- Email: [email protected]
A study of the mechanism of response of liquid ion exchanger calcium selective electrodes
J. Electroanal. Chem., 97 (1979) 163--170
163
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
A S T U D Y O F T H E MECHANISM O F R E S P O N S E O F L I Q U I D ION E X C H A N G E R CALCIUM S E L E C T I V E E L E C T R O D E S P A R T II. C O N D U C T I M E T R I C I N V E S T I G A T I O N O F T H E IONIC D I S S O C I A T I O N IN L I Q U I D ION E X C H A N G E R S
NICOLE DAVION VAN MAU and CLAUDE GAVACH G.R. No. 28 Physicochimze des Interfaces, C.N R.S., Route de Mende, B.P 5051, 34033 Montpellier Cedex (France)
(Received 3 July 1978) ABSTRACT
Measurements of conductivities of didecyl phosphate salts in DOPP have been undertaken using a dielectric constant determination cell. For concentrations greater than 10-4M these salts are highly associated. Dissociated constants have been determined for Li and Na DDP in DOPP in the range concentration where ion pairs are formed (C < 10 -4 M). The values are identical for the two salts studied. The presence of water and inorganic salts enhances the conductivity of ion exchanger solutions. Aggregation under micellar form or even as a pseudo-liquid crystal is discussed.
INTRODUCTION T h e m a t h e m a t i c a l expression o f the steady state m e m b r a n e p o t e n t i a l o f liquid ion exchangers [1--4] takes into consideration the dissociation constants o f the organic electrolytes o f the m e m b r a n e . As the i n t e r f e r e n c e effects o f foreign ions on the response o f liquid m e m b r a n e electrodes can be d e t e r m i n e d using the theoretical expression, it is t h e r e f o r e i m p o r t a n t t o k n o w the exact values o f these dissociation constants. Thus Bagg and Chaung [5] e n d e a v o u r e d to a c c o u n t for the i n t e r f e r e n c e effects o f alkaline ions on an Orion calcium ion electrode b y assuming t h a t the degree o f ionic dissociation was p r o p o r t i o n a l to the electrical resistance o f the m e m b r a n e . Likewise M o o r e [6] d e t e r m i n e d the dissociation constants o f sodium stearate and stearic acid in 1-pentanol in an e f f o r t to elucidate certain e x p e r i m e n t a l results for the t r a n s p o r t o f ions t h r o u g h a liquid ion-exchange m e m b r a n e . As the electrical resistance o f the calcium electrode m e m b r a n e is very high (several m e g o h m s w h e n the thickness is less than 1 m m ) , the c o n d u c t i v i t y cann o t be measured by the usual m e t h o d s . F u r t h e r as it is also necessary to determine the variation o f the c o n d u c t i v i t y o f the organic solutions with the concentration in o r d e r t o establish the dissociation law equations it is essential to have at one's disposal, separately, b o t h solvents and electrolytes, which have been synthesized as previously described in Part I. In this study, which is a c o n t i n u a t i o n of w o r k on the p o t e n t i o m e t n c series
164 of measurements described in Part I, the conductivities of the ion exchangers constituting the membranes of the electrodes studied, have been determined using an electrical set-up adapted to the measurement of very low conductances. The results thus obtained give information both concerning the dissociation of the ion exchangers and the effects due to the penetration of water and inorganic salts. EXPERIMENTAL
