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Ag+-selective electrodes based on lipophilic thioethers
Sensors and Actuators
Ag+-selective
B 24-25 (1995) 183-187
electrodes based on lipophilic thioethers
Wojciech Wrbblewski, Zbigniew Brzdzka * Depo~~?nt
of Analytical
Chemistry, Wamw
Universily of Technology, Noakowskicgo
3, 00-664
Warsaw, Poland
Abstract Membrane silver-selective electrodes based on lipophilic acyclic thioethers 1-6 are described. The and Ag+ responses of these membranes have been determined and compared with membranes based All tested thioethers show selectivity towards Ag+ versus other interfering cations, except mercury. (log k,,M < -5) are obtained for acyclic thioether 4. The highest Ag(I)/Hg(II) selectivities (lp4) based on acyclic thioethers 2 and 4.
potentiometric on macrocyclic However, the are found for
selectivities thioethers. best values membranes
Keywords: Lipophilic thioethers; Silver-selective electrodes
1. Introduction The presently available solid-state silver electrode exhibits a good selectivity towards alkali-, alkaline-earthand some transition-metal ions; however, the strong mercury interference is a weakness of this electrode [l]. This is why a search for new silver-selective sensors has been in progress for the past few years. An alternative might be membrane silver-selective electrodes based on neutral carriers (ionophores). It has been shown theoretically that the potentiometric selectivity of liquid membranes based on neutral carriers depends on the complexation specificity of the carrier molecules involved and on the composition of the membrane. In most instances, the distribution of cation complexes is the predominant factor governing the selectivity of the membrane. The distribution process combines complejr formation in an aqueous phase and transfer of the complexes formed to the membrane phase. Depending on the ligand structure and cation size, complex formation or extractability is the determining factor for selectivity [2]. Selection of carriers is based on the type of donor atoms and on size-match selectivity in macrocycles. However, the role of steric strain in complex formation is an important factor. It is well known that sulphur ligands coordinate with transition-metal cations as exclusive donor atoms. In comparison to aza or oxa ligands, the heavier donor atoms, e.g., S or P when neutral, * Corresponding author. 0925-4005/95/$09.50 SSDI
0
1995 Elsevier
0925-4005(94)01340-N
Science
S.A.
All rights reserved
coordinate well only to the soft metal ions, which tend to be large and often poorly solvated, such as Ag(I) or Hg(I1) [3]. Lai and Shih [4] tested several thiacrown ethers as ionophores in Ag+ ion-selective electrodes @Es) and they found the best sensitivity (40 mV dec-‘) for 1,4dithia-1%crown-5 (PVCidibutyl phthalate), with selectivity coefficients (log kAg,J towards Na+, K+ and Mg” ions higher than - 3.5. The strongest interferent was the Hg(I1) cation (+0.8). Better results were obtained by Oue et al. [5]. ISEs (PVC/d&y1 phthalate (DOP)) with both mono- and di-thiacrown ethers were highly silver selective. The selectivity coefficients for silver with respect to the heavy- and transition-metal ions are < -3 and with respect to alkali- and alkalineearth-metal ions < -4. The mercury ions interfered most seriously (log k,,,, = - 2.2 to - 1.5). The selectivities of the silver electrodes with different thiacrown ethers were quite similar in spite of different numbers and positions of the sulphur atoms in the thiacrown ethers. Casabo et al. [6] have described the performance of several thiabenzocrown ethers as ionophores for 1SEs (PVC/DOP). A Nernstian sensitivity towards silver was found for all the ligands tested. Mercury was the strongest interferent (log kAg,Hg= - 2 to - 2.6), while for other cations log k,,, < -4. Also in this case it was found that despite the different geometries and numbers of sulphur donor atoms in the crown ethers, the selectivity coefficients of the corresponding electrodes were similar.
