DETERMINATION OF ZINC (II) ION USING A SOLUTION OF 1-(2-HYDROXY-1-NAPHTHAZO)-2-NAPHTHOL-4-SULFONIC ACID

ISSN: 2181-3906 2025 International scientific journal «MODERN SCIENCE АND RESEARCH» VOLUME 4 / ISSUE 12 / UIF:8.2 / MODERNSCIENCE.UZ 165 DETERMINATION OF ZINC (II) ION USING A SOLUTION OF 1-(2-HYDROXY-1NAPHTHAZO)-2-NAPHTHOL-4-SULFONIC ACID Tursunqulov Jasur Baxtiyorovich PhD in Chemical Sciences, Associate Professor, Non-State Educational Organization “Alfraganus University” Department of Pharmacy and Chemistry, Republic of Uzbekistan, Tashkent. https://doi.org/10.5281/zenodo.17842268 Abstract. A method for the determination of the zinc (II) ion using a solution of 1-(2hydroxy-1-naphthazo)-2-naphthol-4-sulfonic acid (HNNS) by extraction-spectrophotometric and cyclic voltammetric techniques is proposed. The optimal conditions were studied: distribution coefficient 0.98, choice of organic solvent, the ratio of organic to aqueous phases of 1:8, standard potential -0.51 V, and the detection limit of 0.387 µg/L. The reagent shows light absorption at 500 nm [1], contrast equal to 100 nm, and potential contrast of 0.98 V, which demonstrates the high sensitivity of the developed method. The developed method was compared with other analytical techniques and proved to be not inferior to them. Keywords: extraction-spectrophotometric method, distribution coefficient, standard potential, 1-(2-hydroxy-1-naphthazo)-2-naphthol-4-sulfonic acid (HNNS), zinc. Introduction From the literature, it is known that zinc is a relatively rare element in nature and is often associated with iron, silver, and occasionally gold. Zinc is used as a semi-precious metal in many industries. For example, it finds application in the production of electrical cables, electronic devices, corrosion-resistant alloys, jewelry, and other industrial areas. Currently, it is important to identify and extract zinc ions from environmental objects. For instance, during the extraction of precious metals, copper ions are released as a by-product in technological solutions. Detecting and extracting them is considered a relevant scientific and industrial problem. Zinc ion has been determined using various organic reagents, such as 1-((4-(1-(2hydroxyphenylimino)ethyl)phenyl))diazenyl)naphthalen-2-ol (HPEDN) [3], 1-phenyl-2-(2hydroxy-4-nitrophenylhydrazo)butadione-1,3 [6], 1-(2-pyridylazo)-2-naphthol (PAN) [4], N,N'- bis (salicylidene)-2,3-diaminopyridine (H₂IF) [7], (2,4-dinitrophenol-(6-azo-2)-1-naphthol-3,8disulfonic acid) [2], disodium salt of 4-hydroxy-3-(4-sulfonato-1-naphthylazo)-1naphthalenesulfonate (4-GSNNS) [5], and N-benzoyl-N'-(phenylsulfonyl)hydrazine [1], using optical, electrochemical, colorimetric, ratiometric fluorescent, and other methods. At present, the development of economically accessible and efficient analytical methods is considered essential. The purpose of this article is to develop an extraction-spectrophotometric and cyclic voltammetric method for the detection of zinc (II) ion using the organic reagent 1-(2hydroxy-1-naphthazo)-2-naphthol-4-sulfonic acid. ISSN: 2181-3906 2025 International scientific journal «MODERN SCIENCE АND RESEARCH» VOLUME 4 / ISSUE 12 / UIF:8.2 / MODERNSCIENCE.UZ 166 Experimental Part The Zn(II) ion forms a complex compound with HNSA in 0.1 M sulfuric acid and can be extracted from the solution using organic solvents such as toluene, chloroform, and benzene (“chemically pure” or “analytical grade”). Electronic spectra of the zinc (II) ion complexes with HNSA were recorded using a UVVIS spectrophotometer EMC-30PC-UB. Electrochemical studies were carried out using a potentiostat-galvanostat of the CSPotentiostat/Galvanostat series, which allows cyclic voltammetric measurements. A platinum wire served as the working electrode, and a saturated silver