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УДК 544.3:544.4
THE REVISED POTENTIAL-PH DIAGRAM OF Sc-HO SYSTEM УТОЧНЁННАЯ ДИАГРАММА ПОТЕНЦИАЛ-РН СИСТЕМЫ Sc-HO
P.A. Nikolaychuk П.А. Николайчук
Chelyabinsk State University, 12g Bratiev Kashirinykh St, Chelyabinsk, 454001, Russia Челябинский государственный университет, Россия, 454001, г. Челябинск, ул. Братьев Кашириных, 12g
E-mail: npa@csu.ru
Résumé. The thermodynamic data on the standard Gibbs energies of formation of scandium solid and aqueous species were collected and systematised. The speciation diagram for the dimerisation reaction 2ScOH2+ (aq) ^ Sc2(OH)24+ (aq) was plotted. The equilibria between various hydrolysed scandium (III) species in an aqueous solution were discussed. The potential-pH diagrams of system Sc-H2O at 25°C, air pressure of 1 bar and various activities of ions in a solution were plotted. The influence of possible electrochemical formation of scandium hydride and the effect of non-stoichiometry of scandium sesquioxide on the corrosion-electrochemical properties of scandium were discussed.
Аннотация. Собраны и систематизированы термодинамические данные о стандартных энергиях Гиббса образования различных соединений скандия в твёрдом состоянии и в водном растворе. Построена диаграмма распределения для реакции 2ScOH2+ (aq) ^ Scz(OH)z4+ (aq). Обсуждены равновесия между различными гидратированными формами скандия (III) в водном растворе. Построены диаграммы потенциал-р# системы Sc-H2O при 25°C, давлении воздуха 1 бар и различных активностях ионов в растворе. Обсуждено возможное влияние электрохимического образования гидрида скандия и нестехиометрии сесквиоксида скандия на коррозионно-электрохимические свойства скандия.
Key words: scandium, scandium oxide, scandium hydride, scandium nitride, non-stoichiometry, scandium aqueous species, speciation diagram, activity-pH diagram, potential-pH diagram, corrosion-electrochemical behaviour.
Ключевые слова: скандий, оксид скандия, гидрид скандия, нитрид скандия, нестехиометрия, водные соединения скандия, диаграмма распределения, диаграмма активность-р^, диаграмма потенциал-р#, коррозионно-электрохимическое поведение.
1. Introduction
An interest to the chemistry of scandium significantly increased in the last five decades. With the development of microelectronics, air- and spacecraft engineering, hydrogen energetics and other high-technology branches of industry scandium began to attract an attention of researchers. Various scandium compounds are widely employed for the development of high-temperature ceramics, quantum-mechanical amplifiers, lasers, luminophores, emission materials, dielectrics, solid electrolytes, halogen projectors, catalysts, filters for quasimonochromatic rays of neutrons, emitters of ^-particles et cetera. Scandium-based alloys have a series of valuable properties, including thermal and mechanical stability, corrosion and radiation resistance [Horovitz et al., 1975; Komissarova, 2001]. The aqueous chemistry and the corrosion-electrochemical behaviour of scandium are very similar to that of aluminum [Brookins, 1988; Rayner-Canham, 2013; Schweitzer, Pesterfield, 2010]. The potential-pH diagram for scandium found its application in studying of pitting behaviour of alloys [Santamaria et al., 2013; Wloka, Virtanen, 2007] and characterisation of scandium inorganic compounds [Wang et al., 2013]. However, the existing variants of Pourbaix diagram for scandium [Brookins, 1988; Pourbaix, 1966; Schweitzer, Pesterfield, 2010; Takeno, 2005] take into account only Sc (hcp), Sc3+ (aq), ScOH2+ (aq) and SC2O3 (bcc) and do not consider another scandium aqueous species, including the polimerised forms. The goal of this study is to collect the reliable data on the
Gibbs energies of formation of the various scandium aqueous species, consider the speciation of Sc (III) species and revise the potential-pH diagram of Sc-H2O system.