184
K Wrdbkwski 2. B&.&a I Sensom and Achrafors B 24-25 (1995) 183-187
Silver-selective electrodes (PVC/o-NPOE) based on calixarene derivatives with four sulphur and/or nitrogen donor atoms as ionophores have been described [7]. All exhibited acceptable silver responses, with the best (calix[4]arene substituted with four CH,CO,C,H,SCH, groups) giving a response of 50 mV dec-’ in the Ag(I) ion activity range 10-4-10-’ M. Selectivity coefficients (log kAg,& were higher than -3.3 for C$+, Ni’+, Cc?, Cd’+, and higher than -2.0 for K+, Pb” and Na+. Hg(I1) cations interfered the most (log kA&M = +2.0). Recently Malinowska et al. [S] investigated silver-selective electrodes based on dithioether functionalized calix[4]arenes as ionophores, which exhibited almost theoretical Nernstian slopes in the activity range 10-6-10-’ of silver ions. The selectivity coefficients (log kAg,J were lower than -2.2 for Hg(I1) and lower than -4.6 for other cations tested. We focused on lipophilic thioethers as neutral ionophores for heavy-metal ions. Two groups of lipophilic acyclic thioethers were synthesized and investigated as ionophores in PVC membranes. Ag+ selectivities of ISEs were determined by the fixed interference method (FIM) and separate solutions method (SSM) for different interfering metal cations (Ca’+, K+, Pb’+, Cd*+, Cu’+ and Hg’+) and compared with values for membranes based on macrocyclic thioethers.
2. Experimental 2.1. Chemicals All salts employed were of analytical grade and were purchased from POCH Gliwice, Poland. The standard stock solutions (0.1 M) of metal nitrates were prepared in redistilled water; working solutions were obtained by dilution of the stock solution with redistilled water. The pH was adjusted by the addition of nitric acid or sodium hydroxide solutions. 2.2. Ionophores and membrane materials Ionophores ld were synthesized according to known procedures [9]. The synthesis of ionophores 7-9 has been described previously [lo]. High-molecular-weight poly(viny1 chloride) (PVC), potassium tetrakis(4-chlorophenyl)borate (KTpCIPB), dioctylphthalate (DOP) and o-nitrophenyl octyl ether (o-NPOE) were obtained from Fluka. As a solvent for the membrane components, freshly distilled tetrahydrofuran (THF) p.a. (POCh Gliwice) was used. 2.3. Membrane and electrodepreparation The membranes contained 1 wt.% ionopbore, 50 or 75 mol% KTpClPB (relative to the ionophore), 65-66
wt.% plasticizer and 33 wt.% poly(vinyl chloride) (PVC). The membrane components (200 mg in total) were dissolved in 2 ml of freshly distilled THF. This solution was placed in a glass ring of 24 mm i.d. resting on a glass plate. After solvent evaporation overnight, the resulting membrane was peeled off from the glass mould and discs of 7 mm i.d. were cut out. Membrane discs were mounted in electrode bodies (type IS 561, Philips, Eindhove, The Netherlands) for electromotive force (e.m.f.) measurements. As internal filling solution, a 0.005 M solution of AgNO, and 0.05 M KNO, adjusted to pH=3 by HNO, was used. The electrodes were subsequently conditioned overnight in a solution of 0.005 M KNO, and 0.001 M AgNO,. For each membrane composition two electrodes were prepared. 2.4. E.m.f: measurements All measurements were carried out at 20 “C with cells of the following type:
Ag; AgCl; KCl(O.l M)]O.l M KNO,]sample solutionl(sensor memhrane(]intemal filling solution; AgCI; Ag. The e.m.f. values were measured using a custommade 16-channel electrode monitor. Details of this equipment are described in Ref. [ll]. Potentiometric selectivity coefficients (k,,& were determined by the fixed interference method [2,12]by increasingthe activity of primary ion in the solution in steps of 0.5 log aAg, or for strongly interfering ions by the separate solution method using 0.01 M solutions of metal nitrates at a constant pH 4 (pH 3 for mercury). In these cases the experimentally obtained slope was used for the calculations. The activities of metal ions in aqueous solutions were calculated according to the Debye-Huckel approximation [12]. The performance of the electrodes was examined by measuring the e.m.f.s of the primary ion solutions within the concentration range lo-‘-lo-’ M in solutions stirred with a magnetic stirrer. The response time (t& of the electrode was tested by measuring the time required to achieve a 95% steady potential for a 0.01 M solution, when the Ag(I) ion concentration was rapidly increased by one decade from 0.0001 to 0.01 M.
3. Results and discussion The structures of the compounds examined as silverselective ionopbores are presented in Fig. 1. All ligands used contain at least two sulphur atoms. All tested lipopbilic acyclic tbioetbers l-3 showed Ag+ responses in the presence of lead cations (0.01 M) (see Fig. 2). However, the membrane based on ionopbore 3 displayed a curve with a significant effect
w. Wn5blewski, Z. Brzhka I Sm.wrs and Acluafors B 24-25 (1995) 183-187
185
1 -t+.l
--K
--K
ca =Cd4
cd
-cu
=oUEb -ca
_I%
-acu
4 5oy
KTdlFa
I
7
I
Fig.3. Selectivity coefficients,logk~,,forPVCmembranescontaining ionophores l-3 and blank (without ionophore) membrane. Membrane: 50 mol% KTpClPB, DOP as a plasticizer. FIM method: 0.01 M solutions of nitrates, pH 4, internal electrolyte: 0.005 M AgNO,+O.OS M KNCIs pH 3.
