WO2012057606A1

WO2012057606A1 - Silver and silver chloride reference electrode

Info

Publication number: WO2012057606A1
Application number: PCT/MY2011/000065
Authority: WO
Inventors: Nur Azera Tuhaime; Mohd. Rais Ahmad; Sagir Alva
Original assignee: Mimos Berhad
Priority date: 2010-10-29
Filing date: 2011-06-02
Publication date: 2012-05-03
Also published as: MY183563A

Abstract

The present invention provides an Ag/AgCI reference electrode [10] comprising a rigid hollow structure [12] within which a silver layer [14] such as silver paste coupled with a silver wire [16], an electrolyte layer [18] such as potassium halide saturated in silver chloride is formed on the silver layer [14], a porous layer [20] such as cellulose acetate is formed on the electrolyte layer [18] to physically separate the electrolyte layer with a buffer layer [22] on the porous layer [20], and a polymeric membrane [24] such as PVC is used to seal the rigid tube [12]. The buffer layer [22] is used to maintain stability of pH-sensitive chemical reactions to the electrode and therefore maintain the accuracy of the Ag/AgCI electrode [10]. All components are in solid form of salt mixture to prolong the life of electrode.

Description

SILVER AND SILVER CHLORIDE REFERENCE ELECTRODE

The present invention relates to reference electrodes which are used in electrochemical reactions. More particularly, the invention relates to Ag/AgCl (Silver/Silver Chloride) electrodes, and a method for making such electrode.

BACKGROUND ART

Ag/AgCl (Silver/Silver Chloride) electrode is a type of reference electrode commonly used in electrochemical systems such as a pH meter. It consists of a rigid structure with porous bottom part, such as porous glass, and filled with a solution such as potassium halide, saturated in a silver chloride solution. A silver wire is mounted in the rigid structure, and in contact with the filing solution. The structure's porous surface physically separates the filling and testing solution, but enables ton exchange between the solutions.

Consider in an electrochemical system, when an indicator electrode such as a metal and the reference electrode are in contact with the testing solution, the reaction between the reference electrode, indicator electrode and testing solution will induce a potential in both electrodes. Both electrodes' potential are compared and the relative potential is measured.

Good desirable Ag/AgCl reference electrode has to be stable regardless of pH change or other ionic activity in the testing solution and requires only minor maintenance in field applications.

However, Ag/AgCl electrode is sensitive to halide ions such as chloride ions which will influence the accuracy of potential measurements. The filling solution of currently manufactured Ag/AgCl electrode is in liquid form which promotes leaching of chloride ions into the testing solution and contaminates the testing solution, which thus affects the measurements. Prolonged usage may reduce the Ag/AgCl electrode's life time.

It is an object of the invention to provide a stable Ag/AgCl reference electrode to withstand interference of test environment. The electrode should be stable regardless of pH change or other ionic activity in the test solution. SUMMARY OF INVENTION

The present invention provides an Ag/AgCI electrode which has a buffer layer between filling solution and testing solution to overcome the influences of undesirable chemical reaction to potential measurement. A method for making the Ag/AgCI reference electrode is also described.

The Ag/AgCI reference electrode according to the present invention includes a rigid structure within which: a silver layer such as silver paste; a wire such as silver coupled to the silver layer; an electrolyte layer such as potassium halide saturated in silver chloride is formed on the silver layer; a porous layer such as cellulose acetate is formed on the electrolyte layer to physically separate the electrolyte layer with a buffer layer formed on the porous layer; and a polymeric membrane such as polyvinyl chloride [PVC] is used to seal the rigid structure and to physically separate the buffer layer and testing solution yet to enable ion exchange between them.

A buffer layer consists of a weak acid and its conjugate base. It is added to maintain stability of pH-sensitive chemical reactions and thus can be used to neutralize the influence of pH-sensitive chemical reactions of testing solution to electrolyte layer and therefore maintain the accuracy of the Ag/AgCI electrode.

All components in the Ag/AgCI electrode are in solid form of salt mixture. Such solids reduce the contamination of testing solution through leaching of ions such as chloride and thus prolong the life time of electrode.

According to a method to fabricate the electrode, a silver layer is applied to the rigid structure and the wire is mounted by epoxy. The electrolyte paste is a mixture of silver chloride and potassium halide in 1:1 ratio and glycerol with deionised water in 1 :1 ratio. The mixture is well mixed until homogeneous paste is achieved. The electrolyte paste is gently packed to the structure to avoid air bubbles trapped inside the paste. Excess water is removed from the paste surface with tissue. TwoDlitres of cellulose acetate solution in tetrahydrofuran water is added into the structure and is allowed to dry for 15 minutes. Two litres of cellulose acetate solution is added again and is allowed to dry for 1 hour. Buffer paste is a mixture of 2g lithium acetate and glycerol and deionised water in 1 :1 ratio. 37% high weight PVC, 60% diethyl sebacate plasticizer and 3% of lithium acetate is dissolved in tetrahydrofuran by sonication over 1 hour for polymeric membrane. Ten□ litres of the mixture is added and is allowed to dry for 10 minutes and the second layer is added and is allowed to dry for 20 hours.

