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Influence of the ionic strength on the formation and extraction of the strontium-cryptand 2.2.2-eosin ion-pair complex
Polyhedron Vol. 8, No. 23, pp. 2797-2801, 1989 Printed in Great Britain
0277-5387/89 $3.00+.00 0 1989 Pergamon Press plc
INFLUENCE OF THE IONIC STRENGTH ON THE FORMATION AND EXTRACTION OF THE STRONTIUMCRYPTAND 2.2.2-EOSIN ION-PAIR COMPLEX D. BLANC0
Departamento
GOMIS,* E. EUENTE ALONSO and P. ARIAS ABRODO
de Quimica Fisica y Analitica, Facultad de Quimica, Universidad de Oviedo, 33006 Oviedo, Spain (Received 31 March 1989 ; accepted 20 Jufy 1989)
Abstract-A
systematic study of the factors affecting the extraction of strontium, as an ionpair formed between the positively charged cryptate of strontium with cryptand 2.2.2 and the eosinate anion, into chloroform has been carried out. The pKl and pK2 values of the ligand, the stability constant of the cryptate inclusion complex, as well as the partition coefficient and the extraction constant of the ion-pair, were measured as a function of the ionic strength of the solution. The pK, and KS values remained reasonably constant at ionic strength < 0.1 M. Nevertheless, the partition coefficient values, and consequently the extraction constant values, decrease as the ionic strength increases.
Recently, a large number of papers dealing with the extraction constants of crown- and cryptand-ether complexes of alkali and alkaline-earth ions associated with coloured or fluorescent anions have been published. Much less work has been carried out on analogous systems involving transition metals as well as lanthanide and actinide ions. The overall extraction equilibrium formulated by Frensdorff ’ can be rewritten as M”++L+A”-s(MLA),,
M”++L- K,\
ML”+ +A= ~(MLA),. The two equilibrium constants are KS, the aqueous stability constant of the binary complex, and PC-, the partition coefficient of the complex from separate ions in the aqueous phase into ion-pairs in the organic phase. Thus the overall extraction equilibrium constant is given by
(1)
where M”+, L, A”- and (MLA), are the metal cation, the cyclic polyether, the counter-ion and the ion-pair, respectively. The subscript o and the absence of a subscript denote the organic and the aqueous phase, respectively. The equilibrium constant of eq. (1) was designated as the extraction constant, K,, :
ML”+
%, = K,Rc,. If the ligand is a diamine or cryptand the following acid-base equilibria take place : LH;+&LH++H+ LH +f‘L+H+.
Addition of a metal cation, M”+, will thus affect the basicity of a solution of the free ligand. The role of these equilibria will be considered in detail in the where the brackets denote equilibrium activities. This extraction equilibrium may be analysed in Experimental. However, in more recent publications the extracterms of two constituent equilibria which, when tion constants have been reported using conadded to each other, give eq. (1) : centrations (e) instead of activities, and the *Author to whom correspondence should be addressed. extraction method has been applied at appreciable L
=
[MLAl,/W+I[Y[A”-I,
2791
2798
D. BLANC0 GOMIS et al.
Structure 1. Molecular structure of the dianionic form of eosin. ionic strengths, e.g. for the determination of K+ in blood serum2,3 or Pb2+ in tap water4 and soft drinks,5 where Kz$” differs considerably from K,. In our earlier studies we have observed a critical influence of ionic strength on the extraction of some metal ions, e.g. K+, Sr2+, Ca2+ or Pb2+, with crown- and cryptand-ethers. Thus, while the extraction of potassium with 18-crow~6~ or lead with 18-crown-64 or cryptand 2.2.2’ is not affected by this parameter (probably due to the formation of polymeric species in the organic phase), an increase in the ionic strength promotes a sharp decrease in the extraction of lead with cryptand 2.2.1 6or strontium with cryptand 2.2.2,7 using eosin as the counter-ion, and consequently a decrease in the fluorescence intensity of the organic phase. This factor could be a problem in the analytical application of these extraction systems. For this, we are at present engaged in a detailed study of various factors which affect the extraction constants of potassium, strontium and lead, including extraction of polymeric species and effects of counter-ion and solvents for the extraction. In the present study we report the effect of ionic strength on the extraction of strontium using cryptand 2.2.2 (L) as the ligand, eosinate (A) as the counter-ion and chloroform as the extractant. On this account, we have studied the effect that the variation of the ionic strength causes on the different constants involved in the extraction process: the dissociation constants of the ligand, the stability constant of the cryptate inclusion complex, the partition coefficient and the extraction constant of the ion-pair, and the partition coefficient of the complex. EXPERIMENTAL
Reagents and equipment All reagents were of analytical grade and doubly distilled and demineralized water was used throughout.
