Ion-selective electrode

Details Ion-selective electrode Ion-selective electrode

2002-03-15

2004-09-21

Noguerola, Alex (Department: 1753)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S420000

Reexamination Certificate

active

06793787

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to ion-selective electrodes and comprises an improved pH electrode.
2. Background Information
Various types of ion-selective electrodes are commonly used to sense ions in solution. One such type is the pH electrode, which senses the concentration of hydrogen ions in solution. An especially important example of such electrodes is the Ross™ pH electrode, manufactured and sold by Thermo Orion, Inc.
Among the key characteristics of ion-selective electrodes are their sensitivity, accuracy and reliability. Sensitivity refers to the ability of the electrode to detect low levels of ion in solution. Accuracy refers to the correctness of the resultant reading. Reliability refers to the ability of the electrode to maintain its characteristics over extended periods of time.
Ion-selective electrodes (commonly referred to simply as “ISEs”) typically contain chemical solutions of various types (referred to hereinafter as “electrolyte solutions” or “electrolyte”), both in order to provide specific characteristics to the electrode, as well as to enable electrical contact with the solution containing the ion to be measured (“the test solution”). As a result of such contact, however, the electrolyte solutions become contaminated in the course of time, and the reliability of the ISE degenerates.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an improved ion-selective electrode.
Further, it is an object of the invention to provide an improved ion-selective electrode of the pH type.
Still a further object of the invention is to provide an improved ion-selective electrode that is characterized by extended reliability.
Still a further object of the invention is to provide improved methods for reliably manufacturing ion-selective electrodes.
In accordance with the present invention, an improved ion-selective electrode is characterized by a substantially increased reliability for a given sensitivity. In the preferred embodiment of the invention which is described herein, the invention is spccifically described as applied to a combination electrode that is manufactured and sold by Thermo Orion Corporation as a “Ross™” pH electrode, although it will be understood that the invention is not so limited and one or more aspects thereof can be expected to be applicable generally to ion-specific electrodes.
In the Ross™ pH electrode, a reference half cell body in the form of a narrow helical tube (e.g., from 1 to 2 mm. internal diameter) and filled with a first internal electrolyte solution (e.g., an iodide-triiodide solution) is contained within a larger outer body filled with an internal filling solution that provides a conductive path to the test solution (e.g., potassium chloride). The reference half cell body has upper and lower generally straight segments, and an intermediate helically-formed section. A first conductive lead (e.g., of platinum or the like) extends along the body on the interior thereof, and provides a first electrode for connection into a measuring circuit. A measuring half cell having a body in the form of a generally straight tube is also positioned within the outer enlarged body, and provides a path that connects to a bulbous tip of an ion-selective glass; the glass is sensitive to H
+
ion concentration and enables measurement of the pH of the test solution. A second conductive lead extends along the measuring half cell body on the interior thereof, and provides a second electrode for connection into a measuring circuit.
The pH electrode so far described has proven to be very stable over long periods of use. However, we have found that we have been able to further increase its stability for a given sensitivity, and it is to that enhancement that this invention is directed. Additionally, we have also discovered improved methods of forming conductive interfaces in ion-selective electrodes, and these also are described herein.
In particular, in accordance with the present invention, unlike prior Ross™ pH electrodes, the conductive lead of the reference half cell is removed from the vicinity of the helical coil to a position adjacent the remote end of the cell (i.e., the upper end, distant from the tip which is immersed in the test solution). In particular, the conductive lead itself is looped, preferably coiled, and is positioned within an enlarged chamber at the remote end of the half cell; the chamber is at a location above the immersion level of the electrode in the measuring fluid.
Preferably, the lead coil is itself mounted on, and preferably wound around, a post within the chamber. This construction provides a number of advantages. First, it stably positions the conductive lead of the reference electrode at a location that is most remote from the measuring tip at the sensing end of the electrode, and thus better isolates it from the contaminants that travel backwardly from the sensing end of the electrode to the reference electrode lead. The isolation arises both from the increased path length between the frontal terminus of the inner electrolyte path through which contaiminantions may flow backwardly into the reference half cell body and thence back up to the reference lead itself, and also from the thermal isolation arising from placing the reference lead in a chamber that is both of substantially larger cross-section than the reference body (and thus of substantially greater thermal inertia) and that is also maintained at a location removed from contact with that portion of the electrode that is immersed. Thus, thermal transfer from the test solution to the reference lead is minimized.
Additionally, the construction enables a substantial length of conductive wire to be placed in contact with the reference electrolyte, while yet maintaining an increased separation from the sensing tip. This contributes to an enhanced lifetime for the overall electrode. For example, we have found that by providing a reference lead of length of about 25 mm. of 0.25 diameter and formed in a coil or an otherwise folded configuration, the lifetime of the electrode is significantly increased as compared to a construction in which the reference lead extends essentially axially along the conductive path. Despite its length, however, the entire surface of the electrode is still positioned more remotely from the measuring tip, thus providing the desirable electrical and thermal benefits. In effect, the construction enables the use of an increased length of conductive lead in the reference electrode without concurrently increasing the susceptibility of the electrode to interfering ions from the solution.
As indicated above, the reference half cell construction is accompanied by enlargement of the upper end of the reference body into a chamber of substantially greater cross section, and thus volume per unit length, than the main body of the tube to which it connects For example, in one implementation we have constructed a chamber having an internal diameter of approximately 6-8 millimeters, and a length of approximately 35-40 millimeters, resulting in a volume of 200-300 mm
3
. The lower end of the chamber communicated with the remainder of the reference body in the form of a narrow tube of approximately 1-2 millimeters, and providing a total volume at least several times less than that of the chamber. The reference lead was sealed to the upper end of the chamber. The chamber was further thermally isolated from the sensing tip by enclosing it in a separate housing thermally insulated from the main body of the electrode. In the particular implementation described here, the thermal isolation was provided by gaskets of low thermal conductivity interposed between the main body and the chamber housing. This construction was found to significantly extend the lifetime of electrodes.
A further aspect of the present invention resides in the structure of the interface end of the reference electrolyte body. The interface end is that end at which contact with the internal filling solution is made.

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