Langmuir 1996, 12, 6659-6664
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Dissolution of Chromium Hydroxides Monitored by Turbidimetry Marcelo J. Avena,* Carla E. Giacomelli, Carlos D. Garcı´a, and Carlos P. De Pauli INFIQC, Departamento de Fisicoquı´mica, Facultad de Ciencias Quı´micas, Universidad Nacional de Co´ rdoba, Suc 16, CC 61, 5016 Co´ rdoba, Argentina Received June 5, 1996. In Final Form: September 26, 1996X A turbidimetric method was used to study the dissolution kinetics of different chromium (hydr)oxides in aqueous HClO4. There was a very good agreement between the results obtained with this method and those obtained with a standard one (quantifying the amount of chromium released to solution at different reaction times, after the separation of the solid and liquid phases by centrifugation or filtration), revealing that turbidimetry can be used to monitor the dissolution behavior of the studied materials. Theoretical support for the use of turbidimetry in dissolution studies can be obtained from light scattering theory. Chromium (hydr)oxide particles were composed of a mixture of monomeric and low oligomeric Cr(III) species that dissolves almost instantaneously in HClO4 (rapidly dissolving material) and a more polymerized material that undergoes dissolution at a lower and measurable rate (slowly dissolving material). The dissolution product was also a mixture of monomers and more polymerized Cr(III) species. Apparent activation energies (40-70 kJ/mol) for the dissolution of the slowly dissolving material were high enough to discard diffusion in aqueous solution as the rate-determining step and considerably lower than those corresponding to ligand exchange in Cr(III) species. This suggests that diffusion into the solid could be controlling the dissolution rate, although more direct evidence is needed.
Introduction The dissolution behavior of colloidal metal (hydr)oxide1 particles has important consequences in both natural and technological processes.2,3 Dissolution studies are usually performed through kinetics measurements, and most of them are carried out with standard methods; i.e., solid and liquid phases are separated at different reaction times by filtration or centrifugation to quantify either the remaining solid or the dissolution products. As previously mentioned,4 this kind of treatment, which usually takes several minutes, impedes the study of relatively fast reactions. In a recent work4 it has been shown that fast dissolution of Ni(OH)2 particles could be monitored by turbidimetry, overcoming this limitation. There was a good agreement between the data obtained by turbidimetric and standard methods. Turbidimetry is now applied to study the dissolution behavior of different chromium(III) (hydr)oxides. Although this paper does not intend to explain the dissolution and aging mechanisms of the mentioned materials, some aspects related to these processes are also commented on. Experimental Section All chemicals were of analytical quality, and water was purified using a Millipore Milli-Q system. Chromium(III) (hydr)oxide particles were prepared by adding a concentrated (about 4 M) NaOH solution to a 0.1 M chrome alum (KCr(SO4)2‚12H2O) solution until the desired pH was reached. This treatment is known to produce mainly “active chromium hydroxide” or “active monomeric hydroxide”, solid that dissolves almost instantaneously in acidic media.5-7 To obtain less reactive materials, the active hydroxide was aged in the
mother liquor at different temperatures. This kind of aging produces gradual polymerization of the solid material; thus, several different samples of different reactivities were obtained. Long times or high temperatures of aging result in the formation of the “polymeric chromium hydroxide” (see below), which appears to be the final product of aging and is by far less reactive than the active material. Agings were performed in polyethylene vessels to prevent contamination with silicates. Turbidimetric dissolution studies were performed in the following way: an adequate volume (0.1-0.5 mL, mainly 0.1 mL) of a chromium (hydr)oxide suspension was added to a spectrophotometric cuvette containing 2 mL of a HClO4 solution. The absorbance (A) or turbidity (τ) was then monitored as a function of time (t) in a UV-vis Shimadzu UV-1601 spectrophotometer. In a typical run, for example, data were acquired every 0.5 s for 300 s. To minimize the effects of particle sedimentation, intermittent agitation was applied to the cuvettes during the measurements. In some cases, dissolution data obtained from turbidimetric measurements were compared with results obtained with a standard method. In these experiments a somewhat inert sample (aged at 70 °C) was used. The reaction was started by adding 5 mL of the dispersion to 250 mL of a HClO4 solution. Aliquots were then withdrawn from the reaction vessel at different times, and, after the turbidity measurements, they were filtered through a Nuclepore membrane (pore size, 0.22 µm). The clear supernatants were then treated with a Na2S2O8/AgNO3 solution at 100 °C during 10 min in order to oxidize Cr(III). The resulting Cr(VI) species were spectrophotometrically quantified. All dissolution studies were conducted at room temperature unless otherwise stated.
