METHOD AND DEVICE FOR ELECTROCHEMICAL MEASUREMENTS

Abstract: Method for performing electrochemical measurements in a solution with a low electric conductivity, in which method the current flowing between a test piece and a counter-electrode placed in the solution is measured while the potential of the test piece is controlled in relation to a reference electrobe potential remaining constant. The counter-electrode and the test piece are disposed at a substantially close distance from each other so that in practice a currrent can only flow across the gap between their oppositely placed counterfaces, the potential difference between the reference electode and the test piece being measured outside the electric field between the test piece and the counter-electrode.

Description: METHOD AND DEVICE FOR ELECTROCHEMICAL MEASUREMENTS The present invention relates to a method as defined in the preamble of claim 1 and to an apparatus as defined in the preamble of claim 10 for performing electrochemical measurements. Electrochemical measurements are based on measuring the current flowing between a test piece and a counter-electrode placed in the same solution while the potential of the test piece is controlled in relation to the counter-electrode potential, which remains constant. Of course, in a corresponding manner, the measurements can be based on measuring the potential while the current is controlled. In the following, the invention will be described with reference to the former case, but both alternatives are included in the inventive idea defined by the application. A fundamental prerequisite for a successful measurement is that the solution be electrically con- ductive at least to some extent. If the solution has a large electric resistance, then the product of the current and the electric resistance of the solution, the so-called iR potential fall, in addition to the actual potential difference, will be summed in the measured potential difference between the reference electrode and the test piece. In this case, the control of the test piece potential will be inaccurate. Moreover, the apparatus currently used have an insufficient efficiency, so it is not possible to generate a sufficient current between the test piece and the counter-electrode. This disadvantage can be managed to a limited degree by using an arrangement designed to measure the magnitude of the iR potential fall. In certain practical processes, such as e.g. power plant water and the manufacture of organic compounds, the electric resistance of the solution is so large that the current flowing between the test piece and the counter-electrode with a conventional distance between them is limited to a few microamperes because of the low efficiency of the apparatus, whereas the current level needed to enable the measurements to be carried out is several milliamperes . In thin-layer electrochemistry, a technique is known in which the surface of a test piece is provided with a thin layer of solution e.g. between a glass wall and the test piece. The thickness of this layer of solution may be e.g. of the order of 2- 100 μm. The counter-electrode and the reference electrode are located outside the thin solution layer. The principal aim in thin-layer chemistry is to increase the ratio of the area of the test piece to the amount of solution, which significantly improves the resolution of different electrochemical methods because the concentration of reaction products in the solution increases. This technique has the drawbacks of a very large irregular current distribution and a large iR potential fall even though the counter-electrode and reference electrode are placed as close to the gap containing the thin solution layer as possible. These test arrangements are implemented and workable only at low temperatures and in non-pressurised apparatus, and they do not work in conditions of large pressure and temperature variations. The object of the present invention is to eliminate the drawbacks described above. A specific object of the present invention is to produce a new type of method and apparatus that can be used to carry out electrochemical measurements in solutions at high temperatures and pressures, i.e. in real conditions of use of materials, and so that the disadvantage caused by the iR potential fall is eliminated. As for the features characteristic of the invention, reference is made to the claims. The method and apparatus of the invention are specifically intended for high temperatures (over 100°C) and high pressures (over 1 bar), i.e. for real power plant and process conditions, in which the ex- isting measuring methods are unusable or cannot produce sufficiently accurate results. In the electrochemical measuring method of the invention, the current flowing between a test piece in a solution with a low electric conductivity and a counter-electrode is measured while the potential of the test piece is controlled in relation to a reference electrode potential remaining constant. According to the invention, the counter-electrode and the test piece are disposed at a substantially close distance from each other so that in practice, i.e. within the limits of the measuring accuracy, a current can only flow between them across the gap between their oppositely