- Email: [email protected]
Zero charge potential measurement of solid electrodes by inversion immersion method
ElectrochimicaArta, Vol. 35, No. 9, pp. 1393-1398,
0013-4686/90 53.00+ 0.00 6 1990.PergamonPressplc.
1990
Printedin Great Britain.
ZERO CHARGE POTENTIAL MEASUREMENT ELECTRODES BY INVERSION IMMERSION
OF SOLID METHOD
JERZYS~KOLOWSKI, JAN MACIEJCZAJKOWSKIand MARIA TUROWSKA Department of General Inorganic Chemistry, Institute of Chemistry, University of todi, tedi, Poland (Received 3 June 1987; in revisedform
11 April 1989)
Abstract-A
new apparatus for determination of potentials of zero charge by the immersion method is described. PZC values for Au, Cu, Fe and Ni have been determined by this means, and are compared with those reported in the literature. Key words: PZC measurement,
solid electrode, techniques of electrochemistry.
INTRODUCTION
The immersion method of potential of zero charge (PZC) measurements for liquid metals, developed by Jakuszewski and coworkers[ 11, allows determination of a number of electrochemical relationships, interesting from both theoretical and practical points of view. The application of this technique enables a quick and direct measurement of PZC values on various electrodes. The method is characterized by excellent reproducibility resulting from the easiness of obtaining a clean surface of outflowing stream of liquid metal as well as from good repeatibility of measurement conditions[2]. In addition, as compared to the streaming method, the immersion technique enables a better separation of current component resulting from a double layer from the residual Faraday currents interfering with the measurements. The application of the immersion technique to the measurements of PZC values of solid metal electrodes involves a number of new problems, characteristic for these metals, that have to be solved. The recorded peak value of the immersion impulses, originating at the moment of contact between the dry surface of the electrode and the solution, strongly depends on the conditions of the surface preparation[3], on the way the electrode is immersed into the solution, as well as on the conditions of the measurement itself. Therefore, the solid metal samples should be characterized by a physically clean surface of repeatable structure. It is assumed that such uniform preparation of the electrode surface should enable a determination of the actual relationship between recorded values of potential of zero charge and respective values of electron work functions for the investigated metals. Considering the specific character of the technique, and particularly the fact that the measurement has to be taken in an atmosphere of gas whose pressure exceeds the vapour pressure of the working solution, the conditions of an absolutely physically clean surface of the samples are impossible to maintain. From the chemical point of view the surface of the metal should be free from impurities such as oxides or salts and should not be contaminated with the adsorbed
vapours of the solvent. Suitably prepared samples of pure monocrystals with the specified crystallographic plan exposed at the surface secures the best measurement conditions. As far as polycrystalline metal samples are concerned, at least repeatable standard conditions for surface preparation should be maintained. This also refers to the surface structure. Additionally, in order to reduce the residual Faraday currents, high wetting rates of metal surface with investigated solution should be used. The value of a minimum wetting rate for which the measurement is still correct was determined empirically[2] by means of applying several successive wetting rates of an increasing value and recording the value at which the measured amplitude of immersion impulse becomes constant. Under particular experimental conditions described in our work this value should exceed 3 x 10m3m* s-r. The contact time of the electrode with the solvent vapour should also be limited to a minimum. A circuit for measuring the immersion pulse amplitude should feature a large input impedance over possibly wide frequency range and should allow measurement of a peak value of a single voltage pulse with rise time between a fraction of a microsecond to several milliseconds with sufficient accuracy. The apparatus described below allows us to fulfil these conditions. The preparation of the electrodes, their transfer from one work-stand to another and the measurements are performed entirely automatically in an atmosphere of a dry inert gas. There also exists a possibility of programming the apparatus for continuous operation, eg when a large number of results for one electrode is collected in order to obtain data for statistical analysis.
EXPERIMENTAL The equipment consists of three separate units functionally connected with each other. The operating part, hermetically sealed, includes the electrode preparation stands, a measuring station and auxiliary stands. The control unit includes the electronic and
1393
J.
