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Oxoacidity scale of molten ionic chlorides and bromide
Fluid Phase Equilibria 401 (2015) 77–81
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Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
Oxoacidity scale of molten ionic chlorides and bromide Victor L. Cherginets * , Tatyana P. Rebrova, Vyacheslav A. Naumenko Institute for Scintillation Materials, National Academy of Sciences of Ukraine, Lenin Avenue, 60, Kharkov 61001, Ukraine
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 January 2015 Received in revised form 8 May 2015 Accepted 14 May 2015 Available online 19 May 2015
Determination of NiO and PbO solubilities in molten BaBr2–KBr (0.505:0.495) eutectic at 973 K (of pKs;NiO = 8.06 0.2 and pKs;PbO = 4.04 0.2, molality scale) allowed completing systematic oxide solubility studies in molten ionic bromides. On the basis of the obtained and literature data the acidity scale for chloride and bromide melts (KX–NaX ! BaX2–KX ! KX–LiX sequences, X = Cl, Br) at 973 K was built on the basis of primary medium effects (estimated as oxobasicity indices). The oxoacidic properties of chloride and bromide melts with similar cation composition are close, although the acidity of bromide analogs is always somewhat weaker. The obtained information permits to predict e.g., limits of halide melt purification by precipitating deoxidization or carbohalogenation methods. ã2015 Elsevier B.V. All rights reserved.
Keywords: Oxoacidity Oxide solubility Primary medium effect Potentiometric titration
1. Introduction Investigations of Lux-Flood acid-base interactions in molten salts (ionic melts) are now performed in two obvious directions: determination of acidic (basic) properties of new compounds in the traditional halide melts (e.g., KCl–LiCl, KCl–NaCl etc.) and development of imaginations permitting to extend the use of already obtained physicochemical data on the case of ionic solvents of new compositions. The first direction is practically exhausted since at temperatures of the order of 1000 K the range of stable acidic oxides, oxoanions and metal cations as Lux-Flood acids is rather narrow and the thorough studies of some halide melts (bromides, iodides) especially if they contain constituent cations of high acidity (Li+, cations of alkaline earth and rare earth metals) are very difficult. This forces investigators to develop approaches which allow predicting changes of thermodynamic and kinetic parameters of different Lux-Flood acid-base processes with the alteration of the melt composition. The generally accepted way to obtain such parameters is the estimation of primary medium effects for the ‘carrier’ of acidic or basic properties (M), logg 0;M which, according to equation:
* Corresponding author. Tel.: +380 637119592. E-mail address: [email protected] (V.L. Cherginets). http://dx.doi.org/10.1016/j.fluid.2015.05.025 0378-3812/ ã 2015 Elsevier B.V. All rights reserved.
logg 0;M ¼
AM AM;L 2:3RT
AM
(1)
(where and AM;L the solvation energy of M particle in the reference and the studied (L) solvents, J mol1, respectively, R the universal gas constant, 8.314 J mol1 K1, T the absolute temperature, K) is a measure of work of M particle transfer from infinitely diluted solution in the studied solvent to the infinitely diluted solution in the reference one referred to 1 mole of substance M. Just this resolvation work is the main reason of observed changing constants of a majority of acid-base reactions in different solvents. Recently investigations of the relative melt acidities were fulfilled in our papers [1,2] and they concerned only molten ionic chlorides. Molten bromides are also of interest to investigators as prospective media for electrodeposition of rare-earth metals [3]; they serve as growth material for obtaining single crystals of such novel scintillation materials as Cs2LiYBr6:Ce3+ [4] or CsBa2Br5:Eu2+ [5]. Naturally, to provide high purity of the deposited metals and good performance of the optical crystals the corresponding melts should be thoroughly purified from oxide ion traces. Knowing the primary medium effects of O2 in molten bromides facilitates the problem of choice of the most appropriate purification method. However, the literature data contain only information about acidic properties of melts based on alkali metal bromides (these data were published in our recent work [6]). The purpose of the present work is to determine solubility of nickel and lead monoxides in molten bromide mixture BaBr2–KBr (0.505:0.495, mole fractions) as a typical example of melts based on alkaline earth halides, to estimate the relative oxoacidic
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V.L. Cherginets et al. / Fluid Phase Equilibria 401 (2015) 77–81
Table 1 Chemicals used in the experiments and methods of their treatment. Formula
Supplier
Purity, main substance
Purification method
Ar BaBr2 KBr KOH NH4Br NiCl2 PbCl2
PASS, Ukraine Reakhim, Russia Reakhim, Russia Reakhim, Russia Merck Lankhit, Russia Reakhim, Russia
0.997 0.995 0.995 0.999 0.998 0.995 0.998
Drying Drying Drying Drying None Drying None
properties of this melt on the basis of the solubility data and compare acidic properties in sequences KX–NaX ! BaX2–KX ! KX–LiX (X = Cl, Br). 2. Materials and methods The used reagents and methods of their preparation are presented in Table 1. The stoichiometry of process of the metal cation's titration corresponds to the equation: MeO fi Me2+ + O2.
