- Email: [email protected]
Evaluation of the influence of impurities on the oxygen sensitivity of monocrystalline antimony electrodes
Ekrrochimk.,
Perg~on
Am, Vol. 25, pp. 1585-1590. Prsss Ltd. 1980. Printed in Great Britain
EVALUATION OF THE INFLUENCE OF IMPURITIES ON THE OXYGEN SENSITIVITY OF MONOCRYSTALLINE ANTIMONY ELECTRODES PER-ARNE J~~NGREN and GUNNAR EDWALL Royal Institute of Technology,
Applied Physics Department,
Stockholm,
Sweden
(Receioed 3 January 1980) Abstract - Measurements are presented which show that the oxygen sensitivity of ultra pure (6N) monocrystalhne antimony electrodes presents an improved reproducibility and constancy as compared with monocrystalline electrodes containing impurity inclusions in the electrode surface. The former electrodes also present a higher potential stability. The time to reach a pseudo-stable potential level (drift -C 0.6 mV/h) is also faster for this type of electrodes. The improved characteristics of the ultra pure electrodes are thought to be due to a uniform distribution of the anodic and the cathodic areas over the electrode surfaces. On the electrodes with impurity inclusions exposed in the surfaces, a high degree of local corrosion was noted* The oxygen sensitivity of the ultra pure electrodes, dE/d(logpo,), was found to be 15.3 f 0.7 and 15.7 + 0.8 mV, 5 and 25 h after immersion of the electrodes in the electrolyte respectively.
lNTRODUCTION
The potential of the antimony pH electrode is known to be sensitive to the partial pressure of dissolved A potential shift oxygen, po,. in the electrolyte. towards a more positive voltage level with increasing oxygen partial pressure is a characteristic of the electrodes. In general, their response to poz alternations is sluggish and irreproducible[ 11. On the other hand, the presence of oxygen at the metal surface is claimed to be fundamental for a proper function of this type of pH electrodes[2]. The surface of massive or plated antimony electrodes is slowly etched in solutions containing dissolved oxygen[3]. Consequently, explanations to the electrode behaviour have been made on the basis of the electrochemical corrosion theory. It is reasonable to assume that the metal dissolution takes place at anodic sites where the bare metal is exposed to the electrolyte and that the oxygen reduction process is located to cathodic areas presumably coated with a thin oxide film[4]. The electrode potential is therefore a corrosion potential, representing a dynamic equilibrium between the anodic and the cathodic reactions at the metal surface respectively. However, on a surface of an ultra pure metal the anodic and the cathodic areas are not necessarily stationary with time[5]. Concerning the oxygen sensitivity, dE/d(logp,,) of antimony electrodes much confusion remains. Among previous reports, only three authors have extensively studied the potential-po, relationshipC6, 7, 81. Holmqvist[6] found an S-shaped potential-logp,, relation in 0.1 N HCl and two acetate buffers using massive electrodes Claiming that Holmqvist’s data did not represent equilibrium potentials, Kauko and Knappsberg[7] found a straight potential-log pal relationship in phosphate-borax buffers with an oxy-gen sensitivity equal to 14.5 mV at 25°C. This is very close to the theoretical value at 14.8 mV for a reversible oxygen electrode. However, the electrode response to poI changes was sluggish and the stabilization time
was greater than 15 min. Bishop and Short[B] found a at intermediate linear potential-log po, relationship poz levels with dE/d (log po,) equal to 56mV in unooised -- r- -- 0.07 M KCl solution. In this solution it was also observed that the potential was constant, ie, pot independent in the high and low partial pressure range, respectively. With buffer substances added to the KC1 solution, a lower oxygen sensitivity was obtained and in presence of phosphate ions, the electrodes were sensitive to oxygen variations in the whole po, range used (O.l- 100 kPa). This was suggested to be due to the fact that the oxygen reduction process now occurred at the bare metal. The oxygen sensitivity reported varied between 20 to 60 mV. It was concluded that the oxygen reduction processes at an antimony electrode surface are “condition-dependent”. The above mentioned experimental findings all apply to polyerystalline electrodes. However, monocrystalline antimony electrodes have recently been studied by Edwall[9] in pH 1.9 SbCl, solutions. They proved to have superior potential stability and reproducibility as compared with polycrystalline electrodes. The oxygen sensitivity was found to be 15.9 k 0.3 mV in the 20-1OOkPa oxygen partial pressure range. At these po, levels, the potential was stable. However, during mtrogen bubbling the potential was reported to be unstable. On nitrogen exposure it ran through a minimum and then a drift towards more positive values was observed. This paper presents further investigations on the oxygen sensitivity of monocrystalline antimony electrodes in the po2 range below 20 kPa. The behaviour of ultra pure (6N) and pure (4N) electrodes is compared. EXPFXIMENTAL Monocrystalline antimony electrodes were spark cut from two large single crystals of different puritygrades; pure - 99.990/, (4N) (Materials Research Corp.) and ultm pure-99.9999% (6N) (Studsvik Energiteknik AB) respectively. Ten electrodes of each purity were
1585
PER-ARNEJ~NGREZNAND
GIJNNAREDWALL
