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On the extraction of selenium (IV) with tri-n-octylammonium chloride from chloride solutions
1. inorg, nucl. Chem. Vol. 40. pp. 1793--1796 © Pergamon Press Ltd., 1978. Printed in Great Britain
0022-190217811001-17931502.0010
ON THE EXTRACTION OF SELENIUM (IV) WITH TRI-n-OCTYLAMMONIUM CHLORIDE FROM CHLORIDE SOLUTIONS C. FISCHER, K. GLOE and P. MOHL Central Institute of Solid State Physics and Material Research, Academy of Sciences of the G.D.R., Dresden. G,D.R. and V. V. BAGREEV V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, U.S.S.R. Academy of Sciences, Moscow, U.S.S.R.
(Received 20 September 1977; recdved for publication 3 April 1978) AP~eact--The extraction of selenium (IV) in the system I x 10-~ M tri-n-octylammoniumchloride/benzene-5x 10-2M selenious acid-I x 10-~ (I) and 5 x 10-2 (1I) M sodium chloride-water was studied as a function of the pH value and the chloride ion concentration using the AKUFVE technique. The extraction mechanism can be characterized by the followingreactions: /(o
TOAH+CI~org)+ l/2(H2SeO3)2taq)~ TOAH+CI~-org~+ 1/2(HSeO3-)2(aq)~
TOAH+CI-'H2SeO3(org), TOAH+HSeO~o,,~+ Cl~-aq)
with /~o = (1.36-+0.16)x 10-2 (I) and (3.21- 0.09)× 10-2 (If) resp., and /(~ = (5.29-+0.29)× 10-3 (I) and (3,22-+0.12) x 10-3 (II) resp. each phase was determined by means, of 75Se. The equilibrium pH values were determined with glass electrodes and the pH meter MV 87 (VEB Pr~icitr0nik,Dresden). The pH changes were obtained by addition of small quantifies of sodium hydroxide or hydrochloric acid solutions. The extraction equilibrium in the investigated pH range (1.2-5) was established within 5 rain. The phase separation occurred without any trouble. 50 values for Ds~ were measured per run. The organic phase (1 x 10-1M TOAH+CI- in benzene) was prepared by shaking a 2.5 x 10-~ M solution of tri-n-octylamine (pract., d ~ 0.82; no2° 1.4476:Fluka AG/Switzerland)in benzene with 1 M hydrochloric acid for a period of 10min, followed by subsequent dilution with benzene.
INTRODUCTION
Little is known about the extraction of selenium (IV) from aqueous chloride solutions with long-chain amines or quarternary ammonium salts, particularly about the species involved. In previous papers[l,2], where selenium (IV) was extracted from strong hydrochloric acid solutions, Se(OH)2CI2 and H2SeO2CI2 were assumed to be the extractable compounds. Nothing is known about the species extracted from weak hydrochloric acid solutions. The above mentioned chlorocomplexes must be excluded because these can only arise at hydrochloric acid concentrations of more than 3M[3]. ,This paper investigates the extraction of selenium (IV) with tri-noctylammonium chloride (TOAH÷Cl-)/benzene from aqueous sodium chloride solution as a function of the equilibrium pH value and the chloride ion concentration of the aqueous phase. Using the least squares method the equilibrium constants of four different model extraction reactions were determined from this data on the distribution coefficients Ds~ and the corresponding pH values. Plots Ds~ vs pH were then calculated and compared with the experimentally determined plots to choose the most probable extraction mechanism,
RESULTS AND DISCUSSIONS
EXPERIMENTAL
The distribution measurements were carried out at 25 + 0.2°C using the AKUFVE technique[4], or, in the case of single experiments, separatory funnels. The selenium concentration in
Figure l(a) shows the measured distribution coefficients Ds~ of the selenious acid (ifiitial concentration 5 x 10-2 M) as a function of the equilibrium pH value at two different chloride ion concentrations. The Ds~ values increase initially from pH 1.2, reaching a maxhnum in the pH range of 2.4 to 2.7, and then decrease by more than one order of magnitude towards pH 5. A variation of the initial concentration of the selenious acid over I x 10-4 to 5x 10-2M has no influence on Ds~. On the other hand, Ds~ is clearly influenced by the chloride ion concentration, as is shown by Fig. l(a). The experimental results show that the distribution coefficients depend to a considerable extent on the
1793
1794
C. FISCHER
etal.
