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Prussian blue solid-state films and membranes as potassium ion-selective electrodes
Analytica Chimica Acfa, 239 (1990) 7-12 Elsevier
Science Publishers
B.V., Amsterdam
Prussian blue solid-state films and membranes as potassium ion-selective electrodes Vasanthi Department
Krishnan,
Anthony
ofChemistryKent
L. Xidis and V.D. Neff
*
State University, Kent, OH 44242 (U.S.A.)
(Received
20th April 1990)
Abstract The use of thin films of Prussian blue and heterogeneous Prussian blue membranes as potassium ion-selective electrodes was investigated. All of the heavier group I cations and NH: interfere strongly but there is relatively good selectivity towards Na+ with a selectivity coefficient of ca. 5 x 10m3. The thin-film measurements, based on Prussian blue deposited on platinum, involve conditioning the electrode to a fixed potential according to the method used by Engel and Grabner for copper hexacyanoferrate(II1) films. The membrane electrodes were based on mixing Prussian blue with polymeric supporting films such as polystyrene and epoxy. A particularly simple practical configuration involves Prussian blue membranes deposited directly on copper conductors where one membrane serves as a reference electrode. A reversible cell, without liquid junction, is formed with Prussian blue and Ag/AgCl electrodes and this serves as a means for determining an accurate value for the standard reduction potential of Prussian blue, which is found to be 0.238 V vs. Ag/AgCl at 25 o C.
Keywords: Ion-selective
electrodes;
Prussian
blue; Potassium
The electrochemical behavior of a number of solid-state mixed-valence transition metal hexacyanides is now well known [l]. Thin films of these materials, deposited on conducting substrates, behave like electrodes of the second kind in the sense that the cell potential depends on the activity of certain ions in the electrolytic solution phase [2,3]. It is apparent that this class of compound may be good candidates for practical adaptation as ion-selective electrodes and this aspect has already been studied in the case of copper hexacyanoferrate(II1) (CuHCF) by Engel and Grabner [4], who described a practical method for sensing aqueous potassium ion by direct potentiometry. The voltammetry of CuHCF films was studied initially by Siperko and Kuwana [5], who pointed out that the peak potential for the reduction of KCuFe(CN), was Nemstian with respect to the concentration of potassium ion in the electrolyte. Engel and Grabner introduced the idea of 0003-2670/90/$03.50
0 1990 - Elsevier
Science Publishers
redox conditioning of the electrode prior to potentiometric measurement. This is necessary because the solid film is an electronic semiconductor which is also electroactive and therefore capable, over a sufficient time period, of equilibrating to a potential equivalent to that of an inert conductor in the aqueous electrolyte. The conditioning was accomplished by poising the electrode to the formal half-cell potential in 1 M KC1 and measuring potentials over a short time period (lo-15 s) before any appreciable potential drift can occur. Deakin and Byrd [6] also described a potassium ion detector based on a quartz crystal microbalante coated with Prussian blue (PB). Prussian blue, and some of its transition metal analogues, are ion selective in the sense that certain group I cations can freely migrate into, or out of, the film during electrochemical reduction or oxidation whereas others are excluded [l]. For example, PB and CuHCF will readily admit potassium, B.V.
V. KFUSHNAN
8
rubidium, cesium and ammonium ions whereas sodium and lithium ions are excluded. On the other hand, NiHCF films admit all group I cations [7]. In aqueous solutions the exclusion effect is known to be related to the size of the hydrated ionic radius but the details of ion migration are not yet fully understood [l]. A number of mixedvalence polymeric cyanides, including PB, are inexpensive, chemically and thermally stable, easily synthesized and can be deposited on a great variety of conducting or semiconducting substrates, including ultramicroelectrodes. For these reasons it is believed that further investigation of this class of material as analytical sensors is justified. In this paper a potassium ion-selective electrode based on the prototype mixed-valence hexacyanide Prussian blue (PB) is described. In addition to potentiometry based on methods similar to those used by Engel and Grabner, measurements with heterogeneous solid-state PB membranes are also described.
