improved antimony-antimony (iii) oxide ph electrodes

Med. & Biol. Eng. & Comput., 1978, 16, 661-669

Improved antimony-antimony (111)oxide pH electrodes G. Edwa l l

Department of Applied Physics, Royal institute of Technology, Stockholm, Sweden

A b s t r a c t - - I n routine clinical care rapid results from pH determinations are often desired. This calls for continuous in vivo measurements. Today only intermittent in vitro pH analysis is routinely performed, as the existing transducers do not meet the clinical demands on dimensions, stability, stabilisation time, resolution and ruggedness. Here, some results from a basic experimental study on antimony-antimonyoxide pH electrodes are presented. The purpose of the investigation has been to gain some information on the electrode processes, which govern the electrode response, and to use this information to synthesise more stable and reproducible electrodes than those presently existing. These electrodes are intended, in an appropriate miniaturised form, for in vivo routine pH determinations on homo, e.g. in a continuous acid-base balance surveillance application.

Keywords--Ant imony-ant imonyoxide electrode, pH electrode

1 Introduction

CLINICAL pH determinations are routinely performed in vitro on blood samples withdrawn from patients. Along with determinations of Pco2, P% and the haematocrit, pH measurements provide complete information on the acid-base status of the body.

In the human body, the acid-base regulation constitutes a highly compensated system. Arterial pH deviations as small as 0"05 pH units from the normal value of 7"40 may be clinically important. Early detection of pH changes, corresponding to improper function of the compensatory mechanisms, is essential. For survey purposes, and in situations where immediate results are needed, pH monitoring in vivo is therefore of interest. If an abnormal pH value is encountered, a complete acid-base analysis is indicated. However, if continuous pH monitoring is employed, the need for rapid results from the supplementary determinations may be somewhat reduced.

It has been shown elsewhere (EDWALL, /976) that pH transducers suited for continuous in vivo monitoring shall be able to resolve a pH change of 5/1000 units, have a repeatability of 2/100 units and a long-term drift corresponding to less than 2/100 units between calibrations, which are preferably performed less than once a day. The transducer stabilisation time ought to be less than 1 rain: A small size of transducer is needed in order to have them solely embedded in a tissue of uniform bio- activity. Furthermore, their design must be rugged to allow insertion, e.g. intra-arterially or into soft tissue without a previous incision.

First received 31st August 1977 and in final form 18th January 1978

0140-0118/78/0784-0661 $1.50/0

�9 IFMBE: 1978

2 Present techniques for in vlvo pH monitoring Various techniques for continuous pH measure-

ments in biological fluids have been proposed. These can be divided into methods utilising extracorporeal circulation, reviewed by FRIEDMAN (1967), and in vivo methods with indwelling electrodes reviewed, for instance, by SEKELJ and GOLDBLOOM (1967), GEDDES and BAKER (1968), CATER and SILVER (1961) and KHURI (1967).

When extracorporeal circulation of blood is undertaken, an anticoagulant has to be administered. This increases the risk of uncontrolled bleeding, even from minor wounds, and the patient must be continuously supervised for this reason. Therefore, such methods have not found widespread use in routine clinical care.

In research, microformed glass electrodes have extensively been used in pH determinations in vivo. Usually, due to the design and fragility of the electrodes, a free-dissected measuring structure is needed. For clinical purposes, microbulb glass electrodes encased in cannulas of o.d. down to 0 ' 8 m m have been described for in vivo use (O'DoNNELL, 1975). However, in approaching these electrode dimensions the glass membrane will either have too high an impedance, thus deteriorat- ing the stability and resolution of the electrode response, or be too thin to withstand insertion directly into tissue in a clinical situation.

To avoid problems with fragile glass membranes, metal-metal oxide pH electrodes have been used (CATER and SILVER, 1961). In general, these electrodes, which are polycrystalline, are irreproducible, unstable and susceptible to interference from oxidising and reducing substances or from metal ions

Medical & Biological Engineering & Computing November 1978 661

in the solutions being measured. Accordingly, they are not suited for in vivo pH determinations. How- ever, from a theoretical standpoint, metal-metal oxide electrodes of the antimony type have promis- ing properties for forming a workable pH electrode system (IVES, 1961).

