Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing
Reexamination Certificate
2001-06-19
2003-06-17
Tung, T. (Department: 1753)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
C204S416000, C204S419000, C204S433000, C204S435000, C205S787500
Reexamination Certificate
active
06579440
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention generally relates to electrochemical sensors and more particularly to reference half-cells for use in pH, oxidation/reduction potential, and selective ion activity measurements.
(2) Background Information
Electrochemical potential measurements are commonly used to determine solution pH, other selective ion activities, ratios of oxidation and reduction activities, as well as other solution characteristics. A pH/ion selective electrode/oxidation reduction potential meter (hereafter referred to as a pH/ISE/ORP meter) is typically a modified voltmeter that measures the electrochemical potential between a reference half-cell (of known potential) and a measuring half-cell. These half-cells, in combination, form a cell, the electromotive force (emf) of which is equal to the algebraic sum of the potentials of the two half-cells. The meter is used to measure the total voltage across the two half-cells. The potential of the measuring half-cell is then determined by subtracting the known potential of the reference half-cell from the total voltage value.
The measuring half-cell typically includes an ion selective material such as glass. The potential across the ion selective material is well known by those of ordinary skill in the art to vary in a manner that may generally be described by the Nernst Equation, which expresses the electrochemical potential as a logarithmic function of ion activity (thermodynamically corrected concentration). A pH meter is one example of a pH/ISE/ORP meter wherein the activity of hydrogen ions is measured. pH is defined as the negative logarithm of the hydrogen ion activity and is typically proportional to the measured electrochemical potential.
FIG. 1
is a schematic of a typical, prior art arrangement
20
for measuring electrochemical potential. Arrangement
20
typically includes a measuring half-cell
30
and a reference half-cell
40
immersed in a process solution
24
and connected to an electrometer
50
by connectors
38
and
48
, respectively. Measuring half-cell
30
and reference half-cell
40
are often referred to commercially (as well as in the vernacular) as measuring electrodes and reference electrodes, respectively. Electrometer
50
functions similarly to a standard voltage meter in that it measures a D.C. voltage (electrochemical potential) between measuring half-cell
30
and reference half-cell
40
. Measuring half-cell
30
typically includes a half-cell electrode
36
immersed in a half-cell electrolyte
32
, which is typically a standard solution (e.g., in pH measurements). For some applications, such as pH measurement, measuring half-cell
30
also includes an ion selective material
34
. Alternately, when measuring ORP the half-cell electrode
36
is immersed directly into the process solution
24
.
The purpose of the reference half-cell
40
is generally to provide a stable, constant (known) potential against which the measuring half-cell may be compared. Reference half-cell
40
typically includes a half-cell electrode
46
immersed in a half-cell electrolyte
42
(FIG.
1
). As used herein, the term “half-cell electrode” shall refer to the solid-phase, electron-conducting material in contact with the half-cell electrolyte, at which contact the oxidation-reduction reaction occurs that establishes an electrochemical potential. Half-cell electrolyte
42
(
FIG. 1
) is hereafter referred to as a reference electrolyte. Electrochemical contact between the reference electrolyte
42
(
FIG. 1
) and the process solution is typically established through a reference junction
44
, which often includes a porous ceramic plug or the like (e.g., porous Teflon®, porous kynar®, or wood) for achieving restricted fluid contact. Ideally, the reference junction
44
is sufficiently porous to allow a low resistance contact (which is important for accurate potential measurement) but not so porous that the solutions become mutually contaminated.
However, for many applications, particularly those having a relatively high ion concentration and/or those at a relatively high temperature, ion contamination is a significant difficulty. Both contamination of the reference electrolyte with process solution components and contamination of the process solution with reference electrolyte components are relatively common. Further, clogging of the reference junction with a variety of contaminants (e.g., process solution salts or silver chloride from the reference electrolyte) is also a relatively common difficulty with typical commercial reference electrodes. Both ion contamination and reference junction clogging may lead to unstable and/or erroneous measurements and therefore tend to be undesirable and problematic.
Turning now to the known art, there are several attempts to overcome the above stated difficulties. For example, U.S. Pat. No. 4,495,052 to Brezinski and U.S. Pat. No. 4,495,053 to Souza (hereafter referred to as the '052 and '053 patents, respectively) disclose reference electrodes having a removable and replaceable reference junction, the reference junction typically consisting of a ceramic plug within a glass tube. The '052 and '053 patents, while possibly providing for improved convenience, do not provide an ion-barrier and therefore do not tend to reduce ion contamination. The reference junctions disclosed therein may also be fragile and prone to breakage during removal and insertion.
Brezinski, in U.S. Pat. No. 4,401,548 (hereafter referred to as the '548 patent) and in Analytica Chemica Acta, 134 (1982) 247-262, discloses a double junction type reference electrode having an internal diffusion barrier between the reference electrolyte and the junction electrolyte. The diffusion barrier consists of a porous glass rod (e.g., Vycor®, manufactured by Corning Glass Works, Corning, N.Y.). Because these reference junctions are porous, their effectiveness tends to be limited, especially at elevated temperatures where the diffusion velocity of ions increases greatly. Further, these reference junctions are disposed in an internal cavity within the reference electrode, which tends to substantially complicate replacement. As such, failure of the reference junction may result in the need to replace the entire reference electrode.
Nipkow, et al., in U.S. Pat. No. 5,470,453 (hereafter referred to as the '453 patent) disclose a double junction type silver/silver chloride reference electrode that features a silver ion reducing agent acting as a silver ion-barrier layer to reduce contamination of the junction electrolyte and reference junction with silver ions and/or silver chloride precipitate. As described above with respect to the '548 patent, these reference junctions are disposed in an internal cavity within the reference electrode and, therefore, tend to be difficult to replace. Further, the reference junction disclosed in the '453 patent is not configured to eliminate migration of process solution components (e.g., ions or other mobile species) into the reference electrolyte. Contamination of the reference electrolyte may therefore be problematic in some applications.
Therefore, there exists a need for an improved reference electrode and/or reference electrode junction for use in pH, selective ion activity, oxidation-reduction potential (ORP), and other electrochemical potential measurements that overcomes the aforementioned difficulties.
SUMMARY OF THE INVENTION
In one aspect, the present invention includes a modular reference junction for a reference half-cell. The reference junction includes a body including a reference electrolyte interface portion, a process solution interface portion, and an internal cavity disposed therebetween. The reference junction further includes an ion-barrier membrane disposed at said reference electrolyte interface portion and is sized and shaped for removable receipt within a receptacle of a reference half-cell housing. In one variation of this aspect, the reference junction includes an ion-barrier membrane
Bower Michael M.
Connelly John P.
Barron, Esq. David
Invensys Systems Inc.
Sampson, Esq. Richard L.
Tung T.
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