EXPERIMENTAL
Prussian blue films were deposited on l.O-mm diameter polished platinum and gold discs embedded in Teflon. The method of film deposition has been described previously [2]. The film thickness was ca. 1000 A in all instances. Triply distilled water and analytical-reagent grade chemicals were used throughout. The standard half-cell potential of the PB electrode was determined voltammetrically in a cell consisting of a PB electrode immersed in a KC1 solution at unit activity (1.71 m) which contained a carefully prepared Ag/AgCl wire immersed in the same solution and used as a reference electrode. The PB electrode was dried in air for several days before use. This procedure dramatically improves the adherence of the film to the platinum surface. The electrode was also cycled several times between 0.5 and 0.0 V vs. SCE in 1 M KNO, in order to ensure incorporation of potassium ion into the film. All PB film electrodes were treated in the same way. Several solid-state PB membranes were prepared, all containing solid PB in the form of a ground polycrystalline powder. The PB was pre-
ET AL.
pared by mixing equal amounts of a 0.10 M solution of K,Fe(CN), with a solution 0.10 M in FeSO, and K,SO, adjusted to pH 2 with H,SO,. The sulfate salts were used because it has been reported that chloride ion can be incorporated into the PB lattice [8]. The bulk PB precipitate was filtered and thoroughly washed with triply distilled water. Membranes were prepared with PVC, silicone, polystyrene and epoxy resin (Epon 237) by methods described in the literature [8]. In general, the membranes were used as thin cast films, ca. 0.5 mm thick, deposited at the tip of a standard glass eye dropper. PVC and epoxy membranes were also cast directly on the flat surface of a lo-gauge insulated copper wire with the intention of studying electrodes with no internal filling solution. In all instances the membranes were prepared by forming a paste of ground PB powder with the appropriate casting agent. After setting, the membrane surface was gently abraded with 600-g& emery paper. Potentials were measured with an Orion Model 811 high-impedance voltmeter. Calomel, Ag/AgCl and PB were used as reference electrodes, depending on the type of experiment. Voltammetry was carried out with a PAR Model 173 potentiostat and Model 175 voltage programmer.
RESULTS
AND
DISCUSSION
Thin PB films on metal substrates Prussian blue films which have been voltammetrically cycled in KC1 solution undergo two characteristic solid-state electrochemical reactions: KFeFe(CN),
+ e-+
K+=
K,FeFe(CN), Eve&t’s salt
KFeFe(CN),
= FeFe(CN), + K++ Berlin green
e-
(1) (2)
The half-cell potentials for reactions 1 and 2 are 0.22 and 0.96 V vs. Ag/AgCl, respectively, in 1.0 M KC1 solution [2]. By varying the potential of a PB electrode one can obtain compositions over a continuous range from Everitt’s salt (ES) to Berlin green (BG). In order to establish the formal be-
PRUSSIAN
BLUE
AS POTASSIUM
ION-SELECTIVE
havior of the PB electrode the Ag/AgCl/KCl (aq.)/(PB, ES)/Pt, sents the chemical reaction Ag + PB + KC1 (as.)
= AgCl + ES
9
ELECTRODES
reversible cell which repre-
=
EO
t
y
In
k4%MPB)~(KCl)l
b&Clb@)1
(3)
VS. Ag/AgCI (mV)
Fig. 1. Voltammogram of PB film on Pt in 1.71 m KCI. The reference electrode was Ag/AgCl without a liquid junction. The PB film thickness was 940 A and the scan rate was 5 mV s-1.
/
F
E
100
5 3 <
0
!=? $
-100
6
(4)
applies, where a(PB) and u(ES) refer to the activity of PB and ES in the film which is regarded as a solid solution. Reaction 3 can be used as. a basis for an accurate determination of the standard half-cell potential for the reduction of PB. This values has not been previously reported. The experiment involves the voltammogram shown in Fig. 1. The Pt/PB and Ag/AgCl electrodes were both immersed in a 1.71 m KC1 solution. The scan was run slowly at 5 mV s-r in order to establish
POTENTIAL
/
200 1
is first considered. The proper formulation of the Nernst equation for reactions involving PB (which is an intercalation compound) have been discussed previously in detail [9]. For reaction 3 E
300r
5
-200 -300 -400 .I___
10
6
6
4
2
0
PMCU
Fig. 2. Voltage of a Pt/PB/KCl (a)/AgCl/Ag transference. The PB electrode was preconditioned vs. Ag/AgCl prior to each measurement.
