Method for on-line detection of trivalent chromium ion concentration based on electrochemical characteristic parameter

Abstract

The application belongs to the technical field of electroplating liquid component analysis and electrochemical detection, and discloses a trivalent chromium ion concentration online detection method based on electrochemical characteristic parameters, which comprises the following steps: configuring standard hard chromium plating liquid with different trivalent chromium ion concentrations, measuring the electrochemical response curves of each standard plating liquid and extracting characteristic parameters therefrom, the characteristic parameters are selected from the group consisting of reduction peak current density, reduction peak potential, reduction peak return current density difference, peak current density, peak time and effective nucleation site density; establishing a quantitative calibration relationship between the trivalent chromium ion concentration and the characteristic parameters; and under the same electrochemical test conditions, measuring the electrochemical response curve of the to-be-detected plating liquid and extracting the same characteristic parameters, and based on the calibration relationship, the concentration of trivalent chromium ions in the to-be-detected plating liquid is calculated. 3+ The method uses nucleation kinetics parameters for the first time to detect the concentration of Cr 3+ online, significantly improves the accuracy and robustness of detection, and greatly improves the efficiency of in-situ rapid detection.

Description

Technical Field

[0001] This invention belongs to the field of electroplating solution composition analysis and electrochemical detection technology, specifically relating to a method for online quantitative detection of trivalent chromium ions (Cr) in hexavalent chromium hard chromium plating solutions by extracting characteristic parameters using linear scanning voltammetry or chronoamperometry. 3+ (Concentration method) Background Technology

[0002] Hard chromium plating is widely used in machinery manufacturing, automotive, and aerospace industries due to its high hardness and excellent wear resistance. However, traditional hexavalent chromium plating solutions, in their long-term production, suffer from [a problem with Cr]. 6+ Cathodic reduction inevitably leads to the accumulation of trivalent chromium ions (Cr). 3+ ). Cr 3 + It has a dual effect: "necessary at low concentrations, harmful at high concentrations." Appropriate amounts (typically 1-4 g / L) help form a stable cathode colloidal film, promoting chromium deposition; excessive amounts (>4 g / L) lead to darkening of the coating, coarsening of grains, and deterioration of performance. Therefore, real-time monitoring of chromium... 3+ Concentration is key to achieving plating solution maintenance, quality control, and low-emission production.

[0003] Existing Cr 3+ The main detection methods include chemical titration (such as redox titration), atomic absorption spectrometry (AAS), and inductively coupled plasma optical emission spectrometry (ICP-OES). These methods all require pretreatment such as sampling, dilution, and digestion, which is time-consuming (usually >30 min) and cannot achieve online continuous monitoring. Electrochemical methods such as linear sweep voltammetry (LSV) and chronoamperometry (CA) have the advantages of fast response, no sampling required, and in-situ measurement. However, among the currently published patents and technical documents: CN202323093U discloses a detection device for online detection of components in chromium plating solutions, but it only describes the device structure and does not disclose specific electrochemical measurement parameters or characteristic parameter extraction methods. CN118795085A discloses a method for determining the concentration of trivalent chromium in a solution, which uses titration and is an offline detection method, time-consuming and unable to achieve online continuous monitoring. While there are reports in academic research on the use of LSV to analyze the components of chromium plating solutions (such as determining CrO3 content), continuous and rapid online monitoring is usually not possible, and its application in industrial online detection is not observed.

[0004] Therefore, it is necessary to develop a method for rapid in-situ detection of Cr based on electrochemical characteristic parameters. 3+ Concentration-based methods have both urgent practical needs and room for innovation. Summary of the Invention

[0005] This invention addresses the shortcomings of existing technologies by providing a method for determining the Cr³ content in hard chromium plating solutions based on electrochemical characteristic parameters.+ Online concentration detection method. By measuring the LSV or CA curve of the plating solution within a specific potential range, the concentration of Cr³⁺ can be extracted. + Characteristic parameters that are significantly correlated with concentration (including reduction peak current density i) p Peak current density i m Peak time t m (Effective nucleation site density N0, difference in reduction peak return current density i), establish a segmented calibration model to achieve Cr 3+ Real-time, non-destructive detection of concentration.

