Electrochemical analysis of metal depolarizers in gold electrodeposition.

Abstract

The present disclosure provides a method for determining the concentration of various trace metal ions in an aqueous solution, such as a gold plating solution. At a certain fixed reduction potential, the cathodic current may increase in magnitude rapidly after a certain time (e.g., incubation time) in the presence of a trace metal ion (e.g., Tl(I)), which is inversely proportional to the concentration of the trace metal in the electrolyte. After measuring the incubation time and comparing it to a calibration curve, the concentration of the trace metal can be calculated.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 63 / 316,254, filed March 3, 2022, which is incorporated herein by reference. The present disclosure relates to a method for determining the concentrations of various trace metal ions in an aqueous solution. [Background technology]

[0002] Gold (Au) electrodeposition is widely used in electronics and general metal finishing for decorative and functional coatings. Au deposits have excellent corrosion properties. Electrical contact may be effected by mechanical devices, soldering and wire bonding. One formulation of Au electrodeposition bath contains high levels, i.e., in the g / l range, of Au salts, complexing agents, buffers, surfactants, polarizing agents and other components, and low levels, i.e., in the ppm range, of metal additives, which act as cathodic depolarizers and improve the grain size of the deposit. Several elements have been investigated, including As, Bi, Cd, Cu, Ga, Hg, Pb, Sb, Sn, Tl, Zn, Te, SeCN(-).

[0003] Arsenic salts were used as grain refiners, and later thallium salts were utilized in cyanide and sulfite electrolytes as a safer alternative. + In the voltammetry of gold deposition, it shows the promotion of deposition (i.e., depolarization). The electrodeposition of gold is inhibited by the presence of inhibitory species (SO3 in the gold sulfite bath). 2- ) may be limited by the formation of Tl + Adsorption of metal additives to the surface can release inhibition and therefore destabilize the gold complexes. Adsorption is indicated by hysteretic voltammetric concentration and transport dependence and rising chronoamperometric current transients. The amounts of these metal additives need to be controlled within process limits to achieve good process performance.

[0004] Besides its application in gold electrodeposition, thallium is also used in the pharmaceutical industry, glass manufacturing, and infrared detectors. However, thallium is also a known neurotoxin and environmental contaminant, and was included in the list of 129 so-called "priority pollutants" by the United States Environmental Protection Agency. Both oxidation states of thallium(I) and thallium(III) are toxic, but compounds of thallium(I) are relatively more toxic and reactive. The solubility of the two ions is also different, with thallium(I) being water-soluble while thallium(III) is not. Therefore, it is important to have a quantitative analytical method for trace amounts of Tl, especially in a variety of samples.

[0005] Certain methods have been proposed for the determination of thallium ions. Spectrophotometric methods may be attractive due to the availability of spectrophotometric equipment and the simplicity of the analytical procedure. Some of these methods are based on the ability of anionic chloro or bromo complexes of thallium(III) to interact with cationic dyes to form ion-association complexes, which can be extracted by organic solvents. Examples of such color-forming reagents include brilliant green, pyronine G, 9,10-phenanthaquinone monoethyl thiosemicarbazone, metheomebrazine hydrochloride, N,N-diphenylbenzamidine and rhodamine B. Methods involving these reagents are sensitive but may lack selectivity, so that real samples suffer from serious interferences of foreign ions that are usually present along with thallium, especially in plating samples where the foreign ion levels are several thousand times higher than thallium.

[0006] Furthermore, if Tl(I) is present, it must be oxidized to the trivalent state with a strong oxidizing agent, such as Ce(IV), bromine water or hydrogen peroxide. Because the presence of excess oxidizing agent can destroy the cationic dye, the oxidizing agent can be removed by boiling the solution (in the case of hydrogen peroxide or bromine) or by adding hydroxylammonium chloride (in the case of Ce(IV)). This boiling step makes the procedure more complicated and time-consuming, making it impractical for certain automated monitoring purposes for process control purposes.

[0007] Another method for determining thallium is cathodic and anodic stripping voltammetry (CSV). Certain CSV methods involve a hanging mercury drop electrode or a thin mercury film electrode that tests for the reduction of Tl(I) to metallic Tl. The measured signal can be very small and can only be observed with a mercury electrode due to the very low background current and the increased sensitivity associated with the formation of Tl amalgam. However, mercury is toxic and difficult to apply automatically and maintenance-free, so its use is largely prohibited in industrial laboratories.

