CA1281072C

CA1281072C - Reference electrode

Info

Publication number: CA1281072C
Application number: CA000590303A
Authority: CA
Inventors: Imants R. Lauks
Original assignee: iStat Corp
Current assignee: Abbott Point of Care Inc
Priority date: 1988-02-16
Filing date: 1989-02-07
Publication date: 1991-03-05
Anticipated expiration: 2009-02-07
Also published as: KR0152426B1; JP2521826B2; ATE152520T1; DE68928017D1; DE68928017T2; EP0408575A1; SG47491A1; EP0408575B1; WO1989007758A1; JPH03503677A; EP0408575A4; US4933048A; KR900700877A; HK1007796A1

Abstract

ABSTRACT OF THE DISCLOSURE

An improved reference electrode is disclosed for use in potentiometric measurements of the amount of ions in an aqueous solution. The reference electrode illustratively comprises a metal member on a substrate and a layer of a salt reversible to the ion X overlying the metal member.
The reference electrode is overlaid by a quantity of an electrolyte, which may be a polymer gel having a salt including the ion X dispersed therein. This structure is overlaid by a membrane permeable to water but not to the ion X that extends beyond the portion of the electrolyte overlying the electrode, but leaves a portion of the electrolyte exposed. This assembly may be shipped dry. In use, when the electrode is disposed in an aqueous solution containing an ionic species to be measured, water permeates the membrane and "wets up" the electrolyte relatively quickly. Any changes in ionic concentration, however, have to be made over a substantially longer path through the electrolyte between the electrode and the exposed portion such that the transit time for such changes is substantially longer than the wet-up time of the electrolyte. This reference electrode may he used in combination with a wide variety of potentiometric indicating electrodes or other structures.

Description

~2~7~ `

XFERBNC~e: hECTROD13 FI~D OF T~E INVE~ION
_ A _______ This invention rela~es to an improved reference electrcde. The re~erence electrode may b~ shipped "dry" and may be rapidly l'wet-upl' upon expo~ure to an aqueous ~lution.

BACX~ROUND OF THE__NVENTION

Typical de~ices ~r measuring the io~ic c~n~ent solutions include ~ re~erence ~l~ctrode and a separa~e potentiometric or "working" electrode. When these are immersed in a volume o~ ~olution to be analyzed, the re~erence and working electrodes together constitute an - electroch~mical cell. The re~erence electrode provides a constant potential with respect to which is measured the potential developed by ~he w~rking electrode from the solution. The potential difference across t~e c~ll is proportional to the logarithm o~ the activity of the ion.
This in turn is related to the concentration of the ion in the solution, uch that the co~centratlon can ~a directly determined ~s a function o~ the voltage measured across the re.~erence and work~ng electrodes.
Many documents di~cuss designs for and methods for fabrication o~ ion-sensitive devices for measuring ~he ionic content of solutions. For example, U.S. Pat~nt 4,613,442 issued to the present inventor shows an "Ambient Sensing Device" suitable for use at high temperatures. Other documents include European Patent Application 129,233 to Salman et al..; "A Batch-Processed Reference ~icro Electrode Integrated on a Silicon Substrate"~ Sinsabaugh et al., in Electrochemical Sensors for Biomedical Applications, pp.

