Method and apparatus for electrolytically determining a species in a fluid

July 12, 1966 j. w. ross, jr 3,260,656 ³ METHOD AND APPARATUS FOR ELECTROLYTICALLY

DETERMINING A SPECIES IN A FLUID

Filed Sept. 27, 1962 ² Sheets-Sheet 1

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INVENTOR.

JAMES W ROSS, JR.

BY

St

ATTORNEYS

July 12, 1966 j. w. ross, jr _attv 3,260,656

METHOD AND APPARATUS FOR ELECTROLYTICALLY

DETERMINING A SPECIES IN A FLUID .

Filed Sept. 27, 1962 ³ Sheets-Sheet 2

CURRENT

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F I G. 4

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ATTORNEYS

United States Patent Office Patented July 12, 1966 the concentration gradient, represented by the remainder of broken line 24, across the membrane is approximately linear and its slope is an inverse function of the membrane thickness. _ .

However, during actual operation of the device, as oxygen is consumed at the cathode, there is a continual flow of the gas from the test fluid through the membrane to replenish the oxygen supply being consumed or reduced. If the oxygen in the test fluid adjacent the surface of the membrane is not continually replenished so as to be maintained at a constant level, the concentration gradient will then extend out into the test fluid, and its slope will become non-linear and reduced as shown at broken line 26 in FIG. 1, due to the local depletion of oxygen in a layer shown between lines 22 and 28. With continued operation of the device and no replenishment of oxygen, the local depletion layer will continue to expand further out into the test fluid, distorting the concentration gradient more and more. The distortion of concentration gradient reduces the measurement sensitivity and changes the mass flow rate of oxygen through the membrane to the cathode, making measurement uncertain and even spurious over a period of time. In order to avoid such occurrence, it is customary to provide means for breaking up any depletion layer, as for instance by stirring.

Further, if the fluid under measurement is a minute sample to which access is restricted, as in biological cells for example, depletion will continue until all of the oxygen is consumed. Because access to the fluid is restricted by the cell walls which act as barriers, the consumed oxygen cannot readily be renewed; the measurements in a short time 'become inconclusive. Additionally, even assuming that local depletion layers on the outer memgas concentration at the cathode substantially at zero with reasonable current consumption the input flow rate of the oxygen must be controlled. This is usually done by providing a relatively thick membrane which, however, acts ⁴⁰ to slow the response time of the device to changes in oxygen concentration in the test fluid.

Accordingly, the present invention contemplates the provision of improvements in electrochemical analytic.de„_iLi _______ vices by virtue of which the sensitivity and response time then of the oxygen concentration in. the test 45 _{are en}h_anced, and which improvements minimize material changes in the concentration of the species in the fluid under measurement during the measurement time. A principal object of the present invention is therefore to prove an improved electrochemical analytical device.

Further objects of the present invention are to provide an electrochemical device including enclosure means for an electrolytic medium, at least a portion of the enclosure means being formed of barrier means selectively permeable to a predetermined spaced re⁵⁵ lation to one another within the enclosure means, means for impressing a predetermined potential difference across the electrode, the electrolytic medium being one from which the electro-active species can be liberated by passage of electrical current between said ⁶⁰ electrodes, the electrolytic medium bridging die electrodes and being in contact with the barrier means; to provide such a device in which one electrode is adapted to consume said species and the other electrode is adapted to generate said species; to provide such a device in which the anode is adapted to generate said species; to provide such a device in which the cathode is adapted for reduction of said species and the anode is adapted to generate said species in dependence upon said reduction, said an⁷⁰ ode being adjacent the barrier means and between the latter and the cathode; to provide such a device in which

3,260,656

METHOD AND APPARATUS FOR ELECTROLYTICALLY DETERMINING A SPECIES IN

Jamet W+RoS, Jr., Newton, Mass., assignor to Corning Glass Works, Corning, N.Y., a corporation of New

Filed Sept. 27,1962, Ser. No. 226,533 19 Claims. (Cl. 204—1)