9
a
Fig. 1. Structures of ionophores
Dop
I
1-9.
223
zoo 173 I30 5‘ g
‘23
g
‘m
t
I3
0’ -1
j -6
1 -3
’
10; -’ 1
1
a
1 -2
-I
Ag
Fig. 2. Ag+ responses of electrodes based on ionophores 1-3 and DOP as a plasticizer (with addition of 50 mol% FXpCIPB) in the presence of 0.01 M of Pb(NO&, pH=4.
of sample anions. This deviation from a Nemstian response might be because these ligands form extremely stable complexes with cations. According to the model calculations described by Cobben [13], this S-curve type of response can be caused by the outer boundary potential (membrane-sample solution). The association constant (&) of complex formed between the primary cation and the ionophore should be extremely high (B1= 1013 mol 1-l) to cause such an effect. Ag’ selectivities versus other interfering cations were found for the membranes tested. However, the Ag(I)/ Hg(II) selectivity depends on the structure of the ionophore, e.g., a better value (1v5) was obtained for an electrode based on thioether containing two sulphur atoms (ionophores 1 and 2) (Fig. 3). The Ag+-selective electrode based on ionophore 2 showed a linear response
0’ -1
/ -6
“d
” -3
/
I -2
-I
Fig. 4. Ag+ responses of electrodes based on ionophore 2 and DOP as a plasticizer (with addition of 50 mol% KTpClPB) in the presence of interfering cations: a, 0.01 M Pb2+; n 0.01 M Cu’+; A, 0.01 M Cd”; *, lo-’ M Hg?+, pH=4.
in the range 1.5-5 pAg with slopes of 58.0, 57.8 and 55.1 mV in the presence of 0.01 M of Cd”, Pb*+ and Cu2+ cations, respectively (Fig. 4). The Ag+ response of this electrode in the presence of 10e4 M Hg’ cations is linear in the range 1.5-4 pAg with an almost Nernstian slope (57.6 mV), in contrast to the subNemstian value reported elsewhere [4,5]. Introduction of oxygen or nitrogen as more basic donor atoms into ionophore molecules decreases the Ag+ selectivity over other transition-metal ions, but it can modify the Ag+/H$’ selectivity. We synthesized a group of thioethers with different pendant groups (ionophores 4-6) (see Fig. 1). The potentiometric se-
186
W. Wr66lrwski Z. Bmhka I Sensors and Actualors B 24-2s (199s) 183-187
lectivities and electrode responses of membranes based on these ionophores have been determined and compared under different measurement conditions. The best Ag+ selectivity over mercury cations was shown by membranes based on ionophore 4 in PVC plasticized by o-nitrophenyl octyl ether (o-NPOE) (Fig. 5). An incorporation of ionophore 4 into a polar membrane changed the selectivity pattern drastically in comparison with the so-called ‘blank membrane’. It is known in the literature that silver complexes with aromatic ligands are stabilized by interaction of r-electrons with silver. This effect is minor in the cases of ionophores 5 and 6; it seems that a nitrogen-containing pendant group in the ionophore molecules interacts strongly with Hg cations. A membrane Ag+-selective electrode based on ionophore 4 showed a linear response in the range l-5.5 pAg with a Nernstian slope in the presence of 0.1 M of K’, Pb” and CL?” cations, respectively (Fig. 6). The Ag+ response of this electrode in the presence of 1O-3 M Hg’+ cations is linear in the range 1-3.8 pAg with the Nernstian slope as well. The response time (ts5%) of the electrode based on thioether 4 was found to be less than 10 s and the Ag’ response is not pH sensitive in the range pH 4-8. The macrocyclic thioethers have the ability to discriminate between closely related heavy-metal ions based on the relative fit of the ligand cavity size to the metal-ion radius. All tested macrocyclic thioethers showed a silver response and a selectivity towards Ag‘ and Hg’+ versus other interfering cations. Ionophore 8 with a carbonyl group exhibited Ag+ preference over Hg’+. However, introduction of a carbonyl group causes a lower Ag+ selectivity with respect to all the tested
blark
4
5
6
Fig. 6. Selectivity coefficients, logk,,,M, for PVC membranes containing ionophores 4-6and blank (without ionophore)membrane. Membrane: 75 mol% KTpCIPB, o-NPOE as a plasticizer. SSM method: 0.1 M solutions of nitrates, pH 4, internal electrolyte, 0.005 M AgNO,+O.OS M KNOj, pH 4
r
225
173
1
350
Fig. 7. Ag+ responses of electrodes based on ionophore 9 and DOP as a plasticizer (with addition of 50 mol% KTpCIPB) in the presence of 0.01 M of interfering cations: A, PbZf; 0, Cu’+; +, Cd’+ n, KC, pH=4. ’,
loga
Ag
Fig. 5. Ag’ responses of &xtrodes based on ianophores 4 and oNPOE as a plasticizer (with addition of 75 mol% KTpClPB) in the presence of interfering cations: A, 0.1 M Cu*+; 0, 0.1 M Pb*+; *, 0.1 M K+; W, lo-’ M H$+. pH=3.
