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Variable temperature infrared study of copper sulfate pentahydrate dehydration
Thermochimica Acta 528 (2012) 58–62
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Variable temperature infrared study of copper sulfate pentahydrate dehydration Robert L. White ∗ Department of Chemistry and Biochemistry, University of Oklahoma, Norman, OK 73019, United States
a r t i c l e
i n f o
Article history: Received 31 August 2011 Received in revised form 7 November 2011 Accepted 8 November 2011 Available online 18 November 2011 Keywords: Variable temperature infrared spectroscopy Diffuse reflection infrared spectroscopy Copper sulfate pentahydrate Dehydration
a b s t r a c t Copper sulfate pentahydrate dehydration is studied by using variable temperature diffuse reflection infrared Fourier transform spectroscopy (VT-DRIFTS). In a static environment, dehydration proceeds sequentially from the pentahydrate to the trihydrate and then to the monohydrate. When helium purge is employed while heating samples, the monohydrate is formed directly from the pentahydrate. Infrared spectral features show that in a static environment, lattice water molecules are involved in coordinated motions, which are absent when purge is employed. Differences between crystal structures obtained in the static and purge environments are explained by equilibrium processes that depend on local water vapor concentration and lattice water environments. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The thermal dehydration mechanisms of copper sulfate pentahydrate (CuSO4 ·5H2 O) have been the subject of numerous investigations over the past 80 years. Close scrutiny of the thermal characteristics of this material reveals some interesting properties. For example, the rate of water loss when CuSO4 ·5H2 O is heated is strongly dependent on experimental conditions. In particular, CuSO4 ·5H2 O crystals have been found to exhibit the Topley effect, in which dehydration is affected by the ambient water vapor concentration [1,2]. Dehydration of CuSO4 ·5H2 O under vacuum below 70 ◦ C results in the loss of four water molecules whereas only two are lost in air [3]. Dehydration activation energies measured under vacuum (43 [4] and 76 kJ/mol [5]) are substantially lower than those measured in air (104 [3] and 115 kJ/mol [6]). Thermal analysis studies of CuSO4 ·5H2 O further indicate that dehydration rates are dependent on particle size and heating rate [7,8]. In addition, Saig et al. reported that freshly prepared CuSO4 ·5H2 O exhibits a disordered structure that anneals into a more ordered form after about a week [9]. Four different stable copper sulfate forms have been isolated, each with a different number of water molecules. At room temperature, the pentahydrate is the most stable configuration. Heating the pentahydrate under conditions in which two water molecules are lost produces the trihydrate. Continued heating produces the monohydrate and then finally anhydrous copper sulfate. The crystal structures of all four forms have been reported [10–13]. Two
∗ Tel.: +1 405 325 4811; fax: +1 405 325 6111. E-mail address: [email protected] 0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2011.11.013
non-equivalent copper ions are surrounded by oxygen atoms in distorted octahedral configurations in each of the three hydrated crystal unit cells. The oxygen atoms that coordinate to the copper ions are derived from the sulfate group or water molecules. As dehydration of the crystal progresses, sulfate oxygen atoms replace water molecule oxygen atoms to maintain a distorted octahedral coordination of the copper ions [14]. Structurally, dehydration results in crystal contraction, loss of water, a change in the hydrogen bonding environment for the remaining water molecules, and deformation of the sulfate group. In the pentahydrate, sulfate oxygens have relatively weak bonds to the copper ion, resulting in nearly Td symmetry for SO4 2− [15]. Infrared spectra for the trihydrate indicate that the sulfate group adopts a D2 symmetry, whereas the sulfate group symmetry in the monohydrate is C2v [15,16]. Infrared and Raman spectroscopy have been employed for qualitative structural analysis of each of the four copper sulfate forms [14,15,17–23]. Vibrational spectroscopy provides information regarding the environments of the water molecules and the structure of the sulfate anion. In this paper, it will be shown that variable temperature diffuse reflection infrared Fourier transform spectroscopy (VT-DRIFTS) can provide both qualitative and semi-quantitative temperature-dependent information that provides new insight into CuSO4 ·5H2 O dehydration mechanisms. 2. Experimental Copper sulfate pentahydrate (98% purity) obtained from EMD Chemicals (Gibbstown, NJ) was ground and sieved to produce particles that were smaller than 38 m. A Dupont model 951 TGA was employed for thermogravimetric analyses. Air was purged from
R.L. White / Thermochimica Acta 528 (2012) 58–62
Fig. 1. (a) Thermogravimetric mass loss curves for copper sulfate pentahydrate heated in a static environment (solid line) and with helium purge (dashed line). (b) VT-DRIFTS O–H stretching vibration integrated band intensity versus temperature profiles for copper sulfate pentahydrate heated in a static environment (solid line) and with helium purge (dashed line).
