Electrochemical planar metal/metal oxide electrode

Description of equivalent WO 9623214 A1

ELECTROCHEMICAL PLANAR METAL/METAL OXIDE ELECTRODE

Field of the Invention This invention relates to a planar metal/metal

oxide active elect that may be employed in sensors that

measure pH and/or C0₂.

Background

Electrochemical means for measuring the pH and C0₂

levels of liquid systems are well known. Glass sensors

having membrane type of electrodes are commonly used as

standards for pH and C0₂ measurements because glass

sensors are predictable and provide good measuring

capability. Glass sensors are fabricated as three-

dimensional articles, with many serviceable parts.

Thus, while demonstrating good measuring capability,

these three-dimensional articles are more expensive and

complex to manufacture as well as operate as compared to

planar format sensors.

Planar format sensors have been described in the

literature (such as those taught by Foos et al. , EP-

A0625704, hereby incorporated by reference) and provide an alternative means to measuring pH and C0₂ levels.

Planar sensors are typically smaller than glass sensors

and much cheaper to manufacture as well as operate. The

planar format of the sensors typically comprises

relatively thin layers of materials which are applied to

substrate bases using thick-film or thin-film

techniques, including, for example, silk-screen

printing.

Planar sensors for the measurement of pH and C0₂

are more economically desirable than glass sensors, but

planar sensors have been associated with performance

problems. Limitations of planar pH sensors include, for example, drift in potential requiring frequent

recalibrations; oxygen (0₂) interference (which can give

incorrect answers due to varying levels of 0₂) ;

sensitivity to reduction or oxidation (redox) agents; and short lifetime of sensor, particularly due to

contamination by sample components or impurities present

in other sensor components.

Improvement in the performance of planar sensors

is needed in order to make the sensors more commercially desirable and well accepted as an alternative means for

measuring pH and C0₂.

Summary of Invention

The problem noted above has been solved with the

discovery of an improved active metal/metal oxide

electrode, the active metal/metal oxide electrode

prepared by a process comprising combining a base

component with a metal paste to form a metal paste

mixture, heating the metal paste mixture in the presence

of air for a time sufficient to oxidize a portion of the

mixture to form said metal/metal oxide electrode.

A planar sensor for the measurement of pH is also

provided, the sensor comprising an electrically

nonconductive substrate having applied thereto in a

planar format: an electrically conductive material in at

least one region adjacent to said substrate; a

dielectric coating covering at least a lead portion of

said region of said electrically conductive material but

leaving exposed at least an electrode area and a contact

area on said region of said electrically conductive

material; and a metal/metal oxide electrode in said exposed electrode area, wherein said metal/metal oxide

electrode is prepared by a process comprising combining

a base component with a metal paste to form a metal

paste mixture, heating the metal paste mixture in the

presence of air for a time sufficient to oxidize a

portion of the mixture to form said metal/metal oxide

electrode.

A method for preparing a pH sensor is also

provided, the method comprising: (1) selecting an

electrically nonconductive substrate; (2) adheringly applying thereto in a planar format: an electrically

conductive material in at least one region adjacent to

said substrate; a dielectric coating covering at least a

lead portion of said electrically conductive region but

leaving exposed at least an electrode area and a contact

area of said electrically conductive region; and a metal/metal oxide electrode present on said exposed

electrode area, wherein said metal/metal oxide electrode

is prepared by a process comprising combining a base

component with a metal paste to form a metal paste

mixture, heating the metal paste mixture in the presence of air for a time sufficient to oxidize a portion of the

mixture to form said metal/metal oxide electrode.

A method for measuring pH in a sample has also

been discovered, the method comprising contacting a

liquid sample with the planar pH sensor described above

and a reference, connecting said exposed contact area of

said sensor with a pH sensing instrument; connecting

said reference with said pH sensing instrument;

providing an electrical current from said pH sensing

instrument through said reference and said contact area

of said sensor, and measuring a resulting electrical

signal provided by said pH sensing instrument.