cations. The highest Ag + selectivities (log kAg,M< - 4.5) were obtained for the 14-membered tetrathioether 9 with an exocyclic methylene group. The Ag’ responses of the electrode based on this ionophore in the presence of different interfering cations (CS?‘, Cd*+, Pb” and K+) are displayed in Fig. 7. The macrocyclic thioethers already mentioned were previously applied for the design of Ag+-selective CHEMFETs [14]. Thioether functionalized calix[4]arenes have also been investigated as ionophores [15]. The best results were obtained with membranes containing dithioether functionalized calix[4]arene, potassium tetrakis(4chlorophenyl)borate and bis(l-butylpentyI)adipate as a plas-
W. Wr6biewki, Z. Brz6zka I Sensors and Actuators R 24-25 (1995) 183-187
ticizer. The Ag’ response function exhibited an almost theoretical Nernstian slope in the activity range 10-6-10-’ M of silver ions with selectivity coefficients (log k_,+& of -2.2 for Hg(II) and lower than -4.5 for the other cations tested.
4. Conclusions A plasticized PVC membrane without any ionophore, a so-called ‘blank membrane’, shows Ag+ selectivity due to its higher partition coefficient of monovalent silver cations in comparison with other transition-metal cations. Incorporation of lipophilic acyclic thioether into the membrane causes an almost Nernstian Ag’ response and improves the silver selectivity of membrane versus interfering cations. However, the Ag(I)/Hg(II) selectivity can be improved by modifying acyclic thioether using different pendant groups with heteroatoms.
Acknowledgements
This work was supported by the State Committee for Scientific Research, Project no. 2 0775 9101 sponsored in 1991-1994. We thank Dr J. Wasilewski for his assistance in the preparation of acyclic thioethers 1-6.
References Y. Umezawa, CRC Handbook of Ion-Selective Electrodes: Sel&vi& Gxj@ientr, CRC, Boca Raton, FL, 1990. [2] A. Lewenstam and A. Hula&d, Selectivity coefficients of ionsensing electrodes, Selective Elechode Rev., I2 (1990) 161. [l]
187
I31 R.D. Hancock and A.E. Martell, Ligand design for selective complexation of metal ions in aqueous solution, Chem. Rev., 89 (1989) 1875. t41 M.T. Lai and J.S. Shih, Mercury(I1) and silver(I) ion-selective electrodes based on dithiacrown ethers, Analyst, 111 (1986) 891. PI M. Oue, K Kimura, K. Akama, M. Tanaka and T. Shone, Lipophilic thiacrown ether derivatives as neutral silver-ion selective carriers, Anal. Sci., 5 (1989) 165. WI .I. Casabo, L. Mestres, L. Escriche, F. Teixidor and C. PerezJimenez, Silver(I) ion-selective electrodes based on polythiamacrocycles, J. Chem. Sot., Dabon Tran.~., (1991) 1969. [71 K.M. O’Connor, G. Svehla, S.J. Harris and McKervey, Calixarene-based potentiometric ion-selective electrodes for silver, Tafunhr, 39 (1992) 549. ISI E. Malinowska, 2. Brzdzka, K. Kasiura, R.J.M. Egberink and D.N. Reinhoudt, Silver selective electrodes based on thioether fonctionalized calix[4]arenes ionophores, Anai. Chim. Actu, 298 (1994) 245. PI J.M. Desper, D.R. Powell and S.H. Gellman, Molecular structure and conformation of two acyclic polythioethers; Implications for the design of heav metal chelators, I. Am. Chem. SK, 112 (19%) 4321. WI J. Buter, R.M. Kellogg, F. van Bolhuis, Synthesis, complexation behaviour and reactions of thia-crown ethers incorporating propan-2-one units, 3. Chem. Sot. Chem. Commun., (1991) 