Methods for determining concentration of metal ions

Abstract

This invention provides a method for determining the concentration of various trace metal ions in an aqueous solution, such as a gold plating solution. At a specific fixed reduction potential, after a certain time (e.g., a growing time) in the presence of trace metal ions (e.g., Tl(I)), the amplitude of the cathode current can suddenly increase, wherein the growing time is inversely proportional to the concentration of the trace metal in the electrolyte. The concentration of the trace metal can be calculated by measuring the growing time and comparing it with a calibration curve.

Description

Technical Field

[0001] This invention relates to a method for determining the concentration of various trace metal ions in aqueous solutions. Prior Technology

[0002] Gold (Au) electrodeposition is widely used for general metal surface treatment in electronic products and decorative and functional coatings. Au deposits exhibit excellent corrosion characteristics. Electrical contacts can be provided via mechanical equipment, welding, and wire bonding. One formulation of an Au electrodeposition bath includes high amounts (in the g / L range) of Au salts, chelating agents, buffers, surfactants, polarizing agents, and other components, and low amounts (in the ppm range) of metal additives that act as cathodic depolarizers and improve deposition particle size. Several elements have been studied, 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 used as safer alternatives to cyanide and sulfite electrolytes. Tl+ at ppm levels exhibits accelerated deposition (i.e., depolarization) in gold electrodeposition voltammetry. Gold electrodeposition can be limited by the formation of inhibiting species (e.g., SO32- in a gold sulfite bath), which is relieved by Tl+ adsorption on the tangential surface, thus destabilizing the gold complex. The dependence of concentration and transport on hysteresis voltammetry and rising chronocurrent transients indicates adsorption. The amount of these metal additives must 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, it is also a well-known neurotoxin, an environmental pollutant, and is listed by the U.S. Environmental Protection Agency as one of 129 so-called "priority pollutants." Although both thallium (I) and thallium (III) oxidation states are toxic, thallium (I) compounds are relatively more toxic and reactive. The two ions also differ in solubility, as thallium (I) is soluble in water, while thallium (III) is insoluble. Therefore, quantitative analytical methods for thallium (Tl) are crucial, especially in diverse and trace samples.

[0005] Several methods have been proposed for the determination of thallium ions. Spectrophotometry is attractive due to the availability of spectrophotometric instruments and the simplicity of these analytical procedures. Some of these methods are based on the ability of thallium(III) anionic chloride or bromine complexes to interact with cationic dyes to form ion-associated complexes, which can be extracted with organic solvents. Examples of such chromogenic reagents include Brilliant Green, pyronin G, 9,10-phenanthraquinone monoethylamine thiourea, metheomebrazine hydrochloride, N,N-diphenylbenzomidine, and rhodamine B. While methods involving these reagents are sensitive, they may lack selectivity and are therefore susceptible to severe interference from foreign ions that are often present in the actual sample along with thallium, especially in electroplated samples where the level of foreign ions is thousands of times higher than that of thallium.

[0006] Additionally, if Tl(I) is present, it needs to be oxidized to its trivalent state using a strong oxidizing agent such as Ce(IV), bromine water, or hydrogen peroxide. Since excess oxidizing agent can destroy cationic dyes, such oxidizing agents can be removed by boiling the solution (for hydrogen peroxide or bromine) or by adding hydroxyl ammonium chloride (for Ce(IV)). This boiling procedure makes the process more complex and time-consuming, and is impractical for certain automated monitoring purposes for process control.

[0007] Another method for determining thallium is cathodic and anodic stripping voltammetry (CSV). Some CSV methods involve suspended mercury drop electrodes or mercury thin-film electrodes, which reduce Tl(I) to metallic Tl. Due to the extremely low background current and increased sensitivity associated with the formation of Tl amalgam, the measurement signal can be very small and can only be observed on the mercury electrode. However, due to the toxicity of mercury and the difficulty of automated, maintenance-free applications, the use of mercury 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 alloy with other heavy metals, such as thallium, when reduced or co-deposited on its surface (at a sufficiently negative applied potential). These processes can result in detectable and quantifiable changes in current. However, this may require internal engineering of the working electrode and the use of more sophisticated electrochemical techniques, such as square wave / differential pulse anodic stripping voltammetry. Furthermore, interference from predominantly foreign metal ions, such as gold, can pose challenges.