(2)
In this paper Me designation is referred to Pb or Ni. This equilibrium is described by the dissociation constant, KMeO: K MeO fi mMe2þ
mO2 ; mMeO
(3)
where mMe2þ , mO2 and mMeO the equilibrium molalities (mol kg1, i.e., a number of moles of a substance per 1000 g of the ionic solvent) of metal cation, oxide anion and non-dissociated oxide respectively. In the case of formation of the saturated solutions a certain amount of MeO precipitates and the actual molality of MeO in the saturated solution (designated further as sMeO ) being thermodynamic constant of the oxide deviates from mMeO . Under these conditions equilibrium (2) is described by the solubility product value, K s;MeO : K s;MeO ¼ mMe2þ mO2 :
(4)
Knowing both equilibrium parameters one can estimate the abovementioned molality of non-dissociated oxide in the saturated solution sMeO according to the following equation: sMeO ¼
K s;MeO : K MeO
(5)
over P2O5 under vacuum under vacuum in argon atmosphere at 600 C under vacuum with NH4Cl + sublimation at 1100 K
The indicator oxygen electrode presents itself a test-tube (diameter of 8 mm, wall thickness of 0.5 mm and height of 160 mm) made of YSZ containing inner oxygen electrode Pt(O2). The latter is coiled platinum foil with platinum lead. Dried air (pO2 = 0.21 atm) was the source of the oxygen and it was supplied into the inner electrode space. The silver electrode presents silver wire with platinum lead immersed into the solution containing the constant concentration of corresponding silver bromide (0.1 mol kg1) in the studied bromide melt. The reference electrode is places into a test-tube made of alundum (diameter of 8 mm, wall thickness of 1 mm and height of 160 mm). The walls of the alumina test-tube impregnated by the studied and inner melts serve as excellent salt bridge. Usually the silver electrodes can work for a long time and they are well-reproducible [8]. Each experiment required 50 g of the BaBr2–KBr mixture. It was obtained by mixing of BaBr2 and KBr of reagent quality taken in 0.718:0.282 mass ratio. The melt before the experiment was purified from a majority of oxide ion traces (admixtures of the corresponding hydroxides, carbonates and sulfates in the initial chemicals) by adding NH4Br of reagent quality: 2NH4Br + O2 ! 2NH3" + H2O" + 2Br.
(7)
2
The residual concentration of O in the purified melts was ca. 5 105 mol kg1. Anhydrous NiCl2 was prepared by drying the corresponding hydrate of reagent quality in vacuum (5 Pa) to 673 K with subsequent sublimation in 975–1053 K temperature range. PbCl2 of reagent quality was used without purification. KOH (source of O2, mass fraction of the main substance is 0.999) was melted in alumina crucible and kept for 1 h at 973 K in argon atmosphere for complete removal of absorbed water. Its dissociation in melts proceeds completely according to the following equation: 2KOH ! 2K+ + H2O" + O2.
In molality scale the equilibrium parameters are expressed in mol kg1 (sMeO and Ks,MeO) and mol2 kg2 (K s;MeO ). For different purposes the equilibrium constant indices (inverse decimal logarithms of the constants) such as pKMeO and pKs,MeO are used. The scheme of the used potentiometric cell can be expressed as follows: Ag|Ag+, BaBr2 KBr||BaBr2 KBr + O2|YSZ|(O2)Pt
(6)
(where YSZ is the solid electrolyte membrane of 0.9 ZrO2 + 0.1 Y2O3 composition) and the most detailed description of its construction can be found, e.g., in Ref. [7]. It presents a crucible made of alundum (sintered alumina) tightly closed from top by cover made of chamotte sized by MgO suspension in liquid glass. The smaller crucible made of alundum inserted into the cell serves as a container for the saline mixture. The cover contains holes for the membrane oxygen electrode, the silver reference electrode, tube for supply of solid substances (lead or nickel chlorides, potassium hydroxide), tube for gas supply and control thermocouple. All the tubes and the boot for thermocouple are made from alundum.
(8) 2
This means that 112 g of KOH corresponds to 1 mole of O . To create inert atmosphere in the potentiometric cell we used highpurity Ar (the volume fraction of the main substance was 0.9999), preliminarily dried by passing over P2O5. The residual volume part of oxygen in the gas did not exceed 2 105. To perform the titration experiment the saline mixture was melted in an electrochemical cell and preliminarily calibrated by sequence of known additions of KOH to obtain the dependence of emf of cell (6) vs., the equilibrium molality of O2–, mO2 (mol kg1) or its index pO ¼ logmO2 . These plots were used for pO calculations from the potentiometric data. The titration procedure was as follows. After stabilization of the temperature and emf of cell (5) the weight of MeCl2 corresponding to initial molality of Me2þ m0Me2þ ¼ 0.03–0.05 mol kg1 was added to the melt and the equilibrium emf was measured. The emf measurements were performed each 5 min until three sequential values of emf become the same. Then a sequence of the weights of KOH was added to the melts till m0O2 (the initial molality of oxide
V.L. Cherginets et al. / Fluid Phase Equilibria 401 (2015) 77–81
ion, mol kg1) become considerably greater than m0Me2þ (u > 1.5, where u is the ratio of initial molalities of oxide ion and metal cation, namely, m0 2 =m0Me2þ ) and the equilibrium emf values were
K sx;MeO;L ¼ xMe2þ xO2 ¼ mMe2þ mO2
O