cell without to 0.238 V
reversible conditions and a small peak separation (< 20 mv). Peak potentials were determined accurately by interrupting the scan at the peak maximum. The standard cell potential for the reduction of PB was determined to be E O = 0.238 V vs. Ag/AgCl [ u(KC1) = 11 at 25 o C. This probably represents the most accurate procedure for the determination of the standard potential for this type of intercalation compound, as the cell employed has no liquid junction. Reaction 1 can also be used to establish accurately the Nernstian behavior of the PB electrode. This was done by measuring the potential of the Pt/PB and Ag/AgCl electrodes in solutions of varying KC1 activity, as shown in Fig. 2. Prior to each measurement the PB electrode was adjusted (preconditioned) to 0.238 V vs. AgCl. The leastsquares slope of the voltage curve was determined to be 58.8 mV [pu(KCl)]-‘. Hence it is established that, under ideal conditions, the PB electrode gives an excellent Nernstian behavior and is comparable to an electrode of the second kind. As previously mentioned, the initial work on a potassium ion-selective electrode was done with a CuHCF film on glassy carbon [4]. The electrode
10
V. KRISHNAN
was conditioned by adjusting the potential to 0.7 V vs. SCE and measuring the potential of the analyte solution only a few seconds after the conditioning process. At the above potential the CuHCF film is about half reduced. On open circuit the potential drifted slowly to the same value (ca. 0.22 V) as that of a bare platinum wire in 1 M KCl. With the PB electrode the conditioning potential corresponding to half reduction would be 0.238 V in 1.71 m KCl, as illustrated by the voltammogram shown in Fig. 1. This result is fortuitous because this potential is very close to that of a bare platinum wire in the same solution. This is illustrated in Fig. 3, showing the potential-time curves for a PB film adjusted. initially to 0.6 and 0.0 V vs. Ag/AgCl. As can be seen, the open circuit potential drifts slowly to a value (0.25 V) very close to the standard reduction potential (dashed line) for the same solution. A practical device for potassium ion sensing can be developed from the cell configuration Pt/(PB)/KCl (std.)//KCl (a)/(PB)/Pt. This cell has a liquid junction for which the junction potential can, in principle, be determined from the transference number for the chloride ion. One of the PB electrodes was inserted in the analyte solution and the second PB reference electrode was
Y
I
I
I
(min
I
I
)
Fig. 3. Potential-time curves for a Pt/PB electrode potentiostated to (1) 0.6 and (2) 0.0 V vs. Ag/AgCl in 1.71 m KC1 solution. The solid horizontal line represents the potential of a Pt wire in the same solution and the dashed line represents the standard reduction potential for the PB electrode.
ET AL.
Pt/PB/KCl(xl//KCI(~)IPB/Pt /
I
5
4
2
3
1
0
paw7
Fig. 4. Nernst plot for a PB thin-film electrode using a second PB film as reference electrode. The slope is 57 mV [pn(K+ )I-‘. (1) No NaCl; (2) 0.01 M NaCl; (3) 0.1 M NaCl.
inserted in a fritted glass tube. The measurement was made by potentiostating both electrodes to 0.25 V vs. SCE and measuring in a short time (less than 5 s) before there is any appreciable tendency for the potential to drift. For practical purposes it is found, however, that repeated electrode conditioning is not necessary because, as mentioned previously, the rest potential of PB in KC1 is very close to the formal half-cell potential in KC1 solution. The results of potential measurements for a series of KC1 activities are shown in Fig. 4. The slope of the line is 57 mV [pa(K+) which is close to the value reported by Engel and Grabner [4] for the CuHCF electrode. PB was chosen as the reference electrode in this experiment in order to illustrate a possible practical advantage. A given solution of analyte could be divided and one part saturated with KC1 to be used as the filling solution for the reference electrode. In this way one might hope to achieve some cancelation of effects such as those of spurious chemical reactions which could alter the electrode composition. It is not necessary to use chloride as the anion in the reference electrode described above. The potential of the PB electrode is remarkably insen-
PRUSSIAN
BLUE
AS POTASSIUM
ION-SELECTIVE
sitive to different anions except for the effect on the cation activity. For example, equally good results were obtained with saturated K,SO,. Also, it is not necessary to use platinum as the electrode substrate. PB can be deposited on any conducting or semiconducting material, including glassy carbon or graphite. Thus it appears that a potassium ion-selective device based on PB has the advantage of cheapness and a very fast equilibration time. The main disadvantage is that PB will react directly with strong oxidizing or reducing agents in the analyte and, if these interferents are present in sufficient concentration, the PB electrode will be titrated to the equilibrium concentration dictated by the standard free energy change for the chemical reaction. The effect of sodium ion as an interferent