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On some regularities of metal oxide solubility in molten CsI at T = 973 K
J. Chem. Thermodynamics 43 (2011) 1252–1255
Contents lists available at ScienceDirect
J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct
On some regularities of metal oxide solubility in molten CsI at T = 973 K V.L. Cherginets a,b,⇑, T.P. Rebrova a, Yu.N. Datsko a, V.A. Shtitelman a, E.Yu. Bryleva c a
Institute for Scintillation Materials, National Academy of Sciences of Ukraine, Lenin Avenue, 60, Kharkov 61001, Ukraine National Technical University ‘Kharkiv Polytechnical Institute’, 21 Frunze St., 61002 Kharkov, Ukraine c State Scientific Organization STC ‘Institute for Single Crystals’, National Academy of Sciences of Ukraine, Lenin Avenue, 60, Kharkov 61001, Ukraine b
a r t i c l e
i n f o
Article history: Received 6 November 2010 Received in revised form 12 March 2011 Accepted 14 March 2011 Available online 21 March 2011 Keywords: Ionic melt Cesium iodide Potentiometric titration Metal oxide solubility Oxoacidity
a b s t r a c t Solubility products of CdO (pKs,CdO = 6.80 ± 0.2), ZnO (pKs,ZnO = 10.0 ± 0.5), NiO (pKs,NiO = 11.2 ± 0.2) and EuO (pKs,EuO = 13.1 ± 0.2) in molten CsI at T = 973 K are determined by potentiometric titration of (0.02 to 0.03) mol kg1 solutions of the corresponding metal chlorides by strong base (KOH) using a membrane oxygen electrode Pt(O2)|ZrO2(Y2O3) as an indicator. On the basis of pKs,MeO values, all the oxides studied are referred to practically insoluble in molten CsI. The values of the oxide solubility in CsI melt are lower than the corresponding values in molten alkali metal chlorides. This can be explained by ‘softer’ basic properties of I– as compared with Cl– in the frames of the Pearson ‘hard’ and ‘soft’ acid–base concept. In the oxide samples studied, the values of the solubility fall with the decreasing cation radius. The correlation between pKs,MeO and the polarizing action by Goldshmidt (Zr2 ) of the cation is practiMe2þ cally linear and may be proposed for estimation of the solubility of s- and d- element oxides in molten CsI on the basis of their cation radii. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Melts based on alkali metal iodide are mainly used for production of the most asked-for scintillators such as NaI:Tl, CsI:Na, CsI:Tl, and undoped CsI. Their functional properties, in particular, luminescence characteristics and radiation stability essentially worsen in the presence of oxygen-containing admixtures in the melts used for growing the above-mentioned single crystals [1,2]. Therefore, considerable attention of the investigators is paid to the removal of the said admixtures from iodide melts. A solution of this problem is demonstrated in [3]: the addition of EuI2 to the CsI:Tl melt essentially decreases the afterglow of the said scintillator (this parameter is decreased by an order of magnitude). Although this effect is not explained in [3], one can suppose that it consists in action of Eu2+ or Eu3+ on oxoanions in molten CsI. The ion Eu3+ can be formed due to partial oxidation of Eu2+ under the crystal growth conditions (under a pressure close to 20 KPa in the growth chamber). Both europium ions act as Lux-Flood acids [4] fixing O2– ion as a common Lux-Flood base, e.g.:
Eu2þ ðAcidÞ þ O2 ðBaseÞ ¼ EuO # :
ð1Þ
However, the thermodynamic data on reaction (1), which permit one to predict quantitatively the action of Eu2+ ion on the processes in molten salts, are not available. Similarly, the difficulties of ⇑ Corresponding author at: Institute for Scintillation Materials, National Academy of Sciences of Ukraine, Lenin Avenue, 60, Kharkov 61001, Ukraine. Tel.: +380 63 7119592; fax: +380 57 3404474. E-mail addresses: [email protected], [email protected] (V.L. Cherginets). 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.03.010
the EuI2 synthesis necessitate searching for other reagents to be used as scavengers. The main requirement for the reagents is as follows: the solubility product index of the oxide (MeO) corresponding to the cation (Me2+) in the CsI or NaI melt:
n o n o 2 2 2 2 p K s;MeO =ðmol kg Þ ¼ lg ms;Me2þ ms;O2 =ðmol kg Þ ; ð2Þ where ms;Me2þ and ms;O2 are equilibrium molalities of the metal cation and oxide ion in the saturated solution of the oxide in the melt. The values are not less than 10 to provide reasonably low residual concentration of oxygen-containing impurities. As is known, the solubility product of the metal oxide is dependent on the cation radius. So, the solubility product of EuO should be close to that of SrO, since Sr2+ and Eu2+ radii are practically the same. The pKs,SrO in CsI equals to (5.10 ± 0.3) [5]. However, Sr and Eu belong to different groups of elements in the periodic table (s- and f- elements, respectively), and it is of interest to check the validity of radius-based predictions in this case. As for CdO, ZnO and NiO, the values of their solubility in iodide melts are not known. The determination of these values and establishment of some regularities of the effect of melt composition on the oxide solubility are the goals of the present work. 2. Experimental The CsI (Aldrich, mass fraction purity 0.99999) was used for the experiments without additional purification. After melting, the salt was kept in an argon atmosphere for 1 h, a small amount of NH4I
