Sensitive Electrochemical Detection of the Nitrite Ion Using an ISEM-3 Graphite Electrode and Comparison with Other Carbon-Containing Materials ^†

Abstract

The need for an accurate, rapid, and affordable method for determining nitrite ions arises from their toxic effects on humans at elevated levels in wastewater and drinking water. The electrochemical determination is faster, cheaper, and less labor-intensive. It is based on the study of the electrochemical oxidation of NO₂⁻ ions at different carbon electrodes. In this work, it was established that the cyclic voltammograms for the ISEM-3 graphite electrode have an excellent limit of detection for nitrite ions: 5 × 10⁻⁶ M at pH 3, which makes it possible to determine the NO₂⁻ content below the maximum permissible concentration (6.5 × 10⁻⁵ M) in water.

1. Introduction

Nitrite, an important intermediate in the nitrogen cycle in ecosystems, performs many functions in various industries: It is used in agriculture as fertilizer and in various food products as a preservative in the production and processing of pickled meat and fish products; it is also found in tap water and biological samples [1,2]. Excessive nitrite content harms the environment and has a negative impact on human health [1]. As a result of the interaction of nitrites with amine compounds in the human body, the content of carcinogenic nitrosamines increases, which causes serious health problems, including cancer of the stomach and esophagus, and congenital disabilities of the central nervous system [2]. NO₂⁻ ions also promote the conversion of hemoglobin into methemoglobin, which can lead to a serious illness [3].

Therefore, in recent decades, the determination of nitrites by quantitative analysis has been of considerable interest. The World Health Organization has established that the limit value for nitrite concentration in drinking water is only 3.0 mg L⁻¹, and for fisheries and reservoirs the norm is 0.08 mg L⁻¹ [1]. However, most determination methods have limitations, such as the use of hazardous reagents, the need for a time-consuming sample preparation procedure that requires technical personnel and expensive equipment [3]. Electrochemical methods (cyclic voltammetry, amperometry, coulometry, potentiometry) offer significant advantages over traditional analytical methods, including cost-effectiveness, speed, and ease of operation [2,4]. The electrochemical sensors show high selectivity to NO₂⁻ in the presence of the interference of some common ions (e.g., NO₃⁻, CO₃²⁻, SO₄²⁻, Cl⁻, Ca^2+, and Mg²⁺) and oxidizable compounds, including sodium sulfite and ascorbic acid [5]

In the electrochemical detection of nitrite ions, the most sensitive mechanism is based on the direct electrocatalytic oxidation of the nitrite ion at the electrode surface, resulting in the formation of nitrate [2]. The rate of the oxidation reaction and the oxidation potential depend on the kinetics of electron transfer and the electroactive ability of the electrode materials [6]. The oxidation of nitrite ions is shown by the following reaction in acidic and alkaline environments:

HNO₂ + H₂O − 2e⁻ = NO₃⁻ + 3H⁺   E₀ = 0.94 V vs. RHE

(1)

NO₂⁻ + 2OH⁻ − 2e⁻ = NO₃⁻ + H₂O   E₀ = 0.01 V vs. RHE

(2)

The recent developments for electrochemical detection of nitrate, nitrite, and ammonium are discussed, and the critical examination of current nitrate, nitrite, and ammonium studies as realistic monitoring processes is presented in the short modern review [7].

Due to the use of various functional nanomaterials in electrochemical sensors, the sensitivity and accuracy of electrochemical measurements are significantly increased, which is of great importance for the field of analytical chemistry [6,8,9]. Carbon materials as a nitrite ion sensor have a number of advantages: Good electrical conductivity, low cost, ease of use, wear resistance, and stability in various environmental conditions. Various carbon-based electrodes for the determination of nitrite ions have been extensively studied in recent years: glassy carbon [10], glassy carbon modified with polyvinylimidazole [11], and multi-walled carbon nanotubes [12].

In most studies, the nitrite ion determination reaction was carried out on glassy carbon-based materials, but the oxidation of nitrite on a bare graphite electrode has been less studied. In addition, there is little data on a full-fledged study of graphite across a wide range of nitrite ion concentrations and electrolyte pH.

Thus, the aim of this work is the electrochemical determination of nitrite ions on a graphite electrode, and the tasks included: screening of carbon electrodes (graphite and glassy carbon and graphite modified with Ag particles) using cyclic voltammetry to determine nitrite ions; determination of the detection limit of nitrite ions in solutions; determination of the area of linear dependence of current-concentration at concentrations of nitrite ions in the maximum concentration range.