chloride electrode as the reference electrode. pH-metric measurements and potentiometric titrations were performed using a pH-Ox-Red meter P25 Ecomet (Korea). Results and Discussion It is known that the formation of the complex depends on the nature of the solvent, the acidity of the solution, and the distribution coefficient of Zn(II) between the aqueous and organic phases. Previous studies have shown that the electron density of the organic reagent is 0.203–0.247 for hydroxyl groups in the ring and 0.070–0.042 for the -N=Ngroup [11]. Metal ions form chemical bonds primarily with these functional groups. The reason for using oxygen-free solvents—chloroform, benzene, and toluene—as extractants lies in their immiscibility with water and their differing dissociation constants and complex stability compared to the organic reagent. The maximum light absorption of a 0.1% alcohol solution of the reagent 1-(2-hydroxy-1naphthazo)-2-naphthol-4-sulfonic acid (HNNS) was measured on a spectrophotometer. In a 50 ml separatory funnel, 1.0 ml of Zn(II) solution with a concentration of 65 µg/ml, 2.0 ml of HNNS reagent, 3.0 ml of 0.1 M sulfuric acid, 12.0 ml of distilled water, and 2.0 ml of chloroform, toluene, or benzene (as extractants) were added. The mixture was shaken for 2 minutes and allowed to separate. The organic layer was isolated using the separatory funnel, and the optical density of the resulting complex was measured (λmax = 600 nm, l = 1.0 cm). The obtained data are presented in Table 1. This procedure was repeated with other extractants—toluene and benzene. Table 1. Optical density of the complex compound extracted with various solvents Extractant Chloroform Benzene Toluene Аmax(600 nm) 0,31 0,25 0,28 The degree of extraction of the Zn complex with 1-(2-hydroxy-1-naphthazo)-2-naphthol-4sulfonic acid was found to be the highest in chloroform. For this reason, chloroform was chosen as the extractant for the separation of Zn with 1-(2-hydroxy-1-naphthazo)-2-naphthol-4-sulfonic acid. The volumetric ratio of the aqueous and organic phases from 1:8 to 1:9 does not lead to a decrease in the extraction level of the elements. The degree of extraction of Zn with 1-(2-hydroxy-1-naphthazo)-2-naphthol-4-sulfonic acid was measured and is presented in Table 2. ISSN: 2181-3906 2025 International scientific journal «MODERN SCIENCE АND RESEARCH» VOLUME 4 / ISSUE 12 / UIF:8.2 / MODERNSCIENCE.UZ 167 Table 2. Dependence of the extraction degree (R%) of Zn(II) ion on the volumetric ratio of aqueous and organic phases Ratio of organic to aqueous phases (Vo: Vs) 1:1 1:2 1:6 1:8 1:9 1:10 1:11 1:15 1:20 Zn(II), R% (Extraction degree) 95 96 97 98 98 97 96 95 95 It was found that the optimal ratio of aqueous to organic phases for the extraction of zinc ions using 1-(2-hydroxy-1-naphthazo)-2-naphthol-4-sulfonic acid is 1:8. To study the dependence of the composition of the complex compound on the amount of added reagent, the following procedure was carried out: 1.0 ml of Zn(II) solution with a concentration of 65 µg/ml, various volumes of 0.1% alcoholic solution of 1-(2-hydroxy-1naphthazo)-2-naphthol-4-sulfonic acid (0.5; 1.0; 1.5; 2.0; 2.5; and 3.0 ml), 3.0 ml of 0.1 M sulfuric acid solution, 12.0 ml of distilled water, and 2.0 ml of chloroform solution were placed into a 50 ml separatory funnel. The optical densities of the resulting solutions were studied and are presented in Table 3. Table 3. Effect of reagent amount on optical density (CZn(II) = 65 µg/ml; λmax = 600 nm; l = 1.0 cm; extractant – chloroform; CM = 0.1 M H₂SO₄) Volume of 1-(2-hydroxy-1naphthazo)-2-naphthol-4-sulfonic acid solution (ml) 0,5 1 1.5 2 2.5 3 Optical Density (A) 0,177 0,231 0,281 0,295 0,285 0,278 From the research results, it can be observed that the optical density increased with the increasing amount