tractedbyan anionexchangemechanism.This is supported by the fact that Ds~ decreases by more than one order when the extractant tri-n-octylammonium chloride (1 x 10-1M in benzene) is replaced by the much weaker exchanging tri-n-octylammonium perchlorate (D~, only 8.6 x 10-4 instead of 2.7 x 10-2 at pH 1.83). Next to anionic species the extraction of neutral species, such as undissociated selenious acid, must be taken into consideration under the given conditions. In this case, too, tri-n-octylammonium chloride must be participating in the reaction, possibly by the formation of an adduct, because the extraction of a solution of 5 x 10-2 M selenious acid with pure benzene gave Ds~ values <~3 x 10-4 within the pH range 1.3 to 4.3. All species listed by Barcza and Silltn[5] as a result of e.m.f, titration of different selenium (IV) aqueous solutions were taken into account when drawing up the model extraction reactions. The species are:
3-
//
\
SeO32-, (HSeO3.SeO3) 3-, HSeO3-, (HSeO3-)2, (HSeO3.H2SeO3)-, H2SeO3 and (H2SeO3)2.
3- ~ , \
/\
\\
2-
•
9
\
an
-
Fig. I. Distribution coefficient Ds~ in dependence of the equilibrium pH value of the aqueous phase. (a) Plots of Ds~ vs pH (measured values) Organic phase: I x 10-~ M TOAH÷CI- in benzene; Aqueous phase: 5 x 10-2 M H2SeO3 (0----(3); 1 × 10-~ M (run I); and 5x 10-2M (run II) NaCI resla. (b) Plots of Ds~ vs pH (measured and calculated values, run I) © (3 ...., measured; , calculated acc. to model I; ...... , calculated acc. to model 2; ., calculated acc. to model 3; , calculated acc. to model 4. (c) Plots of Ds~ vs pH (measured and calculated values, run II) Symbols cf. lb.
Table 1. Concentration constants /(o and /(;, calculated according to model 4 CI- concentration (M)
I x 10-~
5 x 10-2
The association of selenious acid and hydrogenselenite in aqueous solution as a result of pronounced hydrogen bonding has been established by previous IR and Raman spectroscopic investigations [7-9]. Trinuclear and higher species can be excluded according to[5]. This is compatible with the results of electrical and optical measurements on aqueous selenium (IV) solutions[6] according to which the dimer (H2SeO3)2 predominates at the concentration of 5 x 10-2 M used in this study. At the same time the neutral selenite ion SeO32- can be neglected because it is not present in our pH range; the same applies to (HSeO3.SeO3) 3-. Since, as already mentioned, the extraction of selenium (IV) is independent of the selenium concentration the binuclear (HSeO3.H2SeO3)-, which has a range of existence between pH 1 and 415], can be excluded as a species participating in the extraction. This leaves HSeO3-, (HSeO3-)2, H2SeO3 and (H2SeO3)2 as possible species participating in the extraction. With these species four models of the extraction mechanism were drawn up. In the first two cases extraction of only one species was assumed in order to check the importance of the individual species and their reactions in relation to the models. The evaluation of the ~ constants is described with reference to model 4. The tri-n-octylammonium compounds arising in the reactions were treated as monomers. For models 1-3 the following reactions are assumed to take place: TOAH+CI~o,g) + HSeO~(,~o ~ TOAH+HSeO ~-(org) +CI~q) (rood. 1) +
/(0x 102 /(~x 103
1.36-+0.16 3.21-+0.09 5.29-+0.29 3.22-+0.12
]
+
(1)
--
TOAH C!~ors)+ ~ (HSeO3-)2~,,o~TOAH HSeO3(ors) + CI~,o (rood. 2)
(2)
TOAH+CIFog) + H2SeO3(~o ~ TOAH+CI -'H2SeO3(orS) composition of the aqueous phase, and consequently on the equilibria between the different species of selenium (IV). The decrease of Ds~ with increasing chloride ion concentration (pH=constant) indicates that anionic species, such as selenite or hydrogenselenite, are ex-
(3)
and TOAH+CI~-org)+ HSeO ~(~,o~ TOAH+HSeO~org) + CI?,,o (rood. 3).