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was added into the melts to provide removal of the oxide ions according to the following equation:
2NH4 I þ O2 ¼ 2NH3 " þH2 O " þ2I :
ð3Þ
After such purification, the melt was appropriate for the solubility investigations. For all the studies, 50 g of CsI were placed into an alumina crucible within a potentiometric cell. Then the electrodes were inserted into the cell and the latter was heated to T = 973 K in an argon atmosphere. The EuI2 was prepared by dissolution of Eu2O3 (Stanford Material Corporation, OX63-5N, mass fraction purity 0.99999) in aqueous HI (reagent quality). Afterwards, NH4I (reagent quality) taken in molar ratio 2:1 to Eu was added to the solution. The solution obtained was evaporated and dried to the formation of a dark-brown powder (EuI3 + NH4I with traces of I2 and water). This powder was heated in vacuum till I2 evaporation started at approximately T = 473 K owing to decomposition of EuI3 followed by the formation of EuI2 and iodine. At the end of the latter process, the sublimation of NH4I commenced and intermediate HI formation in the hot zone provided additional purification of EuI2 from oxygen admixtures. The CdCl2, NiCl2 and ZnCl2 were synthesized by dissolution of the corresponding oxides (mass fraction of each was 0.995) in aqueous HCl of reagent quality. The solutions were mixed with NH4Cl and dried in vacuum at the temperature rising stepwise from 293 K to 673 K. The potentiometric cell with an indicator oxygen electrode Pt(O2)|YSZ (YSZ was 0.9 ZrO2–0.1 Y2O3 ceramics) 1
AgjAgþ ð0:1mol kg Þ þ CsIjjCsI þ O2 jYSZjPtðO2 Þ
ð4Þ
was used for the measurements. It was first calibrated with known weights of KOH as a strong Lux-Flood base
2OH ¼ H2 O # þ O2
ð5Þ
to establish the dependence of the emf of cell (4) against pO (pO ¼ lg mO2 , where mO2 was the equilibrium molality of oxide ions in the melt). The plot obtained was used for recalculation of emf values into equilibrium O2– molalities. To determine the solubility products of EuO, CdO, NiO and ZnO, the amounts of the corresponding chlorides providing approximately (0.02 to 0.03) mol kg–1 molality of Me2+ cation in the melt were added to the molten CsI (this molality is designated further as m0Me2þ ). Then a small (0.001 to 0.002 mol kg–1) weight of the base (KOH) was added to this solution (this molality is denoted as m0O2 ), and the equilibrium molality of O2– was determined. The titration was finished at m0O2 2m0Me2þ . All the calculations were made on the basis of the consumption of oxide ion, i.e. the difference between the added and equilibrium concentrations of O2– in the melt, DmO2
DmO2 ¼ m0O2 mO2 :
3. Results and discussion The curves of the potentiometric titration of Cd2+, Zn2+, Ni2+ and Eu by O2– are presented in figure 1. Let us consider an example of the treatment of the obtained results according to the following reaction: 2+
Me2þ þ O2 ¼ MeO # :
ð9Þ
At first, all the cations demonstrate appreciable acidic properties. Indeed, the usual pO values for pure alkali metal halides are within 3.5 to 4, whereas the initial addition of the metal chlorides results in a noticeable increase of pO because of fixation of O2– ions. The deviation from pO = 4 increases with the strengthening acidity of the metal cation in the sequence Cd2+ ? Zn2+ ? Ni2+. As for the Eu2+ cation, it demonstrates a surprisingly high acidity, since the initial solution of EiI2 possesses pO > 11. Such a fixation results in the formation of non-dissociated MeO in the liquid or solid phase. In the first case, it is described by the dissociation constant of the oxide KMeO or pK MeO ¼ lg K MeO : 1
1
2 pfK MeO =ðmol kg Þg ¼ lgfm2þ Me mO =mMeO =ðmol kg Þg;
ð10Þ
where mMeO is the molality of non-dissociated oxide in the liquid phase (melt). In the case of the precipitation of the oxide deposit from the melt, the equilibrium is characterized by the solubility product, K s;MeO or its index, pK s;MeO , see equation (2). However, since the temperature of the melt is high, visual observations of the presence or absence of the metal oxide deposit are impossible. To elucidate this problem, let us analyze the data in table 1. The conclusion about the saturation may be made from the behavior of the calculated parameters (7) and (8) during the titration process. If the oxide solution remains unsaturated, pK 0MeO values remain constant within some random deviations, whereas pK 0s;MeO values decrease as m0O2 rises; in the case of the saturated solution pK 0s;MeO is constant and pK 0MeO increases [7]. So, one may conclude that all the CdO solutions presented in table 1 were saturated, i.e. even the first small addition of the oxide ion donor results in
ð6Þ pK 0MeO
The experimental values of (dissociation constant index) and pK 0s;MeO (the solubility product index) for statistical treatment were calculated as follows: 1
pfK 0MeO =ðmol kg Þg 1
¼ lgfðm0Me2þ DmO2 Þ m2 O =DmO2 =ðmol kg Þg; 2
2
0
ð7Þ 2
2
2 2 pfK 0s;MeO =ðmol kg Þg ¼ lgfðm2þ Me DmO Þ mO =ðmol kg Þg:
ð8Þ The statistical treatment of the results obtained was performed according to the generally accepted routines described elsewhere [6].