BRIEF DESCRIPTION OF DRAWINGS The invention will now be described in greater detail, by way of an example, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic cross section of an Ag/AgCI reference electrode according to the invention.

FIG. 2 is a schematic cross section of an electrochemical system in which the Ag/AgCI reference electrode shown in FIG.1 is applied.

FIG. 3 is a graph showing the relation between responses of the Ag/AgCI reference electrode shown in FIG. 1 with Nico2000 nitrate ISE.

DESCRIPTION OF EMBODIMENTS In an embodiment of the present invention, an Ag/AgCI reference electrode is as shown in FIG. 1. The Ag/AgCI reference electrode [10] has a rigid hollow structure [12] such as Teflon structure, having a first end and second end. A silver layer [14] such as silver paste is in the hollow structure [12] from first end of the structure. A conductor [16] such as silver wire is coupled with the silver layer [14]. An electrolyte layer [18], such as potassium halide saturated in AgCI, is next to the silver layer [14] towards second end of hollow structure [12]. A porous layer [20] such as cellulose acetate is next to the electrolyte layer [18].

The invention is characterized with a buffer layer [22] of weak acid such as lithium acetate formed next to the porous layer [20], and a polymeric membrane [24] such as PVC formed next to the buffer layer [22] and thus seals the structure [12]. The buffer layer may also use other weak acid such as sodium phosphate dibasic or ammonium sulfate. The polymeric membrane [24] may further contain lithium acetate, potassium halide, potassium nitrate, ammonium sulfate, or sodium phosphate dibasic.

Epoxy is used to couple conductor [16] to silver layer [14]. The reference electrode [10] is coupled to an electrochemical system [25] via conductor [16] which consists of a measuring device [26] and an indicator electrode [28] such as copper as shown in FIG. 2. When both electrodes are immersed into a solution [30] and is in contact with the polymeric membrane [24], a potential characteristic which is measured by measuring device [26] between the reference electrode [10] and indicator electrode varies based on the ion concentration such as pH of the solution [30]. The sensitivity of the reference electrode [10] of present invention is tested by comparing the potential characteristics of the reference electrode [10] with a commercially available electrode such as Nico2000 nitrate ion selective electrode (ISE).

The reference electrode [10] and Nico2000 nitrate ISE are introduced in turn to a standard KN03 test solution (10-4 to 10-1M) in electrochemical system [25]. The responses of the electrodes are shown in FIG. 3.

According to FIG. 3, the sensitivity of the reference electrode [10] is similar to Nico2000 nitrate ISE and thus the reference electrode [10] is suitable for potential measurement. In practice, it has been found that the invention thus described solves the problems noted above.

This solution reduces the problems related to the sensitivity of Ag/AgCI electrode and its short life time.

Claims

1. A silver and silver chloride electrode [10] for use in an electrochemical reaction, comprising: a hollow structure [12] having a first end and a second end; a silver layer [14] in hollow structure [12], from first end of the structure; an electrolyte layer [18], next to silver layer [14] towards second end of hollow structure [12]; a porous layer [20] next to electrolyte layer [18]; characterized in that, a buffer layer [22] of weak acid next to porous layer; and a polymeric membrane [24] next to buffer layer, at second end in hollow structure.

2. An electrode according to claim 1 , further comprising a conductor [16] coupled to the silver layer [14] for electrically coupling said layer.

3. An electrode according to claim 1, wherein said silver layer [14] is silver paste.

4. An electrode according to claim 1 , wherein the electrolyte layer [18] comprises silver chloride and potassium halide.

5. An electrode according to claim 1 , wherein the buffer layer [22] has lithium acetate.

6. An electrode according to claim 1 , wherein the buffer layer [22] has sodium phosphate dibasic.

7. An electrode according to claim 1 , wherein the buffer layer [22] has ammonium sulfate.

8. An electrode according to claim 2, wherein said conductor [16] is a silver wire.

9. An electrode according to claim 2, wherein said conductor [16] is coupled to said silver layer by epoxy.

10. An electrode according to claim 1 , wherein the polymeric membrane [24] is polyvinyl chloride.

11. An electrode according to claim 10, wherein the polymeric membrane [24] further contains lithium acetate, potassium halide, potassium nitrate, ammonium sulfate, or sodium phosphate dibasic.

12. An electrode according to claim 1 , wherein the porous layer [20] is cellulose acetate.

13. A method of making a silver and silver chloride electrode, comprising: disposing a silver layer into a hollow structure having a first end and a second end; disposing an electrolyte layer in said hollow structure on said silver from said second end of said hollow structure; disposing a porous layer in said hollow structure on said silver from said second end of said hollow structure; disposing a buffer layer in said hollow structure on said silver from said second end of said hollow structure; and sealing the second end of said hollow structure with a polymeric membrane.