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Nanoscale magnetite (Fe3O4) and nanocomposite based on it (Fe3O4/SiO2) were synthesized. In solutions of different ionic strength (0.015 and 0.15 M NaCl) by pH-metry, the acid–base properties of the surface were studied: the pH value of the isoin state of the surface (рHIIP), the total concentration, and values of Ki and pK of surface hydroxyl groups active in the pH range 4–9 and are calculated dependencies of the particles of hydroxyl groups of the surface on the pH of the medium. It was found that neutral hydroxyl groups are predominant for the synthesized sample magnetically sensitive nanoparticles (MNPs), which make it possible to form centers of both acidic and base types. Thus, for Fe3O4 to pH = 6 active base centers (C=155.56⋅10−5 − 141.59⋅10−5 mol⋅g−1), the strength of which decreases with decreasing pH (pK = 2.65 ± 0.01–4.48 ± 0.01 (2.59 ± 0.003 − 4.70 ± 0.008 for 0.15 M NaCl solution). Acid centers in a very small amount (4.93⋅10−6 − 2.68⋅10−6 mol⋅g−1) are fixed in the pH range 7–9 and show weakly acidic properties (pK = 10.95 ± 0.21 − 11.31 ± 0.18 (11.34 ± 0.04 − 11.94 ± 0.17 for 0.15 M NaCl solution). For the Fe3O4/SiO2 surface, neutral centers are predominant, which are characterized by the detection of acidic properties, which makes it possible to form acid-type centers, the concentration of which is higher in 0.15 M NaCl solution (C = 237.58⋅10−5 mol⋅g−1), (C for 0.015 M NaCl solution = 150.54⋅10−5 mol⋅g−1), whose strength decreases with increasing pH (pK2=4.69 ± 0.03 − 8.6 ± 0.08 (4.91 ± 0.1 − 9.32 ± 0.09 for 0.15 M NaCl solution)). [ABSTRACT FROM AUTHOR]