ANALYST AUGUST 1986 VOL. 111 891 Mercury( II) and Silver( I) Ion-selective Electrodes Based Crown Ethers Ming-Tain Lai and Jeng-Shang Shih" Department of Chemistry National Taiwan Normal University Taipei Taiwan 1 17, on Dithia Republic of China Mercury (Hg2+) and silver (Ag+) ion-selective PVC membrane electrodes based on 1,4-dithia-I 2-crown-4 and 1,4-dithia-l5-crown-5 as neutral carriers were successfully developed. Both electrodes exhibited good linear responses of 30 and 40 mV decade-1 for Hg2+ and Ag+/ respectively within the concentration ranges 10-2-10-6 M Hg(N03)2 and 10-1-10-6 M AgN03. Some other crown ethers and cryptands were also investigated as neutral carriers for both ions. Both the mercury and silver electrodes exhibited comparatively good selectivities for mercury(l1) and silvertl) ions in comparison with alkali metal alkaline earth metal and some heavy metal ions.These crown ether ion-selective electrodes are suitable for use with aqueous solutions at pH 3 2. They were applied as sensors in titrations of Br- and CI- with Ag+ and of Hg2+ with 1- and Cr2072- and in the determination of the solubility products of AgCl in aqueous solutions. Keywords Dithia crown ethers; mercury(//) and silver(/) ion-selective electrodes Crown ethers have been demonstrated to be highly selective complexing agents for many metal and can be applied in their separation5-7 and determination.sl0 In general a crown ether can form a stable complex with a metal ion that fits well in the cavity of the crown ether. Taking advantage of their ion-discriminating ability crown ethers have been shown11-19 to be suitable neutral carriers for ion-selective electrodes (ISEs) especially for potassium as common crown ethers such as 18-crown-6 and 15-crown-5 can form stable complexes with alkali metal ions especially potassium ions.However crown ether-based ion-selective electrodes for transition metal ions such as Ag+ and Hg2+ have not been reported as they would suffer interference from alkali metal ions. On the other hand by using thia crown ethers in which the oxygen atoms are partly replaced with sulphur the interference from alkali metal ions can be expected to be significantly reduced as Hg2+ and Ag+ ions form stronger complexes with thia crown ethers than do alkali metal ions.20.21 There is no commercially available mercury( 11) ion-selective electrode.Even in the literature there is no report of a specific electrode for Hg2+ ions but a few other electrodes, such as the iodide electrode (AgI - Ag2S) which respond to mercury ions have been described.22-24 For Ag+ ions, homogeneous and heterogeneous silver sulphide solid elec-trodes are commercially available. The compacted crystal silver electrode gives a good performance but it is difficult to prepare; the crown ether silver electrode prepared in this work is more easily fabricated. Systematic studies performed with various crown ethers and cryptands in this study showed that PVC membranes containing 1,4-dithia-12-crown-4 and 1,4-dithia-15-crown-5 were suitable as neutral carriers for mercury(I1) and silver( I) ion-selective electrodes respec-tively.The electrochemical selectivities for various ions and the effects of the membrane matrix crown ether concentration, internal solution and pH for both Hg2+ and Ag+ electrodes were investigated. In addition both electrodes were used in the titration experiments for the determination of Hg2+ and Ag+ ions. Experimental Chemicals All chemicals were of analytical-reagent grade. 