Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-06-08
2003-07-29
Tung, T. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S408000, C204S435000
Reexamination Certificate
active
06599409
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to potentiometric and electrochemical reference electrodes and, in particular, to liquid junction structures such as to be used in electrochemical reference electrodes for electrochemical measurements of solutions. The invention more particularly relates to reference electrodes for use where measurement or control of potential is desired such as with pH or ISE potentiometric sensors in the laboratory, in in-line processing, in the field, or in any environment where the improved precision or extended useful life of the sensor is desirable.
2. Description of the Related Art
The invention is broadly concerned with reference electrodes, such as the reference electrode portion of combination electrodes, and the reference portion of all potentiometric devices that employ a reference electrode to provide the relatively stable reference potential required in various measurements such as electroanalytical measurements, controlled potential coulometry, and polarography, and the like.
Potentiometric measurements are used widely for the determination of pH and the detection of other specific ions in a variety of settings, including chemical processes, environmental monitoring, health care and bio-processes. The accuracy of these measurements depends on the ability to measure the potential difference between a sensing electrode, whose potential varies with the analyte concentration in the measured sample solution, and a reference electrode, which ideally would maintain a constant potential. The physical interface between the reference electrode (typically the electrolyte of the reference electrode) and the sample solution is referred to as the liquid junction. The stability of the reference electrode, and consequently the accuracy of potentiometric measurements, are dependent on the constancy of the liquid junction and more particularly, the constancy of the potential across the liquid junction. However, the liquid junction and more particularly, the potential across the liquid junction are difficult to control and maintain at a constant level. Typically, it is the change in the liquid-junction potential that introduces error into the electrochemical measurement and causes the need for frequent sensor calibration.
The errors observed in currently commercially available reference electrodes include (1) kinetic or transient error; such error refers to relatively slow response between measurements, and slow ability to reach equilibrium, typically of five, ten, or fifteen minutes after exposure to extreme solutions. This response is primarily caused by entrapment of sample solution within the physical junction. Kinetic errors are typically a function of the time required to disperse this entrapped layer of sample solution and obtain a direct interface. The kinetics of this error are determined by the duration of prior immersion. The errors observed in currently commercially available reference electrodes also include (2) static error; such error typically refers to persistent offset after equilibrium is reached. Large static errors are typically caused by irreversible entrapment of sample solution deep within the physical junction structure. The errors observed in currently commercially available reference electrodes include (3) stirring error; such error refers to the shift in potential due to or associated with agitation of the sample solution. Stirring error is typically observed where there is a rate of agitation or flow of the sample. These errors exist in measuring sample solutions, but tend to be suppressed in standard buffers where accuracy is being checked Therefore, users may see no reason to disbelieve the erroneous readings obtained in non-standard environments. See D. P. Brezinski, “Kinetic, Static, and Stirring Errors of Liquid Junction Reference Electrodes”, Analyst 108 (1983) 425-442; see also U.S. Pat. No. 4,495,052. These errors are large enough to be of practical consequence. These errors often correspond to relatively large difference in hydrogen ion (H+) concentration or activity. These errors, including those errors described above, tend to bias the measurements observed on pH meters by as much as 0.5 pH unit.
In typical, currently commercially available electroanalytical measurement systems, the interface between the reference electrolyte and the sample solution is the liquid junction. The junction potential at this sample-reference interface is related to a number of factors; it is an object of every reference electrode design to minimize the effect of the factors that would cause the liquid junction potential to drift or to vary in any way over time. Various materials have been utilized in forming a liquid junction, including porous ceramic rods, porous polymer disks, wood dowls, ground glass sleeves, capillary tubes, agar gels, asbestos fiber bundle, and other porous materials or devices, and the like. These junction structures are, in general, referred to as restriction devices because their finction is to restrict the outward flow or diffusion of electroyte from the reference electrode. However, one important factor that limits the useful lifetime of a reference electrode is that junction structures typically allow the sample solution to enter the junction structure. This transport of sample solution into the junction, whether by diffusion, migration, convection or other mechanism, results in the contamination of the junction structure and a resultant undesirable variation in the liquid junction potential. Such variation typically necessitates re-calibration of the electroanalytical measurement system. If this type of the contamination of the junction continues over time, the junction structure may become fouled or clogged and develop even larger offset potentials and/or potentials that chronically drift despite repeated attempts at re-calibration. In addition, sample solution will often transport past the junction structure and reach the reference half-cell itself, potentially causing additional adverse reactions.
Currently commercially available reference electrodes, especially those used for potentiometric measurements, are typically constructed based on one of two distinct designs. Each of these designs is meant to address one principle limitation encountered when using reference electrodes for making potentiometric measurements. However, each of these designs fails to address a distinct principle limitation encountered when using reference electrodes for making potentiometric measurements.
One design category is often referred to as a flowing junction reference electrode. This design provides a stream of reference electrolyte flowing through a porous junction structure or member, in an attempt to provide a relatively uniform liquid junction potential. While this design is typically effective in providing a liquid junction potential that is more uniform over time than those of the alternate design, flowing junction reference electrodes uniformly require the use of large amounts of electrolyte over relatively short periods of time. Thus, currently commercially available flowing junction reference electrodes require frequent maintenance to replenish the supply of this electrolyte solution. Furthermore, while flowing junctions are often designed to minimize this use of electrolyte by restricting the flow of electrolyte, in such flowing junctions designs the flow velocity is reduced to a velocity that is sufficiently low that the sample solution enters the liquid junction structure, typically via mass transport (diffusion, migration, or convection). The presence of this sample solution in the junction structure causes variable junction potentials, loss of calibration, clogging of the junction structure, and, over time, failure of the reference electrode. See U.S. Pat. No. 5,360,529.
The alternative design category is referred to as a non-flowing, diffusion junction reference electrode. This design depends on the substantially constant diffusion of electrolyte solution through a minimal
Broadley Scott T.
Ragsdale Steven R.
Silverman Herbert P.
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