1394
~KOLOWSKI
pneumatic-electronic systems as well as the measuring circuit. The purpose of this unit is to maintain the repeatable conditions during both the preparation of the electrode and the measurement, to control these conditions and to protect the equipment against malfunctioning. The third, auxiliary unit is designed to supply chemicals, to produce and purify gases, to provide induction heating etc. The operating part, designed as a cylindrically shaped and hermetically sealed chamber, includes six stands: the electrode container, the system for chemical and electrochemical treatment, the rinsing unit, the unit for thermal and electrothermal processing, the measurement stand and the terminal auxiliary stand. The metal rod electrodes are held in iron clamps and moved along the stations by means of the pneumatic-electromagnetic hoist mounted on a rotating arm. The conditions of the thermal and thermochemical treatment had a significant effect on the recorded value of the immersion pulse amplitude. This treatment was carried out in a system which enables us to expose the electrode to an induction heating in a static or dynamic atmosphere of hydrogen and/or argon under pressure reduced to lO-*-1O-3 N rnm2. Present parameters of the process were automatically held constant regardless of the number of the measurement cycles. After the heat treatment had been completed, the electrode was transferred onto the measuring stand, magnetically screened, and also surrounded with a dynamic screen to reduce the effect of parasitic capacitances. The glass measuring vessels containing the electrolyte solution were located at this stand. Figure 1 presents the construction of the measuring vessel. Applying the appropriate pressure pulse of an inert gas to any of the vessels induced an outflow stream of the solution in the direction of the investigated electrode with the linear velocity approx. 10 m SK’. As the reference the silver-silver chloride
et
al.
electrode in the appropriate solution was used in these measurements. When the stream of the solution contacted the electrode surface a voltage pulse appeared, the peak of which was recorded by an electronic system presented in Fig. 2. The investigated electrode was at the point “x”; the reference electrode, on the other hand, was connected through a switch to an input of a high impedance voltage follower. In order to determine precisely the emf value of the pulse the input of the follower was shunted by means of standard resistor of one of the following resistances: 10, 50, 250 and 7000 Mohm. The output of the follower was connected to the oscilloscope and to the input of the peak value detector (Pat. PRL No. 95829). A quick, precise inverter was interconnected between the follower output and the input of a detector to provide the opportunity of measuring pulses of negative polarization. The system presented in Fig. 2 allowed us to sample the value of voltage pulses not shorter than 400 ns of an amplitude not higher than 2000 mV with the accuracy of 0.3%. The sample peak value was measured with a digital voltmeter and printed out along with such information as pulse polarization, sequence number of the electrode, input resistance value etc. The effect of coincidental interference was reduced by means of the system interlocking the operation of the peak value detector after time intervals within the range of 2 x 10-6-10-1 s. Every operation of the equipment was controlled and regulated by the control unit following the respective programme. This included all the electromechanical and pneumatic devices, the induction furnace and all the solenoid valves. Additionally, the auxiliary equipment such as the generator of electrolytic hydrogen, solution and water feeding pumps etc, was also controlled by this unit. All measurements were carried out in the atmosphere of argon with a small addition of hydrogen.
d
Fig. 1. Diagram of the set-up for measuring the PZC: (1) nozzle for solution outflow; (2) tested electrode; (3) measuring cell; (4) electrode holder; (5) non-return valve; (6) pulse pressure connection; (7) container for solution; (8) reference electrode; (9) salt-bridge.
PZC measurement of solid electrodes
6
L___--__-_--------~----------
--------e--4
Fig. 2. Electric diagram of the immersion pulse measuring system: (1) standard resistors; (2) input follower (Analog Devices 46 J); (3) active screen; (4) input; (5) inverter (National Semicond. LM 318); (6) output to the oscilloscope; (7) peak value detector output (to digital voltmeter); (8) peak value detector circuit.
