U.S. patent number RE31,333 [Application Number 06/183,681] was granted by the patent office on 1983-08-02 for electrochemical reference cell with improved liquid junction.
Invention is credited to Theodore R. Barben, II.
United States Patent |
RE31,333 |
Barben, II |
August 2, 1983 |
Electrochemical reference cell with improved liquid junction
Abstract
An electrochemical reference cell has a reference electrode
disposed at one end of a rigid hollow cylinder immersed in an
electrolyte solution of known concentration. Different interlocking
series of plugs, consisting of a material such as wood, with
semipermeable, longitudinally extending capillaries, are fitted
together in abutting relation to fill the hollow interior of the
cylinder between the reference cell and the other end with one
series longitudinally overlapping the other. The abutting end
surfaces of adjacent plugs in each series are sealed to close off
the capillary path between successive plugs, whereas the outer
surface of the last plug in at least one series is left unsealed to
be in contact with the test solution, so that circuitious ion
transfer path is established between the reference cell and the
test solution that passes tranversely through the longitudinal
semipermeable capillary walls from the plugs in one series through
adjacent plugs in the other.
Inventors: |
Barben, II; Theodore R. (Carson
City, NV) |
Family
ID: |
26879425 |
Appl.
No.: |
06/183,681 |
Filed: |
September 3, 1980 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
841169 |
Oct 11, 1977 |
04112352 |
Sep 5, 1978 |
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Current U.S.
Class: |
324/450;
204/435 |
Current CPC
Class: |
G01N
27/401 (20130101) |
Current International
Class: |
G01N
27/30 (20060101); G01N 27/401 (20060101); G01N
27/28 (20060101); G01N 027/28 () |
Field of
Search: |
;324/438,450,446
;204/405,409 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tokar; Michael J.
Attorney, Agent or Firm: Nilsson, Robbins, Dalgarn,
Berliner, Carson & Wurst
Claims
I claim:
1. A reference cell for use in measuring the .[.pH.]. .Iadd.ion
concentration .Iaddend.of a sample fluid stream comprising:
an elongated rigid enclosure having an open end exposed to said
sample fluid and a closed end;
at least two transversely adjacent longitudinal series of plug
means disposed in longitudinally overlapping relationship with an
interlocking fit to fill the open end of said enclosure;
a reference cell containing a reservoir electrolyte solution in
contact with .[.said cell means.]. .Iadd.one of said plugs means in
each of said series .Iaddend.at the .[.opposite.]. .Iadd.close
.Iaddend.end of said enclosure and separated from said sample fluid
by .Iadd.the remaining one of .Iaddend.said plug means;
each of said plug means consisting of successive longitudinally
disposed layers of semipermeable material permeated with said
electrolyte solution;
an impermeable seal between abutting transverse end surfaces of
said plug means in each series and joined to the surrounding
longitudinal surfaces of the overlapping adjacent plug means and
said enclosure at approximately midway between the end portions of
the adjacent series, the outer end surface of at least one plug
means exposed to said sample fluid at the open end being unsealed
to provide fluid communication between said layers;
whereby an ion transfer path between said sample fluid stream and
said electrolyte solution is established transversely through
successive layers of said semipermeable material in passing between
the longitudinal surfaces of the plug means in one series to the
abutting overlapping longitudinal surfaces of adjacent plug means
in the other series.
2. The reference cell of claim 1 wherein:
each of said plug means is solid wood with the grain oriented so
that elongated interior capillary cells are substantially aligned
longitudinally.
3. The reference cell of claim 2 wherein:
said impermeable seal consists of a coating of epoxy adhesive
applied to the .[.abuttiing.]. .Iadd.abuttiing .Iaddend.transverse
end surfaces of adjacent plug means.
4. The reference cell of claim 3 wherein:
said reservoir of electrolyte solution consists of a saturated salt
solution.
5. The reference cell of claim 4 wherein:
said saturated salt solution consists of an aqueous chloride salt
solution with an excess of undissolved salt immersed therein, said
sealed enclosure having its end opposite said open end closed to
form a reference cell cavity surrounding a reference electrode
within.
6. The reference cell of claim 1 wherein:
the two transversely adjacent longitudinal series of plug means
consist of an inner series of solid cylindrical plugs inserted
slidably received into the longitudinal bore of an outer series of
longitudinally aligned .[.toroid.]. .Iadd.annular .Iaddend.shaped
plugs; and,
said rigid enclosure being an elongated tube of impermeable
material for slidably receiving said .[.toroid.]. .Iadd.annular
.Iaddend.shaped plugs.
