Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-11-26
2003-06-03
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S433000, C204S412000, C204S415000, C204S419000, C204S408000, C205S787500, C205S775000
Reexamination Certificate
active
06572748
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns a reference electrode for electrochemical measurements.
It is known to provide reference electrodes in electrochemical measuring circuits as an electrical reference point in a medium to be investigated. Reference electrodes are reversible nonpolarizable half-cells which form the most constant possible potential substantially independent of the components of the medium to be investigated.
The primary reference electrode in electrochemical measuring technology is the standard hydrogen electrode, whose potential is by definition set equal to zero at all temperatures. In practice, however, one uses almost exclusively electrodes of a second type as reference electrodes since these are easier to handle. They consist essentially of a metal which is coated with a layer consisting of a poorly soluble compound of this metal and is immersed in a solution containing a soluble compound with the anion present in the layer [F. Oehme: Ion-Selective Electrodes, Dr. Alfred Huthig Verlag, Heidelberg 1986, p. 67]. Most widespread are reference electrodes of the type Ag/AgCl, Cl, and Hg/Hg
2
Cl
2
. The standard potentials of these reference electrodes are precisely known so that the potentials measured therewith can be easily converted to the potential of the standard hydrogen electrode [K. Schwabe: Physical Chemistry. Volume 2: Electrochemistry. Akademie Verlag, Berlin 1975, p. 177]. Since the mentioned reference electrodes are universally applicable and exhibit high potential constancy, other metallic electrode materials have only slight importance in electrochemical reference electrodes compared to the mentioned metals silver and mercury.
From Lambrechts, M. et al., Biosensors: Microelectrochemical Devices, Katholieke Universiteit Leuven, Institute of Physics Publishing, Bristol, Philadelphia and New York (1992), page 165, one already recognizes a reference electrode of the type mentioned above which exhibits on a substrate of semiconductor material a layer of metallic silver and, on the latter, a layer of silver chloride. However, this reference electrode has the disadvantage that the silver chloride is photosensitive and can be reduced to metallic silver through the action of light. In order to avoid destruction of the reference electrode through the action of light, suitable protective measures are consequently required. Combined with the prior-art reference electrode is a working electrode of gold, which is connected with the reference electrode to form an electrochemical measuring circuit. Working and reference electrodes are arranged on a common substrate. A disadvantage of this electrode arrangement consists, however, in the fact that the reference and working electrodes are produced from different metallic materials, namely from silver and gold, which requires the application of additional lithographic masks in the production of the reference electrode using planar technology.
SUMMARY
The object therefore is to create a reference electrode of the above-noted type in which the previously mentioned disadvantages are avoided.
The solution to this problem lies in the fact that the reference electrode has an electrode component exhibiting metallic palladium, in the fact that this electrode component is covered by a layer which consists of or contains a palladium compound poorly soluble in aqueous media, and in the fact that a reference electrolyte containing anions of this palladium compound in dissolved form is positioned thereon.
A working electrode of palladium can then be combined with the electrode component in an electrochemical measuring circuit such that the reference and working electrodes can be produced from the same metallic starting material. Thus, for example, the electrode component of the reference electrode can be a palladium wire coated with a compound of palladium that is poorly soluble in aqueous media and immersed in an aqueous or gel-like medium containing anions of the palladium compound. An uncoated palladium wire can then be provided as the working electrode. Additionally advantageous is the fact that the reference electrode according to the present invention is highly insensitive to light. Consequently, lightproof shielding of the reference electrode can be foregone.
To be sure, the use of palladium as sensor material is already known from Lundström, I., et al., Hydrogen-sensitive MOS field effect transistor, Appl. Phys. Lett. 26 (1975), p.55-67, for a different kind of hydrogen-sensitive gas sensor. If, in a MOS field effect transistor, the metal on the gate insulator SiO
2
is replaced by a thin layer (<100 nm) of palladium, hydrogen atoms diffuse from the surroundings through this layer to the Pd/SiO
2
phase interface, through which the work function in the palladium layer is changed and a measurable shift occurs in the threshold voltage of the field effect transistor. The prior-art hydrogen-sensitive gas sensor, however, exhibits an entirely different configuration compared to the reference electrode of the present invention and does not serve in obtaining a reference potential.
It has also already been attempted to use palladium in producing industrial hydrogen electrodes as type-one electrodes for pH measurement in hydrofluoric acid-containing etching baths, these electrodes being based on the idea that palladium can take up hydrogen in larger amounts [Jasinski, R., A Palladium Hydride pH Electrode for Use in Buffered Fluoride Etch Solutions, J. Electrochem. Soc. 121 (1974), p. 1579-1584]. However, the service lives of such electrodes are short. For this reason, they have not attained any technical importance up to now [Galster, H., pH measurement, VCH Verlagsgesellschaft, Weinheim (1990), p.69; Shao, M. et al., pH measurements based on a palladium electrode, Electroanalysis 6 (1994), p. 245-249].
The palladium/hydrogen system is also specified in U.S. Pat. No. 4,404,065 as reference system in miniaturizable design, e.g., for coulometric sensors in flow processes or for amperometric sensors of thin-layer design. In spite of the advantage that this reference system is applicable up to high pressures (>4 Mpa), its applicability is substantially limited as a result of the fact that pronounced pH sensitivity occurs.
Also already known from GB-A-2 023 846 is a reference electrode with an electrode component of metallic palladium which is covered by a layer of palladium oxide standing in contact with the reference electrolyte. This reference electrode also exhibits a pronounced dependence of its electrode potential on the pH value of the reference electrolyte.
Also already described in U.S. Pat. No. 4,536,274 is a sensor for transcutaneous measurement of the CO
2
content in blood. The sensor exhibits a measuring electrode with an electrode component of metallic palladium, which is covered by a layer of palladium oxide. Via the measuring electrode, the CO
2
content is indirectly determined through measurement of the pH value. A silver/silver-chloride electrode is assigned as a reference electrode to the measuring electrode. The electric potential at the palladium/palladium-oxide electrode varies as a function of the pH value of the electrolyte standing in contact with the electrode while the potential of the reference electrode remains constant independent of the pH value.
In an especially advantageous embodiment of the present invention, the palladium electrode component is formed as a layer positioned on a substrate formed of semiconductor material, the electrode component particularly being applied to the substrate using a semiconductor-technology production process. The reference electrode can then exhibit especially small dimensions. Beyond this, in producing the reference electrode, additional structures can also be introduced into or applied onto the substrate via the semiconductor-technology production process, for example, structures for an evaluation device for processing a measurement signal, a working electrode, a sensor, and/or similar components.
Herrmann Sigrun
Igel Günter
Kaden Heiner
Oelssner Wolfram
Bell Bruce F.
Micronas GmbH
Volpe and Koenig P.C.
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