Traces of oxygen were removed using active copper, the gas was then dried with potassium hydroxide and anhydrous calcium chloride and passed over active carbon. The purification of argon was carried out in a closed circulation system under pressure slightly exceeding the ambient one. Hydrogen was produced from the aqueous solution of sodium hydroxide by its electrolysis, dried over anhydrous calcium chloride, passed through a scrubber filled with melted sodium and finally through a freezer cooled by the dry ice-acetone mixture. It was then fed into the system through a solenoid valve and used for thermochemical cleaning of the electrodes. Water used for preparation of solutions and for rinsing electrodes was distilled four times in an argon atmosphere using chemically resistant glass distiller. Sodium sulphate (manuf. POCH Gliwice) was crystallized twice before being used for preparation of the solutions. The investigated electrodes were made of metals with the following grades of purity: l
l
l l
Gold: 0.5 mm sheet, 4N purity (State Mint, Warsaw, Poland) Copper: 5 mm rods, 5N purity (Johnson Matthey Chemicals Ltd) Iron: 5 mm rods, 5N purity (Koch-Light Lab.) Nickel: 5 mm rods, 5N purity (Koch-Light Lab.).
The silver-silver chloride electrodes, used as the reference electrodes, were made by anode chlorination of silver, deposited on a platimum base. RESULTS In order to test the equipment the measurements of the amplitude of the immersion pulse originating upon the contact of the working solution with the
steady stream of mercury replacing the solid electrode were carried out. The results obtained for the solutions of potassium fluoride, potassium chloride and sodium sulphate of concentrations of 0.1 mol drne3 and 0.01 mol drne3 did not differ from the values of the PZC for mercury in these solutions by more than 2 mV. This agreement has been considered as entirely satisfying. The determination of the optimum conditions for the preparation of solid electrodes in order to maintain their surface clean and to provide the reproducibility of the immerse pulse measurements led to the conclusion that the preliminary chemical and electrochemical processing of the surface had a negligible effect on the measured values when compared to the thermochemical treatment. Considering that, this preliminary processing was further regarded as an initial preparation of the new electrodes applied only before their first use in the measurements. The purity of hydrogen used for thermochemical cleaning of the electrodes as well as the duration of this treatment both had strong effects on the results of the measurements (Fig. 3). A significant influence of traces of oxygen and water in the argon atmosphere was also observed. The clean samples of iron remaining in argon with a high water vapour content showed the values of immerse pulse amplitude several hundreds of millivolts lower than those maintained in a dry atmosphere (Fig. 4). The opposite effect, that is a shift towards higher values, was observed as a result of the addition of small amounts of oxygen to argon in the case of copper samples. All metals were heated at temperatures close to their melting points. In such conditions the differences in the temperature range of f 100°C did not influence the results of the measurements. In contrast, the treatment at lower temperatures led to the dispersion of the results, showing lack of reproducibility.
J.
1396
~lcoLOwSK1
et al.
- 0.15.
Fig. 3. Dependence of immersion pulse amplitude on electrode preheating time (for iron electrode).
The duration of heating in the stream of hydrogen required for the metal surface to stabilize its properties differed from 10 h (gold electrode) to over 200 h (iron electrode). In order to accelerate this process recurrent cooling of the electrode was employed. Purification of an electrode after a measurement required 15-30 min of heat treatment. When the heating of the electrodes in the atmosphere of hydrogen was followed by their dehydrogenation, that is heating in the stream of argon, no significant differences in the results were observed within the experimental error. The measurements of the immersion pulse amplitude were carried out in an aqueous solution of sodium sulphate (0.01 mol dmv3). The use of fluoride solutions was excluded due to the formation of the non-volatile metal fluorides on the surface of the electrodes during heat treatment. When the multiple measurements were carried out the same layer of the fluoride was accumulated which lead to the systematic error. In order to enable statistical analysis of the results a series of 200 measurements were performed on several samples for each metal. Figure 5 presents an
example of a histogram of immersion pulse amplitude values for copper electrode. Table 1 presents the average values of measured pulse amplitudes for the respective electrodes related to the hydrogen electrode as well as the statistical error for each electrode. According to [l] these values can be regarded as the values of potential of zero charge for the investigated electrodes. The comparison of the PZC values obtained in this work with the literature data (Table 2) shows that our values correspond well with those obtained with different experimental methods in the case of a gold electrode, whereas the results of measurements on a copper electrode are, on average, about 60-80mV lower than the most often cited values. On the other hand, the PAC value for an iron electrode obtained in this work is considerably higher than the literature values, whereas the PZC of nickel corresponds well to the literature data. Such significant disagreement of results in the case of an iron electrode can be explained by the differences in the condition of metal surface at the moment of measurement between our method and other experimental techniques, and in particular the
Fig. 4. Dependence of immersion pulse amplitude on wet inert gas time contact (80% relative humidity).
