- Email: [email protected]
Polycrystalline and monocrystalline antimony, iridium and palladium as electrode material for pH-sensing electrodes
Talm~a. Vol. 33. No. 2. pp. 125-134. 19X6 Prmted m Great Bntam. All rights reserved
CopyrIght
c
0039-9 140186 $3 00 + 0.00 19X6 Pergamon Press Ltd
POLYCRYSTALLINE AND MONOCRYSTALLINE ANTIMONY, IRIDIUM AND PALLADIUM AS ELECTRODE MATERIAL FOR pH-SENSING ELECTRODES EITA KINOSHITA, FOLKE INGMAN, GUNNAR EDWALL* and SIGVARD THULIN* Departments of Analytical Chemistry and Applied Physics *, The Royal Institute of Technology, S-100 44 Stockholm, Sweden STANISLAW Gi.4~ Department of Chemistry, Warsaw University, Warsaw, Poland (Received 4 July 1985. Accepted 25 September 1985)
Summary-Different ways of making pH-sensing electrodes from monocrystalline or polycrystalline antimony, uidmm and palladium have been investigated. Monocrystalline antimony and iridium are superior to the polycrystalline elements with respect to reproducibility between electrodes and stability of the electrode potential over long periods of time. No good palladium/palladium oxide electrode could be obtained by electrochemical oxidation and the thermal preparation method could not take advantage of the properties of the monocrystalline palladium. Therefore, only polycrystalline palladium was used to study this type of electrodes. The different electrodes were compared with respect to the manner of preparation, the pH-response (reproducibility and time response) and the effect that different complexing ligands present in the measuring solutions may have on the electrode response. Also, the redox-response of the electrodes and the effect of different oxygen pressures on the electrode potentials were studied. The monocrystalline antimony electrodes have the best reproducibility and long-term stability but also respond to complexing ligands and to variations in the oxygen pressure. Monocrystalline iridium electrodes can be obtained by continuously cycling the potential between -0.25 and + 1.25 V (SCE) in 0.544 sulphuric acid. They do not respond to the complexing ligands tested, and have fairly good long-term stability, but the reproducibility between electrodes is inferior to that of the monocrystalline antimony electrodes. Polycrystalline antimony and iridium electrodes were inferior to the monocrystalline ones. The properties of the palladium electrodes were similar to those of the iridium ones.
The determination of pH in special situations, for example in uivo applications where the fragility of the glass electrode is a drawback, requires pH-sensors that can easily be miniaturized and built into physically rugged sleeves. Furthermore, the electrodes should have a good pH-response and little or no response to the complex-forming ligands that are present in most biological fluids. One class of potentially suitable micro-electrodes is that of metal/metal oxide electrodes. Recently, electrodes based upon antimony, iridium and palladium have mainly been the subject of scientific interest. The present group of authors has previously studied antimony/antimony oxide’ and palladium/palladium oxide electrodes’ as pH-sensors. Monocrystalline antimony electrodes were introduced by Edwall. The effect of complex-forming ligands in solution on the calibration of electrodes was studied by Glab et al.,’ who recommended procedures for the calibration of the antimony electrode. Various kinds of iridium electrodes are described in the literature. Perley and Godshalk4 were the first to use iridium for measuring pH. De Rooij and Bergveld’ used electrodes made by cycling the potential of the iridium electrode continuously between
-0.25 and + 1.25 V (SCE) for around 200 cycles. Thermal methods of preparation were used by Papeschi et al.’ and by Ardizzone et al.’ The first group used the electrode to monitor pH in biological fluids, and the second made a closer study of the electrode mechanism. Iridium dioxide electrodes were studied by Fog and Buck,’ who used iridium dioxide on an inert electrode of the RdiiEka Selectrode type (Radiometer, Copenhagen). Data on the reproducibility, pH range and some interfering ions were given. A similar mechanism was utilized by Katsube et aI., who used a sputtered film of iridium oxide on a support of steel or tantalum. Electrodes for pH-determination, based on the palladium/palladium oxide couple, have been studied by many workers. Grubb and King” devised a method for preparing electrodes by coating palladium wires with sodium hydroxide and oxidizing them at 800” for 20 min. Chung-Chiun Liu et al.” used electrochemical oxidation in a mixed melt of sodium nitrate and lithium chloride. The present group studied the utility of monocrystalline palladium as a material for palladium/palladium oxide electrodes* but concluded that no advantage was offered by this material since the electrochemically deposited oxide layer did not function well for pH125
EITA KINOSHITA et al.
126
sensing purposes, and the thermally prepared oxide had no defined crystal structure. The aim of the present work was to study different ways of making pH-sensing electrodes from monocrystalline or polycrystalline antimony, iridium or palladium. Also, it was desired to study the reproducibility between electrodes and the stability of the electrode potential over long periods of time. The different electrodes were to be compared with respect to the manner of preparation, the pH-response (reproducibility and time response) and the effect that different complexing ligands present in the solutions may have on the electrode response. Further, the redox-response of the electrodes and the effect of different oxygen pressures on the electrode potentials were to be investigated. EXPERIMENTAL Monocrystalline antimony and iridium electrodes were made by the following method. The pieces of antimony or iridmm, which were 0.6 mm in diameter and 2 mm long, were degreased in trichloroethylene. An electrical contact lead was attached with conductive epoxy resin (MRC 4912, Materials Research).
The electrode was then cast in epoxy resin (Araldite, Ciba-Geigy) to form a plastic cylinder approximately 3 mm in diameter and with the exposed metal surface at the bottom end. The electrodes were ground and polished, with I-pm diamond paste in the final step. The same procedure was used to make polycrystalline tridmm electrodes. However, the polycrystailine iridium wire was 0.25 mm in diameter, and 12 pieces, each 3 mm long, were connected in a bundle, connected by silver-epoxy resin, and cast in one piece of epoxy resin to make an electrode. The palladium electrodes were made as follows. Polycrystalline palladium wire (99.9% pure, 1mm diameter and 10 mm long) was ground to remove any oxide present, and cleaned with acetone, ethanol and concentrated nitric acid. The clean wire was then immersed in a 50% aqueous solution of sodium hydroxide and subsequently dned in a flow of nitrogen. The coated wire was placed in an electrically heated oven at 7.50”.After 20 min, the oxidized wire was rinsed with distilled water. One end of the wire was cleaned from oxide and used for the electrical connection. This part of the wire was covered by silicone rubber sealant (Dow Corning Silastic 734 RTV) and tubing to prevent it from coming into contact with the test solution. Reagent-grade chemicals and doubly distilled water were used for the solutions. All measurements were made at 25” (thermostatic bath). Magnetic stirring was employed. In order to keep the partial pressure’of oxygen constant, air
700 *O-_j 0
t-
A-3
600
> E 2 500
‘2;
ii P i?