A C0₂ planar sensor for the measurement of C0₂ has

also been discovered, the sensor comprising an

electrically nonconductive substrate having adheringly

applied thereto in a planar format: (a) an electrically

conductive material in at least a first and second

region adjacent to said substrate; (b) a dielectric

coating covering at least a lead portion of said first

and second regions of said electrically conductive

material but leaving exposed at least an electrode area and a contact area on said first and second regions; (c)

a silver/silver halide electrode in said exposed electrode area of said first electrically conductive

region; (d) a metal/metal oxide electrode in said

exposed electrode area of said second electrically

conductive region, said metal/ metal oxide electrode

prepared by a process comprising combining a base

component with a metal paste to form a metal paste

mixture, heating the metal paste mixture in the presence

of air for a time sufficient to oxidize a portion of the

mixture to form said metal/metal oxide electrode; (e)

an internal electrolyte layer applied adjacent to and

superimposed over at least said metal/metal electrode

and said silver/silver halide electrode; and (f) a gas

permeable membrane adjacent to and superimposed over said internal electrolyte in at least the portion of

said electrolyte layer covering said metal/metal oxide electrode and said silver/silver halide electrode.

A method for preparing a C0₂ planar sensor is also

provided, the method comprising: (1) selecting an

electrically nonconductive substrate; (2) adheringly applying thereto in a planar format: (a) an electrically conductive contact material in at least

separate first and second regions; (b) a dielectric

coating separating said first and second regions of said

electrically conductive contact material, said

dielectric coating covering at least a lead portion of

said first and second regions of said electrically

conductive material but leaving exposed at least an

electrode area and a contact area on said first and

second regions; (c) a silver/silver halide electrode in

said exposed electrode area of said first electrically

conductive region; (d) a metal/metal oxide electrode in

said exposed electrode area of said second electrically

conductive region, said metal/metal oxide electrode

prepared by a process comprising combining a base

component with a metal paste to form a metal paste

mixture, heating the metal paste mixture in the presence

of air for a time sufficient to oxidize a portion of the

mixture to form said metal/metal oxide electrode; (e)

an internal electrolyte layer applied adjacent to and

superimposed over at least said formed metal/metal electrode and said silver/silver halide electrode; and

(f) a gas permeable membrane adjacent to and

superimposed over said internal electrolyte in at least

the portion of said electrolyte layer covering said

metal/metal oxide electrode and said silver/silver

halide material.

A method for measuring C0₂ in a sample has also

been discovered, the method comprising: contacting a

liquid sample with the C0₂ sensor described above,

connecting said exposed contact area of said first and second electrically conductive regions of said sensor

with a C0₂ sensing instrument, providing an electrical

current from said C0₂ sensing instrument through said contact areas, measuring an electrical signal provided

by said C0₂ sensing instrument.

The invention provides an economical method of manufacturing planar pH and C0₂ sensors capable of

accurate measurement of pH and C0₂ in sample fluids.

Sensors that have been prepared using this electrode

show a good response to pH or C0₂ while demonstrating

minimal drift and 0, interference. Description of Drawings

FIG. 1 shows a metal/metal oxide planar sensor

prepared according to the invention.

FIG. 2 shows a top view (2a) and a side view (2b)

of a sample chamber.

FIG. 3 shows a Severinghaus C0₂ type sensor.

FIG. 4 shows a planar sensor having a metal/metal

oxide electrode and a reference electrode.

FIG. 5 shows a planar sensor having a metal/metal

oxide electrode with a cover membrane and a reference

electrode.

FIG. 6 shows a planar sensor with a metal/metal

oxide electrode covered by an internal electrolyte layer

and a neutral carrier cover membrane placed on top the

electrolyte and a reference electrode.

FIG. 7 shows a Severinghaus C0₂ type sensor.

FIG. 8 shows the electrical circuitry used in the

Examples section.

FIG. 9 is a pH response curve, as described in

Example 2, where line a is from a glass sensor (Type

"A", STANDARD), lines b_λ.₃ are from planar sensors (Type "B", INVENTION) and lines c_x_₂ are from planar sensors

(Type "C", COMPARATIVE) .