PZC measurement of solid electrodes
1397
Fig. 5. Statistical distribution of the immersion pulse amplitude (copper electrode measured USsilver-silver chloride in 0.01 mol dm-3 LiCl aq.).
Table 1. Metal
Copper Gold Iron Nickel
PZC - 0.035 v + 0.060 V -0.16OV -0.19ov
Standard error * + * *
0.005 v 0.020 v 0.015 v 0.010 v
of differential capacity measurement in dilute solutions. During the measurement of differential capacity the iron electrode remains in contact with the solution for a long time which very likely leads to the partial oxidation of its surface. It has to be stressed here that our measurements on iron technique
electrodes maintained in the atmosphere of argon with the small content of water resulted in values of about 450 mV (Fig. 4), that is the values correspond well to the literature data[l7]. The results discussed above strongly suggest that the described method enables the PZC measurements on metals of “clean” surfaces in contrast to other methods which, depending on the activity of metal, involve measurement on surfaces more or less oxidized or contaminated by adsorption. Therefore, in any comparative studies relating the PZC values for metals with their work functions the method described in this work should be used for the determination of the potential of zero charge.
Table 2. Metal Au
cu
PZCIV - 0.06 + 0.05 +0.10 + 0.06 f 0.02
0.1 M KC1 0.001M KC1 l/60 M K, SO4
- 0.04
0.5 M Na,S04
- 0.012 0.0
0.1 M KC1 0.001 M Na, SO, + 0.001 M K,S,O, 0.001 M NaClO,
+ 0.025
+ 0.03
Fe
Ni
Solution
0.01 M Na,SO,
Ref. ;x; Ii this work DO1 PI U 11
1121
0.01 M Na,SO,
113,141
- 0.035 f 0.005
0.01 M Na,SO,
this work
- 0.33 - 0.37 - 0.4 - 0.7 -0.15*0.015
0.05 M H, SO4 0.0005 M H, SO, 0.003 M HCl 0.005 M Na, SO, 0.01 M Na,SO,
P51 [I61 171 (171 this work
- 0.3 - 0.26
0.1 M NaClO., 0.0005 M KClO,
V81 (181
--0.18 0.21 - 0.19 + 0.01
0.01 M M Na,SO, 0.0005 H,SO, 0.01 M Na, SO,
;:z; this work
J. S~KO~OWSKI er al.
1398
Work on the determination of the PZC values of larger groups of metals is now being continued with a view to the adaptation of the method to measurements on metals with low melting point, which cannot be prepared by using the procedure described in this work.
REFERENCES B. Jakuszewski and Z. Kozlowski, Roczn. Chem. 36, 1873 (1962). J. M. Czajkowski, T. Blaszczyk and D. Kazmierczak, Electrochim. Acia 29, 439 (1984). S. Kim, J. phys. Chem. 77,‘2787 (1973). J. Sokolowski. J. M. Czaikowski and B. Jakuszewski. Zjazd Naukoiy PTCh i SiTPCh Zeszyt B, Lodz 5,25i (1978). 5. M. Petit and J. Clavilier, C. r. Acad. Sci., Paris C265, 145 (1967). 6. T. N. Voropaeva et al., Zh. Fir. Khim. 24, 396 (1962).