registered after each addition. The emf value gave us possibility to calculate the corresponding oxygen index, pO. For each experimental point we calculated the assumed dissociation constant of the obtained oxide, K 0MeO , mol kg1, meaning that all the formed oxide remained in liquid phase: 10 K 0MeO ¼ m0Me2þ m0O2 þ 10pO 0 m 2 10pO
79
¼ K s;MeO
ML 1000
!2
ML 1000
!2
;
(13)
or, taking into account pKsx;MeO;L ¼ logK sx;MeO;L ! ML pKsx;MeO;L ¼ pKs;MeO 2log 1000
(14)
pO
(9) Using designations for the studied and the reference melts we can attribute the difference between pKsx;MeO (the reference melt) and pKsx;MeO;L (the studied melt) to the sum of the primary medium
O 0
and its assumed solubility product, K s;MeO , mol2 kg2: K 0s;MeO ¼ m0Me2þ m0O2 þ 10pO 10pO :
(10)
namely,
Statistical treatment of the obtained results was performed using the generally accepted routines [9] and all the confidence ranges are presented at the confidence level 0.95. 3. Results and discussion Estimation of relative oxoacidic properties of BaBr2–KBr melt was performed by oxide solubility method which essence consists in comparison of the oxide solubility products in the studied (denoted as L) and the reference (denoted by superscript *) melts. To unify the conditions of the experiments in melts of different compositions, the solubility parameters should be expressed in mole fraction scale. This is made as follows. The solubility products of MeO oxide in mole fraction scale (K sx;MeO ) in melt of ‘L’ composition is expressed as K sx;MeO;L ¼ xMe2þ xO2
(11)
where xMe2þ and xO2 the mole fractions of Me2þ and O2 in the saturated solution. The said mole fractions values can be obtained from the corresponding molalities by such a manner xMe2þ ¼ mMe2þ
ML ML ; x 2 ¼ mO2 1000 O 1000
effects of MeO (logg 0;MeO ), Me2þ (logg 0;Me2þ ) and O2 (logg 0;O2 ),
(12)
where ML the pseudomolar mass of the solvent. For the studied here BaBr2–KBr mixture this value is MBaBr2 KBr = 168.5 g. Using expressions (12) we can transform Eq. (11) as follows
pKsx;MeO pKsx;MeO;L ¼ logg 0;MeO logg 0;Me2þ logg 0;O2 :
(15)
This relation can be somewhat simplified. The first, since MeO oxide is the solid phase in both melts, the primary effect for this reactant is zero. The second, the formal designation Me2þ actually corresponds to halide complexes of this cation and halide ion (X ) MeX2 4 therefore, in melts possessing the same anion composition its stability can be assumed as the same or close. So, logg 0;Me2þ 0, that yields the final formula: pKsx;MeO pKsx;MeO;L ¼ logg 0;O2 pIL :
(16)
The latter parameter was introduced in our paper [1] as a concentration analog of the primary medium effect for oxide ion and called ‘the oxobasicity index’. So, the starting point of the estimation is to determine in BaBr2– KBr melt the solubilities of oxides which were preliminary studied in the reference melt. In the case of molten chlorides and bromides the reference melt was chosen in [6] to be KCl–NaCl (0.5:0.5) mixture. In this melt the solubilities of SrO, CaO, NiO and PbO were reported [1]. Practice of investigations of oxide solubilities shows, however, that the range of oxides which solubility parameters can be surely determined is restricted with the increase of the oxoacidic properties of melt. Results of the earlier investigations allow excluding oxides of strontium and calcium since the behavior of Ca2+ and Sr2+ ions in the saturated solution should be interfered by their heavier alkaline earth analog (Ba2+ cations) with the possible formation of solid solutions instead of pure alkaline earth metal oxides. Because of this reason only nickel and
Table 2 The experimental data obtained at the titration of Pb2+ cations (0.05 mol kg1) with O2 in molten BaBr2–KBr (0.505:0.495) mixture at 973 K. No
m0 2 , mol kg1
pO
pK0s;PbO
1 2 3 4 5 6 7 8 9
0 0.0074 0.0129 0.0195 0.0285 0.0405 0.0444 0.0508 0.0592
4.09 3.19 2.92 2.67 2.40 2.23 2.17 2.13 2.06
– 4.55 4.33 4.16 3.99b 4.04b 4.08b 4.30
O
pK0PbO – 2.39a 2.40a 2.40a 2.40a 2.58 2.65 2.94
Standard uncertainties of the experimental parameters are: u(p) = 3 103 Pa for pressure, u(T) = 3 K for temperature, u(m0 2þ ) 0.001 mol kg1, u(m0 2 ) = 0.0005 Fig. 1. The dependence of pO vs. u at the titration of Ni2+ (1) and Pb2+ (2) cations with O2 additions in molten BaBr2–KBr (0.505:0.495) mixture at 973 K. Arrow 20 corresponds to the beginning of PbO precipitation.
Pb
mol kg1, u(pO) = 0.04, u(pK0s;PbO ) = 0.02 and u(pK0PbO ) = 0.02. a Points chosen for pK0PbO estimation. b Points chosen for pK0s;PbO estimation.
O
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V.L. Cherginets et al. / Fluid Phase Equilibria 401 (2015) 77–81
lead cations remain to be appropriate for the solubility investigations. The investigation of the oxide solubilities in the molten BaBr2– KBr eutectic was performed by the potentiometric titration method. Going from the obtained results we built the corresponding potentiometric curves in pO ¼ f ðuÞ coordinates where pO the index (negative decimal logarithm) of equilibrium molality of oxide ion (mO2 ). They are presented in Fig. 1. In the case of titration of Ni2+ the potentiometric curve has usual shape. This means that the precipitation of NiO begins after addition of Ni2+ to the initial melt since concentration of oxide ion in this melt due to the presence of traces of oxygen-containing admixtures (sulfates, carbonates etc.) favors running the reaction: Ni2+ + O2 fi NiO#.
