Within the project focused on gold nanoparticles, conducted at the Institute for Medical Research and Occupational Health in Zagreb, two NMR studies were carried out at the Faculty of Pharmacy and Biochemistry in Zagreb. One of the conducted NMR studies investigates the structural changes of citric acid during the synthesis of nanoparticles using citric acid. The second NMR study, which is the subject of this thesis, focuses on examining the stability of citric acid in aqueous solutions under the reaction conditions present during the synthesis of gold nanoparticles. The stability of citric acid was tested under conditions of aeration, acidification, and reflux of the citric acid solution. By recording 1H NMR spectra, it was demonstrated that there were no structural changes in citric acid under the mentioned reaction conditions. Although no structural changes occurred during the acidification of the citric acid solution, a shift in the signals of citric acid towards lower magnetic field regions was observed. The change in the structure of citric acid only occurred after applying highly aggressive reaction conditions, namely, the acidic reflux of the citric acid solution. During the reflux of the acidified citric acid solution, acid-catalyzed and temperature-assisted dehydration of citric acid took place, resulting in the closure of a five-membered anhydride ring. The results of this thesis indicate the stability of citric acid under the reaction conditions of gold nanoparticle synthesis in the presence of citric acid.