0~7~

66-73 (1986); "Characteri~ics of Re~rence ~lectrodes U~ing a Poly~er Gate ~SF~T^~, Matsuo et al.~ in Sensors ~nd Actuators, 5 (1984), ppO 293-305: ~An Inteyra~ed Sensor for Electrochemical Heasurement~", Smith ~ in IE~E
Tran~ac~i~r~ ~n ~lomedical ~nqineer~n~, ~ol. ~ME-33, No. 2, [19B6) pp. 83-90: U.S. paten~ 4,592,824 to Smith et al., the disclosure of which appears to be comparable to that of the Smith et al. paper; "307. Reference IS~ET," in "Chemically Sensitive ~ield Effect Transi~tors", Janata et al., in Ion-Selective ~lectrodes in Anal ~ cal Chemistry, ~ol. 2, - (1980), pp. 161-167; Ion-Selective Electrode ~ethodology, vol. 1, (Covington ed.), pp. 58 62 51979) Ion Selective Electrodes in Analytical Chemistry, vol. 1, ~Freiser ed. ), especially chapter 3.3, "Reference Elec~rodes", pp. 323-33 (19783: and U.S. Patents 4,437,969 to Covington et a~ and 4,274,968 to Battaglia et al. See als~ "Chemically Sensitive Po~entiometric M~cxo~n~rs", by the present inventor, Stan~ord Research Institute (1983), pp. 192-24~.
Typical reference electrodes comprise a layer of a ~aterial reversible to an ion X, that i8, a ~aterial which is capable of undergoing a reversible change in oxidation ~tate in respon~e to the relative presence or absence of the ion X. Such materials include metal halide salts, alloys or compounds. Conveniently this material is formed on the surfac~ of an underlyin~ metallia ~ember. This re~ere~ce electr~de is then overlaid by an electrolyte. The electrolyt~ illustratively contains a quantity o~ the ion X
disper~ed into an aqueous medium, or into a polymeric material. For example, the electrolyte may comprise a gel containing a compound including the ion X. The gel is essentially impervious to mixing with the solution to be analyzed while permitting ion transport therethrough ~y diffusion. Alternatively the electrolyte may be confined behind a membrane, e.g. cel~ulose acetate or a porous glass or c~ramic or the like, which permits ~n transport while ( xestraining flow oP the eolution and the ~lectrolyte itself.
A l'liquid junctlon" i6 thus gor~ed between the electrolyte and the test solution, which allows flow of ions by di~fusion but not ~y convection.
When the compo~itio~ of th~ electrolyte phase i5 ~uitably ad~usted ~o that it ~onta~ns :Lons at relatively high concentrations o~ closely ~imilar mobillties, these ion~ traverse the li~uid iunction bounda~y in ~uch a way as to provide electrical continuity b~twee~ the electrode and the te~t solution Sas require~ to per~orm the potentiometric measur~ment) and maintain a constant (and small3 potenti~l difference across the liquid junction boundary~ reyardless of the composition of the tesS so~ution. The potential di~erence ~etween the ~lec~roda reversi~e to an ion X and its contacting electrolyte depends ~n the concentrat:~on of ion X in this electrolyte. ~herefore, wheD ion X is at a constant concentration, ~h~ electrode potential of this electrode i~ independent o~ the composition of the solution contacting the liguid junction, which i~ the requirement for it to be a properly functioning refer~nce electrode. Sinc~
i~ns must ~reely transport across the liquid junction ~oundary, co~stancy of ion X concentration can only be ~aintained if the elec~rolyte i~ a relatively large reservoir for ion X s~ that ion c~ncentration in the elec~rolyte r~mains ~ub~tantially constant over the time the rQference ele~trode i8 ~ use.
Prior art macro-re~erence electrodes typically consist of ~ sil~er chloride coated silver wire dipped into a concentrated potassium chloride ~olution (or some equ~valent formulation) contained in a tubular sleeve typically one-half inch in d~a~eter by a few inches long.
The ~olume of the electrolyte reservoir is several cm30 In a typical operational arrangement, the working and reference electrodes are sequentially exposed to, for example, a blood ~ample and a reagent containing a known , .

concentration of the ion~ to be ~ asured. By comparison of the potential di~erence between the reference and working electrodes responsive to the s~mple and the reagent, an accurately calibrated value can be d~t~rmined for the concentration og the ion in the blood.
In o.rder to provide a reference electrode which is use~ul in numerous processe~, e.g. ~or ~lood analy~is operations in hospitals, blood chemistry labs and the like, it iæ desirable to provide an electrode which is inexpensive, ~ as to be economically disposable, which is small, to allow use with small samples, and which has a long shel~ life~ The fact ~hat ~ost prior electrodes have emplQyed hydrophilic or aqueous reference electrolyt~s make the long ~helf life goal particularly difficult to achieve.
Typically hydrophilic e7ectrolyteæ ha~e ~een hydrated gels or the like to allow ion tranSportO To s~ip and ~tor~e such "wet~ ele~trvly~es involYes a relatively complex packaging and storage problem. Al ex~atiYely the gels can be 6hipped dry and be hydrated prior to use, but this can caus~ ~urther 2~ operational pro~lems tv ari~e, in particular, because of the size o such dry electrodes, the time it takes to properly hydrate them for use would ~ignificantly detrac$ from their usability. ~ ~urther difficulty is the physical size of prior art reference electrode~

u~Y oP ~æ I~VE~ON

Accordin~ly, it is an object o~ the inv~n~ion to provide a reference electrvde structure which can be shipped 30 dry, thus providing a long shelf life, but which can be "wet-up" ~or use relatively ~uicXly thus maximizing conv~nience to the user.

