Calixarene-based potentiometric ion-selective electrodes for silver

Talanta, Vol. 39, No. 11, pp. 1549-1554, 1992 printed in Great Britain. All rights -cd

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Copyright 0 1992 Pergamon Pleas Ltd

CALIXARENE-BASED POTENTIOMETRIC ION-SELECTIVE ELECTRODES FOR SILVER KATMIRINE M. O’CONNOR and GYULA SVEHLA* Department of Chemistry, University College, Cork, Ireland STEPHEN J. HARRIS

School of Chemical Sciences, Dublin City University, Dublin, Ireland M. ANTHOIWMCKERWY School of Chemistry, Queens’ University, Belfast, Northern Ireland, U.K. (Received 12 March 1992. Accepted 31 March 1992) Summary-Four lipophilic sulphur and/or nitrogen containing calixarene derivatives have been tested as ionophores in Ag(I)-selective poly (vinyl chloride) membrane electrodes. All gave acceptable linear responses with one giving a response of 50 mV/dec in the Ag(1) ion activity range lo-‘-lo-‘M and high selectivity towards other transition metals and sodium and potassium ions. This ionophore was also tested as a membrane coated glassy-carbon electrode where the sensitivity and selectivity of the conventional membrane electrode was found to be repeated. The latter electrode was then used in potentiometric titrations of halide ions with silver nitrate.

Calixzarenes’ are cyclic oligmers of phenolformaldehyde condensates which when derivatixed at the phenolic oxygen group show receptor inophoric activity. Derivatives containing a wide range of functional groups have been synthesized and have been shown to have the ability to complex alkali and alkali-earth metal cations selectively into the cavity present in the cone conformation.* A number of calixarene derivatives have been incorporated as neutral carriers into ion-selective electrodes sensitive to sodium ionqU potassium ions’ and ceasium ions.8*9 The sodium selective electrodes have been used successfully in the determination of sodium in human blood4*10 and recently a sodium selective CHEMFET has been reported.” A polymeric tetrameric calixarene has been used in a modified carbon paste electrode for the stripping voltammetry of copper, lead and mercury. ‘* Due to the success of these calixarene derivatives a number have been patented. However, calixarene based ionselective electrodes sensitive to transition metal ions have not yet been reported as they would suffer interference from alkali metal ions. *Author for correspondence..

Crown compounds containing nitrogen and sulphur atoms in the ring have been incorporated into poly (vinyl chloride) membranes to give electrodes sensitive to Ag(1) ions with some Hg(II) interference. I3914 By using calixarene derivatives with functional groups containing nitrogen and sulphur atoms the interference from alkali metal ions is expected to be significantly reduced.i5 In this paper the performance of four calixarene derivatives, tested as ionophores for silver selective electrodes, is described. The most promising is also tested as a membrane-coated glassy carbon electrode. There are many descriptions on the usage of glassy carbon electrodes in voltammetry, amperometry, potentiometry and coulometq@ but there are a limited number of reports on the usage as a substrate of the sensor phase in ion-selective electrodes. It has been recently used successfully in dip coated enzyme sensors” and tested as a substrate for a variety of sensor phases by Jovanoic et UP. Also carbon rods have been dip coated for use in lead(I1) selective electrodes.19 An application of these types of electrodes in potentiometric titrations of halide ions with silver nitrate is also described.

1549

KATHERINE M. O’CONNOR et al.

1550 EXPERIMENTAL

Materials

The ionophores employed in this work were synthesized by S. J. Harris and M. A. McKervey and are shown in Fig. 1. The membrane components, PVC (ISE grade), potassium tetrakis (4chlorophenyl) borate (KTpClPB), 2nitrophenyl octyl ether (ZNPOE) and AnalaR grade tetrahydrofuran (THF) were obtained from Fluka. The nitrates of sodium, potassium, nickel, copper(II), lead(II), cadmium(II), cobalt(I1) and mercury(I1) were of AnalaR grade as were the potassium chloride, bromide and iodide. The silver nitrate was obtained from Johnson Matthey Chemicals Ltd. All solutions and standards were made up with doubly distilled water. Electrode preparation

