GB2106253A - Ionic activity measuring device - Google Patents

Abstract

In an ionic activity measuring device provided with at least two solid electrodes (0) and a porous bridge (9) extending over the solid electrodes in such a manner that, when droplets are applied (to holes 10, 11) so as to contact the solid electrodes and the porous bridge, an ion contained in the droplets can move through the porous bridge, the porous bridge comprises a porous member (20) of which at least the surface remote from the solid electrodes and extending over the solid electrodes in the direction of the movement of ions through the porous member between the solid electrodes is hydrophobic. The porous member (20) may be made of a variety of materials and may be provided with a non-porous layer (22) on the surface adjacent to the solid electrodes (0). The hydrophobic surface of the porous member (9) may be formed by treating a non-hydrophobic material to impart hydrophobicity thereto. The porous member (9) is provided with liquid receiving holes (10, 11), which are positioned in such a manner that at least one hole is in registry with each solid electrode (0). The solid electrodes are ion selective and of layer construction. <IMAGE>

Description

SPECIFICATION ionic activity measuring device This invention relates to a device for measuring ionic concentration or ionic activity, and more particularly to an ionic activity measuring device useful for potentiometric measurement of the concentration or ionic activity of an ion contained in specimens such as water, body fluids (for example, whole blood, blood plasma, blood serum, urine and the like), and aqueous solutions (for example, service water, irrigation water, river water, rain water, wine beer and the like).

Generally, from the clinical or industrial point of view, it is important to selectively measure the concentration of an inorganic ion, for example K+, Na+, Ca2+, C1 - or HCO3-, contained in body flu ids or aqueous solutions. For this purpose, it has been proposed to use dry tape ion selecting electrodes which are easy to store and operate the measurement. As an example of the dry type ion selecting electrode (half cell or single electrode), it has been proposed in Japanese unexamined Patent Publication No.

52(1977)-i 42584 to stack four functional layers on a substrate and form a film-like dry type solid ion selecting electrode (hereinafter sometimes referred to as a "solid electrode"). To conduct measurement with the film-like solid electrode of this type, a very small amount (e.g. between 5y1 and 50yl) of a specimen is applied to a predetermined position on the ion selecting layer of the solid electrode.

Certain ion measuring devices and electrodes therefor which are known in the art are described hereinafter with reference to Figures 1 to 3 of the accompanying drawings in which: Fig. 1 is a schematic perspective view of a conventional film-like solid electrode; Fig. 2 is a schematic perspective view of a conventional ion measuring device incorporating two electrodes of the type shown in Fig. 1; and Fig. 3 is a schematic sectional view of the device of Fig. 2 taken along line SS.

Referring to Figure 1 , a solid electrode 0 comprises a metal layer 2, an insoluble metal salt layer 3, a reference electrolyte layer 4 and an ion selecting layer 5 sequentially stacked as the functional layers on a substrate 1. For example, when the conventional film-like solid electrode 0 shown in Figure 1 is used for measuring the concentration of potassium ions the metal layer 2 is formed of silver, the insoluble metal salt layer 3 is formed of silver chloride, the reference electrolyte layer 4 is made by dispersing potassium chloride in a hydrophilic organic polymer binder, and the ion selecting layer 5 is an organic ion selecting layer containing an organic compound capable of selecting potassium ions a carrier solvent and an organic polymer binder.

It has also been proposed in Japanese Patent Application No. 55(1 980)-92378 to use a solid electrode comprising three stacked functional layers, in which the reference electrolyte layer 4 shown in Figure 1 is omitted, and the ion selecting layer 5 consisting of organic materials is directly positioned on the insoluble metal salt layer 3. Further, Japanese unexamined Patent Publication No.

48(1 973)-82897 discloses a dry type solid electrode comprising two stacked functional layers, in which the insoluble metal salt layer 3 and the reference electrolyte layer 4 shown in Figure 1 are bmitted, and an ion selecting layer 5 containing an ion exchange material is directly positioned on the metal layer 2.

The conventional ion measuring instrument shown in Figure 2, an instrument such as that disclosed in U.S. Patent No. 4053381, comprises film-like solid electrodes 6 and 7 of the type shown in Figure 1 , which are fixed in a frame 8 so that the functional layers are not short-circuited at their ends. A bridge 9 formed of a porous member extends over the film-like solid electrodes 6 and 7. A potentiometer 12 having a high internal impedance is connected to electric connection terminals on the metal layers 2 of the electrodes 6 and 7 by lead wires 13 and 14. When measurement is conducted with the instrument shown in Figure 2, a specimen and a standard solution are dropped almost at the same time into liquid receiving holes 10 and 11 respectively, which are perforated through the bridge 9 positioned on the electrodes 6 and 7.The specimen and the standard solution then exhibit capillary phenomena and penetrate through the porous member of the bridge 9. When the specimen and the standard solution contact each other approximately at the center of the bridge 9 and ion transfer occurs, the difference in potential between the electrodes 6 and 7 is indicated on the potentiometer 12. By measuring the difference in potential, it is possible to determine the concentration of the activity of an ion contained in the specimen.

Japanese unexamined Patent Publication No. 55(1 980)-20499 disclosed a bridge 9 having a configuration as shown in Figure 3 comprising a non-porous bottom substrate 1 7 (on the face adjacent to the solid ion selecting electrodes) an intermediate porous member layer 18, and a top non-porous hydrophobic layer 1 9 (on the face remote from the solid ion selecting electrodes).In the case where the intermediate porous member layer 18 consists of a filter paper made of a natural vegetable fiber pulp, when a specimen 15 and a standard solution 1 6 are dropped into the liquid receiving holes 10 and 11 in the bridge 9, the specimen 1 5 and the standard solution 1 6 fill up the respective holes 10 and 11 and form a "lid" on the top non-porous hydrophobic layer 19. The specimen 1 5 and the standard solution 1 6 are absorbed by the intermediate porous member layer 18 in less than 30 seconds, and come into contact with each other at approximately equal distances from the liquid receiving holes 10 and 11, i.e.

approximately at the center of the bridge 9. In this way, ion transfer becomes possible, and a potential develops between the electrodes 6 and 7.