is also shown in Fig. 4. The Nemst plots for solutions containing three different concentrations of NaCl are shown superimposed on the curve obtained in the absence of sodium ion. The selectivity coefficient for sodium ion is ca. 5 X 10W3, which is similar to the value reported by Engel and Grabner [4] for the CuHCF electrode. For other univalent cations such as Rb+, Cs+ and NH: the ionic activities are essentially additive and the selectivity coefficient is close to 1. It has also been found that group II cations such as Ca2+ and Mg2+ interfere to about the same extent as Na+. The tendency for most common cations to exchange, to some extent, with K+ implies a difficulty in choosing a salt to buffer the analyte to constant ionic strength. A possible candidate might be H+, as E o for the reduction of PB is remarkably insensitive to changes in pH (21. It has been found, however, that even H+ interferes to some extent and should not be used at concentrations > lo-’ M. For this reason no attempt was made to maintain constant ionic strength in the measurements. Prussian blue solid-state
11
ELECTRODES
membranes
As mentioned previously, a variety of polymeric support materials have been used to fabricate solid-state membranes containing polycrystalline PB. Here only polystyrene- and epoxy resinbased membranes are considered. The results obtained with other support materials such as PVC
O1
L
4
5
4
3
2
1
0
paW+,
Fig. 5. Nemst plot for a solid-state membrane consisting of PB in polystyrene. The internal reference electrode was a PB film in 1.71 m KCl. The slope is 51 mV [pa(K+)
and silicone rubber were in all instances very similar to those obtained with epoxy resin. This is a very good indication that the observed potential response is due fundamentally to the PB and is not related to the substrate. First we consider the cell Ag/AgCl/KCl (1 M)/PB membrane/KNO, (a)//KCl (1 M)/AgCl/Ag. The membrane, which consisted of PB in polystyrene, was formed across the tip of a glass eye dropper with a thickness of ca. 0.5 mm. The Nernst plot for this cell in solutions of various KNO, activity is shown in Fig. 5. The main feature is that curvature begins to set in when the K+ concentration is reduced below ca. lop4 mol 1-l. Also, the Nemst slope is 51 mV (pK+)-‘. Hence the membrane electrode is less sensitive than the thin film considered previously. PB does meet the generally accepted criteria for a solid-state ion sensor. It is highly insoluble, a weak electronic conductor (p = 10’ Q cm [lo]) and a good ionic conductor. Finally, an electrode configuration which may have practical utility in some circumstances is considered. This is a very simple device which involves a PB membrane in direct contact with a copper conductor. In addition, a second PB-KNO, epoxy membrane serves as the reference electrode.
V. KRISHNAN
12
-80
r
ET AL.
analyte electrode consisted of PB only in the epoxy support. The Nernst plot for this configuration is shown in Fig. 6. The slope of 54 mV [pa(K+ is repeatable and similar values, within 3 mV were obtained with several electrodes [pa(K+) prepared by the same method. The equilibration times for the membrane electrodes are acceptable, averaging ca. 30 s. They have the advantage that the PB itself is chemically inert except in strongly basic solutions. Although optimization of their fabrication has not been attempted, it is apparent that PB membranes could be developed for practical use in situations where the interfering heavier group I cations are absent.
REFERENCES Fig. 6. Nemst plot for two solid-state PB-epoxy membrane electrodes deposited on copper. The reference electrode consisted of equal amounts of solid PB and KNOs by weight. The analyte electrode contained no KNO,. The Nemst slope is 54 mV [pn(K+)]-‘.
The reference membrane electrode is prepared by mixing equal parts by weight of bulk PB and KNO, and grinding to a fine powder which is mixed into a paste with epoxy. The membrane is cast on the end of a lo-gauge insulated copper wire and the surface is abraded with emery paper. The electrode is then allowed to stand for several hours in warm water, creating a series of micropores in regions where the KNO, has dissolved. KNO, was used because it was found that the electrode surface turned brown on prolonged standing in hot water when KC1 was used. The
1 K. Itaya, I. Uchida and V.D. Neff, Act. Chem. Res., 19 (1986) 162. 2 D. Ellis, M. Eckhoff and V.D. Neff, J. Phys. Chem., 85 (1981) 1225. 3 K. Itaya, I. Uchida and S. Toshima, J. Am. Chem. Sot., 104 (1982) 4767. 4 D. Engel and E.W. Grabner, Ber. Bunsenges. Phys. Chem., 89 (1985) 982. J. Electrochem. Sot., 130 5 L.M. Siperko and T. Kuwana, (1983) 396. 6 M.R. Deakin and H. Byrd, Anal. Chem., 61 (1989) 290. Chem., 140 7 A.B. Bocarsly and S. Sinha, J. Electroanal. (1982) 167. 8 C.A. Lundgren and R.W. Murray, Inorg. Chem., 27 (1988) 933. (Ed.), Ion Selective Methodology, Vol. I, 9 A.K. Covington CRC, Cleveland, OH, 1979. and V.D. Neff, J. Phys. Chem., 92 (1988) 10 J.W. McCargar 3598. 11 S.J. England, P. Kathirgamanathan and D.R. Rossiensky, J. Chem. Sot., Chem. Commun., (1980) 840.
