OPTIMIZATION OF BUFFER CONDITIONS FOR THE CYCLIC VOLTAMMETRIC DETERMINATION OF Co(II) IONS

ABSTRACT

In this study, the effect of supporting electrolytes on the determination of Co(II) ions in the presence of o-nitrosophenol was investigated using cyclic voltammetry. Experiments were carried out at a silver/mercury amalgam working electrode versus Ag/AgCl. The influence of different types of supporting electrolytes, including acetate, phosphate, and nitrate buffer systems, as well as their pH values, on the formation of the Co(II)–o-nitrosophenol complex was systematically evaluated. The results demonstrated that both the type of supporting electrolyte and the acidity of the medium significantly affect the peak current and peak potential. The highest sensitivity and the most stable voltammetric response were observed in acetate buffer at pH 5.1. Under these conditions, the detection limit of Co(II) was minimized, and repeatability showed high accuracy. These findings indicate that the proper selection of supporting electrolyte is a critical factor in enhancing the sensitivity and selectivity of Co(II) ion determination by electrochemical methods.

АННОТАЦИЯ

В настоящей работе изучено влияние фоновый электролитов на определение ионов Co(II) в присутствии о-нитрозофенола методом циклической вольтамперометрии. Эксперименты проводились на рабочем электроде из амальгамы серебро/ртуть относительно Ag/AgCl. Систематически оценивалось влияние различных типов фоновых электролитов, включая ацетатные, фосфатные и нитратные буферные системы, а также их значения pH на образование комплекса Co(II)–о-нитрозофенол. Результаты показали, что как тип фонового электролита, так и кислотность среды существенно влияют на пик тока и потенциал пика. Наибольшая чувствительность и наиболее стабильный вольтамперометрический сигнал наблюдались в ацетатном буфере при pH 5,1. В этих условиях предел обнаружения Co(II) был минимальным, а повторяемость обеспечивала высокую точность. Полученные данные свидетельствуют о том, что правильный выбор фонового электролита является ключевым фактором для повышения чувствительности и селективности определения ионов Co(II) электрохимическим методом.

Keywords: Cyclic voltammetry, Co(II) ion, supporting electrolytes, buffer solution, o-nitrosophenol complex, peak current.

Ключевые слова: Циклическая вольтамперометрия, ион Co(II), фоновые электролиты, буферная среда, комплекс о-нитрозофенола, пик тока.

Introduction

Cobalt is an essential trace element that belongs to the group of heavy metals [1]. One of the primary sources of cobalt in the biosphere for living organisms is natural water. The need to monitor its concentration in natural waters is associated with several important factors.

In particular, cobalt is a structural component of vitamin B₁₂ and a specific activator of several enzymes. It occupies a central position in the vitamin B₁₂ molecule, accounting for approximately 4.5% of its mass. Cobalt stimulates hematopoiesis, participates in the synthesis of heme from protoporphyrin and iron (Fe), and helps prevent anemia. The concentration of cobalt in human blood ranges from 0.01 to 0.91 ng mL⁻¹. Exceeding these levels may lead to the development of various pathological conditions [2].

The toxicity of cobalt and some of its compounds has been established, and in certain cases the metal exhibits carcinogenic effects [3; 4]. Cobalt can contribute to the development of malignant tumors; therefore, it has been included in the list of carcinogens by the International Agency for Research on Cancer (IARC) of the World Health Organization. At the same time, certain cobalt compounds are used in anticancer therapy[5].

Currently various methods and techniques have been elaborated to detect heavy metals, particularly cobalt, which include flame atomic absorption spectroscopy (FAAS) [6], electrothermal atomic absorption spectrometry [7], colorimetric detection method [8], gravimetric method [9], photometry [10], extended Raman spectroscopy [11], atomic fluorescence spectrometry [12], laser ablation spectrometry [13].

However, these methods require sophisticated instrumentation and specialized laboratory conditions. Electrochemical techniques, particularly cyclic voltammetry, enhance the selectivity and lower the detection limits for metal ions when organic ligands are employed. In this approach, the formation of a metal–ligand complex determines both the shape and the potential of the voltammetric peak. The complexation process, in turn, is directly influenced by the composition of the solution medium, especially the type of supporting electrolyte and its pH.

The supporting electrolyte stabilizes the ionic strength of the solution, establishes the structure of the electrical double layer, and affects the charge distribution in the layer adjacent to the electrodes. The nature and concentration of the supporting electrolyte have been shown to significantly influence the peak current intensity, the half-wave potential, and the diffusion processes.

In cyclic voltammetry, for a diffusion-controlled process, the peak current is described by the Randles–Ševčik equation:

I𝑝= (2.69x10⁵) 𝑛^3/2 𝐴 C 𝐷^1/2𝑣^1/2                                         (1)

Here, D is the diffusion coefficient, which depends on the ionic strength of the solution and the nature of the supporting electrolyte. Therefore, changes in the supporting electrolyte environment lead to shifts in voltammetric parameters and variations in the signal intensity.

Based on the above, a systematic study of the effect of supporting electrolytes on the cyclic voltammetric determination of Co(II) ions is highly relevant.

The aim of the present work is to investigate the influence of supporting electrolytes on the cyclic voltammetric determination of Co(II) ions in the presence of o-nitrosophenol using a silver–mercury amalgam film electrode and to determine the optimal medium for the analysis.