Where the intermediate porous member layer 1 8 used in the bridge is a porous member as disclosed in Japanese unexamined Patent Publication No. 55(1980-20499, it is essential to provide the top non-porous hydrophobic layer 19. Namely, if the top non-porous hydrophobic layer 19 is not provided on the intermediate porous member layer 18, the specimen 1 5 and the standard solution 1 6 dropped into the liquid receiving holes 1 0 and 11 in the bridge not only penetrate through the porous section inside the intermediate porous member layer 18 of the bridge but also tend to overflow and spread on the surface of the intermediate porous member layer 1 8 (and consequentially overflow not only over the upper surface of the layer 18 but also to the edge surfaces thereof).As a result, the specimen 1 5 and the standard solution 16 contact and mix with each other on the surfaces of the bridge (this phenomenon is called external bridging). Therefore, an incorrect or unwanted potential occurs, and the ionic concentration or the ionic activity cannot be determined correctly. External bridging is extremely troublesome particularly in an ionic activity measuring instrument comprising small solid ion selecting electrodes and a small bridge.

Further, when the intermediate porous member layer 18 used in the bridge is constituted of the porous member as disclosed in Japanese unexamined Patent Publication No. 55(1 980) 20499 without being provided with the top non-porous hydrophobic layer 1 9 thereon, water contained as the medium in the specimen 1 5 and the standard solution 1 6 evaporates from the bridge, resulting in a change (an increase) in the concentrations of the specimen 15 and the standard solution 16. In this case, too, it is impossible to correctly determine the ionic concentration or the ionic activity.In addition, if there is no top non-porous hydrophobic layer 1 9, the specimen 1 5 and the standard solution 16 must respectively be applied precisely at the centers of the liquid receiving holes 1 0 and 11 in the bridge. If they are applied to points deviating from the centers of the holes 1 0 and 11, they will not wet the whole areas of the holes 1 0 and 11 but will unevenly penetrate through the inside of the intermediate porous member layer 1 8, or contact each other at a point nearer to one or the other of the liquid receiving holes 10 and 11 instead of approximately at the center of the bridge. Therefore, it becomes impossible to correctly determine the concentration or the activity of a desired ion, or the measured values will fluctuate greatly.

As described above, in the conventional ion measuring instrument shown in Figure 3, it is essential to provide the top non-porous hydrophobic layer 1 9 on the intermediate porous layer 1 8. However, this configuration is disadvantageous in that the process for manufacturing the bridge becomes complicated because the top non-porous hydrophobic layer 1 9 must be formed on the intermediate porous layer 1 8, and in that the layer 1 9 adversely affects the free penetration of the solutions through the layer 1 8.

Particularly when the specimen is highly viscous, it cannot quickly penetrate through the layer 18 and requires a long time before it contacts the standard solution. As a result, it takes a long time for measurement.

On the other hand, studies conducted on the problem of water evaporation from the specimen and the standard solution mentioned in Japanese unexamined Patent Publication No. 55(1 980)-20499 revealed that, when a special porous member is used, water evaporation presents no real problem with regard to ion measurement in the course of a substantial measuring time, for example five minutes. The present invention has been made on the basis these observations.

According to one aspect of the present invention there is provided an ionic activity measuring device comprising at least two solid electrodes and a porous bridge extending therebetween and capable of permitting motion through said porous bridge of ions contained in liquid applied to contact said electrodes and said porous bridge, the said porous bridge essentially consisting of a porous member of which at least the surface remote from said solid electrodes and extending in the direction in which the ion motion in said porous member between said solid electrodes occurs is hydrophobic.

Advantages of ionic activity measuring devices according to the invention include the following: (a) the bridge is capable of being manufactured easily by virtue of the absence of a requirement for a top non-porous hydrophobic layer; (b) the devices can reliably measure the ionic concentration or ionic activity; (c) the range of materials from which the porous member for the bridge may be formed is extended; and (d) the construction of the ionic activity measuring devices may be simple.

In the ionic activity measuring device in accordance with the present invention, the bridge is formed of a porous member free from external bridging removing the need for a top non-porous hydrophobic layer on the porous member. Therefore, the bridge can be manufactured by a simple process, and it is possible to eliminate various problems which occur between the top non-porous hydrophobic layer and the intermediate porous layer in the conventional ion measuring instrument.

Further, external bridging does not occur, the effect being the same as if there were a top non-porous hydrophobic layer on the bridge. Furthermore, the device in accordance with the present invention provides tolerance with respect to the positions at which the specimen and the standard solution are applied, and does not develop changes in measuring time or produce measurement errors due to a difference in specimen viscosities. Accordingly, the device in accordance with the present invention can measure the ionic concentration or the ionic activity with high accuracy and reliability.

In the present invention, the solid electrodes may have the same construction as those generally called half cells or single electrodes.

The term "hydrophobic" as used herein is defined as what is generally considered hydrophobic in the technological field. In the present invention, at least one surface of the porous member constituting the porous bridge must be hydrophobic so that solutions applied to sections of the bridge adjacent to both ends (sections corresponding to the two electrodes) do not move on the outer surface of the bridge to contact each other on the outer bridge surface.