(17)
Simple calculations permit to obtain value of pKs;NiO in molality scale as 8.060.2. The titration curve of Pb2+ with oxide ion additions presents more complicate case proper for oxides with appreciable solubility in molten salts. This kind of curves does not include drop at the equivalence point at all. Therefore, the treatment of the experimental results requires another approach. Let us analyze the experimental and derived data (the values of the pK0PbO and pK0s;PbO ) which are summarized in Table 2. The initial section of the titration curve (points 1–4) corresponds to the formation of unsaturated solution of PbO. This is confirmed by small oscillations of calculated values of the dissociation constant near a definite magnitude since the addition of oxide ions during the titration does no result in removal of formed PbO from the bromide melt. Contrary, the product of Pb2+ and O2 sequentially increases, that corresponds to decrease of pK0s;PbO going from point 2 to point 5. Point 5 (Fig. 1, arrow 20 ) accurately corresponds to the formation of the saturated solution. At this point the value of pK0PbO does not differ from those in points 2–4. Beginning from point 5 the directed shift of pK0s;PbO stops. So, the average value of pK0PbO is 2.39 0.1 and the concentration dissociation constant of PbO is equal to 0.0041. Points 5–9 belong to the saturated solution section in the titration curve and pK0s;PbO is practically unchanged for points 5–7. Using these data the average pK0s;PbO value corresponds to pK0s;PbO = 4.04 0.2. The scatter near the equivalence point (points 8 and 9) is usually not taken into account at all similar investigations. Knowing K s;PbO and K PbO values one can calculate concentration of non-dissociated PbO in its saturated solution (sPbO ¼ 0.022 mol kg1) and degree of dissociation of this oxide (a) in the saturated solution a = 0.30. The value of a shows PbO is rather strong Lux base in the studied bromide melt. Finishing the described above solubility studies we can complete the set of data concerning relative acidities of melts based on chlorides and bromides of alkali and alkaline earth metal Table 3 Data on solubility productsa of PbO and NiO in the studied and the reference melts at 973 K and oxoacidity estimation. Oxide
PbO NiO Average
BaBr2–KBr (0.505:0.495)
KCl–NaCl (0.5:0.5) [2]
pKs;MeO
pKsx;MeO
pKsx;MeO
4.04 8.06 pIL 2.02
5.40 9.42
7.47 11.38
salts. For this purpose let us compare solubility products of PbO and NiO in the melt studied in the present work and the reference melt of KCl–NaCl (0.5:0.5) composition. The necessary data are presented in Table 3. The shifts of pKsx;MeO;L values against the reference melt, DpKsx;MeO , for both oxides calculated according the following formula:
DpKsx;MeO ¼ pKsx;MeO pKsx;MeO;BaBr2 KBr :
(18)
are practically the same (ca. 2). According to Eq. (10) we can consider this value as a measure of the relative oxoacidic properties of the BaBr2–KBr melt. Since the values of pKsx;MeO;BaBr2 KBr are lower than the corresponding parameters obtained the in KCl–NaCl melt, this melt possesses stronger acidity that the reference one. Now let us construct the joint acidity scale for chloride and bromide melts at 973 K using the necessary data obtained in the above-mentioned papers [1,2,6,10] (Fig. 2). From this scheme it is seen that the oxoacidic properties of chloride and bromide melts with same cation composition are close and, as a rule, the bromide melts possess weaker acidity as compared with their chloride analogs. The seeming inverse situation for Ba2+-based halide melts takes place since concentration of the most acidic cation (Ba2+) in the chloride melt is a half of that in the bromide one. It is obvious that twofold decrease of Ba2+ concentration to 0.25 should result in decrease of pIL value by ca. 0.3 (log 1/2 = 0.3). It is interesting that the change of anion in Br ! Cl sequence does not result in considerable changes of the melt acidities. The constructed scale and the oxobasicity indices permit to predict the efficiency of different processes performed to remove oxygen-containing admixtures from molten salts, e.g., the purification limit (pOlim ) of precipitating deoxidization, i.e., treatment of halide melt by metal cations-scavengers can be estimated according to the formula: pOlim ¼ pKs;MeO þ log m0Me2þ m0O2 : (19) However, without knowledge of oxoacidic properties of molten salts such estimation should require, at least, determination of solubility product of oxide formed by the cation-scavenger in the given melt. Knowing the corresponding pKsx;MeO value in the reference melt KCl–NaCl (where ca. 15 cations are studied) and the oxobasicity index pIL of the given melt or melt of similar composition one can perform the estimation by such a manner: pOlim pKs;MeO pIL þ log m0Me2þ m0O2 : (20) The values (pIL) also can be used for predictions of purification limits at treatment of ionic halide melts in reagent gas atmosphere, and under these conditions the estimation becomes extremely simple:
DpKsx;MeO 2.07 1.96
a Values of pKs;MeO are in molality scale and pKsx;MeO ones are expressed via mole fractions; standard uncertainties of the experimental parameters are: u(p) = 3 103 Pa for pressure, u(T) = 3 K for temperature, u(pKs,MeO) = 0.28 and u(pKsx, MeO) = 0.28 in the BaBr2–KBr mixture, u(pKsx,MeO) = 0.06 in the KCl–NaCl mixture and u(DpKsx,MeO) = 0.3.
Fig. 2. Joint oxoacidity scale of chloride and bromide melts based on the salts of alkali and alkaline earth elements.
V.L. Cherginets et al. / Fluid Phase Equilibria 401 (2015) 77–81
pOlim;L pOlim;KClNaCl pIL :
(21)
It should be mentioned that positions of Sr2+-based halides being not considered here are close to those of the corresponding Ba2+-based melts [2], therefore, principal conclusions which can be made from this scale for both kinds of melts are close. The oxoacidic properties of Ca2+-containing halides are close (somewhat stronger) to Li+-based halide melts.
81
purification by precipitating deoxidization or carbohalogenation methods for the case of melts possessing distinguished cation compositions. Acknowledgement The financial support of National Academy of Science of Ukraine is gratefully acknowledged (project No. 0113U001834).
4. Conclusions
References
Solubilities of NiO and PbO in molten BaBr2–KBr (0.505:0.495) eutectic at 973 K were determined as of pKs;NiO = 8.06 0.2 and pKs;PbO = 4.04 0.2 in molality scale, respectively. There results give possibility to construct the oxoacidity scale for chloride and bromide melts (KX–NaX ! BaX2–KX ! KX–LiX sequences, X = Cl, Br) at 973 K using values of the oxobasicity indices (primary medium effects). These scales also can be used for Sr2+-based halide melts since the oxoacidic properties of Sr2+ and Ba2+ in molten halides are very close. Oxoacidic properties of molten bromides are weaker than those of molten chlorides of close cation composition; however, the difference is 0.2-0.4 in pIL units. The obtained information on primary medium effects permits to extend the usability of experimental data on halide melt
[1] V.L. Cherginets, T.P. Rebrova, Electrochem. Commun. 1 (1999) 590–592. [2] V.L. Cherginets, T.P. Rebrova, Electrochim. Acta 46 (2000) 25–30. [3] P.K. Tripathy, T.I. Gutknecht, S.D. Herrmann, G.L. Fredrickson, N.E. Lister, GLOBAL 2013: Nuclear Energy at a Crossroads, International Nuclear Fuel Cycle Conference, 22013, pp. 867–873. [4] A. Bessiere, P. Dorenbos, C.W.E. van Eijk, K.W. Kramer, H. Hüdel, IEEE Trans. Nucl. Sci. 51 (2004) 2970–2972. [5] R. Borade, E.D. Bourret-Courchsne, S.E. Derenzo, Nucl. Instrum. Methods Phys. Res. Sect. A 652 (2011) 260–263. [6] V.L. Cherginets, T.P. Rebrova, V.A. Naumenko, J. Chem. Thermodyn. 74 (2014) 216–220. [7] V.L. Cherginets, T.P. Rebrova, Chemistry of Deoxidization Processes in Molten Ionic Halides, Lambert Academic Publishing, 2014. [8] R. Littlewood, Electrochim. Acta 3 (1961) 270–278. [9] K. Doerffel, Statistik in der analytischen Chemie, Deutscher Verlag für Grundstoffindustrie GmbH, Leipzig, 1990. [10] V.L. Cherginets, O.V. Demirskaya, T.P. Rebrova, J. Chem. Thermodyn. 36 (2004) 115–120.
