ION-SELECTIVE ELECTRODE AND METHOD FOR MAKING SAME

Abstract: An ion-selective electrode including a pellet (12) secured in a housing (11) by a curable resin (13). The front face (14) of the pellet (12) is coated or impregnated with a silicone oil.

Description: ION-SELECTIVE ELECTRODE AND METHOD FOR MAKING SAME Background of the Invention Field of the Invention The invention relates to the field of elec¬ trodes. More particularly, the invention relates to the field of solid-state ion-selective electrodes. In still greater particularity, the invention relates to an ion-selective electrode for use in the assay of electrolyte concentrations of bodily fluids and other aqueous solvents. By way of further characterization but not by way of limitation thereto, the invention is an ion-selective electrode in which the electrode material has been treated with a hydrophobic substance in order to prevent degradation by the fluids to be measured. Description of the Related Art Ion-selective electrodes have been utilized in clinical analysis instruments in order to measure particular characteristics of fluids to allow diagnosis of patient illnesses. The ability to quickly respond to changes in activity of the ion being measured is a highly desirable characteristic of a sensing electrode. The fastest response would be attained by exposing the electrode material directly to the liquid to be meas¬ ured. However, degradation of the electrode material from dissolution or corrosion by direct contact with the test solution greatly reduces the useful life of the electrode. Previous ion-selective electrodes have util¬ ized a membrane material to enhance ion selectivity and protect the active surface of the electrode. This membrane is usually a resin impregnated with an "ionophoric" substance which contributes to ion selec- tivity. Other constituents, such as solvents or plas- ticizers, may also be included within the membrane. The use of such a membrane, while protecting the electrode material and enhancing ion selectivity, often slows the response time of the electrode. Therefore, the speed and accuracy of the measurements are affected. That is, the presence of this finite layer of material generally limits the speed of response of the electrode, possibly because the ions must diffuse some distance into the membrane material. In addition, while the ionophore exhibits great receptivity to the ion of interest, it provides relatively poor response to foreign ions. Ion-selective electrodes utilizing an iono- phoric membrane exhibit selectivity toward a particular ion. This ion selectivity results partially from exposing an ionophoric substance to a solution highly concentrated with respect to the ion. However, reduced sensitivity may result if the membrane is exposed to solutions containing appreciable concentrations of another ion. Reduced sensitivity results because exposure of the ionophoric substance to solutions con¬ taining appreciable concentrations of another ion con¬ verts the ionophoric material to a new species of " ion-selective membrane. As an example, H. Freiser et al. (Analytical Chemistry, Vol. 44, No. 4, April 1972, page 856) utilized a quartemary ammonium compound (Aliquot 366S, an Eastman Kodak product) as an "ion association complex." This "ion association complex" could be converted to a chloride sensitive ionophore by soaking i a 0.5 m to l m solution of sodium chloride, or could be converted to a sulfate sensitive ionophore by exposure to a sodium sulfate solution. A chloride electrode utilizing an ionophoric substance as a membrane could thus be damaged by con¬ tinued exposure to a test reagent containing sodium sulfate. Therefore, such an electrode might only be used with a limited variety of reagent systems. A chloride electrode not utilizing such a membrane could, however, be used with a number of reagent systems containing a relatively large concentration of foreign anions. Such a reagent system could minimize activity versus concentration errors by using a constant ionic strength approach. In summary, fast and accurate measurements may be made with an ion-selective electrode by exposing the electrode material directly to the substance to be tested. However, in so doing, the electrode material is likely to be degraded by dissolution or corrosion by that test solution. An ionophoric membrane, while protecting the electrode material, slows the response time of the electrode and thus negatively affects operation of the instrument. Summary of the Invention The invention is an ion-selective electrode in which the electrode material is protected with a hydrophobic substance. That is, the active