Results and Discussion
* Author to whom correspondence is addressed. Telephone:+5451-334169/334180. Fax:+54-51-334174. E-mail:mavena@fisquim. uncor.edu. X Abstract published in Advance ACS Abstracts, December 1, 1996.
The different green solids resulting from the synthesis were formed of particles having irregular shape and a “diameter” ranging from 20 to 150 nm as seen by transmission electron microscopy. As stated in the previous section and according to the method of synthesis employed here, they were intermediate between two well-
(1) The term (hydr)oxide is used here to denote hydroxides, oxohydroxides, hydrous oxides, oxides, etc. (2) Ludwig, C.; Casey, W. H.; Rock, P. A. Nature 1995, 375, 44. (3) Wieland, E.; Wehrli, B.; Stumm, W. Geochim. Cosmochim. Acta 1988, 52, 1969. (4) Avena, M. J.; De Pauli, C. P. Colloids Surf., A 1996, 108, 181.
(5) Avena, M. J.; Giacomelli, C. E.; De Pauli, C. P. J. Colloid Interface Sci. 1996, 180, 428. (6) Giovanolli, R.; Stadelman, W.; Feitknecht, W. Helv. Chim. Acta 1973, 56, 839. (7) Spiccia, L.; Marty, W. Inorg. Chem. 1986, 25, 266.
S0743-7463(96)00551-3 CCC: $12.00
© 1996 American Chemical Society
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known solids, which were characterized in previous reports by the authors: (i) The active monomeric hydroxide, a nonaged (nonpolymerized) crystalline sample of chemical formula Cr(OH)3‚3H2O. The structure of this solid is based on a layered array of Cr(OH)3(H2O)3 monomers, which are linked through hydrogen bonds between the OH- and H2O ligands of adjacent Cr(III) centers.6 There are no bridging hydroxide ligands connecting these centers so that only monomeric Cr(III) species are present. Active monomeric hydroxide is highly reactive and, upon aging, polymerizes to give an amorphous material.5-9 (ii) The polymeric chromium hydroxide, which is the product of aging of the active hydroxide. It is a highly polymerized and amorphous material of chemical formula Cr(OH)3‚xH2O (x ≈ 0.8) and can be prepared by either aging the active monomeric hydroxide10 or aging diluted Cr(III) aqueous solutions so that polymerization and subsequent precipitation take place.11,12 This solid appears to be formed by cross-linked [Cr(µ-OH)2(OH)(H2O)]n chains that confer particular sorptive properties to the bulk of the particles. It has been shown that polymeric chromium hydroxide particles are capable of absorbing ions and even relatively large molecules such as amino acids.10,13 Since the aging of the active material leads to polymerization, the solids intermediate between the active and the polymeric chromium hydroxide are a mixture of monomeric, oligomeric, and polymeric Cr(III) species. The higher the time and temperature of aging, the greater the proportion of oligomers or polymers. This has been demonstrated in the studies performed by Spiccia et al.,7,14 who detected and quantified the different dissolution products by ion exchange on sephadex gels. These authors have also shown that different aged samples produce different dissolution products, which reflect the composition of the solid phase. For example, the active monomeric hydroxide dissolves instantaneously in acid to form Cr(H2O)63+ monomers. However, the rapid dissolution of a low-aged precipitate constituted by monomers and oligomers generates the respective monomeric and oligomeric species instead of pure Cr(H2O)63+. This is so because the products of polymerization of Cr(III) are kinetically inert to cleavage and survive the acid dissolution of the solid. The mentioned behavior is also shown by the “active dimer” chromium hydroxide,15 which is a hydrogen-bonded array of the hydrolytic dimer Cr2(µ-OH)2(OH)4(H2O)4. Without aging it generates instantaneously the dimeric Cr2(µ-OH)2(H2O)84+ species in acidic media. Upon low aging, higher oligomers are formed and the solid dissolves in acid to give the corresponding protonated oligomeric