placed counterfaces . Thus, the potential difference between the reference electrode and the test piece is measured through a practically cur- rentless part of the solution, i.e. outside the sphere of influence of the electric field generated between the test piece and the counter-electrode. The pressure difference is preferably meas- ured at a certain distance from the gap between the counter-electrode and the test piece, this distance equalling at least the width of the gap. The distance is preferably substantially larger than the gap width, over five or even ten times the gap width, e.g. 20 - 1000 times the gap width. In practice, the gap width at its minimum may be only of an A-magnitude and at its maximum 1 mm. Before the measurement, the counterfaces of the test piece and the counter-electrode are prefera- bly brought into electric contact with each other, whereupon they are withdrawn to a desired small distance from each other. In this way, after the electric contact, the counterfaces can be accurately moved to the desired distance from each other and the gap width can be adjusted to the desired size practically with an accuracy of one nanometre. Since a high level of accuracy and a small distance are to be achieved, mounting the counter- electrode and the test piece with their planar surfaces parallel to each other with an accuracy of one nanometre is very difficult. Therefore, before the surfaces are brought to electric contact with each other, they are preferably made parallel to each other. The parallelling of the surfaces is preferably done by using a suitable abrasion element of uniform thickness, such as a piece of abrasive paper, which is placed between the counterfaces of the counter- electrode and the test piece. By drawing the abrasive paper while the surfaces are pressed against the opposite abrasive surfaces of the paper, the surfaces are rendered very accurately parallel to each other. In an embodiment of the invention, some of the solution in the measurement process is passed into the gap between the oppositely placed counterfaces of the counter-electrode and the test piece. In this way, the reaction products dissolved from the test piece can be removed from the surface of the test piece. In this case, en electrode electrically isolated from the test piece is preferably used outside the test piece, e.g. a ring electrode surrounding the test piece. By adjusting the potential of the ring electrode, the re- action products dissolved from the test piece and their concentrations can be determined. The apparatus of the invention for carrying out electrochemical measurements in solutions with a low electric conductivity comprises a test piece, a counter-electrode and a reference electrode, all placed in the solution. According to the invention, the counter-electrode and the test piece comprise counterfaces corresponding to each other in respect of their shape so that the counterfaces have a constant distance between them over their entire area, and in addition, according to the invention, the reference electrode is located substantially outside the sphere of influence of the electric field between the counter-electrode and the test piece. The counterfaces are preferably straight planar surfaces which have a constant distance between them over their entire area even when the surfaces are moved farther apart from each other or closer to each other. Thus, no current concentrations are formed between the surfaces but a steady electric field prevails in the solution between the surfaces. The counter-electrode or at least its counterface is preferably made of a non-clinging, electrically conductive material, so that when it is brought into contact with the counterface of the test piece at the beginning of the measurement, the counter- electrode will not release any material that could stick or be transferred to the surface of the test piece . The counterface is preferably made of a hard noble metal or a suitable noble metal mixture, e.g. iridium or rodium. The counterface may also be made of a metal oxide that is a relatively good electric conductor. The adjustment of the distance between the test piece and the counter-electrode can be imple- mented in several different ways. For instance, a stepping motor located outside the pressurised space moves a shaft whose displacement is being controlled. The stepping motor is controlled by a computer. The shaft extends through a radial labiate seal in the cover of the pressure vessel to a spiral spring, which converts the motion into a force. The spiral spring transmits the force to a parallelogram spring, to which the counter-electrode has been attached via a rigid screw joint. The force moves the parallelogram spring, allowing the distance between the counter- electrode and the test piece to be changed. The paral- lelogram spring constitutes a part of a U-shaped attachment holding the test piece and the counter- electrode, forming its upper arm. The test piece is fixed to the lower arm, which is of a rigid construction, i.e. the lower arm does not bend elastically. The attachment provides the advantage that the test piece and the counter-electrode can be positioned at exactly the desired distance from each other at room temperature before the test is