A 0 0
400 \
00 ‘A
\
Cl ‘A
A
300
A
A
I
I
I
I
3
4
5
6
I
7
,
a
\,
9
10
ph Fig. 1. Titratton of 0.02M TRIS/O.l4~ NaNO, (to keep the ionic strength at 0.16). A palladium electrode made by thermal oxidation was used for measuring the potentials. The arrows indicate the direction of titration. The following equations were obtained by regression. For the ph-range 2.5-10, E = 821 - 58.6 ph, rz = 0.9982; for the ph-range 2.5-8.3, E = 821.6 - 59.6 ph, r2 = 0.9993. The number of points was 39 in the first case, 34 in the second.
Crystalline
Sb, Ir and Pd electrodes
was bubbled through the test solutions before and during the measurements. The oxygen response measurements utilized mixtures of oxygen and nitrogen obtained from AGA Specialgas AB, Stockholm. Their oxygen partial pressures were2.1 ~0.1,2.3~0.1.3.8+0.1,7.4~0.1, 13.4fO.l and 20.7 f 0.1 kPa (100 kPa = I bar). The equipment used for the titrations has been described previously. All potential values given in this paper are referred to the standard hydrogen electrode unless stated otherwise. Throughout this paper, the electrodes have been calibrated to measure hydrogen-ion concentration (denoted by ph) at the gtven ionic strength instead of hydrogen-ion activity (denoted by pH). The technique has been described elsewhere.’
RESULTS
The ph-response
of the various types of electrode
The monocrystalline antimony electrodes exhibit a very good ph-response in the absence of complexing agents, for example in unbuffered solution or when TRIS [tris(hydroxymethyl)aminomethane] is used to buffer the ph of the solution.’ For six electrodes studied, the E-value measured at ph = 7.4 was - 174.7 mV, S.D. 0.3 mV and the slope was - 52.0 mV/ph, SD. 0.1 mV/ph. The commercially available polycrystalline antimony electrodes showed a less reproducible dependence of potential on ph, and the standard deviation for six electrodes was higher than for the monocrystalline electrodes. Electrodes made by Ingold, Tacussel and Radiometer were tested (two of each make) and the results were: E-value at ph = 7.4 was - 171.8 mV, S.D. 2.7 mV and slope - 52.6 mV/ph, S.D. = 0.1 mV/ph. The polycrystalline
electrodes also had inferior long-term stability in comparison with the monocrystalline antimony electrodes, and were sensitive to the speed of stirring of the solution. The potential is a linear function of ph over the ph range 2-10. For palladium/palladium oxide electrodes made by thermal oxidation for 20 min at 750”, the ph-response was linear over the ph range 2.5-S as shown in Fig. 1. The E”-value for these electrodes decreased by -20 mV during the first two days after preparation, but then remained unchanged for a few months. The E”-values differed between individual electrodes. In our study, fifteen electrodes were tested. Figure 2 shows the range within which all the results lay. The greatest difference in E” between two samples was about 50 mV. The potential values for the thermally prepared palladium/palladium oxide electrodes fell between the values that could be calculated for the reaction PdO+2H++2e-+Pd+H,O from an E”-value of 0.79 V (determined experimentally by Hoare”), and an E”-value of 0.917 V (calculated from thermodynamic data13). Most of the electrodes in the group tested had potentials which were closer to the value calculated from Hoare’s value than to the thermodynamic value. Iridium/iridium oxide electrodes obtained by continuously cycling the potential of the iridium electrode between -0.25 and + 1.25 V (SCE) in 0.5M sulphuric acid, so-called AIROF-electrodes (Anodic Iridium Oxide Film electrodes), exhibited a linear response over the ph-range 2.5-8.7, as can be seen in Fig. 3. At ph-values higher than 8.7, the potential of the electrode drifted away in a positive direction from the straight line. The correlation coefficients for the linear part of the E vs. ph plots were slightly better for monocrystalline (0.9990-0.9997) than for polycrystalline electrodes (~0.999). No clear trend could be observed in E” or slope when electrodes were prepared by cycling voltammetrically for 100, 200 or 400 cycles, other experimental parameters being equal. The slope differed within the group of electrodes and consequently the different electrodes also had different E”-values. In general, the slopes of the E vs. ph plots obtained with polycrystalline AIROFelectrodes were lower (62-68 mV/ph) than the slopes obtained by using monocrystalline AIROFelectrodes (69-74 mV/ph). Response-rates
ph Fig. 2. The dependence of the potential of palladium electrodes, made by thermal oxidation, on ph in 0.02M TRIS/O.l4M NaNO,. the ph was varied by adding either 0.05M HNOJ0.I IM NaNO, or O.O5M- NaOH/O.I IM NaNO,. a and b, lines valid for the data obtained with the thermally oxidized palladium electrodes having the highest and the lowest E” value, respectively; c and d. lines calculated from literature data for the reaction PdO + 2H+ + 2e- $Pd + H,O. Two different E” values were used: line c-790 rnV;12 line d-917 mV.”
127
of the electrodes
From a practical point of view, the speed with which the electrode will respond to a change in ph-value of the solution is important. Our experimental set-up permitted us to make only a relative comparison of the response times of the glass electrode and the metal/metal oxide electrodes. The potential values of the electrodes were recorded for 0.02M orthophosphate/O. 14M sodium nitrate medium subsequent to very fast addition of either strong
EITA KINOSHITAet al.
128
600
z
500
G F 5 & 400 a
300
200 1
I
I
I
I
I
I
I
3
L
5
6
7.
8
9
10
ph Fig. 3. Titration of 0.02M phosphate/O.l4M NaNO,. The electrode was a monocrystalline AIROF electrode obtained by cycling the potential between -0.25 and + 1.25 V (SCE) for 200 cycles, at a scan-rate of 100 mV/sec. The data can be described by the regression line E (mV) = 714 - 71.2 ph, r* = 0.9990. The number of experimental points was 32 and the ph range 2.5-9.5.
(0.05k.f nitric acid/O.1 1M sodium nitrate) or strong base (O.OSMsodium hydroxide/O. 11M sodium nitrate). The ph was changed in steps: from ph 6 to ph 2.5, from ph 2.5 back to ph 6, from ph 6 to ph 8.3 and from ph 8.3 back to ph 6. The response rates were evaluated from the time required for the electrodes to reach 95% of the new steady-state value after change in the composition of the solution. Monocrystalline antimony electrodes have response times that are similar to the response time of the glass electrode used (Ingold, type HA-401 M5, Ingold, Switzerland) over the entire ph-range. Thermally prepared palladium/palladium oxide electrodes have response times similar to the glass electrode in acidic or neutral solution. In the alkaline range, the response times are 6-10 times those for the glass electrode for increase in pH and 24 times longer than for the glass electrode for decrease to ph 6. AIROF-electrodes exhibit response times similar to or slightly longer than those for the glass electrode in alkaline solution. Monocrystalline AIROFelectrodes have 3-10 times longer response times than the glass electrode in acidic solution. The number of acid
cycles used in preparation of the electrode had no clear effect on the response time. Polycrystalline AIROF-electrodes responded more slowly than the monocrystalline ones. The effect response
of complexing
agents
on the electrode
A comparison of the effects of some complexforming ligands on the potential of the various electrode types studied can be based on the results summarized in Fig. 4. The potential of the antimony electrode is non-linear for ph < 8 in the presence of all ligands tested except TRIS. Among the ligands affecting the antimony electrode is orthophosphate, so certain standard buffer solutions cannot be used to calibrate this type of electrode. Orthophosphate had no effect on the potential of the thermally oxidized palladium electrodes, nor on the potential of the AIROF-electrodes. However, oxalate, being a strong reducing agent, will affect the potential of these electrodes. For palladium, a potential shift of between + 1 and +5 mV was observed for 0.02M sodium oxalate (0.14M in sodium nitrate) relative to the response for TRIS of the same concentration.