the carbon dioxide partial pressure (pcoJ was kept constant equal to 5.4 + 0.1 kPa. All gas mixtures were led through thermostated water vessels and thereby 0 SCREW CAP humidified to saturation with water. The voltage @ HGIXlNi IPMMA) measuring system is described in detail in reqlO]. @ SOCKET ,PMMAl Essentially it consists of a digital voltmeter (DVM) @ FLECTraYTC with a high input impedance (10”Q). A digital-to@ SALT BRiOGE analog (D/A) converter acts as an interface between @ ANT,tlONY ELECTROOE theDVM and a stripcart recorder. The potentials were @ ELECTRIC CONNECTION measured with a resolution of 0.1 mV. @ *NTlMCw CRnTAtt The measuring cell was mounted in the electrode @ WACER (NYLWKI @ MEMBRANE (SILICONE1 cuvette of a EMS 3 Mk2 blood gas analyzer (Radio@ ARAtDITE meter A/S). It was thermostated at 25°C. The cell @ PMMA IPOLYMETHYtMETHbCRYLATE , membrane was exposed to the humidified gas mixtures. An equilibrium between the gas phase and the thin electrolyte film at the electrode surface was thereby established by diffusion. Consequently, a change from one gas mixture to another resulted in a po2alteration in the electrolyte film. This po2 alteration was measured as a change in the antimony electrode potential. During the experiments, the pH was equal to 4.6 owing to the constant pco, of all gas mixtures. At a constant poz level equal to 13.4kPa, the antimony electrode potential was continuously recorded during 25 hours for each electrode. The oxygen 1 cm sensitivity was evaluated at the 5th and 25th hour respectively. During a sensitivity measurement, 24po, alterations were made and the potentials were read 4 Fig. 1. The measuring cell. minutes after each po2 change. Linear regression analyses were carried out on the data from each sensitivity measurement. The slopes of the regression lines were used to calculate the mean cast in epoxy resin (Araldite, Ciba-Geigy). The trivalue and the standard error of tht oxygen sensitivity gonal (liO)-surface to be exposed, was ground and within the two electrode groups. polished to optical smoothness using 1 pm diamond paste as the final step. The surface area was 0.8 mm2. The measuring cell is shown in Fig. 1. Its inner part RESULTS consists of a socket of polymethylmethacrylate, in The results of the oxygen sensitivity measurements which an antimony electrode and the end of a salt are presented in Table 1, which shows the sensitivity bridge are mounted. This construction has dimensions similar to those of the Radiometer E.5046 oxygen mean values + SE for the ultra pure and pure electrodes respectively. The ultra pure electrodes preelectrode. The housing for the cell is the conventional sented an oxygen sensitivity of the same magnitude at Radiometer po, electrode jacket. The antimony elecboth sensitivity measurements, 5 and 25 h after immertrode is covered by a nylon spacer (50pm) and a gas sion of the electrodes in the electrolyte respectively. permeable silicone membrane (25pm) (D697, D606, Radiometer A/S) which is stretched over the front end The oxygen sensitivity of these electrodes was not of the electrode jacket. An air-tight seal between the significantly different from 14.8mV/log po2, which is the sensitivity that corresponds to a four electron housing and the socket is obtained with a rubber “O”ring and a screw cap in the back part of the cell. oxygen reduction process at 25°C. The electrode consists of distilled and de-ionized On the other hand, the oxygen sensitivity of the pure water (p = 0.46 M n cm) because a minimal influence electrodes decreased from a higher value than that of dissolved substances on the electrode processes is obtained with the ultra pure electrodes at the first desirable. It is contained in the volume between the sensitivity measurement to a correspondingly lower housing and the electrode socket (- 0.7ml). A thin value at the second occasion. Also a greater spread in electrolyte film at the electrode surface is maintained the oxygen sensitivity, reflected by a larger SE, was by the spacer. obtained in this group at the first sensitivity measureelectrode used was an ments 5 h after immersion of the electrodes in the The reference Ag-AgCl/(3MKCl) - half celt (Ingold), connected electrolyte. through the salt bridge to the measuring cell. The salt The potential-log po, relationship for a reprebridge contained a 3M KC1 solution. It was separated sentative ultra pure and pure electrode is presented in Fig. 2a and b resuectively. These grauhs refer to the first from the electrolyte by a porous plug, as shown in sensitivity measurement. The ultra pure electrodes Fig. 1. Four gas mixtures containing oxygen, carbon dipresented a high reversibility and linearity with respect oxide and nitrogen were used (AGA Specialgas AB). to po, variations (Fig. 2a). The correlation coefficient was typically of the order of0.99. The high reversibility Their oxygen partial pressures (PO,) were 2.3 + 0.1,3.8 + 0.1,7.4 + 0.1 and 13.4 & 0.1 kPa respectively while and linearity was also maintained during the second
Evaluation of the influence of impurities
1587
Table 1. The oxygen sensitivity, dE/d&g PO,), of ultra Pure (6N) and pure (4N) monoerystalline antimony electrodes
f SE (N = 10) Ultra pure electrodes Cmvl
PlUl? electrodes CmVl
After 5 h use
15.3 + 0.7
21.2 f 2.98
After 25 h use
15.7 & 0.8
13.6 + l.l*
* The sensitivities are compensated for the potential drift in electrode group.
