Combination electrode assembly

An electrochemical combination electrode assembly having an inner, tubular glass pH electrode body supported within a tubular outer plastic container, a reference electrolyte reservoir defined in an annular space between the container and the glass electrode, and an annular, pressure contact leakage junction defined between an inclined surface and edge on the glass electrode body and plastic container for establishing electrolytic communication between the reference electrolyte and a test solution. Preferably the leakage junction is established at the junction line of an axially tapering exterior surface of the glass electrode body and a circular edge on the plastic container surrounding the tapered surface. The glass electrode is nested in the plastic container until the tapering surface and circular edge engage. The degree of pressure engagement establishes the flow rate through the junction.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to electrochemical electrodes and, 
more particularly, to combination electrodes for measuring the ion 
concentration of solutions. 
2. Description of the Prior Art 
Combination electrodes comprising a sensing portion and a reference portion 
are well known in the art. One known electrode assembly for measuring pH 
includes a tubular, glass pH electrode coaxially supported within a 
durable, tubular, plastic container to define an annular reference 
electrolyte reservoir between the electrode body and the container. In 
such a structure it is necessary to establish electrolytic contact between 
a reference electrolyte within the annular reservoir and a test solution 
into which the electrode is immersed. Typically this is accomplished by 
means of a wettable material, such as asbestos or linen fibers, which 
provides a minute flow rate leakage path or liquid junction between the 
electrolyte and the test solution. For example, the leakage path of one 
commercial combination electrode assembly, illustrated in copending 
application Ser. No. 629,833, (now U.S. Pat. No. 4,012,308) filed Nov. 7, 
1975, and assigned to the assignee of the present invention, comprises a 
plurality of asbestos fibers spaced around and extending through an 
annular seal between an inner glass pH electrode body and an outer plastic 
container. 
While asbestos fiber liquid junction structures generally function 
satisfactorily, they have several drawbacks and limitations. First, they 
require tedious operations by hand to incorporate in an electrode 
assembly. Second, they do not readily lend themselves to the formation of 
an annular liquid junction as would be desirable in combination electrodes 
having annular electrolyte reservoirs. Third, they can easily become 
clogged during use. 
U.S. Pat. No. 3,492,216 (Riseman et al.) proposes a combination, 
ion-selective electrode assembly incorporating an annular liquid junction 
structure without the use of asbestos fibers or other liquid junction 
materials. In this regard, the electrode aseembly includes inner and outer 
tubular plastic bodies. The exterior surface at one end of the inner body 
is outwardly tapered as a frustoconical end portion. The adjacent interior 
surface of the outer body is conically tapered at the same angle as the 
frustoconical end portion so that the inner body will nest within the 
outer body with the tapered surfaces mating with each other and the ends 
of the bodies defining a planar sensing surface of the electrodes 
assembly. The mating tapered surfaces are roughened to establish an 
annular leakage path between an electrolyte reservoir within the electrode 
assembly and a test solution into which the assembly is to be immersed. 
A similar precision-fit, annular leakage junction is described in U.S. Pat. 
No. 2,058,761 (Beckman et al.) for a single reference electrode and 
comprises a frustoconical plug seated within a glass tube to establish an 
annular leakage path through the contact area between the plug and the 
tube. 
In both of the foregoing approaches, precision grinding and preparation of 
the mating tapered surfaces is required to establish the perfect surface 
area contact therebetween. Obviously, it would be desirable to provide an 
annular leakage path for a combination electrode in a manner which 
eliminates the precision manufacturing steps heretofore employed without 
sacrificing the durability and reliability of the electrode. The present 
invention meets these needs. 
SUMMARY OF THE INVENTION 
Briefly, and in general terms, the present invention resides in a new and 
improved ion sensitive combination electrode assembly of commercially 
practical form and incorporating an annular liquid junction which 
overcomes the disadvantages of the prior electrodes. The electrode 
assembly is simple in construction and reliable in operation and is 
constructed in a straightforward manner without precision manufacturing 
steps heretofore employed. 
To these ends, the present invention comprises a combination electrode 
assembly including an outer tubular container and an inner electrode body 
of nonconductive material. The container includes an open end and an inner 
annular surface adjacent thereto. The electrode body is dimensioned to fit 
coaxially into the container through the open end thereof and includes an 
ion sensitive structure and an annular exterior surface adjacent thereto. 