Crystalline Sb, Ir and Pd electrodes i
J
--5 ;,L,, \ ~ 100 1 ,.2\,~, "\
129
=
I = (oxaIafe) . . . . . . 2 :( EOTA} .......
3o(c,,ra,e,
.... .....
4=( I'ar trafe ) 5=( phosphate )
~~'~k
" ~ "~,~.\\ ~
. . . . . 6=(acefate)
6
1~',\'~
- -
=ITRIS)
'3,~'~
z
'-%
E
-100 z LU I-C) a_
-200
-300 i
I
I
I
2
4
6 ph
8
10
Fig. 4(a).
-----
700
1 =(carbonafe) r=(oxatate ) = TRIS } =(ammonia) I=(ph°spha te ) =(borate }
.
, ~\. 600
> E
500 l-Z
n
400
300
200 i
2
i
4
i
6 ph Fig. 4(b).
I
8
1,0
EITAKINOSHITA et al.
600
> E ;c'500 F 5 k a 400
300
200 2/
I
1
L
6
I
4
8
10
ph
Fig. 4(c). Fig. 4. The dependence of potential on ph for a, antimony/antimony oxide, b, palladium/palladium and c, AIROF electrodes, in 0.14M NaNO,/O.O2M ligand medium.
Also, the response times of the palladium electrodes were longer in the presence of oxalate than in its absence. The potential of neither the monocrystalline nor polycrystalline AIROF electrode was affected by the same concentration of oxalate in the ph range 2.5~K5, but shifts of about + IO mV were observed for neutral solution. The HCO;/CO, system does not affect the phresponse of the antimony electrode but preliminary results indicate that calibration plots of the palladium electrode and the AIROF-electrode are shifted by between +5 and +8 mV in the presence of this system.
The effect of redox systems in the solution Non-linear calibration plots were obtained for all metal/metal oxide electrodes in the presence of a redox buffer (Fig. 5). However, the effect of ferrocyanide on the antimony electrode is different from that on the palladium and AIROF electrodes. The potentials are shifted towards higher values and there is a “bump” in the curve at around ph 5. The potential us. ph plots for the latter two electrode types are the same as for a platinum wire in the solution, indicating that the response is due to slight variations in the redox potential and not to the change in ph. Only at very low ferrocyanide/ferrocyanide concen-
oxide
trations did the potential of the palladium electrode differ from the potential of the platinum wire. The palladium electrode retained its ph-response after being used in the ferricyanide/ferrocyanide buffer, but lost it after being used in a bromine/bromide redox buffer. Dissolution of the oxide phase by formation of palladium bromide complexes is the most probable explanation. The effect of oxygen pressure on the electrode response Various mixtures of oxygen and nitrogen [oxygen content 2, 8 and approx. 20% (air)] were bubbled through a 0.02M orthophosphate/O. 14M sodium nitrate solution at ph 6.5. In this way, the oxygen response of the thermally prepared palladium electrode or the AIROF electrodes could be tested. The results are summarized in Fig. 6. The potential change 2 hr after switching from air to the mixture of 8% oxygen in nitrogen was only 3 mV for the palladium electrode and the monocrystalline AIROF electrode. Consequently, oxygen is probably not directly involved in reactions with the electrode material, or if it is, the reactions proceed very slowly. Thus in practice these electrodes are not sensitive to oxygen. The polycrystalline AIROF electrodes behaved differently. Their oxygen response was both greater and faster. Switching from air to 8% oxygen resulted
Crystalline
Sb, Ir and Pd electrodes
(a) 400-
--
131
(cl
(b) 3
-__e____-__
I: l\,fIj\__[~::_, -LOO2
a
4 ph 6
1 10
200 -
200 -
2
‘P6
a
lo
2
‘P6
a
lo
Fig. 5. The redox sensitivity of the metal/metal oxide electrodes recorded for curves l,O.O2M TRIS/O. 14M NaNO,; curves 2, the same solution, containing also (a) 0.009M Fe(CN):- + O.OOlM Fe(CN):for the Sb-electrode, (6) O.OlM Fe(CN)z- + O.OlM Fe(CN):for the Pd-electrode. and (c) O.OOlM Fe(CN)z- + O.OOlM Fe(CN)ifor the AIROF electrode. A platinum-wire electrode was used as a check of the redox response (curves 3; for Pd, curves 2 and 3 coincided).
to structural differences. The oxidized electrodes displayed obvious differences under an optical microscope (Fig. 7). The monocrystalline electrode appeared uniform, with no visible colour differences, whereas the polycrystalline electrode showed a regu-
in a potential change of around 10 mV after 1 hr and switching from 2% oxygen to air again gave rise to a sharp change in the potential. The difference between the monocrystalline and polycrystalline AIROF electrodes could be attributed
________
a
Y
1
-IO/ ---L_f-
Po,(kPa) k 134
f
74
+
0
Po,IkPa) 120 7f-
0
38 10
7 4
+
23
+
38
TIME lmlnutel
-f-5
2.1 10
TIME
f
74
-
20
-I
207
-
15
( hour)
Fig. 6. The change m the electrode potential of the various types of electrode, on changing the partial pressure of oxygen above the test solution, as a function of time. The oxygen pressures used are indicated. (a) The antimony electrode. (b) the palladium electrode, (c) the polycrystalhne and (d) the monocrystalline AIROF electrode.
132
EITA KINOSHITA et
al.
Fig. I. Photc bmicrographs showing the surfaces of (a) monocrystalline and (b) polycrystallir le A.IR(3F electrodes after oxidation for 200 cycles between -0.25 and + 1.25 V.
lar pattern of two colours (blue and yellow). It is tempting to attribute this pattern to the grain structure of the polycrys~lline electrode. However, attempts to use a scanning electron microscope to analyse different parts of the electrode surfaces showed no difference between the two types of electrode. Antimony electrodes have a definite oxygen response. l4 For the monocrystalline electrodes it is comparatively fast, of the order of 15 set, and reproducible, as seen in Fig. 6. The antimony electrodes were tested in distilled and demineralized water in order to avoid the influence of complexforming ligands (such as orthophosphate). The gas mixtures consisted of carbon dioxide at 5.4 &-0.1 kPa partial pressure to keep the pH constant, various partial pressures of oxygen, and nitrogen to make up the balance.