2. Thermodynamic data on scandium solid and aqueous species
According to the phase diagram of Sc-O system [Kuprashvili et al., 1969; Predel, 1998; Diagrammy sostoâniâ ..., 2001], only one stable oxide - SC2O3 - exists in the system at 298K. It has the crystal structure of body centered cube. It was reported earlier [Dufek et al., 1969, 1967; Petru et al., 1970] about the existence of scandium monoxide, ScO (s), which can be obtained by reduction of Sc2O3 by silicon, but the subsequent studies and the thermodynamic calculations [Work, Eick, 1972; Yudin et al., 1976] disproved this report. Upon interaction with hydrogen scandium form two hydrides, namely, ScH2 (s) and ScH (s) [Bashkin et al., 1978; Horovitz et al., 1975; Jerosch-Herold et al., 1997; Kobayashi, Takei, 1978; Kobayashi, Takei, 1980; Switendick, 1989; Komissarova, 2001]. In aqueous acidic environments scandium exists in form of the cation Sc3+ (aq), which with increase of the basicity of the solution can be hydrolysed to ScOH2+ (aq), Sc(OH)2+ (aq) or Sc(OH)4- (aq) [Baes Jr., Mesmer, 1976; Schweitzer, Pesterfield, 2010]. The hydroxide Sc(OH)3 (s) can be precipitated from a solution, but it is unstable relative to the oxide [Travers et al., 1976].
Table 1 summarises the standard Gibbs energies of formation of various scandium species. In the case when only the standard enthalpies of formation and the standard entropies of the species were present in literature [Bommer, Hohmann, 1941; Huber et al., 1963], the standard entropy of pure scandium were taken from [Gerstein, 1971] and used in calculations of the standard Gibbs energies. The standard Gibbs energy of formation of scandium (III) hydroxide was estimated from the value of the solubility product of Sc(OH)3 (s) [Aksel'rud, 1963; Feitknecht, Schindler, 1963; Moeller, Kremers, 1945; Oka, 1938; Wood, Samson, 2006]. The standard Gibbs energies of formation of various scandium hydroxocations were estimated from the values of hydrolysis constants [Akalin, Ozer, 1971; Antonovich, Nazarenko, 1968; Aveston, 1966; Biedermann et al., 1956; Cole et al., 1969; Haas et al., 1995; Kilpatrick, Pokras, 1953; Kilpatrick, Pokras, 1954; Komissarova et al., 1971; Lindqvist-Reis et al., 2006; Marques et al., 1997; Paul, 1962; Sekine, Hasegawa, 1966; Siqueira et al., 1995; Turkel et al., 1999; Wu et al., 2004; Zhuk, 1954]. The values chosen for further calculations are marked in Italic.
3. The equilibria involving various scandium (III) compounds 3.1. The oxide, the oxyhydrate and the hydroxide
The following reactions involving the oxide, the oxyhydrate and the hydroxide of scandium (III) can be considered:
ScA (s) + H2O (1) ^ 2ScO(OH) (s), (1)
ScO(OH) (s) + H2O (1) ^ Sc(OH)3 (s), (2)
Sc2O3 (s) + 3H2O (1) ^ 2Sc(OH)3 (s). (3)
The Gibbs energy changes in the reactions (1), (2) and (3) are calculated using the values from Table 1, and the following values are obtained: ArG298(1) = 41 940 J • mol 1, ArG098(2) = 10 940 J • mol1 and AG098(3) = 63 8 20 J • mol1. The calculations show that scandium sesquioxide is the most thermodynamically stable compound, and it does not tend to be hydrolysed.
3.2. The polymerisation of aqueous ScOH2+
The ion ScOH2+ (aq) can form a dimer according to the reaction:
2ScOH2+ (aq) ^ Sc2(OHV+ (aq). (4)
The Gibbs energy change in the reaction (4) from the data of Table 1 is AG098 (4) = -15 200 J • mol1, and the equilibrium constant is K(4) = 461.621 • mol 1.