FIGURE 1. Plot of pO against initial molality of O2– (m0O2 ) at titration of Me2+ cations in molten CsI at T = 973 K: (1) Cd2+ (m0Cd2þ = 0.030 mol kg1), (2) Zn2+ (m0Zn2þ = 0.020 mol kg1), (3) Ni2+ (m0Ni2þ = 0.018 mol kg1), (4) Eu2+ (m0Eu2þ = 0.022 mol kg1).
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in iodide melts, that decreases the solubility in the sequence Cl ? I. It should be noted that the said concept gives a clear explanation for different changes of solubility for the oxides studied, going from chloride melt to iodide one. Similarly for ZnO formed by harder acid (Zn2+), the solubility decrease is larger than for CdO formed by the softer acid (Cd2+). A common regularity of oxide solubility in halide melts is a strict dependence of pK s;MeO values on the inverse squared cation radius
TABLE 1 Results of potentiometric titration of Cd2+ (CdCl2, 0.030 mol kg1) by O2– in molten CsI at T = 973 K. 1
m0Me2þ =ðmol kg
Þ
0.0036 0.0074 0.0102 0.0146 0.0223 0.0265 0.0311 0.0331
pO
pK 0CdO =ðmol kg
5.27 5.24 5.18 5.00 4.79 4.19 3.01 2.35
4.37 4.72 4.85 4.92 5.15 4.87 4.21
1
2
pK 0s;CdO =ðmol kg
Þ
2
Þ
6.81 6.85 6.84 6.76 6.80 6.45 5.73
2
00
ð12Þ
where C1 and C2 are constants and rMe2þ is the metal cation radius. For example, the correlation between pK s;MeO and the polarizing action of the cation by Goldshmidt (Zr2 , where Z is the cation Me2þ charge, Q) for the cation contained in the oxide is practically linear. This dependence is characteristic of the solubility of alkaline earth oxides and oxides of transition metals belonging to the first transition row [8]. Such a dependence built using the results obtained in the present work and the values reported in [5] is presented in figure 2. As is seen, the former solubility values are principally in good agreement with the data of [5], since they extend the linear correlation (figure 2) to the region of oxides with slight solubility (pK s;MeO P 7). As for PbO solubility, it falls out from the similar correlations obtained from the oxide solubility data for chloride and bromide melts [5,10]. The EuO solubility does not agree with the regularity for cations of s- and d- elements. Statistical treatment of data presented in figure 2 yields the following equation
CdO precipitation from the melt. The ZnO, NiO and EuO powders are deposited from the cation solution in CsI at the first small addition of O2–, too. The solubility product values obtained are presented in table 2. To check the correctness of these data, the solubilities of NiO and ZnO in molten CsI at T = 973 K are determined by an alternative way. We used the isothermal saturation method with control of the metal ion molality by inductively coupled plasma atomic emission spectrometry (ICP-AES). According to such a determination, the mass fractions of both metals in CsI are of the order of 1 106, that corresponds to 1.2 105 mol kg–1 and K s;MeO = 1.4 1010 (pK s;MeO > 9.76). So, the results of the analytical determination are close to the values obtained by the potentiometric titration method. Somewhat higher values can be explained both by experimental error of the ICP-AES method and by the presence of some amount of the non-dissociated oxide in the melt. Comparing these values with similar data for molten KCl–NaCl eutectic [8] (table 2), it may be assumed that the change of the melt anion composition (the substitution of Cl– by I–) causes pK s;MeO decrease, i.e. the values of the oxide solubility (ms;MeO ) decrease by 10 to 30 times. Such a decrease may be explained by the concept of hard and soft acids and bases (HSAB) proposed by Pearson [9]. The essence of this concept consists in preferable formation of the complexes ‘hard acid–hard base’ and ‘soft acid–soft base’ at the interactions of different acid–base pairs. The dissolution of oxides in molten alkali metal halides is accompanied with the formation of the complexes ‘metal–halide ion’ and ‘alkali metal–oxide ion’ according to the following scheme:
Me00 O þ nMe0 X ¼ ½Me00 Xn 2n ðIÞ þ ½O Me0n n2 ðIIÞ;
2
pfK s;MeO =ðmol kg Þg ¼ C 1 þ C 2 ðr 2 =m2 Þ; Me2þ
2
2
pfK s;MeO =ðmol kg Þg ¼ 0:35ð0:3Þ þ 0:205ð0:01Þ =ðQ m2 Þg; fZr2 Me2þ
ð13Þ
ð11Þ
0
where Me ; Me and X are multi-charged and alkali metal cations and halide ion, respectively. The degree of the shift of equation (11) to the right is dependent on the strength of the above-said interactions. As for the ionic solvents and oxides discussed, the alkali metal cations belong to hard acids and their interaction (I) with O2 hard base is stronger for K+ or Na+ than for Cs+ since K+ (or Na+) is a harder acid than Cs+. The ion Cl– belongs to hard bases and I– is a soft base, therefore, Cd2+ and Zn2+ ions as hard acids interact with chloride ion more strongly than with iodide (II). So, complexes I and II in equation (11) should be more stable in chloride melts than
FIGURE 2. Plot of MeO solubility (pK s;MeO ) in molten CsI at T = 973 K against polarizing action of Me2+ cation Zr2 : (1) BaO [5], (2) SrO [5], (3) CaO [5], (4) CdO, Me2þ (5) ZnO, (6) NiO, (7) PbO [5], (8) EuO.