The potential response was linearly related to the logarithm of the oxygen partial pressure. For ten electrodes of 6N (i.e., 99.9999%) purity, the slope of the response graph was 15.5 f 0.7 mV per unit change in log poi and the correlation coefficient was typically 0.99. The purity of the antimony affected the stability of the E”-value and also the slope of the oxygen-response curve. Electrodes of only 4N purity showed a potential drift of some mV per hr and a response slope of 13.6 t_ 1.I mV/log pW For polycrystalline electrodes, the oxygen response was more sluggish and irreproducible, both between electrodes and with time. When the monocrystalline electrodes were inspected with a scanning electron microscope the 6N purity electrodes were evenly corroded, whereas impurity inclusions, mainly of lead and copper, could be found in the surface of the 4N purity electrodes. The
Crystalline Sb, Ir and Pd electrodes inclusions were standing up like towers in the otherwise generally corroded surface. The inclusions could have acted as cathodic sites for the reduction of oxygen in the corrosion reaction. The inferior stability of less pure electrodes can thus be understood, as the inclusions will in due time be undermined by selective corrosion and thereby the cathodic area will be changed.
DISCUSSION
The results presented in the previous section make it possible to compare the different types of pHsensing electrodes on the basis of the differences in their properties. ~re~a$ut~o~. The monocrystalline antimony electrodes are simple to make. Antimony monocrystals are readily chipped into pieces and easily machined and ground. Orienting the monocrystals with a tightly packed crystal plane towards the solution requires some skill. The palladium and iridium electrodes are not as simple to make. The crystals are hard and difficult to machine. The observation about orientation applies to monocrystalline iridium as bell as to antimony. The ph -response. Monocrystalline antimony electrodes have a good response over the ph range 2-10, The response slope is typically 52 mV/ph. The thermally oxidized palladium~palladium oxide electrodes have a good response in the ph range 2.5-8.2 with a slope which is typically 59.6 mV/ph. AIROF electrodes can be used in the ph range 2.5-8.5. Monocrystalline AIROF electrodes give typical response slopes in the range 69-74 mV,/ph, whereas the slopes for the polycrystalline electrodes are 62-68 mV/ph. Reproducibility of the E”-values between electrodes. Within a group of monocrystalline antimony electrodes, the differences in E” values are very low. A typical standard deviation for six electrodes was 0.3 mV. The differences between electrode samples are substantially greater for the thermally oxidized palladium/palladium electrodes than for the antimony electrodes. However, the E”-value for a particular electrode is quite stable during its life-time. The E”-values of the AIROF electrodes differ, too, because the slope is different for different samptes. The response time. Monoc~stalline antimony electrodes respond as fast as the glass electrode. The palladium electrode has a fast response in acidic and neutral solution, but slower in alkaline medium (ph > 8.2). Monocrystalline AIROF electrodes have response similar to or slightly slower than that of the glass electrode in alkaline solution but substantially slower response in the acidic range. Polycrystalline AIROF electrodes respond more slowly than do the monocrystalline ones. The efkct of complexing ligands. Antimony electrodes are very sensitive to several ligands, some of them components of standard buffers. None of the
133
common ligands except bicarbonate affects the palladium/palladium oxide electrode. The same is true for the AIROF electrode, but it is also slightly affected by oxalate in neutral and alkaline media. The effect of redox systems present in the solution. All three types of electrode are affected by redox systems to an extent that makes their practical use difficult under such circumstances. The effect of oxygen pressure. Palladium and monocrystalline AIROF electrodes display the same kind of slow response towards a change in the oxygen pressure. The polycrystalline AIROF electrodes show a faster and more definite oxygen response. The fastest and also most stable and reproducible oxygen response was, however, found for pure monocrystalline antimony electrodes. These can in fact be used as oxygen electrodes if the pH of the measuring solution can be kept constant. The oxygen response was not significantly different from that for a fourelectron oxygen reduction process.
CONCLUSIONS
The type of metal/metal oxide pH-sensing electrode to choose in a particular case depends on the relative importance of the various properties of the electrodes. Monocrystalline antimony electrodes are the choice when fast response, good reproducibility between electrodes and stability are important. Their potential is, however, sensitive to several ligands that may be present in the solutions tested, and particular care is required in the choice of buffers for the calibration. Thermally oxidized palladium/palladium oxide electrodes are insensitive to most common complexing agents. The slope of the calibration curve varies very little from one electrode to another, which simplifies their calibration. The preparation requires access to a furnace capable of maintaining a temperature of 750”. Monocrystalline palladium does not offer any advantage over the polycrystalline material. Monocrystalline AIROF electrodes are also insensitive to most complexing agents. They are simple to make and to regenerate after polishing off the oxide layer. Both the slopes and the E” values differ between electrodes, which makes calibration difficult. The response time is comparatively long, particularly in acidic medium. The monocrystalline AIROF electrodes are preferable to the polycrystalline ones. The potential of all the electrode types discussed is affected by variations in the oxygen partial pressure, reproducibly for the monocrystalline antimony electrode. Also, all types are affected by the presence of redox buffers in the solution. Acknowledgements-Thanks are due to Professor Adam Hulanicki, University of Warsaw, Poland, for fruitful dlscussions during the course of this work. Financial support from the Swedish Board for Technical Development is gratefully acknowledged.
EITA KINOSHITAet al.
134 REFERENCES
1. S.
Giqb, G.
Edwall,
P.-A. Jijngren and F. Ingman,
Talanra, 198 1, 28, 30 I. 2. E. Kinoshita, S. Glgb, F. Ingman and G. Edwall, Electrochim. Acta, in press. 3. G. Edwall, Med. Eiol. Eng., 1978, 16, 661. 4. G. A. Perley and J. B. Godshalk, U.S. Patent No. 2,416,949, British Patent No. 567,722, June IO,
1942. 5. N. F. de Rooij and P. Bergveld, Ngmegen 1980, pp. 156165. Karger, 6. G. Papeschi, S. Bordi, M. Carla, L. Ledda, J. Med. Eng. Tech., 1981, 5,
7. S. Ardizzone, A. Carugati and S. Trassatti, J. Electroanal. Chem., 1981, 126, 287. 8. A. Fog and R. P. Buck, Sensors and Actuators, 1984, 5, 137. 9. T. Katsube, I. Lauks and J. N. Zemel, ibid., 1982, 2, 399. 10. W. T. Grubb and H. L. King, Anal. Chem., 1980, 52,
270.
Proc. Int. Conf.
11. C.-C. Liu, B. C. Bocchicchio, P. A. Overmyer and M. R. Neuman, Science, 1980, 207, 188. 12. J. P. Hoare, J. Elecfrochem. Sot., 1964, 111, 611. 13. W. M. Latimer, Oxidation Potentials, 2nd Ed., Prentice-
Base], 1981. Criscione and F.