Preferably, the hydrophobic property of the surface of the porous member is such that, when a porous member provided with two through holes of a substantially equal size spaced apart not to contact each other is bonded and fixed to the smooth flat surface of a water-impervious substrate having a smooth flat surface in a manner which does not substantially affect the porpsityofthe porous member and when the ionic activity measuring time has elapsed after an aqueous bovine albumin solution of predetermined volume larger than that of each through hole is added to the respective through holes substantially at the same time, the aqueous bovine albumin solution remains in the respective through holes in an amount allowing the measurement of the ionic activity, the portions of the aqueous bovine albumin solution diffusing from said two through holes through the interior of said porous member contact each other at the interface therebetween, and the overflow widths of the aqueous bovine albumin solution from the marginal sections of said two through holes on the surface of said porous member remote from said water-impervious substrate are within a range wherein the two overflows do not couple and mix with each other.

In the present invention, porous members made from the following materials are particularly suitable: a) paper made of a mixture of pulp consisting of a synthetic polymer fiber and pulp consisting of a natural vegetable fiber, b) paper made of pulp consisting of a synthetic polymer fiber, c) mixed fabric consisting of a synthetic polymer fiber and a natural vegetable fiber, d) plain weave fabric consisting of a natural vegetable fiber, e) membrane filters having an average pore diameter of 2,um or less and consisting of cellulose esters or regenerated cellulose, f) membrane filters having an average pore diameter of 1 Oe4m or less and containing nitrocellulose, and g) paper made of pulp consisting of a natural vegetable fiber and compressed.

Further, preferred aspects of the present invention include the following: 1. An ionic activity measuring device according to the invention wherein said porous member is positioned on said solid electrodes and provided with liquid receiving holes in a number at least equal to the number of said solid electrodes, the said liquid receiving holes being positioned in such a manner that at least one hole is in registry with each solid electrode.

2. A device as defined in item 1 above wherein the surface of said porous member adjacent to said solid electrodes is provided with a non-porous layer having holes therein in registry with said liquid receiving holes.

3. A device as defined in item 2 above wherein said non-porous layer is terminated at the edges of said porous member.

4. A device as defined in item 2 or 3 above wherein said non-porous layer comprises at least one layer of a bonding agent or an adhesive agent.

5. A device as defined in item 2 or 3 above wherein said non-porous layer is formed as an integral part of said porous member by eliminating the porosity of said porous member at said surface adjacent to said solid electrodes and in the vicinity of said surface.

6. A device as defined in item 2 or 3 above wherein said non-porous layer is formed by heating and bonding to said surface adjacent to said solid electrodes a polymer film having a bonding or adhesive property under the action of heat.

7. An ionic activity measuring device wherein the hydrophobic surface of said porous member is formed by treating a non-hydrophobic material forming at least a surface of said porous member to impart hydrophobicity thereto.

According to a further aspect of the present invention there is provided a porous bridge for use in an ion activity measuring device comprising a substantially planar layer of porous material having at least two liquid receiving holes extending through the said layer in the out of plane direction wherein at least one planar integral surface of the said layer is hydrophobic.

Embodiments of the present invention will now be described by way of example with reference to Figures 4A, 4B, 5, 6A and 6B of the accompanying drawings, in which: Figure 4A is a schematic sectional view showing an embodiment of an ionic activity measuring device in accordance with the present invention (having an appearance similar to that of the ion measuring instrument shown in Figure 2), the section being taken through the centers of two liquid receiving holes of the porous bridge; Figure 4B is a schematic plan view showing the porous member of the porous bridge of the embodiment shown in Figure 4A; Figure 5 is a schematic sectional view showing another embodiment of an ionic activity measuring device in accordance with the present invention, the section being taken through the centers of the liquid receiving holes of the porous bridge;; Figure 6A is a schematic view showing a film-like solid electrode comprising three stacked functional layers wherein an ion selecting layer extends around the edges of a metal layer, which can be suitably employed in an ionic activity measuring device in accordance with the present invention; and Figure 6B is a schematic sectional view taken through the electrode of Figure 6A along the line S-S.

Preferred embodiments of the present invention will now be described below with reference to Figures 4A to 6B, in which similar elements are numbered with the same reference numerals as those used in Figures 1 to 3.

In Figure 4A showing an embodiment of an ionic activity measuring device in accordance with the present invention, two film-like solid electrodes 0 each comprising a metal layer 2, an insoluble metal salt layer 3 and an ion selecting layer 5 providied on a substrate 1 are positioned on a frame 8. The solid electrodes 0 may alternatively be any of the other known film-like solid electrodes having two, three or four functional layers, as described above. Further, the solid electrodes 0 may be of the type shown in Figures 6A and 6B in which the ion selecting layer 5 completely or partially extends around the edges of the metal layer 2 indirectly via the insoluble metal salt layer 3.Alternatively, the solid electrodes 0 may comprise the ion selecting layer 5 directly covering all or some of the edges of the metal layer 2 without the intervening insoluble metal salt layer 3. The solid electrodes 0 may alternatively be wire electrodes as disclosed in Japanese Utility Model Publication NO. 40(1 965)-1 4472. In this case, the wire electrode may comprise two, three or four functional layers, including the wire metal serving as the metal layer 2 in the above-mentioned film-like solid electrode.

The ion selecting layer 5 is essential in cases where the ion to be measured is K+, Na+, Ca2+, or HCO3-. In cases where the ion to be measured is CI and the electrode comprises a metal layer 2 made of silver and an insoluble metal salt layer 3 made of silver chloride, it is possible to replace the ion selecting layer 5 with a CI- ion-pervious protective layer made of, for example, cellulose acetate, polymethacrylic acid, polyacrylic acid, or poly(2-hydroxyethyl acrylate) employed in a halogen ionpervious protective layer as disclosed in Japanese unexamined Patent Publication 55(1 980)-89741.