I t can be concluded that clinical pH measulements for the determination of the acid-base status of a patient are certainly desirable and are also commonly performed in a routine fashion. However, today, such measurements are restricted to intermittent in vitro analysis of blood samples withdrawn from the patient due to the lack of suitable electrodes for in vivo use.

3 Antimony pH electrodes

The use of antimony as a base of an electrode system was initiated more than 50 years ago by UrJL and KESTRANEK (1923). Since then, antimony electrodes have been of interest to a large number of investigators. In spite of this, the detailed behaviour of the electrode is still not fully understood.

Being a metal-metal oxide electrode, the antimony electrode should, in principle, be easy to miniaturise. Thus, it is possible to make small and rugged pH electrodes with a low electrical impedance. However, the poor resolution, repeatability and stability of the polycrystalline electrodes commonly used prevent their present use in routine clinical-care applications. They are a/so irreproducibly sensitive to different complex foiming substances present in extracellular fluids and in pH buffers used for standardisation. The electrode sensitivity to stirring of the measuring solution and to changes in its oxygen partial pressure is well documented (Iv~s, 1961). No systematic evaluation of the interfering factors is known, but a survey has been given by EDWALL (1976).

Special forms of antimony-antimony oxide electrodes with improved in vivo stability have been reported by KARLMARK (1973), VIEIRA and MALNIC (1968) and MALNIr and VIEIRA (1972). AS yet, no explanation for the improved behaviour has been given. However, the potentials of these microformed electrodes also seem to be sensitive to the composition of the measuring solutions (KARLMARK and SOHTELL, 1973; GREEN and GIEBISCH, 1974; PUSCHETT and ZURBACH, 1974; QUEHENBERGER, 1977).

The response of antimony electrodes is known to depend on their design and the method of produc- tion used (IrEs, 1961; STOCK et al, 1958). The purity of the antimony metal as well as the method of casting the electrodes have been claimed to be important for the characteristics of the electrode potential. A starting material of high purity and slow cooling during casting are favourable factors. Slow cooling of the metal promotes formation of large crystal grains in the metal bulk. Therefore, in this work, investigations on the influence from

grain size and from the amount of impurities present in the metal surface exposed to the measuring solution have been included. The utmost purpose of the work has been to investigate the fundamental properties governing the antimony electrode response in order to make more stable and reproducible electrodes than those presently existing. These electrodes are intended, in an appropriate miniaturised form, for the alorementioned routine pH determinations in vivo. As the investigation has been centred on the electrochemical properties of antimony, the tests have been performed at ambient conditions, i.e. near room temperature and with measuring solutions normally exposed to air. A detailed discussion of the experiments and results from an electrochemical standpoint has been given elsewhere (EDWALL, 1976, 1977 a-c). Here the implications of these investigations for biomedical pH determinations and for the accuracy of antimony electrodes in such applications are stressed.

4 Experimental

Polycrystalline as well as monocrystalline antimony electrodes have been used in this investigation. All electrodes were made by casting, using 99.95% pure antimony in pieces as starting material.

4.1 Polycrystalline electrodes

Three different modifications of polycrystalline electrodes were employed. The preparation is described elsewhere (EDWALL, 1976, 1977 a). The three types differed mainly with respect to the size of the crystal grains exposed to the measuring solution in the electrode surface. In general, a large number of grains of various sizes and shapes are exposed in the measuring surfaces of polycrystalline electrodes. Therefore, each electrode will have its own individual surface appearance and characteristics. In our electrodes, the typical grain dimension ranged from well below 0-1 mm in one modification to above 1 mm in a modification with only a few crystal grains exposed in the measuring surface.

Measurements on the different types of polycrystalline electrodes were performed at constant temperature in stagnant aerated commercial pH buffer solutions, see Table I, ranging in pH between 1.68 and 8"0. The methodology used has been described by EDWALL (1976, 1977 a). Before use the electrodes were given a smooth and continuous surface by polishing. They were washed in distilled water, dehydrated with absolute alcohol and dried in warm air. No further surface oxidation was attempted.

In general, all our tests indicate that polycrystalline electrodes never attain a stable electrode potential. However, after about 2 to 3 minutes exposure to the measuring solution, a 'quasistable' electrode potential was obtained with most electrodes. On the

662 Medical & Biological Engineering & Computing November 1978

other hand, the poor stability and reproducibility of this potential only allows for pH measurements to within roughly 0- 1 to 0"2 pH units.