(4)
On the extraction of selenium(IV) with tri-n-octylammoniumchloride In the case of model 4 it is assumed that dimer selenious acid and dimer hydrogenselenite are present in the aqueous phase, and that the extraction of selenious acid occurs by formation of an adduct with tri-n-octylammonium chloride, and that anion exchange extraction takes place with the participation of hydrogenselenite.
1795
where the function f is linear in the parameters kl, k2. The quantities kl, k2 can be determined by the least squares method (quasi-linear regression) [11]. Numerical evaluation was carried out by means of a programmable desk calculator. The calculated constants, including the standard deviations, are listed in Table 1. On the basis of the established values /~, the cor1 + responding Dse values were calculated for the given pH TOAH'CI~-or,>+ ~ (H2SeO3)2.,,0~ TOAH Cl-'H2SeO3(or,) values, and then compared with the experimentally (5) established Ds~ values. The corresponding plots Ds~ vs pH for the two chloride ion concentrations are + l + shown in Figs. l(b) and l(c). The plots Ds~ vs pH, using TOAH CI~o~,~+~(HSeO3-h(~)~TOAH HSeO3(o~,~ model 1, are incompatible with the data over the entire + CI ~aq) (6) pH range. On the other hand the curves using model 2, which is based on the sole extraction of dimer hydrogen[TOAH+CI-'H2SeO3]or, Ko = + 1/2 (7) selenite, agreed fairly well in the pH range of 3 to 5. This [TOAH CI ](org)'[(H2SeO3)2](~.) indicates that selenium (IV) is mainly extracted by the K [TOAH+HSeO3-](o~,)[CI-](~q) anion exchange mechanism within this range. On the ,= [ ~ ~ , (8) other hand the lack of conformity at pH values of <3 indicates that a significant process arising during exThe chloride ion concentration remains approximately traction is not taken into account by model 2. In spite of constant in the two phases because Ds~ does not exceed the introduction of a second extraction reaction--the the value 3.2x10 -2 over the entire pH range in- extraction of monomer selenious acid is assumed with vestigated. Consequently, the activity coefficients of all the anion exchange extraction of monomer hydrocomponents of the system, to a first approximation, can genselenite--the conformity of the curves using model 3 be regarded as constant. The calculation of the constants with the experimentally established values is completely Ko and Kt for the reactions (5) and (6) is therefore based unsatisfactory over the entire pH range. This indicates on the concentrations. Thus, Ko and K, represent that the monomer species must be insignificant in the concentration constants which are concentration-in- extraction reaction. Only the assumption, in accordance dependent only over a limited range of concentration so with model 4, that the dimer selenious acid and the dimer that the eqns (7) and (8) can be transformed in the hydrogensetenite are the components participating in the following manner: extraction reactions, produces good conformity of the calculated plots Ds~ vs pH with the experimentally /(o = (TOAH÷CI-'H2SeO3)t°rg) established values. (9) B × [(H2Se03)2] (aq) 1/2 In the given system I x 10-~M tri-n-octylammonium chloride/benzene, 5 x 10-2 M selenious acid 5 x 10-2 and /~ (TOAH+HSeO3-)(o~) (10) 1 × 10-I M NaCI resp. (pH 1.2 to 5), the eqns (5) and (6) ' = A × [(HSeO3-h]~'~) thus represent the most probable extraction mechanism B = (TOAH+C1-)(or,)= 1 x 10-' M (11) for the selenium (IV). The calculation of the distribution of the extractable species on the basis of the eqn (14) and A = TOAH+CI-)(°~*)- 1 (run I) and (12) the calculated values for/~o a n d / ~ of model 4 revealed, (Cl-)(aq) 2 (run II) resp.' that a small decrease of the extraction of selenious acid occurs between pH 1.2 and about pH 2.5, a strong Ds~, as a quotient of the analytical total concentration of decrease, however, from this value to pH 5, while the selenium (IV) in the organic and aqueous phase, can be extraction of hydrogenselenite reaches a maximum represented according to eqns (9) and (10) in the follow- within the pH range of 2.2 to 3. ing manner: The influence of the chloride ion concentration on the Ds~ values at a given pH value, as well as on the position Ds~ = (TOAH+CI-'H2SeO3)(°~*)+ (TOAH+HSeO3-)(°~*) of the maxima of the plots Ds~ vs pH, do not appear to [(H2SeO3)2](aq) + [(HSeO3-)2]taq) be based solely on the equilibria of eqn (6), but also on a (13) correlation between the ion strength of the aqueous phase and the stability of the binuclear species. AccordIntroducing the dissociation constant KDt of selenious ing to [5] the stability of the binuclear species of selenium acid, for which the value (KDI = 1.78x 10-3 at 25°C) (IV) increases with decreasing foreign ion concentration determined by Kawassiades et al.[10] was used, as well in the aqueous solution. The rise of the Ds~ values with as the total concentration c of the selenious acid decreasing chloride ion concentration appears to be due (ctH2s~om=2.5x 10-2M), the following equation is to the two effects acting in the same direction. obtained Finally, the preferred reaction of the dimer species with the extractant--in comparison with the monomers-/~'ox B x [H+] 2 + K, x A x KOl x [H +] D.~ = c m([H+]2 + K~I) (14) is obvious due to the fact, that the larger species with little or no hydration will interact more readily with the amine salt which has an unhydrated and large organic The eqn (14) can be represented in the form of the cation[12]. Consequently, the extraction mechanisms following relation: implied in eqns (5) and (6) are the most probable. Hence, the data for the extraction of selenious acid can be y = / ( x ) = k,l,(x)+kd~(x) (15) described reliably in the given system, over the in-
17%
C. FISCHER et al.
vestigated pH range of 1.2 to 5 by means of the concentration constants Ko and/~I in accordance with model 4. Acknowledgements--The authors wish to express their gratitude to Dr. W. Kluge and G. Maltzahn, Ing., for the mathematical processing of the experimental material and for the helpful discussions in this field. ~ N C ~ 1. G. Nakagava, J. Chem. Soc. Japan. Pure Chem. Soc. 81, 1258 A 87 (1%0). 2. V. F. Borbat and A. V. Bugaeva, Z. Neorg. Chim. 12, 1293 (1967). 3. A. K. Babko and T. T. Mitjureva, Z. Neorg. Chim. 6, 421 (1%1). 4. I. Rydberg, H. Reinhardt and I. O. Liljenzin (Edited by J. A. Marinsky and Y. Marcus), In Ion Exchange and Solvent Extraction, Vol. 3, p. 111-135. Marcel Dekker, New York (1973).
5. L. Barcza and L. H. Sill~n, Acta Chem. Scand. 26, 1250 (1971). 6. G. Narain and L. N. Srivastava, Z. Neorg. Chim. 20, 1184 (1975). 7. A. Simon and R. Paetzold, Z Anorg. AUg. Chem. 301,246 (1959). 8. A. Simon and R. Paetzold, Z. Anorg. Allg. Chem. 303, 46 (1%0). 9. G. E. Walrafen, I. Chem. Phys. 32, 1468(1%2). 10. C. Th. Kawassiades, G. E. Manoussakisand J. A. Tassidis, J. Inorg. Nucl. Chem. 29, 401 (1967). 11. V. Nollau, Statistische Analysen: Mathematische Methoden der Planung und Auswertung yon Versuchen. Fachbuchverlag, Leipzig (1975). 12. Y. Marcus and A. S. Kertes, Ion Exchange and Solvent Extraction of Metal Complexes, p. 738. Wiley-lnterscience, London (i969).
