RELATED APPLICATION

This application is an application under 35 U.S.C. 371 of International Application No. PCT/AU2018/050412 filed on 4 May 2018, which claims priority of AU 2017901642 filed 4 May 2017 the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The invention relates to solid state reference electrodes, and in particular to reference electrodes comprising a reference element embedded in an electrochemically active composite comprising a cross-linked vinyl polymer matrix loaded with an inorganic salt. The invention also relates to methods of producing the reference electrodes, to systems and methods for determining an ionic concentration in an analyte in which the reference electrodes are utilised in combination with an ion-sensing electrode, and to use of the reference electrode in an electrochemical analysis of an acidic analyte.

BACKGROUND OF INVENTION

Electroanalytical techniques, including potentiometric and voltametric electroanalytical measurements, rely on both an indicator electrode and a reference electrode, the performance of which are of equal importance for the accuracy of the measurement. For potentiometric measurements, the potential of the indicator electrode is measured against that of the reference cell. It is thus critical that the reference electrode provides a stable and reproducible potential which is substantially independent of the concentration of species in the analyte, including the target species for analysis.

To provide a constant reference potential, reference electrodes have traditionally included a reference element immersed in a reservoir of liquid or gel electrolyte of known salt concentration, contained within a casing. Silver/silver chloride (Ag/AgCl) and calomel (Hg/Hg2Cl2) reference elements in aqueous potassium chloride (KCl) electrolytes are common. Although the electrolyte is substantially isolated from the analyte by the casing, a liquid junction (salt bridge) between the analyte and the internal electrolyte is required to provide ionic communication, thereby completing the electrochemical cell. The liquid junction typically comprises a small opening or porous plug in the casing.

Although traditional reference electrodes are suitable for many applications, they also have significant disadvantages. These include leakage of the electrolyte through the liquid junction, potentially contaminating the analyte with the reference electrolyte salt, and infiltration of the analyte into the internal electrolyte, affecting the electrolyte salt concentration and introducing extraneous species that interfere with the reference half-cell reaction. Although the junction and electrolyte can be designed to mitigate these concerns, there is an inherent trade-off between the imperatives to minimise liquid communication between electrolyte and analyte and to maintain an acceptable junction potential, which is a source of error in the analytical measurement. Furthermore, traditional electrodes are difficult to miniaturise due to the electrolyte volume and complex construction, and are also generally positionally dependent, fragile, expense and maintenance-intensive.

A number of previous efforts have been made to address one or more of these challenges, using solid state reference electrodes lacking an internal electrolyte and a liquid-liquid junction. Vonau et al., DE 10305005, described a sintered Ag/AgCl reference element embedded in a solidified melt of KCl. The KCl was enclosed within a chemical resistant, ceramic layer, itself sealed in an insulating casing apart from a small opening. The porous ceramic layer at this opening formed a junction, which was required to modulate the ionic communication between the inner electrode and the analyte, and to limit KCl dissolution into the analyte. The solid state reference electrode was used successfully in combination with antimony pH indicator electrode. These electrodes remain reliant on a porous junction between the interior salt and the analyte, resulting in a fragile and complex construction.

Nolan et al., Analytical Chemistry 1997 (69) 1244, fabricated solid state reference electrodes by dip coating Ag/AgCl wires with a solution of NaCl and polyvinylchloride (PVC) to provide an immobilised electrolyte, and subsequently coating the wire with a protective overcoating of permeable polyurethane or Nafion to prevent leaching of the NaCl into an analyte. The electrodes suffered from significant drift due to the low loading of electrolyte, were susceptible to chloride leaching notwithstanding the presence of a protective polymer “casing” and were not satisfactorily stable for many electrochemical applications.

More generally, efforts to protect inner reference electrodes with permeable coatings or membranes may result in increased complexity of construction and the risk that damage, fouling or wear of the protective membrane causes error or failure of the device, for example by exposing the sensitive underlying reference electrode material directly to a corrosive analyte. Restoration of the device in such cases, if practical at all, can generally only be achieved by removing and re-applying the protective coating or membrane.