of reagent in the mixture. As a result of the interaction between 1.0 ml of Zn(II) solution containing 65.00 µg/ml Zn and 2.0 ml of 0.1% solution of 1-(2-hydroxy-1-naphthazo)-2naphthol-4-sulfonic acid, the optical density reached its maximum value. A decrease in optical density was observed when the amount of reagent exceeded 2.0 ml. The molar ratio of the complex compounds was determined using the Bent–French method [4]. The results are shown in Figure 1. ISSN: 2181-3906 2025 International scientific journal «MODERN SCIENCE АND RESEARCH» VOLUME 4 / ISSUE 12 / UIF:8.2 / MODERNSCIENCE.UZ 168 Figure 1. Bent–French plot for determining the molar ratios of the HNNS reagent with Zn(II) ion complexes From Figure 1, it can be concluded that the complex compound of Zn(II) ion with 1-(2hydroxy-1-naphthazo)-2-naphthol-4-sulfonic acid is formed in a 1:1 molar ratio (Me:R = 1:1). The formation of a Zn complex can also be confirmed using cyclic voltammetry, which is one of the most informative electrochemical analysis methods. It allows simultaneous examination of both the anodic and cathodic branches of the polarogram and determination of the potentials of the maximum anodic and cathodic currents. From these values, one can evaluate the reversibility of the electrochemical system and the possible mechanism of electron transfer. It is known that the standard potential of the HNNS reagent is 0.47 V [10], while the standard potential of the complex formed with Zn(II) ion is –0.51 V. The shift of the standard potential toward the negative region confirms that the complex is formed. The half-wave potential slope of 0.058 V indicates that the number of electrons involved in the reaction is 1, i.e., the metal-to-reagent ratio is 1:1. The lower detection limit of Zn²⁺ ions was determined using the following formula: εreal – true molar extinction coefficient; V – volume of the solution (25 ml); B – atomic mass of the element (Zn = 65.00 g); l – optical path length (1.0 cm); M – number of zinc atoms included in the complex; SA – standard deviation (SA=0.001); Qmin– detection limit. Based on the obtained data and calculations, it was established that the lower detection limit (Qmin) for zinc ions is 0.387 µg/L. Table 4. Comparison with previous studies on the determination of Zn(II) № Organic reagent Метод Образцы Предел обнаружения Источник 1 2-aminoterephthalic acid (NH₂-H₂BDC) Highly fluorescent Milk samples 9,01 мкМ [9] 2 Rice husk-based ionimprinted polymer (RHCIIP) Fluorescent chromophore Lake water 4,12 мкг /л [12] 3 1-(2-hydroxy-1naphtoazo)-2-naphthol-4sulfonic acid Extractionspectrophotometric and electrochemical Industrial water 0.387мкг /л This work From the table above, it is evident that the method developed by the authors is not inferior to other existing analogs. ISSN: 2181-3906 2025 International scientific journal «MODERN SCIENCE АND RESEARCH» VOLUME 4 / ISSUE 12 / UIF:8.2 / MODERNSCIENCE.UZ 169 Conclusion The results obtained show that the developed extraction–spectrophotometric and electrochemical methods are recommended for the detection and extraction of Zn(II) ions from solutions using the reagent 1-(2-hydroxy-1-naphtoazo)-2-naphthol-4-sulfonic acid in the presence of H₂SO₄ acid. It was found that chloroform is the best extractant, with a distribution coefficient of 0.98 in chloroform. The lower detection limit for copper ions is sufficient, and the contrast indicates the high sensitivity of the developed method. It has been established that the proposed method is comparable to other existing analogs in terms of effectiveness. References: 1. Kumar S., Patel R. (2022). Spectrophotometric detection of Zn(II) with azo ligands. J. Anal. Chem., 77(4), 621–630. 2. Lv X., Hu H., Yao L. et al. (2023). 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