Article Cite This: J. Chem. Eng. Data 2017, 62, 3848-3853
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Solubility of MoO3 in NaClO4 Solutions at 573 K Tatiana P. Dadze, Galina A. Kashirtseva, Mikhail P. Novikov, and Andrey V. Plyasunov* Institute of Experimental Mineralogy, Russian Academy of Sciences, 142432 Chernogolovka, ul. Osipyana, 4, Moscow Region, Russia S Supporting Information *
ABSTRACT: This study reports the measurements of solubility of crystalline molybdenum trioxide, MoO3, in NaClO4 solutions at 573.2 K and pressures, close to the saturation water vapor. In total, there are 16 data points at NaClO4 molalities between 0.005 and 2.21. The solubility is explained by two dissolution reactions: MoO3(cr) + H2O(l) = HMoO4− + H+, and MoO3(cr) + H2O(l) = H2MoO4(aq). The solubility moderately increases with the sodium perchlorate concentrations. In our view, this effect can be fully explained by the variations of the activity coefficients of H+ and HMoO4− in sodium perchlorate solutions, without the need to invoke a complex formation between Na+ and HMoO4−, as was proposed in the literature. A better knowledge of speciation, association and thermodynamic properties of solutions of typical electrolytes (such as NaCl, NaClO4, etc.) at elevated temperatures is a prerequisite for a more definite and precise interpretation of solubility data for solid compounds in high-temperature solutions.
1. INTRODUCTION Molybdenum is an industrially important metal added to the steel alloys in the manufacture of heat-resistant and corrosionresistant materials for the aerospace and nuclear industries, and in chemical engineering. Its main economic reserves are concentrated in the medium- and high-temperature hydrothermal porphyry Cu−Mo and Mo stockwork deposits, as well as in skarn deposits, mostly as the mineral molybdenite (MoS2). While molybdenite, MoS2, is the most prevalent molybdenumbearing mineral, the species of molybdenum in the +6 oxidation state are the most important in aqueous solutions at various temperatures. For example, the molybdate ion, MoO42−, and its protonated forms coexist with MoO2 (the mineral tugarinovite) even at the oxygen fugacity below that of the Ni−NiO redox buffer (which maintains the oxygen fugacity below 10−20 MPa at T < 800 K), as follows from both the solubility data1 and XANES spectra,2 with no Mo(V) and Mo(IV) aqueous species detected. We expect that the knowledge of aqueous chemistry of Mo(VI) at various temperatures will be useful for optimization of industrial processes involving molybdenum and its compounds. While speciation and complexation of Mo(VI) at ambient temperatures is relatively well understood,3,4 at elevated temperatures, despite a number of relevant studies,1,2,5−11 our knowledge of speciation and thermodynamics of Mo(VI) aqueous forms is fragmentary. Recently, our group investigated the solubility of MoO3 in water and dilute ( 1.5) solutions having such concentrations of molybdenum, the speciation of this metal is dominated2,3 by highly charged polynuclear © 2017 American Chemical Society
species, containing 6, 7, 8, ..., up to 36 Mo atoms per polymer (it is not clear whether consensus on the stoichiometry of Mo polynuclear species is finally reached among researchers). However, there are compelling reasons to expect that at high temperatures simple monomeric species will predominate in aqueous Mo-bearing solutions. Most importantly, the dielectric constant of water, relatively high (∼78) at 298 K rapidly decreases with temperature, becoming equal to ∼20 at 573 K and saturated water vapor pressure. Therefore, highly charged forms become unstable due to the reduction of the dielectric constant of water, and polynuclear forms tend to dissociate into mononuclear constituents with the increase of temperature.13 Indeed, a sharp decrease in the proportion of polymeric species with temperature has been experimentally