Abstract:
A reference half-cell and method includes a reference electrode and a reference electrolyte disposed in mutual electrolytic contact, and a reference junction including a porous member configured to provide controlled flow of the reference electrolyte therein to form a primary electrical pathway extending through the member. A secondary electrical pathway is disposed electrically in parallel with the primary electrical pathway.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (1) Field of the Invention  
         [0002]     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.  
         [0003]     (2) Background Information  
         [0004]     Throughout this application, various publications, patents and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure.  
         [0005]     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.  
         [0006]     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.  
         [0007]      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 .  
         [0008]     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” refers 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® (polytetrafluoroethylene, DuPont), porous KYNAR® (polyvinyldifluoride, Elf Atochem, N.A.), 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.  
         [0009]     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.  
         [0010]     Turning now to the known art, there have been 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 &#39;052 and &#39;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 &#39;052 and &#39;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.  
         [0011]     Nipkow, et al., in U.S. Pat. No. 5,470,453 (hereafter referred to as the &#39;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. This reference junction 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.  
         [0012]     To address this problem, the ceramic rod or plug of many conventional reference junctions is generally provided with a relatively small diameter and pore size to minimize electrolyte flow out of the sensor and into the process solution. However, an unintended effect of this approach has been a tendency for resistance across the reference junction to rise to undesirably high levels in some applications.  
         [0013]     U.S. Pat. No. 6,495,012 (the &#39;012 patent) entitled Sensor for Electrometric Measurement, assigned to The Foxboro Company, discloses an electrode assembly which uses a spring loaded piston to pressurize an electrolyte reservoir and generate electrolyte flow. This flow is taught to prevent backflow of contaminants from the process solution into the electrolyte. While this approach may be effective for many applications, it tends to be relatively complex and costly to manufacture. The &#39;012 patent is fully incorporated herein by reference.  
         [0014]     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 addresses the aforementioned difficulties.  
       SUMMARY  
       [0015]     In accordance with one aspect of the invention, a reference half-cell includes a reference electrode and a reference electrolyte disposed in electrolytic contact therewith. A reference junction is also provided, which includes a porous member configured to provide controlled flow of the reference electrolyte therein to form a primary electrical pathway extending through the member. A secondary electrical pathway is disposed electrically in parallel with the primary electrical pathway.  
         [0016]     In variations of the foregoing, the secondary electrical pathway is independent of any fluid flow through the porous member. Such flow-independence may be provided by provision of a solid state material in the form of an electrically conductive polymeric sleeve disposed concentrically with a porous member in the form of a ceramic plug. Alternatively, a solid state material such as hydrophilic electrolyte-laden gel may be mechanically captured within the pores of the porous member. As a further alternative, the solid state material may comprise a metallic material, carbon based material, and/or dehydrated electrolyte particulate mixed with a ceramic material which is then formed into the porous member to chemically and/or mechanically bond the material of the secondary electrical pathway to the porous member.  
         [0017]     Another aspect of the present invention includes a method for measuring electrochemical potential. This method includes the steps of providing the aforementioned reference half-cell, providing a measuring half-cell, inserting the reference half-cell and the measuring half-cell into a liquid, and electrically connecting the reference half-cell and the measuring half-cell to a meter. The meter is then used to generate a total voltage value, from which is subtracted the potential of the reference half-cell.  
         [0018]     A further aspect of the invention includes a method of fabricating a reference half-cell, which includes providing a reference electrode, disposing a reference electrolyte in electrolytic contact with the reference electrode, and providing a reference junction which includes a porous member configured to provide controlled flow of the reference electrolyte therein to form a primary electrical pathway extending through the member. A secondary electrical pathway is disposed electrically in parallel with the primary electrical pathway. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a schematic representation of a typical electrochemical potential measurement system of the prior art;  
         [0020]      FIG. 2  is a schematic representation of sensor assembly embodying aspects of the present invention;  
         [0021]      FIGS. 3-6  are schematic, not-to-scale representations of alternate embodiments of a portion of the assembly of  FIG. 2 ; and  
         [0022]      FIG. 7  is a graphical representation of test results of a prior art (control) device; and  
         [0023]      FIGS. 8-10  are graphical representations of test results of embodiments of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0024]     Commercially available DolpHin PH10™ (Foxboro Company) pH sensors utilize a porous ceramic rod (e.g., plug) to form a reference junction. A KYNAR® sleeve is melted over the porous ceramic rod to facilitate assembly by press fitting it into a bore within the sensor body. As with the art discussed above, the purpose of this junction is to provide a low resistance, liquid ionic contact between the internal reference electrolyte and the external process solution.  