When a plane polished electrode surface containing only a few crystal grains was exposed to the measuring solution, an electrode potential of higher stability and reproducibility was obtained. A typical potential response in shown in Fig. 1. Thus, a stable voltage level, to within 1 mV or better, was reached in about 2 to 10 minutes on immersion. The reproducibility of that level, using the same electrode, was of the same order of magnitude. The electrode potential remained stable for approximately one day and during that time its typical short-term variations (order of seconds to minutes) were less than _+0.2 mV. After the first day of use, a continuous potential drift was encountered and the electrode surface was found to be corroded. As the corrosion proceeded, thereby developing more sub- stantial corrosion pits and crevices in the surface, the electrode stabilisation time was gradually in- creased. Also, the short-term potential variations were noted to increase and the effect of stirring of the measuring solution was enhanced.

Obviously, in accurate pH measurements, a plane polished electrode surface, which contains only one or a few crystal grains, is to be preferred. On the other hand, our measurements showed that

I , I , 1 , I , I , I , I I - - cont inuous values in s tagnant a e r a t e d s o l u t i o n

-110- o p o t e n t i a l o b t a i n e d in s t i r r e d solution = e l e c t r o d e d r i e d in a i r a n d r e i m m e r s e d

-12l o LU

U_] •165

-130-

< * - .2mV o

~0 L0 6b do ~0 1~0 t i m e (h)

Fig. I Typical potential response of plane pal/crystalline electrodes with only a few crystal grains exposed to a pl-/ 7.0 phosphate buffer at 20~

each individual electrode possessed its own standard equilibrium potential and sensitivity. Therefore, standardisation of the electrodes in solutions of known pH values was necessary. Unfortunately, the composition of these standard solutions turns out to be critical, as some ionic components (e.g. citrate and oxalate ions or phosphate ions in the high-pH region) affect the stability of the electrode potential. The character of the drift obtained in these solutions makes us believe that the drift is connected with a change in the surface layer properties due to the formation of antimony complexes by the interfering buffer ions. This may lead to a dissolution of the antimony surface oxide or to a higher rate of metal dissolution from the bulk.

We assume that the differences observed in standard electrode potential and in sensitivity between different electrodes are connected with the conditions and the crystallographic properties of the exposed antimony surface (EDWALL, 1977 a). Accordingly, after about 50h of immersion in buffer solutions, a grain-selective etching with small but distinct differences in levels between adjacent grains could be seen in the polycrystalline electrodes containing only a few grains. The walls of the etched metal parts were plane, and, in general, not perpendicular to the surface. This etching leads to an enhancement of grain boundary areas. At the same time, electron microprobe investigations of the surface showed that impurities were accumulated in the boundary areas. Consequently, if impurities have any bearing on the electrode potential, grain boundaries must be avoided. This, among other things, means that monocrystalline electrodes can be expected to give still better results.

4.2 Unoriented monocrystalline electrodes The measurements on polycrystalline electrodes

showed that the number of crystal grains exposed in the electrode measuring surface was of principal importance with respect to its potential characteristics. It was therefore natural to extend the study to electrodes with monocrystalline surface character. To our knowledge, no experimental results have earlier been reported on such electrodes.

pH value Constituents Supplier

1 "68 tetroxalate S 1306, Radiometer 2.00 citrate-hydrochloric acid Titrosol, Merck 3"00 citrate-hydrochloric acid Titrosol, Merck 4-00 citrate-hydrochloric acid Titrosol, Merck 4-01 phthalate S 131 6, Radiometer 5.00 phthalate-phosphate P-H Tamm, Uppsala Sweden 6-00 phthalate-phosphate P-H Tamm, Uppsala, Sweden 6- 50 phosphate S 1001, Radiometer 7-00 phosphate P-H Tamm, Uppsala, Sweden 8.00 phosphate P-H Tamm, Uppsala, Sweden

Table 1. Commercial buffer solutions used

Medical & Biological Engineering & Comput ing November 1978 663

An antimony single crystal was made from the aforementioned 99 .95~ pure antimony. The material was purified by repeated zone refining and care was taken to reduce the amount of dissolved oxygen in the melt. A large single crystal (60 x 10 x 5 ram) was grown using the Bridgeman technique. In our case the crystal was of random crystallographic orientation.

