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Study of calcium ion selective electrodes containing Simon ionophores using impedance methods
175
J. Electroanal. Chem., 266 (1989) 175-177 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Short communication
Study of calcium ion selective electrodes ionophores using impedance methods
containing
Simon
Department of Chemistry, Bedson Building, Uniuersity of Newcastle upon Tyne, Newcastle NE1 7R U (Great Britain)
upon Tyne,
Interfacial properties R.D. Armstrong and M. Todd
(Received 13 February 1989)
INTRODUCTION
In an earlier paper [l] we have described the preparation of ion selective electrode membranes based on PVC plasticized with o-nitrophenyl ether and which contained calcium ions, tetraphenylborate ions and either ETHlOOl or ETH129. From measurements of the bulk resistances of the membranes we showed that there was present in the membranes either the complex Ca*+(ETH129), or Ca*+(ETHlOOl), provided that the ligand/Ca*+ ratio exceeded 2. In this present paper we report on the exchange currents at the interface between these membranes and aqueous CaCl, solutions and show that the ligand/Ca*+ ratio has to be greater than 5 before rapid Ca*+ exchange is established. EXPERIMENTAL
The membranes were prepared as described previously [l] by starting with membranes containing NaBPh, + ligand and ion exchanging with aqueous CaCl,. The ligand/NaBPh, ratio was chosen so as to produce ligand/Ca(BPh,), ratios between 2.0 and 6.0 at 0.25 intervals. An equivalent series of membranes was also prepared containing Mg2+ in place of Ca*+. The volume concentration of Ca2+ or Mg 2+ in all the membranes was constant at 1 mM. Four electrode impedance measurements were made on the membranes when they were in contact with either 0.1 M CaCl, or 0.1 M MgCl,, over the frequency range 104-lo-* Hz as described earlier [l]. All measurements were made at 23 + 1°C. 0022-0728/89/$03.50
0 1989 Elsevier Sequoia S.A
176
RESULTS
AND DISCUSSION
All the impedance spectra showed a high frequency semi-circle due to the bulk properties of the membranes, followed by a low frequency part (generally below 10 Hz) due to the interface between the membrane and the contacting aqueous phase. The nature of the interfacial part of the spectrum varied considerably with the membrane being studied, in particular the ligand/ion ratio. For example, for membranes containing Ca*+ and ETHlOOl when the ligand/Ca*+ ratio was 6, the interfacial impedance was too small to be resolved against the bulk impedance (Fig. l), indicating a rapid exchange of Ca 2+ between the aqueous solution and the membrane, hence a low R,,and a relatively high exchange current (i,,), perhaps in excess of 10 IJ.A cmP2. By contrast when the ETHlOOl/Ca*+ ratio was 2, in which case we would expect little free ETHlOOl to be present, a large charge transfer resistance was found (Fig. 2) consistent with an exchange current less than 3 X lo-’ ratio between 2.25 and 4.75, a Warburg A cm-*. For values of the ETHlOOl/Ca*+ impedance appeared to be present (as in Fig. 3). This is presumably associated with the diffusion in the membrane of low concentration intermediate species involved in the overall reaction Cal’(W)
+ 2 ETHlOOl(M)
+ Ca2+(ETH1001);(M)
These low concentration intermediate species may be Ca*+(M) or Ca*+ (ETHlOOl)(M). The values of the Warburg coefficients were consistent with an intermediate species at a concentration of - 10e5 mol dme3 for a ligand/Ca*+ ratio of ratio of 2.25, rising to a value of - 5 x 1O-4 mol dme3 for a ligand/Ca2+ 4.75. For ETH129 and Ca*+ the interfacial behaviour was similar to that for ETHlOOl and Ca*+, except that Warburg impedances were found only over the ligand/Ca2+ ratio 3.75 to 4.75 and rapid transfer was evident only after 5.5
16 1
a G6
.
12.
.
c
1
.
a.
6
a
10
12
14
.
.
16
10-4 Z’I R
10-4Z’ I R Fig. 1. Impedance spectrum for CaZt containing membrane in contact with aqueous CaCl, (0.1 M) with ratio of 6.0. Contact area is 0.78 cm2 on both sides of the the ligand ETHlOOl at a ligand/Ca*+ membrane. Fig. 2. As for Fig. 1 but with a ligand/Ca’+
ratio of 2.0.
177
0
6
8
10
12
14
16
Fig. 3. As for Fig. 1 but with a ligand/Ca*+
ratio of 3.25.
When Ca*+ was replaced by Mg *+ for both ETHlOOl and ETH129 large R,, values (> lo6 !I cm*) were found for a ligand/metal ratio of 2.0. For higher ligand/Mg*+ ratios, R,, gradually became smaller until at 5.5 : 1 for ETHlOOl, %, = 1 x lo4 $I cm* whilst for ETH129 at 6: 1, R,, = 5 X lo4 L? cm*. No Warburg impedances were evident. CONCLUSIONS
These measurements show that when no free ligancl ir; present in a membrane. i.e. when the ligand/Ca”~ ratio ia 2.0. the exchange ()I’ < ‘I hetut it c~cult:iclillg species solution and the calcium complex (C’a(l3I’I-I1001)~ or Ca(ETH129)~~ ) in the membrane is very slow; and that the rate of exchange gradually incrc,lscL ‘15 the ligand/metal ratio is changed from 2.0 to 6.0. ACKNOWLEDGEMENT
We would like to thank
SERC for their support
of this work.