[0006] The complete technical solution of this invention is as follows: An online detection method for trivalent chromium ion concentration based on electrochemical characteristic parameters includes the following steps: (a) Establishing a standard curve: Prepare at least three standard hard chromium plating solutions with different trivalent chromium ion concentrations, and use a three-electrode system to measure the electrochemical response curves of each standard plating solution. The electrochemical response curves are linear sweep voltammetry curves or chronoamperometry curves. Extract at least one characteristic parameter from the electrochemical response curves. The characteristic parameter is selected from: reduction peak current density i p Reduction peak potential E p , Peak-to-back current density difference i, Peak current density i m Peak time t m Effective nucleation site density N0; Establish a quantitative calibration relationship between trivalent chromium ion concentration and the aforementioned characteristic parameters; (b) Online detection: Under the same electrochemical test conditions as in step (a), the electrochemical response curve of the plating solution to be tested is measured, the same characteristic parameters are extracted, and the calibration relationship in step (a) is substituted to calculate the concentration of trivalent chromium ions in the plating solution to be tested.

[0007] Furthermore, the standard hard chrome plating solution concentration covers a range of 0~16 g / L.

[0008] Furthermore, the linear scanning voltammetry has a scanning potential range of -0.9 V to -1.4 V (vs. saturated calomel electrode), a scanning rate of 0.05~0.50 mV / s, preferably 0.10 mV / s, and records the current-potential curve.

[0009] Furthermore, characteristic parameters are extracted from the LSV curve, including the reduction peak potential E. p , reduction peak current density i p The difference in return current density at the reduction peak, i.

[0010] With Cr³ + With concentration c (in g / L) as the independent variable and characteristic parameters as the dependent variable, the calibration curve equation is obtained, and a linear relationship is obtained after fitting.

[0011] Furthermore, the step potential range of the chronoamperometry method is -0.96 V to -1.04 V (vs. saturated calomel electrode), preferably -0.96 V; the sampling time covers at least from 0 to the diffusion control region after the peak appears, and the current-time curve is recorded.

[0012] Furthermore, characteristic parameters are extracted from the CA curve: peak current density i m (Take absolute value), peak time t m The effective nucleation site density N0 is calculated using the following formula:

[0013] in For electron transfer number, It is Faraday's constant. denoted as the molar concentration of hexavalent chromium ions, and D as the diffusion coefficient (determined through independent experiments). For peak current density, This is the peak time.

[0014] Furthermore, with Cr³ + Concentration c is the independent variable, i m Or, with N0 as the dependent variable, establish a calibration curve. At a step potential of -0.96 V, |i m | It exhibits a monotonically increasing relationship with c in the range of 1~4 g / L, and can be fitted by linear or quadratic equations; N0 exhibits a monotonically decreasing relationship with c at a step potential of -1.00 V, and is suitable for detection in the high concentration range.

[0015] Furthermore, step (b) specifically includes: (1) Insert the above three-electrode system directly into the plating bath at the production site to ensure that the working electrode surface is clean and free of bubbles; (2) Under the same electrochemical test parameters as in step 1, measure the LSV or CA curve of the plating solution to be tested; (3) Extract the same characteristic parameters, substitute them into the corresponding calibration curve equation obtained in step (a), and calculate the Cr³ in the plating solution to be tested. + The concentration.

[0016] Furthermore, when establishing the quantitative calibration relationship, segmented fitting is performed based on the trivalent chromium ion concentration range, wherein the low concentration range of 1~4 g / L uses linear or polynomial fitting, and the high concentration range of 4~16 g / L uses linear or power function fitting.

