Replaceable junctions for reference electrodes

A reference electrode comprising an enclosure containing a half-cell electrode, a half-cell electrolyte, and a reference junction positioned in an outlet for the electrolyte, the half-cell electrode being connectable to an external measuring means, in which said reference junction comprises a removable and replaceable reference junction fixedly inserted through said outlet.

CROSS-REFERENCE TO RELATED APPLICATION 
Patent application Ser. No. 218,788, filed in the name of D. B. Brezinski 
on Dec. 22, 1980 and now abandoned, entitled "Improved Replaceable 
Junctions for Reference Electrodes", which application is assigned to the 
assignee of the instant application. 
BACKGROUND OF THE INVENTION 
The present invention is broadly concerned with reference electrodes, and 
the reference electrode portion of combination electrodes, which are 
employed to provide the stable reference potentials required by a variety 
of electroanalytical techniques, such as ion-selective electrode 
measurements, controlled potential coulometry, polarography, and the like. 
More particularly, the present invention is concerned with a reference 
electrode and the reference electrode portion of a combination electrode, 
each having a removable and replaceable junction. 
A reference electrode most frequently is used in conjunction with an 
ion-selective electrode, either separately or in combination, to measure 
the activity (which is a function of concentration) of a given ion in a 
sample solution. Consequently, the discussion which follows primarily 
relates to such use. It is to be understood, however, that such discussion 
is not intended to in any way limit the spirit or scope of the present 
invention. 
The two electrodes, i.e., the reference electrode and the ion-selective 
electrode, both of which are immersed in the sample solution, typically 
are connected to a means for measuring the potential difference between 
the electrodes, e.g., an electrometer. The reference electrode provides a 
constant electromotive force or potential against which the potential of 
the ion-selective electrode is compared. The latter potential consists of 
a constant component from the electrochemical half-cell of the 
ion-selective electrode and a variable component which is the potential 
across the sensing membrane and which is dependent upon the activity 
(concentration) of the ion being measured. The variable component, then, 
is readily correlated with ion activity (concentration) by known means. To 
give accurate results, the potential of the reference electrode should not 
change with the composition of the sample. 
The reference electrode is designed to be minimally sensitive to changes in 
the external, sample ionic environment. It consists of at least three 
components: (1) a half-cell electrode (typically a silver-silver chloride 
mixture), (2) a half-cell electrolyte (typically 4M potassium chloride 
solution saturated with silver ions), and (3) a reference junction. The 
half-cell electrode and half-cell electrolyte constitute an 
electrochemical half-cell having a known, stable, constant electrical 
potential. Direct physical, and therefore electrical, contact between the 
half-cell electrolyte and the sample solution is established through the 
reference junction which usually consists of a porous ceramic plug, metal 
or asbestos fiber bundle, sintered plastic, or like means of achieving a 
fluid mechanical leak. 
As used herein, the term "half-cell electrode" means the solid-phase, 
electron-conducting contact with the half-cell electrolyte, at which 
contact the half-cell oxidation-reduction reaction occurs which 
establishes the stable potential between the half-cell electrolyte and the 
contact. 
Because the junction electrolyte and the measured sample usually differ in 
ionic strength and transference, a "liquid junction potential" typically 
develops across the reference junctions. Variation in this junction 
potential from sample to sample is a source of error in electrode 
measurements, and one goal of reference electrode technology is to make 
the junction potential as small, stable, and reproducible as possible. But 
the reference junction, for various reasons usually involving clogging, 
can become wholly or party inoperable. Clogging of junction pores by 
foreign materials disrupts the direct physical contact which is required 
to establish a stable, reproducible liquid junction potential between the 
internal and measured solutions. Also, clogging typically introduces fixed 
ionic charge into the junction, which causes an anomalous increase in 
junction potential in low-ionic-strength measurements. Also, in many 
reference electrode designs, the internal filling solution flows out of 
the reference electrode into the measured solution. It has been found that 
this flow generally results in faster and more accurate response, since 
the flow serves to flush the previously measured solution more rapidly 
from the junction and also serves to increase the ionic strength at the 
junction surface, thereby reducing anomalies due to fixed space-charge in 
the junction. Clogging blocks this beneficial flow of junction 
electrolyte, leading to slower, less accurate measurements. Finally, 
clogging increases the electrical resistance of the junction, which causes 
a proportionate increase in the electrical noise of the measurement. Thus, 
typical symptoms of a clogged junction include slow, erratic, noisy, and 
often erroneous response. 