(a ) Conductivity and dielectric constant measurements The basic principle underlying these measurements is the determination of the loss resistance of a capacitor constituted by a cell of the type used for the evaluation of dielectric constants (WTW, MFL, i / m s ) when this is filled with the organic solution to be studied. The equivalent series capacitance and the dissipation factor are measured in the 100--20 kHz frequency range by means of a capacitance determination assembly (General Radio type 1620 A). The organic solution is interposed between two cylindrical coaxial non-porous stainless steel electrodes in a cell somewhat similar in design to that proposed by Nichol and Fuoss [7]. The cell constant is determined with a solution of tetrabutyl a m m o n i u m tetraphenyl borate in dichloroethane using the m e t h o d proposed by Fuoss and Krauss [8] in the above frequency range and at 25.00 ° C. It is thus possible to measure the dielectric constant of the solvent after having calibrated the cell with pure solvents whose dielectric constants are exactly known. In this study chromatography grade nitrobenzene e25 ° c 34.82 and dichloroethane e25 o c = 10.387 were used. The value of the cell constant is 0.01 cm -1 . =
(b) Density and viscosity measurements The viscosity of the solvent is determined using an Ubbelohde viscometer immersed in a constant temperature water bath at 25.0 ° C, while the densities of the solvent and the solution are obtained with a DMA 02C "Precision densimeter". RESULTS AND DISCUSSION At 25°C the viscosity of the solvent is 1.74 X 10 -2 kg m -1 s -1 (= 17.4 cP) and the dielectric constant of DOPP is 7.2 for the dry solvent and 9.2 for that saturated with water. Figure 1 shows the variation of the specific conductivities of solutions of Ca(DDP)2 in DOPP with the concentrati~)n, while the densities of these solution are plotted in Fig. 2. The variations of conductivities of LiDDP, NaDDP and HDDP solutions always in DOPP are represented in Fig. 3. Table 1 gives the values of the specific conductivities of the solvent to which have been added a certain a m o u n t of water or solutions of NaC1 and CaC12, as well as those of Ca(DDP)2 in DOPP containing the same amounts of water or of the two aqueous solutions. One notes that these conductivities are more
165 103 K / ..iS cm -1 10
10 3
K
~S cm -1
Y
5 ¢
2
6
4
8 ~ 4 c / m o l I-~
J
4 6 8 1'o lo3 c/tool J-~ Fig. 1. Variation of specific conductivities with concentrations of solutions of didecyl phosphate, Ca(DDP)2, in dioctyl phenyl phosphonate calcium (DOPP) at 25.0 oC.
influenced by the presence of water and inorganic salt solutions than by the addition of Ca(DDP)2. The curves in Fig. 1 and 3 show that, for all electrolytes, there is an abrupt change in the slope for concentrations between 2 and 4 × 10 -4 M, which suggests a high degree of association. On the other hand as the densities of the Ca(DDPh solution remain constant for concentrations greater than 2 × 10 -4 M, this would seem to indicate the association of the DDP salts in a micellar form of the liquid crystal type. At this stage one can recall that the concentration of the liquid ion exchanger in the calcium electrode is greater than the limiting value of 2 × 10 -4 M. For concentrations of alkaline salts less than 2 X 10 -4. M, the variations of
1105
J°t 0 950[/•
1000
2
•
A
4
6 lO4c/mol 1-1
J
ogsc
4
,6
8
103c/tool 1:1 o
Fig. 2. Variation of densities with concentrations of solutions of Ca(DDP)2 in DOPP at 25.00 C.
166 103 K / ff S cm-~
103 K / IJS cm-1 "
"
6
~'~X'-~
_ __ 2
--X-
4
6
104 c/mol 1-1
1C
,;~
-~
(a)
x----
4 --g
~
103~Imo4d
Fig. 3. Variation of the specific conductivities with concentrations of solution in DOPP of: (a) (+) LiDDP, (o) N a D D P ; (b) (x) HDDP.
