2.2. Preparation of Experimental Solutions
This protocol is extracted from research article:
Spectrophotometric Determination of p-Nitrophenol under ENP Interference
J Anal Methods Chem, Jan 7, 2021; DOI: 10.1155/2021/6682722

Stock solutions of PNP (1.000 g/L), NaHCO3 (3.440 g/L), MgSO4 (5.000 g/L), CaCl2 (2.774 g/L), KCl (0.956 g/L), MnSO4 (1.388 g/L), and KNO3 (1.628 g/L) were prepared in ultrapure water in order to match the salt content in natural water. The corresponding working solutions were obtained by diluting stock solutions daily in ultrapure water. The PNP standard solutions of 0.00 mg/L, 4.00 mg/L, 8.00 mg/L, 12.0 mg/L, 16.0 mg/L, and 20.0 mg/L were diluted step by step with the PNP stock solution. It should be noted that the six PNP standard solutions all contained 10% CH₃OH, 5% HCl, and 1 g/L ascorbic acid. Afterwards, the absorbance of PNP standard solutions was determined at 317 nm, which was the unique characteristic UV wavelength of PNP. The PNP standard curve was drawn with the concentration as the abscissa and absorbance as the ordinate, and the linear regression equation was obtained by fitting, as shown in Figure 2(a). It was concluded that the PNP standard curve was linear in the range of 0.00‒20.0 mg/L, and its correlation coefficient, R2, was above 0.999.

(a) PNP standard curve. (b) Tyndall phenomenon of the nano-Fe(OH)3 solution.

The nano-Fe(OH)₃ solution was prepared by extended hydrolysis of Fe3+. In detail, 0.0776 g FeCl3·6H2O was dissolved in 250 mL ultrapure water and hydrolyzed naturally at room temperature for more than four days. Before using the solution, the particle size and ζ potential of the nano-Fe(OH)3 solution were measured by a Particle Sizer and Zeta Potential Analyzer (Nano-ZS, Malvern, UK), as shown in Table 2. The average particle size of Fe(OH)3 was 39.2 nm, which proved that the solution was a nanocolloid. The average ζ potential was 35.3 mV, which implied that the nano-Fe(OH)3 system was very stable after extended hydrolysis [33, 34]. In order to further verify the existence of the colloid, a simple Tyndall experiment was carried out, as shown in Figure 2(b). A bright red “light channel,” which was the Tyndall phenomenon, was obviously observed when a laser was irradiated from the side of the solution. Therefore, it was proven that the nano-Fe(OH)₃, obtained by four-day natural hydrolysis, was suitable for the follow-up study of nanoparticles interfering with PNP tests.

The properties of the nano-Fe(OH)3 solution.

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