One of the inner and exterior annular surfaces is inclined relative to the 
other and the other includes a circular edge dimensioned to engage and 
make line contact with the inclined surface as the electrode body is 
inserted into the container. The circular line contact between the edge 
and the inclined surface defines an annular liquid junction for the 
electrode assembly and the degree of pressure between the container and 
electrode body at the contact line determines at least in part the liquid 
flow rate through the junction between an electrolyte reservoir within the 
assembly and a test solution into which the assembly is immersed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in the drawing for purposes of illustration, the invention is 
embodied in an ion sensitive, combination electrode assembly, indicated 
generally by numeral 10. The electrode assembly 10 comprises an indicating 
electrode 12, of generally tubular configuration, supported near its 
opposite ends to extend coaxially within a generally tubular outer 
container 14 formed of a durable nonconductive material such as 
polyethylene, polypropylene, or a fluorocarbon plastic. Container 14 could 
also be made of glass. As illustrated in FIG. 1, the support for the upper 
end of electrode 12 within container 14 comprises an elastomeric 
sleeve-like structure of silicone rubber including an annular sealing band 
16 and an adhesive layer 18 disposed in an annular space between the 
electrode and the container. The support for the lower end of the 
electrode comprises a novel annular junction, indicated generally by 
numeral 20, between the electrode 12 and container 14. 
For measuring pH, the indicating electrode 12 comprises a generally 
tubular, glass electrode body the lower or sensing end of which is closed 
by a spherical membrane 22 of pH sensitive glass. The interior of the 
indicating electrode 12 defines a first or internal electrolyte reservoir 
24 immediately adjacent membrane 22 and filled with a suitable electrolyte 
26, such as an aqueous solution of potassium chloride. A conventional 
indicating half cell 28 comprising a silver wire conductor coated at its 
lower end with silver chloride extends axially into the internal reservoir 
and is immersed in the electrolyte 26. 
The annular space above the annular junction 20 and between the exterior of 
the electrode 12 and the interior of the container 14 defines a second or 
reference electrolyte reservoir 30 filled with a second or reference 
electrolyte 32, such as potassium chloride. In the preferred embodiment, 
electrolyte 32 includes a gelling agent to minimize electrolyte loss 
during operation of the assembly. A conventional reference half cell 34 
comprising a silver wire conductor coated at its lower end with silver 
chloride extends axially into the reference electrolyte reservoir and is 
immersed in the electrolyte 32. The reference half cell is retained for a 
major portion of its length between electrode 12 and a conductive 
heat-shrink tube 36 coaxially circumscribing and secured by a shrink fit 
to the electrode 12. 
In accordance with a primary aspect of the present invention, the annular 
junction 20 supporting the electrode 12 within the container 14 also 
provides an improved liquid junction structure for establishing a leakage 
path between the reference electrolyte 32 within the electrode assembly 
and a test solution into which the electrode assembly is immersed. To this 
end, and in one form of the invention only, a section 37 (FIG. 2) of the 
tubular glass electrode body 12 tapers outwardly from the electrode axis 
to provide an axially and outwardly tapering surface 38 near the sensing 
end of the glass electrode 12. As illustrated, the enlarged section 37 of 
the electrode 12 presents a substantially frustoconical tapering exterior 
surface 38 extending axially a distance L along the electrode body and 
varying in diameter between first and second values, D.sub.1 and D.sub.2. 
In the preferred embodiment, surface 38 tapers at an angle of about 
10.degree. with respect to the electrode longitudinal axis. 
The glass electrode 12 is supported within tubular container 14 by an edge 
40 on an internal surface of the container coaxially surrounding and 
engaging the tapered surface 38 of the electrode 12. As illustrated in 
FIG. 2, the lower end of tubular container 14 includes an inwardly 
projecting, annular collar 42 which defines a circular opening 43 within 
the end of the container for receiving electrode 12. Significantly, and as 
illustrated in FIG. 3, collar 42 has an axially extending, inwardly 
facing, substantially cylindrical surface 44 inclined with respect to the 
axially tapering surface 38. The cylindrical inner surface 44 terminates 
at a flat lower surface 46 of the collar to define the circular edge 40. 
The tapering surface 38 of electrode 12 is engaged by the edge 40 to 
establish a circular line contact junction between the frustoconical 
section 37 and the container 14. 
Significantly, the annular junction 20 can be located at any point along 
the length L of tapered surface 38. For this purpose, the diameter of the 
opening 43 in the end of container 14 may have any value between the 
diameter values D.sub.1 and D.sub.2 of tapered surface 38. As the diameter 
of opening 43 approaches D.sub.1, the annular junction 20 will be located 
progressively toward the lower end of the tapered surface. Because of the 
tolerance in the diameter of opening 43, no precision machining or 
grinding is required to establish a proper fit between and support for the 
electrode body 12 within the container 14. Representative dimensions in 
the preferred embodiment are L=0.40 in. and D.sub.1 =0.32 in. Collar 42 
extends inwardly 0.06 in. from the interior wall of container 14, and 
cylindrical surface 44 of the collar extends 0.075 in. in the direction of 
the electrode axis. 