the sample chamber by flushing with 60 mL/min helium for at least 10 min after loading samples. For static atmosphere measurements, the helium flow was turned off prior to heating the sample. Approximately 10 mg samples were heated at a rate of 0.5 ◦ C/min. A Harrick Scientific Inc. (Ossining, NY) praying mantis diffuse reflectance accessory and a modified Harrick Scientific Inc. environmental chamber were used for diffuse reflection measurements. A Mattson Instruments Inc. FTIR was employed for infrared spectroscopy. Details of the VT-DRIFTS apparatus are provided elsewhere [24]. Silver powder (100 mesh, 99.95% purity obtained from Alfa AESAR) was used to dilute samples to about 20% (w/w). Approximately 15 mg of diluted samples were loaded into the VT-DRIFTS sample holder and purged with 20 mL/min helium for at least 10 min prior to heating. For static atmosphere measurements, the helium purge was turned off before heating the sample. VT-DRIFTS spectra (8 cm−1 resolution) were measured at 0.5 ◦ C increments by signal averaging 175 interferograms while samples were heated at 0.5 ◦ C/min. Infrared spectra were converted to Kubelka–Munk format. VT-DRIFTS integrated band intensity versus temperature plots were generated by dividing the area under the O–H stretching vibration spectral region (4000–2000 cm−1 ) by the entire mid-infrared spectrum area (4000–650 cm−1 ) and plotting this normalized value as a function of the temperature at which the spectrum was obtained. 3. Results and discussion Fig. 1a shows the dependence of CuSO4 ·5H2 O dehydration (i.e. mass loss) on the atmospheric environment in which the material
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was heated. The solid curve was obtained by heating CuSO4 ·5H2 O in a static environment (i.e. no purge gas flow) and the dashed curve was obtained by heating a similar size sample at the same heating rate while purging with 60 mL/min helium. The solid curve exhibits two distinct mass loss steps, corresponding to the formation of the trihydrate (45–58 ◦ C) and monohydrate species (82–95 ◦ C). In contrast, the dashed line, which represents mass loss while purging the sample with 60 mL/min helium, exhibits a single mass loss step (37–62 ◦ C) followed by gradual mass loss with increasing temperature. The two mass loss steps obtained under static conditions correspond to 14.3% and 13.7% of the initial sample mass, respectively. These values are close to the 14.4% mass loss expected for removing two water molecules from CuSO4 ·5H2 O. Compared to the static dehydration mass loss results, heating CuSO4 ·5H2 O while purging with 60 mL/min helium resulted in a slightly lower mass loss onset temperature, a greater mass loss at 60 ◦ C, and a sample mass above 80 ◦ C that corresponds to a net loss of more than four water molecules from the CuSO4 ·5H2 O starting material. Variable temperature diffuse reflection infrared spectroscopy was employed to monitor the structural changes that occur during CuSO4 ·5H2 O dehydration. The high absorptivity of the neat material necessitated the use of a diluent in order to minimize Reststrahlen effects [25], which resulted in inverted peaks for some spectral features obtained by using neat samples. Silver powder was employed as diluent for this study rather than the more commonly used salts (KCl or KBr) to avoid diluent spectral interferences in the O–H stretching region of infrared spectra. Fig. 1b shows plots of O–H stretching vibration (4000–2000 cm−1 ) infrared band area versus temperature for samples heated in a static environment (solid line) and with a 20 mL/min helium purge (dashed line). A slight increase in integrated intensity was detected for the sample heated in a static environment starting at about 36 ◦ C. At about the same temperature, the integrated O–H stretching band intensity for the sample heated in 20 mL/min helium began to decrease. These onset temperatures are lower than those associated with mass loss determined by thermogravimetry (Fig. 1a). The location of the thermocouple in each apparatus is the most probable cause for this discrepancy. For thermogravimetric analyses, the thermocouple was placed near the sample and in contact with the surrounding gas. For VT-DRIFTS, the thermocouple was in contact with the bottom of the platinum sample pan. Near room temperature, the sample surface exposed to infrared radiation is typically about 10◦ higher than the thermocouple reading due to radiative heating by the FTIR infrared source. The radiative heating contribution to the sample temperature is less important at higher sample temperatures, so correlations between VT-DRIFTS profiles and thermogravimetric mass loss curves improve at higher temperatures. The VT-DRIFTS band intensity temperature profile for the sample heated in 20 mL/min helium was qualitatively similar to the mass loss curve obtained for the sample heated in 60 mL/min helium. It exhibits a step decrease starting at 36 ◦ C and ending at about 46 ◦ C followed by a continuing gradual decrease. In contrast, after the slight increase in the integrated band intensity, the temperature profile for the sample heated in the static environment exhibits a step decrease between 77 and 87 ◦ C, which corresponds to transformation of trihydrate to monohydrate. This VT-DRIFTS transition corresponds to the static environment mass loss step detected over the 82–95 ◦ C temperature range. Diffuse reflection spectra measured at 25 and 60 ◦ C for the sample heated in a static environment are shown in Fig. 2. The 25 ◦ C spectrum was representative of VT-DRIFTS measurements obtained prior to heating samples in either a static or 20 mL/min helium purge environment. Infrared band locations for the 25 ◦ C spectrum closely match those reported by Ferraro and Walker, which were obtained by using the Nujol mull technique [15]. The top spectrum in Fig. 2, which was obtained when the sample
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R.L. White / Thermochimica Acta 528 (2012) 58–62
Fig. 2. VT-DRIFTS spectra for copper sulfate pentahydrate heated in a static environment (top) and the corresponding difference spectrum (bottom).