FIG. 10 is a pH response curve, as described in

Example 3, where line a is from a glass sensor (Type

"A" , STANDARD) , lines b₁.₃ are from planar sensors (Type

"B", INVENTION) , and lines c₂ and C₂ are from planar

sensors (Type "C" , COMPARATIVE) .

FIG. 11 is a pH response curve, as described in

Example 4, where line a is from a glass sensor (Type

"A", STANDARD) and lines d₂.₄ are from a planar sensor

(Type "D", INVENTION) .

Detailed Description

The invention is suitable for use in determining

the pH and/or C0₂ of liquid samples. The invention is

particularly useful for measuring the pH and/or C0₂ of

biological fluids. Non-liquid samples may be prepared

as liquid samples and thereafter tested, as known to

those skilled in the art. The pH range that may be

measured by the invention may vary greatly, with

particular sensitivity realized for a pH measurement of

from a pH of 3 to pH of 11. According to the invention, in preparing the metal/metal oxide electrode, the metal paste may be

purchased or prepared by known techniques. Preferably

the paste is of a thixotropic consistency (such the

paste will flow) but is not of a thin, runny

consistency. The appropriate consistency of the paste

may be easily determined by those skilled in the art of

preparing planar formatted sensors. The metals that may

be used in the preparation of the active electrode are

noble metals as described, for example, in "Metal-Metal

Oxide and Metal Oxide Electrodes as pH Sensor" by S.

Glab, et al, Crit. .Rev. Anal . Chem. 21, 29-47, 1989.

Suitable metals include, but are not limited to,

palladium, rhodium, ruthenium, osmium, iridium,

platinum, tin, antimony, bismuth, alloys thereof, and

mixtures thereof. More preferably the metal is

palladium or iridium, most preferably, palladium.

A base component is combined with the metal paste

to form the metal paste mixture that goes to form the

metal/metal oxide electrode. The base selected must be

capable of generating a metal/metal oxide when the metal paste and base combination is heated. Preferably the

base is a salt, such as, for example salts of an alkali

metal (lithium, sodium, potassium, rubidium, and so on)

and salts of an alkali earth metal (beryllium,

magnesium, calcium, and so on) . More preferably, the

salt is non-hydroscopic, i.e. a salt that remains in a

particulate form at ambient conditions. Preferred non-

hydroscopic salts are represented by "MHB" and "MB",

wherein M is defined as an alkali earth metal or alkali

metal, H is hydrogen, and B is an anion selected from

the group consisting of -C0₃, -OH, and -HC0₃ (including,

for example, Na₂C0₃, NaHC0₃, K₂C0₃, KHC0₃, MgC0₃, and

mixtures thereof) . The most preferred base component is

NaHC0₃. The base component is most preferably a fine

particulate such that when screen printing is the

technique used for fabricating the planar sensor, the

particulate does not exceed the mesh size of the screen

(typically less than about 400 mesh size) . The base

should be used in an amount sufficient to provide an

oxidizing effect when the metal paste mixture are heated

in the presence of air, but not in such an excessive amount as to compromise the consistency of the paste for

planar formatting. Generally, the base is used in an

amount ranging from about 0.1 wt. % to about 50 wt. %,

more preferably, in an amount ranging from 5 wt. % to

25 wt. %, and most preferably about 16 wt. %, with said

weight percentages based on the total weight of the

metal paste and base prior to heating. The metal paste

and base may be combined prior to or during the planar

formatting process. Additionally, the metal paste

mixture may be heated in the presence of air to produce

a metal/metal oxide electrode prior to placement on an

electrically nonconductive substrate. For ease of

manufacturing, preferably the metal paste and base are

combined, mixed, and then applied to an electrically

conductive contact material present on the nonconductive

substrate and thereafter heated. The manner in which

the metal paste mixture is applied to the electrically

conductive contact material on the substrate may be any

suitable technique resulting in a planar format,

including stenciling, and known thin or thick film

processes (such as described in J.L. Pedigo, et al. , Hybrid Circui t Technology, Feb. 1992, pp. 28-31, hereby

incorporated by reference) . Particularly from an ease

of manufacturing standpoint, silk screen printing is a

preferred means to apply the metal paste mixture to the

substrate. The thickness of the metal paste mixture may

be varied.