It is a ~urther ob~ ect of 4he invention o provlde such a 6en~:0r whi¢h can ~e ma~ufactured l2~3inq elec1:ronic circui fabrication technl~ues such that it c~n ~e readily ~iniaturizsd or use ~rl conneotiorl with ~Q.iniature 5 in~truments and other demarlding applicat~ or ~
A re~ererlce ~aleatrode a6~;e~nbly according to a pre~erred ~mbo~iment of ~he invention ~omprises a metallic member which is coated ~ith an elQatrode ~naterial reversible to an ion X a~d a layer oS an ele~tr~lyte containing ion x formed over the electrode. Typ~cally the eleotrolyte m~y comprise a hydrophilic gel. ~ portion o~ the electrolyte extending beyond the perimeter o~ the ele~troda i8 overlaid by a me~brane which is permeable to H20 molecules but not permeable to ion X. ~he m~mbran~ may be fo~med, for example, of polyYlnylc~lor~de tPVC) ~r polytetrafluoroethylene (PTFE) pla~tics, or fiilicone rubber.
The thickness o~ the ~lectrolyte layer under thi~ permeable ~e~bra~e is relatively thin, ~uch that the distance through the elQctrolyte between ~be m~mbrane and a ~u~strate on 2~ which the el~ctrode is ~ormed is ~elati~ely ~h~rt. A
portion of the electrolyte extends through t~e per~eabla membrane or is ~therwise enabl~d to ~or~ a li~uid junction with the solution at a position relatively distant ~rom the electrode. Accordingly, the ions must diffuse along a relatiYely long path through the electrolyte between the liquid junction and the el~atrod~. ~his provides a long time constant ~or ion dif~usion, ~hile the electrolyte may be "wet-up" relatively ~uickly. As a re~ult, there is a period of time after the electrolyte is wet up and be~ore ion dif~usion a~ects ionic conce~ra~ions in the vicinity o~ the electrode during which the potential at the electrode is su~stan~ially c~nstant, This time period is su~ficient ~or ~he working ele~trode t~ taXe good measureme~t~ o~ ionic concentrationæ ~n the te~t solutio~.

17~

RI~ DE8CXIP~ION OF T~ ~RA~ 8 The invention will be better underst~od i~
reference i8 made ~o the aceompanying drawings~ in which:
Fig. l ~hows a s~hemat~c cross sectional view of a conYentional prior art working electrode/reference electrode combination;
Fig. 2 show~ a view comparable to Fig. l, but showing a first embodiment o~ a working electrode/reference electrode assembly according to the present invention;
Fig. 3, comprising Figs. 3~a) through 3~e~, shows successive steps in the fabrication of the ~lectrode of Fig. 2: and Fig. 4 is a diagram illustrating potential versus time useful in understand~ng the invention.

D~8CRIP~IOU O~ ~KE PR~F~RED ~N~ODI~DN~8 Fi~ l show~ a conventi~nal working electrode~eference electrode assembly combination as ; employed in the prior art. A workins electrode lO is formed on a substrate l~ and covered with an overlayer 13. The overlayer 13 i~ a ~brane ~r seri s of membranes that render the working electrode ~peci~ic to a species to be 2~ measured. ~he working electrode may take numerous forms depending on its applicatIon. For example, it may ~e made of a metal such a~ silvex and may include more c~mplex structures consist~ng o~ a ~etal with o~erlayers ~f an electrolyte, an ion se~sitive membrane, an enzyme layer sr the like.
A re~erence electrode 14 is also formed on ~ubstrate 12. Reference electrode 14 comprises a metallic member 16~ ~vercoated with a layer 18 that is overcoated in turn with an electrolyte layer 20. Layer l8 is made of a material reversible to an ion X, that is, a material which . . , ~ , .