In the embodiment shown in Figure 4A, a porous bridge 9 comprising a porous member 20 and a double-faced adhesive tape 22 and having two liquid receiving holes 10 and 11, as best seen in Figure 48, is positioned on the solid electrodes 0. The said adhesive tape 22 terminates at the edges of the porous member 20. The porous member 20 has a hydrophobic surface on the side remote from the solid electrode 0. In another embodiment shown in Figure 5, the porous bridge 9 comprises a bottom non-porous hydrophobic layer 21 made of polyethylene between the porous member 20 and the double-faced adhesive tape 22.

In the ionic activity measuring device in accordance with the present invention, at least the surface of the porous member 20 of the porous bridge 9 remote from the electrodes iq is, hydrophobic. The porosity of the porous member 20 is of such an extent that open cells or pores exist at the surface and in the interior of the porous member 20 to a degree that water contained in the aqueous liquid (aqueous solution or aqueous dispersion) can diffuse and flow through the continuous pores by capillary attraction. The open cells may have any configuration allowing water to diffuse and flow therethrough by capillary attraction. For example, the open cells may form a plurality of hollow paths containing independent or communicating spaces, or may be a group of empty cells partially communicating with one another through openings.They may also be a group of empty cells of various shapes irregularly existing and partially communicating with one another through openings.

The hydrophobic property which at least the surface of the porous member 20 remote from the electrodes exhbits is of such extent that no significant diffusion or flow of water on the surface of the porous member 20 will occur. More preferably, the hydrophobic property should satisfy the following requirement: A porous member provided with two through holes (liquid receiving holes) is fixed on the smooth and flat surface of a water-impervious substrate in such a manner not to substantially clog the pores (open cells) of the porous member by use of a bonding agent impervious to water or a doublefaced adhesive tape impervious to water. Two liquid receiving holes are perforated through the porous member, having a size and hole volume substantially equal to each other, and are appropriately spaced apart from each other so as not to contact each other.Then, droplets of an aqueous bovine albumin solution of equal volume, a volume larger than that of each liquid receiving hole, are applied to the respective liquid receiving holes. After the time has elapsed that would be required for measuring the ionic activity, the overflow widths of the aqueous bovine albumin solution from the marginal sections of the two liquid receiving holes on the surface of said porous member remote from the water-impervious substrate are observed or determined. When the overflow widths thus determined are within a range not giving rise to the aforesaid external bridging, the porous member is, by definition, said to have said at least one hydrophobic surface.

The water-impervious substrate having the smooth and flat surface and used for determining the hydrophobicity may be a plate-like material having a smooth and flat surface (or a mirror surface), for example a water-impervious polymer film such as polyethylene terephthalate (PET) film, bisphenol-A polycarbonate film, polyethylene (PE) film or vinylidene chloride-vinyl chloride copolymer film, a glass plate, or a metal plate such as aluminium plate, copper plate, copper-zince alloy plate or stainless steel plate. The water-imperious bonding agent not substantially clogging the open cells of the porous member may for example be a pressure-sensitive adhesive composition.The double-faced adhesive tape not substantially clogging the open cells of the porous member may for example be a PET film, a polyvinyl chloride film, a cellulose acetate film or a cellophane tape provided with water-impervious, pressure-sensitive adhesive composition layers on both surfaces. In cases where the water-impervious substrate is formed of a film having adhesive surfaces, such as vinylidene choride-vinyl chloride copolymerfilm, it is possible to directly fix the porous member on the water-impervious substrate and determine the hydrophobicity of the surface of the porous member without using a bonding agent or double-faced adhesive tape.From the practical view point, for the purpose of determination or bonding and fixing of the porous member to the surface of te solid electrodes, when the hydrophobicity of the surface of the porous member is determined by bonding and fixing the porous member to the surface of the water-impervious substrate by use of a bonding agent or double-faced adhesive tape, it is possible to perforate the liquid receiving holes in advance through both porous member and adhesive layer or double-faced adhesive tape.

The holes perforated through the porous member should have a diameter approximately equal to that of the liquid receiving holes of the ionic activity measuring device, and can be approximately determined according to the volume of the liquid droplet applied and the thickness of the porous member. When the thickness of the porous member is between 100,um and 2mm, the diameter of the through holes is generally between 1.5mm and 7mm, preferably between 2mm and 4mm. To correctly evaluate the hydrophobicity, it is preferably that the through holes have substantially the same diameter throughout the holes.

The volume of the through hole is defined as the volume of the space defined by two opposing planes of the porous member through which the hole is perforated and the side wall of the through hole.

An aqueous bovine albumin solution should be applied to each through hole in an amount at least equal to the volume of the through hole and allowed to diffuse predominantly from the side wall of the through hole and the marginal section thereoffartherfrom the water-impervious substrate to the interior of the porous member. Therefore, the volume of the aqueous bovine albumin solution applied must be greater than the volume of the through hole but no so much greater as to overflow from the through hole. Generally, the amount of the aqueous bovine albumin solution applied is between two times and 20 times the volume of the through hole, preferably between three times to 15 times the volume of the through hole.The aqueous bovine albumin solution should be carefully applied approximately to the center of the hole by ejecting a droplet from the end of a capillary or a micro pipette. The solution should preferably be applied simultaneously to both through holes but, in some cases, the point of time of application may differ by about three seconds or less.

The concentration of the aqueous bovine albumin solution differs depending on the kind of the aqueous liquid to be measured with the ionic activity measuring device, and is generally between 1% and 15% by weight, preferably between 3% and 10% by weight. To facilitate the observation and determination of the overflow width, is is possible to add a very small amount of a water-soluble dye (e.g. water-soluble bilirubin) to the aqueuous bovine albumin solution.