This invention relates to electrochemical apparatus and more particularly to improved devices for. electrically measuring the concentration of electrochemically active materials in fluids. . . , ,

The term “fluids” as used herein is intended to include gases, liquids, vapors and mixtures thereof. Heretofore, in the determination of the concentration m fluids ot an electrochemically active species (i.e. a substance such as an ion which is capable of being either reduced on oxidized at an electrode), electrochemical devices have been used in which an electrical characteristic of the species is measured and correlated with the concentration. Polarographic devices, for example, usually measure a characteristic potential at which such a species is reduced, and a diffusion current proportional to the concentration. An improvement on the basic polarographic apparatus is the well-known Clark apparatus as disclosed in U.S. Patent No 2,913,386 issued November 17, 1959. The latter employs a dual electrode structure immersed in an electrolyte and encased at least in part in a. membrane which is permeable to a predetermined species, for instance, gaseous oxygen. Typically, when used for. oxygen analysis, the cathode is formed of platinum and is located closely halide solution. In operation of such a device, oxygen in a test fluid outside of the membrane permeates the latter and is presumably reduced at the . cathode to water in accordance with the overall equation

2H++¹AO₂+2e-»H₂O

It will be apparent that the Clark device is intended to completely reduce the oxygen permeating the membrane. The current (2e) necessary to effect this reduction is a - . _j____2.1 _n—_ ' fart

THCclSUrG v/x ~ —j ο— - , . j fluid In determining oxygen concentration this device typically employs the Ag-AgCl anode with a KC1 electrolytic solution. Hence, the anode reaction is

Ag+Cl~-»AgCl+e

As the electrode operates, H+ is consumed at the cathode, changing the pH of the electrolyte in time For prolonged operation, a buffer is therefore required.

While devices of this type have proved satisfactory for many purposes, certain problems arise in some. applications. For instance, referring to FIG. 1, there is shown graphically a concentration diagram for such devices. In this diagram, the ordinate 20 is the scale from zero of the relative concentration of oxygen, and the abscissa represents distance from the cathode-membrane interface toward a test fluid. The position of the cathode-membrane interface (neglecting any small displacement between the two) is at line 20. Line 22, parallel to the ordinate, then represents the membrane-test fluid interfppp The distance between lines 20 and 22 is representa- -----_------, * . .

tive of the membrane thickness. Now it may be assumed ⁶⁵ the cathode is adapted for reduction of said species and that the concentration of oxygen in the test fluid adjacent -----m _sald «nec.ies. to nrovide the outer face of the membrane is constant, as shown by the horizontal portion of broken line 24, and that the consumption of oxygen at the cathode is complete so that the concentration of oxygen at the cathode-membrane interface is substantially zero. Under such circumstances,

3,260,656 the anodic current flow is maintained at the same absolute value as the cathodic current flow; and to provide such a device in which the cathodic current is dependent upon the equilibrium between the species generated at the appropriate electrode and the concentration of the species which is outside of the enclosure means and free to permeate the barrier means.

Other objects are to provide such a device including a reference electrode which is in contact with the electrolytic medium and provides a high stability to the potential at the consuming electrode; to provide such a device which does not substantially affect or deplete the concentration of the electro-active species outside of the enclosure means except (briefly, .if at all, during transient changes in said concentration) for appreciable time periods and in restricted samples of fluid; and to provide such a device in which the sensitivity and response time to changes in the concentration of electro-active species in the test fluid are markedly improved. Yet other objects of the present invention are to provide an electrical circuit for use with 20 an electrochemical analytic device of the type described; and to provide such an electrical circuit which includes a negative feedback loop for stabilizing the potential between a reference electrode and a consuming electrode.

Other objects of the invention will in part be obvious ²⁵and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will ³⁰be .indicated in the claims. For a fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein: . FIG. 1 is a graphical representation of the concentration gradient across the selectively permeable membrane in prior art devices as hereinbefore described;

FIG. 2 is a diagrammatic cross-sectional view of one embodiment of the present invention;

FIG. 3 is a graphical representation of a concentration gradient across a portion of the electrode-membrane structure of the embodiment shown in FIG. 2;.