[0008] Other electrode materials, such as bismuth, have attracted interest as alternatives to mercury in the detection and quantification of heavy metals in samples. Bismuth can form alloys with other heavy metals, such as thallium, when they are reduced (at sufficiently negative applied potentials) or codeposited on its surface. These processes can cause detectable and quantifiable changes in current. However, this may require in-house engineering of the working electrode and the utilization of more complex electrochemical techniques, such as square wave / differential pulse anodic stripping voltammetry. Additionally, interference from major foreign metal ions, such as gold, can be a challenge.

[0009] Other methods for determining thallium ions include inductively coupled plasma mass spectrometry (ICP-MS), atomic absorption spectrometry (AAS), thermal neutron activation analysis and electrothermal atomic absorption spectrometry. Although these can provide good selectivity and reduced interference from high concentrations of extraneous ions, at the same time, the expensive equipment makes these methods impractical for certain routine sample monitoring and automatic detection.

[0010] Certain methods are also known for monitoring organic additives based on their catalytic / inhibitory effect on Cu electroplating, and there are analytical methods based on the gradual decrease or increase in stripping peak area. However, this approach can be problematic for gold electroplating. In particular, Au can be electrochemically deposited but not electrochemically stripped from the surface of a platinum electrode from the same electrolyte. Moreover, the signal is not linear and has poor reproducibility. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] US Patent Application Publication No. 2020 / 0347510 Summary of the Invention [Problem to be solved by the invention]

[0012] Thus, there is a need in the art for a simple, low-cost quantification for thallium and similar elements in gold electrodeposition solutions.Furthermore, there is a need in the art for a method that can provide an accurate, fast and easy-to-use method for the determination of trace levels of Tl(I) and similar elements in aqueous samples, especially in gold electrodeposition solutions where the gold levels are often more than 1000 times higher than the thallium levels. [Means for solving the problem]

[0013] The objects and advantages of the disclosed subject matter will be set forth in and will be obvious from the following description, and will be learned by practice of the disclosed subject matter. Additional advantages of the disclosed subject matter will be realized and attained by the devices particularly pointed out in the specification and claims hereof, as well as the appended drawings.

[0014] To achieve these and other advantages, and in accordance with the objects of the disclosed subject matter as embodied and broadly described, the disclosed subject matter includes a method for determining a concentration of a metal ion, in certain embodiments, the method includes providing an aqueous solution containing a first metal ion, performing a first amperometric scan to obtain a chronoamperometric voltammogram, calculating a first derivative of the chronoamperometric voltammogram, and calculating the concentration of the metal ion by comparing the first derivative of the chronoamperometric voltammogram to a calibration curve.

[0015] In certain embodiments, the first metal ion is selected from As, Bi, Cd, Cu, Ga, Hg, Pb, Sb, Sn, Tl, Zn, Te, SeCN(-). In certain specific embodiments, the first metal ion is Tl(I).

[0016] In certain embodiments, the aqueous solution is a gold plating solution that includes gold(I) ions.

[0017] In certain embodiments, the concentration of the first metal ion is from about 0.1 to about 1000 ppm.

[0018] In certain embodiments, the ratio of the first metal ion to gold(I) ions is from about 1:300 to about 1:5000.

[0019] In certain embodiments, the calibration curve is generated by (i) providing a first test aqueous solution containing a known concentration of a first metal ion, (ii) performing an amperometric scan to obtain a first test chronoamperometric voltammogram, (iii) calculating the first derivative of the first test chronoamperometric voltammogram, and optionally repeating (i)-(iii) using a test aqueous solution having a different concentration than the first test aqueous solution.

[0020] In certain embodiments, the aqueous solution includes sulfate ions, sulfite ions, or a combination thereof.

[0021] In certain embodiments, the methods of the present disclosure further include stripping the plating metal from the working electrode.

[0022] In a particular embodiment, the first amperometric scan is performed at a fixed potential of -0.6 V and a rotation speed of 1000-4000 rpm for 30-300 seconds.

