Analysis of the Optimal Conditions for the Immobilization of Nitroso-R Salt on Synthetic Fibers

Accurate quantification of metal ions is critical in diverse fields including medicine, ecology, and food science [1]. Conventional techniques such as inductively coupled plasma-mass spectrometry, X-ray absorption spectroscopy, electroanalytical methods, and atomic spectroscopy variants are prevalent for metal ion determination. However, these methods are associated with high costs, complex instrumentation, specialized personnel requirements and reliance on centralized laboratory infrastructure [2-4]. Furthermore, the use of specialized, often volatile, toxic and expensive reagents precludes in situ analysis [5]. Consequently, there is a growing demand for rapid, simple testing systems that enable qualitative and quantitative analysis without complex equipment or extensive sample preparation.

A promising approach involves the immobilization of organic reagents onto solid supports, followed by sorption-spectroscopic detection of metal ions. This hybrid methodology enables a reduction in detection limits while enhancing the selectivity and accuracy of metal ion determination.

Previous studies have demonstrated that the incorporation of surfactants, water-soluble polymers, or their immobilized forms into organic reagent solutions can significantly alter reagent properties [6-14]. Optically transparent polymers, including polymethacrylate [15], triacetylcellulose [16], ethylcellulose [15] and gelatin supported on transparent substrates, have garnered particular interest [8,13-17]. Sensor devices fabricated using these polymers hold promise for simplified photometric analysis. A variety of organic reagents have been investigated, including complexes of 4-(2-pyridylazo)resorcinol with divalent cobalt, copper, cadmium, zinc, lead and nickel [18]; arsenazo III and arsenazo B with trivalent lanthanum, tetravalent thorium, hexavalent uranium, and divalent calcium [14]; nichromazo with divalent vanadium [19] and azo dye formation within the transparent gel layer of commercial photographic films [13,20].

Polymeric materials can serve as either co-matrix with conventional porous substrates or as primary solid supports for immobilization. Essential attributes of the polymeric base include optical transparency, substantial absorptive capacity, straightforward synthesis, ease of handling, chemical inertness, mechanical robustness, stability under acidic and alkaline conditions, and heightened sensitivity to analytical reagents. While many existing polymeric sorbents partially fulfil these criteria, a comprehensive investigation into the analytical characteristics of novel fibrous polymer carriers for sorption-spectrophotometric analytical systems is imperative for broadening the scope of analytical chemistry.

Polyacrylonitrile (PAN) fibers were modified with polyethylene polyamine to produce orange polyamine phosphate (PPA) fibers, a cation-exchange material. Significant interest exists in the immobilization of 1-nitroso-2-naphthol-3,6-disulfonic acid disodium salt (nitroso-R-salt, NRS) onto this synthetic support and the elucidation of the underlying polymer-NRS bonding interactions.

Orange polyamine phosphate (PPA) fiber, a cation-exchange material synthesized from polyacrylonitrile fiber modified with polyethylene polyamine by researchers at the Polymer Chemistry department, National University of Uzbekistan. 1-Nitroso-2-naphthol 3,6-disulfonic acid (nitroso-R-salt), a water-soluble analogue of 1-nitroso-2-naphthol [21], is widely employed in analytical chemistry. This study represents the first attempt to immobilize this reagent onto PPA-1 fiber for the development of sorption-spectroscopic methods for the determination of specific metal ions in wastewater. Optimal conditions for this process were investigated.

Carriers were utilized in the form of disks, each with a diameter of 2cm and a mass of 0.2g. The prepared supports were immersed in 10mL of 0.1M hydrochloric acid (HCl) and agitated for 24 hours using a magnetic stirrer. During this process, the amine groups on the fiber bind to the chloride ions present in the hydrochloric acid. Subsequently, the fiber was separated from the solution and rinsed with 50mL of distilled water. The binding of chloride ions to the fiber was quantified by potentiometric titration, measuring the residual hydrochloric acid in the solution with 0.1 M sodium hydroxide (NaOH) (Figure 1). The mechanism of the conversion of the fiber to its chlorinated form is illustrated as follows.