Science Publishers
B.V., Amsterdam
Prussian blue solid-state films and membranes as potassium ion-selective electrodes Vasanthi Department
Krishnan,
Anthony
ofChemistryKent
L. Xidis and V.D. Neff
*
State University, Kent, OH 44242 (U.S.A.)
(Received
20th April 1990)
Abstract The use of thin films of Prussian blue and heterogeneous Prussian blue membranes as potassium ion-selective electrodes was investigated. All of the heavier group I cations and NH: interfere strongly but there is relatively good selectivity towards Na+ with a selectivity coefficient of ca. 5 x 10m3. The thin-film measurements, based on Prussian blue deposited on platinum, involve conditioning the electrode to a fixed potential according to the method used by Engel and Grabner for copper hexacyanoferrate(II1) films. The membrane electrodes were based on mixing Prussian blue with polymeric supporting films such as polystyrene and epoxy. A particularly simple practical configuration involves Prussian blue membranes deposited directly on copper conductors where one membrane serves as a reference electrode. A reversible cell, without liquid junction, is formed with Prussian blue and Ag/AgCl electrodes and this serves as a means for determining an accurate value for the standard reduction potential of Prussian blue, which is found to be 0.238 V vs. Ag/AgCl at 25 o C.
Keywords: Ion-selective
electrodes;
Prussian
blue; Potassium
The electrochemical behavior of a number of solid-state mixed-valence transition metal hexacyanides is now well known [l]. Thin films of these materials, deposited on conducting substrates, behave like electrodes of the second kind in the sense that the cell potential depends on the activity of certain ions in the electrolytic solution phase [2,3]. It is apparent that this class of compound may be good candidates for practical adaptation as ion-selective electrodes and this aspect has already been studied in the case of copper hexacyanoferrate(II1) (CuHCF) by Engel and Grabner [4], who described a practical method for sensing aqueous potassium ion by direct potentiometry. The voltammetry of CuHCF films was studied initially by Siperko and Kuwana [5], who pointed out that the peak potential for the reduction of KCuFe(CN), was Nemstian with respect to the concentration of potassium ion in the electrolyte. Engel and Grabner introduced the idea of 0003-2670/90/$03.50
0 1990 - Elsevier
Science Publishers
redox conditioning of the electrode prior to potentiometric measurement. This is necessary because the solid film is an electronic semiconductor which is also electroactive and therefore capable, over a sufficient time period, of equilibrating to a potential equivalent to that of an inert conductor in the aqueous electrolyte. The conditioning was accomplished by poising the electrode to the formal half-cell potential in 1 M KC1 and measuring potentials over a short time period (lo-15 s) before any appreciable potential drift can occur. Deakin and Byrd [6] also described a potassium ion detector based on a quartz crystal microbalante coated with Prussian blue (PB). Prussian blue, and some of its transition metal analogues, are ion selective in the sense that certain group I cations can freely migrate into, or out of, the film during electrochemical reduction or oxidation whereas others are excluded [l]. For example, PB and CuHCF will readily admit potassium, B.V.
V. KFUSHNAN
8
rubidium, cesium and ammonium ions whereas sodium and lithium ions are excluded. On the other hand, NiHCF films admit all group I cations [7]. In aqueous solutions the exclusion effect is known to be related to the size of the hydrated ionic radius but the details of ion migration are not yet fully understood [l]. A number of mixedvalence polymeric cyanides, including PB, are inexpensive, chemically and thermally stable, easily synthesized and can be deposited on a great variety of conducting or semiconducting substrates, including ultramicroelectrodes. For these reasons it is believed that further investigation of this class of material as analytical sensors is justified. In this paper a potassium ion-selective electrode based on the prototype mixed-valence hexacyanide Prussian blue (PB) is described. In addition to potentiometry based on methods similar to those used by Engel and Grabner, measurements with heterogeneous solid-state PB membranes are also described.