Cryptand 2.2.2. The commercial product (Kryptofix 2.2.2 from Merck) was used as received. An aqueous stock solution (1.689 x lop4 M) of the cryptand was stored in a PVC container and was diluted as required before use. Strontium(I1) stock solution (0.0114 M). Prepared by dissolving 1.2063 g of strontium nitrate in water, diluting to 500 cm3 and standardizing by complexometric titration. All working standard solutions (5.772 x lop5 M) were freshly prepared by appropriate dilution of the stock solution. Eosin acid aqueous solution (8.970 x 10e3 M). Prepared by dissolving 0.1453 g of eosin in 25 cm3 of basic water. Eosin was synthesized by reaction of Br-/BrO; with fluorescein in an acidic wateracetone mixture as described elsewhere.’ Tetramethylammonium hydroxide (4.388 x 10m2 M). The (CH,),N+ cation would be large enough so as not to enter the intramolecular cavity of the ligand. The basic solutions were made from commercial Fluka AG reagent. Preliminary titration showed about 3% of the latter to be carbonated. In order to avoid carbonation of these basic solutions argon was bubbled through them and they were stored in PVC containers. The instrumentation included a UV-vis spectrophotometer (Perkin-Elmer, model 124) with quartz cells (10 mm path), an LS-5 Perkin-Elmer spectrofluorimeter equipped with a Model 3600 data-station, an inductively coupled plasma Perkin-Elmer spectrometer model 5000, a Crison-501 potentiometer and a Dosimat automatic microburet. A thermostatted shaker (Grant) with a time switch was used for horizontal shaking of the stoppered tubes. Potentiometric titrations To determine the protonation and the stability constants (K,, K2 and KS), solutions of the protonated ligands (0.02 M) alone and in the presence of Sr2+ (0.01 M) were titrated with a strong base, (CH3)4NOH (0.073 M). Variable concentrations of supporting electrolyte, (CH,),NBr, in order to maintain the ionic strength (0.01,0.05 and 0.10 M) in the test solution and in the titrant, were used. Calibration of the cell was achieved with HN03 solutions at different ionic strengths by titration with tetramethylammonium hydroxide. The potential of the solutions was measured with the following cell : glass electrode/test solution (I = O.Ol0.10 M, (CH,),NBr)/O.Ol-O.lO M (CH3)4NBr, SCE, in order to avoid diffusion phenomena of potassium from the calomel electrode. All measurements were performed in a glass cell maintained at 25 f O.l"C. Portions of 0.01 cm3 of
Factors affecting the extraction of strontium
2799
Partition coeficient of cryptand 2.2.2
Fig. 1. Potential measurements (E) as a function of the volume of alkali added (v) to solutions containing : (a) strong acid (HNO,) ; (b) strong acid and ligand (cryptand 2.2.2); (c) strong acid, ligand and metal ion (Sr*+); Z = 0.01 M.