The membrane components, [0.66% m/m ionophore, 0.17% m/m KTpClPB, 65.84% m/m 2-NPOE and 33.33% m/m PVC] were

t-m

t-Bu

OR,

OR,

mixed and dissolved by stirring in THF overnight. For the preparation of a conventional membrane electrode the resulting PVC-THF syrup was poured into a glass mouId,20 and the solvent was allowed to evaporate off at room temperature, over a period of 24 hr. A semi-transparent flexible membrane was obtained from which discs of 7 mm diameter were cut using a cork borer. These discs could be pasted, using THF, to an interchangeable PVC tip clipped to the end of the electrode body which consisted of an AgAgCl wire immersed in an internal solution of 5 x 10e4M silver nitrate and 10m2M nitric acid. A membrane-coated glassy-carbon electrode was prepared as follows. Initially, the end of a glassy-carbon electrode (3 mm in diameter) was polished on A&O, slurry and air dried. It was then dip coated to a depth of 10 mm in a PVC-THF syrup and dried at room temperature. This was repeated three times. The PVC-THF syrup was of the same composition as that for the conventional membrane electrodes.

2

Ionophore.

1 2H 52

CH 2b,C

) 2H 52

(ii)

CH *b,,

(iii)

CH,CO,CH,CH,SCH,

a-l&O&H,

(IV)

CH,CO,CHZCH,SCH,

CH,CO,CH,CH,SCH,

Fig. 1. Structure of ionophores.

Caliiarene-based potentiometric ion-selective elewodes for silver

Measurement of electrode potentials All measurements were carried out at 25” in a thermostatted potentiometric cell. The electrochemical systems for the study were as follows:

1551

with ionophore (i) giving the lowest response of 38.26 mV/dec in the limited range of 10m3.*to lo-‘.s M. This trend may be simply attributed to the number and type of “soft” donor atoms present within each ionophore. It should be

Ag,AgCl I int. soln.(5 x 10m4M AgNO, + IO-*M HN03) 1PVC membrane 1sample/ salt bridge (1 M KNO,) I 1M KNO,)l M KNO, I Hg2C12,Hg. Carbon I PVC membrane (sample I salt bridge (1 M KNO,) I Hg,Cl,,Hg. The potential readings were measured using a Metrohm 654 millivot/pH meter and registered on a Linseis LM 24 chart recorder. The reference electrode was a Metrohm saturated calomel. The performance of the electrodes were assessed by measuring parameters such as slope, limit of detection, selectivity, response times and temperature dependence of the electrode. Selectivity coefficients (log K&,J were determined using the separate solution method*’ at a concentration of 0.1 M. Dynamic responses were measured by injection of 90.9 ~1; of 1M silver nitrate solution into 10 ml of 10e3M silver nitrate on a fast chart speed (10 cm/min) thus producing a 1 x 10e3M to 1 x lo-*M change in concentration. The temperature dependence was investigated by noting the potential change in solutions of 10e3M, IO-*M and lo-‘M silver nitrate respectively over a temperature range of 5-45” (27& 318 K). Lifetime of the electrodes was investigated by recalibrating them periodically and calculating the response over the linear range Reproducibility studies were carried out by determining the coefficient of variation for five consecutive potential readings in solutions of 10m3M and lo-*M silver nitrate respectively. The instruments used for the potentiometric titrations were a Metrohm E 536 Potentiograph and E 535 Dosimat.

noted that ionophore (iv) contains S atoms and no N atoms where as in ionophore (i) the opposite is true. When comparing the selectivity coefficients (log ptWM) for the electrodes based on ionophores (i) to (iv) in Fig. 2(b) it can be seen that HgZ+ and Na+ ions are the major (4

I

/

ionophore

(iv) > (iii) > (ii) > (i)

(iii)

x’

/x

x’

/ /

(ii)

JX

~J;,~‘x,x;x,~r;;;

x----+e+r

x,L~x .fl

x--y-y-x

,x

,x’

rA

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I

-6

-5

I -4

I -3

I

I

-2

-I

lop a

m

2

r

,B

-pb

-I-

-M -pb

Four nitrogen and/or sulphur containing calixarene derivatives, shown in Fig. 1 were incorporated into poly (vinyl chloride) membrane electrodes and were shown to give acceptable linear responses to Ag(1) ion activity with limits of detection of between 10e3.* and 10W4M.The calibration graphs of Fig. 2(a) indicate that the degree of linearity of the response occurs in the order of:

J

J

/

)(

Y

RESULTS AND DISCUSSION

,/”

50 mV

-2

-

-I(

-R

-I(

-K -K

-cd -3

-

-co -a

-8

-

NI

-co

x;

---g

-N,

-4

---_$

1 16,

III1

(1111

,,“I

hphors

Fig. 2. (a) Potential responses of con\ientional membrane electrodes incorporating ionophorea (Q-o-(v)to silver (I) ion activity. (i) 38.26 mV/dec, (ii) 45.67 mV/dec, (ii) 47.64 mV/dec and (iv) 50.01 mV/dec. (b) Comparison of sektivity coe&ients (log pw) for conventional membrane ektrodea incorporating ionophores (+0-(v).

1552

KATHERINE M. o’cm4No~ et d.

interferents followed closely by Pb2+ ions. Phase transfer and stability studies on a wide range of tetrameric calixarene derivatives have shown them to be excellent complexing agents for alkali and alkali-earth metal ions with, in the majority of cases, a clear preference for sodium ions2. However, very little work has been reported on tetrameric calixarene derivatives with functional groups containing “soft” donor atoms such as N and S atoms which according to Pearso# are more likely to complex heavy metal ions such as Ag+, Hg2+ and Pb2+. With ionophores (i) to (iv) the soft donor atoms are imparting some selectivity to heavy metals but there is still a significant response to Na+ ions. Ionophore (iii) is a calixarene with mixed ligating functional groups, one being the ethyl ester. In general the trends in extraction and stability constants observed in a homo calixarene containing a certain functional group are still discernible in the mixed calixarene.2 Since the homo calixarene containing the ethyl ester has been incorporated into a poly (vinyl chloride) membrane and has been shown to be a successful sodium ion-selective electrode,6*8J2 it is not surprising that ionophore (iii) has the most severe Na+ interference. The Hg2+ interference is caused most strongly by the S donor atom as Hg2+ is a strongly thiophilic metal ion.23 When the membranes came in contact with Hgr+ and Pb2+ ions the silver response was tested immediately afterwards and also after 3 and 24 hr respectively, the electrodes being stored in O.lM silver nitrate when not in use. The effect of the ions on the membranes was in the following orders;

and the best selectivity towards all the ions tested with the exception of Hg+ ions. A membrane-coated glassy carbon electrode (B) was tested under the same conditions as the conventional membrane electrode (A). The properties of the two types are summarized in Table 1. The sensitivities and selectivities of the two electrodes are very alike with B being slightly more sensitive, with a higher Ag(1) ion response slope. In comparing the full responses (in the concentration range of lO-‘j to 10-‘&f) against all the interfering ions, in Fig. 3, it can be seen that the responses are almost identical. Most of the ions gave practically no response but the Na+ response had a slope of 25 mV/dec and Pb2+ a slope of 17 mV/dec. The HgZ+ response behaved relatively linearly up to a concentration lo-‘.‘M where it became anionic in nature. This is known as the “anion effect”24 and is usually reduced by the addition of incorporated ion exchangers within the membrane. In the case of our membrane which has KTpClPB as an incorporated ion exchanger the transition of the response into an anionic function is obtained only when complete consumption of free Table 1. Properties of silver-selective membrane electrodes based on ionophore (iv). (a) conventional membrane electrodes. (b) membrane-coated glassy carbon electrode.

Detection limit M Slope, m V/dec Log ~t&/M Na+ ;;+ cot+ CU+

Pb2+ effect. ionophore

(ii) >>(iv) > (i) > (iii)

z:

Hg2+ effect. ionophore

(ii)>>(i) > (iv) > (iii)

Hg2+ Response Tiie/sec

Ionophore (ii) did not have its full silver response back after 24 hr after contact with Pb2+ ions but in the case of contact with Hg2+ ions it had almost its full response back. Ionophores (ii) to (iv) were all regenerated in less than 3 hr but the Hg2+ ions in general had a stronger poisoning effect than the Pb2+ ions. It must be borne in mind that these membranes were in contact with interfering ions up to concentration of O.lM. Smaller concentrations would have a less detrimental effect. Na + ions had no poisoning effect on the membrane with it’s response being totally reversible. Further work was carried out on ionophore (iv) as it gave the best Ag(1) ion calibration slope