species. In contrast to low polymerized chromium (hydr)oxides, highly polymerized materials (such as the polymeric hydroxide) are inert to acid dissolution; the cross-linked [Cr(µ-OH)2(OH)(H2O)]n chains cannot pass to solution to give soluble polymers and, in addition, they cannot be easily cleavaged to give soluble species due to the resistance of Cr-OH bonds to ligand exchange. These (8) von Meyenburg, U.; Syroky, O.; Schwarzenbach, G. Helv. Chim. Acta 1973, 56, 1099. (9) Spiccia, L. Inorg. Chem. 1988, 27, 432. (10) Giacomelli, C. E.; Avena, M. A.; Ca´mara, O. R.; De Pauli, C. P. J. Colloid Interface Sci. 1995, 169, 149. (11) Sprycha, R.; Jablonsky, J.; Matijevic´, E. Colloids Surf. 1992, 67, 101. (12) Bell, A.; Matijevic´, E. J. Inorg. Nucl. Chem. 1975, 37, 907. (13) Kumanomido, H.; Patel, R. C.; Matijevic´, E. J. Colloid Interface Sci. 1978, 66, 183. (14) Stu¨nzi, H.; Spiccia, L.; Rotzinger, F. P.; Marty, W. Inorg. Chem. 1989, 28, 66. (15) Spiccia, L.; Stoeckli-Evans, H.; Marty, W.; Giovanolli, R. Inorg. Chem. 1987, 26, 474.
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Figure 1. A vs t data for a chromium (hydr)oxide sample synthesized at pH ) 11.5 and aged for 4 days at room temperature. Dispersions were prepared by adding 0.2 mL of the chromium (hydr)oxide sample to (a) 3 mL of H2O and (b) 3 mL of 2.35 M HClO4. Blank regions in curve b correspond to the time needed for shaking the cuvettes. λ ) 800 nm.
solids, therefore, are highly resistant to acid attack; very concentrated acids and high temperatures are usually needed to produce dissolution.16 The dissolution behavior of the samples studied in this work is exemplified in Figure 1. It compares typical A vs t data obtained by mixing a chromium (hydr)oxide suspension with either water or a HClO4 solution. Curve b appears as a smoothed curve because data were obtained every 0.5 s. There are no data in the first 5-10 s of measurements because this is the time needed to mix the samples. The other blank regions (without data) in the curves correspond to the time at which the cuvettes were removed and shaken to check particle sedimentation. The invariability in A when the dispersion was added to water indicates that neither dissolution nor sedimentation took place in these conditions. In contrast, great changes could be observed when the experiments were performed in HClO4. An almost instantaneous decay in A (about 50%) took place followed by a continuous and measurable decrease in A with increasing time. The continuity of the curve after shaking the cuvettes (after blank spaces) allows one to discard sedimentation, and thus the behavior must be attributed to dissolution of the sample. In fact, a simple visual inspection reveals that the initially turbid suspension becomes progressively clear until no turbidity is detected. The spectral characteristics of the clear solution obtained after dissolution depended on the conditions of preparation and aging of the chromium (hydr)oxide suspension. Figure 2 depicts the vs λ curves (, molar absorbance) of some representative dissolution products compared with that of a chrome alum solution (Cr(H2O)63+). All the curves are characteristic of soluble aqueous Cr(III) species. A comparison with data in the literature suggests that the spectral curves b-d of Figure 2 correspond to oligomeric Cr(III) species, perhaps to a mixture of monomers and oligomers and more polymerized species.17 As the aging time or temperature increased, the maximum of the bands shifted from 407 to 421 nm and from 575 to 587 nm. These shifts, along with an increase in , are evidences for the formation of somewhat polymerized Cr(III) species with aging.17 Since the composition of the dissolution products must reflect the (16) Reartes, G. B.; Morando, P. J.; Blesa, M. A.; Hewlett, P. B.; Matijevic´, E. Langmuir 1995, 11, 2277. (17) Stu¨nzi, H.; Marty, W. Inorg. Chem. 1983, 22, 2145.