Partition coefficients, P(L), at different ionic strengths were determined by equilibration of equal volumes of the cryptand solution in water and in chloroform, followed by spectrofluorimetric determination of the amount of cryptand remaining in the aqueous phase. The determination was carried out by addition of eosin and strontium to the aqueous phase and extraction of the cryptand-strontium-eosinate complex into chloroform, following the general extraction procedure described elsewhere.’ The concentration found in this way in the aqueous phase was subtracted from the initial total concentration of cryptand to deduce the concentration in the organic phase. RESULTS AND DISCUSSION
titrant were added from an automatic microburet. The titration cell was purged with argon which had previously been saturated with water at 25°C. A typical titration curve is shown in Fig. 1. The results were analysed using a general equilibrium-solving program, MINIPOT, to calculate the protonation and the formation constants. Extraction procedure An aliquot (5 cm’) of an aqueous solution containing the metal ion (1.154 x 1O- ’ M), the cryptand solution (5.067 x lo-’ M), the (CH&NOH solution (4.388 x lo- 3 M, pH = 11.5), the eosinate anion for the formation of the ion-pair (9.440 x 10p6-1 .4 x 10m4 M ; this concentration range allows extraction yields ranging from 15 to 85% to be obtained), and variable concentrations of (CH3)4NBr solution in order to keep the ionic strength at 0.01, 0.05 or 0.10 M, was placed in a glass cylindrical tube with a PVC stopper. After the addition of 5 cm3 of chloroform, the mixture was shaken for 60 min at 25 f 0.1 “C. The solvents were saturated with each other before use to prevent volume changes in the phases during extraction. The mixture was centrifuged for 5 min to separate the phases, the pH of the aqueous phase was checked and the concentration of the metal ion was measured by ICP emission photometry at 407.77 nm. Eosin equilibrium concentrations in the aqueous layer were measured spectrophotometrically at 516 nm. Strontium concentrations in the organic phases were determined as follows: 4 cm3 of the chloroform phase were allowed to evaporate, the residue was dissolved in 10% nitric acid solution and determined by ICP emission photometry.
Table 1 shows the results obtained for the concentration equilibrium constants, K,, of complexes of cryptand 2.2.2 with Sr2+ at different ionic strengths, as well as the pK,, pK2 and P(L) values of the ligand. As can be seen, in the ionic strength range 0.01-0.10 M, the concentration formation constant remains reasonably constant and close to values from the literature.“*” In a similar manner, the pK, and P(L) values of the ligand present a similar behaviour. Distribution coejkient When a divalent metal ion M2+, such as Sr2+, forms a complex with a cryptand, L, such as cryptand 2.2.2 and the complex is extractable as an ionpair with large anions, A2-, such as the eosinate ion, the distribution coefficient (D) for the extraction system pM2++qL+rA2--
-
(M,L,EJ,
may be written as D = c
W2+loICW2+l p34.r =
1
P[M,L,W[M~+~+WL2+l + 2 [M,(OH),Z’-3, (2) I
Table 1. K, pK,, pK, and P(L) values for the different ionic strengths Z
log K,
0.01 8.25f0.09 0.05 8.11kO.03 0.10 7.96kO.05
PK,
pK2
P(L)
7.17+0.04 7.27f0.02 7.26f.O.01
9.97kO.03 9.66kO.02 9.71kO.05
89.38kO.62 89.39kO.84 89.42kO.76
D. BLANC0
2800
GOMIS et al. Table 2. PCTand X;, values for the strontium-cryptand 2.2.2-eosin system in chloroform at different ionic strengths
P
50
I
I
4.5
4.0
-log CEOSINI
Fig. 2. Dependence of the distribution ratio for strontium on the eosin concentration in the aqueous phase at different ionic strengths: (a) Z= 0.01 M, [2.2.2] = 5.07x 10m5 M; 0 [Sr*+] = 1.14x 10e5 M, [eosin] = (1.07-8.75) x 1O-5 M; * [Sr’+] = 5.71 x 10m6 M, [eosin] = (0.72-3.87) x lo- 5 M. (b) Z = 0.05 M, [2.2.2] = 5.07 x 10m5 M, [S?] = 1.14x 10m5 M, [eosin] = (1.1-14.0) x 10-S M. (c) Z= 0.10 M, [2.2.2] = 5.07 x 10-5 M, [Sr’+] = 1.14x 10m5 M, [eosin] = (2.9-25.1) x lo- 5 M.
