron energy and with an ionization current of 6.8 lA. Data recording
177
and instrument control were performed by the XcaliburÓ v1.2 software. The identity of the compound analysed compound was assigned by comparison of the mass-spectrometric results with the data in Wiley’s reference spectral databank (Mainlib) and its purity was calculated from the normalized peak areas, without using correction factors to establish abundances. The X-ray powder diffractograms were recorded on a Philips X’Pert PRO diffractometer with automatic data acquisition (X’Pert Data Collector v2.0b), operating in the h–2h mode. The apparatus had a vertical PW 3050/60 goniometer and a X’Celerator detector. A monochromatized Cu Ka radiation source set to 30 mA and 40 kV was used. The diffractograms were recorded at 293 K, in the range 5° < 2h < 35° using the continuous scanning mode with a step size of 0.017° (2h) and a scan step interval of 20 s. The samples were mounted on an aluminium sample holder. The indexation of the powder patterns was performed using the program Celref v3 [11]. Differential scanning calorimetry experiments were performed on a DSC 204 F1-Phoenix from Netzsch at a heating rate of 5 Kmin1. The temperature and heat flow scales of the instrument were calibrated at the same heating rate with the following standards from Netzsch: adamantane (mass fraction > 0.99, Tfus = 208.65 K, Dfus h = 22.0 Jg1), indium (mass fraction = 0.99999, Tfus = 429.75 K, Dfus h = 28.6 Jg1), tin (mass fraction = 0.99999, Dfus h = 60.5 Jg1), bismuth (mass fracTfus = 505.05 K, tion = 0.99999, Tfus = 544.55 K, Dfus h = 53.1 Jg1), zinc (mass fraction = 0.99999, Tfus = 692.65 K, Dfus h = 107.5 Jg1) and cesium chloride (mass fraction = 0.99999, Tfus = 749.15 K, Dfus h = 17.2 Jg1). The MP samples with a mass of 6–9 mg were sealed in aluminium crucibles and weighed with a precision of ±0.1 lg on a Mettler XP2U ultra-micro balance. Nitrogen (Air Liquide N45) was used as both the purge and protective gas, at flow rates of 20 cm3min1 and 50 cm3min1, respectively. The instrument control and data treatment procedures were carried out with the Netzsch Proteus Software v6.1.0.
2.2. Materials Methyl paraben (methyl 4-hydroxybenzoate, Aldrich mass fraction >0.99; CAS number 99-76-3) was purified by sublimation at (338 ± 1) K. Elemental analysis for C8H8O3 (mass percentage): expected C 63.15, H 5.30; found C (63.43 ± 0.08), H, (5.35 ± 0.09) where the assigned uncertainty is twice the mean deviation of two independent determinations. GC–MS analysis gave a mass fraction >0.999 for the purified material. Both PXRD and DSC analysis indicated that the sample corresponds to polymorph I of methyl paraben. The powder pattern, recorded at (296 ± 2) K, was indexed as monoclinic, space group Cc, Z = 12, a = (1350.8 ± 4.9) pm, b = (1691.0 ± 2.0) pm, c = (1243.5 ± 4.4) pm, b = (130.05)±0.18°, crystal density d = (1394 ± 32) kgm3. These values are in good agreement with the following single crystal Xray diffraction data (SCXRD) for form I methyl paraben, at 293 K: space group Cc, Z = 12, a = (1356.8 ± 0.5) pm, b = (1695.9 ± 0.7) pm, c = (1245.8 ± 0.6) pm, b = (130.1 ± 0.3)°, crystal density d = (1383 ± 53) kgm3 [12,13]. Note that the uncertainties quoted for both the PXRD and SCXRD data standard represent standard deviations (u). No phase transitions other than fusion were observed by DSC in the range 283–423 K. The onset, maximum and end temperatures of the fusion peak were Ton = (398.6 ± 0.2) K, Tmax = (402.5 ± 0.4) K, and Tend = (404.1 ± 0.4) K. The corresponding enthalpy of fusion was Dfus Hom = (30.2 ± 0.2) kJmol1. The uncertainties quoted for Ton, Tmax, Tend and Dfus Hom correspond to twice the standard error of the mean of five independent determinations. The Ton value mentioned above is in agreement with previously reported fusion
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R.N. Bento et al. / J. Chem. Thermodynamics 103 (2016) 176–180
temperatures for polymorph I of methyl paraben (399.2 ± 0.4) K [14], 398.6 K [15], (399.2 ± 0.5) K [16]. The corresponding enthalpy of fusion is, however, 4–5 kJmol1 higher than the published results (25.3 ± 0.7) kJmol1 [14], (26.3 ± 0.1) kJmol1 [15]. The sodium hydroxide solutions used in all calorimetric experiments were prepared by dilution of a certified concentrate, Fisher Chemical, molality b = (0.998 ± 0.001 molkg1) using deionized water (conductivity, <0.1 lScm1) from a Millipore system. The KCl sample used to assess the accuracy of the calorimetric system (Fluka, mass fraction 0.999995) through enthalpy of solution measurements in distilled and deionized water was not further purified. Table 1 summarizes relevant information on the provenance and mass fraction purity of the methyl paraben and KCl samples used in this work.