log A with log c are linear (Fig. 4). The solvent conductivity value has been subtracted from the conductivity values of the solutions. The value of the slope is --0.538 which is characteristic of ion pair formation [11]. No minimum suggesting the presence of triple ions is observed for 10 -4 M solutions as could be expected from the empirical law of Fuoss [12] ; see also [9,10]. In addition the product A v ~ does not vary linearly with c as in the case when triple ions are formed [13]. French and Singer [14] have observed a similar phenomenon in the case of conductivity measurements on solutions of amine picrates in tritolyl phosphate, a solvent which has a similar dielectric constant as DOPP and also a strong dipolar character. For alkaline salts LiDDP and NaDDP the dissociation constants K have been determined by the Fuoss and Krauss [11] technique. The initial value for A ° = 0.250 ~2 -1 cm-' was deduced by plotting A versus x/c-. The dissociation TABLE 1
Conduct,vity values x of DOPP and salt solutions in DOPP with and w i t h o u t water
KIpS em -1 DOPP
0.0025
DOPP + 2% H 2 0
0.1500
DOPP + 2 X 10 -3 M CaC12 + 2% H 2 0
0.3350
DOPP + 2 X 10 -3 M NaC1 + 2% H 2 0
0.5485
KIpS c m -1 10 -2 M Ca(DDP)2 in DOPP 1 0 - 2 M Ca(DDP)2 in DOPP + 2% H 2 0
0.0050
10-2 M Ca(DDP)2 in DOPP + 2% H 2 0 + 2 X 10 -3 M CaC12 10 -2 M Ca(DDP)2 in DOPP + 2% H 2 0 + 2 X 10 -3 M NaC1
0.3669
0.167
0.5569
167 log(A) +
-1
-5
-4 5
-4
log (£)
Fig. 4. Variation of log A with log c for Na and Li DDP salts in DOPP. coefficients 7 were obtained using an iteration m e t h o d , and the dissociation constant K is defined by the expression: K
= c,~p/(1
-
~,)
where f, the activity coefficient, is given by --log f = ~ DOPP. Now introducing the variable x defined thus
with ~ = 8.261 for
x = (1 -- 7)/3': = c f ~ K and if one plots f V ~ versus vrx, given the last equation, one m u s t obtain a straight line passing through the origin with a slope x/k. The value of A °, 0.250 £ - ' cm -1, appears to be smaller than it should be since the straight line
10 3
5
lo
15
V7
Fig. 5. Variation f v ~ v e r s u s V/X-for alkaline didecyl phosphate m DOPP with --log f =
and x = (1--7)/72. (o) NaDDP, (A) LiDDP.
168
does n o t pass through the origin but cuts the axis at a negative value of A °. The value corresponding to the required straight line is A ° = 0.500 ~ 4 cm-1 and the dissociation constant K = 3.6 X 10 -7 (Fig. 5). Using A ° = 0.5 ~-1 cm-i we also tried to determine K b y the classical Fuoss method [12] based on the variation of F(Z)/A with cAF 2 when the variable Z is defined by: Z = SA°-3/2x/~A S is the Onsager coefficient. S = aA ° + fi with a = 8.261,/~ = 0.103 and F(Z) is the continued fraction. F(Z) = 1 -- Z(1 -- Z(1 -- ...)-u2)-,/2 A straight line is expected, the slope of which is 1/KA °2- The value of K thus determined is K = 1.6 X 10 -7 (Fig. 6). This last m e t h o d is not as precise as the preceding one because of the difficulty inherent to the graphical determination of the A ° values. This lack of precision of the experimental points is due to the conductance of the solvent as has already been noted b y French and Hart [15]. The low values of the Walden product for NaDDP and LiDDP suggest the existence of dipole atmospheres round the cations particularly when the latter are small. This has been mentioned by French and Hart [15] when considering long chain dialkyl phosphonate solvent molecules which have a low dielectric constant, and a high dipole moment. This property is in line with the results given b y the potentiometric measurements which revealed a higher permeability of the liquid membrane, the smaller the monovalent cation. For the 2-1 Ca(DDP)2 electrolyte the preceding treatment cannot be applied and hence the limiting conductances and the dissociation constants cannot be evaluated. Nevertheless the results obtained do enable one to make certain hypothesis. For instance the sudden change in the slope of the conductivity concentration curves (Fig. 1) for concentration greater than 2 X 10 -~ M corre-
F
5C
2~
lo7 car
F Fig. 6. Variation of e A ~ / F ( Z ) with F / A for Li and Na DDP in DOPP. (©) NaDDP: (A) LiDDP.