electrode material may be impregnated and coated with a hydro¬ phobic substance to protect the electrode material from dissolution or corrosion by a test solution. Because the electrode material is exposed directly to the test solution, fast and accurate response time is preserved. The hydrophobic material eliminates the need for an ionophoric membrane and thus improves the response time and the accuracy of the electrode. The hydrophobic substance employed with the preferred embodiment of the invention is silicone oil and a viscous silicone com- pound. Brief Description of the Drawings Fig. 1 illustrates a preferred embodiment of an ion-selective electrode; and Fig. 2 illustrates an alternate embodiment of an ion-selective electrode. OMPI Description of the Preferred Embodiment Referring to Fig. 1, an ion-selective elec¬ trode includes a housing 11 containing a pellet 12 comprised of active electrode material. Pellet 12 is secured to housing 11 by a curable resin seal 13. A front face 14 of pellet 12 is the area contacted by a test solution. Referring to Fig. 2, an alternate embodiment of an ion-selective electrode is shown. In the alter- nate embodiment housing 11, pellet 12, seal 13, and front face 14 are as described with respect to Fig. 1. The alternate embodiment illustrated in Fig. 2 addi¬ tionally contains a drain wire 15 imbedded in pellet 12 and a conductive material 16 surrounding drain wire 15. Mode of Operation Referring to Fig. 1, pellet 12 is formed by pressing precipitated silver and silver chloride powders of controlled particle size in a mold. Pellet 12 is secured within housing 11 by a curable resin 13 which acts as a seal to control fluid leakage past pellet 12. Front face 14 of pellet 12 forms the active ion-selective area of the electrode. Electrical contact with pellet 12 may be accomplished by means of a spring-loaded gold or gold-plated contact contained in a detachable cable assembly (not shown). This contact could occur on the rear face of pellet 12 opposite to front face 14. After pellet 12 is formed, front face 14 of pellet 12 is finished by machining to remove extraneous electrode material or resin and polishing. Successively finer polishing media is used until front face 14 is smooth and shiny. Pellet 12 is then impregnated with silicone oil under a vacuum. After removal from the vacuum treatment, a coating of viscous silicone compound is applied to front face 14. This treatment renders front face 14 hydrophobic, that is, water repellant, so OMPI that attack by the solution to be measured is greatly minimized, thus extending the lifetime o pellet 12. That is, electrode damage due to corrosion of or deposition onto front face 14, or due to absorption of the solution within the pores of pellet 12, is markedly reduced by filling the pores of pellet 12 and coating front face 14 with the hydrophobic substance while at the same time not affecting the sensitivity to the ion of interest. Referring to Fig. 2, a silver drain wire 15 may be imbedded within pellet 12 at the time of molding to allow an electrical contact to be made to this wire. Electrical contact may be accomplished by soldering wire 15 to another wire to produce a nondetachable cable assembly or by surrounding wire 15 with an electrically conductive (silver-bearing) epoxy compound 16 which in turn contacts a spring-loaded contact assembly. This spring-loaded contact assembly (not shown) may be contained in the detachable cable assembly referred to previously. Silver drain wire 15 and conductive resin contact 16 may be eliminated. Instead, a graded mixture of powders, starting with the desired silver-silver chloride ratio at front face 14 of pellet 12 and ending with a pure silver layer at the opposite face of pellet 12, may be employed. In this arrangement, electrical contact is made by means of the spring-loaded contact assembly (not shown) as discussed previously. Various methods of forming pellet 12 in Fig. 1 may be employed. That is, pellet 12 may be formed by pressing the powder mixture directly into housing 11 with housing 11 acting as a mold. Ultrasonic energy and/or heat may be employed as aids to compaction of the powder to form pellet 12.. Another method of fabricating pellet 12 is to first produce a sintered skeleton structure by means of heat and/or pressure, leaving WΓPO pores of controlled dimensions in pellet 12, which are then filled with the remaining active materials to form a mixture of components with known percentages of composition. Another method for forming pellet 12 is to mix powdered materials with a curable resin which forms a binder possessing appropriate mechanical properties. This mixture