7. The reference cell of claim 6 wherein:
said inner series of solid cylindrical plugs consists of wooden
dowels with the grain oriented to align the interior capillary
cells in a substantially longitudinal direction;
said outer series consists of .[.toroid.]. .Iadd.annular
.Iaddend.shaped wooden sections with a central bore diameter
matching the diameter of the dowels; and,
said elongated sealed enclosure comprises a plastic tube having an
inner diameter matching the outer diameter of said tubular wooden
sections.
8. The reference cell of claim 7 wherein:
said impermeable seal consists of a coating of epoxy adhesive
applied to the abutting transverse end surface .[.os.]. .Iadd.of
.Iaddend.adjacent .[.cell.]. .Iadd.plug .Iaddend.means.
9. The reference of claim 8 wherein:
said reservoir of electrolyte solution consists of a saturated salt
bridge solution.
10. The reference cell of claim 9 wherein:
said saturated salt bridge solution consists of an aqueous chloride
salt solution with an excess of undissolved salt immersed therein,
said rigid enclosure having its closed end defining a reservoir
cavity surrounding a reference electrode with the central bore of
the adjacent .[.toroid.]. .Iadd.annular .Iaddend.shaped wooden
section.
11. The reference cell of claim 1 wherein:
said transversely adjacent longitudinal series of plug means
consist of a first series of .[.toroidal.]. .Iadd.annular
.Iaddend.wooden sections longitudinally aligned with a central bore
for slidably receiving a tubular pH sensitive glass electrode, each
of said .[.toroidal.]. .Iadd.annular .Iaddend.wooden sections
having a pair of longitudinal side apertures uniformly displaced
from and on opposite sides of said central bore, and said adjacent
series of plug means consists of solid wooden dowels slidably
inserted into the longitudinally aligned holes of said
.[.toroidal.]. .Iadd.annular .Iaddend.wooden sections, the grain of
the wood in both series being oriented to lontitudinally align the
elongated capillaries within the wood.
12. The reference cell of claim 11 wherein:
said impermeable seal consists of a coating of epoxy adhesive
applied to the abutting transverse end surfaces of adjacent plug
means.
13. The reference of claim 12 wherein:
said reservoir of electrolyte solution consists of a saturated salt
solution.
14. The reference cell of claim 13 wherein:
said saturated salt solution consists of an aqueous chloride salt
solution with an excess of undissolved salt immersed therein, said
rigid enclosure having its closed end defining a reservoir cavity
surrounding a reference electrode.
15. The reference cell of claim 14 wherein:
said elongated rigid enclosure comprises a sealed plastic tube
having an inner diameter matching the outer diameter of said
.[.toroidal.]. .Iadd.annular .Iaddend.wooden sections.
16. The reference cell of claim 11 wherein:
said solid wooden dowels are inserted midway into the side
apertures of only alternate pairs of adjacent .[.toroidal.].
.Iadd.annular .Iaddend.sections, whereby a conductive bridge path
is established from the dowel on one side through the
.[.toroidal.]. .Iadd.annular .Iaddend.wood section to the dowel on
the other side.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to electrochemical reference cells, and more
particularly, to systems incorporating such reference cells with
improved liquid junctions for use in continuous monitoring of
process streams.
2. Prior Art
Electrochemical reference cells used in pH or other specific ion
meters typically employ calomel or other metal-metal salt reference
electrodes immersed in a suitable electrolyte of known
concentration that communicates through a liquid junction with the
sample fluid being monitored. The liquid junction maintains a
conductive bridge between the reference cell electrolyte and the
liquid sample to provide a common potential in both solutions,
while mixing of the two solutions is restricted to avoid changes in
the electrolyte concentration that would vary the reference cell
potential.
With earlier pH meters used for laboratory testing, the liquid
junction was simply a minute opening in a glass or ceramic barrier
through which ion communication between the two solutions could be
established. However, with prolonged usage, the single opening
junctions could readily be clogged. As a result, larger liquid
junctions were developed using porous ceramic barriers, asbestos
wicks, ground glass joints, or in some cases, an area of thin
cracks produced by fusing together two glass formulations with
different coefficients of expansion. However, to minimize mixing of
the reference cell electrolyte with the sample fluid with these
larger junctions, two or more liquid junctions placed in series
were often required so that the reference cell electrolyte
communicated through an intermediate salt bridge with the sample
liquid. Nevertheless problems were still encountered in maintaining
the liquid junction open for continuous process control use. Even
with the larger, liquid junctions the small openings would
eventually be plugged either by solid impurities in the process
stream or by crystals formed within the reference cell where an
electrolyte or saturated salt solution was used.