169 sponds to the formation of aggregates, in this concentration range the densities of the solution remain constant while for lower concentrations it increases steadily (Fig. 2), for solution of long chain electrolytes the sudden changes in the partial molal volume and in the conductance curves are attributed to micelle formation [16]. In polar solvents the polar group of the ions is oriented towards the outer surface while the long chain forms the core of the micelle. On the contrary for solvents of low dielectric constant such as benzene the long chains are oriented towards the exterior of the micelle. Such solutions, for example long chain sulfonic acids like dinonylnaphthalene sulfonate in benzene are commonly used as liquid ion exchangers for liquid-liquid extraction purposes [17]. In DOPP, a solvent of low dielectric constant, the molecules of which contain two C10 chains and a polar group, the nature of the micelle cannot be established from classical consideration but one must neither eliminate the possibility of the formation of mixed micelles both by the solvent and the DPP- ions nor that of more highly structure associations giving rise to a pseudoliquid crystalline structured solution. Taking into consideration the specific interaction between the phosphate group and the calcium ions the possible existence of a pseudo-liquid crystalline structure of the liquid ion exchanger would lead to an electrode having similar properties as a solid membrane electrode formed by crystals or insoluble salts. The conductance of DOPP containing 2% of water is 60 times higher than the conductance of pure DOPP. This fact may be explained by the electronegative character of this organic molecule which confers to DOPP basic properties [18]. CONCLUSIONS (a) As the concentration of the long chain electrolyte in the ion exchanger falls in the region within which it is highly associated, this must be taken into consideration in any attempt to elucidate the working mechanism of the electrode as well as in the quantitative interpretation of interference effects. (b) No difference is observed in the conductance of Na and Li didecyl phosphoric salts although behaviour with respect to interference effects on the electrode response is not the same. (c) The presence of water brings about an appreciable increase in the conductance of DOPP and Ca(DDP)2 solutions in DOPP. For the m o m e n t it is impossible to establish if this effect is due to the H ÷ and OH- ions in the water or to a hydrolysis of the solvent. Be that as it may, the presence of water in the liquid membrane is of considerable importance and is probably to be related to the electrode potential drift recorded when a freshly prepared electrode is dipped in an aqueous solution. (d) NaC1 and CaCI~ increase considerably the conductance of DOPP + water and solutions of Ca(DDP)2 in DOPP + water, showing that a small penetration of this electrolyte into the membrane modifies greatly the number of free charge carriers in the membrane.
170
REFERENCES
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
F. Contl and G. Eisenman, Blophys. J., 6 (1966) 227. G. Eisenman, Anal. Chem., 40 (1968) 315. J. Sandblom, J. Phys. Chem., 73 (1969) 249. J. Sandblom, J. Phys. Chem., 73 (1969) 257. J. Bagg and W.P. Chaung, J. Chem., 24 (1971) 1963. J.H. Moore, Ph.D., Umversity of Texas at Austin, 1970. J.C. Nlchol and R.M. Fuoss, J. Phys. Chem., 54 (1954) 696. J.J. Zwolenik, R.M. Fuoss, J. Phys. Chem., 68 (1964) 903. C.A. Krauss, J. Chem. Ed., 35 (1957) 325. C.M. French and D.F. Muggleton, J. Chem. Soc., (1957) 5064. R.M. Fuoss and C.A. Krauss, J. Amer. Chem. Soe., 55 (1933) 476. R.M. Fuoss and C.A. Krauss, J. Amer. Chem. Soc., 57 (1935) 488. R.M. Fuoss J. Amer. Chem. Soc., 55 (1933) 2387. C.M. French and N. Stager, J. Chem. Soc., (1957) 5064. C.M. French and P.B. Hart, J. Chem. Soc., (1960) 1671. K. Shmoda, T. Nakagawa, B.I. Tamanushi and T, Isemura, Colloidal Surfaetants, Academic Press, New York, 1963. 17 G.Y. Markowts and G.R. Chopin in J.A. Marinsky and Y. Marcus (Eds): Ion Exchanger and Solvent Extraction, Marcel Dekker, New York, 1973, p. 51. 18 F. Krasovec and C. Klofutar, J. Inorg. NucL Chem., 30 (1968) 1640.