[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. These methods offer good selectivity and low interference from high concentrations of foreign ions, but the instruments are expensive, making them impractical for monitoring and automating certain routine samples.

[0010] Several methods for monitoring the catalytic / inhibitory effects of organic additives on Cu electroplating are known, with analytical methods based on the gradual decrease or increase of the stripping peak area. However, this method may have problems with gold electroplating. Specifically, Au can be electrochemically deposited but cannot be electrochemically stripped from the surface of a platinum electrode derived from the same electrolyte. Furthermore, the signal is not linear and has poor reproducibility.

[0011] Therefore, this technology requires a simple and low-cost method for measuring thallium and similar elements in gold electrodeposition solutions. Furthermore, this technology requires a method that provides accurate, rapid, and easy-to-use information for determining trace amounts of Tl(I) and similar elements in aqueous samples (especially gold plating solutions), where the gold content is typically more than 1000 times higher than the thallium content. Summary of the Invention

[0012] The purpose and advantages of the disclosed subject matter will be set forth in and apparent from the following description, and will be learned by practicing the disclosed subject matter. Additional advantages of the disclosed subject matter will be realized and obtained by means of the means particularly pointed out in the written description of the invention and the claims, and from the accompanying drawings.

[0013] To achieve these and other advantages and in accordance with the purposes of the disclosed subject matter, as implemented and broadly described, the disclosed subject matter includes a method for determining the concentration of a metal ion. In some embodiments, the method includes providing an aqueous solution containing a first metal ion, performing a first current scan to obtain a chronoamperometry plot, calculating the first derivative of the chronoamperometry plot, and calculating the concentration of the metal ion by comparing the first derivative of the chronoamperometry plot with a calibration curve.

[0014] In some embodiments, the first metal ion is selected from As, Bi, Cd, Cu, Ga, Hg, Pb, Sb, Sn, Tl, Zn, Te, SeCN(-). In some specific embodiments, the first metal ion is Tl(I).

[0015] In some embodiments, the aqueous solution is a gold plating solution containing gold (I) ions.

[0016] In some embodiments, the concentration of the first metal ion is from about 0.1 to about 1000 ppm.

[0017] In some embodiments, the ratio of the first metal ion to the gold (I) ion is about 1:300 to about 1:5000.

[0018] In some embodiments, the calibration curve is prepared by: (i) providing a first test aqueous solution containing a first metal ion of known concentration; (ii) performing a current scan to obtain a first test chronoamperometry plot; (iii) calculating the first derivative of the first test chronoamperometry plot; and repeating (i)-(iii) as appropriate using a test aqueous solution with a different concentration than the first test aqueous solution.

[0019] In some embodiments, the aqueous solution includes sulfate ions, sulfite ions, or a combination thereof.

[0020] In some embodiments, the method of the present invention further includes peeling off the plated metal from the working electrode.

[0021] In some embodiments, the first current scan is performed at a fixed potential of -0.6V for 30-300 seconds at a rotational speed of 1000-4000 rpm. Simple Explanation of the Diagram

[0022] The patent or application documents contain at least one color drawing. After the application is filed and the necessary fees are paid, the patent office will provide a copy of the published patent or patent application with the color drawing attached.

[0023] Figure 1 shows the calibration curves used in some embodiments of the present invention.

[0024] Figure 2 shows the first derivative of the chronoamperometry diagram in Figure 1.

[0025] Figure 3 illustrates the linear correlation between the reciprocal of the incubation time and the concentration of Tl(I) according to certain embodiments of the present invention.

[0026] Figure 4 shows a chronoamperometry diagram of measurements performed according to certain embodiments of the present invention.

[0027] Figure 5 shows the first derivative of the chronoamperometry diagram in Figure 4. Implementation

[0028] [Cross-reference to related applications]

[0029] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 316,254, filed March 3, 2022, pursuant to 35 USC § 119(e), which is incorporated herein by reference. []

[0030] This invention provides a method for determining the concentration of metal ions. To meet the aforementioned needs, this invention provides a method for determining trace levels of various metal ions in aqueous samples, such as, but not limited to, gold plating solutions. As used herein, "trace level" includes the range of 0.1 ppm to 1000 ppm.