The substrate materials that make up the base

support upon which the sensors are fabricated may be

selected from any electrically non-conductive material.

These materials are well-known and include, for example,

ceramic, glass, refractory, and polymeric materials, and

combinations thereof, and so on. Particularly preferred

are alumina based substrate chips. Currently, most

preferred substrates are of an alumina and glass binder

combination. Groves may be fashioned in the

nonconductive substrate such that layers can be

specifically applied to sections on the substrate

The electrically conductive material used in the

sensors is preferably a noble metal, with gold most

preferred. The electrically conductive material may be

applied to the substrate in various ways, although most preferably the material is adhered to the substrate,

typically by heating, prior to applying the dielectric

coating and metal paste mixture. Typically for pH

sensors, at least one region of the electrically

conductive material is applied to the substrate. For

C0₂ sensors, at least two separate regions of the

electrically conductive material are applied to the

substrate. Preferably a dielectric coating is present

between the two regions on the C0₂ sensor thus

insulating the conductive regions from each other.

Typically the dielectric coating (as widely commercially

available) is also present on at least the lead portions

of the electrically conductive regions but is not

present on an exposed electrode area and an exposed

contact area in each region of the conductive material.

The metal paste mixture is applied adjacent to the

electrically conductive region in at least the exposed

electrode area.

The metal paste is heated at a temperature and for a period sufficient to oxidize a portion of the metal

paste mixture thus forming a metal/metal oxide electrode. As oxygen may typically be present in the

base component, oxygen does not have to be present

during the heating. Preferably, however, the heating

occurs in the presence of oxygen (most conveniently

air) . The time and temperature of heating may be

adjusted according to the type of heating source

employed, the temperature employed, and the time of

exposure. Preferably, the metal paste mixture is heated

for a period sufficient to provide an active electrode

having an equilibrium of metal to metal oxide content,

such as described in S. Glab, et al . (referenced

previously herein) . More preferably, to transform the

metal oxide paste to a metal/metal oxide electrode

(preferably present on the substrate) , the paste is

heated for a period of from about 1 min. to 30 min.

(more preferably 5 min. to 15 min.) at a temperature of

from about 450 °C to about 600 °C (more preferably from

525 °C to 575 °C) to oxidize a portion (but not all) of

the electrode. Additionally the electrode may be

reheated during the fabrication of the entire sensor if

desired. The active metal/metal oxide electrode may be used

in different types of sensors, most preferably planar pH

and C0₂ sensors. Components described herein, as well

as additional features, may be arranged in the planar

format on non-conductive substrates in various

configurations.

As shown in the FIG. 1, a pH sensor 5 is present

on an electrically nonconductive substrate 10 having

present thereon an electrically conductive material in

one region 15, 20, 25 adheringly applied adjacent to the

substrate 10. The insulating dielectric coating 26 is

applied over the substrate and electrically conductive

lead portion 20 but not over an exposed electrode area

15 and an exposed contact area 25. In the exposed

electrode area 15, the metal paste mixture is applied

and becomes the metal/metal oxide active electrode 30

upon heating in the presence of air. The exposed

contact area 25 is where electrical contact may be made

between the sensor 5 and the pH sensing instrument.

As shown in FIG. 2a (top view) and 2b (side view) ,

a sample chamber 31 may be superimposed upon the active electrode area 15, 30 of the sensor 5 so that the fluid

sample enters and exits through the sample tube 33 and

is confined in a sample chamber contact region 32 such

that contact is made between the sample and the active

electrode 30 (and reference electrode when present in

the sensor chip) . Alternatively, the sensor 5 may

directly immersed in a fluid sample such that a sample

chamber is not part of the configuration of the sensor.