         [0025]     The ceramic rod is provided with a relatively small diameter and pore size to minimize electrolyte flow out of the sensor (and into the process solution). However, in some situations, resistance of the reference junction has been found to undesirably increase (e.g., to 100K-Ohms or more) within a relatively short period of time, often within minutes. Resistance of this magnitude has been found to be often associated with fouling of the reference junction, e.g., with the porous rod becoming coated or otherwise clogged. This level of resistance thus generally triggers alarms in the relatively sophisticated electronic diagnostic systems associated with many process variable transmitters, such as those of the type sold by Invensys Systems, Inc. (Foxboro, Mass.). Moreover, resistances of this magnitude also tend to be beyond the practical limits of older, legacy equipment, which typically do not have high impedance inputs for reference electrodes.  
         [0026]     An aspect of the present invention was the recognition by the present inventors that the rapid increase in resistance often occurred when the reference junction was used in applications involving relatively low ionic strength process solutions under pressure higher than that of the internal reference electrolyte. While not wishing to be tied to any particular theory, it is suspected that the rapid increase in resistance occurs when the low ionic strength process fluid moves, e.g., due to the pressure gradient, into the pores of the porous rod, diluting and/or replacing the reference electrolyte therein. Since the process fluid is of low ionic strength, it is also of relatively high electrical resistance. Moreover, the equivalent cross-sectional area of the ceramic rod is relatively small, e.g., as small as 0.050 inches in diameter. This small cross-sectional area, in combination with the high resistivity of the process fluid disposed within the pores of the porous rod, is thus believed to be at least partially responsible for the relatively high measured resistance of the reference junction.  
         [0027]     This suspected cause of high resistance has been confirmed by tests in which a flow constrictor, such as a paper wick, was inserted into the reference electrode. The constrictor was found to slow the rate of resistance increase ostensibly by slowing the movement of process fluid into the porous rod. However, though the rate of increase was demonstrably slowed, the resistance was still found to reach the unacceptable 100 K-Ohm level in an unacceptably short period of time (e.g., in approximately one hour). It was suspected that this latter increase was due to a different mechanism, i.e., due to a reduction in diffusion of the KCL reference electrolyte from the reservoir into the process solution via the porous rod. It is therefore believed that the paper wick, by effectively slowing the movement of liquid into and out of the porous rod, also inhibited diffusion to an extent by which it became a significant factor affecting KCL concentration, and thus resistance, of the junction. In this regard, those skilled in the art will recognize that a reduction in diffusion rate generally corresponds to an increase in resistance. Thus, while the constrictor advantageously helped to slow the increase in resistance by slowing movement of process fluid into the plug, it also appeared to disadvantageously slow the rate of diffusion of KCl into the plug, which had the opposite effect of increasing the resistance thereof.  
         [0028]     Embodiments of the present invention advantageously maintain the total resistance of the reference junction assembly at an acceptable level, while maintaining the relatively small cross-sectional area of the porous junction (referred to variously herein as a rod or plug) to help prevent excessive mass flow therethrough. These embodiments address the aforementioned drawbacks by using a secondary, flow-independent (e.g., solid state) electrical pathway, with or without a flow constrictor (e.g., wick), to maintain conductivity at levels sufficient for electrode operation. Examples of these flow-independent pathways include one or more of: a conductive holder for the porous plug; a porous plug impregnated with electrolyte-laden gel; and a porous electrically conductive plug.  