0022-190217811001-17931502.0010
ON THE EXTRACTION OF SELENIUM (IV) WITH TRI-n-OCTYLAMMONIUM CHLORIDE FROM CHLORIDE SOLUTIONS C. FISCHER, K. GLOE and P. MOHL Central Institute of Solid State Physics and Material Research, Academy of Sciences of the G.D.R., Dresden. G,D.R. and V. V. BAGREEV V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, U.S.S.R. Academy of Sciences, Moscow, U.S.S.R.
(Received 20 September 1977; recdved for publication 3 April 1978) AP~eact--The extraction of selenium (IV) in the system I x 10-~ M tri-n-octylammoniumchloride/benzene-5x 10-2M selenious acid-I x 10-~ (I) and 5 x 10-2 (1I) M sodium chloride-water was studied as a function of the pH value and the chloride ion concentration using the AKUFVE technique. The extraction mechanism can be characterized by the followingreactions: /(o
TOAH+CI~org)+ l/2(H2SeO3)2taq)~ TOAH+CI~-org~+ 1/2(HSeO3-)2(aq)~
TOAH+CI-'H2SeO3(org), TOAH+HSeO~o,,~+ Cl~-aq)
with /~o = (1.36-+0.16)x 10-2 (I) and (3.21- 0.09)× 10-2 (If) resp., and /(~ = (5.29-+0.29)× 10-3 (I) and (3,22-+0.12) x 10-3 (II) resp. each phase was determined by means, of 75Se. The equilibrium pH values were determined with glass electrodes and the pH meter MV 87 (VEB Pr~icitr0nik,Dresden). The pH changes were obtained by addition of small quantifies of sodium hydroxide or hydrochloric acid solutions. The extraction equilibrium in the investigated pH range (1.2-5) was established within 5 rain. The phase separation occurred without any trouble. 50 values for Ds~ were measured per run. The organic phase (1 x 10-1M TOAH+CI- in benzene) was prepared by shaking a 2.5 x 10-~ M solution of tri-n-octylamine (pract., d ~ 0.82; no2° 1.4476:Fluka AG/Switzerland)in benzene with 1 M hydrochloric acid for a period of 10min, followed by subsequent dilution with benzene.
INTRODUCTION
Little is known about the extraction of selenium (IV) from aqueous chloride solutions with long-chain amines or quarternary ammonium salts, particularly about the species involved. In previous papers[l,2], where selenium (IV) was extracted from strong hydrochloric acid solutions, Se(OH)2CI2 and H2SeO2CI2 were assumed to be the extractable compounds. Nothing is known about the species extracted from weak hydrochloric acid solutions. The above mentioned chlorocomplexes must be excluded because these can only arise at hydrochloric acid concentrations of more than 3M[3]. ,This paper investigates the extraction of selenium (IV) with tri-noctylammonium chloride (TOAH÷Cl-)/benzene from aqueous sodium chloride solution as a function of the equilibrium pH value and the chloride ion concentration of the aqueous phase. Using the least squares method the equilibrium constants of four different model extraction reactions were determined from this data on the distribution coefficients Ds~ and the corresponding pH values. Plots Ds~ vs pH were then calculated and compared with the experimentally determined plots to choose the most probable extraction mechanism,
RESULTS AND DISCUSSIONS
EXPERIMENTAL
The distribution measurements were carried out at 25 + 0.2°C using the AKUFVE technique[4], or, in the case of single experiments, separatory funnels. The selenium concentration in
Figure l(a) shows the measured distribution coefficients Ds~ of the selenious acid (ifiitial concentration 5 x 10-2 M) as a function of the equilibrium pH value at two different chloride ion concentrations. The Ds~ values increase initially from pH 1.2, reaching a maxhnum in the pH range of 2.4 to 2.7, and then decrease by more than one order of magnitude towards pH 5. A variation of the initial concentration of the selenious acid over I x 10-4 to 5x 10-2M has no influence on Ds~. On the other hand, Ds~ is clearly influenced by the chloride ion concentration, as is shown by Fig. l(a). The experimental results show that the distribution coefficients depend to a considerable extent on the
1793
1794
C. FISCHER
etal.