EXPERIMENTAL
Prussian blue films were deposited on l.O-mm diameter polished platinum and gold discs embedded in Teflon. The method of film deposition has been described previously [2]. The film thickness was ca. 1000 A in all instances. Triply distilled water and analytical-reagent grade chemicals were used throughout. The standard half-cell potential of the PB electrode was determined voltammetrically in a cell consisting of a PB electrode immersed in a KC1 solution at unit activity (1.71 m) which contained a carefully prepared Ag/AgCl wire immersed in the same solution and used as a reference electrode. The PB electrode was dried in air for several days before use. This procedure dramatically improves the adherence of the film to the platinum surface. The electrode was also cycled several times between 0.5 and 0.0 V vs. SCE in 1 M KNO, in order to ensure incorporation of potassium ion into the film. All PB film electrodes were treated in the same way. Several solid-state PB membranes were prepared, all containing solid PB in the form of a ground polycrystalline powder. The PB was pre-
ET AL.
pared by mixing equal amounts of a 0.10 M solution of K,Fe(CN), with a solution 0.10 M in FeSO, and K,SO, adjusted to pH 2 with H,SO,. The sulfate salts were used because it has been reported that chloride ion can be incorporated into the PB lattice [8]. The bulk PB precipitate was filtered and thoroughly washed with triply distilled water. Membranes were prepared with PVC, silicone, polystyrene and epoxy resin (Epon 237) by methods described in the literature [8]. In general, the membranes were used as thin cast films, ca. 0.5 mm thick, deposited at the tip of a standard glass eye dropper. PVC and epoxy membranes were also cast directly on the flat surface of a lo-gauge insulated copper wire with the intention of studying electrodes with no internal filling solution. In all instances the membranes were prepared by forming a paste of ground PB powder with the appropriate casting agent. After setting, the membrane surface was gently abraded with 600-g& emery paper. Potentials were measured with an Orion Model 811 high-impedance voltmeter. Calomel, Ag/AgCl and PB were used as reference electrodes, depending on the type of experiment. Voltammetry was carried out with a PAR Model 173 potentiostat and Model 175 voltage programmer.
RESULTS
AND
DISCUSSION
Thin PB films on metal substrates Prussian blue films which have been voltammetrically cycled in KC1 solution undergo two characteristic solid-state electrochemical reactions: KFeFe(CN),
+ e-+
K+=
K,FeFe(CN), Eve&t’s salt
KFeFe(CN),
= FeFe(CN), + K++ Berlin green
e-
(1) (2)
The half-cell potentials for reactions 1 and 2 are 0.22 and 0.96 V vs. Ag/AgCl, respectively, in 1.0 M KC1 solution [2]. By varying the potential of a PB electrode one can obtain compositions over a continuous range from Everitt’s salt (ES) to Berlin green (BG). In order to establish the formal be-
PRUSSIAN
BLUE
AS POTASSIUM
ION-SELECTIVE
havior of the PB electrode the Ag/AgCl/KCl (aq.)/(PB, ES)/Pt, sents the chemical reaction Ag + PB + KC1 (as.)
= AgCl + ES
9
ELECTRODES
reversible cell which repre-
=
EO
t
y
In
k4%MPB)~(KCl)l
b&Clb@)1
(3)
VS. Ag/AgCI (mV)
Fig. 1. Voltammogram of PB film on Pt in 1.71 m KCI. The reference electrode was Ag/AgCl without a liquid junction. The PB film thickness was 940 A and the scan rate was 5 mV s-1.
/
F
E
100
5 3 <
0
!=? $
-100
6
(4)
applies, where a(PB) and u(ES) refer to the activity of PB and ES in the film which is regarded as a solid solution. Reaction 3 can be used as. a basis for an accurate determination of the standard half-cell potential for the reduction of PB. This values has not been previously reported. The experiment involves the voltammogram shown in Fig. 1. The Pt/PB and Ag/AgCl electrodes were both immersed in a 1.71 m KC1 solution. The scan was run slowly at 5 mV s-r in order to establish
POTENTIAL
/
200 1
is first considered. The proper formulation of the Nernst equation for reactions involving PB (which is an intercalation compound) have been discussed previously in detail [9]. For reaction 3 E
300r
5
-200 -300 -400 .I___
10
6
6
4
2
0
PMCU
Fig. 2. Voltage of a Pt/PB/KCl (a)/AgCl/Ag transference. The PB electrode was preconditioned vs. Ag/AgCl prior to each measurement.