7. T. N. Voropaeva et al., In, Akad. Nauk., SSSR, Old. I&m. Nauk., 257 (1963). 8. T. Andersen, R. Perkins and H. Eyring, J. Am. them. Sot. 86,4496 (1964). 9. R. Perkins et al., J. phys. Chem. 69, 3329 (1965). 10. M. Bonnemay et al., C. r. Acad. Sci. 260, 5262 (1965). 11. A. Frnmkin, Z. Elekrrochem. 59, 807 (1955). 12. D. J. Leikis et al.. Electroanal. Chem. Interf. Electrothem. 46, 161 (1973). 13. V. L. Kheifets and B. S. Krasikov, DAN., SSSR 109, 586 (1956). 14. V. L. Kheifets and B. S. Krasikov, Zh. Fiz. Khim. 31, 1992 (1957). 15. E. 0. Aiazian, DAN. SSSR 100, 473 (1955). 16. D. B. Matthews, Ph.D. Thesis, University of Pennsylvania (1965). 17. L. E. Rybalka and D. I. Leikis, Electrokhimiya 10, 1619 (1975). 18. J. Bocris, S. D. Argade and E. Gileadi, Electrochim. Acta 14, 1259 (1969). 19. L. A. Kheifets and L. S. Rejszakrit, Uczen. zatiski L.G.U. (Khim) 13, 173 (1953). 20. W. P. Grigoriev, Z. Meralov 3, 357 (1967).
0013-4686/90 53.00+ 0.00 6 1990.PergamonPressplc.
1990
Printedin Great Britain.
ZERO CHARGE POTENTIAL MEASUREMENT ELECTRODES BY INVERSION IMMERSION
OF SOLID METHOD
JERZYS~KOLOWSKI, JAN MACIEJCZAJKOWSKIand MARIA TUROWSKA Department of General Inorganic Chemistry, Institute of Chemistry, University of todi, tedi, Poland (Received 3 June 1987; in revisedform
11 April 1989)
Abstract-A
new apparatus for determination of potentials of zero charge by the immersion method is described. PZC values for Au, Cu, Fe and Ni have been determined by this means, and are compared with those reported in the literature. Key words: PZC measurement,
solid electrode, techniques of electrochemistry.
INTRODUCTION
The immersion method of potential of zero charge (PZC) measurements for liquid metals, developed by Jakuszewski and coworkers[ 11, allows determination of a number of electrochemical relationships, interesting from both theoretical and practical points of view. The application of this technique enables a quick and direct measurement of PZC values on various electrodes. The method is characterized by excellent reproducibility resulting from the easiness of obtaining a clean surface of outflowing stream of liquid metal as well as from good repeatibility of measurement conditions[2]. In addition, as compared to the streaming method, the immersion technique enables a better separation of current component resulting from a double layer from the residual Faraday currents interfering with the measurements. The application of the immersion technique to the measurements of PZC values of solid metal electrodes involves a number of new problems, characteristic for these metals, that have to be solved. The recorded peak value of the immersion impulses, originating at the moment of contact between the dry surface of the electrode and the solution, strongly depends on the conditions of the surface preparation[3], on the way the electrode is immersed into the solution, as well as on the conditions of the measurement itself. Therefore, the solid metal samples should be characterized by a physically clean surface of repeatable structure. It is assumed that such uniform preparation of the electrode surface should enable a determination of the actual relationship between recorded values of potential of zero charge and respective values of electron work functions for the investigated metals. Considering the specific character of the technique, and particularly the fact that the measurement has to be taken in an atmosphere of gas whose pressure exceeds the vapour pressure of the working solution, the conditions of an absolutely physically clean surface of the samples are impossible to maintain. From the chemical point of view the surface of the metal should be free from impurities such as oxides or salts and should not be contaminated with the adsorbed
vapours of the solvent. Suitably prepared samples of pure monocrystals with the specified crystallographic plan exposed at the surface secures the best measurement conditions. As far as polycrystalline metal samples are concerned, at least repeatable standard conditions for surface preparation should be maintained. This also refers to the surface structure. Additionally, in order to reduce the residual Faraday currents, high wetting rates of metal surface with investigated solution should be used. The value of a minimum wetting rate for which the measurement is still correct was determined empirically[2] by means of applying several successive wetting rates of an increasing value and recording the value at which the measured amplitude of immersion impulse becomes constant. Under particular experimental conditions described in our work this value should exceed 3 x 10m3m* s-r. The contact time of the electrode with the solvent vapour should also be limited to a minimum. A circuit for measuring the immersion pulse amplitude should feature a large input impedance over possibly wide frequency range and should allow measurement of a peak value of a single voltage pulse with rise time between a fraction of a microsecond to several milliseconds with sufficient accuracy. The apparatus described below allows us to fulfil these conditions. The preparation of the electrodes, their transfer from one work-stand to another and the measurements are performed entirely automatically in an atmosphere of a dry inert gas. There also exists a possibility of programming the apparatus for continuous operation, eg when a large number of results for one electrode is collected in order to obtain data for statistical analysis.