Hall, New York, 1952. 14. P. A. Jijngren and’G. Edwall, Electrochim. Acta, 1980,
86.
25, 1585.
CopyrIght
c
0039-9 140186 $3 00 + 0.00 19X6 Pergamon Press Ltd
POLYCRYSTALLINE AND MONOCRYSTALLINE ANTIMONY, IRIDIUM AND PALLADIUM AS ELECTRODE MATERIAL FOR pH-SENSING ELECTRODES EITA KINOSHITA, FOLKE INGMAN, GUNNAR EDWALL* and SIGVARD THULIN* Departments of Analytical Chemistry and Applied Physics *, The Royal Institute of Technology, S-100 44 Stockholm, Sweden STANISLAW Gi.4~ Department of Chemistry, Warsaw University, Warsaw, Poland (Received 4 July 1985. Accepted 25 September 1985)
Summary-Different ways of making pH-sensing electrodes from monocrystalline or polycrystalline antimony, uidmm and palladium have been investigated. Monocrystalline antimony and iridium are superior to the polycrystalline elements with respect to reproducibility between electrodes and stability of the electrode potential over long periods of time. No good palladium/palladium oxide electrode could be obtained by electrochemical oxidation and the thermal preparation method could not take advantage of the properties of the monocrystalline palladium. Therefore, only polycrystalline palladium was used to study this type of electrodes. The different electrodes were compared with respect to the manner of preparation, the pH-response (reproducibility and time response) and the effect that different complexing ligands present in the measuring solutions may have on the electrode response. Also, the redox-response of the electrodes and the effect of different oxygen pressures on the electrode potentials were studied. The monocrystalline antimony electrodes have the best reproducibility and long-term stability but also respond to complexing ligands and to variations in the oxygen pressure. Monocrystalline iridium electrodes can be obtained by continuously cycling the potential between -0.25 and + 1.25 V (SCE) in 0.544 sulphuric acid. They do not respond to the complexing ligands tested, and have fairly good long-term stability, but the reproducibility between electrodes is inferior to that of the monocrystalline antimony electrodes. Polycrystalline antimony and iridium electrodes were inferior to the monocrystalline ones. The properties of the palladium electrodes were similar to those of the iridium ones.
The determination of pH in special situations, for example in uivo applications where the fragility of the glass electrode is a drawback, requires pH-sensors that can easily be miniaturized and built into physically rugged sleeves. Furthermore, the electrodes should have a good pH-response and little or no response to the complex-forming ligands that are present in most biological fluids. One class of potentially suitable micro-electrodes is that of metal/metal oxide electrodes. Recently, electrodes based upon antimony, iridium and palladium have mainly been the subject of scientific interest. The present group of authors has previously studied antimony/antimony oxide’ and palladium/palladium oxide electrodes’ as pH-sensors. Monocrystalline antimony electrodes were introduced by Edwall. The effect of complex-forming ligands in solution on the calibration of electrodes was studied by Glab et al.,’ who recommended procedures for the calibration of the antimony electrode. Various kinds of iridium electrodes are described in the literature. Perley and Godshalk4 were the first to use iridium for measuring pH. De Rooij and Bergveld’ used electrodes made by cycling the potential of the iridium electrode continuously between
-0.25 and + 1.25 V (SCE) for around 200 cycles. Thermal methods of preparation were used by Papeschi et al.’ and by Ardizzone et al.’ The first group used the electrode to monitor pH in biological fluids, and the second made a closer study of the electrode mechanism. Iridium dioxide electrodes were studied by Fog and Buck,’ who used iridium dioxide on an inert electrode of the RdiiEka Selectrode type (Radiometer, Copenhagen). Data on the reproducibility, pH range and some interfering ions were given. A similar mechanism was utilized by Katsube et aI., who used a sputtered film of iridium oxide on a support of steel or tantalum. Electrodes for pH-determination, based on the palladium/palladium oxide couple, have been studied by many workers. Grubb and King” devised a method for preparing electrodes by coating palladium wires with sodium hydroxide and oxidizing them at 800” for 20 min. Chung-Chiun Liu et al.” used electrochemical oxidation in a mixed melt of sodium nitrate and lithium chloride. The present group studied the utility of monocrystalline palladium as a material for palladium/palladium oxide electrodes* but concluded that no advantage was offered by this material since the electrochemically deposited oxide layer did not function well for pH125
EITA KINOSHITA et al.
126
sensing purposes, and the thermally prepared oxide had no defined crystal structure. The aim of the present work was to study different ways of making pH-sensing electrodes from monocrystalline or polycrystalline antimony, iridium or palladium. Also, it was desired to study the reproducibility between electrodes and the stability of the electrode potential over long periods of time. The different electrodes were to be compared with respect to the manner of preparation, the pH-response (reproducibility and time response) and the effect that different complexing ligands present in the solutions may have on the electrode response. Further, the redox-response of the electrodes and the effect of different oxygen pressures on the electrode potentials were to be investigated. EXPERIMENTAL Monocrystalline antimony and iridium electrodes were made by the following method. The pieces of antimony or iridmm, which were 0.6 mm in diameter and 2 mm long, were degreased in trichloroethylene. An electrical contact lead was attached with conductive epoxy resin (MRC 4912, Materials Research).
The electrode was then cast in epoxy resin (Araldite, Ciba-Geigy) to form a plastic cylinder approximately 3 mm in diameter and with the exposed metal surface at the bottom end. The electrodes were ground and polished, with I-pm diamond paste in the final step. The same procedure was used to make polycrystalline tridmm electrodes. However, the polycrystailine iridium wire was 0.25 mm in diameter, and 12 pieces, each 3 mm long, were connected in a bundle, connected by silver-epoxy resin, and cast in one piece of epoxy resin to make an electrode. The palladium electrodes were made as follows. Polycrystalline palladium wire (99.9% pure, 1mm diameter and 10 mm long) was ground to remove any oxide present, and cleaned with acetone, ethanol and concentrated nitric acid. The clean wire was then immersed in a 50% aqueous solution of sodium hydroxide and subsequently dned in a flow of nitrogen. The coated wire was placed in an electrically heated oven at 7.50”.After 20 min, the oxidized wire was rinsed with distilled water. One end of the wire was cleaned from oxide and used for the electrical connection. This part of the wire was covered by silicone rubber sealant (Dow Corning Silastic 734 RTV) and tubing to prevent it from coming into contact with the test solution. Reagent-grade chemicals and doubly distilled water were used for the solutions. All measurements were made at 25” (thermostatic bath). Magnetic stirring was employed. In order to keep the partial pressure’of oxygen constant, air
700 *O-_j 0
t-
A-3
600
> E 2 500
‘2;
ii P i?