910. WI Z. Brzbzka, Multichannel data acquisition workstation for ISE, Pomiary, Au~omalykn, Kon~rola, 5 (1988) 422. WI G.G. Guibault, R.A. Dorst, MS. Frant, H. Freiser, E. Hansen, T. Light, E. Pungor and J.D.R. Thomas, Report from Commission on Analytical Nomenclature: Recommendations for Nomenclature of Ion-selective Electrodes, Pure A&. Chew., 48 (1976) 127. [I31 P.L.H.M. Cobben, Sensors for heavy metal ions based on ISFETs, Ph.D. Thesis, University of Twente, Enschede, 1992. t141 2. Brzbzka, P.L.H.M. Cobben, J.J.H. Edema, J. Buter, R.M. KelIogg and D.N. Reinhoudt, Chemically modified field-effect transistors; potentiometric Ag+-selectivity of PVC membranes based on macrocyclic thioethers, Anal. Chim. Acta, 273 (1993) 139. [1Sl P.L.H.M. Cobben, R.J.M. Egberink, J.G. Bromer, P. Bergveld, W. Verboom and D.N. Reinhoudt, Transduction of selective recogtition of heavy metal ions by chemically modified field effect transistors (CHEMFETs), 3. Am. Chem. Sot., 114 (1992) 10 573.
Ag+-selective
B 24-25 (1995) 183-187
electrodes based on lipophilic thioethers
Wojciech Wrbblewski, Zbigniew Brzdzka * Depo~~?nt
of Analytical
Chemistry, Wamw
Universily of Technology, Noakowskicgo
3, 00-664
Warsaw, Poland
Abstract Membrane silver-selective electrodes based on lipophilic acyclic thioethers 1-6 are described. The and Ag+ responses of these membranes have been determined and compared with membranes based All tested thioethers show selectivity towards Ag+ versus other interfering cations, except mercury. (log k,,M < -5) are obtained for acyclic thioether 4. The highest Ag(I)/Hg(II) selectivities (lp4) based on acyclic thioethers 2 and 4.
potentiometric on macrocyclic However, the are found for
selectivities thioethers. best values membranes
Keywords: Lipophilic thioethers; Silver-selective electrodes
1. Introduction The presently available solid-state silver electrode exhibits a good selectivity towards alkali-, alkaline-earthand some transition-metal ions; however, the strong mercury interference is a weakness of this electrode [l]. This is why a search for new silver-selective sensors has been in progress for the past few years. An alternative might be membrane silver-selective electrodes based on neutral carriers (ionophores). It has been shown theoretically that the potentiometric selectivity of liquid membranes based on neutral carriers depends on the complexation specificity of the carrier molecules involved and on the composition of the membrane. In most instances, the distribution of cation complexes is the predominant factor governing the selectivity of the membrane. The distribution process combines complejr formation in an aqueous phase and transfer of the complexes formed to the membrane phase. Depending on the ligand structure and cation size, complex formation or extractability is the determining factor for selectivity [2]. Selection of carriers is based on the type of donor atoms and on size-match selectivity in macrocycles. However, the role of steric strain in complex formation is an important factor. It is well known that sulphur ligands coordinate with transition-metal cations as exclusive donor atoms. In comparison to aza or oxa ligands, the heavier donor atoms, e.g., S or P when neutral, * Corresponding author. 0925-4005/95/$09.50 SSDI
0
1995 Elsevier
0925-4005(94)01340-N
Science
S.A.
