TECHNICAL FIELD

This invention generally relates to a method for removing silver rust (AgCl) from the surface of a silver substrate such as a silver—silver chloride (Ag/AgCl) electrode. The method involves the use of a solution which composition and formulation is also disclosed.

BACKGROUND ART

Silver-silver chloride (Ag/AgCl) electrode is commonly used as a reference electrode in electrochemical applications. For example, it is employed as an internal reference electrode in pH meters, and ion-selective electrode (ISE) devices. The silver chloride layer may conventional be formed on a silver substrate using ferric chloride solution (FeC ) or electrochemically using solutions of other chloride salts usch as potassium chloride electrolyte.

A state-of-art method of forming AgCl on silver substrate via chlorination process in the formation of ion-selective or ion specific electrode (ISE) which is the basis for fabrication of ion-selective field effect transistor (ISFET) is disclosed in PCT published application No. WO 2010/062162 (MIMOS) which requires a electrochemical setup so that a constant DC supply is provided to imidazole chloride to make it an electrolyte with regenerative imidazolium chloride.

Changes in the electrolyte concentration changes the electrode potential, hence it is important to avoid thinning or stripping the silver chloride off the silver wire or substrate. In addition to the need to remanufacture the electrode's silver chloride surface layer, the precious metal nature of the substrate also makes it desirable to recycle such electrodes and recover its silver material. However, silver chloride shows very low solubility in common solvents thus making it chemically difficult to remove the silver chloride layer and clean the wire for recycling. Silver chloride can be removed physical! by scrubbing with sandpaper but this process is inefficient and results in rough and damaged silver surface. The art of stripping AgCl off Ag substrate has been so far a domestic chore as in polishing silverware from discoloration or tarnished surface. US-2,618,606 (Schaeffer) and US-2,618,608 (Schaeffer) are among the earliest prior art documents in which imidazole compounds or imidazole derivatives have been used in a detergent composition for inhibiting metal discoloration in diningware or kitchenware. Specifically, 4-allyl imidazole, 4-(6-hydroxyethyl)-imidazole, 4- methyl imidazole and 2-methyl imidazole have been disclosed. US-5,480,576 (Lever Brothers) discloses a composition for use in dish-washing machine which prevents tarnishing of silver surface. It includes imidazoles such as as benzimidazole and 2-phenylimidazoles. US-6,147,043 (Henkel) also discloses a dishwashing machine detergent which uses triazoles (which is similar to imidazoles) and where acylated imidazoles being used as bleach activators.

For industrial use, the relevant art has been limited to cleaning the substrate surfaces for solderability, coating or deposition purposes. US-6,444,109 (Redline) and US-6,773,757 (Redline) concerns the use of cleaning additives which includes imidazoles, benzimidazoles and imidazole derivatives.

SUMMARY OF DISCLOSURE

It is desirous to have a method or process for removing AgCl effectively from an Ag substrate such that the article comprising the substrate may be recovered for recycling or reuse. In the present invention, we have also endeavour to provide a solution which formulation is cost-effective using commercially- available compounds, effective is small volume and which may be used repeatedly for many used silver-silver chloride electrodes.

It is further desirous that the method and solution provides chemical treatment to the Ag surfaces in a sufficiently mild effect to be suitable for Ag-AgCl electrodes and articles which have been used, aged or previously misprocessed. Thus, one of our endeavours in this invention is to achieve removal of the AgCl surface layer, which may not be uniform or in patches, to expose a clean and smooth silver substrate surface and avoid the need for physical srubbing. It is also our desire to provide for a chemical process which works faster than conventional physical methods.

Our invention is essentially a method of removing silver chloride (AgCl) from a silver (Ag) substrate by ionic dissociation of said AgCl with an imidazole salt. Preferably, the imidazole salt's ring is in the cation, i.e. an imidazolium salt. Preferably still, the imidazoUum salt is a buffered imidazoUum reagent having the foUowing structure:

wherein X is a haUde (CI, Br or I);

R ¹ is H or low alkyl (Ci— Ce) in straight or branched chain; and R ² is H, methyl, ethyl, butyl, allyl, or benzyl.

In one aspect of our method, the buffered imidazoUum reagent removes the AgCl layer from the Ag substrate in accordance with the following reaction:

Preferably, the imidazole compound is mixed with its Bronsted conjugate acid with chloride counter ion in appropriate ratios to achieve an alkaline solution with pH between 8 and 10 with a concentration in the range of 0.1 M to 8 M.

Preferred solvents include solvent is any one or combination of water, ethanol, methanol, tetrahydrofuran, dimethylsulfoxide or acetonitrile. The article comprising the Ag substrate with AgCl surface may be immersed in the prepared solution without agitation or external heating.

In a preferred embodiment, the treated Ag substrate surface is further subjected to chlorination by ferric chloride (iron (III) chloride or FeCb).

Alternatively, the treated Ag substrate surface is further subjected to

electrochemical treatment with chloride salt solution.

LIST OF ACCOMPANYING DRAWINGS

The drawings accompanying this specification as listed below may provide a better understanding of our invention and its advantages when referred to in conjunction with the detailed description which follows but only as exemplary and non-limiting embodiments thereof, in which:

FIGURE 1 shows a graph plotted of the chloride response trend of fresh silver based on Table I; and

FIGURE 2 illustrates a graph plotted of the chloride response trend of recycled silver recovered with our invention based on Table 2.