TABLE 2 Solubilities of CdO, ZnO and NiO in some molten alkali metal halides at T = 973 K (molality scale). Oxide
KCl–NaCl [8] 2
CsI, this work 2
K s;MeO =ðmol kg CdO ZnO NiO EuO
5
1.0 10 1.2 107 6.3 1012
Þ
1
ms;MeO =ðmol kg 3
3.1 10 3.2 104 3.1 105
Þ
2
2
K s;MeO =ðmol kg 7
1.58 10 1.0 1010 6.3 1012 7.94 1014
Þ
1
ms;MeO =ðmol kg 4
4.0 10 1.0 105 2.5 106 2.82 107
Þ
V.L. Cherginets et al. / J. Chem. Thermodynamics 43 (2011) 1252–1255
for all the above-noted oxides excluding PbO and EuO. The behavior of both oxides does not obey all the rules found for solubility in molten CsI. This plot may be used for estimation of the solubility of alkaline earth oxides and transition metal oxides on the basis the cation radii. Concerning the practical problem of removal of oxide ions from molten salts, it should be noted that Zn2+, Ni2+ and Eu2+ cation dopants can be used for purification of molten CsI due to the extremely low solubility of ZnO, NiO and CdO within the CsI. Moreover, the density of ZnO at room temperature is equal to 5.7 g cm–3, whereas that of molten CsI near the melting temperature is 3.1 g cm–3 [11]. The estimation of density of solid oxides taking into account the typical thermal expansion coefficients (1.1 105, K1) [12] gives a 2% diminution of this parameter; the density of ZnO at the melting temperature of CsI is 5.6 g cm–3. Therefore, the density ratio ZnO/CsI at T = 894 K (the melting point of CsI [11]) is approximately equal to 1.9. This provides easy separation of ZnO from CsI since the former precipitates at the bottom of the crystal growth vessel. Such a purification method is especially suitable while obtaining crystals by the Czochralski or Kyropoulos methods, where the crystals grow from the upper part of the melt. The above speculations remain true for NiO and EuO, whose densities at room temperature are 7.45 and 8.21, respectively. 4. Conclusion The solubility products of CdO (pKs,CdO = 6.80 ± 0.2), ZnO (pKs,ZnO = 10.0 ± 0.5), NiO (pKs,NiO = 11.2 ± 0.2) and EuO (pKs,EuO = 13.1 ± 0.2) in molten CsI at T = 973 K are determined by potentio-
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metric titration using a membrane oxygen electrode Pt(O2)|YSZ as an indicator. The oxide solubility is found to decrease with diminution of the cation radius. The exchange of anion of the ionic melt in the Cl– ? I– sequence results in considerable reduction of the values of oxide solubility. The linear correlation between pKs,MeO and the polarizing action by Goldshmidt (Zr2 ) of the cation is constructed. It may be proMe2þ posed for estimation of oxide solubility on the basis of their cation radii. References [1] N. Balamurugan, A. Arulchakkaravathi, S. Selvakumar, et al., J. Cryst. Growth 286 (2006) 294. [2] B.G. Zaslavsky, J. Cryst. Growth 218 (2000) 277. [3] L.A. Kappers, R.H. Bartram, D.S. Hamilton, C. Brecher, A. Lempicki, V. Gaysinskiy, E.E. Ovechkina, V.V. Nagarkar, Radiat. Measur. 42 (2007) 537. [4] H. Lux, Z. Elektrochem. 45 (1939) 303. [5] V.L. Cherginets, E.G. Khailova, O.V. Demirskaya, Zhurn. Fiz. Khim. 71 (1997) 371. [6] K. Doerffel, Statistik in der analytischen Chemie, Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1990. [7] V.L. Cherginets, E.G. Khailova, Electrochim. Acta 38 (1993) 1481–1485. [8] V.L. Cherginets, V.V. Banik, Rasplavy N1 (1991) 66–69. [9] R.G. Pearson, J. Am. Chem. Soc. 85 (1963) 3533–3539. [10] V.L. Cherginets, E.G. Khailova, O.V. Demirskaya, Electrochim. Acta 41 (1996) 463–467. [11] G.J. Janz, Molten Salts Handbook, Academic Press, NY, London, 1967. p. 588. [12] I.T. Goronovskii, Yu.P. Nazarenko, E.F. Nekryach, Short Handbook on Chemistry, Naukova dumka, Kiev, 1987. p. 830.
