Anđelković, T., Perović, J., Blagojević, S., Purenović, M., Nikolić, R., Bojić, A. and Anđelković, D. (2006) “Acidity of humic acid related to its oxygen-containing functional groups”, Macedonian Journal of Chemistry and Chemical Engineering, 25(2), pp. 131–137. doi: 10.20450/mjcce.2006.297.