[0031] For clarity, but not as a limitation, the embodiments of the subject matter disclosed in this invention are divided into the following sub-parts: I. Definitions II. Methods for detecting trace ions [I.] [definition] []

[0032] The terms used in this specification generally have their ordinary meanings in the context of this disclosure and in the specific context in which they are used. Some terms are discussed below or elsewhere in this specification to provide additional guidance to those skilled in the art in describing the compositions and methods of the invention and how they are made and used.

[0033] For the purposes of interpreting this specification, the following definitions will apply, and where appropriate, terms used in the singular will also include the plural forms and vice versa. In the event of any conflict between any definition set forth below and any reference incorporated herein by reference, the definition set forth below shall prevail.

[0034] As used herein, when used in conjunction with the term "comprising" in the scope of the patent application and / or in this specification, the use of the words "a" or "an" may mean "one (kind)," but it also corresponds to the meanings of "one or more (kinds)," "at least one (kind)," and "one (kind) or more than one (kind)."

[0035] As used herein, the terms “comprise(s)”, “include(s)”, “having”, “has”, “may”, “contain”, and variations thereof are intended to be open-ended transitional phrases, terms, or words that do not exclude additional actions or structures. Whether explicitly stated or not, the invention also contemplates other embodiments that “comprise”, “consist of”, and “substantially consist of” the embodiments or elements presented herein.

[0036] As used herein, the terms "about" or "approximately" mean within an acceptable margin of error for a particular value as determined by someone generally skilled in the art, which will depend in part on how the value is measured or determined, i.e., the limitations of the measurement system. For example, "about" may mean within three or more standard deviations, according to practice in this art. Or, "about" may mean within at most 20%, preferably at most 10%, even better at most 5%, and still even better at most 1% of a given value.

[0037] As used herein, when used in the context of metal ion concentration in solution, the term "high content" refers to a concentration in the range of grams per liter (g / L).

[0038] As used herein, when used in the context of metal ion concentration in solution, the term "low content" refers to a concentration in the range of parts per million (ppm).

[0039] As used herein, grain refiners have their general meaning in electroplating technology; that is, they are additives that help control the electroplating process by attracting "high points" to the plated surface. Without treatment, high points tend to attract more metal deposition than low points, which result in surface roughness. Grain refiners inhibit further deposition. When these points are replaced by other, even higher points, the grain refiner will drift and reposition.

[0040] As used herein, the term "trace" refers to a concentration of less than 1000 ppm. In some embodiments, trace level refers to a concentration range of 0.1 ppm to 1000 ppm.

[0041] As used herein, the term chelation refers to a metal ion that is chemically bonded to an ion or molecule through coordination bonding. [II.] Methods for detecting trace ions []

[0042] This invention provides an accurate, rapid, and easy-to-use method for determining trace levels of various metal ions in aqueous samples. In some embodiments, the trace metal ions are As, Bi, Cd, Cu, Ga, Hg, Pb, Sb, Sn, Tl, Zn, Te, and SeCN(-). In some embodiments, the trace metal ion is Tl. In some embodiments, the trace metal ion is Tl(I).

[0043] In some embodiments, the aqueous sample is a gold plating solution. In some embodiments, the gold content is more than 1000 times higher than the content of trace metal ions such as, but not limited to, Tl(I). As will be readily known to those skilled in the art, the presence of gold ions in the solution interferes with the detection of trace metal ions such as Tl(I). In some embodiments, the gold plating solution contains other species that can interfere with the detection of trace metal ions such as Tl(I), including sulfate ions (SO₄²⁻) and sulfite ions (SO₃²⁻).

[0044] The method of this invention combines multiple elements to determine trace levels of various metal ions. In some embodiments, the method includes deriving the analytical signal from the electrodeposition experiment. This contrasts with methods known in the art that involve stripping as part of an experiment. In some embodiments, the method also includes restoring the electrode surface in a separate electrolyte using electrochemical etching, rather than performing electrochemical stripping from the same metal electrolyte used in the electrodeposition experiment. In some embodiments, the method further includes deriving the analytical signal based on incubation time rather than the inhibition / acceleration ratio, as found in CVS methods commonly described in the art. In some embodiments, the method disclosed herein is sensitive, selective, inexpensive, rapid, and free from various interfering ions. Furthermore, the method disclosed herein does not require procedures that are difficult to automate, such as, but not limited to, organic solvent extraction or sample heating.