All rights reserved
coordinate well only to the soft metal ions, which tend to be large and often poorly solvated, such as Ag(I) or Hg(I1) [3]. Lai and Shih [4] tested several thiacrown ethers as ionophores in Ag+ ion-selective electrodes @Es) and they found the best sensitivity (40 mV dec-‘) for 1,4dithia-1%crown-5 (PVCidibutyl phthalate), with selectivity coefficients (log kAg,J towards Na+, K+ and Mg” ions higher than - 3.5. The strongest interferent was the Hg(I1) cation (+0.8). Better results were obtained by Oue et al. [5]. ISEs (PVC/d&y1 phthalate (DOP)) with both mono- and di-thiacrown ethers were highly silver selective. The selectivity coefficients for silver with respect to the heavy- and transition-metal ions are < -3 and with respect to alkali- and alkalineearth-metal ions < -4. The mercury ions interfered most seriously (log k,,,, = - 2.2 to - 1.5). The selectivities of the silver electrodes with different thiacrown ethers were quite similar in spite of different numbers and positions of the sulphur atoms in the thiacrown ethers. Casabo et al. [6] have described the performance of several thiabenzocrown ethers as ionophores for 1SEs (PVC/DOP). A Nernstian sensitivity towards silver was found for all the ligands tested. Mercury was the strongest interferent (log kAg,Hg= - 2 to - 2.6), while for other cations log k,,, < -4. Also in this case it was found that despite the different geometries and numbers of sulphur donor atoms in the crown ethers, the selectivity coefficients of the corresponding electrodes were similar.
184
K Wrdbkwski 2. B&.&a I Sensom and Achrafors B 24-25 (1995) 183-187
Silver-selective electrodes (PVC/o-NPOE) based on calixarene derivatives with four sulphur and/or nitrogen donor atoms as ionophores have been described [7]. All exhibited acceptable silver responses, with the best (calix[4]arene substituted with four CH,CO,C,H,SCH, groups) giving a response of 50 mV dec-’ in the Ag(I) ion activity range 10-4-10-’ M. Selectivity coefficients (log kAg,& were higher than -3.3 for C$+, Ni’+, Cc?, Cd’+, and higher than -2.0 for K+, Pb” and Na+. Hg(I1) cations interfered the most (log kA&M = +2.0). Recently Malinowska et al. [S] investigated silver-selective electrodes based on dithioether functionalized calix[4]arenes as ionophores, which exhibited almost theoretical Nernstian slopes in the activity range 10-6-10-’ of silver ions. The selectivity coefficients (log kAg,J were lower than -2.2 for Hg(I1) and lower than -4.6 for other cations tested. We focused on lipophilic thioethers as neutral ionophores for heavy-metal ions. Two groups of lipophilic acyclic thioethers were synthesized and investigated as ionophores in PVC membranes. Ag+ selectivities of ISEs were determined by the fixed interference method (FIM) and separate solutions method (SSM) for different interfering metal cations (Ca’+, K+, Pb’+, Cd*+, Cu’+ and Hg’+) and compared with values for membranes based on macrocyclic thioethers.
2. Experimental 2.1. Chemicals All salts employed were of analytical grade and were purchased from POCH Gliwice, Poland. The standard stock solutions (0.1 M) of metal nitrates were prepared in redistilled water; working solutions were obtained by dilution of the stock solution with redistilled water. The pH was adjusted by the addition of nitric acid or sodium hydroxide solutions. 2.2. Ionophores and membrane materials Ionophores ld were synthesized according to known procedures [9]. The synthesis of ionophores 7-9 has been described previously [lo]. High-molecular-weight poly(viny1 chloride) (PVC), potassium tetrakis(4-chlorophenyl)borate (KTpCIPB), dioctylphthalate (DOP) and o-nitrophenyl octyl ether (o-NPOE) were obtained from Fluka. As a solvent for the membrane components, freshly distilled tetrahydrofuran (THF) p.a. (POCh Gliwice) was used. 2.3. Membrane and electrodepreparation The membranes contained 1 wt.% ionopbore, 50 or 75 mol% KTpClPB (relative to the ionophore), 65-66
wt.% plasticizer and 33 wt.% poly(vinyl chloride) (PVC). The membrane components (200 mg in total) were dissolved in 2 ml of freshly distilled THF. This solution was placed in a glass ring of 24 mm i.d. resting on a glass plate. After solvent evaporation overnight, the resulting membrane was peeled off from the glass mould and discs of 7 mm i.d. were cut out. Membrane discs were mounted in electrode bodies (type IS 561, Philips, Eindhove, The Netherlands) for electromotive force (e.m.f.) measurements. As internal filling solution, a 0.005 M solution of AgNO, and 0.05 M KNO, adjusted to pH=3 by HNO, was used. The electrodes were subsequently conditioned overnight in a solution of 0.005 M KNO, and 0.001 M AgNO,. For each membrane composition two electrodes were prepared. 2.4. E.m.f: measurements All measurements were carried out at 20 “C with cells of the following type:
Ag; AgCl; KCl(O.l M)]O.l M KNO,]sample solutionl(sensor memhrane(]intemal filling solution; AgCI; Ag. The e.m.f. values were measured using a custommade 16-channel electrode monitor. Details of this equipment are described in Ref. [ll]. Potentiometric selectivity coefficients (k,,& were determined by the fixed interference method [2,12]by increasingthe activity of primary ion in the solution in steps of 0.5 log aAg, or for strongly interfering ions by the separate solution method using 0.01 M solutions of metal nitrates at a constant pH 4 (pH 3 for mercury). In these cases the experimentally obtained slope was used for the calculations. The activities of metal ions in aqueous solutions were calculated according to the Debye-Huckel approximation [12]. The performance of the electrodes was examined by measuring the e.m.f.s of the primary ion solutions within the concentration range lo-‘-lo-’ M in solutions stirred with a magnetic stirrer. The response time (t& of the electrode was tested by measuring the time required to achieve a 95% steady potential for a 0.01 M solution, when the Ag(I) ion concentration was rapidly increased by one decade from 0.0001 to 0.01 M.