temperature reached 60 ◦ C, has spectral features similar to those previously reported for the trihydrate [14]. As shown by the difference spectrum (60 ◦ C spectrum − 25 ◦ C spectrum) at the bottom of Fig. 2, several spectral changes occurred during the pentahydrate to trihydrate transition. The horizontal dashed line on the difference spectrum plot denotes the zero location for the y-axis. Spectral features above this line represent band intensity that was added to the spectrum when the sample was heated from 25 to 60 ◦ C and features below the dashed line represent band intensity that was lost. Very weakly hydrogen bonded OH functionalities with vibrational frequencies near 3600 cm−1 were lost and more strongly hydrogen bonded OH groups (2750 cm−1 ) were added. Interestingly, even though the pentahydrate to trihydrate transition involved the loss of 2 mol of water from the crystal, the O–H stretching vibration integrated band intensity did not decrease. In fact, a slight increase in integrated band intensity was observed. Thus, there must have been a significant increase in the O–H stretching vibration absorptivity for the water molecules that remained in the crystal compared to those that were removed by heating. Based on numerous examples, Iogansen [26] has shown that integrated O–H band intensity enhancement (A1/2 ) is directly proportional to the hydrogen bond enthalpy. This suggests that the trihydrate water molecules exhibited a substantial increase in hydrogen bond enthalpy compared to those in the pentahydrate. In addition to changes to the O–H stretching vibration, the O–H bending vibration shifted from 1670 cm−1 in the pentahydrate spectrum to about 1630 cm−1 in the trihydrate spectrum. Ab initio calculations reported by Falk et al. indicate that water–anion interactions increase the H–O–H bending force constant whereas water–cation interactions have little effect [27]. Thus, the decrease in the bending vibration wavenumber for the trihydrate compared to the pentahydrate may be interpreted as diminished interactions between lattice water molecules and sulfate anions. In addition to the spectral changes associated with lattice water molecules, the triply degenerate SO4 2− 3 vibration at 1070 cm−1 in the pentahydrate spectrum was split into bands at 1120 and 1065 cm−1 in the trihydrate spectrum and the pentahydrate 1 vibration at 980 cm−1 disappeared in the trihydrate spectrum. This is consistent with the expected decrease in symmetry for the sulfate anion in the trihydrate. Fig. 3 shows VT-DRIFTS spectra obtained when CuSO4 ·5H2 O was heated in 20 mL/min helium. The 25 ◦ C spectrum was indistinguishable from the 25 ◦ C spectrum obtained when a static environment was employed (Fig. 2). However, the spectrum obtained at 60 ◦ C
Fig. 3. VT-DRIFTS spectra for copper sulfate pentahydrate heated in a helium purge (top) and the corresponding difference spectrum (bottom).
was significantly different from the 60 ◦ C spectrum obtained in a static environment, which is apparent by the difference spectrum at the bottom of Fig. 3. When a 20 mL/min helium purge was employed during sample heating, a significant loss in O–H stretching vibration band intensity was detected at 60 ◦ C compared to 25 ◦ C. This loss of water was responsible for the integrated band intensity decrease shown in Fig. 1b. In contrast to the VT-DRIFTS results obtained by using a static environment, the largest decrease in O–H stretching vibration band intensity occurred at a lower wavenumber (3030 cm−1 ), suggesting that water molecules lost while purging were involved in more extensive hydrogen bonding than those that were lost in the static environment. Like the static VT-DRIFTS results, the H–O–H bending vibration decreased from 1670 to 1630 cm−1 when the sample was heated from 25 to 60 ◦ C. The 1200–1000 cm−1 spectral region in the 60 ◦ C spectrum obtained while purging was much broader than the corresponding region of the spectrum obtained under a static environment. In fact, that portion of the infrared spectrum was more similar to the monohydrate spectrum than the trihydrate spectrum. In addition, the 870 cm−1 band that appears in the 25 and 60 ◦ C spectra obtained in the static environment was absent in the 60 ◦ C spectrum obtained in 20 mL/min helium. This band has been assigned to a coordinated H2 O rocking motion associated with linked water molecules. Apparently, this linkage, which was present in the pentahydrate, was broken when the sample was purged while being heated. By correlating thermogravimetric and VT-DRIFTS results, it can be concluded that heating CuSO4 ·5H2 O from 25 to 60 ◦ C in 20 mL/min helium resulted in the loss of more than two water molecules, forming primarily the monohydrate. Fig. 4 shows VT-DRIFTS infrared spectra obtained at 100 ◦ C. The band locations in the spectrum obtained under static conditions (top) closely match those reported by Gamo [18] for CuSO4 ·H2 O and are significantly different from those in the spectrum obtained at the same temperature but with 20 mL/min helium purge (bottom). The O–H stretching vibration band in the spectrum measured while helium was flowing over the sample is shifted toward higher wavenumber relative to the spectrum measured under static conditions. This suggests that the water molecules in the sample that was exposed to helium purge are involved in less hydrogen bonding. The two spectra shown in Fig. 4 also differ significantly in the H2 O bending region. The H2 O bending vibration for the spectrum measured under static conditions has a maximum near 1510 cm−1 , whereas the corresponding peak in the spectrum obtained under
R.L. White / Thermochimica Acta 528 (2012) 58–62
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composition but with a crystal lattice structure that more closely resembles the pentahydrate [28]. Four of the CuSO4 ·5H2 O water molecules exhibit Cu–O bonding and experience similar environments within the crystal lattice. The fifth water molecule is held in place solely by hydrogen bonding to other water molecules and to sulfate groups [10,29]. Loss of this fifth water molecule is likely responsible for the observed decrease in O–H stretching vibration band intensity near 3600 cm−1 when the pentahydrate was heated in a static environment (Fig. 2). In addition to loss of this water molecule, formation of the trihydrate requires the loss of one of the Cu–O bonding waters. The net increase in band intensity at 2750 cm−1 detected by VT-DRIFTS during the pentahydrate to trihydrate transition suggests that the three water molecules that remain in the trihydrate are participating in stronger hydrogen bonding than those in the pentahydrate and are thus more tightly bound within the crystal lattice. References
Fig. 4. VT-DRIFTS spectra obtained at 100 ◦ C (a) in a static environment and (b) with helium purge.
