Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1996-09-17
2003-06-17
Tung, Ta-Hsung (Department: 1743)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C205S778500
Reexamination Certificate
active
06579434
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electronic and electrochemical apparatus and methods for measuring the concentration of fluoride ions in aqueous liquid compositions. The general principles of the apparatus are described in U.S. Pat. No. 3,431,182 of Mar. 4, 1969 to Frant, the entire disclosure of which, except to the extend that it may be contrary to any explicit statement herein, is hereby incorporated herein by reference. This invention is more particularly related to modifications that prolong the service life of the electrodes, particularly when the latter are used in contact with hot acidic aqueous compositions containing surfactants and/or oxidizing agents; such compositions are customarily employed in cleaning aluminum beverage containers in a high speed processing line and in forming protective coatings on aluminum surfaces generally. The invention is still more particularly related to an improved method of securing and sealing the fluoride sensing crystal (hereinafter usually abbreviated as “FSC”) required for a fluoride sensing electrode within the totality of the electrode, without requiring either (i) the use of cement or other adhesive(s) that undergo a liquid to solid phase transition during the course of construction of the electrode or (ii) molding some other part of the electrode in place around the FSC. Along with its other advantages, this makes the sealing of the FSC reversible, so that if desired the FSC can be replaced without the need for replacing any other component of the electrode.
2. Discussion of Related Art
As already noted briefly above, one of the necessary parts of a fluoride ions concentration sensing apparatus according to both this invention and the prior art is a substantially liquid-impermeable solid body that is constituted of ion-sensitive crystalline fluoride(s), most preferably lanthanum fluoride doped with about 0.3% of europium fluoride. This solid body is usually a single crystal and thus is denoted herein as the “fluoride sensing crystal” or “FSC”, even if in some particular instance the solid body might be polycrystalline. In a complete free fluoride ion sensitive electrode, this FSC is physically arranged to have one part of its surface contacted by an ionically conductive liquid in which the concentration of free fluoride ions is to be measured with the aid of the free fluoride ion sensitive electrode. A separate and distinct other area of the FSC surface is in indirect electrical contact, via physical contact with a first liquid ionic solution of known composition, with a first “reference electrode”, i.e., an electrode, such as a silver-silver chloride electrode, that automatically maintains itself at a fixed potential as long as the composition of the ionic solution with which it is in contact does not change and the current density flowing through it is small (Because most commercial FSC's are in the form of a relatively thin membrane, it is customary in the art to refer to the two separate and distinct areas of FSC as “[opposite] sides”. This terminology may be used in the remainder of this specification, but it is to be understood that despite any use of such terminology, no limitation on the shape of the FSC is thereby intended.) The first ionic solution of known composition, the first reference electrode, and the FSC described above are all assembled within and/or on the surface of a container, usually a tubular container, in such a manner that (i) the FSC has one side in contact with the first ionic solution of known composition and its other side on the outside of the container, (ii) the first liquid ionic solution of known composition is sealed within the container, (iii) the first reference electrode is in physical contact with the first liquid ionic solution of known composition and is not in physical contact with the FSC, but is electrically connected thereto via the first liquid ionic solution of known composition; and (iv) the metallic part of the first reference electrode is connected through a metallic electrical conductor lead to the space outside a sealed space within the container, this sealed space containing all of a metallic part of the first reference electrode, the first liquid ionic solution of known composition, and the interior side of the FSC. The Frant reference teaches that the container may be made of any liquid-impervious, substantially rigid, electrically insulating material that is substantially chemically inert to salt solutions containing fluoride ions with which the container might be placed in contact, and in practice most if not all commercially available free fluoride ion sensitive electrodes have plastic containers, usually of poly(phenylene sulfide), with or without inorganic fillers.
When a free fluoride ion sensitive electrode is in use, the electrical lead from the container for the first reference electrode is connected via a metallic conductor to a low current voltage measuring device and through the latter to a second reference electrode, which may or may not be at the same inherent, self-maintained potential as the first reference electrode. The second reference electrode is in contact with a second liquid ionic solution of known composition, which may or may not be the same as the composition of the first liquid ionic solution, and the second liquid ionic solution is not in substantial physical contact with, but is in indirect electrical contact with, the aqueous composition containing the concentration of free fluoride ions to be measured, the indirect contact being established through one of the devices known in the art, such as a glass fiber, gel or ionic solution, porous TEFLON™ polytetrafluoroethylene plug as described, for example, in U.S. Pat. No. 4,128,468 of Dec. 5, 1978 to Bukamier, “flowing junction”, or the like, which permits electrical conductance via ion charge transfer while preventing any substantial physical mixing between the second liquid ionic solution of known composition and the liquid in which the concentration of free fluoride ions is to be measured.
Although fluoride sensitive electrodes of this type have been in use for decades, it has been widely recognized in the art that one of their major disadvantages is an instability of performance. Such instability is usually first manifested as a change in the voltage reading that corresponds to a particular value of free fluoride ions, necessitating frequent recalibration of the particular fluoride sensitive electrode used. Eventually, the electrode may become non-responsive to changes in fluoride concentration within the range in which measurement is needed for the process control of the particular solution being monitored with the electrode, so that the electrode must be replaced. This is a substantial source of expense and also of inconvenience and hazard, inasmuch as many of the solutions in which fluoride concentrations are measured contain hazardous ingredients such as hot hydrofluoric acid, and the free fluoride ion sensitive electrodes are often mounted in the wall of a container vessel for the process solution being monitored, well below the highest liquid level in the vessel; considerable care must be exercised when replacing any component in contact with such a solution in order to avoid injury.
Most prior art attempts to improve the life of free fluoride ion sensitive electrodes have been directed to improving the quality of the seal between the FSC and the container wall in which the FSC is fixed, in order to prevent or at least minimize opportunities for fluid and/or electrical leaks through this desirably completely sealed interface. For example, U.S. Pat. No. 4,049,526 of Sep. 20, 1977 to Maurer teaches that a slow setting type of cement is practically required for sealing the solid fluoride(s) membrane to the insulating container, but that such a slow-setting cement has poor long term resistance to many solutions in which measurement of the free fluoride ion concentration with the free fluoride ion sensitive electrode is desired,
Cornell Kevin H.
Szynal Thaddeus J.
Topping Joseph C.
Harper Stephen D.
Henkel Corporation
Tung Ta-Hsung
Wisdom Norvell E.
LandOfFree
Electrode with reversibly sealed fluoride sensing crystal does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electrode with reversibly sealed fluoride sensing crystal, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electrode with reversibly sealed fluoride sensing crystal will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3115303