Junction clogging may arise from a variety of sources, both extrinsic and 
intrinsic. For example, the proteins and lipids present in many measured 
samples tend, because of electrostatic and hydrophobic forces, to bind to 
and permeate many junction materials. Also, certain components of the 
filling solution tend to precipitate when coming into contact with the 
measured solution within the junction. For example, AgCl and Ag.sub.2 S 
tend to precipitate within the junction of Ag/AgCl reference electrodes 
immersed in dilute chloride- and sulfide-containing samples, respectively. 
In the prior art, failure of the reference junction has usually meant 
replacement of the entire reference electrode, an expensive, undesirable 
solution where the reference junction is often the least expensive 
component of the reference electrode. Attempts to replace the reference 
junction by the laboratory practitioner have usually ended in failure, 
since in most high-quality electrodes, the junction is permanently fused 
or cemented to the electrode body. Even in electrode designs where the 
junction is held within an orifice by friction or pressure alone, the 
junction is typically too fragile to withstand the forces required for 
removal or insertion. Finally, even if the junction could be removed by 
force, some portion of the porous junction material would have to extend 
beyond the electrode body to allow traction. But, it has been found that 
protrusion of the porous material into the measured solution may 
contribute somewhat to slow and inaccurate response by introducing an 
element of spherical rather than planar diffusion. A flat junction surface 
is preferable, which is incompatible with protrusion of the porous member. 
Turning now to the known prior art, a 1979 Graphics Controls catalog shows 
a commercially available renewable junction electrode. A new junction is 
created by pulling the threadlike woven fiber junction to expose a fresh 
increment. When the built-in supply of woven fiber junction is exhausted, 
the entire electrode must be replaced. 
U.S. Pat. No. 4,282,081 discloses a double junction reference electrode 
having a removable sealing plug at the lower end of the lower or external 
junction electrolyte compartment, which plug contains a conduit extending 
axially through the plug and providing a flow-restrictive fluid permeable 
path between the lower compartment and a test system external of the 
electrode, i.e., an outer liquid junction. The conduit or outer junction 
preferably is made of porous ceramic. Thus, the outer junction is 
replaceable only by replacing a portion of the electrode, i.e., the 
removeable sealing plug. According to the patent, the electrode components 
are separable at threaded junctions to allow easy access to the internal 
chambers for thorough cleaning and convenient refilling. There is no 
recognition in the patent regarding outer junction clogging or the 
problems associated therewith, and no indication that an ability to 
replace the outer junction is advantageous. Furthermore, such replacement, 
if actually done, requires replacement of the removable sealing plug which 
contains the outer junction. Moreover, the need to replace the entire plug 
prevents the use of such a concept in a combination electrode, an inherent 
disadvantage. As already noted, however, cleaning and refilling are the 
only reasons cited for dismantling the electrode. 
U.S. Pat. No. 2,058,761 relates to an apparatus for testing acidity. The 
apparatus includes two electrodes, one of which comprises a glass tube 
open at both ends but tapered inwardly at the lower end to constitute a 
seat portion of frustro-conical shape, in which lower end there rests a 
frustro-conical plug. While the plug prevents practically all leakage of 
fluid from the tube, there always is present a thin film of solution 
between the surfaces of the plug and the tube to provide ready conduction 
of current between the solution to be tested and the electrolyte contained 
in the tube. The composition of the plug is not specified. 
U.S. Pat. No. 3,530,056 describes a flexible liquid junction which includes 
a flexible, electrically insulating sheath, within which is disposed at 
one end an electrically insulating porous plug and a wettable wicking 
which extends from the plug to the other end of the sheath. The plug can 
be a porous ceramic. The patent neither teaches nor implies that such 
flexible liquid junction is removeable and replaceable.

SUMMARY OF THE INVENTION 
It is an object of this invention to provide an improved reference 
electrode. 
A more specific object of this invention is to provide a reference 
electrode including a replaceable reference junction. 
Another object of this invention is to provide a reference junction of 
design and strength capable of being removed and replaced by the 
laboratory practitioner or equipment operator with simple laboratory 
tools. 
Still another object of this invention is to provide a replaceable 
reference junction which improves equipment performance. 
Other objects of the invention will be apparent to the skilled artisan from 
the detailed description of the invention hereinbelow. 
In accordance with the present invention, there is provided a reference 
electrode comprising an enclosure containing a half-cell electrode, a 
half-cell electrolyte, and a reference junction positioned in an outlet 
for the electrolyte, the half-cell electrode being electrically 
connectable to an external measuring means, said reference junction being 
removable and replaceable. More specifically in a preferred embodiment, 
the reference electrode comprises a sealing grommet positioned tightly in 
the electrolyte outlet and around which liquid flow essentially does not 
occur, with the grommet having at least one eyelet through which the 
removable and replaceable reference junction can be inserted and removed. 