started and, further, that it makes it possible to ensure that the counter- faces of the counter-electrode and the test piece are parallel to each other by polishing the surfaces after the counter-electrode and test piece have been mounted on the attachment, by using a polishing tool placed between the surfaces. The sensitivity of distance ad- justment can be easily changed for different purposes by choosing a spiral spring with a suitable spring constant. To move the shaft outside the pressure vessel, besides a stepping motor, e.g. devices based on piezoelectric crystals can be used. In an embodiment of the invention, the counter-electrode is provided with a flow channel through which the solution used in the process is passed into the gap between the counter-electrode and the test piece to remove from the gap the material dissolved from the test piece into the solution. In this case, the test piece preferably has outside or around it an electrode electrically isolated from it and permitting adjustment of potential, e.g. a ring electrode, for identification of the substances dis- solved from the test piece. The counterfaces of the counter-electrode and the test piece are preferably circular planar sur- faces, and the flow channel in the counter-electrode opens perpendicularly into the gap at the centre of the circle. Thus, the flow of solution will uniformly wash the entire gap and test piece surface. The ring electrode preferably surrounds the test piece with only an electric insulation between them. As the test piece and ring electrode surfaces are additionally arranged in exactly the same plane, the reaction products will move with the solution near the test piece surface and come close to the ring electrode surface. By selecting a suitable ring electrode potential, the reaction products dissolved from the test piece can be either oxidised or reduced on the surface of the ring electrode. However, the counterfaces of the counter- electrode and the test piece may also have e.g. a rectangular form, in which case the flow channel is a transverse slot at the middle of the counter-electrode rectangle and the solution only flows from this slot perpendicularly in opposite directions. In this case, instead of a ring electrode, straight electrodes transverse to the solution flow are used. The method and apparatus of the invention have significant advantages as compared with prior art. Electrochemical methods are widely used, defined to characterise the properties of materials and their applicability in given service environments. The method and apparatus of the invention make it possible for the first time to use this kind of methods in real working conditions, e.g. in a power plant environment. Moreover, preferred embodiments of the invention can be used to determine the quantities of substances dissolved from a test piece and to identity the various kinds of particles dissolved. In the following, the invention will be described by referring to the attached drawings, wherein Fig. 1 represents a prior-art measuring arrangement, Fig. 2 represents a measuring arrangement according to the invention, and Fig. 3 presents a more detailed view of an embodiment of the invention. Fig. 1 illustrates a conventional measuring arrangement for carrying out electrochemical measurements at high temperatures. In this arrangement, im- mersed in a solution A are a test piece B, a counter- electrode C and a reference electrode D, whose shapes and mutual distances vary from case to case, the distances being of the order of centimetres . As the solution generally has a relatively high electric resis- tance, the product of the current and the electrical resistance of the solution, in addition to the actual potential difference, will be summed in the measured potential difference between the reference electrode and the test piece, which may distort the potential values. In addition, the efficiency of the apparatus currently used for the supply of electricity is insufficient, so it is not possible to set up a sufficient current flow between the test piece and the counter- electrode . Fig. 2 presents an arrangement according to the invention. In this arrangement, immersed in a solution 1 are a test piece 2 and a counter-electrode 3, whose counterfaces 5 and β, i.e. surfaces placed against each other are straight and parallel planar surfaces so that the gap formed between them has a constant width over its entire area. The gap width, i.e. the distance h between the counterfaces 5 and β can be adjusted by means of a mover 7 so as to allow the surfaces to be brought into electrical contact with each other and then to be withdrawn to a suitable distance from each other. The mover itself may be implemented using different known arrangements by which a controlled and accurate movement can be accomplished. Also immersed in the solution is a reference electrode 4, located in the vicinity of the test piece 2 but separated from the gap by a distance substantially larger than the gap width h used in the measurement. In this way, the potential difference between the reference electrode 4 and the test piece 2 can be measured outside the sphere of influence of the elec- trie field prevailing in the gap and its immediate vicinity. In practice, the