Solid State Electrode

Abstract

A solid state electrode for use in determining ion concentration in an aqueous solution. The electrode comprises an electrically conductive inner element with a salt disposed on a surface portion thereof having as a cation a cation form of the element and also having an anion, a hydrophilic layer in intimate contact with the salt and including a water soluble salt of said anion and a hydrophobic layer in intimate contact with the hydrophilic layer whereby the hydrophilic layer is shielded from contact with the ion-containing aqueous solution. This electrode may function as a reference electrode as well as an electrode for identifying specific ions.

Description

BACKGROUND OF THE INVENTION

This invention relates to an electrode for use in measuring ion concentration. More specifically, this invention relates to a totally solid state electrode which may be used as a reference electrode or as an ion specific electrode for use in measuring unknown ion concentrations in a solution.

It is known that variations in electrode potential may be used to determine ion concentration in an aqueous solution. For the purpose of such determination, one practice has been to immerse in the solution containing the unknown ion concentration a reference electrode and an ion specific or indicator electrode. The potential of the reference electrode remains substantially constant regardless of the concentration of the specific ion in the unknown solution. The indicator electrode, on the other hand, is sensitive to variations in the concentration of the specific ion, yielding a change in EMF which may be measured with known electrical devices.

Electrodes for use in determining ion concentrations have many forms, from the well known glass electrodes to the more recent ion selective membrane electrodes. Although the electrode art has advanced in recent years, electrodes still require special handling, particularly when assembling membrane electrodes, and caution in use. Also, these electrodes are expensive and are generally too large to use with micro quantities of solutions. Further, these electrodes share the common structural feature of a liquid inner electrolyte.

SUMMARY OF THE INVENTION

This invention is embodied in a totally solid state electrode for use in determining ion concentration in an aqueous solution. This electrode comprises an electrically conductive inner element having disposed thereon a salt having as a cation a cation form of at least a portion of the inner element and also having an anion. In intimate contact with the salt is a solid hydrophilic layer including a water soluble salt of said anion. In intimate contact with the hydrophilic layer is a solid hydrophobic layer which shields the hydrophilic layer from direct contact with the ion-containing aqueous solution.

Accordingly, it is an object of this invention to provide a totally solid state electrode requiring substantially no special handling during its use.

Another object of this invention is to provide a totally solid state electrode having a simple structure which may be inexpensively manufactured.

A further object of this invention is to provide a miniaturized solid state electrode suitable for use in measuring ion concentrations in micro quantities of sample solutions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electrically conductive inner element of the electrode of this invention may be selected from metals that are commonly used for this purpose. This metal is preferably selected from one which readily forms an insoluble salt and has good electrical properties. Such metals include silver, amalgams and the like. Although the shape of the electrode is not critical, a symmetrical shape, such as a wire, is advantageous in that its size may be very small and it is readily available without additional fabrication. Other configurations such as metalized films on nonconducting substrates, sheets of conducting material, etc., may be used in this invention.

The insoluble salt is in the form of a layer disposed on a surface portion of the inner element and has, as a cation, the cation form of the metal of the element. It also has an anion such as a halogen. The salt layer may be formed by anodizing the element in a suitable solution, by a physical application of a dispersion of the salt in a suitable carrier that will adhere to the element or by other suitable methods. The salt may be formed about an end portion of the inner element or its position may be varied according to the desired structural features of the electrode.

The hydrophilic layer is in intimate contact with the salt disposed on the inner element. The thickness of this layer is not considered critical and may be adjusted in accordance with desired manufacturing techniques. The formation of this layer will be discussed hereinafter and will not, therefore, be set forth at this time. The salt included in this layer is a water soluble salt having a cation selected from alkali metals or alkaline earth metals such as sodium, potassium, magnesium, calcium, and barium; and as anions halogens. Since there is a transfer of charge through ion transfer across this layer the particular salt selected is critical and must be compatible with the insoluble salt. Representative of such salts are NaCl, KCl, KBr, MgCl.sub.2, BaCl.sub.2 and other such halides that may be used with the insoluble halide covering the inner element.