this
sensitivity measurement, except for two electrodes. In these cases, the potentials were unstable and a hysterisis effect was observed’during the poz variations. The potential-log po2 relationship of the pure electrodes was dominated by a drift in the potential level superposed on the potentials corresponding to each po2level as seen in the Fig. 2b. Iiowever, this drift was reduced in magnitude during .the second sensitivity measurement. The potential response of the ultra pure electrodes following each separate poI alteration is shown in Fig. 3a. As is seen in the figure, stable potentials were obtained at each oxygen partial pressure level. The
Fig. 2. The potentm-log p,,, relationship for representative monocrystalline antimony electrodes after 5 h use in distilled water at 25°C. (a) Electrode made of ultra pure material (6N). (b) Electrode made of pure material (4N).
Fig. 3. The potential response to PO2alterations for repm
setnative monocrystalline antimony electrodes after 5 h use. (a) Ultra pure electrode (6N). (b) Pure electrode (4N).
response time (W/$ of the electrodes in the ultra pure group was less than 15 s throughout the whole experiment. The pure electrode potential response to po, alterations, Fig. 3b, was composed of a rapid potential change, as for the ultra pure-electrodes, followed by a slow drift in the potential level. This behaviour was most pronounced during the first sensitivity measurement. The ultra pure electrode potentials reached a state of pseudo-stability approximately one hour after immersion of the electrodes in the electrolyte. This means that the mean value of the drift in the potential levels was - 0.6 mV/h (range + 0.2 - - 1.2 mV/h, N = 10). In contrast to this, none of the pure electrodes reached a psueo-stable potential level within the first 5 hours. At the first oxygen sensitivity measurements the drift was at the order of 6 mV/h. The potential stability of the pure electrodes increased with time and at the second sensitivity measurement, the mean value of the drift was + 1.1 mV/h (range + 7.0 - - 0.4mVjh, N = 10). The surface appearance of some representative electrodes of each purity grade was investigated in a scanning electron microscope (SEM). Figures 4 and 5 show SEM-micrographs of sections near the crystal edge of two ultra pure electrode surfaces. Figure 4 represents the typical ultra pure electrode with stable potential levels and a linear potential-log po, relationship. The whole electrode surface presented a uniform surface structure of the type shown in the micrograph. The surface did contain some cracks but no selective metal dissolution was seen in close vicinity to them. Figure 5 shows the surface of one of the two ultra pure electrodes that had the aforementioned deteriorated oxygen response, This electrode surface presented a corroded zonearound the whole periphery while the rest of the surface had the structure shown in the upper part of the micrograph. In the surfaces of the pure electrodes, impurity inclusions were found. In general, the antimony was
PER-ARNE J~NGREN
AND
GUNNAR EDWALL
Fig3.4. The surface ofa representative ultra pure electrode. The electrode presented a linear potential-log pol relationship. selectively dissolved around such inclusions. This is illustrated in Fig. 6 where an inclusion is undermined by seiectiv ‘e corrosion. Furthermore, the surfaces of all of the pur e electrodes had basically the same surface appearanc :e as that found on the two ultra pure electrodes with the deteriorated oxygen response. The surfaces Hrere divided into two different zones - one
with an etched surface and the other character ,ized by an even surface appearance.
DISCUSSION
Only electrodes made of the ultra pure nnatelial
Fig ,.5. The surface of an ultra pute electrode which presented a deteriorated oxygen response with hysterisis
effects.