helium purge is located at 1630 cm−1 . Grodzicki and Piszczek have explained abnormally low H2 O bending vibrations as being caused by the formation of chains containing water molecules that link neighboring molecules and stabilize the crystal structure [22]. The fact that the H2 O bending vibration occurs at a much higher wavenumber in the spectrum obtained while purging the sample suggests that this chain structure was not formed. The 1200–1000 cm−1 region in the spectrum measured under helium purge conditions was much broader than in the spectrum obtained under the static environment, reflecting a different sulfate group orientation. The overlapping bands at 810 and 850 cm−1 in the monohydrate spectrum have previously been assigned to a coordinated rocking motion of lattice water molecules [14,15]. These bands are absent in the 100 ◦ C spectrum obtained under helium purge, suggesting that the stabilizing coordination between lattice water molecules was unable to form when water was flushed from the crystal surface during sample heating.
4. Conclusions VT-DRIFTS results suggest that employing a purge gas to remove the water liberated when heating CuSO4 ·5H2 O can have a dramatic effect on the structure of the dehydrated crystal. In a static environment, water molecules lost during heating remain in the vicinity of the crystal and are free to return to the lattice. At temperatures near 60 ◦ C, equilibrium between lattice water and water vapor favors the formation of the trihydrate structure. When desorbed water is continuously removed while heating, this equilibrium cannot be established. Previous studies based on mass loss measurements have shown that when CuSO4 ·5H2 O is heated in a vacuum, the product has the empirical formula of the monohydrate. The thermogravimetric results shown here are consistent with these findings. However, although VT-DRIFTS spectra confirm that the monohydrate is the primary product, it does not exhibit the hydrogen bonding coordination between waters that is characteristic of the stable monohydrate. This is consistent with the conclusions of Zagray et al., that heating CuSO4 ·5H2 O under vacuum at 50 ◦ C produces a pseudostructure with a monohydrate
[1] B. Topley, M.L. Smith, Function of water vapour in the dissociation of a salt hydrate, Nature 128 (1931) 302. [2] W.E. Garner, F.R.S. Jennings, T.J. Jennings, Nucleation phenomena arising during the dehydration of solid hydrates, Proc. Roy. Soc. A 224 (1954) 460–471. [3] W.L. Ng, C.C. Ho, S.K. Ng, Isothermal dehydration of copper sulfate pentahydrate and trihydrate, J. Inorg. Nucl. Chem. 34 (1978) 459–462. [4] W.E. Garner, M.G. Tanner, The dehydration of copper sulphate pentahydrate, J. Chem. Soc. (1930) 47–57. [5] M.L. Smith, B. Topley, The experimental study of the rate of dissociation of salt hydrates. The reaction CuSO4 ·5H2 O CuSO4 ·H2 O, Proc. Roy. Soc. 134 (1931) 224–225. [6] V. Marcu, E. Segal, Kinetic analysis of a sequence of two consecutive reactions from thermogravimetric data under non-isothermal conditions. Part II. Dehydration of CuSO4 ·5H2 O, Thermochim. Acta 36 (1980) 235–237. [7] T.I. Taylor, H.P. Klug, Thermal transitions in copper sulphate pentahydrate molecular rotation and the dehydration of hydrates, J. Chem. Phys. (1936) 601–607. [8] K. Nagase, H. Yokobayashi, Effects of heating rate (1–300◦ h−1 ) on the nonisothermal thermogravimetry of CuSO4 –5H2 O, Thermochim. Acta 35 (1980) 99–104. [9] A. Saig, A. Danon, Y. Finkelstein, G. Kimmel, J.E. Koresh, A continuous polymorphic transition of coordinating water molecules in CuSO4 ·5H2 O, J. Phys. Chem. 64 (2003) 701–706. [10] C.A. Beevers, H. Lipson, The crystal structure of copper sulphate pentahydrate, CuSO4 ·5H2 O, Proc. Roy. Soc. A 146 (1934) 570–582. [11] R. Zahrobsky, W.H. Baur, Crystal structure of copper(II) sulfate trihydrate, Naturwissenschaften 52 (1965) 389. [12] G. Giester, The crystal structures of CuSO4 and CuSeO4 ·H2 O, and their relationships to kieserite, Miner. Petrol. 38 (1988) 277–284. [13] P.A. Kokkoros, P.J. Rentzeperis, The crystal structure of the anhydrous sulphates of copper and zinc, Acta Crystallogr. 11 (1958) 361–364. [14] K. Nagase, H. Yokobayashi, Spectrophotometric, Thermal analytical studies on the dehydration of copper (II) sulfate and its double salts, Thermochim. Acta 23 (1978) 283–291. [15] J.R. Ferraro, A. Walker, Comparison of the infrared spectra (4000–70 cm−1 ) of several hydrated and anhydrous salts of transition metals, J. Chem. Phys. 42 (1965) 1278–1285. [16] H.H. Adler, P.F. Kerr, Variations in infrared spectra, molecular symmetry and site symmetry of sulfate minerals, Am. Mineral. 50 (1965) 132–147. [17] J. Fujita, K. Nakamoto, M. Kobayashi, Infrared spectra of metallic complexes. II. The absorption bands of coordinated water in aquo complexes, J. Am. Chem. Soc. 78 (1956) 3963–3965. [18] I. Gamo, Infrared absorption spectra of water crystallization in copper sulfate penta- and monohydrate crystals, Bull. Chem. Soc. Jpn. 34 (1961) 764–766. [19] H. Oki, E. Kyuno, R. Tsuchiya, Some thermal properties of deuterated copper (II) sulfate pentahydrate, Bull. Chem. Soc. Jpn. 43 (1970) 3263–3264. [20] B.K. Srivastava, D.P. Khandelwal, S. Khandelwal, I.D. Bist, Internal modes of and potential environment around water molecules in CuSO4 –5H2 O and its deuterate, Chem. Phys. Lett. 43 (1976) 557–561. [21] B. Soptrajanov, M. Trpkovska, Copper sulfate monohydrate: centrosymmetric or non-centrosymmetric? J. Mol. Struct. 