tractedbyan anionexchangemechanism.This is supported by the fact that Ds~ decreases by more than one order when the extractant tri-n-octylammonium chloride (1 x 10-1M in benzene) is replaced by the much weaker exchanging tri-n-octylammonium perchlorate (D~, only 8.6 x 10-4 instead of 2.7 x 10-2 at pH 1.83). Next to anionic species the extraction of neutral species, such as undissociated selenious acid, must be taken into consideration under the given conditions. In this case, too, tri-n-octylammonium chloride must be participating in the reaction, possibly by the formation of an adduct, because the extraction of a solution of 5 x 10-2 M selenious acid with pure benzene gave Ds~ values <~3 x 10-4 within the pH range 1.3 to 4.3. All species listed by Barcza and Silltn[5] as a result of e.m.f, titration of different selenium (IV) aqueous solutions were taken into account when drawing up the model extraction reactions. The species are:
3-
//
\
SeO32-, (HSeO3.SeO3) 3-, HSeO3-, (HSeO3-)2, (HSeO3.H2SeO3)-, H2SeO3 and (H2SeO3)2.
3- ~ , \
/\
\\
2-
•
9
\
an
-
Fig. I. Distribution coefficient Ds~ in dependence of the equilibrium pH value of the aqueous phase. (a) Plots of Ds~ vs pH (measured values) Organic phase: I x 10-~ M TOAH÷CI- in benzene; Aqueous phase: 5 x 10-2 M H2SeO3 (0----(3); 1 × 10-~ M (run I); and 5x 10-2M (run II) NaCI resla. (b) Plots of Ds~ vs pH (measured and calculated values, run I) © (3 ...., measured; , calculated acc. to model I; ...... , calculated acc. to model 2; ., calculated acc. to model 3; , calculated acc. to model 4. (c) Plots of Ds~ vs pH (measured and calculated values, run II) Symbols cf. lb.
Table 1. Concentration constants /(o and /(;, calculated according to model 4 CI- concentration (M)
I x 10-~
5 x 10-2
The association of selenious acid and hydrogenselenite in aqueous solution as a result of pronounced hydrogen bonding has been established by previous IR and Raman spectroscopic investigations [7-9]. Trinuclear and higher species can be excluded according to[5]. This is compatible with the results of electrical and optical measurements on aqueous selenium (IV) solutions[6] according to which the dimer (H2SeO3)2 predominates at the concentration of 5 x 10-2 M used in this study. At the same time the neutral selenite ion SeO32- can be neglected because it is not present in our pH range; the same applies to (HSeO3.SeO3) 3-. Since, as already mentioned, the extraction of selenium (IV) is independent of the selenium concentration the binuclear (HSeO3.H2SeO3)-, which has a range of existence between pH 1 and 415], can be excluded as a species participating in the extraction. This leaves HSeO3-, (HSeO3-)2, H2SeO3 and (H2SeO3)2 as possible species participating in the extraction. With these species four models of the extraction mechanism were drawn up. In the first two cases extraction of only one species was assumed in order to check the importance of the individual species and their reactions in relation to the models. The evaluation of the ~ constants is described with reference to model 4. The tri-n-octylammonium compounds arising in the reactions were treated as monomers. For models 1-3 the following reactions are assumed to take place: TOAH+CI~o,g) + HSeO~(,~o ~ TOAH+HSeO ~-(org) +CI~q) (rood. 1) +
/(0x 102 /(~x 103
1.36-+0.16 3.21-+0.09 5.29-+0.29 3.22-+0.12
]
+
(1)
--
TOAH C!~ors)+ ~ (HSeO3-)2~,,o~TOAH HSeO3(ors) + CI~,o (rood. 2)
(2)
TOAH+CIFog) + H2SeO3(~o ~ TOAH+CI -'H2SeO3(orS) composition of the aqueous phase, and consequently on the equilibria between the different species of selenium (IV). The decrease of Ds~ with increasing chloride ion concentration (pH=constant) indicates that anionic species, such as selenite or hydrogenselenite, are ex-
(3)
and TOAH+CI~-org)+ HSeO ~(~,o~ TOAH+HSeO~org) + CI?,,o (rood. 3).