When the time required for reading the potential value by use of the ionic activity measuring device (ionic activity measuring time) has elapsed after an aqueous bovine albumin solution has been applied to both through holes, the amounts of the aqueous bovine albumin solution remaining inside the through holes, diffusion of the solution in the interior of the porous member, and diffusion and overflow on the surface of the porous member are observed or determined. The ionic activity measuring time is generally between 30 seconds and 15 minutes, preferably between one minute and 10 minutes.When the ionic activity measuring time has elapsed, the amounts of the aqueous bovine albumin solution remaining in the through holes, the positions of the leading tips of the flows of the aqueous bovine albumin solution diffused from the two through holes to the interior of the porous member by capillary attraction, and spread of the aqueous bovine albumin solution overflowing from the marginal sections of the two through holes to the surface of the porous member farther from the water-impervious substrate or wetting of that surface (the solution overflowing onto or wetting the surface is called the overflow from the marginal sections of the through holes) are observed and determined.It is preferably that the distance between the two through holes, i.e. the shortest distance between the two through holes should be approximately identical with the distance between the liquid receiving holes of the ionic activity measuring device. The distance between the through holes is generally between 6mm and 20mm, preferably between 6mm and 15mm.

In the above-described observation or determination of the hydrophobicity of the porous member, the porous member used in the present invention should exhibit such a hydrophobicity that the amounts of the aqueous bovine albumin solution remaining in the through holes are sufficient to establish electric connection to the solid electrodes and measure the ionic activity, the leading tips of the flows of the aqueous bovine albumin solution diffused through the interior of the porous member contact each other approximately at the middle between the two through holes, and the overflow widths of the aqueous bovine albumin solution overflowing from the margins of the through holes to the surface of the porous member farther from the water-impervious substrate (average distances from the margins of the through holes to the leading tips of the overflows of the aqueous solution in the width region corresponding to the diameter of the through holes along the line corresponding to the shortest distance between the through holes) are within a range not giving rise to external bridging. The range not giving rise to external bridging is that wherein the leading tips of the overflows from the margins of the two holes do not contact and mix with each other.The overflow widths should generally be within 3mm, preferably within 2.5mm. The amounts of the aqueous bovine albumin solution remaining in the through holes should be large enough that thin liquid films are observed on the whole inner walls of the through holes and preferably also on the water-impervious substrate.

Examples of the materials of the porous member in which at least one surface is hydrophobic and which can be used in the porous bridge of the ionic activity measuring device in accordance with the present invention are listed below.

a) Paper made of a mixture of pulp consisting of a synthetic polymer fiber and pulp consisting of a natural vegetable fiber.

Examples of the pulp consisting of a synthetic polymerfiber: pulp consisting of PE fiber, PET fiber and cellulose ester fiber such as cellulose diacetate, cellulose triacetate, cellulose acetate butyrate, cellulose acetate propionate, or cellulose acetate phthalate.

Examples of the pulp consisting of natural vegetable fiber: cotton pulp, linter pulp, linen pulp, hemp pulp, Broussonetia Kazinoki Sieb. pulp, Edgeworthia papyrifera Sieb. et Zucc. pulp, Wikstroemia sikokiana Franch. et Sav. pulp. Musa textilio pulp, Esparto grass pulp, bamboo pulp, hardwood bleached semi-chemical pulp, and conifer sulfite pulp.

The weight ratio of the pulp consisting of natural vegetable fiber (hereinafter referred to as the natural pulp) to the whole pulp is generally 20% or less, preferably 30% or less. Preferably, paper should have smooth surfaces without fiber fluff and should be substantially free from vehicle, paste and sizing agent as in the case of filter paper or absorption paper.

b) Paper made of pulp consisting of a synthetic polymer fiber.

This paper is the same as that in a) except that the pulp is formed only of a synthetic polymer fiber.

c) Mixed fabric consisting of a synthetic polymer fiber and a natural vegetable fiber.

Examples of the synthetic polymer fiber: PET fiber, cellulose ester fiber such as cellulose acetate or cellulose triacetate, polyamide fiber such as polycapramide, polyhexamethylene sebacamide or polyundecaneamide, and regenerated cellulose fiber.

Examples of the natural vegetable fiber: cotton fiber, linter fiber, linen fiber and hemp fiber.

A plain weave fabric such as sheeting, calico, broad cloth or poplin woven with 20-count to 120count union yarn consisting of a synthetic polymer fiber and a natural vegetable fiber (hereinafter referred to as the natural fiber) is preferable. However, other types of fabrics may also be used.

It is possible to blend two or more synthetic polymer fibers and two or more natural fibers.

d) Plain weave fabric consisting of a natural vegetable fiber.

A plain weave fabric such as calico, broad cloth or poplin woven by use of 20-count to 120-count yarn consisting of the natural fibers listed in c) or regenerated cellulose.

It is possible to use union yarn consisting of two or more natural fibers or to use different types of warp and weft.

e) Membrane filters having an average pore diameter of 2,um or less and consisting of cellulose esters or regenerated cellulose.

The average pore diameter is preferably 1 .2m or less, particularly 0.8,ltm or less.

It is also possible to use a membrane filter having an average pore diameter in the range defined above and containing a known plasticizer in cellulose esters or regenerated cellulose.

f) Membrane filters having an average pore diameter of 1 O1im or less and containing nitrocellulose as the main constituent.

The weight ratio of nitrocellulose to the whole filter is between 50% and 100%, preferably between 70% and 95%. The average pore diameter is preferably 8,um or less. The filter may contain subsidiary constituents, for example organic acid esters of cellulose such as diacetyl cellulose and triacetyl cellulose.

g) Paper made of pulp consisting of a natural vegetable fiber and compressed.

The natural pulp is selected from those listed in a). Paper such as filter paper or absorption paper is compressed according to a known procedure and preferably treated to impart smooth surfaces (substantially free from fiber fluff) thereto.

Of the porous members listed above, a), b), e) and f) are preferable.