A 0 0
400 \
00 ‘A
\
Cl ‘A
A
300
A
A
I
I
I
I
3
4
5
6
I
7
,
a
\,
9
10
ph Fig. 1. Titratton of 0.02M TRIS/O.l4~ NaNO, (to keep the ionic strength at 0.16). A palladium electrode made by thermal oxidation was used for measuring the potentials. The arrows indicate the direction of titration. The following equations were obtained by regression. For the ph-range 2.5-10, E = 821 - 58.6 ph, rz = 0.9982; for the ph-range 2.5-8.3, E = 821.6 - 59.6 ph, r2 = 0.9993. The number of points was 39 in the first case, 34 in the second.
Crystalline
Sb, Ir and Pd electrodes
was bubbled through the test solutions before and during the measurements. The oxygen response measurements utilized mixtures of oxygen and nitrogen obtained from AGA Specialgas AB, Stockholm. Their oxygen partial pressures were2.1 ~0.1,2.3~0.1.3.8+0.1,7.4~0.1, 13.4fO.l and 20.7 f 0.1 kPa (100 kPa = I bar). The equipment used for the titrations has been described previously. All potential values given in this paper are referred to the standard hydrogen electrode unless stated otherwise. Throughout this paper, the electrodes have been calibrated to measure hydrogen-ion concentration (denoted by ph) at the gtven ionic strength instead of hydrogen-ion activity (denoted by pH). The technique has been described elsewhere.’
RESULTS
The ph-response
of the various types of electrode
The monocrystalline antimony electrodes exhibit a very good ph-response in the absence of complexing agents, for example in unbuffered solution or when TRIS [tris(hydroxymethyl)aminomethane] is used to buffer the ph of the solution.’ For six electrodes studied, the E-value measured at ph = 7.4 was - 174.7 mV, S.D. 0.3 mV and the slope was - 52.0 mV/ph, SD. 0.1 mV/ph. The commercially available polycrystalline antimony electrodes showed a less reproducible dependence of potential on ph, and the standard deviation for six electrodes was higher than for the monocrystalline electrodes. Electrodes made by Ingold, Tacussel and Radiometer were tested (two of each make) and the results were: E-value at ph = 7.4 was - 171.8 mV, S.D. 2.7 mV and slope - 52.6 mV/ph, S.D. = 0.1 mV/ph. The polycrystalline
electrodes also had inferior long-term stability in comparison with the monocrystalline antimony electrodes, and were sensitive to the speed of stirring of the solution. The potential is a linear function of ph over the ph range 2-10. For palladium/palladium oxide electrodes made by thermal oxidation for 20 min at 750”, the ph-response was linear over the ph range 2.5-S as shown in Fig. 1. The E”-value for these electrodes decreased by -20 mV during the first two days after preparation, but then remained unchanged for a few months. The E”-values differed between individual electrodes. In our study, fifteen electrodes were tested. Figure 2 shows the range within which all the results lay. The greatest difference in E” between two samples was about 50 mV. The potential values for the thermally prepared palladium/palladium oxide electrodes fell between the values that could be calculated for the reaction PdO+2H++2e-+Pd+H,O from an E”-value of 0.79 V (determined experimentally by Hoare”), and an E”-value of 0.917 V (calculated from thermodynamic data13). Most of the electrodes in the group tested had potentials which were closer to the value calculated from Hoare’s value than to the thermodynamic value. Iridium/iridium oxide electrodes obtained by continuously cycling the potential of the iridium electrode between -0.25 and + 1.25 V (SCE) in 0.5M sulphuric acid, so-called AIROF-electrodes (Anodic Iridium Oxide Film electrodes), exhibited a linear response over the ph-range 2.5-8.7, as can be seen in Fig. 3. At ph-values higher than 8.7, the potential of the electrode drifted away in a positive direction from the straight line. The correlation coefficients for the linear part of the E vs. ph plots were slightly better for monocrystalline (0.9990-0.9997) than for polycrystalline electrodes (~0.999). No clear trend could be observed in E” or slope when electrodes were prepared by cycling voltammetrically for 100, 200 or 400 cycles, other experimental parameters being equal. The slope differed within the group of electrodes and consequently the different electrodes also had different E”-values. In general, the slopes of the E vs. ph plots obtained with polycrystalline AIROFelectrodes were lower (62-68 mV/ph) than the slopes obtained by using monocrystalline AIROFelectrodes (69-74 mV/ph). Response-rates
ph Fig. 2. The dependence of the potential of palladium electrodes, made by thermal oxidation, on ph in 0.02M TRIS/O.l4M NaNO,. the ph was varied by adding either 0.05M HNOJ0.I IM NaNO, or O.O5M- NaOH/O.I IM NaNO,. a and b, lines valid for the data obtained with the thermally oxidized palladium electrodes having the highest and the lowest E” value, respectively; c and d. lines calculated from literature data for the reaction PdO + 2H+ + 2e- $Pd + H,O. Two different E” values were used: line c-790 rnV;12 line d-917 mV.”
127
of the electrodes
From a practical point of view, the speed with which the electrode will respond to a change in ph-value of the solution is important. Our experimental set-up permitted us to make only a relative comparison of the response times of the glass electrode and the metal/metal oxide electrodes. The potential values of the electrodes were recorded for 0.02M orthophosphate/O. 14M sodium nitrate medium subsequent to very fast addition of either strong
EITA KINOSHITAet al.
128
600
z
500
G F 5 & 400 a
300
200 1
I
I
I
I
I
I
I
3
L
5
6
7.
8
9
10
ph Fig. 3. Titration of 0.02M phosphate/O.l4M NaNO,. The electrode was a monocrystalline AIROF electrode obtained by cycling the potential between -0.25 and + 1.25 V (SCE) for 200 cycles, at a scan-rate of 100 mV/sec. The data can be described by the regression line E (mV) = 714 - 71.2 ph, r* = 0.9990. The number of experimental points was 32 and the ph range 2.5-9.5.
(0.05k.f nitric acid/O.1 1M sodium nitrate) or strong base (O.OSMsodium hydroxide/O. 11M sodium nitrate). The ph was changed in steps: from ph 6 to ph 2.5, from ph 2.5 back to ph 6, from ph 6 to ph 8.3 and from ph 8.3 back to ph 6. The response rates were evaluated from the time required for the electrodes to reach 95% of the new steady-state value after change in the composition of the solution. Monocrystalline antimony electrodes have response times that are similar to the response time of the glass electrode used (Ingold, type HA-401 M5, Ingold, Switzerland) over the entire ph-range. Thermally prepared palladium/palladium oxide electrodes have response times similar to the glass electrode in acidic or neutral solution. In the alkaline range, the response times are 6-10 times those for the glass electrode for increase in pH and 24 times longer than for the glass electrode for decrease to ph 6. AIROF-electrodes exhibit response times similar to or slightly longer than those for the glass electrode in alkaline solution. Monocrystalline AIROFelectrodes have 3-10 times longer response times than the glass electrode in acidic solution. The number of acid
cycles used in preparation of the electrode had no clear effect on the response time. Polycrystalline AIROF-electrodes responded more slowly than the monocrystalline ones. The effect response
of complexing
agents
on the electrode
A comparison of the effects of some complexforming ligands on the potential of the various electrode types studied can be based on the results summarized in Fig. 4. The potential of the antimony electrode is non-linear for ph < 8 in the presence of all ligands tested except TRIS. Among the ligands affecting the antimony electrode is orthophosphate, so certain standard buffer solutions cannot be used to calibrate this type of electrode. Orthophosphate had no effect on the potential of the thermally oxidized palladium electrodes, nor on the potential of the AIROF-electrodes. However, oxalate, being a strong reducing agent, will affect the potential of these electrodes. For palladium, a potential shift of between + 1 and +5 mV was observed for 0.02M sodium oxalate (0.14M in sodium nitrate) relative to the response for TRIS of the same concentration.