FIG. 4.is a graphical representation of the current-voltage relationship at the electrodes in the structure of FIG. 2;

. FIG. 5 is a diagrammatic cross-sectional representation of another embodiment of the present invention; and

FIG. 6 is a diagram of an electrical measuring circuit particularly adapted for use with the electrode structure of FIG. 5.

To effect the objects heretofore set forth the present invention generally comprises a pair of electrodes electrically coupled through an electrolytic medium of predetermined fixed characteristics. The medium is isolated 55 from the test fluid under measurement by barrier means selectively permeable to the electro-active species which is to be detected and measured in the test fluid. A first electrode is adapted to consume the desired electro-active species, while the other electrode is adapted to produce 60 or generate said species in dependence on the consumption activity of the first electrode. Thus, in operation, there is a steady state equality established between consumption and generation within the electrolyte. The electrode-membrane structure is such that this steady state 65 equality is in turn responsive to an equilibrium between relative concentrations of the species on both sides of the barrier means. It will be seen, therefore, that in the present invention, during steady state conditions, there is i------- ' - · · · during transient changes in the concentration of the species in the test fluid will there be a small net transfer which rapidly decays as an internal steady state is reestablished. Consequently, the consumption of the electro-active species is minimized and changes in the concentration of the 75 _{44 fo}“* instance7adheskes^V which species in the fluid under measurement which may be ascribed to the analytical activity of the apparatus, are minute and quickly and automatically compensated.

The term “electro-active species” as used herein is intended to mean any group of identical chemical entities such as ions, molecules, atoms, etc., which are capable of being electrically oxidized or reduced. Where the species is a gas e.g. oxygen, chlorine or the like, which is dissolved in or mixed with another fluid, rather than referring to concentration, it is perhaps more accurate to refer to the equilibrium partial pressure. This is so because a gas may exhibit different solubilities with respect to different fluids. Hence, although the equilibrium partial pressure (hereinafter referred to as the tension) of the gas in one fluid may be precisely equal to the tension of the same gas in another fluid, the concentrations of gas with respect to each of the fluids are not necessarily the same. However, where the gas tensions on opposite sides of a gas-permeable membrane are in equilibrium, the tension on one side is proportional to the concentration on the other side.

Referring now to FIG. 2, there will be seen one embodiment of the present invention in which there is shown a structure comprising a first electrode or anode 30, a second electrode or cathode 32, an electrolytic medium 34 which electrolytically coupled the anode and cathode, and selectively permeable barrier means, such as membrane 36. Means, such as battery 38, are provided for applying an appropriate potential across the cathode and anode through respective leads 46 and 47. The invention also may include means shown schematically as meter 40, for measuring current flow in the cathodeanode circuit. In order to encase the electrodes and the electrolytic medium there is provided enclosure means 42 which preferably incorporates membrane 36 as a wall portion thereof.

Enclosure means 42 is preferably formed with a substantially hollow chamber 44 interiorly thereof, the chamber having at least one opening to the outside of the enclosure means. Membrane 36 is so disposed as to completely cover the opening in chamber 44. The electrolytic medium (i.e. a medium capable of providing both positive and negative ionic charge transport as by ions provided from a disassociatable compound in a polar solvent, or by ions provided by a molten salt or the like), for example, an aqueous electrolyte, is disposed within the chamber in sufficient quantity to maintain a continuous electrical contract between electrodes. The enclosure means preferably then is formed as a liquid-tight container so that the electrolyte cannot leak out. The electrolyte is preferably selected to be one whose ions require a more negative potential for reduction than does hydrogen and require a more positive potential for oxidation than does water or hydroxyl ion. The difference between these more negative and positive potentials is generally termed the decomposition voltage of the electrolyte.

