Electrochemical Characterization and Voltammetric Determination of Methylisothiazolinone on a Boron-Doped Diamond Electrode

At a Glance

Metadata	Details
Publication Date	2022-12-17
Journal	Molecules
Authors	Magdalena Jakubczyk, Sławomir Michałkiewicz, Agata Skorupa, Kinga Krajcarz
Institutions	Holy Cross University, Jan Kochanowski University
Analysis	Full AI Review Included

Executive Summary

This study presents a novel, highly sensitive, and simplified electrochemical method for determining Methylisothiazolinone (MIT), a widely regulated preservative, using a Boron-Doped Diamond Electrode (BDDE).

Novelty: First reported voltammetric procedure utilizing a BDDE in a citrate-phosphate buffer (C-PB) environment for MIT determination.
Mechanism: MIT undergoes irreversible, diffusion-controlled anodic oxidation involving a two-electron exchange (n=2) at the BDDE surface, yielding sulfoxides and sulfones.
Performance: Achieved a low Limit of Detection (LOD) of 0.24 mg L^-1, which is competitive with or superior to many published chromatographic and voltammetric methods.
Linear Range: The method provides a wide linear concentration range from 0.7 to 18.7 mg L^-1 with excellent linearity (r = 0.9999).
Practical Utility: Successfully applied for accurate and precise determination of MIT in complex matrices (household products) without requiring time-consuming separation or extraction steps.
Green Chemistry: The procedure utilizes robust BDDE material and aqueous buffer solutions, minimizing reagent consumption and waste.

Technical Specifications

Parameter	Value	Unit	Context
Working Electrode Material	Boron-Doped Diamond	-	BDDE (3 mm diameter)
Optimal Buffer Type	Citrate-Phosphate Buffer	-	McIlvaine buffer (C-PB)
Optimal pH	5.6	-	Condition for maximum peak current
Anodic Peak Potential (E_p)	1.535 ± 0.005	V	vs. Ag/AgCl reference electrode
Oxidation Mechanism	Irreversible, Diffusion-controlled	-	Confirmed by CV analysis
Electron Exchange (n)	2	electrons	Total number of electrons transferred
Linear Concentration Range	0.7 to 18.7	mg L^-1	DPV Calibration Range
Limit of Detection (LOD)	0.24	mg L^-1	Calculated using 3.29 S_b/a
Correlation Coefficient (r)	0.9999	-	Linearity of calibration plot
Intra-day Precision (RSD)	0.6	%	Excellent repeatability (n=10)
Inter-day Precision (RSD)	1.1	%	High long-term stability
DPV Amplitude (dE)	50	mV	Optimized measurement parameter
Pulse Width	80	ms	Optimized measurement parameter
Transfer Coefficient (alpha)	0.51 to 0.61	-	Range across tested scan rates (0.025 to 0.500 V s^-1)
Recovery (R) in Real Sample	99.0-99.5	%	Accuracy validation

Key Methodologies

The determination of MIT relied on Differential Pulse Voltammetry (DPV) coupled with rigorous BDDE preparation and optimization in an aqueous buffer system.

Electrode Pre-treatment: The BDDE surface was mechanically polished using 0.01 µm alumina powder slurry, followed by sonication.
Electrochemical Activation: Daily cathodic activation was performed in 1 mol L^-1 H₂SO₄ at a potential of -2.4 V for 5 minutes to ensure maximum surface activity and reproducibility.
Electrolyte Selection: Citrate-Phosphate Buffer (C-PB) at pH 5.6 was selected as the optimal supporting electrolyte based on maximizing the MIT oxidation peak current and achieving the best signal shape.
Voltammetric Optimization: DPV parameters were optimized for sensitivity and resolution: Amplitude (dE) = 50 mV, Potential Step (E_s) = 5 mV, and Pulse Width = 80 ms.
Mechanism Confirmation (CV): Cyclic Voltammetry confirmed the process was irreversible (absence of cathodic peak) and diffusion-controlled (linear relationship between I_p and v^1/2, with log I_p vs. log v slope of 0.41).
Quantification: MIT concentration was determined using the multiple standard addition method, measuring the DPV peak current as a function of added standard MIT solution.
Sample Preparation: Sample preparation was simplified, requiring only the dissolution of the household product (e.g., washing liquid) directly into the C-PB buffer solution, eliminating complex extraction or clean-up steps.

Commercial Applications

The developed BDDE-based voltammetric method offers a robust, fast, and cost-effective alternative to traditional analytical techniques for quality control and environmental monitoring of isothiazolinone preservatives.

Cosmetics and Personal Care QC: Rapid determination of MIT concentrations in rinse-off products (shampoos, soaps) to ensure compliance with strict regulatory limits (e.g., 0.0015% maximum concentration in the EU).
Household and Industrial Product Manufacturing: Quality assurance testing for biocides in water-based paints, adhesives, cooling water systems, and cleaning agents, where MIT is commonly used.
Environmental Analysis: Monitoring water quality and detecting MIT residues in industrial wastewater or effluents, offering a simpler, field-deployable alternative to laboratory-based HPLC-MS/MS systems.
BDDE Sensor Technology: This application highlights the superior stability, low residual current, and wide potential window of BDDEs, making them ideal for the oxidation of difficult organic compounds like sulfur-containing biocides.
Green Analytical Chemistry: Adoption by laboratories seeking methods that minimize solvent use and sample preparation time, aligning with sustainability goals.

View Original Abstract

The electrochemical properties of methylisothiazolinone (MIT), the most widely used preservative, were investigated by cyclic (CV) and differential pulse voltammetry (DPV) to develop a new method for its determination. To our knowledge, this is the first demonstration of a voltammetric procedure for the determination of MIT on a boron-doped diamond electrode (BDDE) in a citrate-phosphate buffer (C-PB) environment. The anodic oxidation process of methylisothiazolinone, which is the basis of this method, proved to be diffusion-controlled and proceeded with an irreversible two-electron exchange. The radical cations, as unstable primary products, were converted in subsequent chemical reactions to sulfoxides and sulfones, and finally to more stable final products. Performed determinations were based on the DPV technique. A linear calibration curve was obtained in the concentration range from 0.7 to 18.7 mg L−1, with a correlation coefficient of 0.9999. The proposed procedure was accurate and precise, allowing the detection of MIT at a concentration level of 0.24 mg L−1. It successfully demonstrated its suitability for the determination of methylisothiazolinone in household products without the need for any separation steps. The proposed method can serve as an alternative to the prevailing chromatographic determinations of MIT in real samples.

Tech Support

Original Source

DOI: https://doi.org/10.3390/molecules27249013

References

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