         [0029]     Referring to  FIG. 2 , one embodiment of the invention includes a low resistance reference junction  100  incorporated within a sensor  120 . In this embodiment, sensor  120  includes a commercially available DolpHing PH10™ (Foxboro Company) pH sensor, which has been modified in accordance with the teachings of the present invention.  
         [0030]     As shown, sensor  120  includes a measuring half-cell  130  and a reference half-cell  140  disposed in a common housing  142 . At least a portion (e.g., a lower portion) of sensor  120  is configured for immersion in a process solution  24  (e.g., a process solution passing through a conduit as shown), and connected to an electronic diagnostic system  150 . Diagnostic system  150  may comprise a conventional process variable transmitter (PVT) coupled to a factory automation network of the type sold by Invensys Systems, Inc., which is configured to measure the electrochemical potential between half-cells  130  and  140 .  
         [0031]     As shown, measuring half-cell  130  is of the type having an electrode  36  immersed in a conventional half-cell electrolyte  32  disposed within an ion selective material  34 , e.g., to effect pH measurement. Alternately, however, electrode  36  may be immersed directly into process solution  24 , such as to measure ORP as discussed hereinabove. Those skilled in the art will recognize that substantially any type of measuring electrode may be used in various embodiments of the present invention.  
         [0032]     Reference half-cell  140  includes a conventional half-cell electrode  46 , typically encased in a cation exchange membrane, such as a NAFION® (sulphonated polytetrafluoroethylene membrane) available from DuPont. Electrode  46  is immersed in reference electrolyte  42  disposed within an electrolyte reservoir  110 .  
         [0033]     Electrochemical contact between the reference electrolyte  42  and process solution  24  is established through reference junction  104 , which includes a porous ceramic plug or the like (e.g., porous Teflon®, porous KYNAR®, wood, or nominally any other porous material) for achieving restricted fluid contact. Reference junction  104  is sufficiently porous to allow a low resistance contact (for accurate potential measurement) but not so porous that the solutions become excessively mutually contaminated. The skilled artisan will recognize that pore size, percent porosity, and effective cross-sectional area of the reference junction  104  must all be balanced, in conjunction with the particular electrolyte used, to achieve the desired restricted fluid contact. In the particular embodiment shown, junction  104  includes a porous ceramic plug of the type conventionally used in the aforementioned DolpHin™ sensor, e.g., having an effective diameter of approximately 0.05 to 0.14 inches, pore sizes between about 1 to 2 μm, and total percent porosity of 20 to 30 volume percent.  
         [0034]     In the embodiment shown, a conductive sleeve  102  fabricated from a conductive polymeric material such as KYNAR® or the like, is melted or otherwise placed concentrically about the porous ceramic rod  104 . Sleeve  102  is thus used in lieu of the conventional non-conductive KYNAR® sleeve commonly used to facilitate assembly. This rod/sleeve assembly is then press-fit into an appropriately sized and shaped bore within sensor body  106 . Optionally, a flow constrictor  108  (shown in phantom), e.g., in the form of a wick of paper, cotton, or other porous material, may also be placed between the rod  104  and the reference electrolyte  42 . Conductive sleeve  102  provides a separate electrical path between the reference electrolyte (e.g., IMole KCl)  42 , and the process fluid  24 , to thus minimize the total resistance of the reference junction.  
         [0035]     The optional flow constrictor  108  may be used to reduce the flow of process fluid  24  through rod  104  and into reservoir  110 , to further minimize any increase in resistance of the reference junction, such as may be due to displacement of reference electrolyte from rod  104  or other contamination by process fluid  24 . Flow constrictor  108  similarly tends to decrease the loss of reference electrolyte  42  into process fluid  24 , to help increase the useful life of the electrode.  
