BACKGROUND OF THE INVENTION
Electrochemical cells form the basis of a variety of analytical sensors. Electrochemical cells generally have two or more electrodes of the cell and are coupled to an analyzer that measures an electrical characteristic of the cell to infer a property of a sample within, or otherwise coupled to, the cell. Many electrochemical cells include a measurement electrode and a reference electrode. The reference electrode will generally include a chamber that houses a reference electrode fill solution. A junction, of some sort, allows electrochemical interaction between a sample solution and the fill solution. Electrochemical cells can be used for oxidation/reduction potential (ORP) sensors, pH sensors, or other suitable sensors.
One type of junction used with reference electrodes of electrochemical cells is known as a liquid junction. A liquid junction uses a relatively small passageway that is in fluidic communication with both the sample solution and the reference fill solution. In order to achieve a stable potential at the liquid junction, it is generally preferred that at least some flow of fill solution through the passageway into the sample solution be induced. With a “flowing” liquid junction, the reference electrode fill solution constantly flows through the liquid junction into the sample solution. However, in order for the flowing liquid junction to work properly, the fill solution must be free to flow through the junction pore(s) to overcome the diffusion of cations and anions in the fill solution. If the passageway becomes blocked, ions in the fill solution will diffuse differently. The electrical resistance of the junction will increase. This can cause electrochemical readings to drift and become noisy. Severe fouling can sometimes completely block the liquid junction, breaking the electrical connection between the electrode and the sample, and making the electrode unusable. Plugging can come from various sources, including: suspended solids in a sample or solids resulting from a chemical reaction involving the fill solution. Plugged junctions are difficult to clean.
Therefore, there is a need to provide electrochemical cells with the longevity advantage of liquid junctions, but without such cells being as susceptible to fouling induced by obstructions within the liquid junction. Such an electrochemical cell would enjoy the advantages of longevity, stable reference potential, and relatively low maintenance requirements.
SUMMARY OF THE INVENTION
An electrochemical cell includes a measuring electrode and a reference electrode. The reference electrode includes a flowing liquid junction between a reference fill fluid and a sample. The flowing liquid junction is configured to inhibit particles from blocking or obstructing it. In one aspect a particle filter is provided before the flowing liquid junction to prevent particles from the fill fluid from entering the flowing liquid junction. In another aspect a particle filter is provided after the flowing liquid junction to prevent particles in the sample from entering the flowing liquid junction. In another aspect the flowing liquid junction has a diameter that generally increases from an aperture proximate the fill fluid to an aperture proximate the sample. One example of such a configuration is a tapered flowing liquid junction.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view of an electrochemical cell environment, with which embodiments of the present invention are particularly useful.
FIG. 2 is a diagrammatic view of a flowing liquid junction of a reference electrode of an electrochemical cell in accordance with the prior art.
FIG. 3 is a diagrammatic view of the junction illustrated in FIG. 2 further illustrating an obstruction to the flow passageway.
FIG. 4 is a diagrammatic view of a flowing liquid junction of a reference electrode of an electrochemical cell in accordance with an embodiment of the present invention.
FIG. 5 is a diagrammatic view of a flowing liquid junction of a reference electrode of an electrochemical in accordance with another embodiment of the present invention.
FIG. 6 is a diagrammatic view of a flowing liquid junction of a reference electrode of an electrochemical cell in accordance with another embodiment of the present invention.
FIG. 7 is a diagrammatic view of a flowing liquid junction of a reference electrode of an electrochemical cell in accordance with yet another embodiment of the present invention.