t~

underqoes rapid exchange of ion X b~tween it and the electrolyte ~o as to maintain therm~dyna~ic equ;librium between it and the electrolyte, resulting i~ a con~tant electrical pot~ntial di~eren~e a~ constan~ concentrations o~ io~ X.
Typical material~ for layer }8 include salt~ (e.g.
alloys or co~pound~) including the io~ X. HereinaftPr reference to a "salt'~ layer should be under~tood to re~er to a layer of such a reversibl~ material. Electrolyte 20 may t~pically comprise a hydrophilic binder, such as ~ gel, having a salt in solid ~oluti~n therein. Qne ion oP the latter salt may be com~on to the s~lt o~ the salt layer 18.
Illustratively, th~ metallic member is s~lver, l~yer 18 is silver chloride and the electroly~e is a gel containing 15 p~tassiu~ chlorLde, In operation electrolyte 20 is permeated by water molecule~ rom an a~ueou~ solution 22 whose chemi~al c~ncentration i~ to be measured by the working electrode.
That is, the electrolyte 20 is selected to allow diffusion ~0 o~ wate.r ~olecules as well as th i~nic species ~n the ; water, but the electrolyt~ 20 does not allow convection, that is, ~low o~ liquid water therethrough. Ionic species present in the electrolyte will al80 diffuse through the electrolyte into the a~eous solution. The junction 24 between the electrolyte 2a and the solution Z2 is geherally referred to as a ~'liquid ~unction."
As described generally ab~ve, the working electrode 10 and the refsrence electrode 14 together comprise an electroche~ical cell. The potential between them ~measured as indicated ~chematically by a voltmeter 26) may be used to derive a value ~or the concentration of the ionic species t~ be measured in the solution 22.
It can be seen fro~ Fig. 1 that the volume of - electrolyte 20 in the reference electrode as~;embly is 35 relatively great. This is to insure that it ta~ces some time 37~

~or changes to c~ccur in the concentrat~ ~n o~ the ionic species in the electrolyte as a result of diffusion through the electrolyte a6 the concentratiorl in the electrolyte see~c~ equilibrium with that in the solution 22. In this way S th~ potelltial dif~erel~ce between l:h~ reference electrode 14 and the working ele~trode 10 will remain constant ~ x ~;ome time, permitting a m~lsurement ~o be made of the ~ onic concentration in the solul:ion 22.
~owever, the requirement t2~at the ~olum~ f the 10 ele~rolyte be relat~rely gr~at, and that it be wet, places certain significant con~traint~ on it~ use. For exa~p}e, if the electroly~e is to be ~hipped dry and i~ to be ~Iwet-up~
prior to use by im~rnersion in water or a sal~ solution, the time requ~red is commen~:urake with the time constant of the t5 ion-diffusiorl process, which i~3 an impediment to th~
conv~nient use of . euch electrodes. on the other hand, if the electrodes are shipped wet then they must ~e stored wet, which i~ clumsy and inconvenient a~ well. Nor would it be po?~sible, according to the teac:hings of the prior art, 20 simply to reduce the volume of the electrolyte 20 ~o that it would l'wet-up~' more s~uickly: this would al~o reduce the time re~ired for the ionio concentration in the electrolyte to reach equilibrium with that of the ~olution 22 under test, w~ich would render the ~easure~ent itself difficlllt i~ not :25 impo~sible.
The prese~3~ invention prc~ides a referenc:e electrode assembly which can be ~;hipped and stored dry, for convenience a~d long shelf li~e, and which proYides a ~uitably long ion diffusion time constant but which can 3~ "wet-up~l quickly.
A pre~erred embodiment of the inventive reference electrode assembly i~ shown in Fiq. 2. A working electrode 10 and ~ reference electrode assembly 38 are formed on a substrate 12 as ~n the pr~or art electrode of Fig. 1. The : 3~ materialæ of the wor~ing electrode 10 and of reference :