1,4-Dithia-15-crown-5 1,4-dithia-12-crown-4 and 1,7-* To whom correspondence should be addressed. dithia-12-crown-4 were synthesised by methods reported in the literature.25 1,4-Dithia-15-crown-5 was prepared by vacuum distillation (200 "C/3 mmHg) of a mixture of l,ll-dichloro-3,6,9-tri-oxundecane (25 g) ethane-1,2-diol (9.5 g) and NaOH (9 g) in ethanol and recrystallisation from a 1 + 1 benzene - hexane mixture.Yield 11%; m.p. 51-52 "C. NMR 6 = 2.63(t), 2.78(s) 3.55(s) and 3.69(t). GC - MS M+ = 252. 1,7-Dithia-12-crown-4 was obtained by vacuum distillation (200 "C/3 mmHg) of a mixture of bis(2-mercaptoethyl) ether (9.2 g) bis(2-chloroethyl) ether (9.5 g) and NaOH (6.0 g) in ethanol. Yield 19%; colourless liquid. NMR 6 = 2.77(t), 1,4-Dithia-12-crown-4 was prepared by vacuum distillation (260 "C/3 mmHg) of a mixture of 1,2-bis(2-chloroethoxy)-ethane (14 g) ethane-1,2-dithiol (7 g) and NaOH (6.0 g) in ethanol and purification by passage through a silica gel column with 3 + 1 benzene - hexane as the eluent. Yield 16%; colourless liquid. NMR 6 = 2.61(t) 2.91(s) 3.56(s) and Cryptand 222 and Cryptand 22 were obtained from E.3.47(t). GC - MS M+ = 208. 3.77(t). GC - MS:M+ = 208. Merck. Electrode Preparation A mixture of about 100 mg of PVC,26,27 30 mg of crown ether and 50 mg of dibutyl phthalate was dissolved in 2 ml of tetrahydrofuran (THF). In some instances especially for the mercury( 11) electrode sodium tetraphenylborate (STPB) was also added. The PVC - THF solution was poured into a glass dish of diameter 5 cm and the THF was evaporated at room temperature for about 24 h. A semi-transparent membrane about 0.03 mm thick was obtained. A good linear correlation between the thickness of the membrane and the amount of PVC was found. A piece about 12 mm in diameter was cut from the PVC membrane and attached to a polyethylene cap by wetting the membrane with the PVC - THF solution mentioned above.The diameter of the exposed membrane was about 7 mm. The polyethylene cap with the membrane was then incorporated into an Ag - AgCl wire electrode or Hg - Hg2C12 electrode. After filling with internal solution contain-ing 5 X 10-4 M Hg(N03)2 or AgN03 and 10-2 M HN03 the electrode was conditioned by soaking in 0.1 M Hg(N03)2 or AgN03 solution for 24 h. A salt bridge containing KN03 was prepared in each instance. The electrochemical system for this study was as follows: Ag - AgCl or Hg - Hg2C12 1 internal solution ( 5 X M AgN03 or Hg(N03)2 + 10-2 M HN03) 1 PVC membrane 892 ANALYST AUGUST 1986 VOL. 111 testing solution 1 salt bridge (1 M KN03) I satd. KCl 1 Hg2C12 -The e.m.f. measurements were made with a Basic Model 321 digital pH/mV meter.The response times of these crown ether electrodes were checked and were fairly short ( S l min). In this work the potential measurements were taken 3 min after the introduction of the test solutions which were stirred during the measurements. Hg Evaluation of Electrode Selectivity The selectivity coefficients (KPAOgfM or for various ions were evaluated graphically by the mixed-solution method.28 The electrical potentials of solutions of the Ag+ or Hg2+ ions alone (10-5 M) and of a mixed solution containing a fixed amount of Ag+ or Hg2+ ions (activity uHg or uAg = M) and a varying amount of the interfering ion M*+ (activity a“) were measured as El and E2 respectively and can be expressed as follows28: (I) KPA”,’M OE = { exp[ ( ~ 2 - E ~ ) F / R T ‘ I } ~ ~ - aAg .. and a$ = {exp[(E2 - EI)F/RT])aHg - aHg * (2) where z is the charge of the interfering ion. KP2iM can be evaluated as the slope of the graph of {exp[(E2 - EI)F/ R g } a A g - U A ~ against a#. miM can be obtained in a similar manner but with a# above being replaced by a%. Results and Discussion Various macrocyclic polyethers containing the crown ethers 1,4-dithia-12-crown-4 1,7-dithia-12-crown-4 1,4-dithia-15-crown-5 and monobenzo-15-crown-5 and cryptands such as cryptand 222 and 22 were tried as neutral carriers for the Ag+ electrode. The e.m.f. potential responses of PVC - Ag+ electrodes based on these macrocyclic polyethers are illus-trated in Fig. 1. It appears that the crown ethers give a better potential response than the cryptands which show almost no response to the presence of the Ag+ ion.It is well known that macrocyclic polyethers are very sensitive to the size of the metal ion. Among these macrocyclic polyethers cryptand 222 and 22 have the most suitable cavity 150 100 > E Li 50 size (0.28 nm)29 to fit the Ag+ ion which has a size of 0.26 nm.30 Even though both crown ethers and cryptands have very similar cavity sizes cryptands generally can form stronger complexes than crown ethers with the Ag+ ion.2 It is reasonable to predict that both the cryptands studied here will form very strong complexes2 with Ag+ ions which results in a difficulty in exchanging the Ag+ ions of the complexes in the PVC membrane with the Ag+ ions in the test solution.Hence the lack of response with both cryptands can be understood. In contrast as shown in Fig. 1 the electrodes based on crown ethers seem to exhibit a linear response to the activity of Ag+ ions. Among these Ag+ - crown ether electrodes that based on 1,4-dithia-15-crown-5 seems to show the best sensitivity with a slope of 40 k 2 mV pAg-1 and the widest linear range within the concentration range 10-1-10-5 M AgN03. Therefore in this work further studies on the Ag+ ion-selective electrode based on 1,4-dithia-15-crown-5 were carried out. Although the sensitivity with a slope of 40 mV pAg-1 for this electrode is good enough the response is non-Nernstian. The non-Nernstian slopes were observed even though we changed the composition of the membrane (e.g.by adding sodium tetraphenylborate which usually increases the sensi-tivity of ion-selective electrodes) changed the matrix of the membrane by using polystyrene and poly(viny1 acetate) instead of PVC and also changed the concentration of all the components in the membrane and the concentration of the internal solution in the electrode. In contrast the response of the Hg2+ electrode based on crown ethers is Nernstian (30 mV pHg-I) as shown in Fig. 2. In general the non-Nernstian slope was considered to be an “anion effect” or “anion response.”31 According to Boles and Buck’s theory,32 the response slope at 25 “C can be expressed by where UAgC+ and Ux- are the mobilities of AgC+ (the complex of Ag+ and crown ether C) and X- (Nos- in this instance) in the membrane.Only when uAgC+ >> UX- or UX-= 0 is the response slope Nernstian (60/n where n is the charge on Ag+). If the N03- ions are partially extracted as \ A IS O I 0 s w 0’ I I 1 I I 7 6 5 4 3 2 1 PAg + Fig. 1. Potential responses of Ag+ ion-selective electrodes based on various crown ethers and cryptands. (A) 1,4-Dithia-15-crown-5; (B) 1,4-dithia-12-crown-4; (C) monobenzo-15-crown-5; (D) 