Overlaying the hydrophilic layer is the hydrophobic layer which shields the hydrophilic layer from direct contact with the ion-containing solution. This layer may be formed from well known hydrophobic polymeric materials such as polyvinyl chloride (PVC), a polymethylmethacrylate, polyvinylidene chloride, polystyrene and the like. Prior to application these materials may be dissolved in well known solvents therefore, such as cyclohexanone, xylene, methylene chloride, ethylene chloride, etc. An increase in the thickness of the hydrophobic layer results in a longer response time for the electrode while also resulting in a longer storage life (wet or dry) and longer useful life. It has been found advantageous to combine with this material additives such as glycerol triacetate, dipentyl phthalate, 1-bromonaphthalene and the like or combinations thereof.

Although the hydrophobic layer shields the hydrophilic layer from direct contact with the aqueous solution, the hydrophobic layer appears to be analogous to the membrane of membrane electrodes or the glass of glass electrodes. It is believed that an osmotic equilibrium is established across the hydrophobic layer similar to that established across semipermeable membranes.

The solid state electrode of this invention may be made ion selective by incorporating with the hydrophobic layer an ion selective material such as valinomycin; crown ethers; magnesium or zinc uranyl acetate; monensin; 6,8-dichlorobenzoylene urea; didecylphosphoric acid-dioctyl phenylphosphonate; monactin; tetraphenylboron; tridodecylhexadecylammonium nitrate in n-octyl-o-nitrophenyl ether; 4-amino-4'-chlorodiphenylhydrochloride, barium salt and other similar ion complexing and selective materials. By incorporating one of the above-mentioned materials in this layer, an ion selective electrode for ions, such as K.sup.+, Ca.sup.+.sup.+, NO.sub.3 .sup.- or SO.sub.4 .sup.-.sup.-, is provided which has the advantageous features of the electrode of this invention.

The solid state electrodes of this invention are prepared by disposing on a surface portion of the inner element a salt layer having as a cation the cation form of the inner element. Preferably, the salt layer is disposed on a small portion of the inner element, such as on a tip portion when it is a wire, so that it may be totally immersed in the solution to be tested. The hydrophilic layer is placed in intimate contact with at least a portion of the salt layer and includes a water soluble salt having the same anion as the salt layer. The hydrophilic layer is readily applied by momentarily dipping the inner electrode into a suitable solution of the components of the hydrophilic layer. Promptly following this dipping, the partially fabricated electrode is momentarily dipped into a suitable solution of the components of the hydrophobic layer such that the hydrophobic layer will totally encompass the hydrophilic layer. The dipping into the hydrophobic layer-forming solution is beneficially repeated from 2 to 6 or more times as required to provide the desired thickness of hydrophobic layer. Throughout the application of the hydrophobic and hydrophilic layers, the environment around the electrode is supported at a predetermined temperature and relative humidity so that a hydrated state of the hydrophilic layer is maintained. This hydrated state is considered important to the proper functioning of the electrode of this invention.

The structure of and the preparation of the solid state electrode of this invention will be further described in the following examples which set forth specific embodiments of this invention which are only representative thereof and do not limit the scope or use of this invention in any way.

EXAMPLE 1

A reference electrode was prepared using as the inner element thereof a straight length of 18 gage silver wire which was first cleaned in isopropyl alcohol to remove organic residue thereon. This wire was further cleaned with deionized water, and then a 2 mm end portion thereof was anodized for one minute at 0.8-1.0 ma. in 0.1N HCl. The silver inner element with the salt layer disposed thereon was dipped momentarily into a polyvinyl alcohol (ELVANOL grade 72-60) -sodium chloride aqueous solution (10% PVA solution in 0.5M NaCl) to form thereon the hydrophilic layer. The element was then promptly dipped momentarily into a glycerol triacetate-polyvinyl chloride (PVC) solution, consisting of 14% glycerol triacetate and 6% PVC dissolved in cyclohexanone, to form the hydrophobic layer. The thus coated element was thereafter similarly dipped into this glycerol triacetate-PVC solution at half hour intervals five more times. The electrode was cured at 37.degree.C and 73% relative humidity during the application of the hydrophobic layer and for at least 18 hours thereafter. The hydrophobic layer totally covered the hydrophilic layer. The balance of the silver wire, with the exception of a portion for its connection to an external circuit, was covered with a wax sealant.