[0045] In some embodiments, when trace metals are detected in a gold plating solution, there are four (4) possible main cathodic reactions: H₂ generation from hydrated hydrogen ions, H₂ generation from water, electrodeposition of trace metal ions, and electrodeposition of gold. Each of these electrochemical reactions is shown below as Equations 1-4. For illustrative purposes, the trace metal is Tl (I), but it may be any of As, Bi, Cd, Cu, Ga, Hg, Pb, Sb, Sn, Zn, Te, and SeCN (-).

[0046] Although electrodeposition of Au(I) appears advantageous based on the reduction potential, this metal ion is typically chelated, which shifts the reduction potential to the negative region. Therefore, at the initial time of polarization, only the release of hydrogen (as shown in Equations 1 and 2) and the deposition of trace metal ions on the electrode surface occur. In some embodiments, as shown in Equation 3, the trace metal ion is Tl(I). The electrodeposition rate is limited by mass transport, and therefore, a rotating electrode is used to induce the analyte flow towards the electrode. The electrodeposition rate of metal ions such as, but not limited to, Tl(I) is described by Levich equation (5) as shown below:

[0047] IL is the Levich current, n is the mole number of electrons transferred in the half-reaction, F is the Faraday constant (C / mol), A is the electrode area (cm²), D is the diffusion coefficient (see Fick's law of diffusion) (cm² / s), ω is the angular rotation rate of the electrode (rad / s), v is the kinematic viscosity (cm² / s), and C is the analyte concentration (mol / cm³). In this form of equation, the constant 0.620 is in rad⁻¹ / ². Note that the current (I) is proportional to the concentration of the trace metal and the electrode surface area.

[0048] Faraday's equation (6) states that the mass of trace metal ions electrodeposited on each electrode surface is proportional to its volume concentration × time. If it is assumed that a monolayer of trace metal is formed on the electrode, then the surface coverage is also proportional to the volume concentration × time.

[0049] In this equation, m is the mass of the electrodeposited metal, z is the electrochemical equivalent, I is the current, and t is the time.

[0050] It has been unexpectedly discovered that there exists a critical value for trace metal coverage, at which the electrodeposition current suddenly accelerates under certain conditions (e.g., appropriate potential). The time required to reach this critical surface coverage is referred to herein as the "growth time". The growth time can be experimentally determined by the duration from the start of electroplating until the first derivative of the current reaches its maximum absolute value in a current scan. It is inversely proportional to the concentration of trace metals (e.g., Tl(I)) in the electrolyte. Therefore, the present invention provides a method for measuring the growth time and relating it to the concentration of trace metals, such as, but not limited to, Tl(I).

[0051] In some embodiments, the present invention provides a method for measuring trace metal ions, such as, but not limited to, Tl(I), with an error of < 5% and a relative standard deviation (RSD) of < 3%.

[0052] In some embodiments, after collecting measurements, the electrode is covered with an electrodeposited metal film, which should be removed to provide a clean surface, enabling a new round of sample analysis. This can be done by one or a combination of mechanical removal, such as by polishing, chemical etching, and electrochemical etching methods. In some embodiments, it may be advantageous to perform chemical / electrochemical etching in a special etching solution rather than the analytical solution used for the analytical scan. Such a cleaning solution may include one or more strong oxidizing agents (unless oxidation is achieved 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, preferably in a 1:3 molar ratio) and hydrogen peroxide. Examples of strong chelating agents suitable for the methods of the present invention include, but are not limited to, thiocyanates and thiourea.

[0053] Although the method for measuring trace materials has been described in conjunction with gold plating solutions, it is also applicable to the general analysis of trace metal ions in other aqueous samples, such as, but not limited to, Tl. [Example] []

[0054] The subject matter disclosed herein will be better understood by referring to the following examples, which are provided as examples of the subject matter disclosed herein and not as limitations. [, Example , ] [, 1 , ] [, Detection of gold plating solution , ] [,Tl , ] [, ( , ] [, I , ] [, ) , ] [, , ]

[0055] Example 1 provides a scheme for determining the concentration of Tl(I) in a gold plating solution according to certain embodiments of the present invention. Example 1 also describes the preparation of a calibration curve that can be used to determine the concentration of Tl(I) ions in an aqueous gold plating solution.