that the association between ML’+ and A’- in the aqueous phase and the dissociation of the ion-pair in the organic phase are negligible. The strontium-cryptand ratio was described as 1 : 1, with strontium occupying a central position in the cavity of the macrobicyclic ligand.” The dissociation constants of eosin’* and the pH value of 11.5 used suggest that only a neutral complex (1 : 1 : 1, strontium-cryptand-eosin) should be extracted. If we suppose that non-polimeric species are formed in the organic phase and that under the assuming
(a)
z
lo!3pcl
log&k;,
0.01 0.05 0.10
4.86+0.05 4.42 +O.Ol 4.17kO.04
13.11 kO.03 12.53 +0.02 12.13f0.06
experimental conditions used one may assume that [ML*+] >>[M*‘] and that the hydroxylation of the metal cation is negligible, eq. (2) can be rewritten by using the partition coefficient of the complex, PCT, 1ogD = log Pcr+log[A2-1. The experimental results are shown in Fig. 2. The slope of the log D vs log [A*-] plot is unity for the same strontium concentration and different ionic strengths tested, which clearly indicates the 1: 1 : 1 metal-ligand-counter-ion stoichiometry. On the other hand, the superposition of the extraction curves (A) for different strontium concentrations at the same ionic strength indicates the non-existence of polymerization reactions. The data obtained from these experiments have been refined by the LETAGROP program version DISTR’ 3 which provides the partition coefficients and extraction constants at different ionic strengths. The results are shown in Table 2 and Fig. 3. As can be seen, the value of the concentration formation constant remains reasonably constant as long as the partition coefficients and obviously the extraction constants decrease appreciably as the ionic strength increases. These results clearly explain the decrease of fluorescence intensity of the ion-pair (A,, = 536
1 (b)
I3 ‘i.__,. 12
i
f
Fig. 3. InIIuence of ionic strength on : (a) The partition coefficients (Pm), the concentration formation constants (KJ and the extraction constants (KJ. (b) The fluorescence intensity of the ion-pair in the organic phase.
Factors affecting the extraction of strontium nm, A, = 552 nm) in the organic phase increasing ionic strength as it can be deduced the same figure.
with from
2801
REFERENCES H. K. Frensdorff, J. Am. Chem. Sot. 1971,93,4684. H. Sumiyoshi, K. Nakahara and K. Ueno, Tulunta
1977,24,763. A. Sanz Medel, D. Blanc0 Gomis and J. R. Garcia
CONCLUSIONS
Our results on stability constants are in accordance with the studies of Lehn and Sauvage,” and Smetana and Popov. I4 They reported a change in log& of less than 0.3 for a variation of ionic strength from 0.005 to 0.3 M. In a recent study on the effects of ionic strength on the extraction of potassium complexed with 1% crown-6, Kolthoff ’ 5 reported the salting-out effect on the partition coefficient of the crown-ether, an effect which is not observed by us in the present study. The partition coefficients of cryptand 2.2.2 are the same for the different ionic strengths tested. From these results, it can be concluded that the ionic strength critically affects the association of ion-pairs in the aqueous phase and its extraction into the organic phase. This trend and the absence of secondary reactions, such as polymerization, observed in other systems,6 promotes the fluorescence decrease observed.
5. 6. I. 8. 9. 10. 11. 12. 13. 14. 15.
Alvarez, Talanta 1981, 28,425. A. Sanz Medel, D. Blanc0 Gomis, E. Fuente Alonso and S. Arribas Jimeno, Talanta 1984, 31, 515. D. Blanc0 Gomis, E. Fuente Alonso and A. Sanz Medel, Talanta 1985, 32, 915. D. Blanc0 Gomis, P. Arias Abrodo, A. M. Picinelli Lobo and A. Sam Medel, Talanta 1988,35, 553. D. Blanc0 Gomis, E. Fuente Alonso and P. Arias Abrodo, Microchim. Acta, in press. D. Fompeydie, F. Onur and P. Levillain, Bull. Sot. Chim. Fr. 1982, II, 5. F. Gaizer and A. Puskas, Talanta 1981,28,565. J. M. Lehn and J. P. Sauvage, J. Am. Chem. Sot. 1975,97, 6700. G. Anderegg, Helv. Chim. Acta 1981,64, 1790. P. Levillain and D. Fompeydie, Analyt. Chem. 1985, 57, 2561. D. H. Siem, Acta Chem. &and. 1971,25,1521. A. J. Smetana and A. I. Popov, J. Chem. Thermodyn. 1979,11, 1145. I. M. Kolthoff, Can. J. Chem. 1981,59, 1548.