1,7-dithia-12-crown-4; (E) cryptand 22; and (F) cryptand 222. Internal solution 5 X lop4 M AgNQ + 5 X M KN03. Silver nitrate solution was used as the test solutio ANALYST AUGUST 1986 VOL. 111 I.,.,, 6 5 4 3 2 1 P M 2 + Fig. 2. Potential responses of Hg2+ ion-selective electrodes based on (A) 1,4-dithia-15-crown-5 (B) 1,7-dithia-12-crown-4 and (C) 1,4-dithia-12-crown-4 with the addition to the membranes of sodium tetraphenylborate (STPB) at an STPB to crown ether ratio of 0.5.Mercury(I1) nitrate solution was used as the test solution t Lu 6 5 4 3 2 1 pHg2+ Fig. 3. Effect of sodium tetraphenylborate (STPB) in membranes on potential rewonses of an Hg2+ ion-selective electrode based on i,4-dithia-12-'crown-4 with STPB to crown ether ratios of (A) 0 (B) 0.5 (C) 1.0 and (D) 2.0 AgC+ - NO3- ion pairs into the membrane this results in a reduction in the average mobility of the Ag+ ion or an increase in the average mobility of the NO3- anion. According to equation (3) this effect eventually leads to a reduction in the slope of the calibration graph Ux- # 0. If this is true UAFc+ = 5UNO3- can be obtained from the calculation according to equation (3) and the slope of 40 mV pAg-1.For the mercury(I1) ion-selective electrode as shown in Fig. 2 among the thia crown ethers examined 1,4-dithia-l2-~rown-4 seems to be the best neutral carrier exhibiting a Nernstian response with a slope of 30 k 1 mV pHg-1 and the widest h e a r range within the concentration range 10-6-10-3 M Hg(N03)2. However sodium tetraphenylborate (STPB) must be added to the membranes of all these Hg2+ electrodes, otherwise very poor electrical responses will be observed as shown in Fig. 3A. The addition of STPB results in a large improvement in the response slope. According to the litera-ture,31-33 the addition of STPB to a membrane can increase the electrical conductivity and the sensitivity of the electrode.t UI I I 1 1 1 I I 7 6 5 4 3 2 1 7 6 5 4 3 2 7 P&l+ Fig 4. Effects of (a) crown ether and (b) PVC contents in 5 cm diameter membranes of Ag+ ion-selective electrodes based on 1,4-dithia-15-crown-5. Amount of crown ether (A) 10; (B) 20; (C) 30; (D) 40; (E) 80; and (F) 100 mg t Lu 11° I I I I 1 1 I 1 7 6 5 4 3 2 1 PA^ + . -Fig. 5. Effect of membrane supports on otential responses of Ag+ ion-selective electrodes based on 1,4-ditka-15-crown-5. (A Poly-(vinyl chloride) (PVC); (B) poly(viny1 acetate) (PVA); (C] poly-styrene In addition according to Boles and Buck's theory [equation (3)] when the TPB- anion replaces NO3- in the membrane, because of its higher relative molecular mass its mobility is expected to be lower than that of NO3- and the reduction in the mobility of the anion ( Ux-) 1x1 the membrane can lead to an improvement of the slope.As illustrated in Fig. 3C and D with STPB to crown ether ratios of 1.0 and 2.0 the slopes are super-Nernstian (>30 mV pHg-1). At an STPB to crown ether ratio of 2.0 (Fig. 3D) the response slope reaches 60 mV pHg-1 which is a typical monovalent response and may be attributed to the formation of Hg(TPB)+ ions in the membrane. However Hg2+ elec-trode membranes with an STPB to crown ether ratio >0.5 exhibited poor reproducibility and salting out occurred after about 1 month whereas the membrane with an STPB to crown ether ratio of 0.5 showed good reproducibility and a Nernstian bivalent (Hg2+) response. In contrast the addition of STPB did not cause any improvement and in fact caused a deterioration