[0017] Furthermore, when Cr is expected 3+ When the concentration is ≤4 g / L, LSV is used to extract the difference in return current density i of the reduction peak or CA is used to extract i.m As a characteristic parameter; when the expected Cr 3+ When the concentration is >4 g / L, CA is used to extract NO as a characteristic parameter.

[0018] Furthermore, two or more feature parameters are extracted simultaneously, and the concentration of trivalent chromium ions is determined jointly by ratio or weighting to eliminate the ambiguity of a single parameter in the non-monotonic range.

[0019] Furthermore, to overcome the uncertainty of a single parameter in the inflection point region (approximately 4 g / L), a dual characteristic parameter joint determination is adopted: i and i' are measured simultaneously. m The concentration is determined by looking up a table using the ratio or weighted average of the two values, with the error controlled within ±5%.

[0020] Furthermore, the working electrode is a platinum disk electrode or a glassy carbon electrode, the reference electrode is a saturated calomel electrode or a silver / silver chloride electrode, and the counter electrode is a platinum sheet electrode.

[0021] Furthermore, the temperature of the hard chrome plating solution is controlled at 50~60℃, and the temperature is kept constant during the test.

[0022] Furthermore, the online detection is performed automatically, and the detection results are used to trigger the plating solution purification device or alarm device.

[0023] Furthermore, an online detection device includes an electrochemical measurement unit with a three-electrode probe and an electrochemical workstation, a data acquisition and control unit, a human-machine interface, and an optional purification device interface, wherein the online detection device for trivalent chromium ion concentration uses any of the methods described above for online detection.

[0024] Compared with the prior art, the present invention has the following obvious advantages: 1. For the first time, the nucleation kinetic parameter N0 was used for Cr. 3+ Online concentration detection. Existing online detection technologies do not address nucleation mechanism parameters; this invention uses the effective nucleation site density N0 from electrocrystallization theory as a quantitative characteristic parameter, establishing a correlation between N0 and Cr. 3+ The quantitative relationship of concentration allows for the direct transformation of fundamental electrochemical theory into industrial detection methods, which is non-obvious.

[0025] 2. Multi-feature parameter coordination and segmented calibration strategy. For i m Based on the non-monotonic relationship with c, this invention designs segmented calibration (monotonically increasing in the low concentration region and monotonically decreasing in the high concentration region) and dual-parameter joint determination, which significantly improves the accuracy and robustness of detection and overcomes the inherent defects of the single-parameter method.

[0026] 3. No sampling required, rapid in-situ detection. The entire detection process can be completed directly in the plating bath, with a single test cycle of ≤10 min (LSV) or ≤1 min (CA), which significantly improves efficiency compared to traditional methods (>30 min) and consumes no chemical reagents.

[0027] 4. Low equipment cost and easy integration. It adopts a standard electrochemical workstation and universal electrodes, eliminating the need for expensive spectroscopic instruments. It can be easily embedded into the plating solution circulation system to achieve automatic early warning and closed-loop control. Attached Figure Description

[0028] Figure 1 For different Cr 3+ LSV curve of the concentration plating solution (scan rate 0.10 mV / s).

[0029] Figure 2 For different Cr 3+ The difference in return current density i between the reduction peaks obtained from the concentration plating solution and the fitted linear graph.

[0030] Figure 3 For different Cr 3+ CA curve of the concentration plating solution (step potential -0.96 V).

[0031] Figure 4 For different Cr 3+ Peak current density i obtained from high-concentration plating solution m And the fitted curve.

[0032] Figure 5 For different Cr 3+ N0 and the fitted straight line obtained from the concentration of the plating solution. Detailed Implementation

[0033] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are merely illustrative and are not intended to limit this application.

[0034] An online detection method for the concentration of trivalent chromium ions in a hard chromium plating solution includes the following steps: (1) Prepare a series of known Cr 3+ Standard hard chrome plating solution of concentration, Cr 3+ The concentration gradient includes at least three different concentration values, covering the range of 0 to 16 g / L.