DETAILED DESCRIPTION OF [SPECIFIC / PREFERRED] EMBODIMENTS

Our method of removing silver chloride (AgCl) from a silver (Ag) substrate involves the use of an imidazole salt to achieve ionic dissociation of the AgCl. It should be noted that imidazole is amphoteric, i.e. it can perform as an acid and as a base. The acidic proton is located on N-l of the ring while the basic site is N-3. Salts of imidazole where the imidazole ring is in the cation is known as

imidazolium salts (e.g. imidazolium chloride) where nitrogen of imidazole is substituted or protonated, which molecular structure is shown below.

Salts wherein a deprotonated imidazole is an anion are known as imidazoHde salts (e.g. sodium imidazoUde). A more preferable embodiment of the imidazolium salt is provided in the form of a buffered imidazolium reagent having the following structure:

wherein X is a halide (CI, Br or I);

R ¹ is H or low alkyl (Ci - Ce) in straight or branched chain; and R ² is H, methyl, ethyl, butyl, allyl, or benzyl.

Preferably, R ¹ is a low molecular weight, straight chain hydrocarbon (Ci— Ce) comprising any one of hydrogen, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl and octyl. Most of these imidazole compounds are commercially available and have good solubility in water.

The buffered imidazolium reagent removes the AgCl layer from the Ag substrate in accordance with the following reaction:

The above proposed mechanism for removal of silver chloride material from silver-silver chloride electrode using buffered imidazolium reagent is made without intention to be bound to any theories or hypothesis. In the disclosed cleaning solution, the chloride ion is presumed to be free and the cation-anion pair is weakly interacted. This gives the chloride ion the strength to pull the silver atom from the silver chloride layer. Likewise, the remaining chloride atom in the silver chloride lattice is extracted by the imidazolium cation. These two ionic actions remove the silver chloride to expose the underlying clean silver surface.

The imidazole compound is preferably mixed with its Bronsted conjugate acid with chloride counter ion in appropriate ratios to achieve an alkaline solution with pH between 8 and 10.

For ease of application of our method described herein, it is preferred that a solution be formulated according to the above description so that a bath may be provided for the article to be treated to immerse therein without the need for agitation or external heating. Specifically, the solution comprises a buffered imidazohum reagent as previously defined and is prepared in a concentration in the range of 0.1 M to 8 M, employing solvent such as water, ethanol, methanol, tetrahydrofuran, dimethylsulfoxide or acetonitrile.

Taking advantage of its amphoteric nature, our solution may preferably be formulated with the imidazole compound mixed with its Bronsted conjugate acid with chloride counter ion in appropriate ratios to achieve an alkaline solution with pH between 8 and 10.

After having the treated Ag substrate surface with the method according to our invention as described herein, the exposed Ag surface is usually in a smooth and shiny condition due to the mild chemical effect of our method. We have found that our method is an economically viable process since the organic compounds used in our solution are all commecially available at low prices. Moreover, only a small volume of the mixture is needed to strip the AgCl layer to give the desired clean silver surface. We have also found that our solution comprising the mixture of the compounds described above can be used repeatedly as a reagent for cleaning many used AgCl-Ag electrodes. (The terms "reactant" and "reagent" are often used interchangeably but a reactant is more specifically a "substance that is consumed in the course of a chemical reaction" according to IUPAC's Compendium of

Chemical Terminology, 1996).

Once the silver chloride material is removed, the clean and smooth the clean and smooth Ag substrate surface may then be further subjected to

chlorination by ferric chloride (iron (III) chloride or FeC ) or electrochemically treated with chloride salt solution. With such chemical post-treatment, damage to the silver surface arising from physical scrubbing with sandpaper, as in

conventional practice, may be avoided in addition to time-saving advantage of the chemical method. The results of silver electrodes in form of silver wires processed according to our method are now described and analysed in the following examples.

Example 1- Preparation of Silver Chloride Removal Solution

using 1-Methylimidazolium Chloride

1-Methylimidazolium chloride (0.60 gram) was mixed with 1.5 mL 1- methylimidazole and 5 mL deionozed water (DIW). The mixture was vigorously stirred or sonicated until all the 1-methylimidazolium chloride crystals were dissolved and homogeneous solution was obtained. The pH of the solution was adjusted to 8.3.

Example 2- Silver Chloride Removal and Silver Chlorination Process

Aged silver-silver chloride electrode was immersed into the imidazolium chloride cleaning solution from Example 1 for 2 - 5 minutes. The removal of silver chloride occurred immediately as the clean, smooth and shiny silver surface was exposed. The silver surface was rinsed with deionozed water (DIW) and wiped with soft, non-abrasive finish and low lint cloth such as Kimwipes® recommended for wiping metal surfaces.

Result: The pre-treatment condition of the electrode, which is in brown aged appearance, has been transformed into a clean and smooth silver surface after the removal of the aged silver chloride layer.

Post-treament: After the cleaning process the recycled silver wire is re- chlorinated with 1 M FeCb solution for 20 second. The post-treated wire is then tested for the response of chloride ion in potassium chloride calibration solutions. The treated silver's chloride response voltages is tabulated and compared with fresh silver's response voltages and graphs therefor plotted as FIGURE 1 and FIGURE 2.

Table 1. Chloride response from fresh silver

Table 2. Chloride response from recycled silver

Logrci-i mV

-1 45.6

-2 100.8

-3 158.8

-4 210.6 Apart from the aforedescribed embodiments of our method and solution for removing AgCl from Ag substrate, there are many aspects or advantages of our invention that is obvious to a skilled person which may be presented in other variations, substitution or modifications to the many compounds and solvents suggested herein without departing from the essence and working principles of the invention. This is especially true for an amphoteric compound such as imidazole given a skilled person's understanding of Bronsted-Lowry conjugate pairs theory. Such suitable variations, alternates, analogs or equivalents are to be considered as falling within the letter and scope of the following claims.

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