Table 1 Таблица 1
The standard Gibbs energies of formation of various scandium species Стандартные энергии Гиббса образования различных соединений скандия
Compound Reference state -AfG098, J• mol"1 Reference
Sc s, hexagonal close packed 0 By convention
ScN s, cubic 253 250 [Alimarin, Yung-Schaing, 1961; Komissarova, 2001]
253150 [Gschneider Jr., 1961; Horovitz et al., 1975]
283 800 [Pankratz et al., 1984]
ScH2 s, face centered cubic 149 280 [Termiceskie konstanty vesestv, 2007]
157170 [Lieberman, Wahlbeck, 1965; Komissarova, 2001]
157 300 [Horovitz et al., 1975]
SC2O3 s, body centered cubic 1 819 360 [Brookins, 1988; Wagman et al., 1982]
1 819 410 [Speight, 2005]
1 819 460 [Horovitz et al., 1975]
1 804 110 [Pankratz et al., 1984]
1 804 250 [Veryagin et al., 1965]
1 819 320 [Ruzinov, Gulzanitskii, 1975; Progress in the Science ..., 2013]
1819 200 [Schweitzer, Pesterfield, 2010]
1 818 930 [Termiceskie konstanty vesestv, 2007]
1 802 030 [Jichang, Ke'ren, 1984; Komissarova, 2001]
1819 380 [Travers et al., 1976]
1 819 370 [Robie et al., 1979]
Sc(OH)3 s, body centered cubic 1 233 300 [Brookins, 1988; Wagman et al., 1982]
1 233 230 [Ruzinov, Gulzanitskii, 1975]
1233 400 [Schweitzer, Pesterfield, 2010]
1 232 980 [Horovitz et al., 1975]
1226 000 [Travers et al., 1976]
1 055 830 [Termiceskie konstanty vesestv, 2007]
ScO(OH) s 1 007 200 [Baes Jr., Mesmer, 1976]
Sc3+ aq 586 600 [Brookins, 1988; Schweitzer, Pesterfield, 2010; Wagman et al., 1982]
586 200 [Schweitzer, Pesterfield, 2010]
583 890 [Termiceskie konstanty vesestv, 2007]
586 650 [Travers et al., 1976]
ScOH2+ aq 801200 [Brookins, 1988; Speight, 2005; Wagman et al., 1982]
799 000 [Schweitzer, Pesterfield, 2010]
799 220 [Travers et al., 1976]
799 200 [Baes Jr., Mesmer, 1976]
794 120 [Termiceskie konstanty vesestv, 2007]
Sc(OH)2+ aq 1005 400 [Schweitzer, Pesterfield, 2010]
1 005 500 [Baes Jr., Mesmer, 1976]
Sc(OH)4- aq 1 370 800 [Brookins, 1988]
1380 700 [Schweitzer, Pesterfield, 2010]
1386 750 [Baes Jr., Mesmer, 1976]
SC2(OH)24+ aq 1649 750 [Termiceskie konstanty vesestv, 2007]
1 613 200 [Baes Jr., Mesmer, 1976]
SC3(OH)45+ aq 2 626 500 [Termiceskie konstanty vesestv, 2007]
SC3(OH)54+ aq 2 838 220 [Termiceskie konstanty vesestv, 2007]
2 852 200 [Baes Jr., Mesmer, 1976]
The ratio of activities of monomelic and dimeric forms is determined by the following system of equations:
a
v _ Sc2(OH)2+ (aq) ,
K(4) " 2 ;
aScOH2+ (aq) (5)
a[Sc] = aScOH2+ (aq) + 2 ' °Sc2(OH)4+ (aq).
Here a[Sc] is the total activity of dissolved scandium in a solution. The dependency of
activities of monomeric and dimeric forms on the total activity of dissolved scandium is
illustrated by Figure 1. Here curves 1 and 2 show the dependencies of the "activity fractions"
a a 4 scoh (aq) and-Sc2 ^ (aq)- on a[Sc], and curve 3 determines the ratio
aScOH2+ (aq) + öSc2 (OH )4+ (aq) 0ScOH2+ (aq) + üSc2 (OH )4+ (aq)
a
ScOH2+ (aq)
a
Sc2 (OH )2+ (aq)
The dimeric form predominates in concentrated solutions. At a[Sc]=1 M the ratio
a 2
—ScOH (aq) = 0.067. With decrease of the total content of dissolved scandium this ratio
a
Sc2 (OH )2+ (aq)
begins to grow. The activities of monomeric and dimeric forms become equal at lg a[Sc]=-2.15, and in more diluted solution the monomeric form predominates. At a[Sc]=10-3 M the
a a
ScOH2+(aq) ~ j . ScOH2+(aq) ^mn
ratio-(-q^ = 3.43 and at a[Sc]=10-6 M-(-q^ = 2170.
aSc2 (OH )4+(aq) aSc2 (OH )4+(-q)
3.3. The aqueous scandium (III) species
The relative stability of other scandium (III) hydroxocomplexes depends on both pH and the activities of species in a solution. This dependence is illustrated in Figure 2.