         [0036]     Any electrochemical effects acting on the conductive sleeve  102  tend to be minimal, but may vary depending on the particular conductive material from which the sleeve is fabricated. In particular embodiments, sleeve  102  is fabricated from the same conductive KYNAR® material commonly used in the solution ground (not shown) of the DolpHin™ sensors, so that any such effects will be nominally identical to those acting on the solution ground. Since measurement (e.g., pH) is a function of the difference of measurement taken between the solution ground and reference half-cell  140 , and between the solution ground and measuring half-cell  130 , any such electrochemical effects tend to advantageously cancel one another.  
         [0037]     Although KYNAR® is described as a representative material for sleeve  102 , nominally any conductive material may be used in various applications, without departing from the spirit and scope of the invention. Conductive KYNAR® was used in this exemplary embodiment primarily for convenience, since it may be conveniently melted onto plug  104  and installed into body  106  in the manner currently used with the non-conductive KYNAR® sleeves of the DolpHin™ sensors. However, those skilled in the art will recognize that any number of other materials may be similarly melt processed.  
         [0038]     Referring now to  FIG. 3 , in an alternate embodiment of the present invention, a conventional non-conductive sleeve (e.g., non-conductive KYNAR®)  102 ′, is disposed concentrically with a conductive junction (e.g., plug)  104 ′. This plug  104 ′ may be fabricated from substantially any porous material having conductive properties, such as electrically conductive alumina based ceramics commercially available from DuPont®, or various ceramics having carbon fiber or other conductive materials, such as salts, disposed therein. The conductive material of plug  104 ′ advantageously serves as a secondary flow-independent (e.g., solid state) conductive pathway which helps maintain the total resistance of the reference junction assembly at an acceptable level, while maintaining the relatively small cross-sectional area of the porous junction to help restrict excessive mass flow therethrough. In the event further restriction of mass flow is desired, this embodiment may also be used with optional flow constrictor  108 , as shown in phantom.  
         [0039]     Exemplary material from which conductive plug  104 ′ may be fabricated includes a mixture of conventional (e.g., non-conductive) ceramic and dehydrated electrolyte. In this regard, water may be evaporated from a KCl solution, and the remaining KCl crystal salts mixed with ceramic (e.g., alumina) powder. The ceramic/KCl mixture may then be formed into the desired shape using conventional ceramic fabrication techniques (e.g., extrusion, or other application of heat and pressure such as molding and furnacing). In this manner, a uniform distribution of embedded KCl sites is provided throughout the porous ceramic plug. These sites provide a conductive pathway through the ceramic to the saturated KCl reference electrolyte  42  in reservoir  110 . Since the sites are effectively embedded and captured within the ceramic, many, if not substantially all, of the sites tend to be flow-independent, i.e., resistant to displacement by process fluid flowing through the porous ceramic.  
         [0040]     Any of various known electrolytes may be mixed with any of various ceramic materials to form plug  104 ′. However, use of the same electrolyte (e.g., KCl) as that used as reference electrolyte  42  (e.g., KCl) tends to advantageously generate, little, if any, unwanted electrical noise, since the electrolyte embedded in the plug will exhibit nominally the same electrolytic activity as the electrolyte  42  disposed within reservoir  110 .  
         [0041]     A variation of the embodiments of  FIG. 3 , involves the use of conductive ceramic plug  104 ′ without holder  102 ′, as shown in  FIG. 4 . Flow constrictor  108  may be optionally used, as shown in phantom.  
         [0042]     Yet another embodiment, shown in  FIG. 5 , is substantially similar to that shown and described with respect to  FIG. 2 , but uses a higher concentration (e.g., 4M) reference electrolyte in reservoir  110 . Prior to use, this relatively highly concentrated electrolyte flows or diffuses from reservoir  110  into the porous plug to create a plug/electrolyte combination (shown as  104 ″) which exhibits relatively high conductivity. This plug  104 ″, including this concentrated electrolyte, tends to further enhance the conductivity provided by conductive holder  102 . Moreover, in some applications, plug  104 ″ may provide sufficient conductivity even when used without conductive holder  102 . As with previous embodiments, optional flow constrictor  108  may be used if desired.  