The reference junction comprises a capillary tube, of length longer than 
the grommet, encasing a porous medium, such as a porous ceramic rod. The 
capillary tube and porous medium are longitudinally coextensive. Liquid 
flow does not essentially occur around the capillary tube positioned 
through the grommet. 
In a more preferred embodiment of the present invention, the capillary tube 
is glass and the porous member is a porous ceramic rod enclosed in the 
capillary tube and coextensive therewith. As used herein, the term "porous 
ceramic" means a porous, rigid, non-metallic solid prepared by the 
sintering or firing of one or more inorganic precursors. 
In another preferred embodiment of the present invention, a pH or other 
ion-selective sensing bulb also extends through a grommet in a combination 
electrode configuration. 
Another preferred embodiment of this invention involves heating the glass 
capillary tube containing the porous member sufficiently to bond or clad 
the glass to the porous member. 
DETAILED DESCRIPTION OF THE INVENTION 
As already noted, the term "porous ceramic" is used herein to mean a 
porous, rigid, nonmetallic solid prepared by the sintering or firing of 
one or more inorganic precursors. In a preferred embodiment, such 
nonmetallic solid is substantially crystalline. In another preferred 
embodiment, such inorganic precursors are particulate. 
The porous ceramic which is used in the present invention generally is 
prepared in accordance with methods which are well known to those having 
ordinary skill in the art. The inorganic precursors also are well known to 
those having ordinary skill in the art. By way of illustration only, some 
examples of suitable inorganic precursors include silica, silica-based 
glasses, feldspar, alumina, clays, and the like. 
FIG. 1 shows the elements of a pH measurement system. A pH electrode 1 and 
reference electrode 3 are partially immersed in sample solution 5 inside 
of container 8 and both electrodes are electrically connected by 
conductors 13 and 15 to electrometer 17. The potential across the glass 
sensing-membrane 7 changes in proportion to differences in pH between 
external sample solution 5 and a pH buffer solution 9 contained within the 
sensor membrane. An electrochemical half-cell 10 is used to establish a 
stable electrical connection between the buffer solution 9 and the wire 
conductor 13 going to the electrometer. This half-cell has a fixed 
potential determined by the chloride-ion concentration of the buffer. The 
difference in potential between the external solution 5 and the positive 
electrometer terminal now changes with pH, and it is this change in 
potential that is monitored. The role of the reference electrode is to 
establish a fixed half-cell potential between the external measured 
solution and the negative electrometer terminal. In measurements of 
unknown solutions, the half-cell cannot be directly immersed in the 
sample, since its potential will vary with the unknown anionic, e.g., 
chloride ion activity, of the solution. Therefore, an indirect reference 
connection is made by immersing the reference half-cell 11 into a known 
electrolyte 19 (usually AgCl-saturated 4M KCl), and then establishing 
physical and electrical contact between this electrolyte and the measured 
solution though a reference junction 21 positioned in outlet 23. The 
reference junction usually consists of a porous ceramic plug, asbestos 
fiber, or other means of achieving a fluid mechanical leak. The reference 
junction functions primarily as a flow restrictor and filtration member 
and also serves to define the shape of the interface between the 
solutions. Ideally, the junction is sufficiently porous to allow a low 
resistance contact, preferably well below 10K ohm, between the external 
and internal solutions, but is not so porous that the solutions become 
mutually contaminated. 
FIG. 2 is a detailed cross-sectional view of a conventional reference 
electrode. 
FIG. 3 is a detailed cross-sectional view of the reference electrode of 
FIG. 2, but including a reference junction as in the present invention. 
In FIGS. 2 and 3, reference electrode 30 includes electrochemical half-cell 
32, electrical conductor 34, electrolyte solution 36 and outlet 38 through 
which the reference junction will communicate with the sample solution, 
not shown. In FIG. 2, ceramic plug 40 is inserted into outlet 38, while in 
FIG. 3, a grommet 42 is fixedly inserted into the outlet, through which 
there is found hollow glass capillary tube 44 having porous ceramic plug 
46 encased therein and being of essentially the same length as the tube. 
The tube and encased plug comprise the replaceable reference junction per 
se of this invention. 
FIG. 4 depicts in cross section a shipping container 39 formed of mating 
screw cap lid 41 and body 43, in which there is placed a plurality of 
replaceable junctions 45 of the present invention. In the shipping 
container, the replaceable junctions are completely immersed in solution 
50, which is preferably the inner electrolyte into which the reference 
junction will be inserted so that an equilibration time is not required 
when the junction is inserted into the reference electrode. 