The cation Sc3+ exhibits the following consecutive reactions with increase of pH depending on the activities:
lg a[Sc] > -7.14: Sc3+ (aq) - ScA (s) - Sc(OH)4- (aq);
71/lsl(W, ^ no«. Sc3+ (aq) - ScOH2+ (aq), Sc2(OH)24+ (aq) - Sc2O3 (s) --7.14 > lg a[Sc] > -9.38: - Sc(OH)4- (aq);
nos^l™ ^ 10 rr. Sc3+ (aq) - ScOH2+ (aq), Sc2(OHV+ (aq) - Sc(OH)2+ (aq) --9.38 > lg a[Sc] > -12.66: - Sc2O3 (s) - Sc(OH)4-(aq);
, , Sc3+ (aq) - ScOH2+ (aq), Sc2(OHV+ (aq) - Sc(OH)2+ (aq) -
lg a[Sc] < -12.66: - Sc(OH)4- (aq).
Accordingly, the cation Sc3(OHV+ has no domain of thermodynamic stability.
lJsl»Soi
4. The potential-p.ff diagram
In accordance with Figure 2 the sequence of equilibria involving various scandium ions differs with alteration of activities of dissolved species in a solution. This also affects the potential - pH diagrams. The diagrams at various activities of ions in solutions from 10-6 M to 10-15 M are shown in Figures 3 through 6. The calculations show that for the environments, in which the activities exceed 10-6 M and which are of the most importance for applied chemistry, the diagrams are in good agreement with the previously published ones [Brookins, 1988; Pourbaix, 1966; Schweitzer, Pesterfield, 2010; Takeno, 2005]. The diagrams at the activities lower than 10-10 M currently have no practical implementation; however they are presented in order to show the influence of the presence or absence of the domains of thermodynamic stability of certain phases on the thermodynamic activities. Probably the thermodynamic prediction of the electrochemical properties of solutions at super-low activities of ions in solutions may become practically important in the future due to developing of analytical techniques and the tendency of decreasing the detection limit of substances in a solution.
The corrosion-electrochemical behaviour of scandium is very similar to that of aluminum. In the highly acidic environments (at pH<4) scandium exhibits the active dissolution with formation of Sc3+ (aq). At higher pH values the protective layer consisting of Sc2O3 (s) is formed on the metal surface. However, in very diluted solutions, where a[Sc]<10-7 M, the hydrolysed aqueous scandium species begin to predominate, and the domain of scandium passivity is rapidly narrowed.
5. The influence of scandium hydride and nitride
In addition to the oxide passivation, scandium can exhibit either hydride or nitride passivation. The layer of scandium hydride can be formed due to electrochemical reduction of water and interaction of metallic scandium with the forming hydrogen. When scandium is exposed in a natural water that stays in equilibrium with atmospheric air, the possible electrochemical formation of scandium nitride due to reduction of the atmospheric nitrogen should also be considered.
Using the data from Table 1 the potentials of reduction of Sc2O3 (s) to metallic scandium, scandium hydride and scandium nitride were calculated:
Sc2O3 (s) + 6H+ + 6e- ^ 2Sc (hcp) + 3H2O (1); £0298=-1.914 V, (6)
Sc2O3 (s) + 10H+ + 10e- ^ 2ScH2 (s) + 3H2O (1); E°298=-0.822 V, (7)
Sc2O3 (s) + N2 (g) + 6H+ + 6e- ^ 2ScN (s) + 3H2O (1), Pm (¡0=0.79 bar; E0298=-1.040 V. (8) It follows from the equations (6), (7) and (8) that both metallic scandium and scandium nitride become thermodynamically unstable in presence of scandium hydride, and that scandium oxide is electrochemically reduced directly to ScH2 (s). Therefore, the potential-pH diagram for scandium with consideration of scandium hydride ScH2 (s) were calculated and presented in Figures 7 through 10. The domain of hydride passivation of scandium is present at these diagrams instead of the domain of thermodynamic stability of pure scandium. However, all the reactions involving dissolution of both metallic scandium and its hydride, occur below the potential of the standard hydrogen electrode, and therefore, do not affect the corrosion-electrochemical properties of scandium in the domain of electrochemical stability of water.
