1. Helfferich, F., Grundlagen Struktur, Herstellung, Theorie, Weinheim: Verlag Chemie, 1959, vol. 1.
2. Altshuler, H.N., Malyshenko, N.V., Lyrshchikov, S.Y., Efimova, O.S., and Altshuler, O.H., Proton transfer in coal of early stage of metamorphism and in SKN-2K carbon sorbent, Coke Chem., 2021, vol. 64, no. 4, pp. 139–143. https://doi.org/10.3103/S1068364X21040037
3. GOST (State Standard) 25543-2013: Brown coals, hard coals and anthracites. Classification according to genetic and technological parameters, Moscow: Standartinform, 2019.
4. GOST (State Standard) 4204-77: Reagents. Sulphuric acid. Specifications, Moscow: Standartinform, 2006.
5. GOST (State Standard) 2184-2013: Sulphuric acid for industrial use. Specifications, Moscow: Standartinform, 2019.