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Figure 2. Spectral characteristics of the clear solutions obtained after dissolution (2.35 M HClO4) of different chromium (hydr)oxide samples compared with those of a chrome alum solution (Cr(H2O)63+). Molarities in units are given in moles of Cr atoms per liter of solution: (a) chrome alum solution; samples prepared at pH ) 11.5 and (b) aged at room temperature for 10 min, (c) aged at 65-70 °C for 5 min, and (d) aged at 65-70 °C for 10 min.
composition of the corresponding solids, it can be concluded, such as is expected, that the proportion of oligomers or more polymerized material increases with the aging time or the aging temperature.17 Analysis of Figure 1 together with Figure 2 indicates, therefore, that the studied material is composed of a mixture of monomeric and low oligomeric Cr(III) species that dissolve almost instantaneously in HClO4 (rapidly dissolving material) and of a more polymerized material that undergoes dissolution at a lower and measurable rate (slowly dissolving material). It could be argued that fast dissolution followed by slow dissolution could also be interpreted as a slowing of the reaction rate as a more or less homogeneous material (not composed of a mixture of solids) undergoes dissolution. However, data obtained at different HClO4 concentrations (not shown here) revealed that the fraction of rapidly dissolving material is independent of the proton concentration in acidic media. In addition, a decrease in the acid concentration did not modify the dissolution rate of this fraction but decreased that of the slowly dissolving material. These results are not expected in the case of a more or less homogeneous sample where a decrease in the dissolution rate should be observed through all the dissolution runs. The effect of pH on the dissolution behavior of chromium (hidr)oxide samples is out of the scope of this paper and will be published separately. In dissolution studies it is useful to present dissolution data as R vs t plots, where R is the dissolved fraction defined by
[solid]0 - [solid] R)
[solid]0
(1)
two cases represent the dissolution of two different solids: the first reaction mechanism could take place with a system formed by equal particles (each particle being a mixture of rapidly and slowly soluble materials) that have the same reactivity and that, therefore, dissolve in the same way. The second mechanism should be observed when the dissolving material is formed by different kinds of particles with different reactivity; e.g., some particles formed by rapidly dissolving chromium (hydr)oxide and others constituted by slowly dissolving chromium (hydr)oxide. In acidic media, the former particles should undergo dissolution first. The relations between R and τ for the two mentioned cases can be easily deduced from light scattering theories.18,19 For a monodisperse suspension of nonabsorbing, spherical particles, the following relation, which gives τ as a function of particle mass mp, can be obtained:
τ)
BNpmp2
(2)
where B is a constant that considers particle geometry and the refractive indexes of the solid and solution phases, F is the particle density, λ is the wavelength of the light in the medium, and Np is the number of particles contained in the volume of dispersion V. The multiplication of both sides of eq 2 by Np/V gives
τNp BNp2mp2 ) 4 2 2 V λFV
(3)
If the ratio Np/V remains constant during a dissolution run (this means that the particles decrease their volume and mass but not their concentration when dissolvingscase 1) and knowing that Npmp/V ) [solid], the following relation results:
[solid] ) C1/2τ1/2
(4)
where C ) Npλ4F2/BV. Substituting eq 4 in eq 1 gives
τ01/2 - τ1/2 R)
τ01/2
(5)
In the other case (case 2) it is assumed that dissolution takes place by decreasing Np without varying the mass of the undissolved particles. If this is so, τ is proportional to [solid] because eq 2 can be rewritten as
[solid] ) Dτ
(6)
where D ) λ4F2/Bmp. Thus, the substitution of eq 4 in eq 1 leads to
R) where [solid] is the concentration of solid at a given reaction time and [solid]0 is the initial concentration. To obtain such plots, A vs t or τ vs t data must be replotted as R vs t data. This can be done with the aid of light scattering theories. The equation relating R and τ depends on the way that dissolution takes place. Two hypothetical, limiting cases can be considered taking into account the characteristics of the solids studied in this work: (i) dissolution proceeds by decreasing the size of the particles without modifying their concentration (particle number per volume unit), and (ii) dissolution takes place by varying the particle number without varying the particles size or mass. These
λ4F2V
τ0 - τ τ0
(7)
Since τ is proportional to the absorbance of the sample, R values can be calculated with eq 5 or eq 7 using either A or τ. Figure 3 presents the results obtained by plotting R vs t data obtained using either eq 5 or eq 7 and compares them with those obtained by measuring spectrophotometrically (after oxidation to Cr(VI)) the amount of (18) Hunter, R. J. Foundations of Colloids Science; Oxford University Press: Oxford, 1993; p 151. (19) Ross, S.; Morrison, I. D. Colloidal Systems and Interfaces; Wiley-Interscience: New York, 1988; p 44.