EXPERIMENTAL The equipment consists of three separate units functionally connected with each other. The operating part, hermetically sealed, includes the electrode preparation stands, a measuring station and auxiliary stands. The control unit includes the electronic and
1393
J.
1394
~KOLOWSKI
pneumatic-electronic systems as well as the measuring circuit. The purpose of this unit is to maintain the repeatable conditions during both the preparation of the electrode and the measurement, to control these conditions and to protect the equipment against malfunctioning. The third, auxiliary unit is designed to supply chemicals, to produce and purify gases, to provide induction heating etc. The operating part, designed as a cylindrically shaped and hermetically sealed chamber, includes six stands: the electrode container, the system for chemical and electrochemical treatment, the rinsing unit, the unit for thermal and electrothermal processing, the measurement stand and the terminal auxiliary stand. The metal rod electrodes are held in iron clamps and moved along the stations by means of the pneumatic-electromagnetic hoist mounted on a rotating arm. The conditions of the thermal and thermochemical treatment had a significant effect on the recorded value of the immersion pulse amplitude. This treatment was carried out in a system which enables us to expose the electrode to an induction heating in a static or dynamic atmosphere of hydrogen and/or argon under pressure reduced to lO-*-1O-3 N rnm2. Present parameters of the process were automatically held constant regardless of the number of the measurement cycles. After the heat treatment had been completed, the electrode was transferred onto the measuring stand, magnetically screened, and also surrounded with a dynamic screen to reduce the effect of parasitic capacitances. The glass measuring vessels containing the electrolyte solution were located at this stand. Figure 1 presents the construction of the measuring vessel. Applying the appropriate pressure pulse of an inert gas to any of the vessels induced an outflow stream of the solution in the direction of the investigated electrode with the linear velocity approx. 10 m SK’. As the reference the silver-silver chloride
et
al.
electrode in the appropriate solution was used in these measurements. When the stream of the solution contacted the electrode surface a voltage pulse appeared, the peak of which was recorded by an electronic system presented in Fig. 2. The investigated electrode was at the point “x”; the reference electrode, on the other hand, was connected through a switch to an input of a high impedance voltage follower. In order to determine precisely the emf value of the pulse the input of the follower was shunted by means of standard resistor of one of the following resistances: 10, 50, 250 and 7000 Mohm. The output of the follower was connected to the oscilloscope and to the input of the peak value detector (Pat. PRL No. 95829). A quick, precise inverter was interconnected between the follower output and the input of a detector to provide the opportunity of measuring pulses of negative polarization. The system presented in Fig. 2 allowed us to sample the value of voltage pulses not shorter than 400 ns of an amplitude not higher than 2000 mV with the accuracy of 0.3%. The sample peak value was measured with a digital voltmeter and printed out along with such information as pulse polarization, sequence number of the electrode, input resistance value etc. The effect of coincidental interference was reduced by means of the system interlocking the operation of the peak value detector after time intervals within the range of 2 x 10-6-10-1 s. Every operation of the equipment was controlled and regulated by the control unit following the respective programme. This included all the electromechanical and pneumatic devices, the induction furnace and all the solenoid valves. Additionally, the auxiliary equipment such as the generator of electrolytic hydrogen, solution and water feeding pumps etc, was also controlled by this unit. All measurements were carried out in the atmosphere of argon with a small addition of hydrogen.
d
Fig. 1. Diagram of the set-up for measuring the PZC: (1) nozzle for solution outflow; (2) tested electrode; (3) measuring cell; (4) electrode holder; (5) non-return valve; (6) pulse pressure connection; (7) container for solution; (8) reference electrode; (9) salt-bridge.