163
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
A S T U D Y O F T H E MECHANISM O F R E S P O N S E O F L I Q U I D ION E X C H A N G E R CALCIUM S E L E C T I V E E L E C T R O D E S P A R T II. C O N D U C T I M E T R I C I N V E S T I G A T I O N O F T H E IONIC D I S S O C I A T I O N IN L I Q U I D ION E X C H A N G E R S
NICOLE DAVION VAN MAU and CLAUDE GAVACH G.R. No. 28 Physicochimze des Interfaces, C.N R.S., Route de Mende, B.P 5051, 34033 Montpellier Cedex (France)
(Received 3 July 1978) ABSTRACT
Measurements of conductivities of didecyl phosphate salts in DOPP have been undertaken using a dielectric constant determination cell. For concentrations greater than 10-4M these salts are highly associated. Dissociated constants have been determined for Li and Na DDP in DOPP in the range concentration where ion pairs are formed (C < 10 -4 M). The values are identical for the two salts studied. The presence of water and inorganic salts enhances the conductivity of ion exchanger solutions. Aggregation under micellar form or even as a pseudo-liquid crystal is discussed.
INTRODUCTION T h e m a t h e m a t i c a l expression o f the steady state m e m b r a n e p o t e n t i a l o f liquid ion exchangers [1--4] takes into consideration the dissociation constants o f the organic electrolytes o f the m e m b r a n e . As the i n t e r f e r e n c e effects o f foreign ions on the response o f liquid m e m b r a n e electrodes can be d e t e r m i n e d using the theoretical expression, it is t h e r e f o r e i m p o r t a n t t o k n o w the exact values o f these dissociation constants. Thus Bagg and Chaung [5] e n d e a v o u r e d to a c c o u n t for the i n t e r f e r e n c e effects o f alkaline ions on an Orion calcium ion electrode b y assuming t h a t the degree o f ionic dissociation was p r o p o r t i o n a l to the electrical resistance o f the m e m b r a n e . Likewise M o o r e [6] d e t e r m i n e d the dissociation constants o f sodium stearate and stearic acid in 1-pentanol in an e f f o r t to elucidate certain e x p e r i m e n t a l results for the t r a n s p o r t o f ions t h r o u g h a liquid ion-exchange m e m b r a n e . As the electrical resistance o f the calcium electrode m e m b r a n e is very high (several m e g o h m s w h e n the thickness is less than 1 m m ) , the c o n d u c t i v i t y cann o t be measured by the usual m e t h o d s . F u r t h e r as it is also necessary to determine the variation o f the c o n d u c t i v i t y o f the organic solutions with the concentration in o r d e r t o establish the dissociation law equations it is essential to have at one's disposal, separately, b o t h solvents and electrolytes, which have been synthesized as previously described in Part I. In this study, which is a c o n t i n u a t i o n of w o r k on the p o t e n t i o m e t n c series
164 of measurements described in Part I, the conductivities of the ion exchangers constituting the membranes of the electrodes studied, have been determined using an electrical set-up adapted to the measurement of very low conductances. The results thus obtained give information both concerning the dissociation of the ion exchangers and the effects due to the penetration of water and inorganic salts. EXPERIMENTAL