As disclosed in U.S. Pat. No. 3,440,525, issued to Charles P.
Cardeiro, this problem was alleviated by the use of relatively
large diameter wooden or porous ceramic plugs that established the
liquid junction through minute capillaries extending longitudinally
between the sample and reference fluids. The wooden plug was most
effective and could be used wherever the cellulose structure was
not dissolved or otherwise incompatible with the chemical solutions
on either side of the junction. The numerous individual capillaries
extending over the entire surface of the plug were not all readily
clogged even under the worse sample process stream conditions,
whereas the conductive bridge between the two liquids was
maintained through the end walls of adjacent capillary cells the
basic dilemma remained. Eventually, either the plug surface became
completely covered with solid deposits or the salt ion exchange
through the cell walls depleted the electrolyte concentration in
the reference cell causing meter drift. Moreover, although the plug
surface area could be increased to retard fouling by impurities,
the resulting higher rate of ion exchange through additional
capillary paths would more rapidly deplete the electrolyte
concentration.
BRIEF SUMMARY OF THE INVENTION
This invention employs a unique liquid junction structure that has
large end surfaces exposing a multitude of semipermeable cell
openings to the sample stream and to the reference cell so as to
resist fouling by solid deposits, while also providing a high
resistance ion exchange path between the two fluids that maintains
a constant electrolyte concentration in the referencee cell. Thus
large diameter plugs can be used without excessive ion exchange
rates diluting the electrolyte concentration in the reference
cell.
In the preferred embodiments, the liquid junction device consists
of longitudinally overlapping series of wooden plugs, or other
semipermeable cell structures, abutting one another to fill the
interior of a rigid elongated plastic cylinder with one closed end.
Each of the plugs is cut so that the capillaries within the wooden
structure extend longitudinally along the path between the opposite
ends of the elongated enclosure, and the abutting surfaces of
adjacent plugs in each series are sealed to close off direct
communication between capillaries in successive blocks. In this
manner, the ion transfer path between the solutions is directed
transversely through the longitudinally aligned cell walls from one
plug in a series through the interface with abutting surfaces of
the overlapping plugs in the adjacent series to define a circuitous
conductive bridge route passing through numerous semipermeable cell
walls.
In one preferred form, the reference cell employing a liquid
junction in accordance with the invention incorporates a
conventional pH sensitive glass electrode in a pH metering assembly
especially suited for prolonged usage with cooling water process
streams. The liquid junction employs a series of .[.toroid.].
.Iadd.annular .Iaddend.shaped hardwood plugs that are inserted to
fit snugly within the rigid plastic cylinder with their central
bores longitudinally aligned to slidably receive the elongated
glass sensing electrode extending through an opening in the closed
end of the container. Each plug also has a pair of side apertures,
axially displaced on opposite sides of the central bore, for
receiving a series of solid cylindrical hardwood plugs or dowels.
The internal capillary structure of the wood in both series of
plugs extends longitudinally, and the abutting end surfaces of
adjacent plugs are sealed with an epoxy resin or other adhesive
sealant to close off the longitudinal fluid path between successive
plugs. The solid plugs are inserted midway into one of the side
apertures in successive pairs of .[.toroidal.]. .Iadd.annular
.Iaddend.plugs on alternate sides of the central bore, and the
resin or other sealant fills the intervening spaces within the side
apertures to seal off the fluid path between successive dowels on
each side.
The metal-metal salt reference electrode, typically a silver-silver
chloride wire, extends through the closed end of the plastic
cylinder into a cavity defined by one of the side apertures in the
.[.toroidal.]. .Iadd.annular .Iaddend.plug at that end. This cavity
is filled with the appropriate electrolyte or salt bridge solution,
such as saturated potassium chloride and, if desired, an excess of
salt crystals to maintain a saturated concentration. The same
solution impregnates the entire wooden structure within the
cylinder to form the conductive bridge between reference cell and
the sample. In this manner, a convoluted ion exchange path is
established from the sample solution in contact with the exposed
outer surface of the plug at the open end of the plastic cylinder,
passing through the first dowel on one side to the next
.[.toroidal.]. .Iadd.annular .Iaddend.plug, and then through the
wood around the glass electrode to the next dowel inserted into the
other side aperture, and so on through each successive
.[.toroidal.]. .Iadd.annular .Iaddend.plug and dowel, finally
reaching the reference cell cavity.