demonstrated for aqueous W(VI) solutions,14 which are similar in properties to Mo(VI) solutions. In addition, in situ X-ray spectroscopic studies2,11,15 show that up to 873 K dense molybdenum-bearing aqueous solutions over a wide pH range, from weakly acidic to alkaline, are dominated by the MoO42− ion and its hydrolysis products, HMoO4− and neutral molybdic acid, H2MoO4(aq) (only in strongly acid chloride solutions is the additional formation of oxo-chloride forms observed at elevated temperatures). Therefore, we interpreted12 our data on the solubility of MoO3 in moderately acid solutions in terms of the following reactions: MoO3(cr) + OH− = HMoO4 −
(1)
with log10 Ko(1) = (7.09 ± 0.2) and MoO3(cr) + H 2O(l) = H 2MoO4 (aq)
(2)
Received: June 9, 2017 Accepted: September 14, 2017 Published: September 25, 2017 3848
DOI: 10.1021/acs.jced.7b00527 J. Chem. Eng. Data 2017, 62, 3848−3853
Journal of Chemical & Engineering Data
Article
with log10 Ko(2) = −(2.22 ± 0.2), see Dadze at al.12 for details. Our preliminary data on the solubility of calcium molybdate in weakly acid solutions, having much lower molybdenum content, confirm the thermodynamic properties of HMoO4− and H2MoO4(aq) deduced from dissolution of molybdenum trioxide. The lower values of the dielectric constant of water at elevated temperatures favor the ionic interactions with the formation of species formed by oppositely charged ions. As a result, the association constants of ionic pairs increase with temperature. If an analogy between the Na+−MoO42−−H2O system and the well studied system Na+−SO42−−H2O16 is valid, then one can expect the formation of the several complex species: NaHMoO4(aq), NaMoO4−, and Na2MoO4(aq), in addition to the molybdate ion and products of its hydrolysis. Indeed, the formation of the complex NaHMoO4(aq) was proposed in a study7 of solubility of MoO2(cr) (the mineral tugarinovite) in NaCl solutions at fixed values of the redox potential at 573−723 K. At 573 and 50 MPa the value of log10 Ko = (1.73 ± 0.4) was obtained for the reaction Na + + HMoO4 − = NaHMoO4 (aq)
molybdenum trioxide both before and after experiments exhibit no peaks other than those for orthorhombic MoO3. Within the limits of sensitivity of INCAEnergy EDXS (energy dispersive X-ray spectroscopy) microanalysis system, the synthesized sample contained, in addition to oxygen, only Mo. Aqueous solutions of NaClO4 were prepared from commercial salt. Anhydrous NaClO4 was stored in a desiccator over silica gel. 2.2. Experimental Methods. Experiments were carried out in autoclaves made of the titanium alloy VT-8 and of the internal volume ∼20 cm3. For experiments, autoclaves were placed in a vertical cylindrical furnace. The temperature gradients were less than 2 K across the autoclave length at a typical loading of 10 autoclaves per furnace. A thermal regulator Miniterm-300 was used to control temperature to an accuracy of ±(1 to 2) K. During the run, temperature was measured with a type K (chromel−alumel) thermocouple using a multilogger thermometer HH506RA (OMEGA Engineering). Temperature variations during the experiments were typically within ±2 K. A weighed quantity of crystalline MoO3 (∼100 mg) was placed in a titanium container, which was suspended in the upper part of the autoclave in such a way that it did not contact the liquid solution unless the latter expanded up to 95% of the volume of the autoclave. Owing to this design, the solid phase was never in contact with the quenched solution. Autoclaves, prepared for experiments and closed, contained about 5·10−5 moles of oxygen, mostly (∼90%) in the gas phase over solution. That means that the runs were carried out at high (at least initially) partial pressures of oxygen, about 10−3 MPa. After the run, the autoclaves were cooled down to room temperature under running cold water for 5−7 min. The duration of experiments was 17−24 h, which, according to