[0056] A gold-plated sample solution containing Tl(I) was added to a beaker containing 25 mL of a supporting electrolyte consisting of 20-80 g / L sulfite, such as sodium sulfite (pH 6-10), and the solution was mixed. Three electrodes were prepared for the current scan: a platinum rotating disk electrode, a stainless steel auxiliary electrode, and an Ag / AgCl reference electrode. The three electrodes were then placed in the beaker, and a first current scan was performed at a fixed potential of -0.6 V for 30-300 seconds at a rotation speed of 1000-4000 rpm. The electrodes were then removed from the beaker and rinsed twice with deionized water (DI water). Au deposits on the Pt working electrode were removed by electrochemical oxidation in the presence of a thiocyanate chelating agent.

[0057] The incubation time (T_sample) was obtained by calculating the first derivative of a chronoamperometry plot, which was obtained from the first current scan after noise reduction of the original signal using a filter such as a Savitzky-Golay filter. The concentration of Tl(I) was then calculated based on T_sample relative to a calibration curve prepared as described below.

[0058] The calibration curve was prepared by scanning a sample containing a known concentration of Tl(I). TlNO3 or Tl2(SO4) was used as the Tl(I) ion source. A solution containing 25 mL of supporting electrolyte Na3[Au(SO3)2] (Au = 1 g / L) and Na2SO3 was combined with a 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. A current scan was performed for 30 seconds at a fixed potential of -0.6 V and a rotation speed of 1000–4000 rpm. As shown in Figure 1, the calibration curve is a linear regression curve of 1 / T relative to the Tl concentration in the supporting electrolyte. The accurate value of the incubation time at the first inflection point was obtained by the first derivative of the chronoamperometry plot shown in Figure 2. In some embodiments, multiple inflection points were observed, which could be used for calculation. In some embodiments, the first inflection point was used. As shown in Figure 3, after adding the sample to the supporting electrolyte, a linear correlation is observed between the inverse of the incubation time (determined by 1) the first inversion point and 2) the time difference between the second and first inversion points and the Tl(I) concentration. When the first inversion point is selected, the R-squared value is 0.9999, and therefore it is used in calibration and analysis.

[0059] After preparing the calibration curves, five (5) samples were prepared as shown in Table 1: Sample labeling Au, g / L (e.g., in Na3[Au(SO3)2]) [SO₃]₂-, g / L (e.g., in Na₂SO₃) Tl,ppm Sample #1 13 20 20 Sample #2 15 40 30 Sample #3 17 90 40 Sample #4 17 90 20 Sample #5 13 20 40 Table 1: Composition of the standard samples used in the test

[0060] The samples were then tested as described above, and the results are shown in Table 2. As shown in Table 2, additional samples were tested, including a composition with a high amount of sulfate, as shown in Sample #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 byproduct of gold sulfite plating baths, formed by the oxidation of sulfite ions. Sample #1 Sample #2 Sample #3 Sample #4 Sample #5 Sample #2 containing 80 g / L SO₄²⁻ Expected value, ppm 20 30 40 20 40 30 Measurement #1, ppm 19.43 30.50 38.80 19.18 38.77 30.04 Measurement #2, ppm 19.85 30.66 40.46 NA NA Average value of measurements, ppm 19.64 30.58 39.63 19.18 38.77 30.04 Accuracy -1.79% 1.93% -0.92% -4.08% -3.08% 0.13% RSD 1.49% 0.38% 2.96% Table 2: Analytical results of standard samples.

[0061] As shown in Table 2, the method of the present invention provides extremely accurate measurement of Tl(I) in gold plating aqueous solution.