[0035] (2) A three-electrode system is adopted: the working electrode is an inert conductive material, preferably a platinum disk electrode or a glassy carbon electrode, the reference electrode is a saturated calomel electrode (SCE) or a silver / silver chloride electrode, and the counter electrode is a platinum sheet; the working electrode is immersed in a standard plating solution at a constant temperature (50~60℃).

[0036] (3) Perform linear scanning voltammetry test: Set the scanning potential range to -0.9 V to -1.4 V (vs. SCE), the scanning rate to 0.05~0.50 mV / s, preferably 0.10 mV / s, and record the current-potential curve; (4) Extract characteristic parameters from the LSV curve: including reduction peak potential E p , reduction peak current density i p The difference in return current density at the reduction peak, i; (5) With Cr 3+ With concentration as the independent variable and characteristic parameters as the dependent variable, the calibration curve equation is obtained, and the fitted curve shows a linear relationship.

[0037] Alternatively, the chronoamperometry method can be used: (3') Perform timing current test: Set the step potential to -0.96 V to -1.04 V (vs. SCE), preferably -0.96 V, record the current-time curve, and the sampling time should at least cover the period from 0 to the post-peak diffusion control region (e.g., 0~10 s). (4') Extract characteristic parameters from the CA curve: peak current density i m (Take absolute value), peak time t m The effective nucleation site density N0 is calculated based on the following model:

[0038] Where n is the number of electrons transferred, F is the Faraday constant, C is the molar concentration of hexavalent chromium ions, and D is the diffusion coefficient, which can be determined through independent experiments.

[0039] (5') with Cr 3+ Concentration c is the independent variable, i m Alternatively, with N0 as the dependent variable, a calibration curve equation can be established. The study found that at a step potential of -0.96 V, |i m The concentration of NO increases monotonically with c in the range of 1 to 4 g / L and can be fitted by a quadratic equation; while NO decreases monotonically with c at a step potential of -1.00 V, making it suitable for detection in the high concentration range.

[0040] Step 2: Online Detection (1) Insert the above three-electrode system directly into the plating bath at the production site to ensure that the working electrode surface is clean and free of bubbles; (2) Under the same electrochemical test parameters as in step 1, measure the LSV or CA curve of the plating solution to be tested; (3) Extract the same characteristic parameters, substitute them into the corresponding calibration curve equation obtained in step 1, and calculate the Cr³ in the plating solution to be tested. + The concentration.

[0041] The preferred detection scheme, when Cr is expected 3+ When the concentration is ≤4 g / L, LSV extraction of the reduction peak return current density difference i or CA extraction of i should be preferred. m As a characteristic parameter; as shown in Table 1. When the expected Cr 3+ When the concentration is >4 g / L, CA is preferred to extract NO as a characteristic parameter.

[0042] Furthermore, to overcome the uncertainty of a single parameter in the inflection point region (approximately 4 g / L), a dual-characteristic parameter joint determination is adopted: i and i' are measured simultaneously. m The concentration is determined by looking up the ratio or weighted average of the two values ​​in a table, and the error is controlled within ±5%, as shown in Table 3.

[0043] Example 1 (LSV method, detection in the low concentration range) The plating solution from a hard chrome plating production line was selected as the test object. The basic composition of the plating solution was: CrO3 250 g / L, H2SO4 2.5 g / L, temperature 55℃. First, a standard curve was established, such as... Figure 1 As shown: Preparation of Cr 3+ Standard solutions (Cr) with concentrations c of 0, 1, 2, 4, 8, and 16 g / L were used. 3+ (Introduced via in-situ reaction of ethanol with CrO3). A three-electrode system was used: a platinum disk electrode (d=3mm) as the working electrode, an SCE as the reference electrode, and a platinum sheet as the counter electrode; the electrochemical workstation was a CHI760e. LSV measurements were performed, with a scan range of -0.9V to -1.4V and a scan rate of 0.10 mV / s. The difference in reduction peak return current density, i, was recorded at each concentration. The results showed that in the range of 1–16 g / L, i and c had a linear relationship, as shown in the figure. Figure 2 As shown: |i| = 0.0019 + 0.00313*c (A / cm²), R² = 0.81801.