Crystalline Sb, Ir and Pd electrodes i
J
--5 ;,L,, \ ~ 100 1 ,.2\,~, "\
129
=
I = (oxaIafe) . . . . . . 2 :( EOTA} .......
3o(c,,ra,e,
.... .....
4=( I'ar trafe ) 5=( phosphate )
~~'~k
" ~ "~,~.\\ ~
. . . . . 6=(acefate)
6
1~',\'~
- -
=ITRIS)
'3,~'~
z
'-%
E
-100 z LU I-C) a_
-200
-300 i
I
I
I
2
4
6 ph
8
10
Fig. 4(a).
-----
700
1 =(carbonafe) r=(oxatate ) = TRIS } =(ammonia) I=(ph°spha te ) =(borate }
.
, ~\. 600
> E
500 l-Z
n
400
300
200 i
2
i
4
i
6 ph Fig. 4(b).
I
8
1,0
EITAKINOSHITA et al.
600
> E ;c'500 F 5 k a 400
300
200 2/
I
1
L
6
I
4
8
10
ph
Fig. 4(c). Fig. 4. The dependence of potential on ph for a, antimony/antimony oxide, b, palladium/palladium and c, AIROF electrodes, in 0.14M NaNO,/O.O2M ligand medium.
Also, the response times of the palladium electrodes were longer in the presence of oxalate than in its absence. The potential of neither the monocrystalline nor polycrystalline AIROF electrode was affected by the same concentration of oxalate in the ph range 2.5~K5, but shifts of about + IO mV were observed for neutral solution. The HCO;/CO, system does not affect the phresponse of the antimony electrode but preliminary results indicate that calibration plots of the palladium electrode and the AIROF-electrode are shifted by between +5 and +8 mV in the presence of this system.
The effect of redox systems in the solution Non-linear calibration plots were obtained for all metal/metal oxide electrodes in the presence of a redox buffer (Fig. 5). However, the effect of ferrocyanide on the antimony electrode is different from that on the palladium and AIROF electrodes. The potentials are shifted towards higher values and there is a “bump” in the curve at around ph 5. The potential us. ph plots for the latter two electrode types are the same as for a platinum wire in the solution, indicating that the response is due to slight variations in the redox potential and not to the change in ph. Only at very low ferrocyanide/ferrocyanide concen-
oxide
trations did the potential of the palladium electrode differ from the potential of the platinum wire. The palladium electrode retained its ph-response after being used in the ferricyanide/ferrocyanide buffer, but lost it after being used in a bromine/bromide redox buffer. Dissolution of the oxide phase by formation of palladium bromide complexes is the most probable explanation. The effect of oxygen pressure on the electrode response Various mixtures of oxygen and nitrogen [oxygen content 2, 8 and approx. 20% (air)] were bubbled through a 0.02M orthophosphate/O. 14M sodium nitrate solution at ph 6.5. In this way, the oxygen response of the thermally prepared palladium electrode or the AIROF electrodes could be tested. The results are summarized in Fig. 6. The potential change 2 hr after switching from air to the mixture of 8% oxygen in nitrogen was only 3 mV for the palladium electrode and the monocrystalline AIROF electrode. Consequently, oxygen is probably not directly involved in reactions with the electrode material, or if it is, the reactions proceed very slowly. Thus in practice these electrodes are not sensitive to oxygen. The polycrystalline AIROF electrodes behaved differently. Their oxygen response was both greater and faster. Switching from air to 8% oxygen resulted
Crystalline
Sb, Ir and Pd electrodes
(a) 400-
--
131
(cl
(b) 3
-__e____-__
I: l\,fIj\__[~::_, -LOO2
a
4 ph 6
1 10
200 -
200 -
2
‘P6
a
lo
2
‘P6
a
lo
Fig. 5. The redox sensitivity of the metal/metal oxide electrodes recorded for curves l,O.O2M TRIS/O. 14M NaNO,; curves 2, the same solution, containing also (a) 0.009M Fe(CN):- + O.OOlM Fe(CN):for the Sb-electrode, (6) O.OlM Fe(CN)z- + O.OlM Fe(CN):for the Pd-electrode. and (c) O.OOlM Fe(CN)z- + O.OOlM Fe(CN)ifor the AIROF electrode. A platinum-wire electrode was used as a check of the redox response (curves 3; for Pd, curves 2 and 3 coincided).
to structural differences. The oxidized electrodes displayed obvious differences under an optical microscope (Fig. 7). The monocrystalline electrode appeared uniform, with no visible colour differences, whereas the polycrystalline electrode showed a regu-
in a potential change of around 10 mV after 1 hr and switching from 2% oxygen to air again gave rise to a sharp change in the potential. The difference between the monocrystalline and polycrystalline AIROF electrodes could be attributed
________
a
Y
1
-IO/ ---L_f-
Po,(kPa) k 134
f
74
+
0
Po,IkPa) 120 7f-
0
38 10
7 4
+
23
+
38
TIME lmlnutel
-f-5
2.1 10
TIME
f
74
-
20
-I
207
-
15
( hour)
Fig. 6. The change m the electrode potential of the various types of electrode, on changing the partial pressure of oxygen above the test solution, as a function of time. The oxygen pressures used are indicated. (a) The antimony electrode. (b) the palladium electrode, (c) the polycrystalhne and (d) the monocrystalline AIROF electrode.
132
EITA KINOSHITA et
al.
Fig. I. Photc bmicrographs showing the surfaces of (a) monocrystalline and (b) polycrystallir le A.IR(3F electrodes after oxidation for 200 cycles between -0.25 and + 1.25 V.
lar pattern of two colours (blue and yellow). It is tempting to attribute this pattern to the grain structure of the polycrys~lline electrode. However, attempts to use a scanning electron microscope to analyse different parts of the electrode surfaces showed no difference between the two types of electrode. Antimony electrodes have a definite oxygen response. l4 For the monocrystalline electrodes it is comparatively fast, of the order of 15 set, and reproducible, as seen in Fig. 6. The antimony electrodes were tested in distilled and demineralized water in order to avoid the influence of complexforming ligands (such as orthophosphate). The gas mixtures consisted of carbon dioxide at 5.4 &-0.1 kPa partial pressure to keep the pH constant, various partial pressures of oxygen, and nitrogen to make up the balance.