In the porous bridge of the ionic activity measuring device in accordance with the present invention, it is possible to use a porous member in which the speed of an aqueous liquid diffusing and advancing through the interior of the porous member by capillary attraction (capillary flow advance speed) is higher, preferably very much higher than the speed of the aqueous liquid diffusing and advancing on the surface of the porous member by wetting (wetting advance speed). A capillarly flow advance speed higher than the wetting advance speed can be achieved by appropriately selecting the material of the porous member or by treating the surface of the porous member with a known water repellent such as silicone or fluorine compound.It is advantageous to treat the surface of the porous member with a water repellent because the treatment can greatly reduce the wetting advance speed and provide a porous member exhibiting a capillary flow advance speed greatly higher than the wetting advance speed. In this case, the water repellant should not be applied to the walls of the liquid receiving holes in the porous member and should not adversely affect the walls.

Measurement of hydrophobicity of porous member: Various porous members were cut to rectangles having a size of 6mm x 5mum, and a doublefaced adhesive tape (Scotch 665 available from 3M) was bonded to one surface of each rectangular piece. Then, two holes having a diameter of 3mm were perforated through the porous member and the double-faced adhesive tape as shown in Figure 4B (diameter r: 3mm, a: 3mm, b: 2.5mm, c: 1 0mum). The surface of the double-faced adhesive tape opposite to the porous member was bonded to the smooth, flat surface of a PET film to fix the porous member piece.

Aqueous bovine albumin solutions having concentration of 4 wt.% and 8 wt.% were prepared, and 1 O/tl portions of each solution were applied simultaneously to the two holes in the porous member bonded and fixed to the PET film at room temperature (about 250C) by use of a micro pipette. After five minutes, spread of the solution in the interior of the porous member and the spread (overflow) of the solution from the margins of the holes on the surface of the porous member farther from the PET film were determined. For each porous member, the experiment described above was conducted on three specimens as shown in Figure 48.

For comparison, experiments were conducted in the same manner as described above by using "WHATMAN CHROMA +2" filter paper provided with a top non-porous hydrophobic layer as disclosed in Japanese unexamined Patent Publication No. 55(1980-20499), and by using "WHATMAN CHROMA &num;2" filter paper.

In each case of the porous members set forth in Table 1 ,five minutes after the aqueous bovine albumin solution was applied to the holes, the solution remained in the two through holes in an amount allowing electric connection with the solid electrodes, and the leading tips of the solution diffused through the interior of the porous member from the two through holes contacted each other.

The results regarding the overflow on the surface of the porous member were evaluated using the following symbols: ++: The overflow width was not more than 2.5mm, and external bridging was not likely to occur.

The material was excellent for use as the porous bridge in the ionic activity measuring device in accordance with the present invention.

+: The overflow width was more than 2.5mm but not more than 3mm, and external bridging was not likely to occur. The material could be used as the porous bridge.

-: The overflow width was more than 3mm, and external bridging occurred or was likely to occur. The specimen could not be used as the porous bridge.

TABLE I

Aqueous bovine Porous member albumin solution Example No. Type Material 4 wt% 8 wt% e-1 Membrane filter FM-120 (Average hole dia. 1.2 m) + + e-2 FM-80 (Average hole dia. 0.8 m) ++ + e-3 FM-30 (Average hole dia. 0.3 um) ++ ++ e-4 "SEPARAX" + ++ e-5 "SEPARAX EF" ++ ++ e-6 Fr-40 (Average hole dia. 0.4 um) + + e-7 FR-20 (Average hole dia. 0.2 ,um) ++ ++ f-l Millipore SCWP (Average hole dia. 8.0 m) ++ ++ f-2 SMWP (Average hole dia. 5.0 pm) ++ ++ f-3 RAWP (Average hole dia. 1.2 pm) ++ ++ a-l Mixed paper Basis weight 153 g/m2 (PE pulp 30%, natural pulp 70%) ++ ++ a-2 Basis weight 80 g/m2 (PE pulp 50%, natural pulp 50%) ++ ++ a-3 Basis weight 115 g/m2 (PE pulp 50%, natural pulp 50%) ++ ++ a-4 Basis weight 144 g/m2 (PE pulp 50%, natural pulp 50%) ++ ++ a-5 Basis weight 115 g/m2 (PE pulp 75%, natural pulp 25%) ++ ++ b-l PE pulp paper Basis weight 55 g/m2 (PE pulp 100%) ++ ++ b-2 Basis weight 110 g/m2 (PE pulp 100%) ++ ++ b-3 Basis weight 120 g/m2 (PE pulp 100%) ++ ++ c-l Mixed fabric 80-count broad cloth (PET 65%, cotton 35%) ++ ++ c-2 100-count broad cloth (PET 65%, cotton 35%) ++ ++ d-1 Cotton fabric 60-count calico (Cotton 100%) ++ ++ d-2 60-count broad cloth (Cotton 100%) ++ ++ d-3 80-count broad cloth (Cotton 100%) ++ ++ 9 1 Calendered paper Calendered impregnation paper (80 pm thick) + ++ 9-2 Calendered impregnation paper (200 pm thick) ++ ++ Comparative Example 1 Filter paper "WHATMAN CHROMA &num; ;2" Comparative Tri-laminate PET adhesive tape "WHATMAN CHROMA &num;2" Example 2 filter paper (having the configuration of double-faced PET ++ ++ adhesive tape) Note: 1. Membrane Filter FM: A membrane filter consisting only of cellulose acetate and available from Fuji Photo Film Co., Ltd 2. Membrane Filter FR: A membrane filter consisting only of regenerated cellulose and available from Fuji Photo Film Co., Ltd.

3. "Separax": A membrane filter consisting of cellulose acetate and a plasticizer.