2. Materials and Methods

The ISEM-3 graphite was used as the main working electrode. The oxidation of nitrite ion was studied by cyclic voltammetry. The synthesis of an additional graphite electrode with electrodeposited Ag particles was carried out at constant potential, and the electrode was characterized by SEM. All reagents (AgNO₃, NaNO₂) used were of analytical grade, unless otherwise indicated. Distilled water was used to prepare all reagents and solutions.

The Autolab 302N potentiostat–galvanostat equipped with Nova 2.1.7 (Metrohm, Herisau City, Netherlands-Switzerland) software was used for all electrochemical experiments. The silver chloride electrode (Ag/AgCl) served as a reference electrode, and the auxiliary electrode was a platinum plate (S = 2 cm²). A three-electrode standard electrochemical cell was filled with 50 mL of electrolyte and degassed with Ar (99.999%) for 30 min. The polarization curves were recorded by cyclic voltammetry (CV) with a scanning rate specified in the following sections. Cyclic voltammograms were measured without mixing.

The carbon-containing materials under study were the working electrode. The geometric surface area (in the form of a disk) was as follows: 0.198 cm² (ISEM-3 graphite), 0.097 cm² (glassy carbon), 0.18 cm² (graphite coated with electrodeposited Ag particles). ISEM-3 graphite and glassy carbon electrodes were used as is, and graphite coated with electrodeposited Ag particles was prepared according to the original technique: Ag particles were deposited potentiostatically (−0.6 V vs. Ag/AgCl) from a solution containing 5 mM AgNO₃ in 0.1 M KNO₃ for 300 s.

3. Results and Discussion

3.1. Determination of Nitrite Ions on Various Types of Carbon Electrodes

This study demonstrates that a graphite electrode is well-suited for the determination of nitrite ion in aqueous solutions using cyclic voltammetry at different pH levels of electrolytes, and the limit of detection (LOD) was 5 µM. The oxidation of nitrite ion is compared with the electrodes: glassy carbon (GC), ISEM-3 graphite, and graphite coated with electrodeposited Ag-particles. The studies showed that the graphite electrode exhibited high sensitivity over a wide range of solution pH (1−10). The best results and the minimum NO₂⁻ detection concentration were achieved at pH 3.

Figure 1 shows the voltammograms for the determination of nitrite ions for three electrode samples at the test boundary concentrations. For both high and low concentrations of nitrite ions, the ISEM-3 graphite sample shows the most pronounced peak. The graphite coated with electrodeposited Ag-particles shows the highest sensitivity at high concentrations but is inferior at low concentrations. For the GC sample, the peaks are not clearly pronounced at any of the concentrations studied.

3.2. Determination of Nitrite Ions on ISEM-3 Graphite Electrode Sensor

An excellent, pronounced peak for nitrite ion oxidation when using the ISEM-3 graphite electrode is shown in Figure 2a. With increasing concentrations, only a slight potential shift occurs, and the clearly linear dependence of the peak current density on an increase in nitrite ion concentrations, which is illustrated in Figure 2b.

3.3. Scan Rate Study

In the final part of our study, cyclic voltammogram measurements were carried out over a wide range of potential scan rates (Figure 3a). The linear dependence of the peak current density on the square root of the potential scanning rate is proof of a diffusion-controlled process (Figure 3a).

The interference of potentially interfering ions in real water is NO₃⁻, CO₃²⁻, SO₄²⁻, Cl⁻. Based on the study of the electrooxidation of nitrite in solutions with different pH, it can be concluded that 100-fold Ca²⁺, SO₄²⁻, K⁺, PO₄³⁻, CO₃²⁻, NO₃⁻, Cl⁻, and plenty of Ac⁻ and Na⁺ do not affect the accuracy of nitrite ion determination. An application to a water sample in this work was not carried out.

4. Conclusions

By using the CV method, it was found that the ISEM-3 graphite electrode has excellent sensitivity to nitrite ions in aqueous solution, and its use for the determination of nitrite ions in solutions is possible in a wide range of pH values (1–10), the best of which was pH 3. The LOD for a graphite electrode is 5 × 10⁻⁶ M at pH 3, which makes it possible to determine the NO₂⁻ content below the maximum permissible concentration (6.5 × 10⁻⁵ M) in water.

As a perspective for our future work, it can be assumed that graphite- and glassy-carbon-based electrodes can be optimized by modifying with carbon nanotubes, which, according to literature data and preliminary experiments, can further enhance sensitivity to nitrite ions.