PZC measurement of solid electrodes
6
L___--__-_--------~----------
--------e--4
Fig. 2. Electric diagram of the immersion pulse measuring system: (1) standard resistors; (2) input follower (Analog Devices 46 J); (3) active screen; (4) input; (5) inverter (National Semicond. LM 318); (6) output to the oscilloscope; (7) peak value detector output (to digital voltmeter); (8) peak value detector circuit.
Traces of oxygen were removed using active copper, the gas was then dried with potassium hydroxide and anhydrous calcium chloride and passed over active carbon. The purification of argon was carried out in a closed circulation system under pressure slightly exceeding the ambient one. Hydrogen was produced from the aqueous solution of sodium hydroxide by its electrolysis, dried over anhydrous calcium chloride, passed through a scrubber filled with melted sodium and finally through a freezer cooled by the dry ice-acetone mixture. It was then fed into the system through a solenoid valve and used for thermochemical cleaning of the electrodes. Water used for preparation of solutions and for rinsing electrodes was distilled four times in an argon atmosphere using chemically resistant glass distiller. Sodium sulphate (manuf. POCH Gliwice) was crystallized twice before being used for preparation of the solutions. The investigated electrodes were made of metals with the following grades of purity: l
l
l l
Gold: 0.5 mm sheet, 4N purity (State Mint, Warsaw, Poland) Copper: 5 mm rods, 5N purity (Johnson Matthey Chemicals Ltd) Iron: 5 mm rods, 5N purity (Koch-Light Lab.) Nickel: 5 mm rods, 5N purity (Koch-Light Lab.).
The silver-silver chloride electrodes, used as the reference electrodes, were made by anode chlorination of silver, deposited on a platimum base. RESULTS In order to test the equipment the measurements of the amplitude of the immersion pulse originating upon the contact of the working solution with the
steady stream of mercury replacing the solid electrode were carried out. The results obtained for the solutions of potassium fluoride, potassium chloride and sodium sulphate of concentrations of 0.1 mol drne3 and 0.01 mol drne3 did not differ from the values of the PZC for mercury in these solutions by more than 2 mV. This agreement has been considered as entirely satisfying. The determination of the optimum conditions for the preparation of solid electrodes in order to maintain their surface clean and to provide the reproducibility of the immerse pulse measurements led to the conclusion that the preliminary chemical and electrochemical processing of the surface had a negligible effect on the measured values when compared to the thermochemical treatment. Considering that, this preliminary processing was further regarded as an initial preparation of the new electrodes applied only before their first use in the measurements. The purity of hydrogen used for thermochemical cleaning of the electrodes as well as the duration of this treatment both had strong effects on the results of the measurements (Fig. 3). A significant influence of traces of oxygen and water in the argon atmosphere was also observed. The clean samples of iron remaining in argon with a high water vapour content showed the values of immerse pulse amplitude several hundreds of millivolts lower than those maintained in a dry atmosphere (Fig. 4). The opposite effect, that is a shift towards higher values, was observed as a result of the addition of small amounts of oxygen to argon in the case of copper samples. All metals were heated at temperatures close to their melting points. In such conditions the differences in the temperature range of f 100°C did not influence the results of the measurements. In contrast, the treatment at lower temperatures led to the dispersion of the results, showing lack of reproducibility.
J.
1396
~lcoLOwSK1
et al.
- 0.15.
Fig. 3. Dependence of immersion pulse amplitude on electrode preheating time (for iron electrode).