JCT 10-419
Contents lists available at ScienceDirect
J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct
On some regularities of metal oxide solubility in molten CsI at T = 973 K V.L. Cherginets a,b,⇑, T.P. Rebrova a, Yu.N. Datsko a, V.A. Shtitelman a, E.Yu. Bryleva c a
Institute for Scintillation Materials, National Academy of Sciences of Ukraine, Lenin Avenue, 60, Kharkov 61001, Ukraine National Technical University ‘Kharkiv Polytechnical Institute’, 21 Frunze St., 61002 Kharkov, Ukraine c State Scientific Organization STC ‘Institute for Single Crystals’, National Academy of Sciences of Ukraine, Lenin Avenue, 60, Kharkov 61001, Ukraine b
a r t i c l e
i n f o
Article history: Received 6 November 2010 Received in revised form 12 March 2011 Accepted 14 March 2011 Available online 21 March 2011 Keywords: Ionic melt Cesium iodide Potentiometric titration Metal oxide solubility Oxoacidity
a b s t r a c t Solubility products of CdO (pKs,CdO = 6.80 ± 0.2), ZnO (pKs,ZnO = 10.0 ± 0.5), NiO (pKs,NiO = 11.2 ± 0.2) and EuO (pKs,EuO = 13.1 ± 0.2) in molten CsI at T = 973 K are determined by potentiometric titration of (0.02 to 0.03) mol kg1 solutions of the corresponding metal chlorides by strong base (KOH) using a membrane oxygen electrode Pt(O2)|ZrO2(Y2O3) as an indicator. On the basis of pKs,MeO values, all the oxides studied are referred to practically insoluble in molten CsI. The values of the oxide solubility in CsI melt are lower than the corresponding values in molten alkali metal chlorides. This can be explained by ‘softer’ basic properties of I– as compared with Cl– in the frames of the Pearson ‘hard’ and ‘soft’ acid–base concept. In the oxide samples studied, the values of the solubility fall with the decreasing cation radius. The correlation between pKs,MeO and the polarizing action by Goldshmidt (Zr2 ) of the cation is practiMe2þ cally linear and may be proposed for estimation of the solubility of s- and d- element oxides in molten CsI on the basis of their cation radii. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Melts based on alkali metal iodide are mainly used for production of the most asked-for scintillators such as NaI:Tl, CsI:Na, CsI:Tl, and undoped CsI. Their functional properties, in particular, luminescence characteristics and radiation stability essentially worsen in the presence of oxygen-containing admixtures in the melts used for growing the above-mentioned single crystals [1,2]. Therefore, considerable attention of the investigators is paid to the removal of the said admixtures from iodide melts. A solution of this problem is demonstrated in [3]: the addition of EuI2 to the CsI:Tl melt essentially decreases the afterglow of the said scintillator (this parameter is decreased by an order of magnitude). Although this effect is not explained in [3], one can suppose that it consists in action of Eu2+ or Eu3+ on oxoanions in molten CsI. The ion Eu3+ can be formed due to partial oxidation of Eu2+ under the crystal growth conditions (under a pressure close to 20 KPa in the growth chamber). Both europium ions act as Lux-Flood acids [4] fixing O2– ion as a common Lux-Flood base, e.g.:
Eu2þ ðAcidÞ þ O2 ðBaseÞ ¼ EuO # :
ð1Þ
However, the thermodynamic data on reaction (1), which permit one to predict quantitatively the action of Eu2+ ion on the processes in molten salts, are not available. Similarly, the difficulties of ⇑ Corresponding author at: Institute for Scintillation Materials, National Academy of Sciences of Ukraine, Lenin Avenue, 60, Kharkov 61001, Ukraine. Tel.: +380 63 7119592; fax: +380 57 3404474. E-mail addresses: [email protected], [email protected] (V.L. Cherginets). 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.03.010
the EuI2 synthesis necessitate searching for other reagents to be used as scavengers. The main requirement for the reagents is as follows: the solubility product index of the oxide (MeO) corresponding to the cation (Me2+) in the CsI or NaI melt:
n o n o 2 2 2 2 p K s;MeO =ðmol kg Þ ¼ lg ms;Me2þ ms;O2 =ðmol kg Þ ; ð2Þ where ms;Me2þ and ms;O2 are equilibrium molalities of the metal cation and oxide ion in the saturated solution of the oxide in the melt. The values are not less than 10 to provide reasonably low residual concentration of oxygen-containing impurities. As is known, the solubility product of the metal oxide is dependent on the cation radius. So, the solubility product of EuO should be close to that of SrO, since Sr2+ and Eu2+ radii are practically the same. The pKs,SrO in CsI equals to (5.10 ± 0.3) [5]. However, Sr and Eu belong to different groups of elements in the periodic table (s- and f- elements, respectively), and it is of interest to check the validity of radius-based predictions in this case. As for CdO, ZnO and NiO, the values of their solubility in iodide melts are not known. The determination of these values and establishment of some regularities of the effect of melt composition on the oxide solubility are the goals of the present work. 2. Experimental The CsI (Aldrich, mass fraction purity 0.99999) was used for the experiments without additional purification. After melting, the salt was kept in an argon atmosphere for 1 h, a small amount of NH4I