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Figure 4. Spectral curves for a chromium (hydr)oxide sample prepared at pH ) 11.5 and aged at 65-70 °C for 5 min: (a) before dissolution, dispersed in H2O; (b) after dissolution in 2.35 M HClO4. This last curve is already shown in Figure 2, curve c.
Figure 3. Comparison of dissolution data for chromium (hydr)oxide samples prepared at pH ) 8.8 and aged at 70 °C, obtained by turbidimetry (λ ) 700 nm) using (fill symbols) eq 7, (open symbols) eq 5, and (open symbols plus crosses) a standard method: (a) sample aged for 20 min and dissolved in 0.34 M HClO4; (b) sample aged for 10 min and dissolved in 0.2 M HClO4; (c) sample aged for 22 min and dissolved in 0.34 M HClO4. Measurements performed at room temperature.
chromium(III) species released to solution. While the evaluation of R through eq 7 overestimates the actual values, eq 5 generates R vs t curves that are in very good agreement with those obtained by measuring Cr(VI). Although the green solids formed in this work are polydisperse and absorbing materials, it is clear from Figure 3 that eq 5 can be used for the analysis of turbidimetric data under our working conditions and that dissolution of chromium (hydr)oxides can be quantitatively monitored by turbidimetry. This conclusion also holds for dissolution of light green Ni(OH)2 particles.4 An explanation to this could arise from the fact that light scattering is so high compared to light absorption that the solid particles can be considered as nonabsorbing in the working conditions. In the case of Ni(OH)2, for example, the turbidity of a suspension at t ) 0 was more than 2000 times higher than the corresponding τ value after complete dissolution, indicating that the turbidity of the suspension was almost exclusively due to light scattering and that very low interferences from colored Ni(II) species existed.4 The chromium (hydr)oxides studied in this work appear to be similar to Ni(OH)2 in this respect. Figure 4 compares the spectral curve of a chromium (hydr)oxide dispersion in water with that of its dissolution products. The high absorbance of the dispersion is mainly due to light scattering; only weak absorption shoulders at around 413 and 580 nm responsible for the green color of the dispersion can be observed. The presence of these shoulders is in agreement with data obtained from diffuse reflectance studies of different solids having Cr(III) species; they absorb light at λ < 800 nm with two absorption maxima at 406-416 nm and at 581-589 nm.20 The relatively low absorbance of Cr(III) species is also apparent from the spectra of the dissolution products. The ratio τ0/τ∞ (τ∞, turbidity after complete dissolution) was always higher than 10 in the working conditions at any λ (τ0/τ∞ ) 19.1 at 330 nm, 23 at 400 nm, 17 at 550 nm, 12 at 600 nm, 56 at 700 nm, 413 at 800 nm, and 411 at (20) Carrado, K. A.; Suib, S. L.; Skoularikis, N. D.; Coughlin, R. W. Inorg. Chem. 1986, 25, 4217.
Figure 5. Turbidimetric data for the dissolution of two chromium (hydr)oxide samples prepared at pH ) 11.5; (A) sample aged at 70 °C for 1.5 min and dissolved in 0.37 M HClO4; (B) sample aged at room temperature for 4 days and dissolved in 2.35 M HClO4. Numbers in the figure indicate λ in nanometers.