FIG. 3 shows a Severinghaus type of sensor 35 that

may be used in the measurement of C0₂. This sensor 35

differs from the pH sensor 5 by having an additional

region of electrically conductive material 37, 38, 39

with an exposed active electrode area 39 (with no dielectric coating) having present thereon a

silver/silver halide material 40 that is used as an

internal reference electrode. The second region of

electrically conductive material also has present an

exposed contact area 37 and a dielectric coating 26 over

a lead portion of the electrically conductive material.

The metal/metal oxide electrode area 15, 30; lead 20;

and contact area 25 are insulated from the silver/silver halide electrode 39, lead 38, and contact area 37 that

form the internal reference electrode by the dielectric

coating. In the Severinghaus sensor 35, the mV output

between the metal/metal oxide electrode 30 and the

silver/silver halide electrode 39 is linearly related to

log pC0₂. Preferably the halide of the internal

reference electrode is chloride. The silver/silver

halide electrode may be pre-formed and plated on at

least the exposed electrode area of the conductive

material of substrate or may be applied as a paste and

thereafter heated to form the silver/silver halide

electrode.

In FIG. 4, a pH sensor is shown where electrical

contact between the active electrode 30, reference

electrode 50, and sensing equipment (or instrument) 55

is made using the electrically conductive regions 15,

20, 25 of the sensor thereby resulting in a measurement

of the pH.

As shown in FIG. 5, the pH sensor may have present

a neutral carrier cover membrane 36. When present on

the pH sensor, the cover membrane functions to protect the active electrode from the outside environment and

materials which degrade the performance of the sensor,

such as redox centers and biological materials. As

defined herein, the neutral carrier cover membrane

itself is capable of sensing the concentration of the

pH, where the protons do not go through during the

measurement of pH but rather establish a potential

across the cover membrane. The cover membrane may be

applied to the planar sensor in various ways, including

dip coating, and so on. Materials that may be used to prepare the cover membrane are well-known water

permeable and gas permeable polymeric materials known to

those skilled in the art. Particularly preferred

polymeric or copolymeric materials that may be used

include polyvinyl halide monomers. More preferably

employed as the major component of the cover membrane is

a polyvinyl chloride (more preferably a plasticized

polyvinyl chloride) . Other components that are typically included in the cover membrane are known

ioniphores and lipophilic salts. FIG. 6 shows a sensor formatted such that an internal electrolyte 27 is superimposed over active

electrode region 15, 30 and neutral carrier cover

membrane 36 is superimposed over the internal

electrolyte layer 27. The internal electrolyte

typically functions to keep the pH constant in the

proximity of the electrode (s) and may be buffered if

desired. Buffers that may be included are well known in the art.

FIG. 7 shows a C0₂ Severinghaus type of sensor

having present a metal/metal oxide electrode area 15, 30

and a silver/silver halide internal reference electrode

area 39, 40. As shown, the C0₂ permeable membrane 28 is

superimposed over the internal electrolyte 27. The C0₂

permeable membrane 28 is not permeable to acid and

separates the sample from the sensor. The permeable

membrane 28 may be applied to the sensor by various

techniques including dip coating, and so on. Materials

that may be used include polymeric and copolymeric

materials, as well known in the art. In the

Severinghaus sensor, the internal silver/silver halide reference electrode provides a stable reference potential. A bicarbonate fill solution is typically

utilized as the internal electrolyte 27 prior to usage.

As C0₂ from the sample diffuses through the membrane and

reaches an equilibrium, the pH of the bicarbonate fill

solution changes, and is detected by the sensor. The

change in the pH is related to the log of the partial

pressure of C0₂.

In addition to using the metal/metal oxide

component in the above-described pH and C0₂ sensors,

other sensor systems may also incorporate the

metal/metal oxide electrode, as may be accomplished by

techniques known to those skilled in the art. The

various components of the sensors are adheringly applied

to the substrate by known techniques. For example, the

electrically conductive material is preferably heated on the substrate prior to applied the metal paste mixture

(and silver/silver halide material) . Additionally, the

silver/silver halide is preferably heated on the

substrate after application to the conductive region.