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was added into the melts to provide removal of the oxide ions according to the following equation:
2NH4 I þ O2 ¼ 2NH3 " þH2 O " þ2I :
ð3Þ
After such purification, the melt was appropriate for the solubility investigations. For all the studies, 50 g of CsI were placed into an alumina crucible within a potentiometric cell. Then the electrodes were inserted into the cell and the latter was heated to T = 973 K in an argon atmosphere. The EuI2 was prepared by dissolution of Eu2O3 (Stanford Material Corporation, OX63-5N, mass fraction purity 0.99999) in aqueous HI (reagent quality). Afterwards, NH4I (reagent quality) taken in molar ratio 2:1 to Eu was added to the solution. The solution obtained was evaporated and dried to the formation of a dark-brown powder (EuI3 + NH4I with traces of I2 and water). This powder was heated in vacuum till I2 evaporation started at approximately T = 473 K owing to decomposition of EuI3 followed by the formation of EuI2 and iodine. At the end of the latter process, the sublimation of NH4I commenced and intermediate HI formation in the hot zone provided additional purification of EuI2 from oxygen admixtures. The CdCl2, NiCl2 and ZnCl2 were synthesized by dissolution of the corresponding oxides (mass fraction of each was 0.995) in aqueous HCl of reagent quality. The solutions were mixed with NH4Cl and dried in vacuum at the temperature rising stepwise from 293 K to 673 K. The potentiometric cell with an indicator oxygen electrode Pt(O2)|YSZ (YSZ was 0.9 ZrO2–0.1 Y2O3 ceramics) 1
AgjAgþ ð0:1mol kg Þ þ CsIjjCsI þ O2 jYSZjPtðO2 Þ
ð4Þ
was used for the measurements. It was first calibrated with known weights of KOH as a strong Lux-Flood base
2OH ¼ H2 O # þ O2
ð5Þ
to establish the dependence of the emf of cell (4) against pO (pO ¼ lg mO2 , where mO2 was the equilibrium molality of oxide ions in the melt). The plot obtained was used for recalculation of emf values into equilibrium O2– molalities. To determine the solubility products of EuO, CdO, NiO and ZnO, the amounts of the corresponding chlorides providing approximately (0.02 to 0.03) mol kg–1 molality of Me2+ cation in the melt were added to the molten CsI (this molality is designated further as m0Me2þ ). Then a small (0.001 to 0.002 mol kg–1) weight of the base (KOH) was added to this solution (this molality is denoted as m0O2 ), and the equilibrium molality of O2– was determined. The titration was finished at m0O2 2m0Me2þ . All the calculations were made on the basis of the consumption of oxide ion, i.e. the difference between the added and equilibrium concentrations of O2– in the melt, DmO2
DmO2 ¼ m0O2 mO2 :
3. Results and discussion The curves of the potentiometric titration of Cd2+, Zn2+, Ni2+ and Eu by O2– are presented in figure 1. Let us consider an example of the treatment of the obtained results according to the following reaction: 2+
Me2þ þ O2 ¼ MeO # :
ð9Þ
At first, all the cations demonstrate appreciable acidic properties. Indeed, the usual pO values for pure alkali metal halides are within 3.5 to 4, whereas the initial addition of the metal chlorides results in a noticeable increase of pO because of fixation of O2– ions. The deviation from pO = 4 increases with the strengthening acidity of the metal cation in the sequence Cd2+ ? Zn2+ ? Ni2+. As for the Eu2+ cation, it demonstrates a surprisingly high acidity, since the initial solution of EiI2 possesses pO > 11. Such a fixation results in the formation of non-dissociated MeO in the liquid or solid phase. In the first case, it is described by the dissociation constant of the oxide KMeO or pK MeO ¼ lg K MeO : 1
1
2 pfK MeO =ðmol kg Þg ¼ lgfm2þ Me mO =mMeO =ðmol kg Þg;
ð10Þ
where mMeO is the molality of non-dissociated oxide in the liquid phase (melt). In the case of the precipitation of the oxide deposit from the melt, the equilibrium is characterized by the solubility product, K s;MeO or its index, pK s;MeO , see equation (2). However, since the temperature of the melt is high, visual observations of the presence or absence of the metal oxide deposit are impossible. To elucidate this problem, let us analyze the data in table 1. The conclusion about the saturation may be made from the behavior of the calculated parameters (7) and (8) during the titration process. If the oxide solution remains unsaturated, pK 0MeO values remain constant within some random deviations, whereas pK 0s;MeO values decrease as m0O2 rises; in the case of the saturated solution pK 0s;MeO is constant and pK 0MeO increases [7]. So, one may conclude that all the CdO solutions presented in table 1 were saturated, i.e. even the first small addition of the oxide ion donor results in
ð6Þ pK 0MeO
The experimental values of (dissociation constant index) and pK 0s;MeO (the solubility product index) for statistical treatment were calculated as follows: 1
pfK 0MeO =ðmol kg Þg 1
¼ lgfðm0Me2þ DmO2 Þ m2 O =DmO2 =ðmol kg Þg; 2
2
0
ð7Þ 2
2
2 2 pfK 0s;MeO =ðmol kg Þg ¼ lgfðm2þ Me DmO Þ mO =ðmol kg Þg:
ð8Þ The statistical treatment of the results obtained was performed according to the generally accepted routines described elsewhere [6].
