Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/28—Electrolytic cell components
  • G01N27/401—Salt-bridge leaks; Liquid junctions

Definitions

• This invention relates to improvemen s in elec rochemical sensors of the kind which utilize ion exchange across a barrier between a conductive liquid and a test fluid, either gas or liquid.
• the basic circuit for ' utilizing electrochemistry to detect and measure the concen ra ion of a particular chemical constituent, or pH , in a test fluid includes a voltaic cell formed by ions of the material to be detected. Charges are transported by those ions to or from a substance which transfers charges selectively to ions of that material. The concentration of the material to be detected is calculated on the basis of the magnitude of the exchange potential, and that requires a complete electrical circuit and electron flow.
• Elec ⁇ trical poten ials are developed at these junctions of electrochemically dissimilar materials.
• the magnitude of those potentials may be of the same order of magnitude as the potentials across the test cell. Because the addi ⁇ tional junction potentials cannot be eliminated, the measuring circuit is arranged so that the additional junctions are created in pairs arranged to cancel one another.
• the most common arrangement is to have the test solution and the ion selective transfer material contact an electrically con ⁇ ductive solution and to connect the conductive solution to the voltmeter wires through a chemical which contains both the metal of the wires and the conducting ion of the con ⁇ ductive solution.
• the copper wire from the voltmeter is joined to one end of a silver wire.
• a silver-silver chloride mixture is fixed to the other end of the silver wire.
• The. silver chloride is immersed in a saturated solution of sodium or potassium chloride which is contained in a vessel where it makes physical contact with the ion selective transfer substance. That substance is disposed to make physical contact with the test fluid.
• the circuit from the other side of the voltmeter is similar. It extends from copper wire to silver wire to silver-silver chloride to saturated salt solution to test fluid.
• the saturated salt solution must make electrical contact with the test fluid without an inter ⁇ vening junction at one side of the sensing junction. That connection is called the reference cell or reference junction.
• the problem is how to complete the electrical contact at the reference junction without a transfer of test fluid to contaminate the saturated salt solution or loss of the saturated salt solution from its container.
• a like problem occurs at the other side of the test cell if the ion transfer material is not impervious to flow of test fluid or the flow of the salt solution.
• a special pH sensitive glass is used as the ion transfer material in most pH test cells. The glass is impervious to both test fluid and salt solution. In that case the separation probably occurs only at the reference junction, and it has become both possible and common to package the pH test cell and the reference cell in a single assembly called a combination electrode.
• the ion transfer material In the case of units for measuring oxygen, potassium, C02, and other substances, the ion transfer material must be incorporated in a carrier material whose physical nature permits it to serve as a barrier to separate the salt solution from the test fluid.
• the carrier materials fall into two classes. In one, the carrier material is impervious to the flow of salt solution and of test fluid. In the other, the salt solution or test fluid, or both, are permitted to migrate into the body of carrier material to contact the ion transfer material jointly whereby to shorten the flow path and minimize motion artifacts and random flow variations.
• Carrier materials of the latter class are porous and, in the current state of the art, they are preferred. They are preferred for use at both the reference junction where they are called salt bridges , and at the measuring junction where they are called carrier or membrane mater ⁇ ials. It has become standard to dispose of them in one end of a tubular housing which serves as the container for the salt solution and the metal-metal salt connection from the solution to the electrode wire.
• Porous salt bridge and carrier materials have included wooden plugs, cotton threads, ceramic plugs, glass frit-filled rubbers , porous elastomers and other materials .
• One of the most successful is shown and des ⁇ cribed in United States Patent No. 4,128,468 to Bukamier.
• the plugs disclosed in that patent are resilient and porous.
• the plug shown in United States Patent No. 4,105,509 is porous but rigid.
• the .salt bridge or carrier material is formed as a plug, and in each case an 0-ri.ng is employed to aid in sealing against the flow of liquid past the plug.
• these units may be subjected to temperature and pressure cycling over a wide range. The forces urging contamination of salt solution by test fluid and leakag.e and weeping of salt solution contribute to shortened useful life.
• a related object is to provide a structure which will permit use o.f more porous plug materials without loss of protection against such contamination or liquid loss.
• a further object is to provide those improve ⁇ ments and advantages at reduced cost.
• the requirement of the sealing structure is to prevent movement of liquid, or gas, between the outer wall of the plug and the inner wall of the electrode body and, in the case of combination electrodes, between the outer wall of the inner element and the wall of the plug opening.
• the theory of the prior art was to accomplish sealing with laterally directed forces - with compression of O-rings and interference fits.
• the invention utilizes forces applied in the longitudinal direction. Greater force can be applied without the kind of compression of the plug that reduces porosity. Sealing, at least primary sealing, is accom ⁇ plished by forming the housing with elements by which he plug can be stressed longi udinally either in compression or in tension. In the case of combination electrodes arranged concentrically, sealing of the opening through the plug is enhanced with longitudinally applied, again without adverse effect on the porosity of the plug.