[0023] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. [Brief description of the drawings]

[0024] [Figure 1] FIG. 2 shows a calibration curve used in certain embodiments of the present disclosure. [Diagram 2] FIG. 2 is a diagram showing the first derivative of the chronoamperometry graph of FIG. 1. [Diagram 3] FIG. 1 shows the linear correlation of the inverse of incubation time to Tl(I) concentration, according to certain embodiments of the present disclosure. [Figure 4] 1 is a chronoamperometry graph from measurements performed according to certain embodiments of the present disclosure. [Diagram 5] FIG. 5 is a diagram showing the first derivative of the chronoamperometry graph of FIG. 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present disclosure provides a method for determining the concentration of metal ions. To address the aforementioned needs, the present disclosure provides a method for determining trace levels of various metal ions in an aqueous sample, such as, but not limited to, a gold plating solution. As used herein, "trace levels" includes a range of 0.1 ppm to 1000 ppm.

[0026] For purposes of clarity, but not limitation, the detailed description of the presently disclosed subject matter is divided into the following subsections: I. Definition II. Methods for Detecting Trace Ions

[0027] I. Definition The terms used herein generally have their ordinary meanings in the art, within the context of this disclosure and in the specific context in which each term is used. Certain terms are discussed below or elsewhere in this specification to provide additional guidance to the practitioner in describing the compositions and methods of the present disclosure, and how to make and use them.

[0028] For purposes of interpreting this specification, the following definitions shall apply and whenever appropriate, terms used in the singular shall include the plural and vice versa. In the event that a definition set forth below conflicts with any document incorporated by reference into this application, the definition set forth below shall control.

[0029] As used herein, the use of the word "a" or "an" when used in conjunction with the term "comprising" in the claims and / or the specification may mean "one," but is also consistent with the meaning of "one or more," "at least one," and "one or more than one."

[0030] As used herein, the terms "comprise(s)," "include(s)," "having," "has," "can," "contain(s)," and variations thereof are intended to be open-ended transitional phrases, terms, or words that do not exclude additional acts or structures. The present disclosure also contemplates other embodiments that "comprise," "consist," and "consist essentially of" the embodiments or elements presented herein, whether or not expressly stated.

[0031] The term "about" or "approximately" as used herein means within an acceptable error range of a particular value as determined by a person skilled in the art, which is determined in part by the limitations of the method of measuring or determining the value, i.e., the measurement system. For example, "about" may mean within 3 or more standard deviations, according to the practice in the art. Alternatively, "about" may mean within a range of up to 20%, preferably up to 10%, more preferably up to 5%, and even more preferably up to 1% of a given value.

[0032] As used herein, the term "high levels" when used in the context of metal ion concentration in solution refers to concentrations in the grams per liter (g / L) range.

[0033] As used herein, the term "low levels" when used in the context of metal ion concentration in solution refers to concentrations in the parts per million (ppm) range.

[0034] Grain refiners, as used herein, have their normal meaning in the art of electroplating, i.e., additives that help control the electroplating process by being attracted to the "high points" of the plating surface. Left untreated, the high points tend to attract more metal deposits than the lower points, resulting in a rough surface. The grain refiners inhibit further deposition. When these points are replaced by other higher points, the grain refiners move away and are relocated.

[0035] As used herein, the term "trace amount" refers to a concentration of less than 1000 ppm. In certain embodiments, trace levels refer to a concentration range of 0.1 ppm to 1000 ppm.

[0036] The term chelation, as used herein, refers to a metal ion being chemically bound to an ion or molecule by a coordinate bond.

[0037] II. Methods for Detecting Trace Ions The present disclosure provides an accurate, fast and easy-to-use method for determining trace levels of various metal ions in an aqueous sample. In certain embodiments, the trace metal ion is As, Bi, Cd, Cu, Ga, Hg, Pb, Sb, Sn, Tl, Zn, Te, SeCN(-). In certain embodiments, the trace metal ion is Tl. In certain embodiments, the trace metal ion is Tl(I).

[0038] In certain embodiments, the aqueous sample is a gold electroplating solution. In certain embodiments, the level of gold is more than 1000 times higher than the level of trace metal ions, such as, but not limited to, Tl(I). As will be readily appreciated by those skilled in the art, the presence of gold ions in the solution will interfere with the detection of trace metal ions such as Tl(I). In certain embodiments, the gold plating solution is free of sulfate ions (SO4 2- ) and sulfite ion (SO32- ) and other species that may interfere with the detection of trace metal ions such as Tl(I).