In the preferred embodiment, the edge 40 engaging the tapered surface 38 is 
located at a lower inner corner of the collar 42. The collar is slightly 
compliant and when the electrode 12 and container 14 are nested, the 
collar flexes slightly around its annular periphery thereby securely 
seating against tapered surface 38. However, if desired, and to further 
simplify construction of tubular container 14, collar 42 may be eliminated 
as illustrated in FIG. 4. In such an embodiment, the diameter of the 
interior wall of container 14 may be reduced to a value intermediate the 
D.sub.1 and D.sub.2 values allowing the lower inner circular edge of the 
container to function as the circular edge 40 engaging the surface 38 of 
electrode 12 and establish the liquid junction 20. 
Applicant has discovered that a controllable liquid leakage path is 
provided by the annular junction 20 between electrolyte 32 within the 
electrode assembly and a test solution. In this regard, the rate of flow 
through the annular junction 20 may be controlled by adjusting pressure 
between surface 38 and edge 40 during assembly of the electrode. The 
degree of pressure between surface 38 and edge 40 is originally 
established by trial and error. With minimal practice, an assembler can 
establish the necessary pressure engagement simply by feel as the parts 
are engaged. Further, it has been found that the flow rate through annular 
junction 20 can be controlled by the characteristics of the junction 
forming surfaces. In this regard, it has been found desirable to abrade 
tapered surface 38. This may be accomplished by rubbing the surface with a 
number 3F carborundum or aluminum oxide emory cloth or by sand blasting or 
chemical etching of the surface. 
To complete the illustrated electrode assembly 10, the upper end thereof is 
closed and electrical connections are made to the indicating and reference 
half cells 28 and 34 in a conventional manner. For this purpose a 
conventional shielded coaxial cable 48 containing a plurality of 
conductors insulated from each other extends through the base of a 
conductive metal cap 50 into the upper end of tubular container 14. The 
cap 50 includes side wall portions coaxially surrounding the upper end of 
container 14 and secured thereto by a layer of epoxy adhesive 52. 
Coaxial cable 48, which connects the electrode assembly 10 to a pH or other 
measuring system, includes a center conductor 54 soldered to indicating 
half cell 28 and a second conductor 56 soldered to reference half cell 34. 
If desired, an insulating shrink tube may be secured in place around the 
solder connections to electrically insulate the same. 
Annular sealing band 16 seals reference electrolyte 32 in the annular 
reservoir 30. If desired, sealing band 16 may directly contact the 
electrolyte to prevent formation of air bubbles in the reservoir. A 
sealing disc or plug 58 closes the top of the internal electrolyte 
reservoir 24 and may similarly contact electrolyte 26 therein, if desired. 
A space within glass electrode body 12 above sealing disc 58 is filled 
with an epoxy sealing material 60 which is cured to secure the sealing 
disc in place and to insulate and mechanically strengthen the solder 
connection between half cell 28 and center conductor 54 of the coaxial 
cable. In a similar manner, the sealing material 18 at the upper end of 
the annular reservoir 30 above band 16 holds the band in place and 
insulates and mechanically strengthens the remaining electrical 
connections to the coaxial cable. In addition, this sealing material 
further serves to secure electrode 12 and container 14 in a fixed relative 
axial position after establishment of the desired annular junction 20 at 
the sensing end thereof. 
In accordance with another aspect of the present invention, the electrode 
assembly 10 includes an integral thermal sensing element 62. The element 
combines with external circuitry (not shown) to compensate for variations 
in temperature of the test solution. As is well known, the voltage 
developed by a pH electrode is a function of temperature. Thermal sensing 
element 62 includes a conventional thermistor covered with a protective 
layer of epoxy and disposed adjacent the sensing end of the electrode 
assembly 10. For this purpose, the wall of tubular container 14 includes 
an axially extending slot or passage 64 extending most of the length of 
the tubular container for receiving first and second leads of the 
thermistor 62. At its lower end, slot 64 communicates with a bore 66 
through the remainder of the axial length of the container wall. 
Thermistor 62 is disposed at the bottom of container 14 with first and 
second conductor leads (only one such conductor 67 is illustrated) 
extending therefrom upwardly through bore 66 and slot 64 to the upper end 
of the electrode assembly. Bore 66 is filled with an epoxy adhesive 68 for 
securing the thermistor in place. Preferably, each conductor of the 
thermistor is sheathed within an insulating plastic tube to ensure 
adequate insulation between the conductors. The conductors extend through 
a small port 70 in the wall of tubular container 14 and are solder 
connected to respective insulated conductors of the coaxial cable assembly 
48. The connection for conductor 67 is illustrated in FIG. 1. After the 
thermistor 62 is electrically connected to the coaxial cable 48, slot 64 
may be filled with a sealing compound, such as epoxy, to protect the 
conductors therein. 