0277-5387/89 $3.00+.00 0 1989 Pergamon Press plc
INFLUENCE OF THE IONIC STRENGTH ON THE FORMATION AND EXTRACTION OF THE STRONTIUMCRYPTAND 2.2.2-EOSIN ION-PAIR COMPLEX D. BLANC0
Departamento
GOMIS,* E. EUENTE ALONSO and P. ARIAS ABRODO
de Quimica Fisica y Analitica, Facultad de Quimica, Universidad de Oviedo, 33006 Oviedo, Spain (Received 31 March 1989 ; accepted 20 Jufy 1989)
Abstract-A
systematic study of the factors affecting the extraction of strontium, as an ionpair formed between the positively charged cryptate of strontium with cryptand 2.2.2 and the eosinate anion, into chloroform has been carried out. The pKl and pK2 values of the ligand, the stability constant of the cryptate inclusion complex, as well as the partition coefficient and the extraction constant of the ion-pair, were measured as a function of the ionic strength of the solution. The pK, and KS values remained reasonably constant at ionic strength < 0.1 M. Nevertheless, the partition coefficient values, and consequently the extraction constant values, decrease as the ionic strength increases.
Recently, a large number of papers dealing with the extraction constants of crown- and cryptand-ether complexes of alkali and alkaline-earth ions associated with coloured or fluorescent anions have been published. Much less work has been carried out on analogous systems involving transition metals as well as lanthanide and actinide ions. The overall extraction equilibrium formulated by Frensdorff ’ can be rewritten as M”++L+A”-s(MLA),,
M”++L- K,\
ML”+ +A= ~(MLA),. The two equilibrium constants are KS, the aqueous stability constant of the binary complex, and PC-, the partition coefficient of the complex from separate ions in the aqueous phase into ion-pairs in the organic phase. Thus the overall extraction equilibrium constant is given by
(1)
where M”+, L, A”- and (MLA), are the metal cation, the cyclic polyether, the counter-ion and the ion-pair, respectively. The subscript o and the absence of a subscript denote the organic and the aqueous phase, respectively. The equilibrium constant of eq. (1) was designated as the extraction constant, K,, :
ML”+
%, = K,Rc,. If the ligand is a diamine or cryptand the following acid-base equilibria take place : LH;+&LH++H+ LH +f‘L+H+.
Addition of a metal cation, M”+, will thus affect the basicity of a solution of the free ligand. The role of these equilibria will be considered in detail in the where the brackets denote equilibrium activities. This extraction equilibrium may be analysed in Experimental. However, in more recent publications the extracterms of two constituent equilibria which, when tion constants have been reported using conadded to each other, give eq. (1) : centrations (e) instead of activities, and the *Author to whom correspondence should be addressed. extraction method has been applied at appreciable L
=
[MLAl,/W+I[Y[A”-I,
2791
2798
D. BLANC0 GOMIS et al.
Structure 1. Molecular structure of the dianionic form of eosin. ionic strengths, e.g. for the determination of K+ in blood serum2,3 or Pb2+ in tap water4 and soft drinks,5 where Kz$” differs considerably from K,. In our earlier studies we have observed a critical influence of ionic strength on the extraction of some metal ions, e.g. K+, Sr2+, Ca2+ or Pb2+, with crown- and cryptand-ethers. Thus, while the extraction of potassium with 18-crow~6~ or lead with 18-crown-64 or cryptand 2.2.2’ is not affected by this parameter (probably due to the formation of polymeric species in the organic phase), an increase in the ionic strength promotes a sharp decrease in the extraction of lead with cryptand 2.2.1 6or strontium with cryptand 2.2.2,7 using eosin as the counter-ion, and consequently a decrease in the fluorescence intensity of the organic phase. This factor could be a problem in the analytical application of these extraction systems. For this, we are at present engaged in a detailed study of various factors which affect the extraction constants of potassium, strontium and lead, including extraction of polymeric species and effects of counter-ion and solvents for the extraction. In the present study we report the effect of ionic strength on the extraction of strontium using cryptand 2.2.2 (L) as the ligand, eosinate (A) as the counter-ion and chloroform as the extractant. On this account, we have studied the effect that the variation of the ionic strength causes on the different constants involved in the extraction process: the dissociation constants of the ligand, the stability constant of the cryptate inclusion complex, the partition coefficient and the extraction constant of the ion-pair, and the partition coefficient of the complex. EXPERIMENTAL
Reagents and equipment All reagents were of analytical grade and doubly distilled and demineralized water was used throughout.