The large single crystal was spark cut in two directions perpendicular to each other with a continuous wire spark erosion machine (Servomet, Metals Research). As the crystallographic structure of antimony is represented by a hexagonal or a rhombohedral unit cell, the plane surfaces thereby exposed represent crystallographic planes of different orientations. Their absolute orientation was not determined, but no low-index crystal plane should have been exposed in these surfaces (EDwALL, 1977 b). Electrodes representing the two orientations are in the following text referred to as type A and type B, respectively.

Monocrystalline pieces approximately 5 x 5 x 3 mm were made with the larger area representing one of the two surfaces mentioned. The pieces were degreased in trichloroethylene and an electrical contact lead was attached with conductive epoxy resin (MRC 4912, Materials Research). They were then cast in epoxy resin (Araldite, Ciba-Geigy) to form 25 mm diameter plastic cylinders with the antimony measuring surface exposed at the bottom end, see Fig. 2. This surface was ground and polished to optical smoothness, using 1/zm diamond paste as the final step. The electrodes were examined in a stereo microscope (x80) and all defects (e.g. cracks) found on the metal surface were coated with a plastic lacquer (edge protecting lacquer, Struers).

The experimental setup has been described by EDWALL (1976, 1977 b). The major part of experiments were performed at constant temperature in stagnant unbuffered solutions notably consisting of HC1 exposed to air. The pH of the solutions was chosen near 2 to oppose pH changes induced by the electrode reactions or from ions leaking to the measuring solution. They were also saturated with respect to antimony ions by adding SbOC1 pre- cipitate in excess. A possibility for dispersive bubbling of different water-saturated preheated gases was also included.

CO~er wire ~Z~--L~-- teflon ~epoxy acquer insulo@d) ~ ~ f~--.--.--.--.~icylinder onductive epoxy ! rnonocrystoIIine | ,,silicone ~antirnony antimony ~ / grease exposed at

~surfece to be ~ bottom end polished % fLat plastic surface

a b e

Fig, 2 Steps in manufacturing of plastic cylinder with monocrystalline antimony electrode

The potentials of monocrystalline electrodes were more stable, both regarding the short-term and the long-term stability, than those of polycrystalline electrodes. Typical short-term variations were +0-1 mV for a monocrystalline electrode shortly after polishing compared with _+ 1 mV for ordinary polycrystalline electrodes. For the monocrystalline electrodes, the potential attained a stable voltage level and remained stable at that level for typically

164"

162

E 160

L U " I -

LL~ 158

t55

154-

n I n I I I i I

continuous values in stagnant aerated solution

o potential obtained in air stirred solution . f

. . . . potential not registered . . ' o . ,

' 160 2bo ' 3bo ' ~bo 5'oo t ime [h}

Fig. 3 Typical potential response of best type un- orientated monocrystalline electrodes (type A, see text) in pH 1.9 HC 1- Sb 0 C/ solution at 20 ~ C

100h or more. Moreover, this voltage level was reproducibly obtained with different electrodes, and with the same electrode following surface polishing, to within better than 1 mV, as long as the surface had monocrystalline properties and was even and continuous. In this respect the crystallographic orientation of the electrode surface was of minor importance.

The time needed for the electrode potential to stablise was shorter for monocrystalline than for large-grain polycrystalline electrodes. In this respect, a dependency on the crystallographic orientation could be seen. In the HC1-SbOC1 solutions (p i l l - 9), the typical stabilisation time for al l mona- crystalline electrodes was less than 10 rain to within 1 mV of the stable voltage level. However, with one of the surface orientations employed (type A), a voltage reading differing from the stable to within less than 1 mV could in most cases be obtained within 30 s or even less. Generally, a longer stabilisa- Lion time was also found when measurements were undertaken in buffer solutions, e.g. phosphate buffers than in HC1-SbOC1 solutions.

The time during which the potential was stable and the subsequent drift could be seen to differ between electrodes of each of the two orthogonal orientations, respectively. A typical result for the best orientation (type A) is shown in Fig. 3. For the type B electrodes, the potential remained stable

664 Medica l & Biological Engineer ing & C o m p u t i n g N o v e m b e r 1978

during a shorter time period and the subsequent drift typically amounted to + 2 mV/day. In Fig. 3 the effect on the electrode potential from stirring (i.e. air bubbling) the measuring solution can also be seen. This phenomenon increases with time of electrode use.