The potential response was linearly related to the logarithm of the oxygen partial pressure. For ten electrodes of 6N (i.e., 99.9999%) purity, the slope of the response graph was 15.5 f 0.7 mV per unit change in log poi and the correlation coefficient was typically 0.99. The purity of the antimony affected the stability of the E”-value and also the slope of the oxygen-response curve. Electrodes of only 4N purity showed a potential drift of some mV per hr and a response slope of 13.6 t_ 1.I mV/log pW For polycrystalline electrodes, the oxygen response was more sluggish and irreproducible, both between electrodes and with time. When the monocrystalline electrodes were inspected with a scanning electron microscope the 6N purity electrodes were evenly corroded, whereas impurity inclusions, mainly of lead and copper, could be found in the surface of the 4N purity electrodes. The
Crystalline Sb, Ir and Pd electrodes inclusions were standing up like towers in the otherwise generally corroded surface. The inclusions could have acted as cathodic sites for the reduction of oxygen in the corrosion reaction. The inferior stability of less pure electrodes can thus be understood, as the inclusions will in due time be undermined by selective corrosion and thereby the cathodic area will be changed.
DISCUSSION
The results presented in the previous section make it possible to compare the different types of pHsensing electrodes on the basis of the differences in their properties. ~re~a$ut~o~. The monocrystalline antimony electrodes are simple to make. Antimony monocrystals are readily chipped into pieces and easily machined and ground. Orienting the monocrystals with a tightly packed crystal plane towards the solution requires some skill. The palladium and iridium electrodes are not as simple to make. The crystals are hard and difficult to machine. The observation about orientation applies to monocrystalline iridium as bell as to antimony. The ph -response. Monocrystalline antimony electrodes have a good response over the ph range 2-10, The response slope is typically 52 mV/ph. The thermally oxidized palladium~palladium oxide electrodes have a good response in the ph range 2.5-8.2 with a slope which is typically 59.6 mV/ph. AIROF electrodes can be used in the ph range 2.5-8.5. Monocrystalline AIROF electrodes give typical response slopes in the range 69-74 mV,/ph, whereas the slopes for the polycrystalline electrodes are 62-68 mV/ph. Reproducibility of the E”-values between electrodes. Within a group of monocrystalline antimony electrodes, the differences in E” values are very low. A typical standard deviation for six electrodes was 0.3 mV. The differences between electrode samples are substantially greater for the thermally oxidized palladium/palladium electrodes than for the antimony electrodes. However, the E”-value for a particular electrode is quite stable during its life-time. The E”-values of the AIROF electrodes differ, too, because the slope is different for different samptes. The response time. Monoc~stalline antimony electrodes respond as fast as the glass electrode. The palladium electrode has a fast response in acidic and neutral solution, but slower in alkaline medium (ph > 8.2). Monocrystalline AIROF electrodes have response similar to or slightly slower than that of the glass electrode in alkaline solution but substantially slower response in the acidic range. Polycrystalline AIROF electrodes respond more slowly than do the monocrystalline ones. The efkct of complexing ligands. Antimony electrodes are very sensitive to several ligands, some of them components of standard buffers. None of the
133
common ligands except bicarbonate affects the palladium/palladium oxide electrode. The same is true for the AIROF electrode, but it is also slightly affected by oxalate in neutral and alkaline media. The effect of redox systems present in the solution. All three types of electrode are affected by redox systems to an extent that makes their practical use difficult under such circumstances. The effect of oxygen pressure. Palladium and monocrystalline AIROF electrodes display the same kind of slow response towards a change in the oxygen pressure. The polycrystalline AIROF electrodes show a faster and more definite oxygen response. The fastest and also most stable and reproducible oxygen response was, however, found for pure monocrystalline antimony electrodes. These can in fact be used as oxygen electrodes if the pH of the measuring solution can be kept constant. The oxygen response was not significantly different from that for a fourelectron oxygen reduction process.
CONCLUSIONS
The type of metal/metal oxide pH-sensing electrode to choose in a particular case depends on the relative importance of the various properties of the electrodes. Monocrystalline antimony electrodes are the choice when fast response, good reproducibility between electrodes and stability are important. Their potential is, however, sensitive to several ligands that may be present in the solutions tested, and particular care is required in the choice of buffers for the calibration. Thermally oxidized palladium/palladium oxide electrodes are insensitive to most common complexing agents. The slope of the calibration curve varies very little from one electrode to another, which simplifies their calibration. The preparation requires access to a furnace capable of maintaining a temperature of 750”. Monocrystalline palladium does not offer any advantage over the polycrystalline material. Monocrystalline AIROF electrodes are also insensitive to most complexing agents. They are simple to make and to regenerate after polishing off the oxide layer. Both the slopes and the E” values differ between electrodes, which makes calibration difficult. The response time is comparatively long, particularly in acidic medium. The monocrystalline AIROF electrodes are preferable to the polycrystalline ones. The potential of all the electrode types discussed is affected by variations in the oxygen partial pressure, reproducibly for the monocrystalline antimony electrode. Also, all types are affected by the presence of redox buffers in the solution. Acknowledgements-Thanks are due to Professor Adam Hulanicki, University of Warsaw, Poland, for fruitful dlscussions during the course of this work. Financial support from the Swedish Board for Technical Development is gratefully acknowledged.
EITA KINOSHITAet al.
134 REFERENCES
1. S.
Giqb, G.
Edwall,
P.-A. Jijngren and F. Ingman,
Talanra, 198 1, 28, 30 I. 2. E. Kinoshita, S. Glgb, F. Ingman and G. Edwall, Electrochim. Acta, in press. 3. G. Edwall, Med. Eiol. Eng., 1978, 16, 661. 4. G. A. Perley and J. B. Godshalk, U.S. Patent No. 2,416,949, British Patent No. 567,722, June IO,
1942. 5. N. F. de Rooij and P. Bergveld, Ngmegen 1980, pp. 156165. Karger, 6. G. Papeschi, S. Bordi, M. Carla, L. Ledda, J. Med. Eng. Tech., 1981, 5,
7. S. Ardizzone, A. Carugati and S. Trassatti, J. Electroanal. Chem., 1981, 126, 287. 8. A. Fog and R. P. Buck, Sensors and Actuators, 1984, 5, 137. 9. T. Katsube, I. Lauks and J. N. Zemel, ibid., 1982, 2, 399. 10. W. T. Grubb and H. L. King, Anal. Chem., 1980, 52,
270.
Proc. Int. Conf.
11. C.-C. Liu, B. C. Bocchicchio, P. A. Overmyer and M. R. Neuman, Science, 1980, 207, 188. 12. J. P. Hoare, J. Elecfrochem. Sot., 1964, 111, 611. 13. W. M. Latimer, Oxidation Potentials, 2nd Ed., Prentice-
Base], 1981. Criscione and F.
Hall, New York, 1952. 14. P. A. Jijngren and’G. Edwall, Electrochim. Acta, 1980,
86.
25, 1585.