The dielectric coating may be heated/and or dried. The 4

various membranes are typically dried and/or heated onto

the substrate.

It is to be understood that various modifications

to the invention will be apparent to and can readily be

made by those skilled in the art, given the disclosure

herein, without departing from the scope and materials

of this invention. It is noted that the following

examples given herein are intended to illustrate and not

to limit the invention thereto.

EXAMPLES

The electrical circuitry utilized for the all of

the examples is shown in FIG. 8. Further description of

circuitry may be found in Jon-Selective Electrode

Methodology, Vol. I, Ed. Arthur K. Covington, CRC Press,

1979, pp. 32-33. (hereby incorporated by reference).

Three dimensional, flow-through glass pH sensors ("Type A") were used as standards (as available from Ciba

Corning Diagnostics Corp., Medfield, MA, 278 series), as

identified as sensor Type "A" . Three other types of

sensors (all planar) were prepared and are identified as

Types "B", "C" and D" . A silk screen printing technique was used in the fabrication of the planar sensors in the format shown in FIG. 3. As shown in FIG. 3, a

metal/metal oxide and a Ag/AgCl electrode were present

on the substrate chip, however, in the present set of

examples, the Ag/AgCl electrode was not used, rather a

Ag/AgCl external reference was used.

The substrate chips upon which each type of planar

sensors were fabricated were 2" by 2" wafers perforated

to form a total of 40 sensors. The wafers were made of

approximately 96 % alumina and approximately 4 % glass binder, as purchased from Coors Ceramic Company, Grand

Junction, Colorado. As shown in FIG. 3, a gold (Au)

electrically conductive region was applied to 15, 20, 25. Parallel to this strip, a gold strip was applied to

region 37 and the upper portion of 38. The lower region

of 38 continuous to and including 39 had applied thereto

a silver paste (obtained from Metech, Elverson PA) . The

gold was purchased from E.I. DuPont DeNemours & Company

of Wilmington, Delaware. Upon depositing the conductive

regions on the substrate chips 10, the chips were heated

at 850° C for 6 minutes. A dielectric insulating material 26 (Cat. No. 9615 from E.I. DuPont DeNemours &

Co.) was applied over the substrate and conductive

regions with the exception of an exposed first portion

15 and 25 on the first gold strip and an exposed second

region 39 and 37 on the second strip. The chips were

then reheated at 850 °C for 6 minutes. Thereafter, the

different metal pastes, as described below, were applied

to the other gold strip in the exposed electrode area.

A preformed Ag/AgCl electrode plating was deposited on

the exposed silver region 39 to go to form the internal

reference region 40.

In the Type B and D sensors (both representing the

INVENTION) , the metal paste was a base and metal paste

mixture prepared by mixing approx. 5 g of palladium

paste (purchased as Catalogue No. PC10141 from Metech

Company of Elverson, PA.) and approx. 1 g of NaHC0₃

(fine powder particulate) . In the Type C sensor chips

(representing a COMPARATIVE) , the metal paste applied to

the gold strip was an already formed palladium oxide

paste purchased from Metech under Catalogue No. PC10119. The metal paste was applied to the exposed electrode area 15 of the gold strip on the substrate by

silk screening and thereafter the entire chip was then

heated at about 550° C for approx. 10 min. in the

presence of air, thereby forming the Pd/Pd oxide active

electrode.