Evaluation of the influenceof impurities
Fig. 6. An impurity inclusion,underminedby selectivecorrosion, found in a pure electrode surface.
presented a surface with a regular distribution of areas where antimony metal dissolution had taken place (Fig. 4). We think that an almost uniform distribution of dissolved reaction products was established over the whole electrode surface. The precise positions of the areas where the cathodic oxygen reduction reactions occurred can not be visualized in this case. However, the uniformity of the distribution of the anodic area makes us believe that a corresponding distribution exists for the cathodic reactions. An even distribution of reaction products dissolved in the electrolyte layer might be the reason for the high stability of these electrodes. The conditions for the oxygen reduction at the ultra pure electrodes were almost the same at the first sensitivity measurement and at the second as reflected by the constancy in the oxygen sensitivity. As claimed by Edwall[ll], general corrosion is promoted at an ultra pure crystal surface. This means that the corrosion current density is low thus leading to a slow variation of the surface properties with time. Furthermore, the oxidation rate of a monocrystalline crystal surface exposed to air was found to be extremely low[9], and even so during pure oxygen exposure. This means that only a very thin oxide film might be present (a few Angstroms thick, if any) when the electrode is first immersed into the electrolyte. Thus, due to the low oxidation tendency of water and of the low degree of air oxidation of the electrodes, the oxide film is thought to besthin in the beginning of the measurements and that the growth during the measurements will proceed with a slow rate. Impurity inclusions present in the pure electrode surfaces were found to promote a corrosion form
where the anodic areas were restricted to certain parts of the electrode surfaces. The corrosion form noted is thought to be induced by localized galvanic cells[5] created between the impurities and the antimony metal. The selective dissolution of antimony around certain impurities (Fig. 6) indicates that the impurity acted as a local cathode and the antimony metal in the close vicinity to the impurity as the anode. The high oxygen sensitivity noted for the pure electrode group during the first sensitivity measurement may then be caused by a different oxygen reduction mechanism at the impurities than on the antimony surface. The low oxygen sensitivity of the pure electrodes at the second sensitivity measurement is not well understood. It might be due to an increased thickness of the oxide film as the amount of dissolved antimony metal is thought to be larger at these electrodes than at the ultra pure ones. According to Gatty and Spooner[4], the oxygen reduction can take place at thin oxide films if the oxide is able to pass electrons from the metal bulk to the oxygen molecules. This ability will certainly decrease with increasing thickness of the IiIm. If the oxide film becomes too thick for electron conduction and thereby more or less acts like an insulator, the oxygen reduction may be restricted to the vicinity of anodic areas where the oxide film might be thinner or to the bare metal. The unstable potential levels of the two ultra pure electrodes during the second sensitivity measurement may be due to that the corrosion form at these electrodes after 25 hours use was of the same type as that of the pure electrodes described above. The anodic areas at these two electrodes were restricted to the periphery, ie, a local corrosion form existed. The
1590
PFX-ARNEJ~NGRENAND GUNNAR EDWALL
reason for this anomalQus behaviour as compared with the other ultra pure electrodes, may be due to the fact that an oxygrah concentration cell was established at the surfaces. Thus, in order to obtain stablepotential levels at each pal value during oxygen partial pressure variations, we believe -that general corrosion of the electrode surface is a prerequisite. Kurella[lZ] SugseSted that the oxygen sensitivity of antimony electrodes was due to impurities in the metal. This view is not supported by this investigation since the ultra pure electrodes had a clearly pronounced oxygen sensitivity. Furthermore, Caflisch et 4133 found that the antimony electrode potentials were displaced in the negative direction during nitrogen exposure. They concluded hat this was a “true effect of nitrogen” and that nitrogen should participate in the electrod&’ re@ons. In our view the nitrogen effect mentioned is an effect of a very low oxygen partial pressure as a result of the nitrogen bubbling.
Acknowledgement-The authors gratefully acknowledge the technical assistance of Rune Perssbn and Arnold Olofsson. This work was partly support&l by the National Swedish Board for Teebnical Development (STU).