respectively, where Ac is the area of the calibration curve; Vi and Ii are the ith voltage and current readings by the power supply and the digital multimeter, respectively; Dt i is the time difference between two consecutive data acquisitions during calibration; A represents the area of the curve obtained in the main experiment; and m and M are the mass and molar mass of the sample, respectively. The obtained calibration constants were: e= (6.97566 ± 0.02640) lWmV1 at 298.02 K, e = (6.90676 ± 0.00879) lWmV1 at 303.04 K, and e = (6.83374 ± 0.00773) lWmV1 at 310.01 K. The indicated uncertainties are the standard errors of the mean of 6, 12, and 7 determinations, respectively. It is interesting to note that the e values show an approximate linear dependency with temperature. Indeed, a least squares fit to these values led to:
e=ðlW mV1 Þ ¼ ð1:1762 0:1802Þ 102 ðT=KÞ þ 10:477 0:547
2.3. Isothermal microcalorimetry
ð4Þ 2
Isothermal microcalorimetry experiments were carried out at 298.02 K, 303.04 K, and 310.01 K using a LKB 2277 Thermal Activity Monitor (TAM), with a 15.0 cm3 stirred stainless steel cell containing in-house designed systems for electrical calibration and solid sample dissolution. In a typical experiment, a MP sample with mass m 30 mg was dropped into 12 g of a (0.496 ± 0.001) molkg1 NaOH aqueous solution (NaOH:112H2O) and the output of the thermopiles was monitored with an Agilent 34420A nanovoltmeter until the calorimetric signal returned to the baseline. Completion of the hydrolysis reaction under the concentration and temperature conditions of the calorimetric experiments had been previously reported [8]. This was nevertheless confirmed in the present work by performing spectrophotometric runs where the absorbance of the solution was followed at 272 nm. This wavelength corresponds to the formation of the final MP-hydrolysis product, identified in preliminary scans of the reaction mixture over time. Electrical calibrations were performed before initiation or after completion of the calorimetric process. The electrical calibration circuit consisted of a glass encased 22 X manganin resistance connected in a four-wire configuration to an Agilent 34401A multimeter and an Agilent 6611C DC power supply. The calorimeter was kept in an air-conditioned room whose temperature was regulated to (295 ± 1) K. Instrument control and data acquisition were performed with the CBCAL 3.0 program [17]. The energy equivalent, e, of the calorimeter and the enthalpy of the calorimetric process, Dr Hom , were obtained from:
e¼
P
Dr Hom
i V i I i Dt i Ac
ð2Þ
M ¼ eA m
ð3Þ
for 95% probability, with a regression coefficient R = 0.994. The accuracy of the electrical calibration was tested by using the enthalpy of solution of KCl in water to yield a KCl:1525H2O solution. The obtained value was Dr Hom = (17.39 ± 0.12) kJmol1, where the assigned uncertainty is twice the standard error of the mean of seven independent determinations. This result is in excellent agreement with the corresponding value, Dr Hom = (17.48 ± 0.08) kJmol1, calculated from data in the NBS tables [18] after correction for the molar mass used in this work (M = 74.548 gmol1). 3. Results and discussion All molar quantities were based on the molar mass of methyl paraben M = 152.149 gmol1, calculated from the conventional atomic masses recommended by the IUPAC Commission in 2013 [19]. The enthalpy of hydrolysis of methyl paraben corresponding to step 2 in Fig. 1, which is given by reaction (5), can be derived by combining the enthalpy of reaction (6) obtained in this work by isothermal calorimetry and the enthalpies of processes (7)–(9): O
O OCH3
−
O−
(aq) + OH -(aq)
−
O O
ð5Þ
O OCH 3
O−
(cr) + 2OH− (aq)
−
HO
(aq) + CH3 OH(aq) + H 2O(l)
O
ð6Þ O
O OCH3
OCH 3
(cr) + H 2O(l)
HO
Material
CAS number
Supplier
Mass fraction purity
Methyl 4-hydroxybenzoate (cr I) Potassium chloride (cr)
99-76-3 7447-40-7
Aldrich Fluka
>0.999a 0.999995b
Mass fraction purity from GC–MS analysis after purification carried out in this work by sublimation and identification of the sample as polymorph I of methyl paraben (cr I) from X-ray powder diffraction analysis. The initial mass fraction indicated by the supplier was >0.99. b Given by the supplier.