[0062] The process was then repeated with another sample ("process sample") having an unknown Tl(I) concentration. Three samples were collected: the first sample was tested as is (process sample (PS)); the second sample was doped with 10 ppm Tl(I) ions (PS doped with 10 ppm Tl); and the third sample was diluted 75% with deionized water (PS diluted with DIW_75%), as shown in Table 3. Process Sample (PS) Add 10 ppm of PS with Tl PS diluted with DIW (75%) Expected value, ppm NA 42.48 24.36 Measurement value, ppm 32.48 42.86 24.26 Accuracy NA 0.90% -0.44% Table 3: Analytical results of process samples, process samples doped with pure Tl(I), and process samples diluted with DI water.

[0063] As shown in Table 3, the process sample measured to have a Tl(I) concentration of 32.48 ppm. The sample with 10 ppm Tl(I) added had a concentration of 42.86 ppm, which differed from the expected value by approximately 0.90%. When the process sample was diluted 75% with DI water, the measured concentration was 24.26 ppm, which differed from the predicted 24.36 ppm by only 0.44%.

[0064] The chronoamperometry diagrams and their first derivatives of the samples shown in Tables 2 and 3 are shown in Figures 4 and 5, respectively.

[0065] Table 4 shows the analytical results of a group of gold-plated samples, in which the variation in Au content is much greater than that of the sample group in Table 1, and the Tl concentration is lower than that of the samples shown in Table 1. The samples were tested using the above procedure. It was observed that the effect of Au content on incubation time is much smaller than that of Tl. It was found that a 75% variation in Au concentration (Au range / midpoint value) in the samples caused a deviation of approximately 9% in the reciprocal of the incubation time. When the variation in Au in a sample group is less than 45%, the analytical error is less than 5%; therefore, the effect of Au is negligible. An example is the sample group in Table 1. Otherwise, signal compensation should be made based on the Au analysis results.

[0066] Different methods can be used, such as adding an appropriate amount of Au to the supporting electrolyte to maintain a constant Au concentration in different samples, or using Au concentration as a multivariate analysis excluding incubation time. The latter method was applied to sample group 2 in Table 4: Sample #6 Sample #7 Sample #8 Sample #9 Sample #10 Preparation of Au, g / L 10 16 twenty two twenty two 10 Expected Tl value, ppm 5 12 20 5 20 Tl measurement #1, ppm 5.00 12.41 19.93 4.84 19.82 Tl measurement #2, ppm 5.15 12.33 20.21 4.79 19.65 The average value of Tl measurements, ppm 5.08 12.37 20.07 4.81 19.73 Accuracy 1.58% 3.11% 0.36% -3.76% -1.33% RSD 2.07% 0.45% 1.00% 0.68% 0.61% Table 4: Analytical results of samples with increased Au content. ⁎ ⁎ ⁎

[0067] Although the subject matter and advantages of the present invention have been described in detail, it should be understood that various changes, substitutions, and modifications may be made herein without departing from the spirit and scope of the disclosed subject matter. Furthermore, the scope of this application is not intended to be limited to the processes, machines, manufactures, compositions of matter, methods, and specific embodiments of processes described herein. As will be readily apparent from the subject matter disclosed herein, processes, machines, manufactures, compositions of matter, methods, or procedures currently existing or to be developed herein can perform substantially the same functions or achieve substantially the same results as the corresponding embodiments described herein. Therefore, the appended claims are intended to include such processes, machines, manufactures, compositions of matter, methods, or procedures within their scope.

[0068] Throughout this application, various patents, patent applications, publications, product descriptions, designs and serial numbers are cited, and their inventions are incorporated herein by reference in their entirety for all purposes.

Claims

1. A method for determining the concentration of a metal ion, the method comprising: providing an aqueous solution containing a first metal ion; performing a first current scan to obtain a chronoamperometry plot; calculating the first derivative of the chronoamperometry plot to obtain a growth time; and calculating the concentration of the metal ion by comparing the growth time of the chronoamperometry plot with a calibration curve.

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, SeCN(-).

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

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

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

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

7. The method of claim 1, wherein the calibration curve is prepared by: (i) providing a first test aqueous solution containing a known concentration of the first metal ion; (ii) performing a current scan to obtain a first test chronoamperometry; (iii) calculating the first derivative of the first test chronoamperometry; and repeating (i)-(iii) as appropriate using a test aqueous solution having a different concentration than the first test aqueous solution.

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

9. The method of claim 1, wherein the method comprises stripping the plated metal from the working electrode.

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

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