SEALED SALT BRIDGE

Abstract: The present invention is a sealed salt bridge (10) consisting of two connected, electrolyte-filled sections; an electrolyte conduit (14) that is relatively long and narrow, the distal end of which contacts a test solution (50) or another salt bridge and serves as a liquid junction (16); and the proximal end of which is connected to an electrolyte chamber (12) that is significantly wider than the electrolyte conduit, and contacts an electrode or second liquid junction (18). The conduit, by reason of its length and relatively small cross-section, serves to limit the rate that foreign ions from a test solution or another electrolyte compartment can diffuse from the liquid junction and contaminate the electrolyte chamber. The electrolyte chamber, by reason of its larger cross-section and concomitantly larger volume per unit length, serves to limit, by means of dilution, the concentration of foreign ions which eventually diffuse into the chamber.

Description: SEALED SALT BRIDGE FIELD OF THE INVENTION The present invention relates to salt bridges used in electrochemical sensors. A salt bridge is an electrolyte-filled compartment that serves to isolate two parts of an electrochemical cell from each other while maintaining an electrolytic connection between them. BACKGROUND OF THE INVENTION When used in a potentiometric cell, a salt bridge can eventually become contaminated by foreign ions or other species that diffuse from the primary liquid junction to the vicinity of the electrode or secondary liquid junction and cause a shift in potential. Previous methods of retarding the onset of a potential shift can be found in U.S. Patent Nos. 4,959, 138; 5, 147,524; and 5,346,606. In U.S. Patent No. 4,959, 138, the inner electrolyte is gelled in situ to form an ion-permeable polymer and ion permeation is further retarded by incorporation of solid silica gel particles which absorb contaminating ions. This patent describes a measuring probe in which the electrolyte is in the form of a highly viscous gel produced in situ from an ion-permeable polymer and a neutral salt suspension. Homogeneously suspended neutral salt particles in the gel cause the gel to have a turbid appearance which disappears progressively as the suspended neutral salt particles pass into solution, thus indicating at all times the state of aging of the electrolyte. The additional presence of silica gel renders the probe potential resistant to pressures in excess of 10 bars. In U.S. Patent No. 5,147,524, an ion-permeable wooden plug impedes diffusion of ions, and the plug is furthermore notched in a manner that requires the ions to follow a tortuous path. This patent describes a pH sensor that includes a unitary cylindrical semi-porous plug which is formed with a central bore and is also formed with a cavity near a first end of the plug. A pH electrode is positioned in the central bore and extends outwardly from the second end of the plug for contacting a specimen fluid, and a reference electrode is positioned in the cavity. The plug is saturated with an electrolyte to establish electrochemical conductivity between the reference electrode and specimen fluid. To prevent ions from the specimen fluid from migrating through the plug and contaminating the reference electrode, a plurality of notches are radially machined part way through the plug and filled with ion-impermeable epoxy. The notches are preferably oriented at oblique angles relative to the axis of the plug and filled with ion- impermeable epoxy. The notches are preferably oriented at oblique angles relative to the axis of the plug to form "dead end" ion traps that will immobilize and impede a substantial proportion of contaminating ions, thus prolonging the life of the electrode. U.S. Patent No. 5,346,606 is similar to U.S. Patent No. 5, 147,524 in that a wooden plug is used, however, in this case a spiral is cut out of the wood, forcing the ions to follow a spiral path. This patent describes an electrochemical sensor wherein the salt bridge is a unitary plug of semipermeable material which is saturated with an electrolyte. The plug has a spiral cut from its outer surface to the central bore or the plug. The spiral has at least one complete turn and starts at or near one end of the plug and stops at or near the other end of the plug. A layer of impermeable material is deposited in the plug to thereby form an ion impermeable spiral barrier. The disclosures of these references are hereby incorporated herein by reference. SUMMARY OF THE INVENTION The present invention is a sealed salt bridge consisting of two connected, electroyte-filled sections; an electrolyte conduit that is relatively long and narrow (as defined below), the distal end of which contacts a test solution or another salt bridge and serves as a liquid junction; and the proximal end of which is connected to an electrolyte chamber that is significantly wider (as defined below) than the electrolyte conduit, and contacts an electrode or second liquid junction. The conduit, by reason of its combination of long length and relatively small cross-section, serves to limit the rate that foreign ions from a test solution or another electrolyte compartment can diffuse from the liquid junction and contaminate the electrolyte chamber. The electrolyte chamber, by reason of its larger cross-section and concomitant larger volume per unit length, serves to limit, by means of dilution, the concentration of foreign ions which eventually diffuse into the chamber. BRIEF DESCRIPTION OF THE DRAWING The Figure is a schematic diagram of a preferred embodiment