FIG. 8 is a diagrammatic view of a flowing liquid junction of a reference electrode of an electrochemical cell in accordance with yet another embodiment of the present invention.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
FIG. 1 is a diagrammatic view of an electrochemical cell being used to analyze pH. Those skilled in the art will recognize that embodiments of the present invention can be practiced with various types of chemical analysis, and embodiments of the present invention are certainty not limited to the illustrative pH analyzer application illustrated in FIG. 1. Meter 10 is coupled to measuring electrode 12, working electrode 14 and temperature sensor 16. Each of measurement electrode 12, reference electrode 14 and temperature sensor 16 are disposed within sample 18. In most applications, pH is simply determined by measuring the voltage of electrochemical cell 20. The voltage of cell 20 is generally directly proportional to the pH of sample liquid 18. Meter 10 measures the voltage and uses a temperature-dependent factor to convert the voltage to pH. Because cell 20 has a relatively high internal resistance, meter 10 has a very high input impedance. Although FIG. 1 illustrates separate measuring and reference electrodes 12, 14, such electrodes are sometimes combined, along with temperature sensor 16 within a single body. Such sensors are called combination electrodes.
The voltage of cell 20 is the algebraic sum of the potentials of measuring electrode 12, reference electrode 14, and the liquid junction. The potential of measuring electrode 12 depends primarily on the pH of sample 18. The potential of reference electrode 14 is unaffected by pH, so it provides a stable reference voltage. The liquid junction potential depends in a complex way on the identity and concentration of the ions in sample liquid 18. The liquid junction potential is always present, but is usually small and relatively constant if cell 20 is properly designed. All three potentials depend on temperature.
Measuring electrode 12 is generally application-specific. In an application such as that illustrated in FIG. 1, where pH is measured, measuring electrode will include a thin piece of pH-sensitive glass, which is blown onto the end of a length of glass tubing. Sealed inside of electrode 12 is a solution of potassium chloride buffered at pH 7. A piece of silver wire plated with silver chloride contacts the solution. However, where electrochemical cell 20 is used for other applications, measuring electrode 12 may take any suitable form.
Reference electrode 14 includes a piece of silver wire plated with silver chloride in contact with a concentrated solution of potassium chloride held in a glass or plastic tube. However, the selection of material for the reference electrode wire as well as the fill solution can also vary depending on the application. Reference electrode 14 includes flowing liquid junction 22 which fluidically couples fill solution 24 disposed within reference electrode 14 to sample solution 18. The use of flowing liquid junction 22 allows cations and anions to pass evenly allowing reference electrode 14 to have a fairly stable junction potential.
FIG. 2 is a diagrammatic view of a flowing liquid junction of a reference electrode of an electrochemical cell in accordance with the prior art. FIG. 2 illustrates a portion of an endcap 26 having a narrow flow passageway 28 that fluidically communicates between an interior of the reference electrode containing fill solution 24 and an exterior of the reference electrode exposed to sample 18. The flowing liquid junction is in the form of a relatively constant diameter bore, such as provided by a capillary. It is also known to provide a plurality of such passageways using a porous material having a pore diameter that allows for fluidic communication between the reference fill solution 24 and the sample solution 18. However, in both such circumstances, the passageway itself is believed to have a relatively constant or irregular diameter hole(s) that extends from fill solution 24 to sample solution 18.
FIG. 3 is a diagrammatic view illustrating the flowing liquid junction of FIG. 2 in a clogged, or fouled condition. Particles, or solids 30 have aggregated within flow passageway 28 and obstructed flow passageway 28 such that fluid communication therethrough is no longer possible. Plugging can generally come from at least two sources. Suspended solids in sample 18 can flow into passageway 28 and clog it. Additionally, plugging can be generated by solids resulting from a chemical reaction involving reference fill solution 24. One way that fouling has been addressed in the past is to provide a flowing liquid junction with a relatively large surface area. However, to achieve a relatively long service life of fill solution 24, the inner diameter of the flowing junction should generally be in the range of micrometers, or smaller. This design goal increases the clogging problems associated with flowing liquid junctions of the prior art.