The resulting electrode was about 2 mm in length and 2.5 mm in diameter. The silver wire extended from the electrode an additional 1.5 cm of which about 5 mm adjacent to the electrode was covered with the wax sealant. This electrode maintained a substantially constant potential for normal testing periods when evaluated in solutions of varying sodium and potassium ion concentration.

EXAMPLE 2

The procedure of this Example was the same as Example 1 with the exception that KCl was substituted for NaCl in the hydrophilic layer.

The resulting electrode had an appearance and performance substantially identical with the electrode of Example 1.

EXAMPLE 3

The procedure of this Example was the same as Example 2 with the exception that the KCl concentration was reduced to 0.005 M and the solution used to form the hydrophobic layer consisted of 6% PVC, 14% dipentyl phthalate and 0.1% valinomycin in cyclohexanone.

The resulting electrode had an appearance substantially the same as the electrodes of Examples 1 and 2. This electrode was observed to have a high selectivity for potassium ions as set forth in the following Example 4.

EXAMPLE 4

An electrode prepared according to Example 3 was evaluated in aqueous solutions of different concentrations of mixtures of potassium and sodium chloride as set forth in Table 1. Replicate determinations were made in each solution in a random manner with the six solutions consisting of two KCl concentrations of 3 meq/liter and 6 meq/liter in combination respectively with NaCl concentrations of 120, 140 and 160 meq/liter. All electrode measurements (.DELTA.E.sub.mv) were made versus a Standard Calomel Electrode (SCE) (K401, Radiometer, Denmark) through a saturated lithium trichloracetate salt bridge. The electrodes were dry blotted between measurements. Measurements were made with a Hewlett-Packard (3440A) Digital Voltmeter and an Analog Device 311 J follower as a preamplifier.

TABLE 1 ______________________________________ Electrode Potassium Response in Mixed Salt Solutions NaCl .DELTA.E.sub.mv vs SCE .DELTA.E.sub.mv vs SCE (meq/liter) KCl (3 meq/liter) KCl (6 meq/liter) ______________________________________ 120 0.4, 0.4 17.8, 17.7 140 0.7, 0.6 17.5, 17.5 160 0.5, 0.0 18.4, 17.7 ______________________________________

It was observed that the differences between the EMF measurements did not exceed 0.7 mv. for a given potassium ion concentration regardless of the sodium ion concentration. Such a difference was not considered significant and accordingly demonstrated the selectivity of this electrode for K.sup.+ over Na.sup.+. It was also observed that there was a greater than 25 fold difference in .DELTA.E.sub.mv between the 3 meq/liter solution and the 6 meq/liter solution. Although electrode response was completed after 30 seconds, readings were taken after 2 minutes to insure that equilibrium was reached.

Claims

What is claimed is:

1. A solid state electrode for use in determination of ion concentration in an aqueous solution comprising, an electrically conductive inner element,

a salt having as a cation a cation form of at least a portion of said inner electrode material and also having an anion, said salt being disposed on a surface portion of said inner electrode,

a solid hydrophilic layer in intimate contact with said salt, said solid hydrophilic layer including polyvinyl alcohol and a water soluble salt of said anion, and

a solid hydrophobic layer in intimate contact with said solid hydrophilic layer shielding said solid hydrophilic layer from direct contact with the ion-containing aqueous solution when said electrode is immersed therein.

2. An electrode as described in claim 1 wherein said salt is a layer in intimate contact with said inner element.

3. An electrode as described in claim 1 wherein said anion is a halogen.

4. An electrode as described in claim 1 wherein said hydrophilic layer includes water and a water soluble salt.

5. An electrode as described in claim 1 wherein said hydrophobic layer includes at least one polymer selected from the group consisting of polyvinyl chloride, polymethylmethacrylate and polyvinylidene chloride.

6. An electrode as described in claim 1 wherein said hydrophobic layer consists essentially of a combination of polyvinyl chloride and glycerol triacetate.

7. An electrode as described in claim 1 wherein an ion selective material is included in said hydrophobic layer.

8. An electrode as described in claim 1 wherein said hydrophobic layer consists essentially of a combination of polyvinyl chloride, dipentyl phthalate and valinomycin.

9. An electrode as described in claim 1 wherein said hydrophobic layer consists essentially of a combination of polyvinyl chloride, 1-bromonaphthalene and valinomycin.