• the preferred form uses a plug that is both resilient and porous. Resilience lends an ability to
• a major advantage in the longitudinal compres ⁇ sion form of the invention is that the shape of the plug can be simplified and manufacturing cost reduced. The need for closely held plug dimensions is eliminated and assembly can be simplified.
• Figure 1 is an isometric view of electro ⁇ chemical sensor electrode in which the invention is embodied
• Figure 2 is a cross-sectional view taken on the vertical midplane containing the axis of the unit of Figure 1 as indicated by line 2-2 of Figure 1;
• FIG. 3 is an isometric view of the porous plug of the unit of Figures 1 and 2;
• Figure 4 is an enlarged cross-sectional view of the screw front of the unit of Figures 1 and 2;
• Figure 5 is an enlarged view, the lower half in section, of a fragment of the sensing end of said uni ;
• Figure 6 is an isometric view of a sensing electrode of another form in which the invention is embodied
• Figure 7 is an enlarged cross-sec iona 1 view taken on line 7-7 of Figure 6 of the sensor end of the unit.
• FIG. 8 is an isometric view of the porous plug of the unit of Figure 6. Description of the Preferred Embodiment
• the sensing electrode 10 of Figure 1 is a combi ⁇ nation electrode arranged to sense pH of a fluid in which the sensor element 12 and the end of the porous plug 14 of the reference element are immersed.
• the exterior shape and appearance of the unit are common, even standard.
• the body 16 is an elongated cylindrical tube terminating at its sensor end in scallops 18 which are designed to permit fresh fluid to flow past and contact the sensor element and porous plug while keeping them free of any solids which may be contained in the test .fluid.
• the body contains saturated salt solution which is sealed in place by the porous plug 14 at the sensor end and by several layers of sealing material at the connector end 20. That end is closed, and a connector of coaxial cable end is held in place by an end cap 22. In this case the cap holds a BNC-type electrical connector 24 which extends axially from the body 16.
• the plug 14 is held in place and compressed in the direction of the longitudinal axis of the unit by a shoulder formed on the inner periphery of the clamping section .
• the shoulder is formed by an annular flange 32 which extends in ardly into the central opening of the clamping section 30.
• the second flange 34 is formed on the in ⁇ terior of the main body section 28. It is preferred that the flanges 32 and 34 be molded integrally with the
• OMPI clamping section 30 and main body 28, respectively are possible and acceptable.
• the shoulders may be the consequence of a change in diameter of the clamping section or main body, or both, or they might be formed by separate elements which are fixed to the interior of the body.
• the preferred construction is shown in Figure 2, 3 and 4.
• the sensor end of the main body 28 has reduced diameter. Its inner bore is smooth and its outer wall is formed with threads. The threads are threaded into threadways formed on the inner surface of the clamping section 30.
• the unit is conventional. This particular one has its reference cell divided into two cells in series by a porous plug 36 held in place by a pair of 0-rings at its outer surface, and by tight embracement of the inner pH cell body 38. That arrangement is taught in Bukamier United States Patent No. 4,128,468. Division of the reference elec ⁇ trode, and of the sensing electrode for that matter, into a series of cells is used when there are likely to be large differentials between interior and exterior pres ⁇ sures or where there is a possibility that constituents of the test fluid might migrate into the reference fluid (the salt solution) of the reference electrode.
• Both of the cavities 40 at the right of plug 36, and 42 at the left are filled with a conductive liquid. In this case both are filled with saturated potassium chloride solution.
• the body 38 of the pH sensing elec ⁇ trode is also filled with a saturated potassium chloride
• the bulb 16 is made of pH sensitive glass. It is bonded to the glass tube which serves as the body 38. Contact with the reference electrode fluid is made through a silver wire 44 and a body of silver and silver chloride mixture 46 which is bonded to the wire and is disposed, in cavity 40. Contact with the sensing electrode -is made by silver wire 48 and a glob of silver-silver chloride 50. At the right, both electrode bodies are sealed by several layers 52 of sealing material. The two-conductor connec ⁇ tor 24 is held in place by the sealing material and the end cap 22. One of the silver wires is connected to one terminal of the connector and the other wire is connected to the other terminal.
• the plug itself is not new. It is made of a porous material which is inert to the materials with which it does or might come into contact. In the past selection of plug material required a compromise between porosity and rigidity. Increased porosity reduces the need for pre-soaking and increases response to change in test fluid chemistry, but it complicates the problem of sealing the unit to prevent flow or migration of test fluid or the electrode solution. Rigid materials simplify the sealing problem, but make the problem of achieving uniform poro ⁇ sity at a reasonable cost more complex.