900 nm). However, this ratio is not as high as in the case of Ni(OH)2, and the effects of light absorption must be checked. The possible interferences of light absorption in turbidity measurements were checked through dissolution studies conducted at different wavelengths. The results obtained for two different samples are depicted in Figure 5. The plots indicate that the calculated R value varies with λ in the 330-700 nm range but is independent of wavelength at λ g 700 nm. In other words, the curves do not superimpose in the range where the solid absorbs light (low τ0/τ∞) but coincide quite well where the absorption is negligible, τ0/τ∞ > 50. In these last cases the solid seems to behave as a nonabsorbing material. Some dissolution studies were also performed at 800 nm using different amounts of solid. The results are shown in Figure 6. The independence of R with the initial solid concentration reinforces the applicability of eq 5. This result also demonstrates the usefulness of turbidimetry for these kinds of studies because it minimizes the
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Figure 8. Possible pathways for the dissolution of the studied chromium (hydr)oxide samples. The qualitative composition of the particles is depicted in the ellipses.
Figure 6. Effects of varying the initial concentration of solid in turbidimetric data obtained by dissolving a chromium (hydr)oxide sample prepared at pH ) 11.5 and aged at room temperature for 4 days. Dissolutions performed in 2.35 M HClO4 solutions: (A) A vs t data; (B) R vs t obtained by applying eq 5 to A vs t data of A. Numbers in A represent the volume (in milliliters) of the chromium (hydr)oxide suspension added to 3 mL of the HClO4 solution. λ ) 800 nm.
Figure 7. Temperature dependence of the dissolution rate (in 1 M HClO4) for different studied samples: (a) sample prepared at pH ) 11.8 and aged at 60 °C for 9 min, Ea ) 70 kJ/mol; (b) sample prepared at pH ) 9.5 and aged at room temperature for 17 days, Ea ) 46 kJ/mol; (c) sample prepared at pH ) 11.8 and aged at 60 °C for 22 min, Ea ) 65 kJ/mol.
problems associated with the weighing of a very low amount of solid and other sampling procedures. Figure 7 depicts the results obtained by dissolving some samples at different temperatures. Data are plotted as the logarithm of the dissolution rate as a function of 1/T. Dissolution rates of the slowly dissolving material (the unique ones that could be monitored under our experimental conditions because the other fraction dissolved almost instantaneously) were calculated as dβ/dt where β is defined as β ) (R - R0)/(1 - R0). In the previous definition, R0 represents the fraction of the solid material that undergoes instantaneous dissolution, i.e., the value of R obtained by extrapolating R vs t curves to t ) 0 (see Figure 6). Thus, the value of β represents the fraction of slowly dissolving material that dissolves at a given t. If the initial dissolution rate, (dβ/dt)0, is proportional to the rate constant of the process, the slope of the curves in Figure 7 is equal to -Ea/R where Ea is the apparent
activation energy for dissolution. Typical Ea values were in the 40-70 kJ/mol range. These relatively high values suggest that diffusion of Cr(III) species in the aqueous solution is not the rate-determining step. Two main mechanistic pathways appear to be possible for the dissolution of the studied chromium (hydr)oxides (Figure 8). (i) In the first step of the first pathway, all monomers and all low oligomers protonate to give the corresponding soluble species, which pass rapidly to solution. Solid particles remain constituted by insoluble high oligomers. The second step (the rate-controlling step) takes place by cleavage of high oligomers into soluble monomers and soluble low oligomers. The high activation energy of the process could support this pathway. However, there are some other data that are not consistent with this pathway. For example, although the cleavage of some oligomers of chromium could be somewhat rapid (half-lifes of 0.5-3 h for the cleavage of the tetramer into dimers in 1 M HClO4 at 25 °C), most oligomers produce species of lower nuclearity at very much lower rates (t1/2 ) 7 days for the cleavage of the dimer and t1/2 ) 21 days for the cleavage of the trimer, both in 1 M HClO4 at 25 °C),17 which is not consistent with the fact that most of our samples dissolve in minutes. On the other hand, if the cleavage of Cr(III) species is rate controlling, the apparent activation energy of the dissolution process should be similar to that of ligand exchange in Cr(III) centers (around 100 kJ/mol21,22 ). This is so because ligand exchange must take place during cleavage; for example, the Cr* center forming a hypothetical tCrOHCr*t bond in an oligomer can be considered as bonded to a tCrOH- ligand; therefore, the Cr* center must lose the tCrOH- ligand to end up coordinated to H2O or OH- and give a cleavaged H2OCr*t or OHCr*t group, which is a ligand exchange reaction. (ii) In the first step of the second pathway some of the monomers and low oligomers constitute the solid protonate to give the corresponding soluble species and pass rapidly to solution. The others, however, remain trapped in the network formed by the insoluble high oligomers or polymers. In this case, the slow step of the dissolution process is postulated to be the diffusion of trapped monomers and low oligomers in the bulk of the particles. This assumption is supported by several facts. High oligomers and polymers confer gel properties to chromium (hydr)oxide particles, which were revealed by their great (21) Hunt, J. P.; Plane, R. A. J. Am. Chem. Soc. 1954, 76, 5960. (22) Hunt, J. P. In Progress in Inorganic Chemistry; Lippard, S. J., Ed.; John Wiley and Sons: New York, 1983; Vol. 30, p 359.