' , ~; ( C~2 electr~de 14, that i8, the ~etal layer 16 and the overlying salt layer 18, ~ay be t~e same as in tbe device o~ the Fig. 1, and of course their cDnnection to the volt~eter 26 is the same. ~he material o~ the electrolyte layer 30 can also be the same as that of electrolyte layer 20 of ~ig. 1 A~ in the ca~e ~f Fig~ 1, layer 18 o~ Fig. 2 is made of a material that undergoes rapid exchange of an ion X between it and electrolyte layer 3~.
Accord$~g to the in~ention, a portion of the tO electrolyte layer 30a extending beyond ~ayers 16 and 18 i~
covered by a membrane 320 Membrane 32 comprises a permselective material which is selectivPly permeable to water (H20) molecule~ but is~impermeable to tha ion X and, - in qener~l~ is imper~eable to any ion present in electrolyte 3D sr present in the 601ution in which the reference electrode is to be used whlch can ~ect the potent:ial developed at electrode 14. ~embrane 32 may be made of PVC
or PTFE pla~tio ~aterLals or of 6ilicone rubber, A second portion 30b of the electrolyte that is di~placed from layers 16 and 18 may be expo6ed to tbe æolution 22 directly, as shown. Accordin~ly, a liquid ~unction 34 is formed between the 601ution 22 and portion 30b ~ the electrolyte 30, ~ n practice, the electrode assembly shown in Fig. 2 is shipped dry, such that the electrolyte co~tains an insufficient amount of water to support ion difusion. Such an assembly i~ ~table and has a long shelf life. When the ~ssembly is brought nto contact with an a~ueo~s ~olut~on 22, indi~idual water ~olecules penetrate the permselective ~embrane 32 and wet up the electrolyte and b~th water 30 molecules and ions X cross the liquid junction. Since the electrolyte layer 30 is r~latively thin compared to its length, the electrolyte 30 ~wets-up" relatively quickly~ At the same time, ionic species in ~oth the electrolyte ~nd the ; water begin to di~use across the liguid junction, with 35 changes in the concentrations o~ these species mo~in~ at ~2~ 7~

essentially con~tant rates from the exposed port~on 30b of tAe electrolyte leftward irl the~ diagra~ of Fig. 2. Since the water ~olecules "wet-up" the electrolyte layer over the much shorter distance of top to botto~ in the layer, the electrolyta i~ :iEully '1wet-up" prior to the time ~he ions dif ~using ~rom the solution 22 reach the violnity o~ the elec:trode layers 16, 18. A a result, there is a substantial time lag between the ti~e the electroly~e is fully "wet-up" and therefore operational and the time change~: b~gin to appear in the concentration of the ions (and hence the electrode potential) in the region of the electrolyte adjacent the electrode because of diffusion of ion~ across the liquid junction. During thls time lag, electric continuity is provided by ion transport ac:ros~3 the 15 liquid junct:ion, but the electrode potential remains constant since the changes in ioll concentration will not have reached the electrodec As a result, an accurate measurernen~ an be made of i~n concentration by mea~;uring the potential differenc~ between the working electrode and : ~0 the reference electrode.
This per~ormance o~ the reference ~lectrode of Fig. 2 is illustrated in the plot of Fig. 4 of voltaga against ~ime. As shown therein,-in the time period from o to a, the reference elec~rode wets-up and the voltage changes rapidly. In the period a-b, the voltage remains substan~i~lly constant until at time ~ the ion~c concentration a~jacent th~ electrode begins to change. I~
practice, time period a-b can be made to be a minuta or more with reference electrodes of ~he present in~ention, whioh allows adequate time for the working electrode to make the necessary measurements.
Those.of skill in the art will recognize that the refere~ce elsctrode structure shown can be used in a number of di~ferent applications, and in a number of different 35 experimen~al arrangements. Accordingly, disclosure herein .

. .
- . . . , .... . .

, o~ a particular method of use of the electrode assemb~y accordlng to the invention should n~t be deemed to limit the invention thereto.
Figs. 3(a) through 3(e3 disc}ose typical stages in the formation of the reference ~lectrode as~embly shown in F~g~ 2. In Fig. 3(a~ the working ~lectrode 10, overl~yer ~3, the matal layer 16, the overlying galt layer 1~ and patterned photoresist areas 40 and 42 h~ve been deposited on the sub~trate 12. This p~otoresist is referred to as a "positive resist" so a~ to di6tinguish it from a ~'negative resist" used later. Essentially the ~wo re ists need to be separately removable.
In Fig. 3(b) the electxclyte material 30 is shown haYing been deposited over the ent:ire assembly of F.ig. 3~a~.
It will be appreciated by those of ~Xill in the art that a polymer gel or other no~metallic electrolyte material can readily be depo~ited ~y castin~ or the like over the entire surface o~ a substrate having a large number of reference electrodes and worXing electrQdes, as well as other pos~ible structures ~ormed tharaon. H~wever, patterned layers of such electrolyte material~ cannot be formed using the usual microcircuit fabri¢ation techniques, that i8, they are not suitahle for sputterin~, vapor disposition or other : techniques used ~o deposit ~etallic l~yers. A~cording to 2~ one aspect of the invention, this difficulty is avoided.
I~ Fig. 3tc~ there i~ 6hown a ~ubsequent stage of fabricat~on ~f the device cf the invent~on, in which a pattern 46 ~f a negative resist material, that is, which may ~e re~oved separately ~r~m the positive resist 42, is 30 deposited over the area~ in which the electrolyte material 30 is to be retained as part o~ the re~erence electrode a~sembly 38 according to the invention. That is, tha negative re~ist material is deposited, exposed and '.