REFERENCES 1. D. J. G. Ives, In Refmence Electrodes (Edited by D. J. G. Ives and G. J. Janz), p. 336. Academic Press, New York (1961). 2. A. R. Tourky and A. A. Mousa. Chem. Sot. J. 756 (1948). 3. J. P. Hoax, The Electrochemistry ofOxygen, p. 71. Wiley, New York (1968). 4. 0. Gattv and E. C. R. Swoner. The Electrode Potential Behovi&r of Corroding Metals’ in Aqueous S&t&, i. 331. Oxford Univ. Preso, London (1938). 5. 1. O’M. Bock& and A. K. N. l&ddy; Modern Elecnochemistry, Vol. 2, p. 1273. Plenum Press, New York, (1970). 6. A. Holmqvist, Undersiikningar av antimons och vismuts elekrromotoriskafdhdllanden i vattenliisningar, pp. l-58, Gleerup. Land (1956). 7. Y. Kauko and L. Knappsberg. 2. Elektrochem. 45, 760 (1939). I?. Bishop and G. D. Short, Talunta 11, 393 (1964). :: G. Edwall, High Stability Antinomy Electrodes, TRITA-DISS 1064, Ph.D. Thesis, Royal Institute of Technology, Stockholm, (1976). 10. G. Edwall, Rev. Sci. Instrum. 48, 1545 (1977). 11. G. Edwall, Electrochim. Acta 24, 613 (1979). 12. G. A. Kucella as cited in ref.[13]. 13. C. 8. Caflisch, L. R. Pucacco and N. W. Carter, Kidney Znt. 14, 126 (1978).
Perg~on
Am, Vol. 25, pp. 1585-1590. Prsss Ltd. 1980. Printed in Great Britain
EVALUATION OF THE INFLUENCE OF IMPURITIES ON THE OXYGEN SENSITIVITY OF MONOCRYSTALLINE ANTIMONY ELECTRODES PER-ARNE J~~NGREN and GUNNAR EDWALL Royal Institute of Technology,
Applied Physics Department,
Stockholm,
Sweden
(Receioed 3 January 1980) Abstract - Measurements are presented which show that the oxygen sensitivity of ultra pure (6N) monocrystalhne antimony electrodes presents an improved reproducibility and constancy as compared with monocrystalline electrodes containing impurity inclusions in the electrode surface. The former electrodes also present a higher potential stability. The time to reach a pseudo-stable potential level (drift -C 0.6 mV/h) is also faster for this type of electrodes. The improved characteristics of the ultra pure electrodes are thought to be due to a uniform distribution of the anodic and the cathodic areas over the electrode surfaces. On the electrodes with impurity inclusions exposed in the surfaces, a high degree of local corrosion was noted* The oxygen sensitivity of the ultra pure electrodes, dE/d(logpo,), was found to be 15.3 f 0.7 and 15.7 + 0.8 mV, 5 and 25 h after immersion of the electrodes in the electrolyte respectively.
lNTRODUCTION
The potential of the antimony pH electrode is known to be sensitive to the partial pressure of dissolved A potential shift oxygen, po,. in the electrolyte. towards a more positive voltage level with increasing oxygen partial pressure is a characteristic of the electrodes. In general, their response to poz alternations is sluggish and irreproducible[ 11. On the other hand, the presence of oxygen at the metal surface is claimed to be fundamental for a proper function of this type of pH electrodes[2]. The surface of massive or plated antimony electrodes is slowly etched in solutions containing dissolved oxygen[3]. Consequently, explanations to the electrode behaviour have been made on the basis of the electrochemical corrosion theory. It is reasonable to assume that the metal dissolution takes place at anodic sites where the bare metal is exposed to the electrolyte and that the oxygen reduction process is located to cathodic areas presumably coated with a thin oxide film[4]. The electrode potential is therefore a corrosion potential, representing a dynamic equilibrium between the anodic and the cathodic reactions at the metal surface respectively. However, on a surface of an ultra pure metal the anodic and the cathodic areas are not necessarily stationary with time[5]. Concerning the oxygen sensitivity, dE/d(logp,,) of antimony electrodes much confusion remains. Among previous reports, only three authors have extensively studied the potential-po, relationshipC6, 7, 81. Holmqvist[6] found an S-shaped potential-logp,, relation in 0.1 N HCl and two acetate buffers using massive electrodes Claiming that Holmqvist’s data did not represent equilibrium potentials, Kauko and Knappsberg[7] found a straight potential-log pal relationship in phosphate-borax buffers with an oxy-gen sensitivity equal to 14.5 mV at 25°C. This is very close to the theoretical value at 14.8 mV for a reversible oxygen electrode. However, the electrode response to poI changes was sluggish and the stabilization time