cell without to 0.238 V
reversible conditions and a small peak separation (< 20 mv). Peak potentials were determined accurately by interrupting the scan at the peak maximum. The standard cell potential for the reduction of PB was determined to be E O = 0.238 V vs. Ag/AgCl [ u(KC1) = 11 at 25 o C. This probably represents the most accurate procedure for the determination of the standard potential for this type of intercalation compound, as the cell employed has no liquid junction. Reaction 1 can also be used to establish accurately the Nernstian behavior of the PB electrode. This was done by measuring the potential of the Pt/PB and Ag/AgCl electrodes in solutions of varying KC1 activity, as shown in Fig. 2. Prior to each measurement the PB electrode was adjusted (preconditioned) to 0.238 V vs. AgCl. The leastsquares slope of the voltage curve was determined to be 58.8 mV [pu(KCl)]-‘. Hence it is established that, under ideal conditions, the PB electrode gives an excellent Nernstian behavior and is comparable to an electrode of the second kind. As previously mentioned, the initial work on a potassium ion-selective electrode was done with a CuHCF film on glassy carbon [4]. The electrode
10
V. KRISHNAN
was conditioned by adjusting the potential to 0.7 V vs. SCE and measuring the potential of the analyte solution only a few seconds after the conditioning process. At the above potential the CuHCF film is about half reduced. On open circuit the potential drifted slowly to the same value (ca. 0.22 V) as that of a bare platinum wire in 1 M KCl. With the PB electrode the conditioning potential corresponding to half reduction would be 0.238 V in 1.71 m KCl, as illustrated by the voltammogram shown in Fig. 1. This result is fortuitous because this potential is very close to that of a bare platinum wire in the same solution. This is illustrated in Fig. 3, showing the potential-time curves for a PB film adjusted. initially to 0.6 and 0.0 V vs. Ag/AgCl. As can be seen, the open circuit potential drifts slowly to a value (0.25 V) very close to the standard reduction potential (dashed line) for the same solution. A practical device for potassium ion sensing can be developed from the cell configuration Pt/(PB)/KCl (std.)//KCl (a)/(PB)/Pt. This cell has a liquid junction for which the junction potential can, in principle, be determined from the transference number for the chloride ion. One of the PB electrodes was inserted in the analyte solution and the second PB reference electrode was
Y
I
I
I
(min
I
I
)
Fig. 3. Potential-time curves for a Pt/PB electrode potentiostated to (1) 0.6 and (2) 0.0 V vs. Ag/AgCl in 1.71 m KC1 solution. The solid horizontal line represents the potential of a Pt wire in the same solution and the dashed line represents the standard reduction potential for the PB electrode.
ET AL.
Pt/PB/KCl(xl//KCI(~)IPB/Pt /
I
5
4
2
3
1
0
paw7
Fig. 4. Nernst plot for a PB thin-film electrode using a second PB film as reference electrode. The slope is 57 mV [pn(K+ )I-‘. (1) No NaCl; (2) 0.01 M NaCl; (3) 0.1 M NaCl.
inserted in a fritted glass tube. The measurement was made by potentiostating both electrodes to 0.25 V vs. SCE and measuring in a short time (less than 5 s) before there is any appreciable tendency for the potential to drift. For practical purposes it is found, however, that repeated electrode conditioning is not necessary because, as mentioned previously, the rest potential of PB in KC1 is very close to the formal half-cell potential in KC1 solution. The results of potential measurements for a series of KC1 activities are shown in Fig. 4. The slope of the line is 57 mV [pa(K+) which is close to the value reported by Engel and Grabner [4] for the CuHCF electrode. PB was chosen as the reference electrode in this experiment in order to illustrate a possible practical advantage. A given solution of analyte could be divided and one part saturated with KC1 to be used as the filling solution for the reference electrode. In this way one might hope to achieve some cancelation of effects such as those of spurious chemical reactions which could alter the electrode composition. It is not necessary to use chloride as the anion in the reference electrode described above. The potential of the PB electrode is remarkably insen-
PRUSSIAN
BLUE
AS POTASSIUM
ION-SELECTIVE
sitive to different anions except for the effect on the cation activity. For example, equally good results were obtained with saturated K,SO,. Also, it is not necessary to use platinum as the electrode substrate. PB can be deposited on any conducting or semiconducting material, including glassy carbon or graphite. Thus it appears that a potassium ion-selective device based on PB has the advantage of cheapness and a very fast equilibration time. The main disadvantage is that PB will react directly with strong oxidizing or reducing agents in the analyte and, if these interferents are present in sufficient concentration, the PB electrode will be titrated to the equilibrium concentration dictated by the standard free energy change for the chemical reaction. The effect of sodium ion as an interferent