The duration of heating in the stream of hydrogen required for the metal surface to stabilize its properties differed from 10 h (gold electrode) to over 200 h (iron electrode). In order to accelerate this process recurrent cooling of the electrode was employed. Purification of an electrode after a measurement required 15-30 min of heat treatment. When the heating of the electrodes in the atmosphere of hydrogen was followed by their dehydrogenation, that is heating in the stream of argon, no significant differences in the results were observed within the experimental error. The measurements of the immersion pulse amplitude were carried out in an aqueous solution of sodium sulphate (0.01 mol dmv3). The use of fluoride solutions was excluded due to the formation of the non-volatile metal fluorides on the surface of the electrodes during heat treatment. When the multiple measurements were carried out the same layer of the fluoride was accumulated which lead to the systematic error. In order to enable statistical analysis of the results a series of 200 measurements were performed on several samples for each metal. Figure 5 presents an
example of a histogram of immersion pulse amplitude values for copper electrode. Table 1 presents the average values of measured pulse amplitudes for the respective electrodes related to the hydrogen electrode as well as the statistical error for each electrode. According to [l] these values can be regarded as the values of potential of zero charge for the investigated electrodes. The comparison of the PZC values obtained in this work with the literature data (Table 2) shows that our values correspond well with those obtained with different experimental methods in the case of a gold electrode, whereas the results of measurements on a copper electrode are, on average, about 60-80mV lower than the most often cited values. On the other hand, the PAC value for an iron electrode obtained in this work is considerably higher than the literature values, whereas the PZC of nickel corresponds well to the literature data. Such significant disagreement of results in the case of an iron electrode can be explained by the differences in the condition of metal surface at the moment of measurement between our method and other experimental techniques, and in particular the
Fig. 4. Dependence of immersion pulse amplitude on wet inert gas time contact (80% relative humidity).
PZC measurement of solid electrodes
1397
Fig. 5. Statistical distribution of the immersion pulse amplitude (copper electrode measured USsilver-silver chloride in 0.01 mol dm-3 LiCl aq.).
Table 1. Metal
Copper Gold Iron Nickel
PZC - 0.035 v + 0.060 V -0.16OV -0.19ov
Standard error * + * *
0.005 v 0.020 v 0.015 v 0.010 v
of differential capacity measurement in dilute solutions. During the measurement of differential capacity the iron electrode remains in contact with the solution for a long time which very likely leads to the partial oxidation of its surface. It has to be stressed here that our measurements on iron technique
electrodes maintained in the atmosphere of argon with the small content of water resulted in values of about 450 mV (Fig. 4), that is the values correspond well to the literature data[l7]. The results discussed above strongly suggest that the described method enables the PZC measurements on metals of “clean” surfaces in contrast to other methods which, depending on the activity of metal, involve measurement on surfaces more or less oxidized or contaminated by adsorption. Therefore, in any comparative studies relating the PZC values for metals with their work functions the method described in this work should be used for the determination of the potential of zero charge.
Table 2. Metal Au
cu
PZCIV - 0.06 + 0.05 +0.10 + 0.06 f 0.02
0.1 M KC1 0.001M KC1 l/60 M K, SO4
- 0.04
0.5 M Na,S04
- 0.012 0.0
0.1 M KC1 0.001 M Na, SO, + 0.001 M K,S,O, 0.001 M NaClO,
+ 0.025
+ 0.03
Fe
Ni
Solution
0.01 M Na,SO,
Ref. ;x; Ii this work DO1 PI U 11
1121
0.01 M Na,SO,
113,141
- 0.035 f 0.005
0.01 M Na,SO,
this work
- 0.33 - 0.37 - 0.4 - 0.7 -0.15*0.015
0.05 M H, SO4 0.0005 M H, SO, 0.003 M HCl 0.005 M Na, SO, 0.01 M Na,SO,
P51 [I61 171 (171 this work
- 0.3 - 0.26
0.1 M NaClO., 0.0005 M KClO,
V81 (181
--0.18 0.21 - 0.19 + 0.01
0.01 M M Na,SO, 0.0005 H,SO, 0.01 M Na, SO,
;:z; this work
J. S~KO~OWSKI er al.
1398
Work on the determination of the PZC values of larger groups of metals is now being continued with a view to the adaptation of the method to measurements on metals with low melting point, which cannot be prepared by using the procedure described in this work.
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