The basic chemical and electrochemical equilibria in Sc-H2O system are listed in Table 2.
pH
Fig. 3. The potential-pH diagram of Sc-H2O system at 298K, air pressure of 1 bar and
activities of ions in solutions ai=10-6 M Рис. 3. Диаграмма потенциал-рЯ системы Sc-H2O при 298K, давлении воздуха 1 бар и
активностях ионов в растворе a;=10-6 M
-3 -I-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-
0 2 4 6 S 10 12 14 16
ph
Fig. 4. The potential-pH diagram of Sc-H2O system at 298K, air pressure of 1 bar and
activities of ions in solutions ai=10-9 M Рис. 4. Диаграмма потенциал-рЯ системы Sc-H2O при 298K, давлении воздуха 1 бар и
активностях ионов в растворе a;=10-9 M
pH
Fig. 5. The potential-pH diagram of Sc-H2O system at 298K, air pressure of 1 bar and
activities of ions in solutions ai =10-12 M Рис. 5. Диаграмма потенциал-рЯ системы Sc-H2O при 298K, давлении воздуха 1 бар и
активностях ионов в растворе ai=10-12 M
0 2 4 6 8 10 12 14 16
ph
Fig. 6. The potential-pH diagram of Sc-HO system at 298K, air pressure of 1 bar and
activities of ions in solutions ai=10-15 M Рис. 6. Диаграмма потенциал-рЯ системы Sc-H2O при 298K, давлении воздуха 1 бар и
активностях ионов в растворе a;=10-15 M
Table 2 Таблица 2
Basic chemical and electrochemical equilibria in Sc—H20 system at 298K and air pressure of 1 bar Основные химические и электрохимические равновесия в системе Sc—H^O при 298К и давлении воздуха l bar
No of line in Figure s 2-10 and 12 Electrode reaction E, V(s. h. e.) or pH of the solution
1 2 3
a 2Я+ (aq) + 2e- ^ Я2 (g); P,h (g) = 5-10-7 bar E = 0.186-0.0591-pH
1 Г ScOH'2+ (aq) + Я+ (aq) ^ Scз+ (aq) + Я20 (1); L. 5с2(ОЯ)24+ (aq) + 2Я+ (aq) ^ 2Sc3+ (aq) + 2Я20 (1) TT _ , "ScOH^aq) /sc2(0Hf(aq) pH = 4.266+lg-= 2.934+О.5-lg---±- flSc3+(aq) flSc3+(aq)
2 ГSc{OH)A+ (aq) + Я+ (aq) ^ ScO№+ (aq) + Я20 (1); 25с(ОЯ)2+ (aq) + 2Я+ (aq) ^ 5с2(ОЯ)24+ (aq) + 2Я20 (1) pH = 5.387+lg£'Sc(OHE(aq) =6.719+0.5-lg ^ScOH2+ (aq) flSc2(OH g+(aq)
3 Sc(OH)4- (aq) + 2Я+ (aq) ^ 5с(ОЯ)2+ (aq) + 2Я20 (1) pH = 8.673+0.5-1ёЯ8с(ОНиа<1) ^ScfOHgCaq)
4 Sc203 (s) + 6Я+ (aq) ^ 2Sc3+ (aq) + 3Я20 (1) pH=i.887-i-lg«Sc3+(aq)
5 J~Sc203 (s) + 4Я+ (aq) ^ 2ScO№+ (aq) + Я20 (1); \Sc203 (s) + 4Я+ (aq) ^ 5с2(ОЯ)24+ (aq) + Я20 (1) pH= 0.698-0.5 lg «ScOH2+(aq)= 1.364-0.25-lg «Sc2(OHr(aq)
6 Sc203 (s) + Я20 (1) + 2Я+ (aq) ^ 2Sc(OH)2+ (aq) pH = -3.99I-lg«Sc(OHE(aq)
7 2Sc(OH)4- (aq) + 2Я+ (aq) ^ Sc203 (s) + 5Я20 (1) pH = 21.337 + lg«Sc(OH)_(aq)
00 о
End of table 2 Окончание таблицы 2
1 2 3
8 Sc3+ (aq) + зе- ^ Sc (hcp) E =-2.025+ 0.0197 dg flSc3+(aq)
9 f ScOH2+ (aq) + H+ (aq) + зе~ ^ Sc (hcp) + H20 (1); \Sc2(0H)24+ (aq) + 2H+ (aq) + 6e~ ^ 2Sc (hcp) + 2H20 (1) E = -1.941 - 0.0197 • pH + 0,0197 dg flgcOH2+ (aq) = =-1.967 - 0.0197 • pH + 0.0099 dg «Sc2(OH g+(aq)