The walls of chamber 44, other than membrane 36, are preferably made of a material, such as glass, ceramic, solid polymer or the like, which is substantially impermeable to fluids, is a high resistance material (i.e. an electrical insulator) and is substantially chemically inert to the electrolyte and decomposition products of the latter. Membrane 36, in the form of a sheet with a substantially uniform thickness, for example, not more than approximately 5 mils, is selected from materials which exhibit selective permeability to the electro-active species which one desires to measure. For instance, where the species is gaseous oxygen, membrane 36 may be of a polymer such as polyethylene, polyvinyl chloride, rubber, or other 1, „. ’ . _a solid barrier selectively pervious to the gas. Barrier materials are known which are selectively permeable to other electro-active species.. Appropriate means (not shown) are provided for sealing the membrane across the opening to chamber , ------------ -----u are insoluble in the no net transfer of species through the barrier means: only 7n i ^aS Ρ°^^6ί11Υ^1βη®&gt; polyvinyl ch during transient changes in the concentration of ih_P known substances which provide

3,260,656 ⁵ electrolyte and test fluid, an O-ring, or other known techniques or structures.

Anode 30 is formed as a substantially sheet-like element typically having a thickness of about 3 mils and being porous to both the electrolyte and electro-active species. The anode is made of an electrically conductive material, preferably a noble metal such as platinum, gold or the like. To provide porosity, the anode may be provided as a mesh or screen as shown, as a spongy mass, or in other known configurations. In the form shown, anode 30 is disposed within electrolyte 34 and.is superposed adjacent membrane 36, i.e. arranged either in face-to-face contact with the latter, or, if the anode and membrane are in separated condition, then there is approximate parallelism of the adjacent surfaces of the anode and membrane with a stratum of electrolyte between them.

Cathode 32, in the form shown, is also a substantially sheet-like element which may be either porous or solid, preferably the latter, and its thickness is determined by cost and considerations of structural strength. The cathode is also preferably made of a noble metal, for instance the same metal used as the anode. Cathode. 32 is also disposed within electrolyte 34 and is disposed in approximately parallel face-to-face relationship to the anode, but separated from the latter a short distance by appropriate •means such as spacer 46. .....

⁶.

according to the same equation heretofore used in describing the cathodic reduction in the Clark apparatus.

Referring now to FIG. 4 there is graphically shown the general current-voltage relationship in. the present invention during operation. The ordinate is shown as the current axis, the voltage axis being the abscissa. While the curve 48 is shown as being symmetrical about the origin, this is only for purposes of illustration and is not actually the case. At a point where there is a predetermined negative voltage —E, the oxygen at the cathode is electrically reduced. This produces a current I whose amplitude is a function of the concentration of gaseous oxygen present in the electrolyte. With a negative current —I, equal in magnitude to I, now flowing at the anode, the latter is at a positive voltage +E, shown at point 51, predetermined by the nature of the anode material and electrolyte. With proper choice of parameters in the example described above, at the anode the current oxidizes the water in the electrolyte, generating oxy20 gen according to the following:

H₂O—2e-^ 1/2 Ο²+2H+

Hence the system consumes the species at one electrode and tends to generate a like quantity thereof at 25 the other electrode, without changing the electrolyte pH. This leads rapidly to an internal or interelectrode steady state condition between the consumption and generation cnHctantiallv sheet- state condition Between me consumption auu _scuw&lt;lUv_U

In the preferred form spacer 46 is^a sut t^a« ζ¹¹^ _{of the spec}ies, and the current flow is therefore a measure like porous element such as a wovemfabric. The spacer _{present system}. _{The steady gtate equality} is preferably formed of matena which·« ^β1®«^Γ^^alJ 30 between generation and consumption is, however, responconductive, relatively ^che™fUy «ert to ffie electrolyt in concentration of the oxygen Outand its decomposition products, and exhibits good di e membrane. This is largely because of the side of the membrane. This is largely because of the geometry of the system wherein the generating electrode lies between the consuming electrode and membrane.