[0044] The same test was performed on the plating solution to be tested, and the measured value of i was 0.212 A / cm². Substituting this into the linear equation, the calculated value of c was 8.9 g / L. Simultaneously, ICP-OES verification was performed, and the result was 8.8 g / L, with a relative error of 1.1%. The standard deviation of six repeated measurements was 0.07 g / L.

[0045] Example 2 (CA Method) The chronoamperometry method was used. The standard plating solution was the same as in Example 1, with the step potential set to -0.96 V (vs. SCE). The current-time curves from 0 to 10 s were recorded, as follows: Figure 3 As shown. Extract the peak current density i. m (Absolute value). Within the range of 1~16 g / L, |i m|Increases and then decreases with increasing c, and the fitted result is i m = -0.167 + (-0.015)*c + 0.00115*c 2 R² = 0.93, as Figure 4 As shown. The plating solution to be tested (the production bath solution that has been running for 3 months) was tested, i m = -0.14 A / cm², C = 14.65 g / L. Compared with the traditional titration method (16 g / L), the relative error is 7%. The CA method has a single test time of about 5 s (peak appearance time < 1 s), which is faster than LSV.

[0046] Example 3 (CA method, using N0) As shown in Table 2, for Cr 3+ For plating solutions with concentrations >2 g / L, the effective nucleation site density N0 was used as a characteristic parameter. The step potential was set to -1.00 V (vs. SCE), and different Cr values ​​were recorded. 3+ CA curves at concentrations (1, 2, 4, 8, 16 g / L) were used to extract i. m and t m N0 was calculated using the aforementioned formula (the D value was determined through a transient experiment and taken as 2.3 × 10⁻⁶). -6 cm² / s). The fitted result is N0 / (10 5 cm -2 The relationship between N0 and c: N0 = y = 3.72348 0.12813*c (unit: 10) 5 cm -2 ), R²=0.97, such as Figure 5 As shown. The plating solution to be tested (with known Cr content) 3+ The concentration was 11.5 g / L, and the calculated N0 was 2.25·10⁻⁶. 5 cm -2 Substituting this into the equation, we get c = 12.25 g / L, with an error of 6.5%.

[0047] Example 4 (Joint Determination by Two Parameters) For samples with unknown concentrations, simultaneously measure the current hysteresis difference i of LSV and the i of CA. m Calculate the ratio r=i m Studies have shown that r changes most rapidly near c=4 g / L, which can be used to accurately determine the inflection point. A lookup table for rc is established within the range of c=1~16 g / L. The dual-parameter method effectively avoids the ambiguity in the inflection point range caused by a single parameter.

[0048] Industrial Application Examples An online detection system was installed on the hard chrome plating production line of a certain electroplating plant: a three-electrode probe was inserted into the bypass of the main plating tank circulation pipeline, and the system was automatically controlled by a PLC to perform 6 LSV scans per day (once every 4 hours), with the scanning parameters the same as in Example 1. The measured i p The value is transmitted to the central control room via a 4~20 mA signal, and the system automatically calculates Cr based on the built-in segmented calibration model. 3+ The system monitors the concentration and issues an alarm when it exceeds 4.5 g / L, simultaneously activating the electrolytic purification device. After 90 days of continuous operation, the maximum deviation compared to weekly manual ICP analysis results was ≤5%, meeting process control requirements. The application of this system extends the plating bath life by approximately 30%, due to the reduced Cr³⁺ content. + Exceeding the standard resulted in a decrease of more than 50% in the quality of the plating.