Materials and Methods

o-Nitrosophenol (L) (Sigma-Aldrich, China), acetic acid (70.0%), sodium hydroxide (99.0%), cobalt(II) nitrate hexahydrate (99.95%), nitric acid (99.95%), metallic mercury (P-00), and potassium chloride (99.0%) were used.

All other chemicals were of analytical grade and used without further purification. The pH of the working solutions was adjusted using acetic acid or sodium hydroxide.

A CS350 potentiostat, 30 mL quartz electrochemical cell, silver–mercury amalgam working electrode (AmE), Ag/AgCl reference electrode, platinum counter electrode (Pt), pH meter (Bante 210), digital magnetic stirrer (BIOBASE MS7-H550-S), bidistilled water system (EASYpure LF, Barnstead), and analytical balance (BIOBASE) were used.

Results and Discussion

Experimental Conditions

Cyclic voltammetric measurements were performed using a three-electrode electrochemical system: a silver–mercury amalgam film working electrode (AmE), an Ag/AgCl reference electrode in 1 M KCl, and a platinum wire counter electrode. Experiments were conducted under an inert nitrogen atmosphere.

A 0.01 M Co²⁺ standard solution was prepared from Co(NO₃)₂·6H₂O crystals and standardized by complexometric titration with EDTA. Working solutions were obtained by appropriate dilution of the stock solution. A 0.1 M acetate buffer was prepared using acetic acid and sodium hydroxide.

Cyclic voltammograms were recorded at room temperature in 0.1 M acetate buffer at pH 5.1 with the Co²⁺ standard solution (total volume 25 mL). The potential was scanned in the range of 40–500 mV at a scan rate of 5–11 mV/s, with a cobalt ion accumulation time of 10 s. The solution was refreshed after every 10 measurements, and the results demonstrated good reproducibility.

Supporting Electrolytes and the Electrochemical Behavior of Co²⁺ Ions

Supporting electrolytes provide the solution with electrical conductivity, reduce ohmic resistance, and maintain the stability of the actual potential between the electrodes. To evaluate the electrochemical behavior of cobalt(II) ions, cyclic voltammograms were recorded in various supporting electrolyte media: NaOAc (pH ~5.1–5.2), HCl (pH ~1.2), H₂SO₄ (pH ~2), NH₄Cl (pH ~9), (NH₄)₂SO₄ (pH ~6), and a H₃PO₄ + KNO₃ mixture (pH ~7.2).

The results indicated that the highest and most well-defined analytical signal was observed in 0.1 M acetate buffer (pH 5.1). Under slightly acidic conditions, Co²⁺ ions remain stably solvated in the solution and accumulate efficiently on the electrode surface, resulting in more intensive redox processes and an increased peak current. As the pH increased to 6.0–6.5, the signal decreased, which can be attributed to the formation of cobalt hydroxo-complexes or partial precipitation of Co²⁺ ions at higher pH values (Equations 2 and 3).

This significantly reduces the formation of the complex with the reagent.

In strongly acidic media (HCl, H₂SO₄), the decrease in signal is explained by the high proton concentration, which limits the formation of reactive Co²⁺ species. In media containing NH₄⁺ and PO₄³⁻ ions, the electron transfer process slows down, manifesting as voltammetric peaks with small amplitudes.

To evaluate the effect of acetate buffer pH on the voltammetric response of Co²⁺ ions, cyclic voltammograms were recorded in the pH range of 5.0–6.5. The experimental results are presented in Figures 1 and 2.

Figure 1. Influence of pH on the complexation of cobalt ions under CV conditions (accumulation time (τₐcc): 10 s, preconcentration potential 50 mV). Experimental parameters: Co²⁺ ion concentration of 0.040 μM, o-nitrosophenol concentration of 2.0 μM, with a metal-to-ligand molar ratio of 1:50, and an acetate buffer as the supporting electrolyte with pH values of 5.1, 5.2, 5.5, 6.0, 6.2, and 6.5.

The optimal pH range for the determination of cobalt ions was found to be 5.0–5.5, corresponding to a slightly acidic medium. As the pH increased from 5.0 to 6.5, a gradual decrease in the peak current was observed [14]. At pH 5.1, the analytical signal reached its maximum, indicating favorable conditions for the stable electrochemical behavior of Co²⁺ ions. The significant decrease in signal at higher pH values (6.0–6.5) is attributed to the precipitation of Co²⁺ ions and the limited kinetics of the complexation reaction.

Figure 2. Effect of pH on the peak current (Iₚ) under cyclic voltammetry conditions.

Therefore, to optimize the kinetics of complex formation between Co²⁺ ions and the analytical reagent, as well as to ensure reliable electrochemical analysis, an acetate buffer at pH 5.1 and an accumulation time of 10 s were selected as the optimal conditions.

Conclusion

The electrochemical behavior of cobalt(II) ions was studied using cyclic voltammetry. The results showed that 0.1 M acetate buffer at pH 5.1 is the most suitable supporting electrolyte for the determination of Co²⁺ ions, providing stable accumulation on the electrode surface and a high peak current. The decrease in signal under higher or lower pH conditions is associated with the precipitation of Co²⁺ ions or kinetic limitations. Thus, the results obtained at pH 5.1 are recommended as the optimal conditions for the determination of cobalt(II) ions.

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