Our measurements indicate that the electrode potential depends on the condition of the electrode surface. Thus, after a few hundred hours of use, the long-term potential drift of the type B electrodes (with measuring surfaces crystallographically perpendicular to that of type A) changed character from the regular type of drift shown in Fig. 3 to a more irregular one, confined within the 170 to 230 mV interval when using pH 1.9 solutions. Sub- sequent scanning electron microscope (s.e.m.) in- spection of the surfaces of type B electrodes revealed them to he heavily corroded and covered with an almost compact surface layer which we believe to consist of antimony oxide, see Fig. 4. On the other hand, even after several hundred hours of use, only traces of the surface phase could be seen on the type A electrodes. The major part of their surfaces was unaffected by corrosion and only at a few discrete

points pit corrosion had occurred, see Fig. 4. The electrode potential of these electrodes was still regularly drifting.

The tests on unoriented monocrystalline electrodes strongly support the idea that the crystallographic surface orientation is of prime importance for the electrode's potential characteristics. In all respects mentioned, the B type electrode turned out to have inferior qualities.

4.3 Oriented monocrystalline electrodes

According to present knowledge, the mechanical properties of a metal, as well as its resistance to corrosion, depend on the binding energy of the atoms in different crystallographic planes. In experiments on other metals than antimony, the crystal plane has been shown to be an important factor in connection with, for example, the oxidation of the metal surface in air and the etching of the surface with various liquids (LEIDHEISER and GWATHMEY, 1947). The surface oxide formation is of great importance for the electrode behaviour

Fig. 4 Typical surface appearance of monocrystalline electrodes after approximately 400 h of use in pH 1.9 HCI-SbOCI solution at 20~

(a-c) Type B electrode (crystallo graphically orientated perpendicular to type A)

(d) Type A electrode (same magni- fication asc)


(IrEs, 1961), and the tests on the unoriented monocrystalline electrodes indicate that the resistance of the metal to etching should be important for its stability as an electrode. In order to preserve an intact electrode surface for long periods of time, a corrosion resistant surface should obviously be preferred. Therefore, we found it appropriate to study crystallographically oriented monocrystalline electrodes of high surface atomic densities.

Oriented monocrystalline electrodes representing the six rhombohedral planes ( l I0) , (111), ( l l i ) , (001), (110) and (211) of the antimony structure were used in this study. These surfaces represent the most close-packed planes in the antimony lattice. A single crystal, made as previously described, was oriented by X-ray diffraction and subsequently cut in the appropriate directions. The monocrystalline pieces were degreased, contacted, cast in epoxy (this time forming 10 mm cylinders) and polished as previously described with the unoriented electrodes. The misalignment of each electrode surface from the intended crystallographic plane was determined, again with X-ray diffraction. Deviations greater than 1.5 ~ were corrected for by surface polishing. Surface defects were coated with plastic lacquer. Accordingly, all electrodes had an even monocrystalline measuring surface without exposed defects aligned to a known crystallographic plane to within better than 1" 5 ~

The experimental setup and the measuring pro- cedure used is described in full by EDWALL (1976, 1977 c). Essentially, the conditions were the same as during the measurements on unoriented electrodes.

The stabilisation time for the potentials of the oriented electrodes to within 1 mV of their final stable value was always less than 1 rain. For all

E

150 -

1/ .0-

130-

Fig. 5

~ I + . L , I ~ I ~ I ~ I , I ~ I

, ' 211 i + i y I + ' t | | | I I

2O0 400 600 O00 time (h)

L ong-term potential behaviour of virgin, oriented monocrystalline electrodes in pH 1.9 HCI- SbOCI solution at 24~ (the I10 electrode had an initial surface flaw which later became covered with occluding corrosion products, while in the 211 electrode a surface flaw developed during use)

electrodes, unless a surface defect existed or developed during use, the potentials were then stable to within 1 mV at a level of 150 mV (SHE) during at least 100 h. A typical result is shown in Fig. 5. The reproducibility of the potential to that of other oriented electrodes was to within + 1 mV or better for more than 100 h of continuous use, while its repeatability was to within 0"2mV. Short-term potential variations were of the order of +__ 0.1 mV for long periods of time. After the initial 100 h of relative potential stability, a continuous potential drift was experienced, Fig. 5. However, this drift amounted to less than - 0.8 mV ]day for all orientations studied during at least 800 h of use provided no surface defect developed. No stilring effect above _+ 0" 1 mV of magnitude was noted, as tested when the electrode potentials were stable. During that period, a reproducible temperature sensitivity, amounting to - 1 . 4 m V ] d e g r e e was found, and the electrodes responded reproducibly to oxygen and nitrogen bubbling. The sensitivity to changes in oxygen partial pressure in the Po2 range between 20 and 100kPa [expressed as dE/d(log Po2)] was 15"9 mV/decade in the solution studied. At low Po~ levels no linear potential/Po~ relation existed. The pH sensitivity was preliminarily found to be - 55.1 mV/pH unit.

