Polymeric compositions for ion-selective electrodes

Details Polymeric compositions for ion-selective electrodes Polymeric compositions for ion-selective electrodes

2000-08-30

2002-08-13

Warden, Sr., Robert J. (Department: 1744)

Electrolysis: processes, compositions used therein, and methods

Electrolytic analysis or testing

For ion concentration

C205S778500, C205S781500, C205S789500, C205S793000, C204S414000, C204S415000, C204S418000, C204S435000

Reexamination Certificate

active

06432296

ABSTRACT:

FIELD OF THE INVENTION
The present invention is broadly directed to ion-selective electrodes (ISE's). More specifically, it relates to polymeric compositions for internal reference electrodes of dry-operative ISE's.
BACKGROUND OF THE INVENTION
A wide variety of ion-selective electrodes are known for measuring the amount of an ion in solution. Typically devices for obtaining such measurements include a reference or standard electrode and a separate ion-selective electrode. When simultaneously contacting a solution to be analyzed, the reference or standard electrode and the ion-selective electrode together constitute an electrochemical cell across which a potential difference develops in proportion to the logarithm of the activity of the ion to which the ion-selective electrode is sensitive. The activity is related to the concentration of the ion in the solution. The relationship between potential difference and ion activity in solution is described by the well-known Nernst equation. An electrometric device, usually either a direct reading circuit or a null-balance potentiometric circuit, is employed for measuring the potential between the electrodes.
In principle, an ion-selective electrode can be constructed which is sensitive to any cationic or anionic substance. Cations that can be determined include, but are not limited to, group IA ions, such as sodium, potassium, lithium (alkali metals) and hydrogen; group IIA metal ions such as calcium and magnesium (alkaline earths); metal ions from groups VIA, VIIA, VIIIA, IB, IIB and IIIB; and lead ion from group IVB. Anions include, but are not limited to, halide ions, chloride and fluoride being of particular interest. Carbon dioxide, which is non-ionic, can be determined using an ISE sensitive to hydrogen ion.
Ionic substances are present in a wide range of sample types, including but not limited to, industrial effluents, tap water, rain water, sewer water, biological sources, such as plant and animal derived fluids, and so forth. Human biological fluids are of particular interest and include whole blood, serum, plasma, saliva, sweat, bronchial fluid, vaginal excretions, and so on.
Electrodes based on ion-selective glass membranes are well-known. Solid-state electrodes are also known, such as those described in U.S. Pat. No. 3,856,649 to
Genshaw et al. (the '649 patent) and in a paper entitled “Miniature Solid State Potassium Electrode for Serum Analysis” in Analytical Chemistry, v45, pp 1782-84 (1973). An advance in solid-state electrodes has been achieved with the “dry-operative” electrodes described in U.S. Pat. No. 4,214,968 to Battaglia et. al. (the '968 patent), U.S. Pat. No. 4,053,381 to Hamblen et al. and U.S. Pat. No. 4,487,679 to Stare.
Metal, insoluble metal salt solid-state electrodes comprise an electrically conductive inner element, a metal, having disposed thereon an insoluble salt of the metal. The metal, insoluble metal salt combination represents a half-cell or in the context of the present invention, an internal reference electrode, which can be used directly by contacting it with a solution containing an ion to be measured. Alternatively, the internal reference electrode can comprise in intimate contact with the metal and insoluble metal salt, a water-soluble salt dispersed in a hydrophilic “binder” capable of forming a solid matrix. The anion of the water-soluble salt has the same identity as the anion of the insoluble metal salt. In intimate contact with the matrix of the internal reference electrode is a hydrophobic zone which shields the internal reference electrode from direct contact with the ion-containing solution. The hydrophobic zone generally comprises an ion-specific carrier.
The '649 patent describes the use of polyvinyl alcohol as binder to form a hydrophilic layer which includes a water-soluble salt.
The '968 patent lists several binders for the “dried” reference electrolyte solution: polyvinyl alcohol, gelatin, agarose, deionized gelatin, polyacrylamide, polyvinyl pyrrolidone, poly(hydroxyethyl acrylate), poly(hydroxyethyl methacrylate) and poly(acrylic acid). Deionized gelatin is a preferred binder in the '968 patent. Unfortunately, in dry-operative ISE's, the layer comprising the internal reference electrode having gelatin as binder, is brittle, it does not adhere well to the hydrophobic overlayer, and is sensitive to variations in humidity. As a result, the integrity and performance of these ISE's are adversely affected.
SUMMARY OF THE INVENTION
We have found that improved dry-operative ion-selective electrode performance can be achieved using as binder, copolymers prepared from a hydrophilic monomer having at least one carboxylic acid group or salt thereof and a hydrophobic monomer. The resulting copolymer has a glass transition temperature lower than the acid homopolymer, provides good interlayer adhesion as well as high salt tolerance.
In one aspect the present invention relates to dry-operative ion-selective electrodes comprising:
a) an internal reference electrode comprising a water-soluble salt dispersed in a polymer consisting essentially of 60 to 99 weight percent of a monomer having at least one carboxyl group or salt thereof and 1 to 40 weight percent of a hydrophobic monomer; and
b) a hydrophobic zone in contact with the internal reference electrode, said hydrophobic zone having distributed therein a carrier selective for the ion.
The internal reference electrode can be a metal salt, insoluble metal salt type electrode, or an oxidation-reduction type electrode comprising a metal and a redox salt couple. The dry-operative ion-selective electrode may further comprise a support wherein the internal reference electrode is disposed between the support and the hydrophobic zone. In preferred embodiments, the binder polymer comprises 70 to 95 weight percent of a monomer having at least one carboxyl group or salt thereof and 5 to 30 weight percent of a hydrophobic monomer.
Preferred polymers of the invention consist essentially of a monomer of formula
wherein R
1
is H or —COOM; R
2
is H, Cl or —COOM; R
3
is —COOM, —CH
2
CH
2
COOM, —CHCONHC(CH
3
)
2
CH
2
COOM, or
wherein R
4
, R5, R
6
, R
7
are independently H, methyl, or ethyl, and a monomer of formula
wherein R
8
is H or methyl; R
9
is methoxy, ethoxy, propoxy, butoxy, hexoxy or —NHCH
3
.
In another aspect the present invention relates to a method for determining the presence or amount of an ion in a liquid comprising:
A) contacting a dry-operative ion-selective first electrode with a sample of the liquid wherein the dry-operative first electrode comprises
a) an internal reference electrode comprising a water-soluble salt dispersed in a polymer consisting essentially of 60 to 99 weight percent of a monomer having at least one carboxyl group or salt thereof and 1 to 40 weight percent of a hydrophobic monomer; and
b) a hydrophobic zone in contact with the internal reference electrode, said hydrophobic zone having distributed therein a carrier selective for the ion,
B) contacting a second electrode with a solution comprising a known or constant amount of an ion to which said second electrode is selective, and wherein said dry-operative first electrode and said second electrode are in electrochemical contact or are capable of being in electrochemical contact; or
C) contacting the dry-operative ion-selective first electrode and the second electrode with the same sample of the liquid wherein the sample comprises a known or constant amount of an ion to which said second electrode is selective; and
D) measuring the potential difference between the dry-operative first electrode and the second electrode as a determination of the presence or amount of the ion in the liquid sample.
The second electrode in the above method of determining an ion can be any suitable reference electrode, such as a calomel electrode or others known in the art, or a solid-state electrode, such as a dry-operative electrode. It can be identical in structure and composition to the dry-operative ion-selective first electrode

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