FIGURE 1. Plot of pO against initial molality of O2– (m0O2 ) at titration of Me2+ cations in molten CsI at T = 973 K: (1) Cd2+ (m0Cd2þ = 0.030 mol kg1), (2) Zn2+ (m0Zn2þ = 0.020 mol kg1), (3) Ni2+ (m0Ni2þ = 0.018 mol kg1), (4) Eu2+ (m0Eu2þ = 0.022 mol kg1).
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in iodide melts, that decreases the solubility in the sequence Cl ? I. It should be noted that the said concept gives a clear explanation for different changes of solubility for the oxides studied, going from chloride melt to iodide one. Similarly for ZnO formed by harder acid (Zn2+), the solubility decrease is larger than for CdO formed by the softer acid (Cd2+). A common regularity of oxide solubility in halide melts is a strict dependence of pK s;MeO values on the inverse squared cation radius
TABLE 1 Results of potentiometric titration of Cd2+ (CdCl2, 0.030 mol kg1) by O2– in molten CsI at T = 973 K. 1
m0Me2þ =ðmol kg
Þ
0.0036 0.0074 0.0102 0.0146 0.0223 0.0265 0.0311 0.0331
pO
pK 0CdO =ðmol kg
5.27 5.24 5.18 5.00 4.79 4.19 3.01 2.35
4.37 4.72 4.85 4.92 5.15 4.87 4.21
1
2
pK 0s;CdO =ðmol kg
Þ
2
Þ
6.81 6.85 6.84 6.76 6.80 6.45 5.73
2
00
ð12Þ
where C1 and C2 are constants and rMe2þ is the metal cation radius. For example, the correlation between pK s;MeO and the polarizing action of the cation by Goldshmidt (Zr2 , where Z is the cation Me2þ charge, Q) for the cation contained in the oxide is practically linear. This dependence is characteristic of the solubility of alkaline earth oxides and oxides of transition metals belonging to the first transition row [8]. Such a dependence built using the results obtained in the present work and the values reported in [5] is presented in figure 2. As is seen, the former solubility values are principally in good agreement with the data of [5], since they extend the linear correlation (figure 2) to the region of oxides with slight solubility (pK s;MeO P 7). As for PbO solubility, it falls out from the similar correlations obtained from the oxide solubility data for chloride and bromide melts [5,10]. The EuO solubility does not agree with the regularity for cations of s- and d- elements. Statistical treatment of data presented in figure 2 yields the following equation
CdO precipitation from the melt. The ZnO, NiO and EuO powders are deposited from the cation solution in CsI at the first small addition of O2–, too. The solubility product values obtained are presented in table 2. To check the correctness of these data, the solubilities of NiO and ZnO in molten CsI at T = 973 K are determined by an alternative way. We used the isothermal saturation method with control of the metal ion molality by inductively coupled plasma atomic emission spectrometry (ICP-AES). According to such a determination, the mass fractions of both metals in CsI are of the order of 1 106, that corresponds to 1.2 105 mol kg–1 and K s;MeO = 1.4 1010 (pK s;MeO > 9.76). So, the results of the analytical determination are close to the values obtained by the potentiometric titration method. Somewhat higher values can be explained both by experimental error of the ICP-AES method and by the presence of some amount of the non-dissociated oxide in the melt. Comparing these values with similar data for molten KCl–NaCl eutectic [8] (table 2), it may be assumed that the change of the melt anion composition (the substitution of Cl– by I–) causes pK s;MeO decrease, i.e. the values of the oxide solubility (ms;MeO ) decrease by 10 to 30 times. Such a decrease may be explained by the concept of hard and soft acids and bases (HSAB) proposed by Pearson [9]. The essence of this concept consists in preferable formation of the complexes ‘hard acid–hard base’ and ‘soft acid–soft base’ at the interactions of different acid–base pairs. The dissolution of oxides in molten alkali metal halides is accompanied with the formation of the complexes ‘metal–halide ion’ and ‘alkali metal–oxide ion’ according to the following scheme:
Me00 O þ nMe0 X ¼ ½Me00 Xn 2n ðIÞ þ ½O Me0n n2 ðIIÞ;
2
pfK s;MeO =ðmol kg Þg ¼ C 1 þ C 2 ðr 2 =m2 Þ; Me2þ
2
2
pfK s;MeO =ðmol kg Þg ¼ 0:35ð0:3Þ þ 0:205ð0:01Þ =ðQ m2 Þg; fZr2 Me2þ
ð13Þ
ð11Þ
0
where Me ; Me and X are multi-charged and alkali metal cations and halide ion, respectively. The degree of the shift of equation (11) to the right is dependent on the strength of the above-said interactions. As for the ionic solvents and oxides discussed, the alkali metal cations belong to hard acids and their interaction (I) with O2 hard base is stronger for K+ or Na+ than for Cs+ since K+ (or Na+) is a harder acid than Cs+. The ion Cl– belongs to hard bases and I– is a soft base, therefore, Cd2+ and Zn2+ ions as hard acids interact with chloride ion more strongly than with iodide (II). So, complexes I and II in equation (11) should be more stable in chloride melts than
FIGURE 2. Plot of MeO solubility (pK s;MeO ) in molten CsI at T = 973 K against polarizing action of Me2+ cation Zr2 : (1) BaO [5], (2) SrO [5], (3) CaO [5], (4) CdO, Me2þ (5) ZnO, (6) NiO, (7) PbO [5], (8) EuO.