3. Results and discussion The structures of the compounds examined as silverselective ionopbores are presented in Fig. 1. All ligands used contain at least two sulphur atoms. All tested lipopbilic acyclic tbioetbers l-3 showed Ag+ responses in the presence of lead cations (0.01 M) (see Fig. 2). However, the membrane based on ionopbore 3 displayed a curve with a significant effect
w. Wn5blewski, Z. Brzhka I Sm.wrs and Acluafors B 24-25 (1995) 183-187
185
1 -t+.l
--K
--K
ca =Cd4
cd
-cu
=oUEb -ca
_I%
-acu
4 5oy
KTdlFa
I
7
I
Fig.3. Selectivity coefficients,logk~,,forPVCmembranescontaining ionophores l-3 and blank (without ionophore) membrane. Membrane: 50 mol% KTpClPB, DOP as a plasticizer. FIM method: 0.01 M solutions of nitrates, pH 4, internal electrolyte: 0.005 M AgNO,+O.OS M KNCIs pH 3.
9
a
Fig. 1. Structures of ionophores
Dop
I
1-9.
223
zoo 173 I30 5‘ g
‘23
g
‘m
t
I3
0’ -1
j -6
1 -3
’
10; -’ 1
1
a
1 -2
-I
Ag
Fig. 2. Ag+ responses of electrodes based on ionophores 1-3 and DOP as a plasticizer (with addition of 50 mol% FXpCIPB) in the presence of 0.01 M of Pb(NO&, pH=4.
of sample anions. This deviation from a Nemstian response might be because these ligands form extremely stable complexes with cations. According to the model calculations described by Cobben [13], this S-curve type of response can be caused by the outer boundary potential (membrane-sample solution). The association constant (&) of complex formed between the primary cation and the ionophore should be extremely high (B1= 1013 mol 1-l) to cause such an effect. Ag’ selectivities versus other interfering cations were found for the membranes tested. However, the Ag(I)/ Hg(II) selectivity depends on the structure of the ionophore, e.g., a better value (1v5) was obtained for an electrode based on thioether containing two sulphur atoms (ionophores 1 and 2) (Fig. 3). The Ag+-selective electrode based on ionophore 2 showed a linear response
0’ -1
/ -6
“d
” -3
/
I -2
-I
Fig. 4. Ag+ responses of electrodes based on ionophore 2 and DOP as a plasticizer (with addition of 50 mol% KTpClPB) in the presence of interfering cations: a, 0.01 M Pb2+; n 0.01 M Cu’+; A, 0.01 M Cd”; *, lo-’ M Hg?+, pH=4.