In long-term experiments (800 h) the potential drift was towards more negative values, Fig. 5. The following s.e.m, investigations of the surfaces showed no surface cover (oxide) on these electrodes. Only minor corrosion had taken place, chiefly in the form of pit corrosion. Moreover, the surfaces appeared to be less corroded for electrodes possessing more negative potentials. On the other hand, in other test runs on oriented electrodes, a connection between the gradual coverage of the antimony surface with an irregular surface structure, presum- ably consisting of oxide, and a positive offset of the electrode potential was found. When this surface structure was nearly compact, the electrode potential started to drift irregularly also with these electrodes, although still within the limited voltage range ex- tending 170 to 230 mV in the pH 1-9 solution. Furthermore, when the potential was drifting, no reproducible temperature and oxygen partial pre- sure sensitivities could be obtained. However, the initial electrode characteristics could again be restored by surface polishing.

5 D i s c u s s i o n

The results obtained with oriented monocrystalline electrodes in the pH 1"9 HCI-SbOC1 solutions are compared with the requirements on pH electrodes intended for in vivo pH determinations on human arterial blood in Table 2. It is clear that the antimony electrodes have characteristics, as measured in the pH 1' 9 solution, which well meet the physiological requirements. However, it is to be remem-

666 M e d i c a l & B i o l o g i c a l Eng ineer ing & C o m p u t i n g N o v e m b e r 1978

bored that the results were obtained during labora- tory conditions especially designed for stability tests on the antimony electrode system. If similar results can be obtained in blood at 37~ and under physiological conditions is presently under study.

The pH resolution for the electrodes is determined by their short-term potential stability. With electrodes previously unused after polishing this corresponds to a resolution of 2/1000 pH units. This value is valid, at least during the period of relative voltage stability, for all monocrystalline electrodes. However, for the oriented electrodes, it was applicable even after 800 h of continuous use, corresponding to our longest test run. We believe that a loss of resolution is connected with exposure of crystallographic planes, which have exchange current densities differing from the original. In monocrystalline electrodes this takes place in corrosion crevices, pits and flaws and therefore electrodes with corrosion resistant properties shall be employed.

The rePeatability of a monocrystalline electrode is of the same order of magnitude as its short-term stability. To obtain the highest repeatability, the electrodes should only be used during their period of relative potential stability. After that time, the electrode surface should be renewed by polishing.

Initially, the reproducibility between different monocrystalline electrodes with high relative surface atomic densities corresponds to 2/100 pH units. However, after some hours use an even better reproducibility was found corresponding to better than 1/100 pH unit (1 standard deviation). These values were obtained irrespective of the absolute surface orientation among the orientations tested. The reproducibility deteiiorated when the potential drift started but it could again be restored by surface polishing.

A detailed discussion on possible mechanisms responsible for the electrode potential response, as presented here, has been given by EDWALL (1976, 1977 a-c). The electrode potential is, in fact, a corrosion potential. In practice, this means that a stable and reproducible potential can only be obtained provided the conditions for the anodic and

cathodic reactions, respectively, do not change. The formation of a covering surface structure, of corrosion pits or of the other defects in the surface, must therefore be hindered or in any case re- tarded. Obviously, a plane monocrystalline electrode surface of high surface atomic density partly fulfils these requirements. The relatively small differences in behaviour noted between the close- packed oriented electrodes as compared to that of the unoriented type B electrodes is expected to be due to the fact that all oriented electrodes represented low Miller index crystallographic planes, i.e. they were all of a relatively high surface atomic density. Although the corrosion as such was small on these electrodes, those which were most close- packed were also less corroded. With regard to the relatively severe corrosion found on the type B electrodes, tl:e corresponding surface should represent the least close-packed orientation studied (EDWALL, 1977 c).