In a simplier embodiment involving only a reference cell assembly
without an integral sensing electrode, a series of simple
.[.toroid.]. .Iadd.annular .Iaddend.shaped hardwood plugs are
slidably received within the rigid plastic cylinder with their
central bores aligned to receive a series of solid cylindrical
wooden plugs or dowels. An epoxy adhesive seals off the abutting
end surfaces of the successive wooden plugs in each series. The
dowels arranged in a overlapping relationship to extend
approximately midway into the central bore on either side of the
two successive .[.toroid.]. .Iadd.annular .Iaddend.shaped plugs,
and the reference electrode extends into the opening provided
within the cavity formed by the outer half of the central bore of
the .[.toroid.]. .Iadd.annular .Iaddend.plug at the closed end of
the cylinder. As in the previously described embodiment, the entire
wooden structure is impregnated with the appropriate salt bridge
solution and the reference cell cavity filled with an appropriate
electrolyte solution of known concentration. The ion exchange paths
is thus established from the exposed surface of the dowel at the
open end of the cylinder, through the adjacent .[.toroid.].
.Iadd.annular .Iaddend.shaped plug into the next dowel in the
series, and so on, until reaching the reference cell cavity at the
closed end.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is in an exploded perspective view showing the individual
components of one preferred form of the invention that incorporates
a conventional pH sensitive electrode arranged in the proper order
for assembly;
FIG. 2 is a cross-sectional view of the preferred embodiment of the
invention employing the components illustrated in FIG. 1; and,
FIG. 3 is a cross-sectional view of another preferred form of the
invention for providing an electrochemical reference cell with a
separate sensing electrode.
DETAILED DESCRIPTION
Referring now to FIGS. 1 and 2, the preferred form of pH meter
assembly has a conventional glass sensing electrode 10 that extends
through a rigid cylindrical container 12 which houses the other
components. Typically, the container 12 is formed with high density
polyethylene plastic or other material that has the desired
structural rigidity and is inert or otherwise chemically compatible
with the electrochemical system. The container 12 has an open end
14 that communicates with the sample fluid and a closed end 16 is
sealed around the glass electrode 10 inserted through a central
opening. Also a reference electrode wire 18, which is typically the
conventional silver-silver chloride or a calomel type electrode,
extends through a small opening in the closed end 16 and is sealed
in place.
A liquid junction between the reference electrode 18 and the sample
stream at the open end 14 is established through two overlapping
series of interlocking plugs that fill the interior of the
container 12 surrounding the glass electrode 10. In the specific
embodiment illustrated, the first series consist of three thick
walled hollow cylindrical or .[.toroid.]. .Iadd.annular
.Iaddend.shaped larger plugs 20 that fit snugly within the cylinder
12, each having a central bore 22 that slidably receives the
axially disposed glass electrode 10 at the center of the housing.
Each of these .[.toroidal.]. .Iadd.annular .Iaddend.plugs 20 also
has a pair of longitudinally extending side apertures 24 axially
displaced on opposite sides of the central bore 22.
The second series of plugs are solid cylinders 26 and 28 that are
slidably insertable into the side apertures 24 of the
.[.toroidal.]. .Iadd.annular .Iaddend.plugs 20. Two of solid
cylindrical plugs 26 have approximately the same longitudinal
dimensions as the larger .[.toroidal.]. .Iadd.annular
.Iaddend.plugs 20 for insertion midway into the side apertures 24
thus overlapping the longitudinal extent of adjacent pairs of
.[.toroidal.]. .Iadd.annular .Iaddend.blocks 20. Only one solid
plug 26 is used between each adjacent pair of .[.toroidal.].
.Iadd.annular .Iaddend.blocks 20 with successive ones on alternate
sides of the central bore 22. The shorter cylindrical plugs 28 are
only half as long and are inserted into the outer portion of both
side apertures 24 in the .[.toroidal.]. .Iadd.annular
.Iaddend.block 20 located at the open end 14 of the rigid cylinder
housing 12.