[0049] Table 1 Peak current density i m With Cr 3+ Concentration Relationship Table

[0050] Table 2. Effective nucleation site density N0 at different step potentials / (10 5 cm -2 ) With Cr 3+ Concentration relationship

[0051] Table 3. Lookup table for joint determination using dual characteristic parameters

[0052] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the present invention. Any simple modifications, alterations, or equivalent structural changes made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for online detection of trivalent chromium ion concentration based on electrochemical characteristic parameters, characterized in that, Includes the following steps: (a) Establishing a standard curve: Prepare standard hard chromium plating solutions with different trivalent chromium ion concentrations, and use a three-electrode system to measure the electrochemical response curves of each standard plating solution. The electrochemical response curves are linear sweep voltammetry curves or chronoamperometry curves. Extract at least one characteristic parameter from the electrochemical response curves. The characteristic parameter is selected from: reduction peak current density i p Reduction peak potential E p Peak current density i m Peak time t m The effective nucleation site density N0 and the difference in reduction peak return current density i are used to establish a quantitative calibration relationship between the trivalent chromium ion concentration and the aforementioned characteristic parameters. (b) Online detection: Under the same electrochemical test conditions as in step (a), the electrochemical response curve of the plating solution to be tested is measured, the same characteristic parameters as in step (a) are extracted, and the quantitative calibration relationship in step (a) is substituted to calculate the concentration of trivalent chromium ions in the plating solution to be tested.

2. The online detection method for trivalent chromium ion concentration based on electrochemical characteristic parameters according to claim 1, characterized in that: The linear scanning voltammetry has a scanning potential range of -0.9 V to -1.4 V (vs. saturated calomel electrode) and a scanning rate of 0.05 to 0.50 mV / s.

3. The online detection method for trivalent chromium ion concentration based on electrochemical characteristic parameters according to claim 1, characterized in that: The step potential range of the timing current is -0.96 V to -1.04 V (vs. saturated calomel electrode), and the sampling time covers at least from 0 to the diffusion control region after the peak occurs.

4. The online detection method for trivalent chromium ion concentration based on electrochemical characteristic parameters according to claim 3, characterized in that: The effective nucleation site density N0 is calculated using the following formula: in For electron transfer number, It is Faraday's constant. denoted as , where is the molar concentration of hexavalent chromium ions in the hard chromium plating solution, and D is the diffusion coefficient. For peak current density, This is the peak time.

5. The online detection method for trivalent chromium ion concentration based on electrochemical characteristic parameters according to claim 1, characterized in that: In step (a), when establishing the quantitative calibration relationship, the trivalent chromium ion concentration range is used for segmented fitting. The low concentration range of 1~4 g / L is fitted with linear or polynomial fitting, and the high concentration range of 4~16 g / L is fitted with linear or power function fitting.

6. The online detection method for trivalent chromium ion concentration based on electrochemical characteristic parameters according to claim 1, characterized in that: Simultaneously extract two or more feature parameters and jointly determine the trivalent chromium ion concentration by ratio or weighting method to eliminate the ambiguity of a single parameter in the non-monotonic range.

7. The online detection method for trivalent chromium ion concentration based on electrochemical characteristic parameters according to claim 1, characterized in that: In the three-electrode system, the working electrode is a platinum disk electrode or a glassy carbon electrode, the reference electrode is a saturated calomel electrode or a silver / silver chloride electrode, and the counter electrode is a platinum sheet electrode.

8. The online detection method for trivalent chromium ion concentration based on electrochemical characteristic parameters according to claim 1, characterized in that: In step (a), the temperature of the standard hard chrome plating solution is controlled at 50~60℃, and the temperature is kept constant during the measurement process.

9. The online detection method for trivalent chromium ion concentration based on electrochemical characteristic parameters according to claim 1, characterized in that: The online detection described in step (b) is performed in an automated manner, and the detection results are used to trigger the plating solution purification device or alarm device.

10. An online detection device for trivalent chromium ion concentration based on electrochemical characteristic parameters, characterized in that: The device includes an electrochemical measurement unit with a three-electrode probe and an electrochemical workstation, a data acquisition and control unit, a human-machine interface, and an optional purification device interface. The online detection device for trivalent chromium ion concentration uses the method described in any one of claims 1-9 for online detection.

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