293 (1993) 109–112. [22] A. Grodzicki, P. Piszczek, A new interpretation of abnormal shift of water molecules’ bending vibration frequencies in kieserite family monohydrates, J. Mol. Struct. 443 (1998) 141–147. [23] E. Widjaja, H.H. Chong, M. Tjahjono, Use of thermo-Raman spectroscopy and chemometric analysis to identify dehydration steps of hydrated inorganic samples – application to copper sulfate pentahydrate, J. Raman Spectrosc. 41 (2010) 181–186. [24] D.R. White, R.L. White, Isoconversion effective activation energy profiles by variable temperature diffuse reflection infrared spectroscopy, Appl. Spectrosc. 62 (2008) 116–120.
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[25] T. Eickhoff, P. Grosse, W. Theiss, Diffuse reflectance spectroscopy of powders, Vib. Spectrosc. 1 (1990) 229–233. [26] A.V. Iogansen, Direct proportionality of the hydrogen bonding energy and the intensification of the stretching v(XH) vibration in infrared spectra, Spectrochim. Acta A 55 (1999) 1585–1612. [27] M. Falk, H.T. Flakus, R.J. Boyd, An ab initio SCF calculation of the effect of water–anion and water–cation interactions on the vibrational frequencies of water, Spectrochim. Acta 42A (1986) 175–180.
[28] A.I. Zagray, V.V. Zyryanov, N.Z. Lyakhov, A.P. Chupakhin, V.V. Boldyrev, Study of the products of the dehydration reaction of CuSO4 ·5H2 O in X-shaped and eliptic nuclei, Thermochim. Acta 29 (1979) 115–119. [29] A.D. Harris, L.H. Kalbus, Decomposition of copper (II) sulfate pentahydrate, J. Chem. Educ. (1979) 417–418.
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Variable temperature infrared study of copper sulfate pentahydrate dehydration Robert L. White ∗ Department of Chemistry and Biochemistry, University of Oklahoma, Norman, OK 73019, United States
a r t i c l e
i n f o
Article history: Received 31 August 2011 Received in revised form 7 November 2011 Accepted 8 November 2011 Available online 18 November 2011 Keywords: Variable temperature infrared spectroscopy Diffuse reflection infrared spectroscopy Copper sulfate pentahydrate Dehydration
a b s t r a c t Copper sulfate pentahydrate dehydration is studied by using variable temperature diffuse reflection infrared Fourier transform spectroscopy (VT-DRIFTS). In a static environment, dehydration proceeds sequentially from the pentahydrate to the trihydrate and then to the monohydrate. When helium purge is employed while heating samples, the monohydrate is formed directly from the pentahydrate. Infrared spectral features show that in a static environment, lattice water molecules are involved in coordinated motions, which are absent when purge is employed. Differences between crystal structures obtained in the static and purge environments are explained by equilibrium processes that depend on local water vapor concentration and lattice water environments. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The thermal dehydration mechanisms of copper sulfate pentahydrate (CuSO4 ·5H2 O) have been the subject of numerous investigations over the past 80 years. Close scrutiny of the thermal characteristics of this material reveals some interesting properties. For example, the rate of water loss when CuSO4 ·5H2 O is heated is strongly dependent on experimental conditions. In particular, CuSO4 ·5H2 O crystals have been found to exhibit the Topley effect, in which dehydration is affected by the ambient water vapor concentration [1,2]. Dehydration of CuSO4 ·5H2 O under vacuum below 70 ◦ C results in the loss of four water molecules whereas only two are lost in air [3]. Dehydration activation energies measured under vacuum (43 [4] and 76 kJ/mol [5]) are substantially lower than those measured in air (104 [3] and 115 kJ/mol [6]). Thermal analysis studies of CuSO4 ·5H2 O further indicate that dehydration rates are dependent on particle size and heating rate [7,8]. In addition, Saig et al. reported that freshly prepared CuSO4 ·5H2 O exhibits a disordered structure that anneals into a more ordered form after about a week [9]. Four different stable copper sulfate forms have been isolated, each with a different number of water molecules. At room temperature, the pentahydrate is the most stable configuration. Heating the pentahydrate under conditions in which two water molecules are lost produces the trihydrate. Continued heating produces the monohydrate and then finally anhydrous copper sulfate. The crystal structures of all four forms have been reported [10–13]. Two
∗ Tel.: +1 405 325 4811; fax: +1 405 325 6111. E-mail address: [email protected] 0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2011.11.013