The Type D sensor chips differed from Type B

sensors by the Type D sensors having an internal

electrolyte layer adjacent to the electrodes as well as

a neutral carrier cover membrane superimposed over the

internal electrolyte layer, as shown in FIG. 6. The

internal electrolyte layer was prepared as a coating

using a solution formed by first dissolving approx. 2 %

wt./vol. of 100 % hydrolyzed polyvinyl alcohol in 100 ml

of water and thereafter adding 0.8 ml of a 0.1 .

solution, of NaHC0₃ and KCl. This coating solution was

then deposited over both the active electrode and the

silver electrode on the chip. Thereafter, the material

was dried at 80° C for about 1 hour thus forming the

internal electrolyte layer on the chip. A cover

membrane was then applied on top of the internal electrolyte membrane. The cover membrane was a polymer

solution in 5 % wt./vol. tetrahydrofuran of 68.9 wt.%

dioctyl phthlate (a plasticizer) , 26.0 wt.% polyvinyl

chloride (PVC) , 3.1 wt.% (TDDA, ioniphore for pH) , 2 wt.

% potassium tetra (p-chloro phenyl) borate (KTpClPB) .

An aliquot of this solution was spread evenly over the

entire chip and the solvent allowed to evaporate to form

the PVC-based cover membrane. When electrical contact

was made between the Type D sensors and the sensing

instrument, the cover membrane was scratched off the

electrical contact area 25 prior to usage. Acrylic

sample chambers, as shown and described in FIG. 2 were

employed for all planar sensors used.

Example 1 This example demonstrates a linear mV response of

the standard glass sensors (Type A) and two types of the

planar sensors (Types B and C) when tested in aqueous

solution containing bicarbonate. Variations in pH level

were produced by varying the percent of C0₂ in solution

and the concentration of bicarbonate, with the solution

kept at approx. 37 °C. Carbon dioxide was used to generate the pH values. The 0₂ levels were also varied

with the C0₂ levels to test for 0₂ interference. A

total of three different pH conditions and levels of C0₂

gas in aqueous samples containing bicarbonate were

tested.

The output of the pH sensors was measured as the

potential (volts) between the active electrode and an

external reference. The mV output of the sensors was

measured, with results summarized in TABLE I below and

illustrated in FIG. 9. As shown in the figure, line a

is the mV output from the glass sensors (Type A) , lines

b₂.₃ are from the Type B planar pH sensors (INVENTION) ,

and lines c_x_₂ are from the Type C planar pH sensors

(COMPARATIVE) . Results indicated that all sensors

provided adequate slopes and linearity to give

acceptable measurement for pH.

_____

pH Linearity

-____& TYPE A TYPE B TYPE B TYPE B TYPE C TYPE

SJ__ -60.87 -74.03 -73.06 -73.62 -73.94 -72.9

RS£ 0.9998 0.9995 0.9995 0.9995 0.9997 0.999 Example 2 The amount of oxygen (0₂) was varied to test for drift induced by the changes in oxygen levels (i.e. 0₂

interference) . The pH and C0₂ present were kept at a constant 3% during all tests.

As shown in FIG. 10, a Type A glass sensor (line

a) was tested as well as three Type B planar sensors

(INVENTION, lines b₁.₃) and two Type C planar sensors

(COMPARATIVE, lines c₂.₂) . The data show little to no

response changes in the output of the Type B sensors in

going from 0 to 26 % 0₂ (at constant 3% C0₂) . In

contrast, the Type C sensors show undesirable drifts in

the mV output induced by the changes in 0₂ levels, i.e.

0₂ interference.

Example 3 Using the Type D planar sensors ( INVENTION) ,

samples at three different pH conditions were tested.

Type A glass sensors were also tested under the same

conditions to provide a standard, with the method of

collecting data as described in Examples 1 and 2. Results are summarized in TABLE II and illustrated in

FIG. 11. As shown, the Type D planar sensors provided

comparable slopes and linearity as those provided by the

standard.

_TABLE -.-.

pH Linearity

SENSOR TYPE A TYPE D TYPE D TYPE D TYPE D TYPE D

SLOPE -61.9 -54.6 -56.9 -56.8 -58.4 -53.5

Rsq. 0.99995 0.99986 0.99984 0.99994 0.99992 0.9998

It is not intended that the scope of the claims

appended hereinafter are limited to the description as

set forth herein, but rather that the claims be

construed as encompassing all features of patentable

novelty which reside in the present invention, including

all features which would be treated as equivalents

thereof by those skilled in the art to which the

invention pertains.