Contents lists available at ScienceDirect
Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
Oxoacidity scale of molten ionic chlorides and bromide Victor L. Cherginets * , Tatyana P. Rebrova, Vyacheslav A. Naumenko Institute for Scintillation Materials, National Academy of Sciences of Ukraine, Lenin Avenue, 60, Kharkov 61001, Ukraine
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 January 2015 Received in revised form 8 May 2015 Accepted 14 May 2015 Available online 19 May 2015
Determination of NiO and PbO solubilities in molten BaBr2–KBr (0.505:0.495) eutectic at 973 K (of pKs;NiO = 8.06 0.2 and pKs;PbO = 4.04 0.2, molality scale) allowed completing systematic oxide solubility studies in molten ionic bromides. On the basis of the obtained and literature data the acidity scale for chloride and bromide melts (KX–NaX ! BaX2–KX ! KX–LiX sequences, X = Cl, Br) at 973 K was built on the basis of primary medium effects (estimated as oxobasicity indices). The oxoacidic properties of chloride and bromide melts with similar cation composition are close, although the acidity of bromide analogs is always somewhat weaker. The obtained information permits to predict e.g., limits of halide melt purification by precipitating deoxidization or carbohalogenation methods. ã2015 Elsevier B.V. All rights reserved.
Keywords: Oxoacidity Oxide solubility Primary medium effect Potentiometric titration
1. Introduction Investigations of Lux-Flood acid-base interactions in molten salts (ionic melts) are now performed in two obvious directions: determination of acidic (basic) properties of new compounds in the traditional halide melts (e.g., KCl–LiCl, KCl–NaCl etc.) and development of imaginations permitting to extend the use of already obtained physicochemical data on the case of ionic solvents of new compositions. The first direction is practically exhausted since at temperatures of the order of 1000 K the range of stable acidic oxides, oxoanions and metal cations as Lux-Flood acids is rather narrow and the thorough studies of some halide melts (bromides, iodides) especially if they contain constituent cations of high acidity (Li+, cations of alkaline earth and rare earth metals) are very difficult. This forces investigators to develop approaches which allow predicting changes of thermodynamic and kinetic parameters of different Lux-Flood acid-base processes with the alteration of the melt composition. The generally accepted way to obtain such parameters is the estimation of primary medium effects for the ‘carrier’ of acidic or basic properties (M), logg 0;M which, according to equation:
* Corresponding author. Tel.: +380 637119592. E-mail address: [email protected] (V.L. Cherginets). http://dx.doi.org/10.1016/j.fluid.2015.05.025 0378-3812/ ã 2015 Elsevier B.V. All rights reserved.
logg 0;M ¼
AM AM;L 2:3RT
AM
(1)
(where and AM;L the solvation energy of M particle in the reference and the studied (L) solvents, J mol1, respectively, R the universal gas constant, 8.314 J mol1 K1, T the absolute temperature, K) is a measure of work of M particle transfer from infinitely diluted solution in the studied solvent to the infinitely diluted solution in the reference one referred to 1 mole of substance M. Just this resolvation work is the main reason of observed changing constants of a majority of acid-base reactions in different solvents. Recently investigations of the relative melt acidities were fulfilled in our papers [1,2] and they concerned only molten ionic chlorides. Molten bromides are also of interest to investigators as prospective media for electrodeposition of rare-earth metals [3]; they serve as growth material for obtaining single crystals of such novel scintillation materials as Cs2LiYBr6:Ce3+ [4] or CsBa2Br5:Eu2+ [5]. Naturally, to provide high purity of the deposited metals and good performance of the optical crystals the corresponding melts should be thoroughly purified from oxide ion traces. Knowing the primary medium effects of O2 in molten bromides facilitates the problem of choice of the most appropriate purification method. However, the literature data contain only information about acidic properties of melts based on alkali metal bromides (these data were published in our recent work [6]). The purpose of the present work is to determine solubility of nickel and lead monoxides in molten bromide mixture BaBr2–KBr (0.505:0.495, mole fractions) as a typical example of melts based on alkaline earth halides, to estimate the relative oxoacidic
78
V.L. Cherginets et al. / Fluid Phase Equilibria 401 (2015) 77–81
Table 1 Chemicals used in the experiments and methods of their treatment. Formula
Supplier
Purity, main substance
Purification method
Ar BaBr2 KBr KOH NH4Br NiCl2 PbCl2
PASS, Ukraine Reakhim, Russia Reakhim, Russia Reakhim, Russia Merck Lankhit, Russia Reakhim, Russia
0.997 0.995 0.995 0.999 0.998 0.995 0.998
Drying Drying Drying Drying None Drying None
properties of this melt on the basis of the solubility data and compare acidic properties in sequences KX–NaX ! BaX2–KX ! KX–LiX (X = Cl, Br). 2. Materials and methods The used reagents and methods of their preparation are presented in Table 1. The stoichiometry of process of the metal cation's titration corresponds to the equation: MeO fi Me2+ + O2.