TABLE 2 Solubilities of CdO, ZnO and NiO in some molten alkali metal halides at T = 973 K (molality scale). Oxide
KCl–NaCl [8] 2
CsI, this work 2
K s;MeO =ðmol kg CdO ZnO NiO EuO
5
1.0 10 1.2 107 6.3 1012
Þ
1
ms;MeO =ðmol kg 3
3.1 10 3.2 104 3.1 105
Þ
2
2
K s;MeO =ðmol kg 7
1.58 10 1.0 1010 6.3 1012 7.94 1014
Þ
1
ms;MeO =ðmol kg 4
4.0 10 1.0 105 2.5 106 2.82 107
Þ
V.L. Cherginets et al. / J. Chem. Thermodynamics 43 (2011) 1252–1255
for all the above-noted oxides excluding PbO and EuO. The behavior of both oxides does not obey all the rules found for solubility in molten CsI. This plot may be used for estimation of the solubility of alkaline earth oxides and transition metal oxides on the basis the cation radii. Concerning the practical problem of removal of oxide ions from molten salts, it should be noted that Zn2+, Ni2+ and Eu2+ cation dopants can be used for purification of molten CsI due to the extremely low solubility of ZnO, NiO and CdO within the CsI. Moreover, the density of ZnO at room temperature is equal to 5.7 g cm–3, whereas that of molten CsI near the melting temperature is 3.1 g cm–3 [11]. The estimation of density of solid oxides taking into account the typical thermal expansion coefficients (1.1 105, K1) [12] gives a 2% diminution of this parameter; the density of ZnO at the melting temperature of CsI is 5.6 g cm–3. Therefore, the density ratio ZnO/CsI at T = 894 K (the melting point of CsI [11]) is approximately equal to 1.9. This provides easy separation of ZnO from CsI since the former precipitates at the bottom of the crystal growth vessel. Such a purification method is especially suitable while obtaining crystals by the Czochralski or Kyropoulos methods, where the crystals grow from the upper part of the melt. The above speculations remain true for NiO and EuO, whose densities at room temperature are 7.45 and 8.21, respectively. 4. Conclusion The solubility products of CdO (pKs,CdO = 6.80 ± 0.2), ZnO (pKs,ZnO = 10.0 ± 0.5), NiO (pKs,NiO = 11.2 ± 0.2) and EuO (pKs,EuO = 13.1 ± 0.2) in molten CsI at T = 973 K are determined by potentio-
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metric titration using a membrane oxygen electrode Pt(O2)|YSZ as an indicator. The oxide solubility is found to decrease with diminution of the cation radius. The exchange of anion of the ionic melt in the Cl– ? I– sequence results in considerable reduction of the values of oxide solubility. The linear correlation between pKs,MeO and the polarizing action by Goldshmidt (Zr2 ) of the cation is constructed. It may be proMe2þ posed for estimation of oxide solubility on the basis of their cation radii. References [1] N. Balamurugan, A. Arulchakkaravathi, S. Selvakumar, et al., J. Cryst. Growth 286 (2006) 294. [2] B.G. Zaslavsky, J. Cryst. Growth 218 (2000) 277. [3] L.A. Kappers, R.H. Bartram, D.S. Hamilton, C. Brecher, A. Lempicki, V. Gaysinskiy, E.E. Ovechkina, V.V. Nagarkar, Radiat. Measur. 42 (2007) 537. [4] H. Lux, Z. Elektrochem. 45 (1939) 303. [5] V.L. Cherginets, E.G. Khailova, O.V. Demirskaya, Zhurn. Fiz. Khim. 71 (1997) 371. [6] K. Doerffel, Statistik in der analytischen Chemie, Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1990. [7] V.L. Cherginets, E.G. Khailova, Electrochim. Acta 38 (1993) 1481–1485. [8] V.L. Cherginets, V.V. Banik, Rasplavy N1 (1991) 66–69. [9] R.G. Pearson, J. Am. Chem. Soc. 85 (1963) 3533–3539. [10] V.L. Cherginets, E.G. Khailova, O.V. Demirskaya, Electrochim. Acta 41 (1996) 463–467. [11] G.J. Janz, Molten Salts Handbook, Academic Press, NY, London, 1967. p. 588. [12] I.T. Goronovskii, Yu.P. Nazarenko, E.F. Nekryach, Short Handbook on Chemistry, Naukova dumka, Kiev, 1987. p. 830.
JCT 10-419