FIG. 5 depicts a preferred embodiment of the replaceable junction of the 
present invention in which glass capillary tube 52 of about two 
millimeters in outside diameter, about one millimeter in internal diameter 
and about twelve millimeters long is employed as the encasing sheath. A 
porous ceramic plug 54, about one millimeter in diameter and of about the 
same length as the tube, is inserted into one end of the 12 millimeter 
long tube. A high flame, say about 1250.degree. C., is run over the 
outside of the tube encasing the ceramic to seal or clad the glass to the 
ceramic. Thereafter, end 56 of the tube is ground flat to the ceramic and 
end 58 is fire-polished or beveled to facilitate insertion into the 
grommet. Where desired, annealing of the cladding can be carried out. 
FIG. 6 is a cross section of the lower end of a combination electrode 60 
where both glass electrode 62 and reference junction 70 extend through 
grommet 66. Junction electrolyte 68 is contained within the combination 
electrode. Reference half-cell 64 extends into the electrolyte. Reference 
junction 70 extends through the grommet. 
Furthermore, since a general feature of the present invention is the 
encasing of the porous junction member within an encasing body which 
facilitates its removal and insertion into the body of the reference 
electrode, this may be achieved by means other than those illustrated 
above. For example, rather than affixing the grommet to the electrode 
body, grommet means could be incorporated as an integral part of the 
removable encasing body. 
In a further example, no grommet would be used, but either the removable 
encasing body itself or the reference electrode orifice would be made of a 
slightly compressible material, e.g., polypropylene, allowing a tight, 
leak-free fit between the junction-encasing body and electrode body. In 
FIG. 7, removable polypropylene encasing body 80 for porous ceramic member 
82 fits tightly into glass orifice 84 formed in electrode body 86. 
D. P. Brezinski, in application Ser. No. 218,788, has invented a 
replaceable reference junction which differs from the present invention in 
that in Brezinski's invention the filtration member is substantially 
shorter in length than the glass capillary tube; that is, the filtration 
member is not coextensive from end to end with the capillary tube. 
In the present invention, the long capillary tube provides sufficient 
strength for removal and insertion of the reference junction through the 
grommet without breaking. 
A method for preparing reference junctions of the present invention is set 
forth below. 
A length of glass tubing, say about 18 millimeters long and say of outside 
diameter of about 2 millimeters to fit snugly through the grommet and of 
inner diameter of say about one millimeter, is filled along its entire 
length or part of its length with a reference junction porous medium, such 
as a plurality of short, say each about 1.6 millimeters long, porous 
ceramic plugs, or asbestos fibers. The porous medium is packed into the 
tube so that its diameter is about the same as the inner diameter of the 
tube; that is, it fills the cross-section of the tube. 
Next, a high flame, say about 1250.degree. C. is run over the length of the 
tube containing the filtration medium to clad or bond the tube to the 
filtration medium. Then the cladding is cut to length for use as a 
reference junction and, where desired, can be annealed. Any portion of the 
tubing not containing the filtration medium is discarded. 
A suitable filtration medium is Corning Glass Works high flow ceramic No. 
003789. 
Variations of the present invention will be apparent to the skilled 
artisan. For example, any porous member used as a junction in reference 
electrodes in the art should be usable in the present invention, as long 
as a compatible casing is known. For example, some porous members might 
not be usable where the glass tube is to be fired, but a plastic casing 
flowing at lower temperature or sealable by means of solvent gluing or 
epoxy cement could be employed, for example, a porous polypropylene or 
polyvinylidene plug encased in a non-porous tube of the same composition. 
It is possible for the porous member to extend out of the encasing sheath, 
although such a configuration is not preferred. Preferably, when used, the 
reference junction extends beyond the grommet in both directions, so that 
one end of the reference junction can easily be grasped by tweezers or 
other simple laboratory tool for removal, while the other end of the 
reference junction extends into the internal electrolyte solution. 
Furthermore, although the present invention has been illustrated by means 
of single junction reference electrode embodiments, the present invention 
is also applicable to double junction reference electrodes where the 
reference junction of the present invention would preferably be used as 
the reference junction between the electrode and sample solution (external 
junction), although it could also be used as the internal reference 
junction, particularly in pull-apart electrode designs. In addition, 
different types of electrochemical half-cells, such as silver-silver 
chloride, calomel and so on can be employed with the present invention. 
The reference junction of the present invention could also be used with 
gelled internal electrolytes.