10 Sc(0H)2+ (aq) + 2H+ (aq) + зе~ ^ Sc (hcp) + 2H20 (1) E = -1.835 - 0.0394 • pH+ 0.0197 -Iga^ (aq)
li Sc203 (s) + 6Я+ (aq) + 6e~ ^ 2Sc (hcp) + 3H20 (1) E = -1.914-0.0591-pH
12 Sc(OH)4- (aq) + 4Я+ (aq) + зе~ ^ Sc (hcp) + 4Я20 (1) E = -1.493 - 0.0788 • pH+ 0.0197 -Iga^ (aq)
13 Sc:-+ (aq) + 2H+ (aq) + 5e~ ^ ScH2 (s) E = -0.889 - 0.0236 • pH + 0.0118 dg a^
14 Г ScOH^ (aq) + 3Я+ (aq) + se~ ^ ScH2 (s) + H20 (1); \ Sc2(0H)24+ (aq) + 6Я+ (aq) + юе- ^ 2ScH2 (s) + 2H20 (1) E = -0.839 - 0.0355 • pH + 0.0118 dg flScOH2+ (aq) = = -0.855 - 0.0355 • pH + 0.0059 • 1 g ^^ (>q)
15 Sc(0H)2+ (aq) + 4Я+ (aq) + se- ^ ScH2 (s) + 2H20 (1) E = -0.775 - 0.0473 • pH + 0.0118 dg %OH)+ (aq)
16 Sc203 (s) + ЮЯ+ (aq) + юе- ^ 2ScH2 (s) + 3H20 (1) E =-0.822-0.0591-pH
17 Sc(OH)4- (aq) + 6Я+ (aq) + se- ^ ScH2 (s) + 4Я20 (1) E = _0.570 - 0.0790 • pH + 0.0118 dg %OH)_ (aq)
18 2Sc3+ (aq) + xH20 (1) + (6-2x)e~ ^ Sc20x (s) + 2xH+ (aq); 2.89<X<3 ^ -0.419-x2 -0.656-x + 6.076 0.0591-x TT 0.0591, E" 'PH 'lgfl,Sc3+(aq) x-3 x-3 x-3 ÖC iaqj
19 Sc20x (s) + 2хЯ+ (aq) + 2xe~ ^ 2Sc (hcp) + xH20 (1); 2.89<x<3 E = -0.419 • x - 0.656 - 0.0591 • pH
20 25с(ОЯ)4- (aq) + (8-2х)Я+ (aq) + (6-2x)e- ^ Sc20x (s) + (8-х)Я20 (1); 2.89<x<3 ^ -0.419-х2 -0.656-X + 4.495 0.0591-(x-4) u 0.0591 , x-3 x-3 p x-3 lg£,Sc(OHHaq)
Fig. 7. The potential--pH diagram of Sc-HO system at 298K, air pressure of 1 bar and activities of ions in solutions а;=10-6 M with consideration of scandium hydride Рис. 7. Диаграмма потенциал-рЯ системы SC-H2O при 298K, давлении воздуха 1 бар и активностях ионов в растворе а;=10-6 M с учётом гидрида скандия
Fig. 8. The potential-pH diagram of SC-H2O system at 298K, air pressure of 1 bar and activities of ions in solutions a;=10-9 M with consideration of scandium hydride Рис. 8. Диаграмма потенциал-рЯ системы SC-H2O при 298K, давлении воздуха 1 бар и активностях ионов в растворе а;=10-9 M с учётом гидрида скандия
Fig. 9. The potential-pH diagram of Sc-HO system at 298K, air pressure of 1 bar and activities of ions in solutions a;=10-12 M with consideration of scandium hydride Рис. 9. Диаграмма потенциал-рЯ системы SC-H2O при 298K, давлении воздуха 1 бар и активностях ионов в растворе ai=10-12 M с учётом гидрида скандия
Fig. 10. The potential-pH diagram of SC-H2O system at 298K, air pressure of 1 bar and activities of ions in solutions ai=10-15 M with consideration of scandium hydride Рис. io. Диаграмма потенциал-рЯ системы SC-H2O при 298K, давлении воздуха 1 бар и активностях ионов в растворе ai=10-15 M с учётом гидрида скандия
6. The influence of non-stoichiometry of scandium sesquioxide
It was reported [Arkharov, Kichigina, 1966; Horovitz et al., 1975; Young Jr., Sienko, 1972] that scandium sesquioxide has a considerable degree of homogeneity Sc2Ox, 2.89<x<3.