counter-electrode and the test piece may be e.g. bars of a circular cross- section with a diameter of the order of 1 mm. As the gap used between them in the measurements is of a μm magnitude, the edges of the gap, i.e. the edges of the counter-electrode and the test piece will be rounded with a radius of curvature considerably larger than the gap width. Therefore, no current concentrations are generated at the gap edges, but the electric field prevailing in the gap is very uniform throughout its area. The measurement according to the invention is performed as follows. The apparatus and method of the invention can be used in various electrochemical measurements in which either a current or a potential is controlled, for instance in impedance spectroscopy. In the arrangement illustrated by Fig. 2, a suitable abrasive or polishing tool is placed between the test piece 2 and the counter-electrode 3 at the beginning of the measurement and accurate parallelism of the counterfaces is ensured by drawing the polishing tool against the surfaces. After this, the surfaces are brought into electric contact with each other, in other words, they are moved by means of a mover 7 towards each other until the disappearance of the resistance of the medium between the counter-electrode and the test piece is detected. In this way, the position of the counterfaces 5 and 6 relative to each other is established. After this, the surfaces are moved away from each other by means of the mover 7 to a desired distance, e.g. 5-10 μm. As the counterfaces are now exactly at the desired distance from each other in the entire gap area, the measurements can be carried out. Now, between the test piece 2 and the counter-electrode 3 there flows in the solution 1 a current, practically all of which flows only through the narrow gap between the counterfaces. The electric field extends in the solution only to a minimal distance outside the gap, and even at a distance equal to the gap width from the gap it can be said, within the limits of the measuring accuracy, that the solution 1 is currentless. The reference electrode 4 is therefore placed at a suitable distance from the gap, so it can be used to measure the potential difference between the reference elec- trode and the test piece without the measurement being in any way disturbed by the electric field between the test piece and the counter-electrode. Fig. 3 is a diagram presenting a sectional view of an apparatus according to the invention which makes it possible to measure the particle types dissolved from a test piece under investigation directly from a process at a high temperature. The apparatus comprises a test piece 2 and a counter-electrode 3, both immersed in a solution 1. Both have a circular cross-sectional form and they have a gap 10 of uniform width between them. The counter-electrode 3, made e.g. of iridium, has a central hole opening into the gap 10 and forming a flow channel 8 through which the solution 1 can be passed into the gap 10. The counter-electrode 3 is radially surrounded by an electric insulation 11. The surface of the round, bar-like test piece 2 is also covered by an electric insulation 12, outside which there is a ring electrode 9 surrounding the test piece, the ring electrode being again surrounded by a ring-like insulation 13. The ring electrode used is preferably a ring made e.g. of gold or platinum, because these materials do not develop an insulating film on their surface. However, other electrically conductive materials can also be used. The electri- cally conductive surfaces of the test piece 2 and the ring electrode 9 adjoining the gap 10 are as precisely parallel to each other as possible and as straight and planar as possible. Moreover, immersed in the solution 1 is a reference electrode 4, which, within the limits of the measuring accuracy, is located outside the sphere of influence of the electric field prevailing in the gap 10. The reference electrode 4 and the counter-electrode 3 are used for the polarisation of the test piece 2. In the apparatus presented in Fig. 3, the potential of the ring electrode 9 and that of the test piece 2 can be adjusted separately. Consequently, by selecting a given ring electrode potential, the reaction products dissolved from the test piece can be ei- ther reduced or oxidised on the surface of the ring electrode. The magnitude of the oxidising or reducing current indicates the concentration of the substances dissolved from the test piece. By appropriately varying the ring electrode potential, the particle types dissolved can be identified. The dissolved reaction products are conveyed from the test piece 2 to the ring electrode 9 along with the solution, which is continuously pumped through the flow channel 8 in the counter-electrode 3. By varying the rate at which the solution is pumped or by varying the distance between the counter-electrode 3 and the test piece 2, i.e. by varying the gap 10, the rate of flow of the solution on the surface of the test piece under investigation can be substantially changed. In the foregoing, the invention has been de- scribed by way of example by the aid of the attached drawing, but different embodiments of the invention are possible within the framework of the inventive idea defined by the claims.