(aq)
ð7Þ
HO
O
Table 1 Provenance and mass fraction purity of the material used in this work.
a
(aq) + CH3 OH(aq)
O
O OCH 3
(aq)
HO
OCH 3 −O
(aq) + H+(aq)
H2 OðlÞ ! Hþ ðaqÞ þ OH ðaqÞ
ð8Þ
ð9Þ
by using the following equations:
Dr Hom ð5Þ ¼ Dr Hom ð6Þ Dsln Hom ð7Þ Dr Hom ð8Þ þ Dr Hom ð9Þ Dsln Hom ð7Þ=kJ mol
1
¼ 0:26086ðT=KÞ 49:96
ð10Þ ð11Þ
R.N. Bento et al. / J. Chem. Thermodynamics 103 (2016) 176–180
Dr Hom ð8Þ=kJ mol
1
¼ 0:15800ðT=KÞ þ 65:20
ð12Þ
Dr Hom ð9Þ=kJ mol
1
¼ 0:20409ðT=KÞ þ 116:68
ð13Þ
Eq. (11) was derived from a linear least squares regression to Dsln Hom (7) values calculated from critically analysed solubility data [20]. Eqs. (12) and (13) were analogously obtained from literature data for the enthalpies of acid dissociation of MP [5] and autoionization of water [21], respectively. The values of Dr Hom (5) to Dr Hom (9) at the three temperatures studied in this work are summarized in Table 2. The uncertainty assigned to Dr Hom (6) corresponds to twice the standard error of the mean of seven, ten and six determinations, for T = 298.02 K, T = 303.04 K, and T = 310.01 K, respectively. In the case of Dr Hom (7) and Dr Hom (8) a 1% uncertainty was assumed in this work. The uncertainty of Dr Hom (9) is that given by the authors [21]. As shown in Fig. 2, a very good linear relationship was found when the Dr Hom (6) results obtained in this work (Table 2) were plotted as a function of temperature. A linear least squares regression to those values led to (95% probability): 1
Dr Hom ð6Þ=ðkJ mol Þ ¼ ð0:4219 0:0324ÞðT=KÞ 186:75 9:86
ð14Þ
with a regression coefficient R2 = 0.998. From the slope of Eq. (14) the heat capacity change of the reaction in the temperature range studied can be derived as Dr C op;m ± U c = (422 ± 32) JK1mol1. Given the linear variations of Dr Hom (6) to Dr Hom (9) with temperature, a linear relationship of Dr Hom (5) with temperature was necessarily found (95% probability): 1
Dr Hom ð5Þ=ðkJ mol Þ ¼ ð0:1142 0:0334ÞðT=KÞ 85:08 10:16
ð15Þ
the regression coefficient being R2 = 0.98 and Dr C op;m ± U c = (114 ± 33) JK1mol1. It should, nevertheless, be noted that the observed variation of Dr Hom (5) with temperature is covered by the uncertainty of the determinations, as can be concluded from Table 2. The standard molar enthalpy of the base catalysed hydrolysis of methyl paraben here obtained at 298.02 K, Dr Hom (5) = (50.99 ± 0.61) kJmol1, is in good agreement with the previously recommended benchmark value at 298 K temperature, Dr Hom (5) = (50.5 ± 4.3) kJmol1 [3], within their combined uncertainty intervals. An approximately seven fold improvement in precision was, however, achieved in the present work. Finally, the small temperature dependence found for Dr Hom (5) in the range 298.02– 310.01 K also lends support to the assumption that the enthalpy of the base catalysed hydrolysis of methyl paraben is approximately constant around 298 K [3], particularly if the ±4.3 kJmol1 uncertainty of the previously recommended benchmark is considered.