non-equivalent copper ions are surrounded by oxygen atoms in distorted octahedral configurations in each of the three hydrated crystal unit cells. The oxygen atoms that coordinate to the copper ions are derived from the sulfate group or water molecules. As dehydration of the crystal progresses, sulfate oxygen atoms replace water molecule oxygen atoms to maintain a distorted octahedral coordination of the copper ions [14]. Structurally, dehydration results in crystal contraction, loss of water, a change in the hydrogen bonding environment for the remaining water molecules, and deformation of the sulfate group. In the pentahydrate, sulfate oxygens have relatively weak bonds to the copper ion, resulting in nearly Td symmetry for SO4 2− [15]. Infrared spectra for the trihydrate indicate that the sulfate group adopts a D2 symmetry, whereas the sulfate group symmetry in the monohydrate is C2v [15,16]. Infrared and Raman spectroscopy have been employed for qualitative structural analysis of each of the four copper sulfate forms [14,15,17–23]. Vibrational spectroscopy provides information regarding the environments of the water molecules and the structure of the sulfate anion. In this paper, it will be shown that variable temperature diffuse reflection infrared Fourier transform spectroscopy (VT-DRIFTS) can provide both qualitative and semi-quantitative temperature-dependent information that provides new insight into CuSO4 ·5H2 O dehydration mechanisms. 2. Experimental Copper sulfate pentahydrate (98% purity) obtained from EMD Chemicals (Gibbstown, NJ) was ground and sieved to produce particles that were smaller than 38 m. A Dupont model 951 TGA was employed for thermogravimetric analyses. Air was purged from
R.L. White / Thermochimica Acta 528 (2012) 58–62
Fig. 1. (a) Thermogravimetric mass loss curves for copper sulfate pentahydrate heated in a static environment (solid line) and with helium purge (dashed line). (b) VT-DRIFTS O–H stretching vibration integrated band intensity versus temperature profiles for copper sulfate pentahydrate heated in a static environment (solid line) and with helium purge (dashed line).
the sample chamber by flushing with 60 mL/min helium for at least 10 min after loading samples. For static atmosphere measurements, the helium flow was turned off prior to heating the sample. Approximately 10 mg samples were heated at a rate of 0.5 ◦ C/min. A Harrick Scientific Inc. (Ossining, NY) praying mantis diffuse reflectance accessory and a modified Harrick Scientific Inc. environmental chamber were used for diffuse reflection measurements. A Mattson Instruments Inc. FTIR was employed for infrared spectroscopy. Details of the VT-DRIFTS apparatus are provided elsewhere [24]. Silver powder (100 mesh, 99.95% purity obtained from Alfa AESAR) was used to dilute samples to about 20% (w/w). Approximately 15 mg of diluted samples were loaded into the VT-DRIFTS sample holder and purged with 20 mL/min helium for at least 10 min prior to heating. For static atmosphere measurements, the helium purge was turned off before heating the sample. VT-DRIFTS spectra (8 cm−1 resolution) were measured at 0.5 ◦ C increments by signal averaging 175 interferograms while samples were heated at 0.5 ◦ C/min. Infrared spectra were converted to Kubelka–Munk format. VT-DRIFTS integrated band intensity versus temperature plots were generated by dividing the area under the O–H stretching vibration spectral region (4000–2000 cm−1 ) by the entire mid-infrared spectrum area (4000–650 cm−1 ) and plotting this normalized value as a function of the temperature at which the spectrum was obtained. 3. Results and discussion Fig. 1a shows the dependence of CuSO4 ·5H2 O dehydration (i.e. mass loss) on the atmospheric environment in which the material
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was heated. The solid curve was obtained by heating CuSO4 ·5H2 O in a static environment (i.e. no purge gas flow) and the dashed curve was obtained by heating a similar size sample at the same heating rate while purging with 60 mL/min helium. The solid curve exhibits two distinct mass loss steps, corresponding to the formation of the trihydrate (45–58 ◦ C) and monohydrate species (82–95 ◦ C). In contrast, the dashed line, which represents mass loss while purging the sample with 60 mL/min helium, exhibits a single mass loss step (37–62 ◦ C) followed by gradual mass loss with increasing temperature. The two mass loss steps obtained under static conditions correspond to 14.3% and 13.7% of the initial sample mass, respectively. These values are close to the 14.4% mass loss expected for removing two water molecules from CuSO4 ·5H2 O. Compared to the static dehydration mass loss results, heating CuSO4 ·5H2 O while purging with 60 mL/min helium resulted in a slightly lower mass loss onset temperature, a greater mass loss at 60 ◦ C, and a sample mass above 80 ◦ C that corresponds to a net loss of more than four water molecules from the CuSO4 ·5H2 O starting material. Variable temperature diffuse reflection infrared spectroscopy was employed to monitor the structural changes that occur during CuSO4 ·5H2 O dehydration. The high absorptivity of the neat material necessitated the use of a diluent in order to minimize Reststrahlen effects [25], which resulted in inverted peaks for some spectral features obtained by using neat samples. Silver powder was employed as diluent for this study rather than the more commonly used salts (KCl or KBr) to avoid diluent spectral interferences in the O–H stretching region of infrared spectra. Fig. 1b shows plots of O–H stretching vibration (4000–2000 cm−1 ) infrared band area versus temperature for samples heated in a static environment (solid line) and with a 20 mL/min helium purge (dashed line). A slight increase in integrated intensity was detected for the sample heated in a