<p>In this study, the effect of Mg addition on the composition and aging of water samples treated with nitrogen plasma was analyzed. The study focused on the measurements of the pH of the samples and the comparison of chosen oxygen and nitrogen reactive species (RONS) concentrations formed as a result of the plasma reaction and their changes over time. The results showed that the addition of Mg increased the concentration of RONS compared to the control samples. The pH changes resulting from the reaction of magnesium with water were also observed. Stability studies of the resulting composition also showed that the addition of Mg improved the stability of H₂O₂, NO₃⁻, and NO₂⁻ ions in water samples. The results suggest that it may be useful in water purification processes, environmental decontamination, and in analytical techniques requiring accurate control of chemical components in solutions. Additionally, it can be used in medicine and agriculture, where accurate analysis and stabilization of the composition of solutions are crucial.</p> Graphical Abstract <p></p>