Claims: CLAIMS 1. Method for performing electrochemical measurements in a solution with a low electric conductivity, in which method the current flowing between a test piece and a counter-electrode placed in said solution is measured while the potential of the test piece is controlled in relation to a reference electrode potential remaining constant or, correspondingly, the potential of the test piece is measured in relation to a reference electrode potential remaining constant while the current flowing between the test piece and the counter-electrode is controlled, characte ri s ed in that the counter-electrode and the test piece are disposed at a substantially close distance from each other so that in practice a current can only flow across a gap between their oppositely placed counterfaces, the potential difference between the reference electrode and the test piece being measured outside the electric field between the test piece and the counter-electrode. 2. Method as defined in claim 1, char a ct e r i sed in that the potential difference is measured at a distance from the gap, said distance being at least equal to that determined by the width of the gap. 3. Method as defined in claim 1, cha ract e r i sed in that the potential difference is measured at a distance from the gap that is over 5 times, preferably over 10 times the gap width, e.g. 20 - 1000 times the gap width. 4. Method as defined in any one of claims 1 - 3, characte ri sed in that, before the measurement, the counterfaces of the test piece and the counter-electrode are brought into electric contact with each other, whereupon they are moved apart to a desired distance from each other. 5. Method as defined in claim 4, cha r acte ri s ed in that, before being brought to electric contact with each other, the counterfaces of the test piece and the counter-electrode are brought into parallelism relative to each other. 6. Method as defined in claim 5, characte ri s ed in that the parallelling is done by using an abrasive element of uniform thickness which is moved between the counterfaces. 7. Method as defined in any one of claims 1 - 6, cha racte r i s ed in that some of the solution in the measurement process is passed into the gap between the oppositely placed counterfaces of the counter-electrode and the test piece to remove the re- action products dissolved from the test piece from the surface of the test piece and to maintain a constant electrolyte composition. 8. Method as defined in claim 7, chara ct eri s ed in that the solution is passed into the gap between the counterfaces in a centric and symmetric manner. 9. Method as defined in claim 7 or 8, characte ri s ed in that en electrode electrically isolated from the test piece is used outside the test piece to determine the reaction products dissolved from the test piece. 10. Apparatus for performing electrochemical measurements in a solution with a low electric conductivity, said apparatus comprising a test piece (2), a counter-electrode (3) and a reference electrode (4) , all placed in the solution (1), in which apparatus the current flowing between the test piece and the counter-electrode is measured while the potential of the test piece is controlled in relation to a counter- electrode potential remaining constant or, correspondingly, the test piece potential is measured in relation to a constant reference electrode potential while the current flowing between the test piece and the counter-electrode is controlled, cha ra ct e ri s ed in that the counter-electrode (3) and the test piece (2) comprise counterfaces (5, 6) corresponding to each other in respect of their shape so that the distance between them is constant throughout the counterface area and that the reference electrode (4) is located substantially outside the sphere of influence of the electric field between the counter-electrode and the test piece. 11. Apparatus as defined in claim 10, chara cte ri s ed in that the counterfaces are straight planar surfaces. 12. Apparatus as defined in claim 10, character i s ed in that the counterface (6) of the counter-electrode is made of a non-clinging, electrically conductive material. 13. Apparatus as defined in claim 12, cha racte ri s ed in that the counterface (6) is made of a hard noble metal or a mixture of hard noble metals, e.g. iridium or rodium. 14. Apparatus as defined in claim 12, characte ri s ed in that the counterface is made of a metal whose oxide is a good electric conductor. 15. Apparatus as defined in claim 10, character i s ed in that the apparatus comprises a mover (7) for adjusting the distance between the counterfaces within the range of 0 - 1 mm. 16. Apparatus as defined in any one of claims 10 - 15, characte r i s ed in that the counter- electrode (3) is provided with a flow channel (8) for passing the solution (1) into the gap between the counterfaces (5, β) of the counter-electrode (3) and the test piece (2) . 17. Apparatus as defined in claim 16, cha racte ri s ed in that it comprises a ring electrode (9) with adjustable potential, placed around the test piece (2) and electrically isolated from it to allow identification of the substances dissolved from the test piece.

Inventors: BOJINOV MARTIN, LAITINEN TIMO, MAEKELAE KARI, MAEKELAE MATTI, SAARIO TIMO, SIRKIAE PEKKA, BOJINOV, MARTIN, LAITINEN, TIMO, MAEKELAE, KARI, MAEKELAE, MATTI, SAARIO, TIMO, SIRKIAE, PEKKA
Applicants: BOJINOV MARTIN, LAITINEN TIMO, MAEKELAE KARI, MAEKELAE MATTI, SAARIO TIMO, SIRKIAE PEKKA, VALTION TEKNILLINEN, BOJINOV, MARTIN, LAITINEN, TIMO, MAEKELAE, KARI, MAEKELAE, MATTI, SAARIO, TIMO, SIRKIAE, PEKKA, VALTION TEKNILLINEN TUTKIMUSKESKUS
Application year: 1998
Main classification: G01N27
IPC codes: G01N27