static environment starting at about 36 ◦ C. At about the same temperature, the integrated O–H stretching band intensity for the sample heated in 20 mL/min helium began to decrease. These onset temperatures are lower than those associated with mass loss determined by thermogravimetry (Fig. 1a). The location of the thermocouple in each apparatus is the most probable cause for this discrepancy. For thermogravimetric analyses, the thermocouple was placed near the sample and in contact with the surrounding gas. For VT-DRIFTS, the thermocouple was in contact with the bottom of the platinum sample pan. Near room temperature, the sample surface exposed to infrared radiation is typically about 10◦ higher than the thermocouple reading due to radiative heating by the FTIR infrared source. The radiative heating contribution to the sample temperature is less important at higher sample temperatures, so correlations between VT-DRIFTS profiles and thermogravimetric mass loss curves improve at higher temperatures. The VT-DRIFTS band intensity temperature profile for the sample heated in 20 mL/min helium was qualitatively similar to the mass loss curve obtained for the sample heated in 60 mL/min helium. It exhibits a step decrease starting at 36 ◦ C and ending at about 46 ◦ C followed by a continuing gradual decrease. In contrast, after the slight increase in the integrated band intensity, the temperature profile for the sample heated in the static environment exhibits a step decrease between 77 and 87 ◦ C, which corresponds to transformation of trihydrate to monohydrate. This VT-DRIFTS transition corresponds to the static environment mass loss step detected over the 82–95 ◦ C temperature range. Diffuse reflection spectra measured at 25 and 60 ◦ C for the sample heated in a static environment are shown in Fig. 2. The 25 ◦ C spectrum was representative of VT-DRIFTS measurements obtained prior to heating samples in either a static or 20 mL/min helium purge environment. Infrared band locations for the 25 ◦ C spectrum closely match those reported by Ferraro and Walker, which were obtained by using the Nujol mull technique [15]. The top spectrum in Fig. 2, which was obtained when the sample
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Fig. 2. VT-DRIFTS spectra for copper sulfate pentahydrate heated in a static environment (top) and the corresponding difference spectrum (bottom).
temperature reached 60 ◦ C, has spectral features similar to those previously reported for the trihydrate [14]. As shown by the difference spectrum (60 ◦ C spectrum − 25 ◦ C spectrum) at the bottom of Fig. 2, several spectral changes occurred during the pentahydrate to trihydrate transition. The horizontal dashed line on the difference spectrum plot denotes the zero location for the y-axis. Spectral features above this line represent band intensity that was added to the spectrum when the sample was heated from 25 to 60 ◦ C and features below the dashed line represent band intensity that was lost. Very weakly hydrogen bonded OH functionalities with vibrational frequencies near 3600 cm−1 were lost and more strongly hydrogen bonded OH groups (2750 cm−1 ) were added. Interestingly, even though the pentahydrate to trihydrate transition involved the loss of 2 mol of water from the crystal, the O–H stretching vibration integrated band intensity did not decrease. In fact, a slight increase in integrated band intensity was observed. Thus, there must have been a significant increase in the O–H stretching vibration absorptivity for the water molecules that remained in the crystal compared to those that were removed by heating. Based on numerous examples, Iogansen [26] has shown that integrated O–H band intensity enhancement (A1/2 ) is directly proportional to the hydrogen bond enthalpy. This suggests that the trihydrate water molecules exhibited a substantial increase in hydrogen bond enthalpy compared to those in the pentahydrate. In addition to changes to the O–H stretching vibration, the O–H bending vibration shifted from 1670 cm−1 in the pentahydrate spectrum to about 1630 cm−1 in the trihydrate spectrum. Ab initio calculations reported by Falk et al. indicate that water–anion interactions increase the H–O–H bending force constant whereas water–cation interactions have little effect [27]. Thus, the decrease in the bending vibration wavenumber for the trihydrate compared to the pentahydrate may be interpreted as diminished interactions between lattice water molecules and sulfate anions. In addition to the spectral changes associated with lattice water molecules, the triply degenerate SO4 2− 3 vibration at 1070 cm−1 in the pentahydrate spectrum was split into bands at 1120 and 1065 cm−1 in the trihydrate spectrum and the pentahydrate 1 vibration at 980 cm−1 disappeared in the trihydrate spectrum. This is consistent with the expected decrease in symmetry for the sulfate anion in the trihydrate. Fig. 3 shows VT-DRIFTS spectra obtained when CuSO4 ·5H2 O was heated in 20 mL/min helium. The 25 ◦ C spectrum was indistinguishable from the 25 ◦ C spectrum obtained when a static environment was employed (Fig. 2). However, the spectrum obtained at 60 ◦ C
Fig. 3. VT-DRIFTS spectra for copper sulfate pentahydrate heated in a helium purge (top) and the corresponding difference spectrum (bottom).