(a ) Conductivity and dielectric constant measurements The basic principle underlying these measurements is the determination of the loss resistance of a capacitor constituted by a cell of the type used for the evaluation of dielectric constants (WTW, MFL, i / m s ) when this is filled with the organic solution to be studied. The equivalent series capacitance and the dissipation factor are measured in the 100--20 kHz frequency range by means of a capacitance determination assembly (General Radio type 1620 A). The organic solution is interposed between two cylindrical coaxial non-porous stainless steel electrodes in a cell somewhat similar in design to that proposed by Nichol and Fuoss [7]. The cell constant is determined with a solution of tetrabutyl a m m o n i u m tetraphenyl borate in dichloroethane using the m e t h o d proposed by Fuoss and Krauss [8] in the above frequency range and at 25.00 ° C. It is thus possible to measure the dielectric constant of the solvent after having calibrated the cell with pure solvents whose dielectric constants are exactly known. In this study chromatography grade nitrobenzene e25 ° c 34.82 and dichloroethane e25 o c = 10.387 were used. The value of the cell constant is 0.01 cm -1 . =
(b) Density and viscosity measurements The viscosity of the solvent is determined using an Ubbelohde viscometer immersed in a constant temperature water bath at 25.0 ° C, while the densities of the solvent and the solution are obtained with a DMA 02C "Precision densimeter". RESULTS AND DISCUSSION At 25°C the viscosity of the solvent is 1.74 X 10 -2 kg m -1 s -1 (= 17.4 cP) and the dielectric constant of DOPP is 7.2 for the dry solvent and 9.2 for that saturated with water. Figure 1 shows the variation of the specific conductivities of solutions of Ca(DDP)2 in DOPP with the concentrati~)n, while the densities of these solution are plotted in Fig. 2. The variations of conductivities of LiDDP, NaDDP and HDDP solutions always in DOPP are represented in Fig. 3. Table 1 gives the values of the specific conductivities of the solvent to which have been added a certain a m o u n t of water or solutions of NaC1 and CaC12, as well as those of Ca(DDP)2 in DOPP containing the same amounts of water or of the two aqueous solutions. One notes that these conductivities are more
165 103 K / ..iS cm -1 10
10 3
K
~S cm -1
Y
5 ¢
2
6
4
8 ~ 4 c / m o l I-~
J
4 6 8 1'o lo3 c/tool J-~ Fig. 1. Variation of specific conductivities with concentrations of solutions of didecyl phosphate, Ca(DDP)2, in dioctyl phenyl phosphonate calcium (DOPP) at 25.0 oC.
influenced by the presence of water and inorganic salt solutions than by the addition of Ca(DDP)2. The curves in Fig. 1 and 3 show that, for all electrolytes, there is an abrupt change in the slope for concentrations between 2 and 4 × 10 -4 M, which suggests a high degree of association. On the other hand as the densities of the Ca(DDPh solution remain constant for concentrations greater than 2 × 10 -4 M, this would seem to indicate the association of the DDP salts in a micellar form of the liquid crystal type. At this stage one can recall that the concentration of the liquid ion exchanger in the calcium electrode is greater than the limiting value of 2 × 10 -4 M. For concentrations of alkaline salts less than 2 X 10 -4. M, the variations of
1105
J°t 0 950[/•
1000
2
•
A
4
6 lO4c/mol 1-1
J
ogsc
4
,6
8
103c/tool 1:1 o
Fig. 2. Variation of densities with concentrations of solutions of Ca(DDP)2 in DOPP at 25.00 C.
166 103 K / ff S cm-~
103 K / IJS cm-1 "
"
6
~'~X'-~
_ __ 2
--X-
4
6
104 c/mol 1-1
1C
,;~
-~
(a)
x----
4 --g
~
103~Imo4d
Fig. 3. Variation of the specific conductivities with concentrations of solution in DOPP of: (a) (+) LiDDP, (o) N a D D P ; (b) (x) HDDP.