[0039] The disclosed method combines multiple elements to determine trace levels of various metal ions. In certain embodiments, the method includes deriving an analytical signal from an electrodeposition experiment. Such methods are in contrast to methods known in the art that perform stripping as part of the experiment. In certain embodiments, the method also includes the use of electrochemical etching for electrode surface recovery in a separate electrolyte rather than electrochemical stripping from the same metal electrolyte for the electrodeposition experiment. In certain embodiments, the method further includes deriving an analytical signal based on incubation time rather than inhibition / acceleration ratios found in CVS methods commonly described in the art. In certain embodiments, the methods disclosed herein are sensitive, selective, inexpensive, rapid, and free of various interfering ions. Additionally, the presently disclosed methods do not require procedures that are difficult to automate, such as, but not limited to, organic solvent extraction or sample heating.

[0040] In certain embodiments, when trace metals are detected in gold electroplating solutions, there are four main possible cathodic reactions: production of H2 from hydronium ions, production of H2 from water, electrodeposition of trace metal ions, and electrodeposition of gold. Each of these electrochemical reactions is shown below as Equations 1-4. For purposes of illustration, the trace metal is Tl(I), but may be any one of As, Bi, Cd, Cu, Ga, Hg, Pb, Sb, Sn, Zn, Te, and SeCN(-). 2H + +2e - →H2E0=0V (1) 2H2O+2e - →H2+2OH - E0=-0.83V (2) Tl + +e - →TlE0=-0.34V (3) Au + +e - →AuE0=1.691V (4)

[0041] Based on the reduction potential, electrodeposition of Au(I) seems favorable, but this metal ion is typically chelated, which shifts the reduction potential to the negative region. Thus, initially during polarization, only hydrogen evolution (as shown in Equations 1 and 2) and deposition of trace metal ions onto the electrode surface occurs. In a particular embodiment, the trace metal ion is Tl(I), as shown in Equation 3. Because the electrodeposition rate is mass transport limited, a rotating disk electrode is used to induce the flux of analyte to the electrode. The electrodeposition rate of metal ions, such as, but not limited to, Tl(I), is described by the Lebitsch equation (5) shown below: I L =(0.62)nFAD 2 / 3ω 1 / 2ν -1 / 6C (5)

[0042] I L is the Lewitch current, n is the number of moles of electrons transferred in the half reaction (number), F is the Faraday constant (C / mol), and A is the electrode area (cm 2 ), and D is the diffusion coefficient (see Fick's law of diffusion) (cm 2 / s), ω is the angular rotation speed of the electrode (rad / s), and v is the kinetic viscosity (cm 2 / s), C is the analyte concentration (mol / cm 3 In this form of equation, the constant with the value 0.620 is rad -1 / 2 Note that the current (I) is proportional to the trace metal concentration and the electrode surface area.

[0043] Faraday's equation (6) states that the mass of deposited trace metal ions per electrode surface area is proportional to their bulk concentration. * It is assumed that a monolayer of the trace metal forms on the electrode, and the surface coverage is proportional to the bulk concentration. * Proportional to time. m=zIt (6)

[0044] In this formula, m is the mass of the deposited metal, z is the electrochemical equivalent, I is the current, and t is the time.

[0045] It has been unexpectedly found that there exists a critical value of trace metal coverage at which the electrodeposition current suddenly accelerates under certain conditions (e.g., appropriate potential). The time to reach this critical surface coverage is referred to herein as the "incubation time". The incubation time can be experimentally determined by the time from the start of electroplating until the first derivative of the current reaches its maximum absolute value in an amperometric scan. It is inversely proportional to the concentration of the trace metal (e.g., Tl(I)) in the electrolyte. Thus, the present disclosure provides a method to measure the incubation time and correlate it to the concentration of the trace metal, such as, but not limited to, Tl(I).

[0046] In certain embodiments, the present disclosure provides methods for measuring trace metal ions, such as, but not limited to, Tl(I), with an error of less than 5% and a relative standard deviation (RSD) of less than 3%.