of the response slopes of the silver( I) ion-selective electrodes based on thia crown ethers.This could be due to the formation of AgC+TBP- ion pairs in the membrane which leads to an increase in the mobility of anions or a decrease in the mobility of Ag+ ions and eventually a reduction in the response slope. Maximum electrical responses are found in all Hg2+ electrodes and some Ag+ electrodes based on thia crown ethers. According to Simon's theory,31 this maximum concen-tration (amax) at the maximum response is inversely propor-tional to the stability constant (KeJ of the complex of the metal ion (ML+) with the ligand (crown ether in this instance) and can be expressed as amax = constant x Kex-l/(z + 1).I 894 ANALYST AUGUST 1986 VOL. 111 t Lu I I 5 4 3 2 1 PH Fig. 6. Effect of pH of test solutions on potential responses of an Hg2+ ion-selective electrode based on 1,4-dithia-12-crown-4. (A) [Hg2+] = 10-3 M; and (B) [Hg2+] = M in solutions + Table 1. Selectivity coefficients ( K T t J for various ions with a silver(1) ion-selective electrode based on 1 .f-dithia- 15-crown-5 KPot Ag M Ion Li + . . . . <10-4 Na+ . . . 5 x 10-2 K+ . . . . 5 x 10-3 Ca2+ . . . . 4 0 - 4 sr2+ . . . . 4 0 - 4 Ba2+ . . . . 4 0 - 4 Mg*+ . . . . 2.8 x 10-2 Fe3f . . . . 0.87 Ion Pb2+ . . . . co2+ . . . . zn2+ . . . . Cd2+ . . . . cu2+ . . . . Ni2+ . . . . Hg2+ . . . . K g M < 10-4 < l o - 4 2.5 x 10-4 4 x 10-4 4 x 10-4 < 10-4 0.77 Table 2.Selectivity coefficients (KFgtM) for various ions with a mercury(I1) ion-selective electrode based on 1,4-dithia-l2-crown-4 Li + Na + K+ Rb+ cs+ Mg2+ Ca2+ Sr2+ Ba2+ Ion K g M . . . . 4 . 7 ~ 10-4 . . . . 7 . 3 ~ 10-4 . . . . 1.2 x 10-4 . . . . 1 . 2 ~ 10-3 . . . . <lo-5 . . . . <10-5 * . . . 7.2 x 10-4 . . . . 1.6 x 10-3 . . . . 6 . 0 ~ 10-5 Fe3 + cu2+ Cd2+ Zn2 + Pb2+ Ni2+ C02+ Ag+ Ion Kf$ M . . . . 3.5 x 10-2 . . . . 8.9 x 10-4 . . . . 4.4 x 10-4 . . . . 7.5 x 10-4 . . . . < 10-5 . . . . 2.5 x 10-4 . . . . 7.6 x 10-4 . . . . <10-5 7 6 5 4 3 2 1 PM Fig. 7. Selectivity measurements for various metal ions with an Ag+ ion-selective electrode based on 1,4-dithia-15-crown-5.[Ag+] = 10-5 M in solutions t UI t Lu ,x-x \ x/ v a/ 0 h C02+ Ag+ VZn l2+ YNi2, I I I 1 I 6 5 4 3 2 1 PM Fig. 8. Selectivity measurements for various metal ions with an Hg2 + ion-selective electrode based on 1,4-dithia-12-crown-4. [Hg2+] = 10-5 M in solutions other words among these thia crown ethers 1,4-dithia-12-crown-4 may form the weakest complex with Hg+ ions and gives the largest value of amax and the widest linear concentra-tion range. Unfortunately insufficient experimental data in the literature could be applied to confirm this suggestion. The effect of the composition of the PVC membranes containing PVC crown ethers and dibutyl phthalate as a plasticiser for both Ag+ and Hg2+ electrodes was also investigated.For example the effects of the contents of crown ether and PVC in a membrane with a diameter of 5 cm for the Ag+ electrode are shown in Fig. 4(a) and ( b ) respectively. As shown in Fig. 4(a) the electrode membranes containing 20-30 mg of crown ether exhibit better responses. In addition the pxA X 6 5 4 3 2 1 P M Fig. 9. Potential responses of an Hg'+ ion-selective electrode based on 1,4-dithia-12-crown-4 for the testing solutions containing (A) Hg(NOd2 (B) HgN03 and (C) HgCl, electrode with 30 mg of crown ether seems to have the lowest detection limit (10-6 M). Similar results were found for the