4. Millipore SCWP, SMWO, RAWP: A membrane filter consisting of a mixture of nitrocellulose (about 90%) and diacetyl cellulose (about 10%), available fròrMillipore Corp.

5. The natural pulp constituent of the mixed paper was LBKP (hardwood bleached kraft pulp). Both mixed paper and PE pulp paper were plain paper.

6. Impregnation paper: The paper was of the type used as impregnated with oil or the like. In the examples described above, unimpregnated paper was calendered and compressed to reduce fiber fluff on the surface.

7. Trilaminate paper: This had the same configuration as that used as a bridge in Japanese unexamined Patent Publication No. 55(1980)---20499.

In Comparative Example 1 in which "WHATMAN CH ROMA i2" filter paper (175,um thick) availab!e from Wand R Balston Ltd. was used, the aqueous bovine albumin solution very quickly spread immediately when it was applied to the through holes. Thus the spreading of the solution was too fast.

Further, the solution oozed out and flowed on the surface farther from the PET film, and two portions of the solution flowing from the two through holes coupled instantly. It was presumed that almost all specimens would develop external bridging. In addition, only small amounts of the solution remained in the through holes, and it was presumed that it would be difficult to measure the electrical potential.

EXAMPLE 1 For each of the 23 kinds of the porous members which were evaluated as the ++ grade in the measurements of the hydrophobicity either with the 4 wt.% aqueous bovine albumin solution or with the 8 wt.% solution in Table 1, a porous bridge was prepared in the same way as in the examples of the measurement of the hydrophobicity of the porous member surface (hereinafter referred to as the hydrophobicity measurement examples). Instead of bonding the porous bridge to a PET film having a smooth surface, the porous bridge was bonded and fixed to two potassium ion selecting solid electrode films, which had the configuration described below and were fixed on a polystyrene frame, so that the through holes in the porous bridge aligned with the solid electrodes. In this way, 23 kinds of devices for measuring the activity of potassium ion as shown in Figure 4A were prepared.

Configuration of potassium ion selecting solid electrode film: The potassium ion selecting solid electrode film had a configuration as shown in Figures 6A and 6B, and consisted of a 1 OOum-thick PET film 1 having a smooth surface, a silver deposited layer 2, a silver chloride layer 3 formed by partially converting the surfaces of the silver layer into silver chloride, and a potassium ion selecting layer 5 containing "Valinomycin".

For comparison, a device for measuring the activity of potassium ion similar to the abovedescribed 23 kinds of devices was prepared by forming the porous member by use of trilaminate filter paper similar to that used in Comparative Example 2 for the measurement of hydrophobicity described above.

To each of the 23 kinds of the potassium ion activity measuring devices and the one device for comparison, 1 OFcl of commercial pseudo-serum having a potassium ion concentration different from the standard and 1 owl of a standard potassium ion solution were applied. The potential was measured at 30-second intervals until seven minutes elapsed after the specimen and the standard solution were applied. The 23 kinds of the potassium ion activity measuring devices and the one device for comparison indicated nearly the same change in potential on nearly the same potential level.In this way, it was found that the ionic activity measuring device provided with the porous bridge consisting of a porous member having no top non-porous hydrophobic layer in accordance with the present invention exhibits a performance equivalent to that provided by a porous bridge comprising a top non-porous hydrophobic layer.

EXAMPLE 2 A porous member was prepared by forming a low-density PE layer (bottom non-porous hydrophobic layer) 21 on one side of a layer 20 made of mixed paper (basis weight 1 44g/m2, thickness 249,um) consisting of 50% of PE pulp and 50% of LBKP (hardwood bleached kraft pulp), and bonding a double-faced adhesive tape 22 (Scotch 665 available from 3M) to the PE layer 21. Then, the porous member thus prepared was cut into a rectangular shape having a size of 6mm x 1 5mm, and holes were perforated through all of the mixed paper layer 20, the PE layer 21 and the double-faced adhesive tape 22 of each rectangle as shown in Figure 4B (diameter r: 3mm, a: 3mm, b: 2.5mm, c: 1 Omm).Each porous member piece obtained as described above was bonded and fixed on the potassium ion selecting layer of a potassium ion selecting solid electrode film, in which two solid electrodes as shown in Figures 6A and 6B had a common potassium ion selecting layer 5 as shown in Figure 5, so that the centers of the through holes 1 0 and 11 in the porous member piece aligned with the centers of the silver chloride layers 3 of the solid electrodes. In this way, potassium ion activity measuring devices in which the sectional view taken along the centers of the two through holes (liquid receiving holes) 10 and 11 in the porous bridge was as shown in Figure 5 was prepared.

To the liquid receiving holes 10 of the devices thus obtained, 2Ol of control blood serum (pseudo-serum) "Versatol" (R.T.M.) was applied as the standard solution. At the same time, 20Fl of "Versatol A" or "Versatol AA" was appplied as the specimen to the liquid receiving holes 11 ("Verstol", "Verstol A" and "Verstol AA" was available from Generai Diagnostics). The potential was measured two minutes and five minutes after the standard solution and the specimen were applied to the holes. This operaton was repeated 20 times, and the average and the square root of variance (a)* of the measured potential values were calculated. The results were as shown in Table 2.

TABLE 2

Potential value (mV) Specimen After 3 min. After 5 min.

"Verstol AA" * (K0 ion concentration: -9.9+0.49 -9.7+0.48 3.1 meq/l) "Verstol A" (Ke ion concentration: 9.4+0.30 9.4+0.34 7.2 meq/l) The results shown in Table 2 clearly showed that the ionic activity measuring device in accordance with the present invention exhibits excellent measurement reproducibility.A comparison of the potential values measured after 3 minutes with those measured after 5 minutes showed that, in the porous bridge provided with no top non-porous layer, no measurement error occurred due to water evaporation from the surface of the porous bridge if the potential value was measured within 5 minutes after the specimen and the standard solution were applied to the holes.