[0047] In certain embodiments, after collecting measurements, the electrodes are covered with a film of electrodeposited metal that must be removed to provide a clean surface, thereby allowing a new sample analysis. This can be done by one or a combination of the following: mechanical removal, e.g. by polishing, chemical etching, and electrochemical etching methods. In certain embodiments, it may be beneficial to perform the chemical / electrochemical etching with a special etching solution rather than the analytical solution used for the analytical scan. The composition of such a cleaning solution may include one or more strong oxidizing agents (unless oxidation is accomplished electrochemically) and strong chelating agents to keep the etched metal soluble. Examples of strong oxidizing agents include, but are not limited to, aqua regia (a mixture of nitric acid and hydrochloric acid, optimally in a 1:3 molar ratio) and hydrogen peroxide. Examples of strong chelating agents suitable for the methods of the present disclosure include, but are not limited to, thiocyanide and thiourea.

[0048] The method for the determination of trace substances is described in relation to a gold plating solution, but is also suitable for the general analysis of trace metal ions in other aqueous samples, such as, but not limited to, Tl. EXAMPLES

[0049] The presently disclosed subject matter may be better understood by reference to the following examples, which are provided as illustrations of the presently disclosed subject matter, and not as limitations thereof.

[0050] Example 1 Detection of Tl(I) in gold plating solutions Example 1 provides a protocol for determining the Tl(I) concentration in a gold plating solution according to certain embodiments of the present disclosure. Example 1 also describes the creation of a calibration curve that can be used to determine the concentration of Tl(I) ions in an aqueous gold plating solution.

[0051] The Au plating sample solution with Tl(I) was added to a beaker containing 25 mL of supporting electrolyte consisting of 20–80 g / L of a sulfite such as sodium sulfite (pH 6–10), and the solution was mixed. Three electrodes were prepared for the amperometric scan: a platinum rotating disk electrode, a stainless steel auxiliary electrode, and an Ag / AgCl reference electrode. The three electrodes were placed in the beaker, and the first amperometric scan was performed at a fixed potential of -0.6 V for a period of 30–300 s with a rotation speed of 1000–4000 rpm. The electrodes were then removed from the beaker and washed twice with deionized water (DI water). The Au deposits on the Pt working electrode were removed by electrochemical oxidation in the presence of a thiocyanide chelating agent.

[0052] The incubation time (T_sample) was obtained by calculating the first derivative of the chronoamperometric voltammogram obtained from the first amperometric scan after noise reduction of the raw signal using a filter such as a Savitzky-Golay filter. The Tl(I) concentration was then calculated based on T_sample against a calibration curve constructed as described below.

[0053] A calibration curve was created by scanning samples containing known Tl(I) concentrations. TlNO3 or Tl2(SO4) was used as the source of Tl(I) ions. 25 mL of a solution containing the supporting electrolyte Na3[Au(SO3)2] (Au=1 g / L) and Na2SO3 was combined with the test solution containing Tl(I) ions. A platinum rotating disk electrode, a stainless steel auxiliary electrode, and an Ag / AgCl reference electrode were placed in the solution. Amperometric scans were performed at a fixed potential of -0.6 V for 30 seconds at a rotation speed of 1000-4000 rpm. As shown in Figure 1, the calibration curve is a linear regression curve of 1 / T versus the Tl concentration in the supporting electrolyte. The exact value of the incubation time at the first inflection is obtained by the first derivative of the chronoamperometric graph, as shown in Figure 2. In certain embodiments, multiple inflections are observed, which can be used for the calculation. In certain embodiments, the first inflection is used. As shown in Figure 3, a linear correlation was found for the Tl(I) concentration after adding the sample to the supporting electrolyte with 1) the first inflection and 2) the inverse of the incubation time, determined by the time difference between the second inflection and the first inflection. When selecting the first inflection, the R-squared was 0.9999, so this is used for calibration and analysis.

[0054] After constructing the calibration curve, five samples were prepared as shown in Table 1: [Table 1]

[0055] Each of the samples was then tested as described above, and the results are shown in Table 2. Additional samples were tested, including compositions as shown in the "Sample #2" sample, which was spiked with a large amount of sulfate, as shown in Table 2. As shown in Table 2, the measurements were not affected by the presence of sulfate ions, which is important because sulfate ions are a major by-product of sulfite gold plating baths formed by the oxidation of sulfite ions. [Table 2]

[0056] As shown in Table 2, the method of the present invention provided highly accurate measurements of Tl(I) in aqueous gold plating solutions.