Hg*+ electrode and therefore in subsequent work 30 mg of the crown ether was generally applied. In general the thickness of the membrane depends on the content of PVC in the membrane.A good correlation between the thickness and the PVC content of the membrane was found. As shown in Fig. 4(b) the sensitivity and the detection limit of the electrode seem to increase with a decrease in the PVC content. However if the membrane is too thin it is easily broken. Therefore in this study 100 mg of PVC were used to prepare the membranes for all membranes for both the Ag+ and Hg2+ electrodes. Polystyrene (PS) and poly(viny1 acetate) (PVA) were tried as alternatives to PVC but as shown in Fig. 5 PVC was much better. The effect of the pH of test solutions on these ion-selective electrodes was also studied. As illustrated in Fig. 6 for the Hg2+ electrode the potential responses of 10-3 and 10-4 M Hg2+ in solutions seem to indicate no significant changes at pH > 2 but at pH d 2 the potential responses are drastically reduced.Similar results were also found with the Ag+ selective electrode This could be due to protonation of the crown ether in the membrane at pH d 2 which results in a los ANALYST AUGUST 1986 VOL. 111 895 (a) I T 1 2 3 4 5 6 7 1 2 3 4 5 1 6 7 8 9 1 0 Titrant volumeiml Fig. 10. A plications of Ag+ and Hg2+ ion-selective electrodes based on ditiia crown ethers in titrations of (a) Br- (5 mmol) with Ag+ M); ( c ) Hg2+ (250 mrnol) with I- (0.1 M); and ( d ) Hg2+ (250 mmol) with Cr2072- (0.1 M) M); ( b ) Ag+ (5 mmol) with C1-of their complexing ability with the metal ions (Ag+ or Hg2+). The selectivities of both the Hg2+ and Ag+ ion-selective electrodes were investigated by making potential measure-ments in solutions containing a fixed amount of Hg2+ or Ag2+ ions (10-5 M) and varying amounts of interfering ions.The interferences of alkali metal alkaline earth metal Pb2+ and some common transition metal ions such as Fe3+ Co2+ Ni2+, Cu2+ Zn2+ and Cd2+ were studied. As shown in Figs. 7 and 8, except for Fe3+ these common transition metal ions (M”+) at [Mn+] < 10-3 M in 10-5 M Hg2+ or Ag+ solutions cause virtually no interferences with either the Hg2+ or Ag+ electrodes. The selectivity coefficients (WlM or for various ions (Mz+) were evaluated as the slopes of the graphs of {exp[(E2 - E1)FIRTl}aAg - aAg or {exp[E2 - E#’l RTJ}aHg - aHg against a# or a$ (for the Ag+ and Hg2+ electrodes respectively) as mentioned above and in previous papers.18Jg As shown in Tables 1 and 2 for both electrodes the selectivity coefficients for most metal ions are fairly small (10-2-10-5).In other words most common ions cause only small interferences with either Hg2+ or Ag+ electrodes. The Hg2+ electrode based on 1,4-dithia-l2-crown-4 was very sensitive to the anion as shown in Fig. 9; nearly no potential response to the Hg2+ ion was found for HgC12 (C), whereas a Nernstian response was observed for Hg(N03)~ (A). This could be attributed to the formation of C1- Hg2+ C1- or Hg(OH)+CI- ion pairs34 in HgCI2 solution (C). In addition the Hg2+ electrode also showed a response to the Hg+ ion not only to the Hg2+ ion. However as illustrated in Fig. 9B the electrical response of the Hg2+ electrode to Hg+ ions is non-Nernstian with a slope of 28 mV pHg+-l compared with 30 mV (pHg2+)-1 for the Hg2+ ion.The non-Nernstian (bivalent) response for the Hg+ ion can be understood as this ion is well known to exist as Hg22+ in aqueous