Thermal sensing element 62 and its associated circuitry (not shown) provide 
rapid thermal compensation of the electrode assembly 10 since the thermal 
element directly contacts the test solution in which the electrode 
assembly is immersed and responds directly and rapidly thereto. 
Significantly, the thermal sensing element is provided integral with the 
electrode assembly without any attendant increase in size or complexity of 
the assembly. 
While the basic structure of the electrode assembly 10 has been described 
hereinabove, an even clearer appreciation of the simplicity and novel 
features of the invention and its manufacture may be achieved from a 
consideration of the manner of assembly of the invention. In this regard, 
thermistor 62 is first dipped in epoxy and the epoxy allowed to cure. 
Thereafter, two pieces of polyethylene tubing (60 clay adams) are slipped 
over the respective thermistor leads and the thus insulated leads are fed 
through bore 66 in tubular container 14 until the sensing element is 
positioned at the end of the container. Epoxy adhesive 68 is injected into 
the bore 66 and allowed to cure to secure the sensing element 62 in place. 
The two conductors are then pulled tight and the upper ends thereof 
inserted through passage 70 into the interior of container 14. 
Next, glass electrode body 12 with its outwardly tapered section 38 is 
formed using well known glass blowing techniques. Thereafter, pH sensitive 
glass bulb 22 is fused to the lower end of body 12. 
Conventional indicating and reference half cells 28 and 34 are provided. 
Half cell conductor 34 is positioned on the outside of glass electrode 12 
with its lower end just short of tapered surface 38. Conductive heat 
shrink tubing 36 is slipped over the electrode 12 and the half cell 34 and 
is shrunk in place with a portion of the half cell protruding from the 
bottom of the shrink tube. 
The internal electrolyte reservoir 24 of electrode 12 is filled with a 
potassium chloride electrolyte solution. Indicating half cell 28 is 
inserted through sealing disc 58 and the half cell 28 and the sealing disc 
are inserted together into the electrolyte reservoir 24 with the half cell 
immersed in the electrolyte. Part of the volume within the glass tube 
above sealing disc 58 is filled with a sealing adhesive 60 which is cured 
to hold sealing disc 58 in place. 
Next the tubular plastic container 14 is slipped over the upper end of the 
glass electrode 12 and telescoped axially therealong until the edge 40 on 
inwardly projecting collar 42 engages tapered surface 38 in proper 
pressure engagement therewith. As indicated, this is a simple and 
straightforward operation for forming annular pressure contact junction 20 
and is readily mastered by a semi-skilled assembler. 
Annular reservoir 32 is then filled with a potassium chloride electrolyte 
and sealing band 16 is positioned between electrode 12 and container 14 to 
center the electrode within the container and to seal the reference 
electrolyte reservoir 30. A small volume above sealing band 16 is filled 
with epoxy which is cured to hold the band in place. 
Finally, electrical connection is made with the electrode assembly 10. In 
this regard, the indicating half cell 28 is soldered to center conductor 
54 of coaxial cable 48. Reference half cell conductor 34 is soldered to 
twined strands 56 from the shielding layer of the cable. The conductive 
leads from thermistor 62 are similarly soldered to respective conductors 
of the coaxial cable. Each solder connection may be covered with an 
insulating shrink tube to adequately electrically insulate the connection. 
The remaining volume within the top of electrode 12 is filled with epoxy 
and the cable 48 and soldered center conductor 54 thereof are pushed into 
the epoxy filled end of electrode 12. Thereafter, the remaining cavity 
within the upper end of tubular container 14 above sealing band 16 is 
filled to the top of container 14. The epoxy thus introduced into 
electrode 12 and container 14 is allowed to cure while keeping cable 48 
straight. After curing, cap 50 is located over the end of the electrode 
assembly 10 and secured by adhesive 52. As assembled, the electrode 
assembly is approximately 5.5 inches in length and 0.65 inch in diameter. 
From the foregoing it is evident that the electrode assembly 10 is 
extremely simple in design and in manner of assembly. The annular junction 
20 is provided as a circular pressure contact junction between a circular 
edge and an inclined surface on different ones of the inner electrode body 
12 and the tubular container 14. The junction is, in effect, selfadjusting 
during assembly to seat and locate the annular junction 20 at a position 
along the length L of the tapered surface 38 within a range of possible 
positions. The exact location is not critical and precision machining or 
grinding of corresponding surfaces to critically locate the annular 
junction is unnecessary. Moreover, the electrode assembly provides an 
integral thermal sensor for directly contacting the test solution to 
compensate for temperature variations of the solution. 
While a preferred embodiment of the invention has been illustrated and 
described, various modifications can be made therein without departing 
from the scope of the invention as defined in the following claims.