;

' ' " ' ,' ''' ' ' ' 1'72 dev~loped, leavinq behind patterns 4~ over the areas in whlch the electrolyte 30 ~s to appear in the completed product.
In Fig. 3(d) the re~ult o~ several ~ubsequent steps are shown, The~e include a pla~a etching or othPr ~tep to remove the exposed electrolyte, leaving behind the portion dispo6ed under the negative re8~8t pattern 46 o~
Fiq. 3~c). The positive and negatiYe re~ists are then remo~ed, leaving the electrode st~ucture shown in Fig. 3d, in which the reference electrode 14 i~ covered by the electrolyte 30.
Finally, Fig. 3(e) shows the re~ult of the last step, in which memhrane 32 i8 added over the electrolyte 3~, leaving a portion 30b of the electrolyte exposea as ~hown.
A~ mentioned, th~ mem~rane 32 may ~e formed of PVC or PTFE
plastics or of ~ilicone rubber. Other ~uitable matarials may appear to those skilled in the art. In another embodiment o~ th~ inYent~n, the membrane 32 could ext~nd over the entire electrolyte 30, and holPs sized t~ permit passage o~ ions as well as water c~uld be formed, e.g. by laser per~oration or otherwise, in the portion of the ~embrane 32 covering the portion 30b Qf the electrolyte disposed beyond the perimeter o~ the referenc~ electrode 14.
In another e~bbdiment of the in~ention, the electrolyte 30 is patterned in the form o~ a meander line or spiral with one end 30a over the electrode 14 and the other end 30b, exposed to the 601ution. Per~eable membran~e 32 covers the entirety of the m~ander line except at 30b.
Thus~ the distance for di~fusion of ions laterally along the layer 30 can be made very larqe.
; In ~iew of the ~oregoing descript~on other methods of fabrica$ion of the wor~inq electrode/reference electrode assembly may occur to those of skil~ in the art.

: 35 , ,:

f', (:' While several pref~rred embodim~nts of the invention hav~ been shown and dascribed, i~ will be realized by those skilled in the art ~hat others are possible as well. In each case, an i~portant aspect o~ the invention is that prior to use tbe reference electrode, ~llustratively compri~ing a metal co~ere~ by a salt layer, is covered by a dry electrolyte. A path for water molecules to diffuse throu~h the electrolyte to reach ~he reference electrode is provided which is shorter than any path by which chan~es in ~o ion concentration can reach the reference electrode. In this way the electrolyte in the vicini~ty of the electrode "wets-up" more quickly than the changes in ion concentration reach the electrode.
It will be understood by tho~e of s~ill in thP art that the raference electrode a~sembly of the invention can be used in combinat~on with a wide variety of additional electxodes, which ~ay be er~ed "potentiometric indicating"
or simply :'working" electrode6. These ~ay include electrodes which are actually ~on-selective. ~he reference electrode of the invention may also be used in connection with other ~ypes o~ structures or devices.
~ ccordingly, while a nu~b~r of preferred embodiments of the invention have been desoribed, the invention is not to be limited thereby but only by the 2~ following claims.

. 35 -~3 - ' '

Claims

1. A reference electrode assembly comprising:
an electrode comprising an outer layer of a material reversible to a chemical species, said electrode being disposed on a substrate;
a first layer of an electrolyte material covering all portions of said electrode not in contact with said substrate and extending beyond the perimeter of said electrode; and a second layer of material impermeable to said chemical species but permeable to water extending over a portion of said electrolyte layer beyond the perimeter of said electrode.
such that when said electrode contacts an aqueous solution a first portion of the surface of the electrolyte over the electrode and extending beyond the perimeter of said electrode is exposed to water but is sealed with respect to said chemical species and a liquid junction is formed at a remaining second portion of the surface of the electrolyte;
and wherein the distance in the electrolyte between the exposed second portion of the surface of said electrolyte material and the electrode is large compared to the distance in the electrolyte between the impermeable layer and the substrate.