During our tests, the measuring surfaces seemed to be unaffected during the period of relative potential stability. The following period, character- ised by a continuous potential drift, corresponds to a slow change in the conditions for one or both of the surface reactions, e.g. by covering corrosion products or by surface pitting. When the period of uncontrolled potential drift within a restricted voltage region is encountered, the anodic metal dissolution reaction is thought to be severely polarised, thereby possibly giving access to a third potential controlling reaction. This reaction might be the formation of Sb204, which has an expected potential of 221 mV at pH 1.9 (EDWALL, 1976). Surface polishing of used and corroded monocrystalline electrodes obviously restores the initial plane undisturbed surface. Thereby, the initial conditions for the electrode reactions are regained. In physiological fluids at 37~ the anodic metal dissolution reaction can be expected to be accel- erated, e.g. by complex-forming substances. I t is also possible that the electrode surfaces might be partly passivated, e.g. by proteins, or that the conditions for the surface-oxide formation will change. Therefore, the time for relative potential stability is

Required in blood Obtained in pH 1.90 solution

Resolut ion 0 -3 mV Repeatab i l i ty a 1 -2 mV Reproduc ib i l i t y b 1 �9 2 mV ~ Abso lu te accuracy 4 -3 mV Stabi l i ty ( l ong- te rm) 1 .2 mV be tween ca l ibrat ions

Response t ime 1 min

0.1 mV 0 .2 mV

< 4-1 mV + 3 mV

1 m V / 1 0 0 h then < 0- 8 mY/day < 1 min

Table 2. Comparison of clinically required accuracy data for in v i vo pH determinations in arterial blood with those obtained with monocrystalline antimony electrodes in pH 1.9 HCI-SbOCI solution

a With the same electrode on the same pat ient or solut ion, respect ively

b With dif ferent electrodes on the same pat ient or solut ion

c Af ter ind iv idual cal ibrat ion against standard solut ions fo r each electrode employed


expected to decrease in physiological fluids. How- ever, our early preliminary results in such solutions do not indicate any drastically shortened useful time. Therefore, we believe that there is a fair chance to obtain similar results to those reported here in the HCI-SbOC1 solutions in physiological fluids as well.

Antimony electrodes are sensitive to changes in the oxygen partial pressure of the solution. In general, the potential response to Po2 changes is sluggish and irreproducible (IvEs, 1961). A dE/d log Po2 response of 14.5 mV/decade has earlier been reported by KAUKO and KNAPPSBERG (1939) and IVES 0961) concluded that this value was close to the theoretical value of 14.8 mV/decade for the oxygen reduction reaction. During our investigations we found both stable, reproducible and repeat- able potentials during air and oxygen bubbling for all monocrystalline electrodes when their potentials in stagnant solution wete stable. This makes us believe that the oxygen sensitivity can be corrected for by separately determining or controlling the Po2 of the measuring solution. Since the electrode potential is a corrosion potential, a Po2 sensitivity slightly lower than the theoretical value for the pure oxygen reduction reaction is expected. Con- trary to this, we found a larger sensitivity thereby suggesting more complicated reaction mechanisms (EDWALL, 1977 b).

The long-term potential stability reported here indicates that the transducers can be continuously used for more than 100 h without calibration in standard solution. Provided that similar results can be obtained in vivo, long-term pH-monitoring should be feasible. The relaaarkable reproducibility obtained between electrodes might then also make standardisation of the individual electrodes un- necessary. This constitutes quite a new measuring situation for pH determinations as compared with the existing glass-electrode technique, which is commonly used in in vitro blood pH determinations and in highly accurate industrial or chemical pH determinations.

Acknowledgment--The author is greatly indebted to Prof. Sigvard Thulin for putting the resources of the Applied Physics Department at his disposal and also for his encouraging discussions during the course of this work. The author also wishes to thank G. Eklund for his help and advice during the metallographic parts of the work.

References

CATER, D. B. and SILVER, I. A. (1961) Microelectrodes and electrodes used in biology. In Reference electrodes (Eds. Ives, D. J. G. and Janz, G. J.), Academic Press, 464.

EDWALL, G. (1976) Stable and reproducible antimony- antimonyoxide electrodes. TRITA-DISS 1064. Thesis. Royal Inst. Techn., Stockholm.