log A with log c are linear (Fig. 4). The solvent conductivity value has been subtracted from the conductivity values of the solutions. The value of the slope is --0.538 which is characteristic of ion pair formation [11]. No minimum suggesting the presence of triple ions is observed for 10 -4 M solutions as could be expected from the empirical law of Fuoss [12] ; see also [9,10]. In addition the product A v ~ does not vary linearly with c as in the case when triple ions are formed [13]. French and Singer [14] have observed a similar phenomenon in the case of conductivity measurements on solutions of amine picrates in tritolyl phosphate, a solvent which has a similar dielectric constant as DOPP and also a strong dipolar character. For alkaline salts LiDDP and NaDDP the dissociation constants K have been determined by the Fuoss and Krauss [11] technique. The initial value for A ° = 0.250 ~2 -1 cm-' was deduced by plotting A versus x/c-. The dissociation TABLE 1
Conduct,vity values x of DOPP and salt solutions in DOPP with and w i t h o u t water
KIpS em -1 DOPP
0.0025
DOPP + 2% H 2 0
0.1500
DOPP + 2 X 10 -3 M CaC12 + 2% H 2 0
0.3350
DOPP + 2 X 10 -3 M NaC1 + 2% H 2 0
0.5485
KIpS c m -1 10 -2 M Ca(DDP)2 in DOPP 1 0 - 2 M Ca(DDP)2 in DOPP + 2% H 2 0
0.0050
10-2 M Ca(DDP)2 in DOPP + 2% H 2 0 + 2 X 10 -3 M CaC12 10 -2 M Ca(DDP)2 in DOPP + 2% H 2 0 + 2 X 10 -3 M NaC1
0.3669
0.167
0.5569
167 log(A) +
-1
-5
-4 5
-4
log (£)
Fig. 4. Variation of log A with log c for Na and Li DDP salts in DOPP. coefficients 7 were obtained using an iteration m e t h o d , and the dissociation constant K is defined by the expression: K
= c,~p/(1
-
~,)
where f, the activity coefficient, is given by --log f = ~ DOPP. Now introducing the variable x defined thus
with ~ = 8.261 for
x = (1 -- 7)/3': = c f ~ K and if one plots f V ~ versus vrx, given the last equation, one m u s t obtain a straight line passing through the origin with a slope x/k. The value of A °, 0.250 £ - ' cm -1, appears to be smaller than it should be since the straight line
10 3
5
lo
15
V7
Fig. 5. Variation f v ~ v e r s u s V/X-for alkaline didecyl phosphate m DOPP with --log f =
and x = (1--7)/72. (o) NaDDP, (A) LiDDP.
168
does n o t pass through the origin but cuts the axis at a negative value of A °. The value corresponding to the required straight line is A ° = 0.500 ~ 4 cm-1 and the dissociation constant K = 3.6 X 10 -7 (Fig. 5). Using A ° = 0.5 ~-1 cm-i we also tried to determine K b y the classical Fuoss method [12] based on the variation of F(Z)/A with cAF 2 when the variable Z is defined by: Z = SA°-3/2x/~A S is the Onsager coefficient. S = aA ° + fi with a = 8.261,/~ = 0.103 and F(Z) is the continued fraction. F(Z) = 1 -- Z(1 -- Z(1 -- ...)-u2)-,/2 A straight line is expected, the slope of which is 1/KA °2- The value of K thus determined is K = 1.6 X 10 -7 (Fig. 6). This last m e t h o d is not as precise as the preceding one because of the difficulty inherent to the graphical determination of the A ° values. This lack of precision of the experimental points is due to the conductance of the solvent as has already been noted b y French and Hart [15]. The low values of the Walden product for NaDDP and LiDDP suggest the existence of dipole atmospheres round the cations particularly when the latter are small. This has been mentioned by French and Hart [15] when considering long chain dialkyl phosphonate solvent molecules which have a low dielectric constant, and a high dipole moment. This property is in line with the results given b y the potentiometric measurements which revealed a higher permeability of the liquid membrane, the smaller the monovalent cation. For the 2-1 Ca(DDP)2 electrolyte the preceding treatment cannot be applied and hence the limiting conductances and the dissociation constants cannot be evaluated. Nevertheless the results obtained do enable one to make certain hypothesis. For instance the sudden change in the slope of the conductivity concentration curves (Fig. 1) for concentration greater than 2 X 10 -~ M corre-
F
5C
2~
lo7 car
F Fig. 6. Variation of e A ~ / F ( Z ) with F / A for Li and Na DDP in DOPP. (©) NaDDP: (A) LiDDP.