Vonau et al., DE19533059, reported reference electrodes comprising an Ag/AgCl reference element embedded in a hardened resin composite. The polyester resin, loaded with KCl, was hardened with peroxide hardeners to produce the cured electrode body. However, hardened resin composite electrodes may have high impedance due to poor ionic communication between the reference element and the analyte.

Mousavi et al.,Analyst2013 (138) 5216 andJournal of Solid State Electrochemistry2014 (18) 607, prepared reference electrodes comprising an Ag/AgCl reference element embedded in a polyvinyl acetate (PVAc) homopolymer loaded with particulate KCl. The composite materials could be formed either by injection moulding of mixed PVAc/KCl powder or by chemical polymerisation of a suspension of particulate PVAc and KCl in vinyl acetate monomer. Good potential stability was exhibited in a range of different analytes. However, the resistance to chemical and mechanical degradation of these electrodes may be unsatisfactory, particularly in corrosive or abrasive environments.

Despite progress to date, previously reported solid state reference electrodes generally fail to provide a satisfactory balance of properties; in particular resistance to chemical and or mechanical degradation, rapid conditioning and low impedance. There is therefore an ongoing need to provide solid state reference electrodes which provide an improved balance of these properties, particularly in harsh use conditions, while preferably also providing one or more of a low rate of salt leakage into the analyte, long electrode lifetime, simplicity of construction and suitability for miniaturisation and a variety of electrode configurations.

A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

SUMMARY OF INVENTION

The inventors have discovered that a solid state reference electrode, comprising an electrochemically active composite with a matrix comprising a cross-linked vinyl polymer of a vinyl monomer containing a heteroatom, exhibits excellent resistance to chemical and/or mechanical degradation, including in some embodiments in highly acidic use conditions. These reference electrodes may also provide one or more of rapid conditioning, low impedance and high electrochemical stability. Vinyl polymers cross-linked with hydrophilic cross-linking groups may provide a particularly favourable balance of properties, which is believed to be due to enhanced ionic conductivity between the reference electrode and the analyte while maintaining excellent resistance to degradation. Moreover, in some embodiments, it is believed that fouling or degradation of the reference electrode after a use period can be addressed by simply removing a surface layer of the composite to expose fresh composite beneath.

In accordance with a first aspect the invention provides a solid state reference electrode comprising a reference element embedded in an electrochemically active composite, the electrochemically active composite comprising a polymeric matrix loaded with a solid inorganic salt, wherein the polymeric matrix comprises a cross-linked vinyl polymer of a vinyl monomer containing a heteroatom.

In some embodiments, the cross-linked vinyl polymer is cross-linked with hydrophilic cross-linking groups.

In some embodiments, the cross-linked vinyl polymer is a co-polymer of the vinyl monomer and a cross-linking agent, wherein the cross-linking agent comprises at least two co-polymerisable vinyl functionalities. In some embodiments, the vinyl polymer comprises from 0.2 to 3.5 mol % of the cross-linking agent.

In some embodiments, the vinyl monomer is selected from the group consisting of vinyl esters, vinyl amides and acrylates. The vinyl polymer may comprise at least 50% by mass of the vinyl monomer containing a heteroatom.

In some embodiments, the composite is dimensionally stable in an aqueous analyte, such as an acidic analyte. The composite may be dimensionally stable in such analytes for periods of time longer than 1 week, preferably longer than one month.

In an embodiment, the invention provides a solid state reference electrode comprising a reference element selected from an Ag/AgCl, Ag/Ag2SO4and Cu reference element, the reference element embedded in an electrochemically active composite comprising a polymeric matrix loaded with a solid inorganic salt selected from a chloride salt, a sulfate salt and copper sulfate, wherein the polymeric matrix comprises a cross-linked vinyl polymer which is a co-polymer of a hydrophilic cross-linking agent and a vinyl monomer selected from a vinyl ester, a vinyl amide and an acrylate.