(4)
On the extraction of selenium(IV) with tri-n-octylammoniumchloride In the case of model 4 it is assumed that dimer selenious acid and dimer hydrogenselenite are present in the aqueous phase, and that the extraction of selenious acid occurs by formation of an adduct with tri-n-octylammonium chloride, and that anion exchange extraction takes place with the participation of hydrogenselenite.
1795
where the function f is linear in the parameters kl, k2. The quantities kl, k2 can be determined by the least squares method (quasi-linear regression) [11]. Numerical evaluation was carried out by means of a programmable desk calculator. The calculated constants, including the standard deviations, are listed in Table 1. On the basis of the established values /~, the cor1 + responding Dse values were calculated for the given pH TOAH'CI~-or,>+ ~ (H2SeO3)2.,,0~ TOAH Cl-'H2SeO3(or,) values, and then compared with the experimentally (5) established Ds~ values. The corresponding plots Ds~ vs pH for the two chloride ion concentrations are + l + shown in Figs. l(b) and l(c). The plots Ds~ vs pH, using TOAH CI~o~,~+~(HSeO3-h(~)~TOAH HSeO3(o~,~ model 1, are incompatible with the data over the entire + CI ~aq) (6) pH range. On the other hand the curves using model 2, which is based on the sole extraction of dimer hydrogen[TOAH+CI-'H2SeO3]or, Ko = + 1/2 (7) selenite, agreed fairly well in the pH range of 3 to 5. This [TOAH CI ](org)'[(H2SeO3)2](~.) indicates that selenium (IV) is mainly extracted by the K [TOAH+HSeO3-](o~,)[CI-](~q) anion exchange mechanism within this range. On the ,= [ ~ ~ , (8) other hand the lack of conformity at pH values of <3 indicates that a significant process arising during exThe chloride ion concentration remains approximately traction is not taken into account by model 2. In spite of constant in the two phases because Ds~ does not exceed the introduction of a second extraction reaction--the the value 3.2x10 -2 over the entire pH range in- extraction of monomer selenious acid is assumed with vestigated. Consequently, the activity coefficients of all the anion exchange extraction of monomer hydrocomponents of the system, to a first approximation, can genselenite--the conformity of the curves using model 3 be regarded as constant. The calculation of the constants with the experimentally established values is completely Ko and Kt for the reactions (5) and (6) is therefore based unsatisfactory over the entire pH range. This indicates on the concentrations. Thus, Ko and K, represent that the monomer species must be insignificant in the concentration constants which are concentration-in- extraction reaction. Only the assumption, in accordance dependent only over a limited range of concentration so with model 4, that the dimer selenious acid and the dimer that the eqns (7) and (8) can be transformed in the hydrogensetenite are the components participating in the following manner: extraction reactions, produces good conformity of the calculated plots Ds~ vs pH with the experimentally /(o = (TOAH÷CI-'H2SeO3)t°rg) established values. (9) B × [(H2Se03)2] (aq) 1/2 In the given system I x 10-~M tri-n-octylammonium chloride/benzene, 5 x 10-2 M selenious acid 5 x 10-2 and /~ (TOAH+HSeO3-)(o~) (10) 1 × 10-I M NaCI resp. (pH 1.2 to 5), the eqns (5) and (6) ' = A × [(HSeO3-h]~'~) thus represent the most probable extraction mechanism B = (TOAH+C1-)(or,)= 1 x 10-' M (11) for the selenium (IV). The calculation of the distribution of the extractable species on the basis of the eqn (14) and A = TOAH+CI-)(°~*)- 1 (run I) and (12) the calculated values