of the present invention. DETAILED DESCRIP ION OF THE PREFERRED EMBODIMENT As described above, and as shown in the prior art, a salt bridge is an electrolyte-filled compartment that serves to isolate two parts of an electrochemical cell from each other while maintaining an electrolytic connection between them. The two parts of the cell that are isolated may be electrodes or other electrolyte compartments. One purpose of a salt bridge is to prevent or retard contamination of one part of the cell which can occur by diffusion of components from a test solution or other part of the cell. In some salt bridges, hereafter referred to as open salt bridges, prevention of contamination is aided by a continuous replenishment of the salt bridge electrolyte by gravity-induced flow or other pressurization means, such replenishment serving to flush contaminants out of the system. Bulk mixing of the salt bridge electrolyte with test solutions or other electrolytes it contacts in the cell may be prevented by restriction of liquid-to- liquid contact points to small orifices or porous barriers. Hereafter, any point of contact between different liquids will be referred to as a liquid junction, whether the point of contact is a small orifice or porous barrier or not. The present invention is not an open salt bridge but instead is referred to hereafter as a sealed salt bridge. In a sealed salt bridge, transport of contaminants is retarded without the aid of replenishing flow or flushing but only by interposing electrolyte through which the contaminants must diffuse. The greater the distance the contaminants must traverse by diffusion, the greater the time required for a given level of contamination to occur. The Figure accompanying this specification depicts one preferred embodiment of a sealed salt bridge of the present invention. As illustrated therein, the sealed salt bridge 10 is filled with an electrolyte 30. The sealed salt bridge 10 further comprises the following elements; electrolyte chamber 12, electrolyte conduit 14, primary liquid junction 16 and an electrode or secondary liquid junction 18. As illustrated in the Figure, the electrolyte chamber 12 is connected at one end to an electrolyte conduit 14, which terminates at a primary liquid junction 16. The primary liquid junction 16 is the point of contact between the sealed salt bridge 10 and a test solution 50. At the opposite end of the electrolyte chamber 12 there is located an electrode or secondary liquid junction 18. In the present invention, the onset of a potential shift is retarded by means of the conduit's long, narrow diffusion path from the primary liquid junction to the electrolyte chamber and by the large volume of electrolyte in the electrolyte chamber. The rate at which contaminants reach the electrolyte chamber is inversely proportional to the conduit length and square of its diameter. At a given rate of contaminant ingress, the rate at which the contaminant concentration in the electrolyte chamber increases in inversely proportional to the volume of the electrolyte chamber. Thus, in a preferred embodiment, the present invention is directed to a sealed salt bridge consisting of two connected, electrolyte-filled sections: an electrolyte conduit that is relatively long and narrow (relative to the electrolyte chamber), the distal end of which contacts a test solution or another salt bridge and serves as a liquid junction, and the proximal end of which is connected to an electrolyte chamber that is significantly wider than the electrolyte conduit and contacts an electrode or second liquid junction. In especially preferred embodiments, the electrolyte conduit is between 1 mm and 100 cm in length and between 10 μm and 2 mm in cross-section with the length always at least five times the diameter. In especially preferred embodiments, the electrolyte chamber cross-section is between 5 and 100 times the cross-section of the conduit, and its length is between one-half and one-hundredth that of the conduit. Generally, the electrolyte employed in the salt bridge of the present invention may be selected from the group consisting of aqueous salt solutions, non-aqueous salt solutions, solid salts, or gels comprising salts. If desired, the salt bridge of the present invention can be modified, for example wherein all or parts of the conduit or chamber contain an inert filler in addition to the electrolyte. If desired, the salt of the present invention can be formed using any shapes and materials commonly employed in such devices. For example, the conduit or chamber can be provided with cross-sectional shapes that are round, oval, or multi-sided with high or low aspect ratios. The conduit or chamber can be straight, curved, coiled, spiral, serpentine, or otherwise tortuous. The materials used to form these portions of the device can be glass, ceramic, or other chemically inert, electrically insulating materials. The conduit, by reason of its length and relatively small cross-sectional area, serves to limit the rate that foreign ions from a test solution or another electrolyte compartment can diffuse from the liquid junction and contaminate the electrolyte chamber. The electrolyte chamber, by reason of its larger cross- sectional area and concomitant larger volume per unit length, likewise serves to limit, by means of dilution, the concentration of foreign ions which eventually diffuse into the chamber. The present invention will be further illustrated with reference to the following examples which aid in the understanding of the present invention, but which are not to be construed as limitations