15 Coated-Wire Ion-Selective Electrodes
Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch015
L. Cunningham and H. Freiser Strategic Metals Recovery Research Facility, Department of Chemistry, University of Arizona, Tucson, AZ 85721
Coated wire ion selective electrodes were first developed in 1971, and comprise a film of PVC or other suitable polymeric matrix substrate containing a dissolved electro-active species, coated on a conducting substrate (generally a metal, although any material whose conductivity is substantially higher than that of the film can be used.) Electrodes of this sort are simple, inexpensive, durable and capable of -1
reliable response in the concentration range of 10 M to 10 M for a wide variety of both organic and inorganic cations and anions. The principles on which these electrodes are based, as well as their application to a variety of analytical problems, will be discussed. -6
When faced with the problem of a trace level determination of an inorganic ion, the analyst often considers use of an ion-selective electrode (ISE) for the species of interest. This approach is advantageous because of the speed and ease of ISE procedures In which little or no sample is required. Further, they possess wide dynamic ranges, and are relatively low in cost. These characteristics have inevitably led to sensors for several ionic species, and the l i s t of available electrodes has grown substantially over the past two decades. In most cases, the traditional barrel configuration has been utilized. However, the large size of this type of ISE along with the requirement that it be used in a nearly upright position renders i t somewhat cumbersome to use and unnecessarily expensive. In our laboratory, these disadvantages have been overcome with the development of the coated wire electrode (OWE) . This sensor, having response characteristics equal to and occasionally better than conventional types, is only 1-2 mm in diameter (further size reduction can be easily achieved), can be used at any angle, and costs only a few pennies to make. Indeed, they can be considered "disposable", though with proper handling lifetimes of over six 0097-6156/ 86/ 0309-O256S06.00/ 0 © 1986 American Chemical Society
In Fundamentals and Applications of Chemical Sensors; Schuetzle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
15. CUNNINGHAM AND PREISER
Coated- Wire Ion-Selective Electrodes
257
months have been realized. During the course of COTE investigations here, the l i s t of analyte species has been lengthened to include not only most common inorganic ions of interest, but also organic species which are anionic or cationic under appropriate solution conditions (see Table 1). This paper w i l l review this research from the inception of the OWE to the present. The f i r s t electrode of this type was based on the Ca -didecyl-
Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch015
i ^
phosphate/dioctylphenyl phosphonate system (JJ. An effective Ca selective CWE resulted when a 6:1 mixture of 5* PVC in cyclohexanone and 0.1 M. Ca didecylphosphate in dioctylphenylphosphonate was dried on the end of a platinum wire. Favorable comparison of this electrode response characteristics against the commercial counterpart (Table 2) encouraged further studies with other membrane components. The C a electrode response relied upon the complexation of aqueous + +
Ca by didecylphosphate dispersed i n the organic, or membrane, phase. In a similar manner, incorporation of methyltricaprylammonium (Aliquat 336S) salts in polymer membranes produced CWEs for their respective anions (2J. A 60*(v/v) solution of Aliquat 336S i n decanol was f i r s t converted to the desired anionic form v i a shaking +
with 1 M aqueous solution of the appropriate Na s a l t . A 10:1 mixture of PVC i n cyclohexanone and this decanol solution was then used to coat copper wires by repeated dipping and drying until a small bead completely encapsulated their ends. Listed i n Tables 3 and 4 are anionic species for which Aliquat based electrodes were prepared. Applications ranged from c r i t i c a l micelle determination using laurylsulfonate sensors (3), analyses of atmospheric N0 x
pollutants with nitrate electrodes (4J, and assay of phenobarbital tablets using a phenobarbital anion CWE (5J. In many cases, poly (methyl methacrylate) or epoxy resin could be substituted for PVC with retention response. This, along with absence of the t r a d i t i o n a l internal reference electrode, raised fundamental questions surrounding the charge conduction mechanism occurring in the membrane and at the polymer-substrate interface. Calculation of activation energies from the temperature dependence of conduction suggested that an electronic mechanism was operative, such as that observed in organic semiconductors (6). Later studies of the pressure dependence of conduction gave strong evidence for ionic conduction because much larger activation volumes than could be expected from an electronic mechanism were obtained (7_r JU • As such, the existence of some redox couple at the substrate-polymer interface probably functions as an "internal reference". This hypothesis i s further reinforced when one considers that conditional standard potentials shift by significant and reproducible amounts from one type of metal substrate to another. Our attention next turned to the development of cation selective electrodes in order to develop methods for protonated alkylammonium ions. Initial studies i n this area were aimed at improving s e l e c t i v i t y among s i m i l a r l y charged cations by utilizing a mobile exchange site, facilitating membrane response to changing counter ions (9) . These membranes were comprised of dinonylnaphthalene sulfonic acid (DNNS), a lipophilic anionic extractant, dissolved i n
In Fundamentals and Applications of Chemical Sensors; Schuetzle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
258
FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS
Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch015
Table 1 .
Coated Wire Electrodes
Lipophilic Cation-Based Halide: Oxyanion:
Cl~, Br", i " , CNS~ NOg, C 1 0 ~
Organic Anion:
RCOO ,
RS03
Amino Acid: HN-C(-R)-C * OH Neutral Carrier-Based K
+
Dlnonylnapthalenesul fonate-Based Quaternary Ammonium Ions Drugs of Abuse, e.g., POP, speed, methadone 3-Adrenergic Drugs, e.g., acebutalol Ca-Blockers, e.g., Verapamil Phenothiazines, e.g., chloropromazine
In Fundamentals and Applications of Chemical Sensors; Schuetzle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch015
15. CUNNINGHAM AND PREISER
Coated- Wire Ion-Selective Electrodes
259
a
Table 2. Selectivity Coefficients, k^°j# of Various Divalent Cations [2] Interferent
Ni
a
2 +
Orion Electrode
Coated Wire Electrode
0.026
0.0039
CU
2+
0.24
0.15
M3
2+
0.033
0.014
Ba
2+
0.016
0.0036
Sr
2 +
0.029
0.021
Pb
2+
0.23
1.86
Zn
2+
1.44
32.3
Calculated from AE = 30 log (1 + k?°*a,/a- )
In Fundamentals and Applications of Chemical Sensors; Schuetzle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
260
FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS
Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch015
Table 3. Response Characteristics of Coated Wire Electrodes
Electrode
Slope, mV/log a
Perchlorate
58
Chloride
55
Bromide
59
Iodide
60
Thiocyanate
59
Oxalate
28
a
Acetate
50
a
Benzoate
53
a
Sulfate
28
Salicylate
53
a
Phenylalanine
54
a
Leucine
52
a
a
Concn. Range of Useful Concn. Linear Response, M Range, M jL
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