FIG. 4 is a diagrammatic view of a flowing liquid junction of a reference electrode of an electrochemical cell in accordance with an embodiment of the present invention. FIG. 4 illustrates a flowing liquid junction having an internal passageway 28 that has a relatively constant internal diameter as it fluidically couples reference fill solution 24 to sample solution 18. However, disposed within the reference electrode, proximate an internal surface of endcap 26 of the electrode is filter 32. Filter 32 is a particle filter that is placed in the flow path before passageway 28. The flow path is illustrated diagrammatically at reference numeral 34. Filter 32 can be any suitable porous material as long as the pore size of filter 32 is smaller than the inner diameter of passageway 28. In a preferred embodiment, filter 32 is a sub-micron porous polymer particle filter sheet such as that provided under the trade designation Supor® supplied by Pall. Any particle that passes through filter 32 will not be sized to clog, or otherwise obstruct passageway 28, on its own.
FIG. 5 is a diagrammatic view of a flowing liquid junction of a reference electrode of an electrochemical cell in accordance with another embodiment of the present invention. FIG. 5 bears many similarities to FIG. 4, and reference numerals are numbered similarly. In the embodiment illustrated in FIG. 5, a second porous particle filter 36 is disposed proximate an external surface of endcap 26. Filter 36 is in contact with sample solution 18 and with passageway 28. Filter 36 may be identical to filter 32, or may be of any other suitable design, as long as it also has a pore size that is smaller than the inner diameter of passageway 28. Again, it is preferred that filter 36 be formed of a sub-micron porous polymer sheet material such as Supor®.
FIG. 6 is a diagrammatic view of a flowing liquid junction of a reference electrode of an electrochemical cell in accordance with another embodiment of the present invention. Endcap 126 is similar to endcap 26 (shown in FIG. 1) but is given a different reference numeral because its physical configuration proximate the flowing liquid junction is different. In particular, an internal surface 128 of endcap 126 has an aperture 130 of a first diameter. Additionally, endcap 126 has an external surface 132 having an aperture 134 with a diameter that is larger than aperture 130. Preferably, the internal diameter varies linearly between aperture 130 and aperture 134 to create a tapered flowing liquid junction. However, other configurations can be practiced in accordance with embodiments of the present invention as long as the diameter of the internal passageway at any point beyond aperture 130 is always larger than aperture 130. In a preferred embodiment, however, the flowing liquid junction is a tapered flowing liquid junction. Since the wall of the junction is tapered, aperture 134 is larger than aperture 130. Accordingly, if any particle manages to enter through aperture 130, the tapered wall makes it relatively easy for the flowing junction to essentially perform self-cleaning.
FIG. 7 is a diagrammatic view of flowing liquid junction of a reference electrode of an electrochemical cell in accordance with another embodiment of the present invention. The embodiment illustrated in FIG. 7 bears many similarities to that illustrated in FIG. 6, and like components are numbered similarly. FIG. 7 illustrates the flowing liquid junction also including a particle filter 32 disposed proximate internal surface 128. Filter 32 can be identical to that described above with respect to FIGS. 4 and 5. Filter 32 has a pore size that is selected to be smaller than the diameter of aperture 130. Thus, filter 32 helps to ensure that no particles that are larger than aperture 130 even reach aperture 130. However, if such as a particle does manage to get through filter 32, the tapered design of the flowing liquid junction helps ensure that the particle is swept cleanly through the passageway into the sample 18.
FIG. 8 is a diagrammatic view of a flowing liquid junction of a reference electrode of an electrochemical cell in accordance with yet another embodiment of the present invention. The embodiment illustrated in FIG. 8 bears many similarities to that described with respect to FIG. 7, and like components are numbered similarly.
The primary difference between the embodiment of FIG. 8 and that of FIG. 7 is that the embodiment illustrated in FIG. 8 utilizes yet another filter 136 disposed proximate surface 132. Preferably, filter 136 is a sub-micron porous polymeric sheet material, such as Supor®, that has a maximum pore size that is smaller than the diameter of aperture 130.
Embodiments of the present invention are believed to provide the benefits of the extended operating lifetimes associated with reference electrodes that use flowing liquid junctions, while also providing the stability of the reference potential enjoyed by such flowing liquid junctions. Moreover, embodiments of the present invention are less susceptible to clogging, or similar forms of fouling than liquid junctions of the prior art.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
Patent Application
20060278529