is also shown in Fig. 4. The Nemst plots for solutions containing three different concentrations of NaCl are shown superimposed on the curve obtained in the absence of sodium ion. The selectivity coefficient for sodium ion is ca. 5 X 10W3, which is similar to the value reported by Engel and Grabner [4] for the CuHCF electrode. For other univalent cations such as Rb+, Cs+ and NH: the ionic activities are essentially additive and the selectivity coefficient is close to 1. It has also been found that group II cations such as Ca2+ and Mg2+ interfere to about the same extent as Na+. The tendency for most common cations to exchange, to some extent, with K+ implies a difficulty in choosing a salt to buffer the analyte to constant ionic strength. A possible candidate might be H+, as E o for the reduction of PB is remarkably insensitive to changes in pH (21. It has been found, however, that even H+ interferes to some extent and should not be used at concentrations > lo-’ M. For this reason no attempt was made to maintain constant ionic strength in the measurements. Prussian blue solid-state
11
ELECTRODES
membranes
As mentioned previously, a variety of polymeric support materials have been used to fabricate solid-state membranes containing polycrystalline PB. Here only polystyrene- and epoxy resinbased membranes are considered. The results obtained with other support materials such as PVC
O1
L
4
5
4
3
2
1
0
paW+,
Fig. 5. Nemst plot for a solid-state membrane consisting of PB in polystyrene. The internal reference electrode was a PB film in 1.71 m KCl. The slope is 51 mV [pa(K+)
and silicone rubber were in all instances very similar to those obtained with epoxy resin. This is a very good indication that the observed potential response is due fundamentally to the PB and is not related to the substrate. First we consider the cell Ag/AgCl/KCl (1 M)/PB membrane/KNO, (a)//KCl (1 M)/AgCl/Ag. The membrane, which consisted of PB in polystyrene, was formed across the tip of a glass eye dropper with a thickness of ca. 0.5 mm. The Nernst plot for this cell in solutions of various KNO, activity is shown in Fig. 5. The main feature is that curvature begins to set in when the K+ concentration is reduced below ca. lop4 mol 1-l. Also, the Nemst slope is 51 mV (pK+)-‘. Hence the membrane electrode is less sensitive than the thin film considered previously. PB does meet the generally accepted criteria for a solid-state ion sensor. It is highly insoluble, a weak electronic conductor (p = 10’ Q cm [lo]) and a good ionic conductor. Finally, an electrode configuration which may have practical utility in some circumstances is considered. This is a very simple device which involves a PB membrane in direct contact with a copper conductor. In addition, a second PB-KNO, epoxy membrane serves as the reference electrode.
V. KRISHNAN
12
-80
r
ET AL.
analyte electrode consisted of PB only in the epoxy support. The Nernst plot for this configuration is shown in Fig. 6. The slope of 54 mV [pa(K+ is repeatable and similar values, within 3 mV were obtained with several electrodes [pa(K+) prepared by the same method. The equilibration times for the membrane electrodes are acceptable, averaging ca. 30 s. They have the advantage that the PB itself is chemically inert except in strongly basic solutions. Although optimization of their fabrication has not been attempted, it is apparent that PB membranes could be developed for practical use in situations where the interfering heavier group I cations are absent.
REFERENCES Fig. 6. Nemst plot for two solid-state PB-epoxy membrane electrodes deposited on copper. The reference electrode consisted of equal amounts of solid PB and KNOs by weight. The analyte electrode contained no KNO,. The Nemst slope is 54 mV [pn(K+)]-‘.
The reference membrane electrode is prepared by mixing equal parts by weight of bulk PB and KNO, and grinding to a fine powder which is mixed into a paste with epoxy. The membrane is cast on the end of a lo-gauge insulated copper wire and the surface is abraded with emery paper. The electrode is then allowed to stand for several hours in warm water, creating a series of micropores in regions where the KNO, has dissolved. KNO, was used because it was found that the electrode surface turned brown on prolonged standing in hot water when KC1 was used. The
1 K. Itaya, I. Uchida and V.D. Neff, Act. Chem. Res., 19 (1986) 162. 2 D. Ellis, M. Eckhoff and V.D. Neff, J. Phys. Chem., 85 (1981) 1225. 3 K. Itaya, I. Uchida and S. Toshima, J. Am. Chem. Sot., 104 (1982) 4767. 4 D. Engel and E.W. Grabner, Ber. Bunsenges. Phys. Chem., 89 (1985) 982. J. Electrochem. Sot., 130 5 L.M. Siperko and T. Kuwana, (1983) 396. 6 M.R. Deakin and H. Byrd, Anal. Chem., 61 (1989) 290. Chem., 140 7 A.B. Bocarsly and S. Sinha, J. Electroanal. (1982) 167. 8 C.A. Lundgren and R.W. Murray, Inorg. Chem., 27 (1988) 933. (Ed.), Ion Selective Methodology, Vol. I, 9 A.K. Covington CRC, Cleveland, OH, 1979. and V.D. Neff, J. Phys. Chem., 92 (1988) 10 J.W. McCargar 3598. 11 S.J. England, P. Kathirgamanathan and D.R. Rossiensky, J. Chem. Sot., Chem. Commun., (1980) 840.