Figure 1:Integral (a) and differential (b) curves of potentiometric titration of PPA-1 fiber in aqueous medium.

The washed fiber was maintained in a moist state within Petri dishes. The objective of converting PPA fiber to its chlorinated form is to facilitate the formation of a strong bond between the sulfonate group of the organic reagent and the amine group of the chlorinated fiber. The mechanism underlying this process is as follows.

The effects of pH, buffer mixture and reagent concentration in the solid phase were investigated at a solution flow rate of 10mL/ min. The concentration of the reagent in the carrier was quantified spectrophotometrically by measuring the change in absorbance of the solutions at the optimal wavelength for each reagent, both before and after immobilization [22].

From the experiments, it was observed that treating the fiber in a 0.1M hydrochloric acid solution for 24 hours significantly increased the SOE capacity. Increasing the concentration from 25% to 50% resulted in an approximately threefold increase in the degree of polymer conversion. After treatment with hydrochloric acid, PPA- 1 fibers were kept moist in Petri dishes (Figure 2) and subsequently immobilized with a solution of the organic reagent nitroso-R salt.

The optimum immobilization conditions, organic reagent concentration, and optical density were measured using a Shimadzu- 1900 spectrophotometer. The optimal reaction conditions were determined with a Five Easy pH/mV meter

We reviewed the literature to analyze the active state of the organic reagent in solution. 1-Nitroso-2-naphthol-3,6-disulfonic acid (nitroso-R-salt) is extensively used in analytical chemistry as a water-soluble analogue of 1-nitroso-2-naphthol. However, there is limited information on the optical characteristics of its various ionic forms, and the dissociation constant of its hydroxyl group is reported inconsistently, ranging from 6.57 to 7.89 (pK units) due to variations in temperature, ionic strength, and ionic environments [23]. This variability complicates the calculation of complex stability constants. The literature examines the optical and colorimetric characteristics of nitroso-R salt at pH 4-10, determining the dissociation constant of its hydroxyl group using colorimetry at an ionic strength of 0.1-0.5 M, and compares these findings with our colorimetric results. Based on this literature review, we conducted experiments. Initially, we studied the molar absorption coefficient of the organic reagent at a selected concentration in solution and compared our findings with the literature (Table 1).

Figure 2:PPA-1 fiber in acidic medium in 0.1M HCl acid solution (a) and fiber treated with 0.1M HCl acid in Petri dish (b).

Table 1:Physical constants of nitroso-R salt.

Dissociation constants ΔS/ΔpH and pH were determined colorimetrically at a concentration of 1.0 × 10^-4 M and an ionic strength of 0.1−0.5 (where ) and ΔS/ΔpH is the specific color difference).

The absorption spectra of the solutions are presented in Figure 3. At pH 5.09, the solutions exhibit absorption maxima at wavelengths less than 220nm, 266nm, and 370nm (curve 1). As the pH increases, the absorbance at 266nm, corresponding to the absorption maximum of the HR2- form, increases. Concurrently, the absorbance at 370nm decreases slightly, and a more pronounced maximum peak emerges at 425nm.

Figure 3:UV spectra of the nitroso-R salt.

After determining the physical constants of the organic reagent, immobilization processes were conducted on the fiber. The capacity of the fiber was then assessed using a spectrophotometric method (Figure 4). The percentage concentration of the organic reagent immobilized on the fiber was measured in a spectrophotometer and calculated using the formula R=Δ𝐴/𝐴_before∙100%.

Immobilization onto 0.2g of fiber resulted in a binding efficiency of 99.76% for the organic reagent at a concentration of 3.2 x 10-4M, pH 5.09 and room temperature. This immobilized fiber has potential application in the determination of metal ions in various samples, which is currently under investigation. Further studies are ongoing to elucidate the mechanisms of the immobilization process using techniques such as infrared spectroscopy.

Figure 4:Immobilized nitroso-R salt.

We would like to express our sincere gratitude to our professors Gafurova Dilfuza Anvarovna, Shahidova Dilbar Nematovna, professors of the “Polymer Chemistry” department of the National University of Uzbekistan, who contributed to the synthesis of various polymer fibers for our scientific research.

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