(2)
In this paper Me designation is referred to Pb or Ni. This equilibrium is described by the dissociation constant, KMeO: K MeO fi mMe2þ
mO2 ; mMeO
(3)
where mMe2þ , mO2 and mMeO the equilibrium molalities (mol kg1, i.e., a number of moles of a substance per 1000 g of the ionic solvent) of metal cation, oxide anion and non-dissociated oxide respectively. In the case of formation of the saturated solutions a certain amount of MeO precipitates and the actual molality of MeO in the saturated solution (designated further as sMeO ) being thermodynamic constant of the oxide deviates from mMeO . Under these conditions equilibrium (2) is described by the solubility product value, K s;MeO : K s;MeO ¼ mMe2þ mO2 :
(4)
Knowing both equilibrium parameters one can estimate the abovementioned molality of non-dissociated oxide in the saturated solution sMeO according to the following equation: sMeO ¼
K s;MeO : K MeO
(5)
over P2O5 under vacuum under vacuum in argon atmosphere at 600 C under vacuum with NH4Cl + sublimation at 1100 K
The indicator oxygen electrode presents itself a test-tube (diameter of 8 mm, wall thickness of 0.5 mm and height of 160 mm) made of YSZ containing inner oxygen electrode Pt(O2). The latter is coiled platinum foil with platinum lead. Dried air (pO2 = 0.21 atm) was the source of the oxygen and it was supplied into the inner electrode space. The silver electrode presents silver wire with platinum lead immersed into the solution containing the constant concentration of corresponding silver bromide (0.1 mol kg1) in the studied bromide melt. The reference electrode is places into a test-tube made of alundum (diameter of 8 mm, wall thickness of 1 mm and height of 160 mm). The walls of the alumina test-tube impregnated by the studied and inner melts serve as excellent salt bridge. Usually the silver electrodes can work for a long time and they are well-reproducible [8]. Each experiment required 50 g of the BaBr2–KBr mixture. It was obtained by mixing of BaBr2 and KBr of reagent quality taken in 0.718:0.282 mass ratio. The melt before the experiment was purified from a majority of oxide ion traces (admixtures of the corresponding hydroxides, carbonates and sulfates in the initial chemicals) by adding NH4Br of reagent quality: 2NH4Br + O2 ! 2NH3" + H2O" + 2Br.
(7)
2
The residual concentration of O in the purified melts was ca. 5 105 mol kg1. Anhydrous NiCl2 was prepared by drying the corresponding hydrate of reagent quality in vacuum (5 Pa) to 673 K with subsequent sublimation in 975–1053 K temperature range. PbCl2 of reagent quality was used without purification. KOH (source of O2, mass fraction of the main substance is 0.999) was melted in alumina crucible and kept for 1 h at 973 K in argon atmosphere for complete removal of absorbed water. Its dissociation in melts proceeds completely according to the following equation: 2KOH ! 2K+ + H2O" + O2.
In molality scale the equilibrium parameters are expressed in mol kg1 (sMeO and Ks,MeO) and mol2 kg2 (K s;MeO ). For different purposes the equilibrium constant indices (inverse decimal logarithms of the constants) such as pKMeO and pKs,MeO are used. The scheme of the used potentiometric cell can be expressed as follows: Ag|Ag+, BaBr2 KBr||BaBr2 KBr + O2|YSZ|(O2)Pt
(6)
(where YSZ is the solid electrolyte membrane of 0.9 ZrO2 + 0.1 Y2O3 composition) and the most detailed description of its construction can be found, e.g., in Ref. [7]. It presents a crucible made of alundum (sintered alumina) tightly closed from top by cover made of chamotte sized by MgO suspension in liquid glass. The smaller crucible made of alundum inserted into the cell serves as a container for the saline mixture. The cover contains holes for the membrane oxygen electrode, the silver reference electrode, tube for supply of solid substances (lead or nickel chlorides, potassium hydroxide), tube for gas supply and control thermocouple. All the tubes and the boot for thermocouple are made from alundum.
(8) 2
This means that 112 g of KOH corresponds to 1 mole of O . To create inert atmosphere in the potentiometric cell we used highpurity Ar (the volume fraction of the main substance was 0.9999), preliminarily dried by passing over P2O5. The residual volume part of oxygen in the gas did not exceed 2 105. To perform the titration experiment the saline mixture was melted in an electrochemical cell and preliminarily calibrated by sequence of known additions of KOH to obtain the dependence of emf of cell (6) vs., the equilibrium molality of O2–, mO2 (mol kg1) or its index pO ¼ logmO2 . These plots were used for pO calculations from the potentiometric data. The titration procedure was as follows. After stabilization of the temperature and emf of cell (5) the weight of MeCl2 corresponding to initial molality of Me2þ m0Me2þ ¼ 0.03–0.05 mol kg1 was added to the melt and the equilibrium emf was measured. The emf measurements were performed each 5 min until three sequential values of emf become the same. Then a sequence of the weights of KOH was added to the melts till m0O2 (the initial molality of oxide
V.L. Cherginets et al. / Fluid Phase Equilibria 401 (2015) 77–81
ion, mol kg1) become considerably greater than m0Me2þ (u > 1.5, where u is the ratio of initial molalities of oxide ion and metal cation, namely, m0 2 =m0Me2þ ) and the equilibrium emf values were
K sx;MeO;L ¼ xMe2þ xO2 ¼ mMe2þ mO2
O