tend to reach an equilibrium with one another inasmuch as a high tension outside the membrane will result in net diffusion of the gas throughout the latter toward the anode, and conversely a higher tension inside the mem40 brane would result in net gaseous diffusion through the membrane from the anode to the test fluid. Thus, any change in gas tension outside the membrane will upset the internal steady state of the system, forcing it to a new steady state by either increase or decreased consumption generation. Each change in gas generation is thus in a direction tending to establish equilibrium between the gas tensions on opposite sides of and across the membrane. Every change in the internal steady state is, of ⁿ ' r _______ _____ _ ) between the electrodes'. It will thus be seen that in the equilibrium condition of gas tension across the membrane, the current flow is constant and there is no net transfer of gas through the membrane. On the other

L transient condition will exist until a new steady state is established internally which again provides a gas tension equivalency on both faces of the membrane. Hence, depletion of oxygen is minimized at points adjacent the outside surface of the membrane i.e. the surface contacting the test fluid.

Referring now to FIG. 3, there is shown graphically a concentration diagram taken across the electrode-membrane structure of the embodiment of FIG. 2. As in ₆₅ FIG. 1, the ordinate 52 represents the relative concentration of the particular electroactive species. The absicca 54 represents distance from the cathode-electrolyte interface which is at the origin of the graph and hence, ______ _r______________ -- in a sense, the latter interface is line 52. Lines 56 and oxygen will diffuse ^through the latter to its inner ₇₀ 58 then represent the respective surfaces of the anode -...... facing the cathode and the membrane; and line 60 is the outer surface of the membrane in contact with the test fluid. For the sake of clarity, no displacement between anode and membrane is shown, hence line 58 can

TvS mat3 from 35 The gas tensions on both sides of the membrane will ,. Typical materials irom „ pnnlUhrinm with another inasmuch lie C1UVL1M * ,.. , .

preferably 45 ^{of gas and a} corresponding increase or decrease in gas . in me oprinration is thus in Ά

Tn chinntinn 50 course, accompanied by a chance in the current flowing • ® 1__J..,.___ t-Kn Tf ITflll tllll c 1ϊ o coon tliat in flip.

sional stability with respect to variables such as temperature, pressure, solvents and the like, whereby the spacing between the anode and the cathode is maintained at a substantially constant value. Τ/Γ¹which such spacer may be made are nylon, polytetrafluorethylene, and the like. It will be seen then that the electrode structure of FIG. 2 is that of a sandwich in which the anode is spaced from the cathode and retained in between the latter and the membrane, other portions of the volume between the membrane and cathode being substantially filled with electrolyte. _

In describing the operation of the embodiment of FIG. 2, it will be exemplary to consider oxygen as the electroactive species. In such case, the electrolyte is an aqueous solution of a base, such as KOH, because in the operation of the device it is desired to avoid evolution of gaseous hydrogen at the anode. Hence, the pH of the electrolyte should preferably be basic and generally not less than approximately pH 2 or 3. Li the cathode and anode in this instance, it is preferred to use materials which will respectively reduce oxygen and generate oxygen with maximum current efficiencies. Because of their low oxygen over-voltages and relative chemical inertness, the noble metals are t— _x-------both anode and cathode can be made of, for example, platinum. If the potential applied across the anode and cathode is established well below the decomposition voltage of the electrolyte, and there is no oxygen available in the electrolyte (as from diffusion into the electrolyte ⁶⁰through the membrane, or from being dissolved in the latter), only a virtually constant, minute, residual current will flow in the cathode-anode circuit. If, however, a supply of oxygen is presented to the outer surface of membrane 36, as for instance, by contacting the latter with a liquid having a dissolved oxygen content or by contacting the latter with a gas which comprises oxygen, because of the selective permeability of the membrane some c.._JO— — face and thence into the electrolyte to the cathode. . If the potential across the electrodes is above the reduction potential of oxygen, although below the decomposition potential of the electrolyte, oxygen present at the cathode .

will be reduced. The reduction process is believed to be 75 also be considered the anode-membrane interface. The thm: referred and 55 hand, if the gas tension in the test fluid should change, a