In another embodiment, the invention provides a solid state reference electrode comprising an Ag/AgCl reference element embedded in an electrochemically active composite, the electrochemically active composite comprising a polymeric matrix loaded with a solid inorganic chloride salt, wherein the polymeric matrix comprises a cross-linked vinyl polymer which is a co-polymer of vinyl acetate and/or vinyl caprolactam and a cross-linking agent selected from the group consisting of ethylene glycol di(meth)acrylate, poly(ethylene glycol) di(meth)acrylate, and glycerol propoxylate triacrylate.

In accordance with a second aspect, the invention provides a method of producing a solid state reference electrode, the method comprising: dispersing a solid inorganic salt in a vinyl monomer containing a heteroatom and/or a polymer thereof; producing a cross-linked vinyl polymer by co-polymerising the vinyl monomer and a cross-linking agent or by cross-linking the polymer with a cross-linking agent, thereby forming an electrochemically active composite comprising a polymeric matrix loaded with the solid inorganic salt; and embedding a reference element in the electrochemically active composite.

In some embodiments, the cross-linked vinyl polymer is cross-linked with hydrophilic cross-linking groups.

In some embodiments, the solid inorganic salt is dispersed in a mixture comprising the vinyl monomer and a cross-linking agent, wherein the cross-linking agent comprises at least two co-polymerisable vinyl functionalities and wherein the cross-linked vinyl polymer is produced by co-polymerising the vinyl monomer and the cross-linking agent. The cross-linking agent may be a hydrophilic cross-linking agent.

In some embodiments, the vinyl monomer is selected from the group consisting of vinyl esters, vinyl amides and acrylates.

In an embodiment, the invention provides a method of producing a solid slate reference electrode, the method comprising: dispersing a solid inorganic salt selected from a chloride salt, a sulfate salt and copper sulfate in a mixture comprising a hydrophilic cross-linking agent and a vinyl monomer selected from a vinyl ester, a vinyl amide and an acrylate; co-polymerising the hydrophilic cross-linking agent and the vinyl monomer to produce a cross-linked vinyl polymer, thereby forming an electrochemically active composite comprising a polymeric matrix loaded with the solid inorganic salt; and embedding a reference element selected from an Ag/AgCl, Ag/Ag2SO4and Cu reference element in the electrochemically active composite.

In an embodiment, the invention provides a method of producing a solid state reference electrode, the method comprising: dispersing a solid inorganic chloride salt in a mixture comprising vinyl acetate and/or vinyl caprolactam and a cross-linking agent selected from the group consisting of ethylene glycol di(meth)acrylate, poly(ethylene glycol) di(meth)acrylate, and glycerol propoxylate triacrylate; co-polymerising the cross-linking agent and the vinyl acetate and/or vinyl caprolactam to produce a cross-linked vinyl polymer, thereby forming an electrochemically active composite comprising a polymeric matrix loaded with the solid inorganic chloride salt; and embedding an Ag/AgCl reference element in the electrochemically active composite.

In accordance with a third aspect, the invention provides a solid state reference electrode produced by a method according to any of the embodiments disclosed herein.

In accordance with a fourth aspect, the invention provides a system for determining an ionic concentration in an acidic and/or abrasive analyte, the system comprising: a solid state reference electrode according to any of the embodiments disclosed herein; an ion-selective electrode; and means for measuring the difference in electric potential between the ion-selective electrode and the reference electrode.

In accordance with a fifth aspect, the invention provides a method of determining an ionic concentration in an acidic and/or abrasive analyte, the method comprising: immersing an ion-selective electrode and a solid state reference electrode according to any of the embodiments disclosed herein in an analyte; and measuring the difference in electrochemical potential between the ion-selective electrode and the reference electrode.

In accordance with a sixth aspect, the invention provides use of a solid state reference electrode according to any of the embodiments disclosed herein in an electrochemical analysis of an acidic analyte.

Where the terms “comprise”, “comprises” and “comprising” are used in the specification (including the claims) they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

Further aspects of the invention appear below in the detailed description of the invention.