thereof. All percentages reported herein, unless otherwise specified, are percent by weight. All temperatures are expressed in degrees Celsius. SEALED SALT BRIDGE: EXAMPLE 1 Eight combination pH electrodes were built. Electrodes 1-4 had salt bridges without the large electrolyte chamber (12) of the present invention, only the narrow electrolyte conduit (14), whereas numbers 5-8 had the large chamber in conformance with description of this invention. In all electrodes the salt bridge electrolyte (30) was a solution composed of 4 M potassium iodide 0.0069 M. iodine, 0.01 M. boric acid, adjusted to pH 7.15 with potassium hydroxide, then mixed thoroughly with 35 g/L fumed silica (CAB- O-SIL M-5, Cabot Corporation, Billerica, MA) as an inert thickener. In electrodes 1-4, a platinum wire electrode was sealed directly into the proximal end of the electrolyte conduit (14) which was constructed from a glass tube with inner diameter 1.5 mm, outer diameter 2.5 mm, 300 mm long and coiled in 10 turns to with inner coil diameter 10 mm. In electrodes 5-8, the proximal end of the conduit (14) was connected to an electrolyte chamber (12) with a length of 20 mm and an inner diameter of 10 mm. In this case the electrolyte chamber contained a platinum wire electrode. All electrodes had a porous ceramic plug at the distal end of the conduit to provide a liquid junction (16) to a second salt bridge with 3 M KCI electrolyte. A second ceramic plug provided a liquid junction between the 3 M KCI and a test solution. The reference half-cell was constructed integrally with a glass pH bulb half-cell as a complete combination pH probe. The pH half-cell had a platinum wire internal electrode and was filled with the same solution as the inner salt bridge except without the inert filler. When new, all probes had potentials in pH 7 puffer solution of approximately 0 mV, since the pH of the solution in the pH bulb was nearly 7, and the potentials at each platinum wire in each half-cell were approximately equal. In other words, the cell was symmetrical, and the reference half-cell potentials and glass membrane potentials cancelled out. Referring to Tables 1 and 2 , it can be seen that the potentials of electrodes 1-4 changed in a negative direction an average of nearly -23 mV in approximately four months of storage in pH 7 buffer, while under the same conditions, electrodes 5-8 changed less than 1 mV. The change in electrodes 1 -4 was due to diffusion of 3 M KCI solution to the platinum wire at the proximal end of the electrolyte conduit. In electrodes 5-8, the effect of KCI diffusion was rendered undetectable because the large electrolyte chamber diluted the incoming KCI. Table 1. Electrodes 1-4 1 2 3 4 Avg 17-Oct -5.0 0.3 -7.8 -5.3 -4.5 23-Oct -5.3 -4.1 -7.6 -7.8 -6.2 7-Nov -13.9 - 11.0 -9.6 - 12.2 -11.7 19-Feb -28.0 -29.3 -24.5 -27.2 -27.3 Change -23.0 -29.6 - 16.7 -21.9 -22.8 Table 2. Electrodes 5-8 1 2 3 4 Avg 17-Mar 1.4 -4.3 0.4 1.6 -0.2 23-Mar 2.0 -4.0 0.4 1.2 -0.1 6-Apr 2.6 -4.9 0.4 0.9 -0.3 17-Jul 4.0 -3.7 - 1.6 -0.9 -0.6 Change 2.6 0.6 -2.0 -2.5 -0.3 The present invention has been described in detail, including the preferred embodiments thereof. However, it will be appreciated that those skilled in the art, upon consideration of the present disclosure, may make modifications and/ or improvements on this invention and still be within the scope and spirit of this invention as set forth in the following claims.

Claims: HAT IS CLAIMED IS: 1. A sealed salt bridge consisting of two connected, electrolyte-filled sections: an electrolyte conduit that is relatively long and narrow, the distal end of which contacts a test solution or another salt bridge and serves as a liquid junction, and the proximal end of which is connected to an electrolyte chamber that is significantly wider than the electrolyte conduit and contacts an electrode or second liquid junction, wherein the conduit length is at least five times the cross-sectional diameter; wherein, the conduit, by reason of its long length and relatively small cross-section, serves to limit the rate that foreign ions from a test solution or another electrolyte compartment can diffuse from the liquid junction and contaminate the electrolyte chamber; and wherein the electrolyte chamber, by reason of its larger cross-section dimension and concomitantly larger volume per unit length, serves to limit by means of dilution the concentration of foreign ions which eventually diffuse into the chamber. 2. A salt bridge as in Claim 1, where the electrolyte conduit is between about 1 mm and about 100 cm in length and between about 10 μm and about 3 mm in cross-section diameter. 3. A salt bridge as in Claim 1, where the electrolyte chamber cross- section is between from about 3 to about 100 times the cross-section of the conduit, and its length is between from about one-half to about one-hundredth that of the conduit. 4. A salt bridge as in Claim 1, where the electrolyte is an aqueous solution, a non-aqueous solution, a solid, or a gel. 5. A salt bridge as in Claim 1 , where all or parts of the conduit or chamber contain inert filler in addition to the electrolyte. 6. A salt bridge as in Claim 1, where the conduit or chamber have cross-sections that are round, oval, or multi-sided with high or low aspect ratios. 7. A salt bridge as in Claim 1, where the conduit or chamber are straight, curved, coiled, spiral, serpentine, or otherwise tortuous.

Inventors: HIRSHBERG MOSHE, WEST STEPHEN J, HIRSHBERG, MOSHE, WEST, STEPHEN, J.
Applicants: ORION RESEARCH, ORION RESEARCH, INC.
Application year: 2000
Main classification: G01N27
IPC codes: G01N27