was greater than 15 min. Bishop and Short[B] found a at intermediate linear potential-log po, relationship poz levels with dE/d (log po,) equal to 56mV in unooised -- r- -- 0.07 M KCl solution. In this solution it was also observed that the potential was constant, ie, pot independent in the high and low partial pressure range, respectively. With buffer substances added to the KC1 solution, a lower oxygen sensitivity was obtained and in presence of phosphate ions, the electrodes were sensitive to oxygen variations in the whole po, range used (O.l- 100 kPa). This was suggested to be due to the fact that the oxygen reduction process now occurred at the bare metal. The oxygen sensitivity reported varied between 20 to 60 mV. It was concluded that the oxygen reduction processes at an antimony electrode surface are “condition-dependent”. The above mentioned experimental findings all apply to polyerystalline electrodes. However, monocrystalline antimony electrodes have recently been studied by Edwall[9] in pH 1.9 SbCl, solutions. They proved to have superior potential stability and reproducibility as compared with polycrystalline electrodes. The oxygen sensitivity was found to be 15.9 k 0.3 mV in the 20-1OOkPa oxygen partial pressure range. At these po, levels, the potential was stable. However, during mtrogen bubbling the potential was reported to be unstable. On nitrogen exposure it ran through a minimum and then a drift towards more positive values was observed. This paper presents further investigations on the oxygen sensitivity of monocrystalline antimony electrodes in the po2 range below 20 kPa. The behaviour of ultra pure (6N) and pure (4N) electrodes is compared. EXPFXIMENTAL Monocrystalline antimony electrodes were spark cut from two large single crystals of different puritygrades; pure - 99.990/, (4N) (Materials Research Corp.) and ultm pure-99.9999% (6N) (Studsvik Energiteknik AB) respectively. Ten electrodes of each purity were
1585
PER-ARNEJ~NGREZNAND
GIJNNAREDWALL
the carbon dioxide partial pressure (pcoJ was kept constant equal to 5.4 + 0.1 kPa. All gas mixtures were led through thermostated water vessels and thereby 0 SCREW CAP humidified to saturation with water. The voltage @ HGIXlNi IPMMA) measuring system is described in detail in reqlO]. @ SOCKET ,PMMAl Essentially it consists of a digital voltmeter (DVM) @ FLECTraYTC with a high input impedance (10”Q). A digital-to@ SALT BRiOGE analog (D/A) converter acts as an interface between @ ANT,tlONY ELECTROOE theDVM and a stripcart recorder. The potentials were @ ELECTRIC CONNECTION measured with a resolution of 0.1 mV. @ *NTlMCw CRnTAtt The measuring cell was mounted in the electrode @ WACER (NYLWKI @ MEMBRANE (SILICONE1 cuvette of a EMS 3 Mk2 blood gas analyzer (Radio@ ARAtDITE meter A/S). It was thermostated at 25°C. The cell @ PMMA IPOLYMETHYtMETHbCRYLATE , membrane was exposed to the humidified gas mixtures. An equilibrium between the gas phase and the thin electrolyte film at the electrode surface was thereby established by diffusion. Consequently, a change from one gas mixture to another resulted in a po2alteration in the electrolyte film. This po2 alteration was measured as a change in the antimony electrode potential. During the experiments, the pH was equal to 4.6 owing to the constant pco, of all gas mixtures. At a constant poz level equal to 13.4kPa, the antimony electrode potential was continuously recorded during 25 hours for each electrode. The oxygen 1 cm sensitivity was evaluated at the 5th and 25th hour respectively. During a sensitivity measurement, 24po, alterations were made and the potentials were read 4 Fig. 1. The measuring cell. minutes after each po2 change. Linear regression analyses were carried out on the data from each sensitivity measurement. The slopes of the regression lines were used to calculate the mean cast in epoxy resin (Araldite, Ciba-Geigy). The trivalue and the standard error of tht oxygen sensitivity gonal (liO)-surface to be exposed, was ground and within the two electrode groups. polished to optical smoothness using 1 pm diamond paste as the final step. The surface area was 0.8 mm2. The measuring cell is shown in Fig. 1. Its inner part RESULTS consists of a socket of polymethylmethacrylate, in The results of the oxygen sensitivity measurements which an antimony electrode and the end of a salt are presented in Table 1, which shows the sensitivity bridge are mounted. This construction has dimensions similar to those of the Radiometer E.5046 oxygen mean values + SE for the ultra pure and pure electrodes respectively. The ultra pure electrodes preelectrode. The housing for the cell is the conventional sented an oxygen sensitivity of the same magnitude at Radiometer po, electrode jacket. The antimony elecboth sensitivity measurements, 5 and 25 h after immertrode is covered by a nylon spacer (50pm) and a gas sion of the electrodes in the electrolyte respectively. permeable silicone membrane (25pm) (D697, D606, Radiometer A/S) which is stretched over the front end The oxygen sensitivity of these electrodes was not of the electrode jacket. An air-tight seal between the significantly different from 14.8mV/log po2, which is the sensitivity that corresponds to a four electron housing and the socket is obtained with a rubber “O”ring and a screw cap in the back part of the cell. oxygen reduction process at 25°C. The electrode consists of distilled and de-ionized On the other hand, the oxygen sensitivity of the pure water (p = 0.46 M n cm) because a minimal influence electrodes decreased from a higher value than that of dissolved substances on the electrode processes is obtained with the ultra pure electrodes at the first desirable. It is contained in the volume between the sensitivity measurement to a correspondingly lower housing and the electrode socket (- 0.7ml). A thin value at the second occasion. Also a greater spread in electrolyte film at the electrode