EDWALL, G. (1977 a) Influence of crystallographic properties on antimony electrode potential--I. Poly- crystalline material. Electrochim. Acta. (submitted for publication)

EDWALL, G. (1977 b) Influence of crystallographic properties on antimony electrode potential--II. Mono- crystalline material, ibm (submitted for publication)

EDWALL, G. (1977 c) Influence of crystallographic properties on antimony electrode potential--III. Oriented monocrystalline material, ibid. (submitted for publication).

FRIEDMAN, S. M. (1967) H + and cation analysis of biological fluids in the intact animal. In Glass electrodes for .hydrogen and other cations (Ed. Eisenman, G.), Arnold, 442.

GEDDES, L. A. and BAKER, L. E. (1968) Biomedical instrumentation, Wiley, 125.

GREEN, R. and GIEBISCH, G. (1974) Some problems with the antimony microelectrode. In Ion selective microelectrodes (Eds. Berman, H. J. and Hebert, N. C.), Plenum Press, 43.

IRES, D. J. G. (1961) Oxide, oxygen, and sulfide electrodes. In Reference Electrodes (Eds. Ives, D. J. G. and Janz, G. J.), Academic Press, 322.

KARLMARK, B. (1973) Determination of titratable acid and ammonium ions in picomole amounts. Analyt. Biochem. 52, 69.

KARLMARK, B. and SOHTELL, M. (1973) The determination of bicarbonate in nanoliter samples, ibid. 53, 1.

KAUKO, Y. and KNAPPSBERG, L. (1939) ~ber die Anti- monelectrode. Z. Electrochem 45, 760.

KHURI, R. N. (1967) Glass microelectrodes and their uses in biological systems. In Glass electrodes for hydrogen and other cations (Ed. Eisenman, G.), Arnold, 478.

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668 Medical & Biological Engineering & Computing November 1978

Electrodes de pH antimoine/oxyde d'antimoine (111) amalior4es Sommaire--Dans les traitements cliniques routiniers, on d6sire souvent determiner rapidement le pH. Pour

cela, il faut effectuer des mesures continues in vivo. Aujourd'hui, seule une analyse de pH intermittente in vitro est effectu6e de mani~re routini~re, 6tant donn6 que les transducteurs existants ne peuvent r6pondre aux exigences cliniques concernant les dimensions, la stabilit6, te temps de stabilisation, la r6solution et la robustesse. Cet article pr6sente certains rdsultats d 'une 6tude exp6rimentale fonda- mentale sur des 61ectrodes de pH du type Sb/monoxyde de Sb. Le but de l'6tude 6tait d 'obtenir certains renseignements sur les processus auxquels sont soumises les 61ectrodes et qui gouvernent leur r6ponse, pour pouvoir utiliser ces renseignements en rue de la synth~se d'dlectrodes p lu s stables et donnant des rdsultats plus reproductibles que celles disponibles actuellement. Ces 61ec- trodes sont destin6es sous une forme suffisamment miniaturis6e/t d6terminer le pH in vivo sur l 'homme de mani~re routini~re, c'est-5_-dire dans des applications de surveillance continue de l'6quilibre acides / bases.

Verbesserte PH Elektroden aus Antimon-Antirnonoxid Zusammenfassung--13ei der klinischen Routinebehandlung sind pH-Bestimmungen oft wtinschenswert.

Hierzu werden andauernde in vivo Messungen ben6tigt. Heutzutage werden nut unterbrochene in vitro Analysen als Routineuntersuchungen durchgefiJhrt, da die vorr~tigen MeBwandler nicht den klinischen Anforderungen mit Hinsicht auf Gr6Be, Stabilit/it, Stabilisationszeit, Resolution und Robustheit nachkommen. Hier werden einige Resultate aus einer experimentellen Grundstudie an Antimon-Antimonoxid-pH-Elektroden dargestellt. Der Zweck dieser Untersuchung war, einige Informationen fiber Elektrodenverfahren zu sammeln, die den Elektrodenfrequenzgang bestimmen, und diese Information ftir die Synthese stabilerer und reproduzierbarerer Elektroden als die zur Zeit vorhandenen zu benutzen. Diese Elektroden sind in angemessen verkleinerter Form vorgesehen fiir in vivo routinem~igige pH-Bestimmungen im Menschen, wie z.B. bei einer kontinuierlichen Gleichgewichtsbeobachtung auf S~iurebasis.


improved antimony-antimony (iii) oxide ph electrodes

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