169 sponds to the formation of aggregates, in this concentration range the densities of the solution remain constant while for lower concentrations it increases steadily (Fig. 2), for solution of long chain electrolytes the sudden changes in the partial molal volume and in the conductance curves are attributed to micelle formation [16]. In polar solvents the polar group of the ions is oriented towards the outer surface while the long chain forms the core of the micelle. On the contrary for solvents of low dielectric constant such as benzene the long chains are oriented towards the exterior of the micelle. Such solutions, for example long chain sulfonic acids like dinonylnaphthalene sulfonate in benzene are commonly used as liquid ion exchangers for liquid-liquid extraction purposes [17]. In DOPP, a solvent of low dielectric constant, the molecules of which contain two C10 chains and a polar group, the nature of the micelle cannot be established from classical consideration but one must neither eliminate the possibility of the formation of mixed micelles both by the solvent and the DPP- ions nor that of more highly structure associations giving rise to a pseudoliquid crystalline structured solution. Taking into consideration the specific interaction between the phosphate group and the calcium ions the possible existence of a pseudo-liquid crystalline structure of the liquid ion exchanger would lead to an electrode having similar properties as a solid membrane electrode formed by crystals or insoluble salts. The conductance of DOPP containing 2% of water is 60 times higher than the conductance of pure DOPP. This fact may be explained by the electronegative character of this organic molecule which confers to DOPP basic properties [18]. CONCLUSIONS (a) As the concentration of the long chain electrolyte in the ion exchanger falls in the region within which it is highly associated, this must be taken into consideration in any attempt to elucidate the working mechanism of the electrode as well as in the quantitative interpretation of interference effects. (b) No difference is observed in the conductance of Na and Li didecyl phosphoric salts although behaviour with respect to interference effects on the electrode response is not the same. (c) The presence of water brings about an appreciable increase in the conductance of DOPP and Ca(DDP)2 solutions in DOPP. For the m o m e n t it is impossible to establish if this effect is due to the H ÷ and OH- ions in the water or to a hydrolysis of the solvent. Be that as it may, the presence of water in the liquid membrane is of considerable importance and is probably to be related to the electrode potential drift recorded when a freshly prepared electrode is dipped in an aqueous solution. (d) NaC1 and CaCI~ increase considerably the conductance of DOPP + water and solutions of Ca(DDP)2 in DOPP + water, showing that a small penetration of this electrolyte into the membrane modifies greatly the number of free charge carriers in the membrane.
170
REFERENCES
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
F. Contl and G. Eisenman, Blophys. J., 6 (1966) 227. G. Eisenman, Anal. Chem., 40 (1968) 315. J. Sandblom, J. Phys. Chem., 73 (1969) 249. J. Sandblom, J. Phys. Chem., 73 (1969) 257. J. Bagg and W.P. Chaung, J. Chem., 24 (1971) 1963. J.H. Moore, Ph.D., Umversity of Texas at Austin, 1970. J.C. Nlchol and R.M. Fuoss, J. Phys. Chem., 54 (1954) 696. J.J. Zwolenik, R.M. Fuoss, J. Phys. Chem., 68 (1964) 903. C.A. Krauss, J. Chem. Ed., 35 (1957) 325. C.M. French and D.F. Muggleton, J. Chem. Soc., (1957) 5064. R.M. Fuoss and C.A. Krauss, J. Amer. Chem. Soe., 55 (1933) 476. R.M. Fuoss and C.A. Krauss, J. Amer. Chem. Soc., 57 (1935) 488. R.M. Fuoss J. Amer. Chem. Soc., 55 (1933) 2387. C.M. French and N. Stager, J. Chem. Soc., (1957) 5064. C.M. French and P.B. Hart, J. Chem. Soc., (1960) 1671. K. Shmoda, T. Nakagawa, B.I. Tamanushi and T, Isemura, Colloidal Surfaetants, Academic Press, New York, 1963. 17 G.Y. Markowts and G.R. Chopin in J.A. Marinsky and Y. Marcus (Eds): Ion Exchanger and Solvent Extraction, Marcel Dekker, New York, 1973, p. 51. 18 F. Krasovec and C. Klofutar, J. Inorg. NucL Chem., 30 (1968) 1640.