REFERENCE 1 R.D. Armstrong and M. Todd, J. Electroanal. Chem.. 257 (1988) 161
J. Electroanal. Chem., 266 (1989) 175-177 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Short communication
Study of calcium ion selective electrodes ionophores using impedance methods
containing
Simon
Department of Chemistry, Bedson Building, Uniuersity of Newcastle upon Tyne, Newcastle NE1 7R U (Great Britain)
upon Tyne,
Interfacial properties R.D. Armstrong and M. Todd
(Received 13 February 1989)
INTRODUCTION
In an earlier paper [l] we have described the preparation of ion selective electrode membranes based on PVC plasticized with o-nitrophenyl ether and which contained calcium ions, tetraphenylborate ions and either ETHlOOl or ETH129. From measurements of the bulk resistances of the membranes we showed that there was present in the membranes either the complex Ca*+(ETH129), or Ca*+(ETHlOOl), provided that the ligand/Ca*+ ratio exceeded 2. In this present paper we report on the exchange currents at the interface between these membranes and aqueous CaCl, solutions and show that the ligand/Ca*+ ratio has to be greater than 5 before rapid Ca*+ exchange is established. EXPERIMENTAL
The membranes were prepared as described previously [l] by starting with membranes containing NaBPh, + ligand and ion exchanging with aqueous CaCl,. The ligand/NaBPh, ratio was chosen so as to produce ligand/Ca(BPh,), ratios between 2.0 and 6.0 at 0.25 intervals. An equivalent series of membranes was also prepared containing Mg2+ in place of Ca*+. The volume concentration of Ca2+ or Mg 2+ in all the membranes was constant at 1 mM. Four electrode impedance measurements were made on the membranes when they were in contact with either 0.1 M CaCl, or 0.1 M MgCl,, over the frequency range 104-lo-* Hz as described earlier [l]. All measurements were made at 23 + 1°C. 0022-0728/89/$03.50
0 1989 Elsevier Sequoia S.A
176
RESULTS
AND DISCUSSION
All the impedance spectra showed a high frequency semi-circle due to the bulk properties of the membranes, followed by a low frequency part (generally below 10 Hz) due to the interface between the membrane and the contacting aqueous phase. The nature of the interfacial part of the spectrum varied considerably with the membrane being studied, in particular the ligand/ion ratio. For example, for membranes containing Ca*+ and ETHlOOl when the ligand/Ca*+ ratio was 6, the interfacial impedance was too small to be resolved against the bulk impedance (Fig. l), indicating a rapid exchange of Ca 2+ between the aqueous solution and the membrane, hence a low R,,and a relatively high exchange current (i,,), perhaps in excess of 10 IJ.A cmP2. By contrast when the ETHlOOl/Ca*+ ratio was 2, in which case we would expect little free ETHlOOl to be present, a large charge transfer resistance was found (Fig. 2) consistent with an exchange current less than 3 X lo-’ ratio between 2.25 and 4.75, a Warburg A cm-*. For values of the ETHlOOl/Ca*+ impedance appeared to be present (as in Fig. 3). This is presumably associated with the diffusion in the membrane of low concentration intermediate species involved in the overall reaction Cal’(W)
+ 2 ETHlOOl(M)
+ Ca2+(ETH1001);(M)
These low concentration intermediate species may be Ca*+(M) or Ca*+ (ETHlOOl)(M). The values of the Warburg coefficients were consistent with an intermediate species at a concentration of - 10e5 mol dme3 for a ligand/Ca*+ ratio of ratio of 2.25, rising to a value of - 5 x 1O-4 mol dme3 for a ligand/Ca2+ 4.75. For ETH129 and Ca*+ the interfacial behaviour was similar to that for ETHlOOl and Ca*+, except that Warburg impedances were found only over the ligand/Ca2+ ratio 3.75 to 4.75 and rapid transfer was evident only after 5.5
16 1
a G6
.
12.
.
c
1
.
a.
6
a
10
12
14
.
.
16
10-4 Z’I R
10-4Z’ I R Fig. 1. Impedance spectrum for CaZt containing membrane in contact with aqueous CaCl, (0.1 M) with ratio of 6.0. Contact area is 0.78 cm2 on both sides of the the ligand ETHlOOl at a ligand/Ca*+ membrane. Fig. 2. As for Fig. 1 but with a ligand/Ca’+
ratio of 2.0.
177
0
6
8
10
12
14
16
Fig. 3. As for Fig. 1 but with a ligand/Ca*+
ratio of 3.25.
When Ca*+ was replaced by Mg *+ for both ETHlOOl and ETH129 large R,, values (> lo6 !I cm*) were found for a ligand/metal ratio of 2.0. For higher ligand/Mg*+ ratios, R,, gradually became smaller until at 5.5 : 1 for ETHlOOl, %, = 1 x lo4 $I cm* whilst for ETH129 at 6: 1, R,, = 5 X lo4 L? cm*. No Warburg impedances were evident. CONCLUSIONS
These measurements show that when no free ligancl ir; present in a membrane. i.e. when the ligand/Ca”~ ratio ia 2.0. the exchange ()I’ < ‘I hetut it c~cult:iclillg species solution and the calcium complex (C’a(l3I’I-I1001)~ or Ca(ETH129)~~ ) in the membrane is very slow; and that the rate of exchange gradually incrc,lscL ‘15 the ligand/metal ratio is changed from 2.0 to 6.0. ACKNOWLEDGEMENT
We would like to thank
SERC for their support
of this work.
REFERENCE 1 R.D. Armstrong and M. Todd, J. Electroanal. Chem.. 257 (1988) 161