kinetic series12 is sufficient to reach the steady-state solubility values. The pressure in autoclaves was determined by the degree of the filling of the autoclave. To avoid the vapor phase in the experimental vessel, the degree of filling slightly exceeded the density, ρ, of solution in equilibrium with the vapor phase at the temperature of experiments, 573 K. The saturated vapor pressure of water at this temperature is ∼8.6 MPa, the calculated pressures of our experiments are ∼10 MPa. Regrettably, the densities of NaClO4 solutions at 573 K and various pressures are measured only up to molalities of ∼1 m,19 so, some extrapolations are necessary. We found that at 473−573 K the ratio of densities of NaCl solutions20 to densities of NaClO4 solutions19 of the same molality, m, can be described by the following empirical equation:
(3)
For the K analogue of reaction 3 log10 K = (1.97 ± 0.5). It should be noted that at the saturated water vapor pressure log10 Ko will be somewhat larger, as the stability constants increase with the decreasing pressure (or the density) of water.17,18 Such values of equilibrium constants suggest that the neutral form NaHMoO4(aq) would make a significant contribution to the material balance of molybdenum and strongly promote the solubility of molybdenum phases in Na-bearing hydrothermal solutions. The dissolution of MoO2 with the formation of Mo(VI) aqueous species requires the control of redox conditions, which strongly complicates the experimental setup and methods. Therefore, to check the possible formation NaHMoO4(aq), we decided to investigate the solubility of molybdenum trioxide, MoO3 (the mineral molybdite) in NaClO4 solutions at 573 K. Sodium perchlorate, a typical inert electrolyte at ambient conditions, was preferred to sodium chloride due to literature indications of possible formation of oxo-chloride complexes of Mo(VI) in hydrothermal solutions. +
o
2. EXPERIMENTAL METHODS 2.1. Materials. All reagents used in this study are listed in Table 1 and were used without further purification. Bidistillate Table 1. Sample Description
ρ(NaCl)/ρ(NaClO4 )
chemical name
source
mass-fraction purity
ammonium molybdate sodium perchlorate anhydrous
REACHIM REACHIM
>0.98 >0.98
purification method none none
= 1 + (0.12083T − 78.045) × 10−3m
(4)
This equation was used to estimate the filling coefficients for experiments on solubility of MoO3 in NaClO4 solutions. Prior to experiments, autoclaves were loaded with 20 wt % nitric acid solution with the degree of filling of 0.72 and kept for a day at 573 K to form a protective TiO2 layer. Although the titanium alloy VT-8 contains molybdenum, up to 3 wt %, because of the formation of the protective TiO2 layer this alloy has a high corrosion resistance, and Mo concentrations in tested aqueous solutions (H2O, 0.10 m HClO4, 0.51 m NaClO4, and 0.001 m NaOH, see Dadze et al.12) after blank runs carried out at 573 K and 10 MPa were below the detection limit of our analyses, see below. There may be questions related to the stability of perchlorate ions in our experiments. Indeed, it is known21 that concentrated
water was used to prepare aqueous solutions. Crystals of MoO3 (molybdenum trioxide, the mineral molybdite, CASRN 1313-27-5) were prepared by the three-stage calcining of ammonium molybdate: first, 3 h of drying at 373−393 K, then calcining for 2 h at 723 K and for 3 h at 973 K. The synthesized phase was hydrothermally treated at 573 K for 7 days in pure water to remove small particles, edges, defects, etc. MoO3 formed elongated crystals with sizes of 1 to 50 μm. Patterns of X-ray powder diffraction were measured both before and after the experiments using a Bruker D8 Discovery diffractometer with Cu Kα and Co Kα radiation. Powder diffraction patterns of 3849
DOI: 10.1021/acs.jced.7b00527 J. Chem. Eng. Data 2017, 62, 3848−3853
Journal of Chemical & Engineering Data
Article