may be extruded to shape or machined after curing to form pellet 12. The hydrophobic substance, such as silicone oil, may be applied to a pellet fabricated by any of the above-described processes. Advantages of the invention include allowing the electrode to be presented more intimately to the solution to be evaluated than is possible with a mem¬ brane of appreciable thickness while protecting the electrode material from degradation. More intimate contact with the solution allows maximum speed of re¬ sponse- The hydrophobic substance eliminates the need for a physical membrane to be used between the pellet and the solution to be measured. Thus, poisoning effects due to chemical changes in the membrane material caused by foreign ions tend to be eliminated. In addition, deterioration of response speed of the electrode material caused by absorption of water or solution is greatly reduced. The protective hydrophobic substance may be readily renewed as needed to further extend the working life of the electrode. While the invention has been disclosed with respect to a preferred embodiment thereof, it is not to be so limited as changes and modifications may be made which are within the full intended scope of the inven¬ tion. For example, various methods of fabricating the pellet may be employed. A few of these methods have been disclosed. However, it may be possible to form the pellet by other methods. Treatment of the pellet with the hydrophobic substance may be accomplished by methods other than the vacuum condition described. Any method of saturating the pellet with the hydrophobic substance and coating the active surface thereof may be employed and are within the full intended scope of the claims.

Claims: What is claimed is; 1. In an ion-selective electrode including an active electrode material contained in an electrode housing, the improvement comprising: a hydrophobic substance in intimate contact with said active electrode material. 2. Device according to claim 1 wherein said active electrode material is impregnated with said hydrophobic substance. 3. Device according to claim 1 wherein at least a portion of said electrode material is coated with said hydrophobic substance. 4. Device according to claim 1 wherein said hydrophobic substance includes silicone. 5. Device according to claim 2 wherein said hydrophobic substance includes silicone oil. 6. Device according to claim 3 wherein said hydrophobic substance includes a viscous silicone compound. 7. Method for making a pellet for an ion- selective electrode comprising the steps of: forming an electrode material into said pellet; finishing at least one surface of said pellet; impregnating said pellet with a hydrophobic substance; and coating at least said finished surface with a hydrophobic substance. 8. Method according to claim 7 wherein said step of forming includes pressing an electrode material into a mold. 9. Method according to claims 7 or 8 wherein said electrode material includes silver and silver chloride powders of controlled particle size. 10. Method of claim 7 further including the step of sealing said pellet to an electrode housing. 11. Method according to claim 10 wherein said step, of sealing includes employing a curable resin as said seal. 12. Method according to claim 11 wherein said curable resin includes a polymer resin. 13. Method according to claim 12 wherein said polymer resin includes an epoxy resin. 14. Method according to claim 12 wherein said polymer resin includes an acrylic resin. . 15. Method according to claim 7 wherein said step of finishing includes: machining at least one surface of said pellet to remove extraneous electrode material; and polishing said at least one surface. 16. Method according to claim 7 wherein said step of impregnating includes saturating said electrode material with a hydrophobic substance under vacuum conditions. 17. Method according to claim 16 wherein said hydrophobic substance includes silicone oil * . 18. Method according to claim 7 wherein said step of coating includes applying a viscous hydrophobic sub¬ stance to said at least one surface. 19. Method according to claim 18 wherein said viscous hydrophobic substance includes a silicone compound. 20. Method according to claim 7 wherein said step of forming includes imbedding a drain wire in said pellet. 21. Method according to claim 7 further including the step of renewing said hydrophobic substance.

Inventors: AHRENS KENNETH OTTO, CULLERTON JAMES FRANCIS, AHRENS, KENNETH, OTTO, CULLERTON, JAMES, FRANCIS
Applicants: BECKMAN INSTRUMENTS INC, BECKMAN INSTRUMENTS, INC.
Application year: 1982
Main classification: G01N27
IPC codes: G01N27