P _ francianf nnnrlit inn V7l11 AYISt Until f) HP.W StfifldV State

3,260,656 distance between the various lines is representative of the thickness of the various elements but is not to be considered as necessarily showing the actual proper proportions. Using again the example of oxygen and assuming for simplicity, that the oxygen exhibits the same tension in both electrolyte and test fluid and the concentration of oxygen in the test fluid is constant, the latter concentration is indicated by the horizontal portion of broken line 62 which extends from the test fluid into the membrane. The concentration gradient, represented by the remainder of broken line 62, extends from the anode-membrane interface to the cathode-electrolyte interface. The slope and position of line 62 will not change during steady state operation because of the equilibria established as heretofore described which results in no net transfer of oxygen across the membrane. However, during a transient change in oxygen concentration in the test fluid, although the horizontal portion of line 62 extending into the test fluid from membrane surface 60 will shift its level, this only results in a temporaray departure from a zero slope 20 of the concentration level across the membrane. This zero slope is quickly re-established by the corresponding change in generation of oxygen at the anode. Hence, substantially no depletion layer will be created in the test fluid.

If the membrane is made very thin, for example, in the nature of 0.5 mils, the response time is considerably reduced inasmuch as diffusion time through the membrane in enhanced. Presupposing constant diffusion coefficients for the membrane, electrolyte and anode, the 30 steady state current is approximately r_ KC (H+L) avoid transient from extraneous

Referring now to FIG. 6 there will be. seen an electronic circuit for use with the embodiment of FIG. 5 to determine the current flow between anode and cathode as well as to provide high stability to the operation of the system. The circuit of FIG. 6 comprises means for electrically connecting the circuit to electrodes of FIG. 5, and such means may comprise leads 68, 46 and 47 respectively adapted to be connected to reference electrode 66, cathode 32 and anode 30. Lead 68 is also connected to lead 46 through a series resistor 70 and an electrical power source, such as battery 72, the latter being poled so that lead 46 is negative. A point intermediate resistor 70 and the reference electrode is connected, as by lead 74, to the input of D.C. voltage amplifying device or amplifier 76, the output of the latter being coupled to the anode through lead 47. A measuring instrument such as ammeter 78 is located preferably in the circuit between the amplifier and anode. The amplifier and positive side of the battery should be appropriately grounded in order to ' phenomena.

In operation of the device shown in connection with FIGS. 5 and 6, the presence of a species in the electrolyte permits current flow between the cathode and reference electrode, creating a potential across resistor 70 due to the IR drop therein. The voltage across resistor 70 is amplified by the gain of amplifier 76 and applied to anode 30. It will thus be apparent that amplifier 76 is a D.C. inverting amplifier which preferably exhibits high gain (in the order of thousands) and is also preferably stabilized as by internal negative feedback, chopper-stabilization or the like, all as well known in the art. Thus the reference electrode is positive with respect to cathode 32, but negative with respect to anode 30. If the gain of amplifier 76 where K is a constant depending, inter alia, on the diffu- ³⁵ is sufficiently high, the coupling of its output through /t. . . anode 30 to reference electrode 66 constitutes a negative feedback loop which will tend to create a virtual ground at the input of the amplifier. The potential of the reference electrode is therefore fixed at the virtual ground, and the potential between cathode 32 and the reference electrode is consequently maintained at a very stable level at which only oxgen will be reduced at the cathode.

The following example is illustrative of the construction and operation of a device of the type shown in connection with FIGS. 5 and 6. A hollow container was formed of polymethylmethacrylate. In the container was placed a cathode formed of 1 mil platinum metal foil of approximately 0.3 square centimeter per surface, a platinum metal anode formed of 80 mesh gauze, 3 mil diameter wire, and having a mean thickness of 5 mils, was placed in the container and spaced from the cathode with a nylon fabric spacer having a thickness of approximately 5 mils. In contact with the electrolyte there was also installed an Ag-AgO reference electrode in the form of a very short piece of No. 16 gauge wire. The remainder of the container was filled with a IM KOH aqueous solution, and the opening into the container was capped with a polytetrafluoroethylene membrane having a thickness of 0.5 mil. In operation, the cathode potential was set at —0.7 volt with respect to the Ag-AgO electrode, and the device was operated at a temperature of 20° C.