In the devices produced for cooling water control systems, the
plugs 20, 26 and 28 are all preferably formed of a suitable
hardwood, such as ash or birch with the grain oriented so that the
elongated capillary cells extend substantially longitudinally in
parallel with the central axis of the cylinder 12. In a typical
assembly, the diameter of the .[.toroidal.]. .Iadd.annular
.Iaddend.plugs 20 has been three-quarters inch with the smaller
overlapping plugs 26 and 28 being 1/4 to 1/8 inch in diameter,
depending upon the available rim thickness. For the most part,
available wood dowel stock can be employed if care is taken in
selecting the proper grain orientation. Each plug is cut to the
desired length, and the central bore 22 and side apertures 24 in
the .[.toroidal.]. .Iadd.annular .Iaddend.plugs 20 drilled to fit
the glass electrode 10 and the smaller dowels 26 and 28,
respectively.
During assembly, each of the plugs 20, 26 and 28 has its interior
transverse surfaces coated with an appropriate epoxy or other
adhesive sealant 30 that forms a fluid type seal between adjacent
plugs in each series. Also, the space within the side apertures 24
opposite those containing the overlapping dowel plugs 26 are filled
with the epoxy or adhesive 30 to prevent direct fluid communication
between the adjacent .[.toroidal.]. .Iadd.annular .Iaddend.plugs
20, as shown in FIG. 2. However, the end of one side aperture 24 in
the .[.toroidal.]. .Iadd.annular .Iaddend.plug 20 at the closed end
of the rigid cylinder 12 is left unfilled to define a reference
cell cavity 32 into which the reference cell electrode 18 extends
through the closed end 16 of the cylinder 12. The remainder of this
side aperture 24 defining the reference cell cavity 32 is filled
with the epoxy sealant 30, but the other side aperture 24 has one
of the longer solid plugs 26 inserted midway, and the unfilled
space between the sealed end of the solid plug 26 and the closed
end of the rigid container 12 provides a reservoir cavity for
storing additional electrolyte or salt bridge solution. In most pH
meter assemblies, the .[.convention.]. .Iadd.conventional
.Iaddend.reference half cell has a silver-silver chloride electrode
immersed in a saturated potassium chloride electrolyte, and an
excess of undissolved potassium chloride salt can be stored within
the vacant portion of the side aperture 24 opposite the one
containing the reference electrode 18, thereby maintaining the
saturation of the reference cell electrolyte by ion exchange
through the capillary structure of the plug 20. Of course, other
available reference half cell systems such as those employing the
mercury-calomel electrode can be similarly implemented.
Referring now to FIG. 3, a simplier form of the invention is useful
in improving the operational characteristics and longevity of a
reference cell with a separate sensing electrode. In this instance,
an elongated rigid cylindrical container 36 has its open end 38
communicating with the sample fluid and a reference electrode 42
inserted and sealed in place through its closed end 40. The
interior of the cylindrical container 36 is filled by a first
series of hollow thick walled cylinders or .[.toroidal.].
.Iadd.annular .Iaddend.shaped plugs 44 with a second series of
solid cylindrical plugs or dowels 46 and 48 slidably inserted into
their central bores to longitudinally overlap successive
.[.toroidal.]. .Iadd.annular .Iaddend.plugs 44. The overlapping
solid plugs 46 each have a longitudinal dimension substantially the
same as that of the .[.toroidal.]. .Iadd.annular .Iaddend.plugs 44
to overlap midway on either side. A final solid plug 48 about half
as long is inserted into the outer portion of the central bore in
the .[.toroidal.]. plug 44 at the open container end 38 with its
outer surface exposed to the sample fluid. Preferably the
surrounding outer surface of the outer .[.toroidal.]. .Iadd.annular
.Iaddend.plug 44 is coated with a layer of epoxy or other adhesive
sealant to close off direct fluid communication of the sample with
the interior cell structure. Similarly, the abutting end surfaces
between both series of overlapping plugs 44, 46 and 48 are coated
with the epoxy or adhesive sealant to prevent direct fluid
communication between the interior cell structures of successive
plugs. At the closed end of the rigid tubular container 36, the
final solid dowel plug 46 is inserted only about halfway into the
central bore of the innermost .[.toroidal.]. .Iadd.annular
.Iaddend.plug 44, so that the remaining half of the central bore
form an electrolyte reservoir cavity surrounding the reference
electrode 42, and the adjacent end surface of the dowel 46 is
coated with the epoxy or adhesive sealant to prevent direct fluid
communication from this reservoir into the interior cell structure
of the final dowel 46.