Cryptand 2.2.2. The commercial product (Kryptofix 2.2.2 from Merck) was used as received. An aqueous stock solution (1.689 x lop4 M) of the cryptand was stored in a PVC container and was diluted as required before use. Strontium(I1) stock solution (0.0114 M). Prepared by dissolving 1.2063 g of strontium nitrate in water, diluting to 500 cm3 and standardizing by complexometric titration. All working standard solutions (5.772 x lop5 M) were freshly prepared by appropriate dilution of the stock solution. Eosin acid aqueous solution (8.970 x 10e3 M). Prepared by dissolving 0.1453 g of eosin in 25 cm3 of basic water. Eosin was synthesized by reaction of Br-/BrO; with fluorescein in an acidic wateracetone mixture as described elsewhere.’ Tetramethylammonium hydroxide (4.388 x 10m2 M). The (CH,),N+ cation would be large enough so as not to enter the intramolecular cavity of the ligand. The basic solutions were made from commercial Fluka AG reagent. Preliminary titration showed about 3% of the latter to be carbonated. In order to avoid carbonation of these basic solutions argon was bubbled through them and they were stored in PVC containers. The instrumentation included a UV-vis spectrophotometer (Perkin-Elmer, model 124) with quartz cells (10 mm path), an LS-5 Perkin-Elmer spectrofluorimeter equipped with a Model 3600 data-station, an inductively coupled plasma Perkin-Elmer spectrometer model 5000, a Crison-501 potentiometer and a Dosimat automatic microburet. A thermostatted shaker (Grant) with a time switch was used for horizontal shaking of the stoppered tubes. Potentiometric titrations To determine the protonation and the stability constants (K,, K2 and KS), solutions of the protonated ligands (0.02 M) alone and in the presence of Sr2+ (0.01 M) were titrated with a strong base, (CH3)4NOH (0.073 M). Variable concentrations of supporting electrolyte, (CH,),NBr, in order to maintain the ionic strength (0.01,0.05 and 0.10 M) in the test solution and in the titrant, were used. Calibration of the cell was achieved with HN03 solutions at different ionic strengths by titration with tetramethylammonium hydroxide. The potential of the solutions was measured with the following cell : glass electrode/test solution (I = O.Ol0.10 M, (CH,),NBr)/O.Ol-O.lO M (CH3)4NBr, SCE, in order to avoid diffusion phenomena of potassium from the calomel electrode. All measurements were performed in a glass cell maintained at 25 f O.l"C. Portions of 0.01 cm3 of
Factors affecting the extraction of strontium
2799
Partition coeficient of cryptand 2.2.2
Fig. 1. Potential measurements (E) as a function of the volume of alkali added (v) to solutions containing : (a) strong acid (HNO,) ; (b) strong acid and ligand (cryptand 2.2.2); (c) strong acid, ligand and metal ion (Sr*+); Z = 0.01 M.