In order to establish the backgriund current, the device was first operated with pure helium disposed against the surface of the membrane opposite the surface contacting the electrolyte, and a steady state current of 4.3 microamperes was obtained. Subsequently the outer surface of the membrane of the device was exposed to a number of various oxygen-containing fluids in samples of 0.2 cc.

sion coefficients, C is the concentration of the species in the test fluid and H and L are respectively the anode (generating here) thickness and inter-electrode spacing.

It is apparent that the spacing between the cathode and anode affects the sensitivity and response time, hence 40 should be as small as possible without creating a dead short circuit. Typically, a spacing of 1 to 10 mils has proved quite satisfactory. Similarly, the spacing between the anode and membrane, if any, will also affect the response time and can be minimized without con- 45 cern for short circuits.

Referring now to FIG. 5 there will be seen another embodiment of the present invention similar in many respects to the embodiment of FIG. 2, and wherein like numerals denote like parts. The device shown in FIG. 5 also includes, in a configuration similar to FIG. 2, enclosure means 42 having a hollow 44 therein open to the outside of the enclosure means, the opening being sealed by membrane 36. A generating electrode, such as anode 30, is disposed inside hollow 44 against the membrane. A consuming electrode, such as cathode 32, is also disposed within hollow 44 and separated, as by separator 46, from the anode 36. Appropriately fluid-sealed electrically conductive leads 46 and 47 are respectively attached to cathode 32 and anode 30 to provide electrical connection to the electrodes from the exterior of the enclosure means. An electrolytic medium, such as a fluid electrolyte 34, substantially occupies the remaining space within hollow 44. Contacting the electrolyte and spaced from the other electrodes and membrane is a third or 65 reference electrode 66 to which another electrically conductive lead 68 is attached to provide an electrical connection between electrode 66 and the exterior of the enclosure means.

Where this latter embodiment is intended to measure 70 ^each, with the following results: concentration of, for example, oxygen, the elements thereof can be selected and formed in the manner and of the materials heretofore described in connection with the embodiment of FIG. 2, except that electrode 66 is preferably an Ag-AgO electrode.

Sample:

Air saturated water

Air_____________

90% O₂_________

Current in microamperes

----------------- 17.3

----------------- 17.3

-----------------59.6

All readings reached a constant level within a maximum of one minute after exposure to the sample. Each reading stayed constant for more than 3 hours after the initial transient. Despite the different mobilities of the oxygen in water and in air, the readings achieved with re- 5 spect to air and air saturated water were nevertheless identical and remarkably stable, indicating that in fact there was no net oxygen diffusion through the membrane during steady state operation.

If one uses a similarly constructed apparatus in which jq however the electrolyte is aqueous KC1 and the cathode is Ag-AgCl (thereby ensuring that no oxygen generation would occur within the apparatus), after 40 minutes the concentration of oxygen in a sample of similar size will be reduced approximately 70%. On the other hand, in the 15 present invention, when operated for the same period of time there was no detectable change in oxygen concentration in the test sample.

It will be apparent to those skilled in the art that many modifications of the invention as thus described can be 20 made. For example, a large number of electrolytes may be used. Indeed, under some circumstances the electrolyte may be a molten salt, and the enclosure means a membrane selected accordingly to have appropriate temperature resistant characteristics. While the anode and 25 cathode of the present invention have been shown as substantially flat plates, of course other configurations which preserve the relative positioning of the electrodes and membrane may also be used. For example, the device may be constructed with the ancde and cathode as concentric cylinders, the membrane lying along a portion of the exterior cylindrical surface of the anode. It will also be apparent that where the species to be measured is reducible, the generating electrode is an anode and the consuming electrode is a cathode. However, if the species ⁰⁰to be measured is oxidizable, the generating electrode will be a negative electrode placed adjacent the membrane and the consuming electrode will be an anode or positive electrode. . ₄₀

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted in an illustrative and not in a limiting sense. 45