registered after each addition. The emf value gave us possibility to calculate the corresponding oxygen index, pO. For each experimental point we calculated the assumed dissociation constant of the obtained oxide, K 0MeO , mol kg1, meaning that all the formed oxide remained in liquid phase: 10 K 0MeO ¼ m0Me2þ m0O2 þ 10pO 0 m 2 10pO
79
¼ K s;MeO
ML 1000
!2
ML 1000
!2
;
(13)
or, taking into account pKsx;MeO;L ¼ logK sx;MeO;L ! ML pKsx;MeO;L ¼ pKs;MeO 2log 1000
(14)
pO
(9) Using designations for the studied and the reference melts we can attribute the difference between pKsx;MeO (the reference melt) and pKsx;MeO;L (the studied melt) to the sum of the primary medium
O 0
and its assumed solubility product, K s;MeO , mol2 kg2: K 0s;MeO ¼ m0Me2þ m0O2 þ 10pO 10pO :
(10)
namely,
Statistical treatment of the obtained results was performed using the generally accepted routines [9] and all the confidence ranges are presented at the confidence level 0.95. 3. Results and discussion Estimation of relative oxoacidic properties of BaBr2–KBr melt was performed by oxide solubility method which essence consists in comparison of the oxide solubility products in the studied (denoted as L) and the reference (denoted by superscript *) melts. To unify the conditions of the experiments in melts of different compositions, the solubility parameters should be expressed in mole fraction scale. This is made as follows. The solubility products of MeO oxide in mole fraction scale (K sx;MeO ) in melt of ‘L’ composition is expressed as K sx;MeO;L ¼ xMe2þ xO2
(11)
where xMe2þ and xO2 the mole fractions of Me2þ and O2 in the saturated solution. The said mole fractions values can be obtained from the corresponding molalities by such a manner xMe2þ ¼ mMe2þ
ML ML ; x 2 ¼ mO2 1000 O 1000
effects of MeO (logg 0;MeO ), Me2þ (logg 0;Me2þ ) and O2 (logg 0;O2 ),
(12)
where ML the pseudomolar mass of the solvent. For the studied here BaBr2–KBr mixture this value is MBaBr2 KBr = 168.5 g. Using expressions (12) we can transform Eq. (11) as follows
pKsx;MeO pKsx;MeO;L ¼ logg 0;MeO logg 0;Me2þ logg 0;O2 :
(15)
This relation can be somewhat simplified. The first, since MeO oxide is the solid phase in both melts, the primary effect for this reactant is zero. The second, the formal designation Me2þ actually corresponds to halide complexes of this cation and halide ion (X ) MeX2 4 therefore, in melts possessing the same anion composition its stability can be assumed as the same or close. So, logg 0;Me2þ 0, that yields the final formula: pKsx;MeO pKsx;MeO;L ¼ logg 0;O2 pIL :
(16)
The latter parameter was introduced in our paper [1] as a concentration analog of the primary medium effect for oxide ion and called ‘the oxobasicity index’. So, the starting point of the estimation is to determine in BaBr2– KBr melt the solubilities of oxides which were preliminary studied in the reference melt. In the case of molten chlorides and bromides the reference melt was chosen in [6] to be KCl–NaCl (0.5:0.5) mixture. In this melt the solubilities of SrO, CaO, NiO and PbO were reported [1]. Practice of investigations of oxide solubilities shows, however, that the range of oxides which solubility parameters can be surely determined is restricted with the increase of the oxoacidic properties of melt. Results of the earlier investigations allow excluding oxides of strontium and calcium since the behavior of Ca2+ and Sr2+ ions in the saturated solution should be interfered by their heavier alkaline earth analog (Ba2+ cations) with the possible formation of solid solutions instead of pure alkaline earth metal oxides. Because of this reason only nickel and
Table 2 The experimental data obtained at the titration of Pb2+ cations (0.05 mol kg1) with O2 in molten BaBr2–KBr (0.505:0.495) mixture at 973 K. No
m0 2 , mol kg1
pO
pK0s;PbO
1 2 3 4 5 6 7 8 9
0 0.0074 0.0129 0.0195 0.0285 0.0405 0.0444 0.0508 0.0592
4.09 3.19 2.92 2.67 2.40 2.23 2.17 2.13 2.06
– 4.55 4.33 4.16 3.99b 4.04b 4.08b 4.30
O
pK0PbO – 2.39a 2.40a 2.40a 2.40a 2.58 2.65 2.94
Standard uncertainties of the experimental parameters are: u(p) = 3 103 Pa for pressure, u(T) = 3 K for temperature, u(m0 2þ ) 0.001 mol kg1, u(m0 2 ) = 0.0005 Fig. 1. The dependence of pO vs. u at the titration of Ni2+ (1) and Pb2+ (2) cations with O2 additions in molten BaBr2–KBr (0.505:0.495) mixture at 973 K. Arrow 20 corresponds to the beginning of PbO precipitation.
Pb
mol kg1, u(pO) = 0.04, u(pK0s;PbO ) = 0.02 and u(pK0PbO ) = 0.02. a Points chosen for pK0PbO estimation. b Points chosen for pK0s;PbO estimation.
O
80
V.L. Cherginets et al. / Fluid Phase Equilibria 401 (2015) 77–81
lead cations remain to be appropriate for the solubility investigations. The investigation of the oxide solubilities in the molten BaBr2– KBr eutectic was performed by the potentiometric titration method. Going from the obtained results we built the corresponding potentiometric curves in pO ¼ f ðuÞ coordinates where pO the index (negative decimal logarithm) of equilibrium molality of oxide ion (mO2 ). They are presented in Fig. 1. In the case of titration of Ni2+ the potentiometric curve has usual shape. This means that the precipitation of NiO begins after addition of Ni2+ to the initial melt since concentration of oxide ion in this melt due to the presence of traces of oxygen-containing admixtures (sulfates, carbonates etc.) favors running the reaction: Ni2+ + O2 fi NiO#.