surface is maintained the oxygen sensitivity, reflected by a larger SE, was by the spacer. obtained in this group at the first sensitivity measureelectrode used was an ments 5 h after immersion of the electrodes in the The reference Ag-AgCl/(3MKCl) - half celt (Ingold), connected electrolyte. through the salt bridge to the measuring cell. The salt The potential-log po, relationship for a reprebridge contained a 3M KC1 solution. It was separated sentative ultra pure and pure electrode is presented in Fig. 2a and b resuectively. These grauhs refer to the first from the electrolyte by a porous plug, as shown in sensitivity measurement. The ultra pure electrodes Fig. 1. Four gas mixtures containing oxygen, carbon dipresented a high reversibility and linearity with respect oxide and nitrogen were used (AGA Specialgas AB). to po, variations (Fig. 2a). The correlation coefficient was typically of the order of0.99. The high reversibility Their oxygen partial pressures (PO,) were 2.3 + 0.1,3.8 + 0.1,7.4 + 0.1 and 13.4 & 0.1 kPa respectively while and linearity was also maintained during the second
Evaluation of the influence of impurities
1587
Table 1. The oxygen sensitivity, dE/d&g PO,), of ultra Pure (6N) and pure (4N) monoerystalline antimony electrodes
f SE (N = 10) Ultra pure electrodes Cmvl
PlUl? electrodes CmVl
After 5 h use
15.3 + 0.7
21.2 f 2.98
After 25 h use
15.7 & 0.8
13.6 + l.l*
* The sensitivities are compensated for the potential drift in electrode group.
this
sensitivity measurement, except for two electrodes. In these cases, the potentials were unstable and a hysterisis effect was observed’during the poz variations. The potential-log po2 relationship of the pure electrodes was dominated by a drift in the potential level superposed on the potentials corresponding to each po2level as seen in the Fig. 2b. Iiowever, this drift was reduced in magnitude during .the second sensitivity measurement. The potential response of the ultra pure electrodes following each separate poI alteration is shown in Fig. 3a. As is seen in the figure, stable potentials were obtained at each oxygen partial pressure level. The
Fig. 2. The potentm-log p,,, relationship for representative monocrystalline antimony electrodes after 5 h use in distilled water at 25°C. (a) Electrode made of ultra pure material (6N). (b) Electrode made of pure material (4N).
Fig. 3. The potential response to PO2alterations for repm
setnative monocrystalline antimony electrodes after 5 h use. (a) Ultra pure electrode (6N). (b) Pure electrode (4N).
response time (W/$ of the electrodes in the ultra pure group was less than 15 s throughout the whole experiment. The pure electrode potential response to po, alterations, Fig. 3b, was composed of a rapid potential change, as for the ultra pure-electrodes, followed by a slow drift in the potential level. This behaviour was most pronounced during the first sensitivity measurement. The ultra pure electrode potentials reached a state of pseudo-stability approximately one hour after immersion of the electrodes in the electrolyte. This means that the mean value of the drift in the potential levels was - 0.6 mV/h (range + 0.2 - - 1.2 mV/h, N = 10). In contrast to this, none of the pure electrodes reached a psueo-stable potential level within the first 5 hours. At the first oxygen sensitivity measurements the drift was at the order of 6 mV/h. The potential stability of the pure electrodes increased with time and at the second sensitivity measurement, the mean value of the drift was + 1.1 mV/h (range + 7.0 - - 0.4mVjh, N = 10). The surface appearance of some representative electrodes of each purity grade was investigated in a scanning electron microscope (SEM). Figures 4 and 5 show SEM-micrographs of sections near the crystal edge of two ultra pure electrode surfaces. Figure 4 represents the typical ultra pure electrode with stable potential levels and a linear potential-log po, relationship. The whole electrode surface presented a uniform surface structure of the type shown in the micrograph. The surface did contain some cracks but no selective metal dissolution was seen in close vicinity to them. Figure 5 shows the surface of one of the two ultra pure electrodes that had the aforementioned deteriorated oxygen response, This electrode surface presented a corroded zonearound the whole periphery while the rest of the surface had the structure shown in the upper part of the micrograph. In the surfaces of the pure electrodes, impurity inclusions were found. In general, the antimony was
PER-ARNE J~NGREN
AND
GUNNAR EDWALL
Fig3.4. The surface ofa representative ultra pure electrode. The electrode presented a linear potential-log pol relationship. selectively dissolved around such inclusions. This is illustrated in Fig. 6 where an inclusion is undermined by seiectiv ‘e corrosion. Furthermore, the surfaces of all of the pur e electrodes had basically the same surface appearanc :e as that found on the two ultra pure electrodes with the deteriorated oxygen response. The surfaces Hrere divided into two different zones - one
with an etched surface and the other character ,ized by an even surface appearance.