6)

@I

1o-4 50.01

10-d 51.74

-1.16 -2.01 -3.08 -2.85 -3.3 -1.81 -2.57 +1.93

-1.21 -2.14 -3.02 -3.02 -2.59 -1.86 N/A +1.79

3

2

Temp. Coeff. IE/BT. IO-‘A4 lo-2M lo-‘M

-0.180 +0.116 +0.397

+.031 - 0.057 + 0.028

Slope/mV/dec day 14 day 22 day1 I1

46.04 N/A 38.61

N/A 42.42 N/A

0.400% 0.218%

0.474% 0.212%

Coeff. of. var. (n = 5) Day 1 lo-’ M 1O-2 M Day 6 lo-’ M 1O-2 M Day 111 lo-’ M in-2

k.4

N/A N/A

3.087% 16.609%

0.109% 0.123%

N/A N/A

Calixarene-based potentiometric

ion-selective electrodes for silver

carriers has occurred.” In the linear range of the H&+ response a slope of 80-90 mV/dec is obtained compared to the expected Nemstian slope of 29.6 mV/dec for a divalent metal cation. This response characteristic may be due to the formation of charged associates within the membranez6 and has been noticed previously in Hg(I1) and Ag(1) selective poly (vinyl chloride) membrane electrodes based on dithia crown ethers13 where this unusual response was attributed to the formation of Hg(TPB)+ ions in the membrane so perhaps the response in this case is due to Hg(TPClB) + ions. On preparation of a membrane containing no ion exchanger there is a large increase in anion interference, the Ag(1) ion response has a slope of < 10 mV/dec and the membrane has a very dark brown/yellow colour (characteristic of a very old stored membrane) after a few days. However, the Hg*+ response is nearer to divalent in nature. Electrode type B has a marginally shorter response time than type A which is probably due to the much thinner film (0.2 mm) formed on the carbon surface. Temperature studies indicate that both electrodes are affected very little by temperature, with type B being slightly more stable. Examining the lifetime calibration slopes in Table 1 it can be seen that at day 22 electrode type B gives a slope of 42.42

1553

nV/dec but the potential readings for this slope iave shifted up by 230 mV from the original lotential readings. On day 111 electrode A had Lcalibration slope which differed by only 20 mV iom the original potential readings. Reproduci,ility studies on new and old membranes of 30th types show that type A membranes have steady readings even after 111 days whereas ifter only 6 days type B membranes have erratic .eadings. The initial readings of both are very similar. Membrane electrodes of type B were used in >otentiometric titrations of halide ions against silver nitrate as these electrodes were easier and Iuicker to prepare and did not require long :onditioning times. Figure 4 shows that the nembrane was successful in resolving a mixture )f Cl, Br and I ions. The reversal of potential occurring in the presence of iodide and to a esser extent of bromide ions is reproducible md does not affect the accuracy of the titra:ions. Though we did not investigate this natter further, it can probably be attributed to :he adsorption of precipitate particles on the :lectrode surface, causing increased cell resisttnce. By physically removing the precipitate Yom the electrode surface, potential readings :an be restored more or less to their expected Jalues. 450r

I

z

601 -6

-6

I

-5

I -5

I -4

I -3

I -4

I -3

I -2

I -2

I

I -I

I -I

m-6

-6

P

I

I

I

-4

I -3

I

-5

-2

-I

-5

-4

-3

-2

-I

log c

Fig. 3. Response of silver-selective electrodes to metal cations. (A) Na, (B) K, (C) Ni, (D) Co, (E) Cu, (F) Pb and (G) Hg. (unlabelled response) Ag. (a) conventional membrane electrode. (b) membrane-coated glassy carbon electrode.

K.QTHWINEM.O'CONNOR et al.

1554

4. K. Kimura, T. Miura, M. Matsuo and T. Shona, AnaZ. Chem., 1990, 62, 1510. 5. K. Cunningham, G. Svehla, M. A. McKervey and S. J. Harris, Anal. Proc., 1991, 2B, 294.