in the range 1.5-5 pAg with slopes of 58.0, 57.8 and 55.1 mV in the presence of 0.01 M of Cd”, Pb*+ and Cu2+ cations, respectively (Fig. 4). The Ag+ response of this electrode in the presence of 10e4 M Hg’ cations is linear in the range 1.5-4 pAg with an almost Nernstian slope (57.6 mV), in contrast to the subNemstian value reported elsewhere [4,5]. Introduction of oxygen or nitrogen as more basic donor atoms into ionophore molecules decreases the Ag+ selectivity over other transition-metal ions, but it can modify the Ag+/H$’ selectivity. We synthesized a group of thioethers with different pendant groups (ionophores 4-6) (see Fig. 1). The potentiometric se-
186
W. Wr66lrwski Z. Bmhka I Sensors and Actualors B 24-2s (199s) 183-187
lectivities and electrode responses of membranes based on these ionophores have been determined and compared under different measurement conditions. The best Ag+ selectivity over mercury cations was shown by membranes based on ionophore 4 in PVC plasticized by o-nitrophenyl octyl ether (o-NPOE) (Fig. 5). An incorporation of ionophore 4 into a polar membrane changed the selectivity pattern drastically in comparison with the so-called ‘blank membrane’. It is known in the literature that silver complexes with aromatic ligands are stabilized by interaction of r-electrons with silver. This effect is minor in the cases of ionophores 5 and 6; it seems that a nitrogen-containing pendant group in the ionophore molecules interacts strongly with Hg cations. A membrane Ag+-selective electrode based on ionophore 4 showed a linear response in the range l-5.5 pAg with a Nernstian slope in the presence of 0.1 M of K’, Pb” and CL?” cations, respectively (Fig. 6). The Ag+ response of this electrode in the presence of 1O-3 M Hg’+ cations is linear in the range 1-3.8 pAg with the Nernstian slope as well. The response time (ts5%) of the electrode based on thioether 4 was found to be less than 10 s and the Ag’ response is not pH sensitive in the range pH 4-8. The macrocyclic thioethers have the ability to discriminate between closely related heavy-metal ions based on the relative fit of the ligand cavity size to the metal-ion radius. All tested macrocyclic thioethers showed a silver response and a selectivity towards Ag‘ and Hg’+ versus other interfering cations. Ionophore 8 with a carbonyl group exhibited Ag+ preference over Hg’+. However, introduction of a carbonyl group causes a lower Ag+ selectivity with respect to all the tested
blark
4
5
6
Fig. 6. Selectivity coefficients, logk,,,M, for PVC membranes containing ionophores 4-6and blank (without ionophore)membrane. Membrane: 75 mol% KTpCIPB, o-NPOE as a plasticizer. SSM method: 0.1 M solutions of nitrates, pH 4, internal electrolyte, 0.005 M AgNO,+O.OS M KNOj, pH 4
r
225
173
1
350
Fig. 7. Ag+ responses of electrodes based on ionophore 9 and DOP as a plasticizer (with addition of 50 mol% KTpCIPB) in the presence of 0.01 M of interfering cations: A, PbZf; 0, Cu’+; +, Cd’+ n, KC, pH=4. ’,
loga
Ag
Fig. 5. Ag’ responses of &xtrodes based on ianophores 4 and oNPOE as a plasticizer (with addition of 75 mol% KTpClPB) in the presence of interfering cations: A, 0.1 M Cu*+; 0, 0.1 M Pb*+; *, 0.1 M K+; W, lo-’ M H$+. pH=3.
cations. The highest Ag + selectivities (log kAg,M< - 4.5) were obtained for the 14-membered tetrathioether 9 with an exocyclic methylene group. The Ag’ responses of the electrode based on this ionophore in the presence of different interfering cations (CS?‘, Cd*+, Pb” and K+) are displayed in Fig. 7. The macrocyclic thioethers already mentioned were previously applied for the design of Ag+-selective CHEMFETs [14]. Thioether functionalized calix[4]arenes have also been investigated as ionophores [15]. The best results were obtained with membranes containing dithioether functionalized calix[4]arene, potassium tetrakis(4chlorophenyl)borate and bis(l-butylpentyI)adipate as a plas-
W. Wr6biewki, Z. Brz6zka I Sensors and Actuators R 24-25 (1995) 183-187
ticizer. The Ag’ response function exhibited an almost theoretical Nernstian slope in the activity range 10-6-10-’ M of silver ions with selectivity coefficients (log k_,+& of -2.2 for Hg(II) and lower than -4.5 for the other cations tested.
4. Conclusions A plasticized PVC membrane without any ionophore, a so-called ‘blank membrane’, shows Ag+ selectivity due to its higher partition coefficient of monovalent silver cations in comparison with other transition-metal cations. Incorporation of lipophilic acyclic thioether into the membrane causes an almost Nernstian Ag’ response and improves the silver selectivity of membrane versus interfering cations. However, the Ag(I)/Hg(II) selectivity can be improved by modifying acyclic thioether using different pendant groups with heteroatoms.
Acknowledgements
This work was supported by the State Committee for Scientific Research, Project no. 2 0775 9101 sponsored in 1991-1994. We thank Dr J. Wasilewski for his assistance in the preparation of acyclic thioethers 1-6.
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