(17)
Simple calculations permit to obtain value of pKs;NiO in molality scale as 8.060.2. The titration curve of Pb2+ with oxide ion additions presents more complicate case proper for oxides with appreciable solubility in molten salts. This kind of curves does not include drop at the equivalence point at all. Therefore, the treatment of the experimental results requires another approach. Let us analyze the experimental and derived data (the values of the pK0PbO and pK0s;PbO ) which are summarized in Table 2. The initial section of the titration curve (points 1–4) corresponds to the formation of unsaturated solution of PbO. This is confirmed by small oscillations of calculated values of the dissociation constant near a definite magnitude since the addition of oxide ions during the titration does no result in removal of formed PbO from the bromide melt. Contrary, the product of Pb2+ and O2 sequentially increases, that corresponds to decrease of pK0s;PbO going from point 2 to point 5. Point 5 (Fig. 1, arrow 20 ) accurately corresponds to the formation of the saturated solution. At this point the value of pK0PbO does not differ from those in points 2–4. Beginning from point 5 the directed shift of pK0s;PbO stops. So, the average value of pK0PbO is 2.39 0.1 and the concentration dissociation constant of PbO is equal to 0.0041. Points 5–9 belong to the saturated solution section in the titration curve and pK0s;PbO is practically unchanged for points 5–7. Using these data the average pK0s;PbO value corresponds to pK0s;PbO = 4.04 0.2. The scatter near the equivalence point (points 8 and 9) is usually not taken into account at all similar investigations. Knowing K s;PbO and K PbO values one can calculate concentration of non-dissociated PbO in its saturated solution (sPbO ¼ 0.022 mol kg1) and degree of dissociation of this oxide (a) in the saturated solution a = 0.30. The value of a shows PbO is rather strong Lux base in the studied bromide melt. Finishing the described above solubility studies we can complete the set of data concerning relative acidities of melts based on chlorides and bromides of alkali and alkaline earth metal Table 3 Data on solubility productsa of PbO and NiO in the studied and the reference melts at 973 K and oxoacidity estimation. Oxide
PbO NiO Average
BaBr2–KBr (0.505:0.495)
KCl–NaCl (0.5:0.5) [2]
pKs;MeO
pKsx;MeO
pKsx;MeO
4.04 8.06 pIL 2.02
5.40 9.42
7.47 11.38
salts. For this purpose let us compare solubility products of PbO and NiO in the melt studied in the present work and the reference melt of KCl–NaCl (0.5:0.5) composition. The necessary data are presented in Table 3. The shifts of pKsx;MeO;L values against the reference melt, DpKsx;MeO , for both oxides calculated according the following formula:
DpKsx;MeO ¼ pKsx;MeO pKsx;MeO;BaBr2 KBr :
(18)
are practically the same (ca. 2). According to Eq. (10) we can consider this value as a measure of the relative oxoacidic properties of the BaBr2–KBr melt. Since the values of pKsx;MeO;BaBr2 KBr are lower than the corresponding parameters obtained the in KCl–NaCl melt, this melt possesses stronger acidity that the reference one. Now let us construct the joint acidity scale for chloride and bromide melts at 973 K using the necessary data obtained in the above-mentioned papers [1,2,6,10] (Fig. 2). From this scheme it is seen that the oxoacidic properties of chloride and bromide melts with same cation composition are close and, as a rule, the bromide melts possess weaker acidity as compared with their chloride analogs. The seeming inverse situation for Ba2+-based halide melts takes place since concentration of the most acidic cation (Ba2+) in the chloride melt is a half of that in the bromide one. It is obvious that twofold decrease of Ba2+ concentration to 0.25 should result in decrease of pIL value by ca. 0.3 (log 1/2 = 0.3). It is interesting that the change of anion in Br ! Cl sequence does not result in considerable changes of the melt acidities. The constructed scale and the oxobasicity indices permit to predict the efficiency of different processes performed to remove oxygen-containing admixtures from molten salts, e.g., the purification limit (pOlim ) of precipitating deoxidization, i.e., treatment of halide melt by metal cations-scavengers can be estimated according to the formula: pOlim ¼ pKs;MeO þ log m0Me2þ m0O2 : (19) However, without knowledge of oxoacidic properties of molten salts such estimation should require, at least, determination of solubility product of oxide formed by the cation-scavenger in the given melt. Knowing the corresponding pKsx;MeO value in the reference melt KCl–NaCl (where ca. 15 cations are studied) and the oxobasicity index pIL of the given melt or melt of similar composition one can perform the estimation by such a manner: pOlim pKs;MeO pIL þ log m0Me2þ m0O2 : (20) The values (pIL) also can be used for predictions of purification limits at treatment of ionic halide melts in reagent gas atmosphere, and under these conditions the estimation becomes extremely simple:
DpKsx;MeO 2.07 1.96
a Values of pKs;MeO are in molality scale and pKsx;MeO ones are expressed via mole fractions; standard uncertainties of the experimental parameters are: u(p) = 3 103 Pa for pressure, u(T) = 3 K for temperature, u(pKs,MeO) = 0.28 and u(pKsx, MeO) = 0.28 in the BaBr2–KBr mixture, u(pKsx,MeO) = 0.06 in the KCl–NaCl mixture and u(DpKsx,MeO) = 0.3.
Fig. 2. Joint oxoacidity scale of chloride and bromide melts based on the salts of alkali and alkaline earth elements.
V.L. Cherginets et al. / Fluid Phase Equilibria 401 (2015) 77–81
pOlim;L pOlim;KClNaCl pIL :
(21)
It should be mentioned that positions of Sr2+-based halides being not considered here are close to those of the corresponding Ba2+-based melts [2], therefore, principal conclusions which can be made from this scale for both kinds of melts are close. The oxoacidic properties of Ca2+-containing halides are close (somewhat stronger) to Li+-based halide melts.
81
purification by precipitating deoxidization or carbohalogenation methods for the case of melts possessing distinguished cation compositions. Acknowledgement The financial support of National Academy of Science of Ukraine is gratefully acknowledged (project No. 0113U001834).
4. Conclusions
References
Solubilities of NiO and PbO in molten BaBr2–KBr (0.505:0.495) eutectic at 973 K were determined as of pKs;NiO = 8.06 0.2 and pKs;PbO = 4.04 0.2 in molality scale, respectively. There results give possibility to construct the oxoacidity scale for chloride and bromide melts (KX–NaX ! BaX2–KX ! KX–LiX sequences, X = Cl, Br) at 973 K using values of the oxobasicity indices (primary medium effects). These scales also can be used for Sr2+-based halide melts since the oxoacidic properties of Sr2+ and Ba2+ in molten halides are very close. Oxoacidic properties of molten bromides are weaker than those of molten chlorides of close cation composition; however, the difference is 0.2-0.4 in pIL units. The obtained information on primary medium effects permits to extend the usability of experimental data on halide melt
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