• Resilience in the plug material reduces the need for close dimensional control, but it, too, complicates the sealing problem. While the renitence of the material aids sealing, too much compression collapses the internal voids and reduces porosity, and the resilient plug is deformed by pressurized fluid to permit leakage.
• the invention makes it possible to utilize softer, more porous plugs than was possible in prior
• the plug material is stressed and the reaction force is utilized to accomplish sealing, but the stress is in the longitudinal direction.
• the sealing is accomplished at the ends of the plug instead of or in addition to sealing at the sides of the plug.
• the flange 32 and 34 are continuous whereby a continuous end sealing surface is provided around the margins of both ends of the plug. If neither end sealing surface is continuous, end compression must be sufficiently great to force the plug material laterally to accomplish sealing at the outer wall of the plug and at the inner wall around the opening which is numbered 54 in Figure 3.
• OMPI Figures 4 and 5 respectively.
• the thread grooves 56 of the compression section are visible in Figure 4. They are engaged by the thread 58 of the reduced end portion 60 of the main body section 28. Except for the threads and thread grooves, and the scallops 18 of the- compression section,' both sections are symmetrical about their central longitudinal axis.
• the flanges 32 and 34 are continuous and extend inwardly only enough to engage the outer peripheral margin of the plug ends. As best seen in Figure 5, the plug is seated against the flange 34 at the right and it extends beyond the end of the end section 60 at the left. The right face of flange 32 being flat will bear against and compress the plug longitudinally when the compression section is screwed on to the main body. Other configurations can have the same result.
• the screw connection affords adequate mechanical advantage to permit tightening the compression section by hand.
• This unit is designed to permit easy removal and reassembly of the compression section 30 in the event that it is desired to clean the pH glass and the plug face.
• the lower half of Figure 2 is sectioned to make visible another feature of the invention.
• the surface area at the interface between the plug and the inner sensor body or tube 38 is much less than the area at the interface between the outer body an the outer surface of the plug. If a resilient plug which fits snugly on the inner body is compressed radially enough to accomplish sealing at the outer surface, more than enough force is applied to the inner body to seal the inner opening . However, in the invention the radial forces are usually less, especially if the plug is quite porous and resil ⁇ ient. In that case, while the renitence of the plug is still enough to affect the inner seal, it is preferred to
• the bulb 12 is held in place part way into the plug by the tube 38 and the tube is held relative to the main body section 28 by the closure material 52.
• the longitudinal force of the bulb on the plug is transmitted to the flange 34 at the other end of the plu .
• the end result of that construction is an effec ⁇ tive seal against the flow or migration of fluid past the plug 14 notwithstanding that the plug is more resilient and more porous than the standard plugs of the past.
• the invention is not limited to the use of softer, more porous plugs, but it makes their use practical and a feature of the invention.
• the range of resilience and porosity when those qualities constitute a feature of the invention are:
• FIGs 6, 7 and 8. This is a mono-electrode design for measuring potassium concentration.
• the plug 100 is resil ⁇ ient. It is porous, and it contains an ion transfer agent, phe ly nomycene, which is specific to potassium. It is produced in pads or sheets which are sliced into rec ⁇ tangular plugs, as shown in Figure 8, requiring no further shaping or processing.
• the sensor body is divided into a compression section 106 and a main body section 108.
• the mounting bracket 110 is shown to be integrally formed with the main body section in Figure 6. The right end is closed by a cap 112 from which a coaxial cable 114 extends.
• the internal construction at the sensor end is shown in Figure 7. Not unlike the unit of Figures 1 through 5, the sensor end of the main body section has reduced outer dimensions at 116.
• the plug 100 is pressed into that reduced sized end section against an inwardly directed flange 118 which is continuous around the inner perimeter of the main body section. Longer than the distance from flange 118 to the sensor end of the main body section, the plug is squeezed in compression when the compression section 108 is telescoped on the reduced size section of the main body and forced into place.
• the com ⁇ pression section 106 and the main body section 108 are held in assembled condition with the plug compressed by co-acting locking conformatio s.
• a latch projection triangular in cross-section, fits into a complemen tally shaped keeper depression.
• the keeper is formed in the compression section 106 and the latch element projects from the reduced end section of the main body.
• the body parts and the plug are distorted in shape to permit assembly, but return to the shape shown when the latch is seated in the keeper.
• the flange 122 at the sensor end of the compression section 106 bears on the periphery of the plug 100 at face 102 and forces it into compression against the flange 118 at its rear face 124.

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• 1984
  • 1984-06-11 EP EP19840902558 patent/EP0182783A1/de not_active Withdrawn
  • 1984-06-11 WO PCT/US1984/000894 patent/WO1986000137A1/en not_active Ceased

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