was significantly different from the 60 ◦ C spectrum obtained in a static environment, which is apparent by the difference spectrum at the bottom of Fig. 3. When a 20 mL/min helium purge was employed during sample heating, a significant loss in O–H stretching vibration band intensity was detected at 60 ◦ C compared to 25 ◦ C. This loss of water was responsible for the integrated band intensity decrease shown in Fig. 1b. In contrast to the VT-DRIFTS results obtained by using a static environment, the largest decrease in O–H stretching vibration band intensity occurred at a lower wavenumber (3030 cm−1 ), suggesting that water molecules lost while purging were involved in more extensive hydrogen bonding than those that were lost in the static environment. Like the static VT-DRIFTS results, the H–O–H bending vibration decreased from 1670 to 1630 cm−1 when the sample was heated from 25 to 60 ◦ C. The 1200–1000 cm−1 spectral region in the 60 ◦ C spectrum obtained while purging was much broader than the corresponding region of the spectrum obtained under a static environment. In fact, that portion of the infrared spectrum was more similar to the monohydrate spectrum than the trihydrate spectrum. In addition, the 870 cm−1 band that appears in the 25 and 60 ◦ C spectra obtained in the static environment was absent in the 60 ◦ C spectrum obtained in 20 mL/min helium. This band has been assigned to a coordinated H2 O rocking motion associated with linked water molecules. Apparently, this linkage, which was present in the pentahydrate, was broken when the sample was purged while being heated. By correlating thermogravimetric and VT-DRIFTS results, it can be concluded that heating CuSO4 ·5H2 O from 25 to 60 ◦ C in 20 mL/min helium resulted in the loss of more than two water molecules, forming primarily the monohydrate. Fig. 4 shows VT-DRIFTS infrared spectra obtained at 100 ◦ C. The band locations in the spectrum obtained under static conditions (top) closely match those reported by Gamo [18] for CuSO4 ·H2 O and are significantly different from those in the spectrum obtained at the same temperature but with 20 mL/min helium purge (bottom). The O–H stretching vibration band in the spectrum measured while helium was flowing over the sample is shifted toward higher wavenumber relative to the spectrum measured under static conditions. This suggests that the water molecules in the sample that was exposed to helium purge are involved in less hydrogen bonding. The two spectra shown in Fig. 4 also differ significantly in the H2 O bending region. The H2 O bending vibration for the spectrum measured under static conditions has a maximum near 1510 cm−1 , whereas the corresponding peak in the spectrum obtained under
R.L. White / Thermochimica Acta 528 (2012) 58–62
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composition but with a crystal lattice structure that more closely resembles the pentahydrate [28]. Four of the CuSO4 ·5H2 O water molecules exhibit Cu–O bonding and experience similar environments within the crystal lattice. The fifth water molecule is held in place solely by hydrogen bonding to other water molecules and to sulfate groups [10,29]. Loss of this fifth water molecule is likely responsible for the observed decrease in O–H stretching vibration band intensity near 3600 cm−1 when the pentahydrate was heated in a static environment (Fig. 2). In addition to loss of this water molecule, formation of the trihydrate requires the loss of one of the Cu–O bonding waters. The net increase in band intensity at 2750 cm−1 detected by VT-DRIFTS during the pentahydrate to trihydrate transition suggests that the three water molecules that remain in the trihydrate are participating in stronger hydrogen bonding than those in the pentahydrate and are thus more tightly bound within the crystal lattice. References
Fig. 4. VT-DRIFTS spectra obtained at 100 ◦ C (a) in a static environment and (b) with helium purge.
helium purge is located at 1630 cm−1 . Grodzicki and Piszczek have explained abnormally low H2 O bending vibrations as being caused by the formation of chains containing water molecules that link neighboring molecules and stabilize the crystal structure [22]. The fact that the H2 O bending vibration occurs at a much higher wavenumber in the spectrum obtained while purging the sample suggests that this chain structure was not formed. The 1200–1000 cm−1 region in the spectrum measured under helium purge conditions was much broader than in the spectrum obtained under the static environment, reflecting a different sulfate group orientation. The overlapping bands at 810 and 850 cm−1 in the monohydrate spectrum have previously been assigned to a coordinated rocking motion of lattice water molecules [14,15]. These bands are absent in the 100 ◦ C spectrum obtained under helium purge, suggesting that the stabilizing coordination between lattice water molecules was unable to form when water was flushed from the crystal surface during sample heating.
4. Conclusions VT-DRIFTS results suggest that employing a purge gas to remove the water liberated when heating CuSO4 ·5H2 O can have a dramatic effect on the structure of the dehydrated crystal. In a static environment, water molecules lost during heating remain in the vicinity of the crystal and are free to return to the lattice. At temperatures near 60 ◦ C, equilibrium between lattice water and water vapor favors the formation of the trihydrate structure. When desorbed water is continuously removed while heating, this equilibrium cannot be established. Previous studies based on mass loss measurements have shown that when CuSO4 ·5H2 O is heated in a vacuum, the product has the empirical formula of the monohydrate. The thermogravimetric results shown here are consistent with these findings. However, although VT-DRIFTS spectra confirm that the monohydrate is the primary product, it does not exhibit the hydrogen bonding coordination between waters that is characteristic of the stable monohydrate. This is consistent with the conclusions of Zagray et al., that heating CuSO4 ·5H2 O under vacuum at 50 ◦ C produces a pseudostructure with a monohydrate
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