6. A. Cadogan, D. Diamond. M. R. Smvth. M. Deasv. M. A. McKervey and S. J.-Harris, Arudyst. 1989, ll& 1551. I. A. Cadogan, D. Diamond, S. Cremin, M. A. McKervey and S. J. Harris. Anal. Proc., 1991, 28, 13. 8. D. Diamond, in M. R. Smyth and J. G. Vos, (eds.),

n

Electrochemistry,

Fig. 4. Potentiometric titration of a mixture of 4.5 ml KI, 5.5 ml KBr and 6.5 ml KC1 (all O.lM concentration) with 0. 1M AgNO, using a membrane-coated glassy carbon electrode incorporating ionophore (iv).

Sensors

and

Analysis.

Analytical

Chemistry Symposium Series. Volume 25, p. 155. Elsevier, Amsterdam, 1986. 9. A. Cadogan, D. Diamond, M. R. Smyth, G. Svehla, E. M. Seward, M. A. McKervey and S. J. Harris, Analyst, 1990, 115, 1207. 10. M. Telting-Diaz, D. Diamond, M. R. Smyth, G. Svehla, E. M. Seward, M. A. McKervey and S. J. Harris, Anal. Proc., 1989, 26, 29.

CONCLUSIONS

Conventional membrane electrodes and membrane-coated glassy-carbon electrodes based on ionophore (iv) were found to be successful Ag(1) selective electrodes. Both types exhibited good linear responses and good sensitivity and selectivity. They responded quickly and reversibly and gave good temperature stability. However conventional membrane electrodes were more beneficial for long-term use as they were found to be more stable and reproducible with time.

11. J. Brunink, J. R. Haak, J. G. Bomer, D. N. Reinhoudt, M. A. McKervey and S. J. Harris, Anal. Chim. Acta., 1991, 254, 75. 12. D. W. M. Arrigan, G. Svehla, M. A. McKervey and S. J. Harris, Anal. Proc., 1992, 29, 27. 13. M. Lai and J. Shih, Analyst, 1986, 111, 891. 14. M. Oue, K. Kimura, K. Akama, M. Tanata and T. Shona, Chem. Lett., 1988, 409. 15. P. R. Pearson, .I. Am. Chem. Sot., 1963, 85, 22, 3533. 16. W. E. Van der Linden and J. W. Dieker, Anul. Chtm. Acta, 1980, 119, 1. 17. C. Chen, Y. Sakai, J. Hasebe, J. Anzai, A. Ueno and T. Osa, Chem. Pharm. Bull., 1989, 37, 3316. 18. S. M. Stankovic, V. M. Jovanovic and M. S. Jovanovic, J. Serb. Chem. Sot., 1990, 55, 125. 19. S. Kamata and K. Onoyama, Anal. Chem., 1991, 63, 1295.

REFERENCES 1. C. D. Gutsche, Calixarenes: Monographs in Supramolecular Chemistry, No. 1, RSC, Cambridge (U.K.),

1989. 2. Marie-Jo&

Schwing and M. Anthony

McKervey,

Chemically Mod#ied Calixarenes as New Selective Receptors for Monovalent Cations, in J. Vicens and V. Biihmer (eds.), Calixarenes: A Versatile Class of Macrocylic Compounds. Kluwer Academic Publishers,

Dordrecht 1991, p. 139. 3. K. Kimura, M. Matsuo and T. Shona, Chem. L.&t., 1988, 615.

20. G. J. Moody, J. D. R. Thomas and T. E. Edmonds, (ed.) Chemical Sensors, Blackie, London, 1988. 21. G. G. Guilbault, R. A. Durst, M. S. Frant, H. Freiser, E. H. Hansen, T. S. Light, E. Pungor and J. D. R. Thomas, Pure Appl. Chem., 1976, 48, 127. 22. D. Diamond, G. Svehla, E. M. Seward and M. A. McKervey, Anal. Chim. Acta., 1988, 204, 223. 23. G. Wu, W. Jiang, J. D. Lamb, J. S. Bradshaw and R. M. Izatt, J. Am. Chem. Sot., 1991, 113, 6538. 24. W. E. Morf, The Principles of Zen-selective Electrodes and of Membrane Transport, Chap. 12, p. 296. Elsevier, Amsterdam. 1981. 25. Ref. 24, p. 311. 26. Ref. 24, p. 307.