DISCUSSION
Only electrodes made of the ultra pure nnatelial
Fig ,.5. The surface of an ultra pute electrode which presented a deteriorated oxygen response with hysterisis
effects.
Evaluation of the influenceof impurities
Fig. 6. An impurity inclusion,underminedby selectivecorrosion, found in a pure electrode surface.
presented a surface with a regular distribution of areas where antimony metal dissolution had taken place (Fig. 4). We think that an almost uniform distribution of dissolved reaction products was established over the whole electrode surface. The precise positions of the areas where the cathodic oxygen reduction reactions occurred can not be visualized in this case. However, the uniformity of the distribution of the anodic area makes us believe that a corresponding distribution exists for the cathodic reactions. An even distribution of reaction products dissolved in the electrolyte layer might be the reason for the high stability of these electrodes. The conditions for the oxygen reduction at the ultra pure electrodes were almost the same at the first sensitivity measurement and at the second as reflected by the constancy in the oxygen sensitivity. As claimed by Edwall[ll], general corrosion is promoted at an ultra pure crystal surface. This means that the corrosion current density is low thus leading to a slow variation of the surface properties with time. Furthermore, the oxidation rate of a monocrystalline crystal surface exposed to air was found to be extremely low[9], and even so during pure oxygen exposure. This means that only a very thin oxide film might be present (a few Angstroms thick, if any) when the electrode is first immersed into the electrolyte. Thus, due to the low oxidation tendency of water and of the low degree of air oxidation of the electrodes, the oxide film is thought to besthin in the beginning of the measurements and that the growth during the measurements will proceed with a slow rate. Impurity inclusions present in the pure electrode surfaces were found to promote a corrosion form
where the anodic areas were restricted to certain parts of the electrode surfaces. The corrosion form noted is thought to be induced by localized galvanic cells[5] created between the impurities and the antimony metal. The selective dissolution of antimony around certain impurities (Fig. 6) indicates that the impurity acted as a local cathode and the antimony metal in the close vicinity to the impurity as the anode. The high oxygen sensitivity noted for the pure electrode group during the first sensitivity measurement may then be caused by a different oxygen reduction mechanism at the impurities than on the antimony surface. The low oxygen sensitivity of the pure electrodes at the second sensitivity measurement is not well understood. It might be due to an increased thickness of the oxide film as the amount of dissolved antimony metal is thought to be larger at these electrodes than at the ultra pure ones. According to Gatty and Spooner[4], the oxygen reduction can take place at thin oxide films if the oxide is able to pass electrons from the metal bulk to the oxygen molecules. This ability will certainly decrease with increasing thickness of the IiIm. If the oxide film becomes too thick for electron conduction and thereby more or less acts like an insulator, the oxygen reduction may be restricted to the vicinity of anodic areas where the oxide film might be thinner or to the bare metal. The unstable potential levels of the two ultra pure electrodes during the second sensitivity measurement may be due to that the corrosion form at these electrodes after 25 hours use was of the same type as that of the pure electrodes described above. The anodic areas at these two electrodes were restricted to the periphery, ie, a local corrosion form existed. The
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PFX-ARNEJ~NGRENAND GUNNAR EDWALL
reason for this anomalQus behaviour as compared with the other ultra pure electrodes, may be due to the fact that an oxygrah concentration cell was established at the surfaces. Thus, in order to obtain stablepotential levels at each pal value during oxygen partial pressure variations, we believe -that general corrosion of the electrode surface is a prerequisite. Kurella[lZ] SugseSted that the oxygen sensitivity of antimony electrodes was due to impurities in the metal. This view is not supported by this investigation since the ultra pure electrodes had a clearly pronounced oxygen sensitivity. Furthermore, Caflisch et 4133 found that the antimony electrode potentials were displaced in the negative direction during nitrogen exposure. They concluded hat this was a “true effect of nitrogen” and that nitrogen should participate in the electrod&’ re@ons. In our view the nitrogen effect mentioned is an effect of a very low oxygen partial pressure as a result of the nitrogen bubbling.
Acknowledgement-The authors gratefully acknowledge the technical assistance of Rune Perssbn and Arnold Olofsson. This work was partly support&l by the National Swedish Board for Teebnical Development (STU).
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