[0057] The process was then repeated with another sample with an unknown Tl(I) concentration (the "Process Sample"). As shown in Table 3, three samples were taken: one was tested as is (Process Sample (PS)), a second was spiked with 10 ppm Tl(I) ions (PS spiked with 10 ppm Tl), and a third was diluted 75% with DI water (PS_DIW 75% Dilution). [Table 3]

[0058] As shown in Table 3, the process sample was measured to have 32.48 ppm Tl(I). The sample spiked with 10 ppm Tl(I) was measured to have a concentration of 42.86, which is about a 0.90% difference from the expected value. When the process sample was diluted 75% with DI water, the measured concentration was 24.26 ppm, which was only 0.44% different from the predicted 24.36 ppm.

[0059] The chronoamperometric graphs and their first derivatives for the samples shown in Tables 2 and 3 are shown in Figures 4 and 5, respectively.

[0060] Table 4 shows the analytical results of a set of gold-plated samples, in which the variation in Au levels was much greater than in the sample set of Table 1, and the Tl concentration was lower than in the samples shown in Table 1. The samples were tested using the procedure described above. It was observed that the Au level in the samples affects the incubation time to a much smaller extent than Tl. It was found that a 75% variation in the Au concentration in the samples (range / midpoint value of Au) would result in a deviation of about 9% in the reciprocal of the incubation time. If the variation in Au in the set of samples is less than 45%, the analytical error is less than 5% and therefore the effect of Au can be neglected. An example is the sample set of Table 1. If this is not the case, the signal needs to be corrected based on the analytical results of Au.

[0061] Different approaches can be used, such as adding a suitable amount of Au to the supporting electrolyte to keep the Au concentration constant among different samples, or multivariate analysis using Au concentration as a variable in addition to incubation time. The latter approach was applied to the second set of samples in Table 4: [Table 4]

[0062] Although the presently disclosed subject matter and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosed subject matter. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, method and process described herein. As one skilled in the art will readily appreciate from the disclosed subject matter of the presently disclosed subject matter, any currently existing or later developed process, machine, manufacture, composition of matter, method or procedure that performs substantially the same function or achieves substantially the same result as the corresponding embodiment described herein may be utilized in accordance with the presently disclosed subject matter. Accordingly, it is intended that the appended claims include within their scope such processes, machines, manufacture, compositions of matter, methods or procedures.

[0063] Various patents, patent applications, publications, product descriptions, protocols, and sequence accession numbers are cited throughout this application, the inventions of which are incorporated herein by reference in their entireties for all purposes.

Claims

1. 1. A method for determining a concentration of a metal ion, comprising: Providing an aqueous solution containing a first metal ion; performing a first amperometric scan to obtain a chronoamperometric voltammogram; calculating the first derivative of said chronoamperometric voltammogram; calculating the concentration of the metal ion by comparing the first derivative of the chronoamperometric voltammogram to a calibration curve; The method according to claim 1, further comprising:

2. 2. The method of claim 1, wherein the first metal ion is selected from the group consisting of As, Bi, Cd, Cu, Ga, Hg, Pb, Sb, Sn, Tl, Zn, Te, and SeCN(-).

3. 2. The method of claim 1, wherein the first metal ion is Tl(I).

4. 2. The method of claim 1, wherein the aqueous solution is a gold plating solution containing gold(I) ions.

5. 10. The method of claim 1, wherein the concentration of the first metal ion is from about 0.1 to about 1000 ppm.

6. 5. The method of claim 4, wherein the ratio of said first metal ions to said gold(I) ions is from about 1:300 to about 1:5000.

7. 2. The method of claim 1 , wherein the calibration curve comprises: (i) providing a first test aqueous solution containing a known concentration of said first metal ion; (ii) performing an amperometric scan to obtain a first test chronoamperometric voltammogram; (iii) calculating the first derivative of the first test chronoamperometric voltammogram; and Optionally, repeating (i)-(iii) using a test aqueous solution having a different concentration than said first test aqueous solution. A method comprising:

8. 10. The method of claim 1, wherein the aqueous solution comprises sulfate ions, sulfite ions, or a combination thereof.

9. 10. The method of claim 1, comprising stripping the plating metal from the working electrode.

10. 2. The method of claim 1, wherein the first amperometric scan is performed at a fixed potential of -0.6 V and a rotation speed of 1000 to 4000 rpm for 30 to 300 seconds.

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