         [0043]     A still further embodiment of the present invention, shown in  FIG. 6 , uses a porous plug  104 ′″ which has been impregnated with electrolyte-laden gel to provide it with a secondary electrical pathway therein. The gel may, for example, comprise a hydrophilic material such as cellulose, which expands when in contact with water to form a substantially solid state material that remains captured within the pores of the plug during operation. In this manner, the electrolyte-laden gel provides a secondary electrical pathway that is flow-independent, i.e., that by virtue of its capture within the pores, resists displacement due to fluid flow through the plug.  
         [0044]     In particular embodiments, plug  104 ′″ may be fabricated by providing ceramic plugs of conventional diameters (e.g., about 0.05 to 0.14 inches), but having relatively large pores, e.g., 5-10 μm or more to facilitate receipt of the gel therein. A cellulose based powder is mixed with a KCl solution (e.g., 4 Mole). The plugs are then placed in a dish with the cellulose/KCl solution, and placed under a vacuum to force the Cellulose/KCl solution through the pores of the plugs where it solidifies. This electrolyte-laden gel thus forms a substantially solid state electrolytic path between the reference half cell and the process solution, through which diffusion may advantageously take place, but which may limit or nominally eliminate ingress of the process fluid. Again, this embodiment may be used with or without optional flow constrictor  108 , shown in phantom.  
         [0045]     The following illustrative examples are intended to demonstrate certain aspects of the present invention. It is to be understood that these examples should not be construed as limiting.  
       EXAMPLES  
     Example 1  
     Control  
       [0046]     Conventional reference junctions, having a conventional porous ceramic plug  104  fabricated from 244B ceramic (244B porous 70% alumina ceramic from Homexx International) of 0.050 inch diameter, approximately 1 μm pore size, 26 percent pore volume, and using a KCL reference electrolyte, were tested using a process solution of tap water at 20 psi. Resistance measurements were captured at 15 second intervals over approximately 130 minutes. As shown in  FIG. 7 , the resistance measured by these conventional devices routinely exceeded 100 k-Ohm and often reached or exceeded 300-400 k-Ohm.  
       Example 2  
       [0047]     Reference junctions in accordance with the subject invention were fabricated substantially as described in Example 1, but also having a conductive (KYNAR®) sleeve  102  as described above with respect to  FIG. 5  (without a flow constrictor  108 ). These devices were tested under conditions substantially similar to those of Example 1, for approximately 32 hours. As shown in  FIG. 8 , the resistance measured by these inventive devices remained well below 100 k-Ohm, and seldom exceeded 35 k-Ohm.  
       Example 3  
       [0048]     A reference junction in accordance with the subject invention was fabricated substantially as described in Example 1, but using a porous ceramic plug  104 ′ formed by mixing ceramic alumina powder with dehydrated KCl, which was then formed into plugs by conventional extrusion as described hereinabove with respect to  FIG. 3 . This device was tested under conditions substantially identical to those of Example 1, for approximately six hours. As shown in  FIG. 9 , the resistance measured by this inventive device remained well below 100 k-Ohm, rising to a maximum of about 51.7 k-Ohm before decreasing.  
       Example 4  
       [0049]     A reference junction in accordance with the subject invention was fabricated substantially as described in Example 1, but using a porous ceramic plug  104 ′″ having pores of about 10 μm, which were impregnated with KCl-laden cellulose gel as described hereinabove with respect to  FIG. 6 . This device was tested under conditions substantially identical to those of Example 1, using a process solution of tap water at 15 psi for over 50 hours. As shown in  FIG. 10 , the resistance measured by this inventive device remained below 11 k-Ohm for the duration of the test.  
         [0050]     While several embodiments of the present invention have been shown and described with various characteristics, it should be understood that one or more of these characteristics of one embodiment may be substituted or added to characteristics of other embodiments without departing from the spirit and scope of the present invention.  
         [0051]     The modifications to the various aspects of the present invention described hereinabove are merely exemplary. It is understood that other modifications to the illustrative embodiments will readily occur to persons with ordinary skill in the art. All such modifications and variations are deemed to be within the scope and spirit of the present invention as defined by the accompanying claims.