aqueous solutions of perchloric acid at temperatures above 523 K decompose according to the reaction H+ + ClO4− = 0.5 Cl2 + 0.5 H2O + 1.75 O2. However, the rate of decomposition quickly decreases with the fall of temperature and the molality of acid, and Henderson et al.21 concluded that “The initial rate of decomposition of dilute aqueous HClO4, decreases··· ten-fold for each halving of the initial concentration of HClO4. ...it can be stated that the rate is negligibly small···for the 0.2 m acid at 300°, as far as experiments of about a day’s duration are concerned.” Our experience is in agreement with these findings. Previously, we12 verified that the 0.011 m solution of perchloric acid (no solid phase present) with the initial pH = 1.98 ± 0.03 did not show any change of pH values, within errors of measurements, after being held at 573 K and 10 MPa, at durations from 1 to 3 days. Therefore, we are confident that the perchlorate solutions used in our experiments, with acidity far less than 0.1 m, remained stable over the course of autoclave runs. In any case, as we previously12 experienced when working at a higher temperature, 623 K, it is hardly possible to miss the decomposition of perchloric acid due to a strong smell of chlorine after opening the autoclave. 2.3. Analytical Methods. The concentration of molybdenum in quenched solutions after the experiment was determined colorimetrically on the spectrophotometer Spekol-11 at λ = 453 nm with the thiocyanate method, based on the formation of yellow color thiocyanate complexes of Mo(V),22 and by using thiourea as a reducing agent. The detection limit for Mo is 5 μg in a sample; that is, at the largest aliquot, 10 mL, the minimal measurable molality of Mo is ∼5 × 10−6 m. At 10 μg of Mo in a sample, the analytical uncertainty is about 20%, and at 20 μg and above it is about 10%. Values of pH (in the activity scale) of solutions before the runs and in quenched solutions were measured by the ion selective electrode analyzer ECOTEST-120 (Econix, Russia), which was calibrated using commercial standard buffer solutions.
Table 2. Experimental Results on the Solubility of MoO3 in NaClO4 Solutions at 573 K and Pressure ∼ 10 MPaa pH of solution no. of run
m NaClO4 (mol kg−1)
log10 m(Mo) (mol kg−1)
initial
quenched
307 308 309 310 311 313 314 315 567 568 569 570 587 588 589 590
0.005 0.005 0.01 0.01 0.02 0.05 0.05 0.10 0.51 1.05 1.62 2.21 0.30 0.73 1.39 1.85
−1.78 −1.80 −1.78 −1.78 −1.79 −1.74 −1.69 −1.66 −1.50 −1.44 −1.52 −1.48 −1.63 −1.57 −1.51 −1.49
5.51 5.51 6.32 6.32 5.60 5.85 5.85 6.18 6.03 n/da n/da n/da 6.95 7.51 n/da n/da
2.20 2.23 2.21 2.20 2.15 2.11 2.03 2.05 1.96 1.83 1.47 1.28 2.05 1.92 1.34 1.25
a Notation: n/da, no data. Standard uncertainties u are u(T) = 2 K, ur(P) = 0.10, ur(m) = 0.02, u(log10(m)) = 0.10, u(pH) = 0.05.
Table 3. Thermodynamic Data, the Molar Gibbs Energy, Gom, at 573 K, Ps (in kJ mol−1), of Basic Species Used in Modeling of Solubility Results Gom
species H2O(l) H+ OH− Na+
−263.86 0 −138.70 −282.23
ref a a a a
species −
ClO4 HMoO4− H2MoO4(aq) MoO3(cr)
Gom
ref
−57.02 −913.18 −936.10 −696.64
a b b c
a The database Unitherm, built-in into the HCh program,23,24 based on the HKF-model.25,26 bDadze et al.12 cThe database Unitherm, built-in into the HCh program,23,24 based on the handbook by Glushko et al.27
3. RESULTS AND DISCUSSION 3.1. Solubility Results for MoO3. The experimental solubility data obtained in this study are presented in Table 2 in molality units, where molality m is defined as a number of moles of a substance per 1 kg of water. The uncertainty of experimental results is estimated to be within 0.1 log10 units. The typical run duration was 17−24 h, based on earlier kinetic runs,12 which showed the achievement of steady-state concentrations within 10 h. Pressures were not measured, but calculated from the filling coefficients, and the values of pressure are approximate. The solubility at small (