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water absorption capacity, by the reversibility of the process, and by the absorption of large molecules (amino acids) and ions (H+, Ca2+, SO42-, etc.).10,11,13 The only explanation for these facts is that molecules and ions diffuse into the particles to become absorbed. Moreover, Ea values in the range 40-70 kJ/mol are commonly found for diffusion in gels or viscous solvents.23 On the other hand, the second pathway indicates that some highly insoluble polymeric material must remain, forming the particles after diffusion of the trapped monomers and low oligomers. This is consistent with the fact that complete dissolution could not be achieved in the time of measurements, especially for samples aged for a relatively long time or at a relatively high temperature. Although the results in this paper are best accommodated by the second pathway, evidence given in the last paragraph is indirect. For example, it is know that Ea for dissolution of aluminum oxides where the cleavage of Al-O bonds is rate determining is around 56 kJ/mol,3 whereas those corresponding to water exchange of Al(III) are around 100 kJ/mol.22 It is possible, therefore, that the fact that Ea for dissolution of chromium (hydr)oxides is lower than those for ligand exchange cannot be used as a valid argument for discarding the first pathway. On the other hand, it is known that chromium (hydr)oxide particles synthesized from chrome alum solutions can be contaminated with sulfate ions that coordinate Cr centers by replacing H2O or OH ligands.10 Then, it could be possible that sulfate labilizes some Cr-OH-Cr bonds, increasing the dissolution rate and decreasing the apparent activation energy of the dissolution process. In fact, it was previously informed that sulfuric acid is more efficient in the dissolution of chromium (hydrous) oxides.16 In a previous paper,16 the rate-determining step for the dissolution of chromium (hydrous) oxides having polymeric nature appeared to be the cleavage of a µ-hydroxo bond tCrOHCrt after the protonation of the corresponding (23) Brandrup, J.; Immergut, E. H. Polymer Handbook; John Wiley and Sons: New York, 1975; Vol. III, p 230.
Avena et al.
µ-oxo bond tCrOCrt. Since these materials were prepared at rather high temperatures and for relatively long times, they may be assumed to be highly polymerized. The absence of monomeric or low oligomeric species in the particles makes the cleavage of Cr-O(H) bonds appear as rate determining. Unfortunately, no Ea data were reported in this paper to compare with those obtained here. Conclusions The very good correlation between results obtained by turbidimetry and those obtained with the standard method reveals that turbidimetry can be used to study the dissolution kinetics of chromium (hydr)oxides because these solids behave as nonabsorbing materials at λ g 700 nm. The good agreement found in this work together, with that found by studying dissolution of Ni(OH)2 particles,4 opens the possibility that other materials (especially nonabsorbing or low-absorbing materials) could be monitored by turbidimetry, which appears as a simple and useful tool for this kind of study. Dissolution data indicated that the studied samples were formed by at least two phases of different reactivity. One of them, formed by monomeric and low oligomeric Cr(III) species, dissolved almost instantaneously in acidic media, whereas the other, composed of more polymerized species, dissolved at a slower rate. The dissolution products were, in both cases, monomeric and oligomeric Cr(III) species. Although more direct evidence is needed, apparent activation energies for the acid dissolution of the less reactive phase are consistent with diffusion into the solid as the rate-determining step of the process. This mechanism is different from that proposed for highly polymerized materials. Acknowledgment. The authors thank Miss L. P. Falco´n for language assistance and Dr. P. I. Ortiz for helpful suggestions. This study was partially supported by CONICET, CONICOR, and SECYT. LA960551W