for/~o a n d / ~ of model 4 revealed, (Cl-)(aq) 2 (run II) resp.' that a small decrease of the extraction of selenious acid occurs between pH 1.2 and about pH 2.5, a strong Ds~, as a quotient of the analytical total concentration of decrease, however, from this value to pH 5, while the selenium (IV) in the organic and aqueous phase, can be extraction of hydrogenselenite reaches a maximum represented according to eqns (9) and (10) in the follow- within the pH range of 2.2 to 3. ing manner: The influence of the chloride ion concentration on the Ds~ values at a given pH value, as well as on the position Ds~ = (TOAH+CI-'H2SeO3)(°~*)+ (TOAH+HSeO3-)(°~*) of the maxima of the plots Ds~ vs pH, do not appear to [(H2SeO3)2](aq) + [(HSeO3-)2]taq) be based solely on the equilibria of eqn (6), but also on a (13) correlation between the ion strength of the aqueous phase and the stability of the binuclear species. AccordIntroducing the dissociation constant KDt of selenious ing to [5] the stability of the binuclear species of selenium acid, for which the value (KDI = 1.78x 10-3 at 25°C) (IV) increases with decreasing foreign ion concentration determined by Kawassiades et al.[10] was used, as well in the aqueous solution. The rise of the Ds~ values with as the total concentration c of the selenious acid decreasing chloride ion concentration appears to be due (ctH2s~om=2.5x 10-2M), the following equation is to the two effects acting in the same direction. obtained Finally, the preferred reaction of the dimer species with the extractant--in comparison with the monomers-/~'ox B x [H+] 2 + K, x A x KOl x [H +] D.~ = c m([H+]2 + K~I) (14) is obvious due to the fact, that the larger species with little or no hydration will interact more readily with the amine salt which has an unhydrated and large organic The eqn (14) can be represented in the form of the cation[12]. Consequently, the extraction mechanisms following relation: implied in eqns (5) and (6) are the most probable. Hence, the data for the extraction of selenious acid can be y = / ( x ) = k,l,(x)+kd~(x) (15) described reliably in the given system, over the in-
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C. FISCHER et al.
vestigated pH range of 1.2 to 5 by means of the concentration constants Ko and/~I in accordance with model 4. Acknowledgements--The authors wish to express their gratitude to Dr. W. Kluge and G. Maltzahn, Ing., for the mathematical processing of the experimental material and for the helpful discussions in this field. ~ N C ~ 1. G. Nakagava, J. Chem. Soc. Japan. Pure Chem. Soc. 81, 1258 A 87 (1%0). 2. V. F. Borbat and A. V. Bugaeva, Z. Neorg. Chim. 12, 1293 (1967). 3. A. K. Babko and T. T. Mitjureva, Z. Neorg. Chim. 6, 421 (1%1). 4. I. Rydberg, H. Reinhardt and I. O. Liljenzin (Edited by J. A. Marinsky and Y. Marcus), In Ion Exchange and Solvent Extraction, Vol. 3, p. 111-135. Marcel Dekker, New York (1973).
5. L. Barcza and L. H. Sill~n, Acta Chem. Scand. 26, 1250 (1971). 6. G. Narain and L. N. Srivastava, Z. Neorg. Chim. 20, 1184 (1975). 7. A. Simon and R. Paetzold, Z Anorg. AUg. Chem. 301,246 (1959). 8. A. Simon and R. Paetzold, Z. Anorg. Allg. Chem. 303, 46 (1%0). 9. G. E. Walrafen, I. Chem. Phys. 32, 1468(1%2). 10. C. Th. Kawassiades, G. E. Manoussakisand J. A. Tassidis, J. Inorg. Nucl. Chem. 29, 401 (1967). 11. V. Nollau, Statistische Analysen: Mathematische Methoden der Planung und Auswertung yon Versuchen. Fachbuchverlag, Leipzig (1975). 12. Y. Marcus and A. S. Kertes, Ion Exchange and Solvent Extraction of Metal Complexes, p. 738. Wiley-lnterscience, London (i969).