EXAMPLE 3 Potential values were measured in the same way as described in Example 2 except that the centers of the liquid receiving holes 10 and 11 of the porous bridge were shifted about 0.5mm from the centers of the silver chloride layers 3 of the solid electrodes to the side farther from the connection terminal sections (exposed silver layers 2) of the solid electrodes, or the liquid receiving holes 10 and 11 were shifted in Figure 5 to the left or right. The measurement was repeated 10 times in each case. In this example, substantially the same results as those shown in Table 1 were obtained.

EXAMPLE 4 Potential values wera measured in the same way as described in Example 2 except that the application points of one or both of the specimen and the standard solution were shifted about 0.5mm in an arbitrary direction from the centers of the liquid receiving holes. The measurement was repeated 10 times in each case. In this example, substantially the same results as those shown in Table 1 were obtained.

From the results obtained in Examples 3 and 4, it was found that the ionic activity measuring device in accordance with the present invention exhibits a wide tolerance with respect to the position of the porous bridge and the application points of the specimen and the standard solution. This greatly facilitates the manufacture and the ionic activity measuring operation of the ionic activity measuring device.

EXAMPLE 5 The time required for the specimen and the standard solution to spread and contact each other in the interior of the porous bridge and develop a potential was measured 1 0 times in each case by using a potassium ion activity measuring device similar to that used in Example 2 and a potassium ion activity measuring device provided with a trilaminate porous bridge similar to that used for comparison in Example 1. "Versatol A" and "Versatol AA" each containing 12 wt.% of bovine albumin were used as high-viscosity specimens. The time was 10 seconds or less with the ionic activity measuring device in accordance with the present invention, and between 30 seconds and 45 seconds in the case of the device for comparison. The results showed that the-ionic activity measuring device in accordance with the present invention can greatly reduce the time required for the measurement of the ionic activity.

Claims (13)

1. An ionic activity measuring device comprising at least two solid electrodes and a porous bridge extending therebetween and capable of permitting motion through said porous bridge of ions contained in liquid applied to contact said solid electrodes and said porous bridge, the said porous bridge essentially consisting of a porous member of which at least the surface remote from said solid electrodes and extending in the direction in which in use ion motion in said porous member between said solid electrodes occurs is hydrophobic.

2. A device as claimed in claim 1 wherein the hydrophobic property of said remote surface of said porous member is such that, when a porous member provided with two through holes of a substantially equal size spaced apart so as not to contact each other is bonded and fixed to the smooth flat surface of a water-impervious substrate having a smooth flat surface in a manner not to substantially affect the porosity of the porous member and when the ionic activity measuring time has elapsed after an aqueous bovine albumin solution of predetermined volume larger than that of each through hole is added to the respective through holes substantially at the same time, the aqueous bovine albumin solution remains in the respective through holes in an amount allowing the measurement of the ionic activity, the portions of the aqueous bovine albumin solution diffusing from said two through holes through the interior of said porous member contact each other at the interface therebetween, and the overflow widths of the aqueous bovine albumin solution from the marginal sections of said two through holes on the surface of said porous member remote from said water-impervious substrate are in the range not allowing the two overflows to couple and mix with each other.

3. A device as claimed in either of claims 1 and 2 wherein said porous member is formed from material selected from: a) paper made of a mixture of pulp consisting of a synthetic polymer fiber and pulp consisting of a natural vegetable fiber; b) paper made of pulp consisting of a synthetic polymer fiber; c) mixed fabric consisting of a synthetic polymer fiber and a natural vegetable fiber, d) plain weave fabric consising of a natural vegetable fiber; e) membrane filters having an average pore diameter of 2ym or less and consisting of cellulose esters or regenerated cellulose; f) membrane filters having an average pore diameter of 10ym or less and containing nitrocellulose; and g) paper made of pulp consisting of a natural vegetable fiber and compressed.

4. A device as claimed in any of Claims 1 to 3 wherein said porous member is positioned on said solid electrodes and provided with liquid receiving holes in a number at least equal to the number of said solid electrodes, the said liquid receiving holes being positioned in such a manner that at least one hole is in registry with each solid electrode.

5. A device as claimed in Claim 4 wherein the surface of said porous member adjacent to said solid electrodes is provided with a non-porous layer having holes therein in registry with said liquid receiving holes.

6. A device as claimed in Claim 5 wherein said non-porous layer is terminated at the edges of said porous member.

7. A device as claimed in Claim 5 or 6 wherein said non-porous layer comprises at least one layer' of a bonding agent or an adhesive agent.

8. A device as claimed in Claim 5 or 6 wherein said non-porous layer is formed as an integral part of said porous member by eliminating the porosity of said porous member at said surface adjacent to said solid electrodes and in the vicinity of said surface.

9. A device as claimed in Claim 5 or 6 wherein said non-porous layer is formed by heating and bonding to said surface adjacent to said solid electrodes a polymer film having a bonding or adhesive property under the action of heat.

1 0. A device as claimed in any one of the preceding claims wherein the hydrophobic surface of said porous member is formed by treating a non-hydrophobic material forming at least a surface of said porous member to impart hydrophobicity thereto.

11. A device for measuring ionic activity substantially as herein described with reference to Figures 4A,4B,5,6A and 6B of the accompanying drawings.

12. A porous bridge for use in an ion activity measuring device comprising a substantially planar layer of porous material having at least two liquid receiving holes extending through the said layer in the out of plane direction, wherein at least one planar integral surface of the said layer is hydrophobic.

13. A porous bridge for use in an ion activity measuring device substantially as herein described and having an integral planar hydrophobic surface.