Partition coefficients, P(L), at different ionic strengths were determined by equilibration of equal volumes of the cryptand solution in water and in chloroform, followed by spectrofluorimetric determination of the amount of cryptand remaining in the aqueous phase. The determination was carried out by addition of eosin and strontium to the aqueous phase and extraction of the cryptand-strontium-eosinate complex into chloroform, following the general extraction procedure described elsewhere.’ The concentration found in this way in the aqueous phase was subtracted from the initial total concentration of cryptand to deduce the concentration in the organic phase. RESULTS AND DISCUSSION
titrant were added from an automatic microburet. The titration cell was purged with argon which had previously been saturated with water at 25°C. A typical titration curve is shown in Fig. 1. The results were analysed using a general equilibrium-solving program, MINIPOT, to calculate the protonation and the formation constants. Extraction procedure An aliquot (5 cm’) of an aqueous solution containing the metal ion (1.154 x 1O- ’ M), the cryptand solution (5.067 x lo-’ M), the (CH&NOH solution (4.388 x lo- 3 M, pH = 11.5), the eosinate anion for the formation of the ion-pair (9.440 x 10p6-1 .4 x 10m4 M ; this concentration range allows extraction yields ranging from 15 to 85% to be obtained), and variable concentrations of (CH3)4NBr solution in order to keep the ionic strength at 0.01, 0.05 or 0.10 M, was placed in a glass cylindrical tube with a PVC stopper. After the addition of 5 cm3 of chloroform, the mixture was shaken for 60 min at 25 f 0.1 “C. The solvents were saturated with each other before use to prevent volume changes in the phases during extraction. The mixture was centrifuged for 5 min to separate the phases, the pH of the aqueous phase was checked and the concentration of the metal ion was measured by ICP emission photometry at 407.77 nm. Eosin equilibrium concentrations in the aqueous layer were measured spectrophotometrically at 516 nm. Strontium concentrations in the organic phases were determined as follows: 4 cm3 of the chloroform phase were allowed to evaporate, the residue was dissolved in 10% nitric acid solution and determined by ICP emission photometry.
Table 1 shows the results obtained for the concentration equilibrium constants, K,, of complexes of cryptand 2.2.2 with Sr2+ at different ionic strengths, as well as the pK,, pK2 and P(L) values of the ligand. As can be seen, in the ionic strength range 0.01-0.10 M, the concentration formation constant remains reasonably constant and close to values from the literature.“*” In a similar manner, the pK, and P(L) values of the ligand present a similar behaviour. Distribution coejkient When a divalent metal ion M2+, such as Sr2+, forms a complex with a cryptand, L, such as cryptand 2.2.2 and the complex is extractable as an ionpair with large anions, A2-, such as the eosinate ion, the distribution coefficient (D) for the extraction system pM2++qL+rA2--
-
(M,L,EJ,
may be written as D = c
W2+loICW2+l p34.r =
1
P[M,L,W[M~+~+WL2+l + 2 [M,(OH),Z’-3, (2) I
Table 1. K, pK,, pK, and P(L) values for the different ionic strengths Z
log K,
0.01 8.25f0.09 0.05 8.11kO.03 0.10 7.96kO.05
PK,
pK2
P(L)
7.17+0.04 7.27f0.02 7.26f.O.01
9.97kO.03 9.66kO.02 9.71kO.05
89.38kO.62 89.39kO.84 89.42kO.76
D. BLANC0
2800
GOMIS et al. Table 2. PCTand X;, values for the strontium-cryptand 2.2.2-eosin system in chloroform at different ionic strengths
P
50
I
I
4.5
4.0
-log CEOSINI
Fig. 2. Dependence of the distribution ratio for strontium on the eosin concentration in the aqueous phase at different ionic strengths: (a) Z= 0.01 M, [2.2.2] = 5.07x 10m5 M; 0 [Sr*+] = 1.14x 10e5 M, [eosin] = (1.07-8.75) x 1O-5 M; * [Sr’+] = 5.71 x 10m6 M, [eosin] = (0.72-3.87) x lo- 5 M. (b) Z = 0.05 M, [2.2.2] = 5.07 x 10m5 M, [S?] = 1.14x 10m5 M, [eosin] = (1.1-14.0) x 10-S M. (c) Z= 0.10 M, [2.2.2] = 5.07 x 10-5 M, [Sr’+] = 1.14x 10m5 M, [eosin] = (2.9-25.1) x lo- 5 M.
