Solid state reference electrode

1. 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 particulate solid inorganic salt, the solid inorganic salt being dispersed in the polymer matrix,

wherein the polymeric matrix comprises a cross-linked vinyl polymer of a vinyl monomer containing a heteroatom.

2. A solid state reference electrode according to claim 1 , wherein the cross-linked vinyl polymer is cross-linked with hydrophilic cross-linking groups.

3. A solid state reference electrode according to claim 1 , wherein the vinyl monomer containing a heteroatom is selected from the group consisting of vinyl esters, vinyl amides and acrylates.

4. A solid state reference electrode according to claim 1 wherein the vinyl monomer containing a heteroatom is selected from the group consisting of vinyl acetate, vinyl caprolactam, methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate and hexyl (meth)acrylate.

5. A solid state reference electrode according to claim 1 wherein the electrochemically active composite comprises at least 30% by mass of the solid inorganic salt.

6. A solid state reference electrode claim 1 wherein the reference element is selected from the group consisting of an Ag/AgCl reference element, an Ag/Ag 2 SO 4 , reference element and a Cu reference element.

7. A solid state reference electrode according to claim 1 , wherein the reference electrode is an Ag/AgCl reference element and wherein the solid inorganic salt is a chloride salt.

8. A solid state reference electrode according to claim 7 , wherein the chloride salt is selected from the group consisting of potassium chloride, sodium chloride and lithium chloride.

9. A solid state reference electrode according to claim 1 , wherein a surface of the electrochemically active composite is at least partially exposed for direct contact with an analyte.

10. A solid state reference electrode according claim 1 wherein the composite is dimensionally stable in an aqueous analyte.

11. A system for determining an ionic concentration in an acidic and/or abrasive analyte, the system comprising:

a solid state reference electrode according to claim 1 :

an ion-selective electrode; and

means for measuring the difference in electric potential between the ion-selective electrode and the reference electrode.

12. 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 claim 1 in an acidic and/or abrasive analyte; and

measuring the difference in electrochemical potential between the ion-selective electrode and the reference electrode.

13. A method according to claim 12 wherein the analyte is a leachate or a leach slurry from a mineral leaching process or an acid mine drainage stream.

14. A method according to claim 12 , wherein the analyte has a pH of below 4.

15. A solid state reference electrode according to claim 1 , wherein the solid inorganic salt is a water-soluble salt.

16. A solid-state reference electrode according to claim 1 wherein the cross-linked vinyl polymer is a copolymer of the vinyl monomer and a cross-linking agent, wherein the cross-linking agent comprises at least two copolymerisable vinyl functionalities.

17. 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, the solid inorganic salt being dispersed in the polymer matrix,

wherein the polymeric matrix comprises a cross-linked vinyl polymer of a vinyl monomer containing a heteroatom

wherein 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.

18. A solid state reference electrode according to claim 17 , wherein the cross-linking agent is selected from the group consisting of ethylene glycol, dimethacrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, poly(ethylene glycol) di(meth)acrylate, N,N′-methylene bis-acrylamide, 1,3-butanediol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, bisphenol A glycerolate di(meth)acrylate, diurethane di(meth)acrylate, divinylbenzene, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, glycerol propoxylate triacrylate, trimethylolpropane ethoxylate triacrylate and trimethylolpropane propoxylate triacrylate.

19. A solid state reference electrode according to claim 17 , wherein the cross-linking agent has the formula:

wherein each R 1 and R 2 is independently selected from —H and —CH 3 , wherein each n is an integer from 1 to 50, wherein m is an integer from 2 to 6, and wherein X is an organic radical linking group having a valency equal to the value of m.

20. A solid state reference electrode according to claim 17 , wherein the vinyl polymer comprises from 0.2 to 3.5 mol % of the cross-linking agent.