Table 2 Standard (p° = 100 kPa) temperatures. Reaction
(5) (6) (7) (8) (9)
molar
enthalpies
of
reactions
(5)–(9)
at
different
Dr Hom /(kJmol1) T = 298.02 K
T = 303.04 K
T = 310.01 K
(50.99 ± 0.61) (60.96 ± 0.50) 27.78 ± 0.28 18.11 ± 0.18 55.86 ± 0.10
(50.60 ± 0.82) (59.02 ± 0.74) 29.09 ± 0.29 17.32 ± 0.17 54.83 ± 0.10
(49.64 ± 0.90) (55.92 ± 0.82) 30.91 ± 0.31 16.22 ± 0.16 53.41 ± 0.10
179
Fig. 2. Standard molar enthalpy of reaction (6) as a function of temperature.
4. Conclusions Results from an isothermal microcalorimetry study together with literature data allowed the determination of the standard molar enthalpy of the base catalysed hydrolysis of methyl paraben according to reaction (5), Dr Hom (5), at 298.02 K, 303.04 K and 310.01 K. Unlike previous determinations, the adopted strategy avoided the need of kinetic information, an aspect that may be at the root of the significant discrepancies found in the reported MP enthalpy of hydrolysis data [3,7–10]. Furthermore, it also allowed a ca. 7-fold improvement in precision relative to the previously proposed benchmark value of Dr Hom (5) at 298 K [3], which is, nevertheless, in excellent agreement with the present determinations. Finally, the small temperature dependence found for Dr Hom (5) in the range 298.02–310.01 K, also supports the assumption [3] that the enthalpy of the base catalysed hydrolysis of methyl paraben is approximately constant around 298 K. Eq. (15) is, nevertheless, recommended for the calculation of benchmark Dr Hom (5) values used to determine the effective volume of flow-through calorimetric apparatus, within the approximate range 298–310 K. Acknowledgments This work was supported by FCT, Portugal, through Projects PEst-OE/QUI/UI0612/2013 and PEst-OE/QUI/UI0100/2013, and a post-doctoral grant awarded to C.E.S. Bernardes (SFRH/ BPD/101505/2014). Thanks are also due to PARALAB (Portugal) and NETZSCH (Germany) for providing the Netzsch DSC 204 F1Phoenix apparatus. We finally thank Prof. C. Teixeira (CQE-IST, Portugal) for all her help during the set up of the TAM apparatus at CQB-FCUL and Prof. M.C. Oliveira (CQE-IST, Portugal) for the GC– MS analysis. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jct.2016.07.042. References [1] A.M. James, Thermal and Energetic Studies of Cellular Biological Systems, IOP Publishing, Bristol, 1987. [2] A.E. Beezer, Biological Microcalorimetry, Academic Press, London, 1980. [3] M.A.A. O’Neill, A.E. Beezer, C. Labetoulle, L. Nicolaides, J.C. Mitchell, J.A. Orchard, J.A. Connor, R.B. Kemp, D. Olomolaiye, Thermochim. Acta 399 (2003) 63–71. [4] M.A.A. O’Neill, A.E. Beezer, G.J. Vine, R.B. Kemp, D. Olomolaiye, P.L.O. Volpe, D. Oliveira, Thermochim. Acta 413 (2004) 193–199. [5] A. Kroflicˇ, A. Apelblat, M. Bešter-Rogacˇ, J. Phys. Chem. B 116 (2012) 1385–1392.
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JCT 16-461