As in the other previous embodiment of FIGS. 1 and 2, the reference
cell structure of FIG. 3 typically employs hardwood plugs 44, 46
and 48 that may be selected from available dowel stock with
appropriate care in selecting the grain orientation so that the
interior capillaries are substantially longitudinally aligned. In
both embodiments, the inner diameter of the various components are
made slightly larger than the outer diameter of the component to be
received within so that the parts are easily slidable into position
within one another. In the assembly, the components are assembled
together with the epoxy or adhesive sealant applied to the
indicated surfaces. Almost any conventional two part epoxy can be
used as a sealant, as well as most sorts of glue, as long as they
are chemically compatible with the electrolyte and sample stream
solution and cured to form a uniform fluid tight layer. A few that
have proved successful are Hobby Formula #2 by Petit Paint Company
of Belleville, N.J.; Resin #86 with catalyst #C-321 from Epoxylite
Corporation of Buffalo, N.Y.; and the adhesive product sold under
the trademark "E-POX-E" from Woodhill Chemical of Cleveland,
Ohio.
After assembly and adequate curing of the sealant material, the
entire structure is placed for relatively long period in a vacuum
chamber to insure that substantially all of the residual moisture
and gas within the wood is removed. Thereafter, the entire assembly
is immersed, preferably under high pressure, in a bath containing
the reference cell electrolyte or salt bridge solution until the
wood is thoroughly impregnated throughout the entire length of the
container and the reference cell cavity is filled. Depending upon
the dimensions and the type of wood involved, the vacuum and
pressure immersion process might require anywhere from a few hours
to several days to obtain complete impregnation. If solid salt
crystal are desired in the reservoir, these must be introduced
during the initial assembly.
The hardwood plug described in connection with the preferred
embodiments used in cooling water control systems are ideal because
of the internal capillary structure wherein the closely packed
elongated cells provide multiple semipermeable layers overlying one
another in the transverse direction. Normally, cooling water
control systems operate to maintain a substantially neutral pH to
avoid acidic corrosion of the pipes or scaling from too alkaline a
condition. Of course, wood may not be compatible with other sorts
of process streams where a more acid or alkaline condition, or
other chemical properties of the fluid, could attack the wood. In
such instances, the plugs might be formed with a closed cell,
hydrophillic plastic foam material or similarly inert material with
an internal cell structure. Materials like cellulose acetate or
butyrate, phenolic, or polyurethane foams could be adapted using
known techniques to simulate the effects of the wood in providing
successive semipermeable layers.
In operation, the conductive bridge for achieving the liquid
junction ion exchange is established through the exposed surface of
one or more of the plugs at the open end of the container but are
exposed to the sample stream. Since the end of the capillaries or
other cell structure is closed off by the sealant, the path must be
directed transversely through the multiple semipermeable layers
across the interface between the overlapping plugs. The conductive
bridge is thus directed through numerous separate layers of
semipermeable material in traveling a circuitous path from a plug
in one series transversely to the overlapping plug in the adjacent
series with each successive layer adding to the total resistence,
whereas the exposed surface of the outermost plug in contact with
the sample stream affords a multitude of tiny capillary openings
that resist fouling by impurities.
Additionally, with the wooden plugs described in connection with
the preferred embodiments herein, the absorption of the salt bridge
solution causes swelling of the wood that expands its dimensions in
the transverse direction thus causing the plugs to be tightly
pressed against one another, as well as against the central glass
electrode in the embodiment of FIG. 2, and against the inside of
the rigid cylindrical container 12, or 36 for the second embodiment
of FIG. 3. Significantly, the impregnation with the fluid causes
little or no longitudinal expansion of the plug elements.
In comparing the embodiment of FIGS. 1 and 2 with that of FIG. 3,
it should be noted that the former provides significant advantages
in achieving a high resistance conductive bridge path with a
shorter series of individual plugs. This is because of the
alternate placement of the dowel plugs 26 so that the path is
directed through a substantial thickness of wood and the large
.[.toroidal.]. .Iadd.annular .Iaddend.plugs 20 surrounding the
glass electrode 10 and the central bore 22. By this means, an
enhanced effect can be achieved with reduced overall dimension.
While the cylindrical shapes of the interlocking plugs and
container is most advantageous in achieving a tight fit between the
components, other interlocking plug shapes might be employed, such
as rectangular blocks or curved plugs with the congruent convex and
concave abutting surfaces.
* * * * *