The standard Gibbs free energy of formation of Sc2Ox can be estimated according to the Gorichev's method [Gorichev, Klyushin, 1971] based on Waring - Lagrange interpolation polynomial [Meijering, 2002]. However, the implementation of this method requires reliable data on at least two different oxides of the same metal with different oxidation states. As only data on Sc2O3 (s) are available in literature, the standard Gibbs energy of formation of ScO (s) should be estimated.
V.A. Kireev proposed [1970] that an approximate relationship exists between the standard Gibbs energies of formation of similar compounds of transition metals and the atomic number of elements forming these compounds. Using this method, the standard Gibbs energy of formation of hypothetical scandium monoxide ScO (s) was estimated basing on the data on the standard Gibbs energies of formation of TiO (s), VO (s), MnO (s) and ZnO (s) taken from [Wagman et al., 1982]. The following relationship was found: AfG°8 (MeO)
f 29 -- = -233.4• z^e +13500-zMe -20555Q J• mot1 , (9)
ZMe
where 21 < zMe < 30 is the atomic number of metal. The dependence is illustrated in Figure 11.
From this relationship the standard Gibbs energy of formation of ScO (s) was found to be equal to AfG098 (ScO) = -525550 J• m ol-1. Using this value and that of ScO (s) from
Table 1, the standard Gibbs energy of formation of non-stoichiometric scandium oxide was estimated by the following relationship:
AfG098 (Sc2Ox) = -80900-x2 -363700-x, J• mol-1; 2.89<x<3. (10)
The hypothetical potential-pH diagram of Sc-H2O system at 298 K, air pressure of 1 bar and activities of ions in solutions ai=10-6 M taking into consideration the non-stoichiometry of scandium sesquioxide is presented in Figure 12. Thermodynamic prediction shows that the domain of thermodynamic stability of Sc2Ox decreases with decrease of x.
7. Conclusions
The dimerisation of the ion ScOH2+ (aq) was considered, and it was shown that the dimeric form predominates in concentrated solutions, whereas the monomeric form predominates in diluted media. The polynuclear ions Sc3(OH)45+ (aq) and Sc3(OHV+ (aq) have no domains of thermodynamic stability.
The revised potential-pH diagrams of Sc-H2O system presented in this study, take into account the possible formation of various hydrolysed scandium species with alteration of the activities of ions in a solution. For the solutions with moderate content of scandium these diagrams are consistent with the previously published ones.
The corrosion properties of scandium are similar to that of aluminum; in most environments the passivation layer on the surface of metal consists of ScO (s). The possible formation of scandium hydride ScH2 (s) shifts the borders of scandium active dissolution, but does not affect its properties in the domain of electrochemical stability of water. The scandium nitride ScN (s) is not thermodynamically stable in aqueous solutions in presence of pure scandium and scandium hydride.
An attempt to estimate the corrosion properties of non-stoichiometric scandium oxide Sc2Ox (2.89<x<3) was performed. Using Kireev's method the standard Gibbs energy of formation of ScO (s) was estimated and then, using Gorichev's method the dependency of AG09s (Sc2Ox) on x was also estimated. Thermodynamic prediction shows that with
increase of the degree of homogeneity of Sc2Ox its domain of passivity in the potential-pH diagram narrows.
Fig. 11. The relationship between the standard Gibbs energies of formation of some transition metals monoxides and the atomic numbers of metals Рис. 11. Зависимость между стандартными энергиями Гиббса образования моноксидов некоторых переходных металлов и атомными номерами металлов
0 2 4 б 8 10 12 14 16
рН
Fig. 12. The hypothetical potential-pH diagram of SC-H2O system at 298K, air pressure of 1 bar and activities of ions in solutions a,=10-6 M with consideration of non-stoichiometry of
scandium sesquioxide Рис. 12. Гипотетическая диаграмма потенциал-рЯ системы SC-H2O при 298K, давлении воздуха 1 бар и активностях ионов в растворе a;=10-6 M с учётом нестехиометрии сесквиоксида скандия
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