2. A reference electrode of claim 1 wherein said electrode is formed by thin film fabrication techniques.

3. The reference electrode assembly of claim 1 wherein said electrode is generally planar with respect to said substrate.

4. The reference electrode assembly of claim 1 wherein said electrode comprises a metal portion in contact with said substrate and an overlying outer layer of a salt, alloy, or compound of said metal which is reversible to an ion X.

5. The reference electrode assembly of claim 3 wherein ion is Cl-, the metal portion is Ag and the overlying outer layer is AgCl.

6. The reference electrode assembly of claim 4 wherein said electrolyte is a material having KCl dispersed therein.

7. The reference electrode assembly of claim 1 wherein said second layer is formed of PVC, PTFE, or silicone rubber materials.

8. The reference electrode assembly of claim 1 in combination with a working electrode.

9. The combination of claim 8 wherein said reference and working electrodes are formed on a common substrate.

10. The combination of claim 8 wherein said electrodes are formed using thin film fabrication techniques.

11. A reference electrode assembly comprising:
an electrode comprising an outer layer of a material reversible to a chemical species, said electrode being disposed on a substrate;

a first layer of an electrolyte material covering all portions of said electrode not in contact with said substrate and extending beyond the perimeter of said electrode; and a second layer of material permeable to water but impermeable to any chemical species present in said electrolyte material or in a solution in which the reference electrode is used whose concentration affects a potential developed at said electrode, said second layer extending over a portion of said electrolyte layer extending beyond the perimeter of said electrode, such that when said electrode contacts an aqueous solution a first portion of the surface of the electrolyte over the electrode and extending beyond the perimeter of said electrode is exposed to water but is sealed with respect to said chemical species and a liquid junction is formed at a remaining second portion of the surface of the electrolyte;
and wherein the distance in the electrolyte between the exposed second portion of the surface of said electrolyte material and the electrode is large compared to the distance in the electrolyte between the impermeable layer and the substrate.

12. A method for measuring a signal responsive to the concentration of a chemical species in an aqueous solution, comprising the steps of:
manufacturing a reference electrode assembly, said assembly comprising an electrode on a substrate, said electrode being surrounded by a dry electrolyte, a portion of said electrolyte extending over said electrode beyond the perimeter of said electrode being covered by a layer of material permeable to water and impermeable to any chemical species present in said solution or in said electrolyte whose concentration affects potential developed at said electrode, another portion of said electrolyte being exposed, such that the distance in said electrolyte from said layer of material to said substrate is less than the distance in said electrolyte from the exposed portion of the electrolyte to the electrode;
assembling the reference electrode assembly together with a working electrode and connecting said electrodes to means for measuring the voltage therebetween;
placing said reference electrode assembly and said working electrode in contact with an aqueous solution containing the chemical species whose concentration is to be measured, whereby the electrolyte wets-up faster than concentration changes take place in the electrolyte as a result of migration between the exposed portion of the electrolyte and the electrode; and monitoring the potential difference between said reference and working electrodes.

13. The method of claim 12 comprising the further step of calibrating said assembly by exposure thereof to a reagent having a predetermined concentration of said chemical species.

14. A method of manufacture of a working electrode and reference electrode assembly, comprising the steps of:
depositing layers forming a working electrode and a conductor for the reference electrode on a substrate;
forming a layer of material reversible to a chemical species over the conductor of the reference electrode, thus forming a reference electrode;
depositing a layer of a dry electrolyte over said reference electrode; and forming a layer of a material permeable to water and impermeable to said chemical species over said layer of electrolyte, such that a portion of said layer of electrolyte extending beyond the perimeter of the reference electrode is not covered by said layer or material;

wherein the distance through said electrolyte between said portion of said layer of material overlying said reference electrode and said substrate is small compared to the distance through said electrolyte between the surface of the portion of said electrolyte which is not covered by said layer of material and said reference electrode.