solutions. In other words the Hg2+ ion-selective electrode shows a similar degree of response to both Hg2+ and Hg+ ions. Both the Ag+ and Hg2+ ion-selective electrodes were applied as sensors in titrations of Br- with Ag+ and of Ag+ with C1- [Fig. lO(a) and (b)] and of Hg2+ with I- and Cr2072-[Fig. 1O(c) and ( d ) ] . In addition the Ag+ electrode was also applied in the evaluation of the solubility products (K:p) of AgCl in aqueous solution [Fig. 10(b)] as 1.6 x 10-10 which is in good agreement with the literature value35 of 1.7 X lo-*().Because of the low solubilities of the thia crown ethers in water these crown ether - PVC membranes can be used repeatedly for at least 1 month. It can be concluded that both silver and mercury ion-selective electrodes based on dithia crown ethers exhibit good sensitivities detection limits (10-5 M for Ag+ and 10-6 M for Hg2+) reproducibilities and selectivities. The authors express their appreciation to the National Science Council of the Republic of China for financial support of this study. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. References Pedersen C.J. J . Am. Chem. SOC. 1967,89 7017. Christensen J. Eatough D. J . and Izatt R. M. Chem. Rev., 1974 74 351. Frensdorff H. K. J. Am. Chem. SOC. 1971,93 600. Kolthoff I. M. Anal. Chem. 1979 51 1R. Danesi P. R. J. Inorg. Nucl. Chem. 1975 37 1479. Blasius E. Jansen K. P. Adrian W. Klantke G . . Dorschneider R. Mauner G. P. Nguyen V. B. and Strockenen J. Fresenius 2. Anal. Chem. 1977 284 337. Jepson B. E. and Dewitt R. J. Inorg. Nucl. Chem. 1976,38, 1175. Pedersen C. J. and Frensdorff H. K. Angew. Chem. Int. Ed. Engl. 1972 11 16. Hogen-Esch T. E. Kopolow S . and Smid J . Macromol-ecules 1973 6 133. Kopolow K. Machacek Z . Takaki Z . and Smid J. J. Macromol. Sci. Chem. 1973,7 1015. Ryba O. and Petranek J. J . Electroanal. Chem. Interfacial Electrochem.1973,44 425. Petranek J. and Ryba O. Anal. Chim. Acta 1974 72 375. Ryba O. and Petranek J . J. Electroanal. Chem. 1976 67, 321. Tamura H . Kirnura K. and Shono T. J. Electroanal. Chem. 1980 115 115. Kirnura K. J. Electroanal. Chem. 1979 95 91. Shono T. Okahara M. Ikeda I . Kirnura K. and Tamura, H. J . Electroanal. Chem. 1982 132 99. Kamata S . Higo M. Kamibeppn T. and Tanaka I . Chem. Lett. 1982 287. Jeng J. and Shih J. S. Analyst 1984 109 641. Wang D . and Shih J . S . Analyst 1985 110 635. Vogtle F. and Weber E. Angew. Chem. 1974,89 126. Sedvic D. and Meider H. J. Inorg. Nucl. Chem. 1971 39, 1409. Orion Newsl. 1970 9a 10. “Analytical Method Guide,” Fifth Edition Orion Research, Cambridge MA 1972. Oehme F. and Dolzalova L. Fresenius Z . Anal. Chem., 1970 251 1. Bradshaw J. S . Hui J. Y. Haymore B. L. Christensen J. J., and Izatt R. M. J . Heterocycl. Chem. 1973 10 1. Shatkay A . Anal. Chem. 1967 39 1056. Moody G. J. Oke R. B. and Thomas J. D. R. Analyst, 1970 95 910. Srinivasan K. and Rechnitz G. A . Anal. Chem. 1969 41, 1203. Dietrich B. Lehn J. M. and Sauvage J. P. Tetrahedron Lett. 1969 2885. Cotton F. A . and Wilkinson F. R. “Advanced Inorganic Chemistry,” Fourth Edition Wiley New York 1980 p. 14. Simon W. Anal. Lett. 1974 7 9. Boles J. H. and Buck R. P. Anal. Chem. 1973 45 2057. Koryta J . Anal. Chim. Acta. 1977 61 329. Lamb J. D. J. Am. Chem. SOC. 1980 102 3039. Peters D. G . Hayes J. M. and Hieftje G . M . “Chemical Separations and Measurements,” Saunders Philadelphia, 1974 p. 205. Paper A51280 Received July 30th 198.5 Accepted January loth 198