that the association between ML’+ and A’- in the aqueous phase and the dissociation of the ion-pair in the organic phase are negligible. The strontium-cryptand ratio was described as 1 : 1, with strontium occupying a central position in the cavity of the macrobicyclic ligand.” The dissociation constants of eosin’* and the pH value of 11.5 used suggest that only a neutral complex (1 : 1 : 1, strontium-cryptand-eosin) should be extracted. If we suppose that non-polimeric species are formed in the organic phase and that under the assuming
(a)
z
lo!3pcl
log&k;,
0.01 0.05 0.10
4.86+0.05 4.42 +O.Ol 4.17kO.04
13.11 kO.03 12.53 +0.02 12.13f0.06
experimental conditions used one may assume that [ML*+] >>[M*‘] and that the hydroxylation of the metal cation is negligible, eq. (2) can be rewritten by using the partition coefficient of the complex, PCT, 1ogD = log Pcr+log[A2-1. The experimental results are shown in Fig. 2. The slope of the log D vs log [A*-] plot is unity for the same strontium concentration and different ionic strengths tested, which clearly indicates the 1: 1 : 1 metal-ligand-counter-ion stoichiometry. On the other hand, the superposition of the extraction curves (A) for different strontium concentrations at the same ionic strength indicates the non-existence of polymerization reactions. The data obtained from these experiments have been refined by the LETAGROP program version DISTR’ 3 which provides the partition coefficients and extraction constants at different ionic strengths. The results are shown in Table 2 and Fig. 3. As can be seen, the value of the concentration formation constant remains reasonably constant as long as the partition coefficients and obviously the extraction constants decrease appreciably as the ionic strength increases. These results clearly explain the decrease of fluorescence intensity of the ion-pair (A,, = 536
1 (b)
I3 ‘i.__,. 12
i
f
Fig. 3. InIIuence of ionic strength on : (a) The partition coefficients (Pm), the concentration formation constants (KJ and the extraction constants (KJ. (b) The fluorescence intensity of the ion-pair in the organic phase.
Factors affecting the extraction of strontium nm, A, = 552 nm) in the organic phase increasing ionic strength as it can be deduced the same figure.
with from
2801
REFERENCES H. K. Frensdorff, J. Am. Chem. Sot. 1971,93,4684. H. Sumiyoshi, K. Nakahara and K. Ueno, Tulunta
1977,24,763. A. Sanz Medel, D. Blanc0 Gomis and J. R. Garcia
CONCLUSIONS
Our results on stability constants are in accordance with the studies of Lehn and Sauvage,” and Smetana and Popov. I4 They reported a change in log& of less than 0.3 for a variation of ionic strength from 0.005 to 0.3 M. In a recent study on the effects of ionic strength on the extraction of potassium complexed with 1% crown-6, Kolthoff ’ 5 reported the salting-out effect on the partition coefficient of the crown-ether, an effect which is not observed by us in the present study. The partition coefficients of cryptand 2.2.2 are the same for the different ionic strengths tested. From these results, it can be concluded that the ionic strength critically affects the association of ion-pairs in the aqueous phase and its extraction into the organic phase. This trend and the absence of secondary reactions, such as polymerization, observed in other systems,6 promotes the fluorescence decrease observed.
5. 6. I. 8. 9. 10. 11. 12. 13. 14. 15.
Alvarez, Talanta 1981, 28,425. A. Sanz Medel, D. Blanc0 Gomis, E. Fuente Alonso and S. Arribas Jimeno, Talanta 1984, 31, 515. D. Blanc0 Gomis, E. Fuente Alonso and A. Sanz Medel, Talanta 1985, 32, 915. D. Blanc0 Gomis, P. Arias Abrodo, A. M. Picinelli Lobo and A. Sam Medel, Talanta 1988,35, 553. D. Blanc0 Gomis, E. Fuente Alonso and P. Arias Abrodo, Microchim. Acta, in press. D. Fompeydie, F. Onur and P. Levillain, Bull. Sot. Chim. Fr. 1982, II, 5. F. Gaizer and A. Puskas, Talanta 1981,28,565. J. M. Lehn and J. P. Sauvage, J. Am. Chem. Sot. 1975,97, 6700. G. Anderegg, Helv. Chim. Acta 1981,64, 1790. P. Levillain and D. Fompeydie, Analyt. Chem. 1985, 57, 2561. D. H. Siem, Acta Chem. &and. 1971,25,1521. A. J. Smetana and A. I. Popov, J. Chem. Thermodyn. 1979,11, 1145. I. M. Kolthoff, Can. J. Chem. 1981,59, 1548.