The variation of the magnetic properties of 2,2,5,5,tetrametyhl-4,methyl-imidazoline nitroxide with the pH of the embedding medium is investigated by means of quantum mechanical calculations. The hyperfine coupling constants of the nitroxide are determined by mixed QCISD/PBE0 calculations, while the geometry of the neutral and protonated nitroxide and the relative dissociation constants are calculated at the PBE0 level. Solvent effects are modelled both by including a few explicit solvent molecules (strongly bound to the solute) and by using the CPCM method. The agreement between the calculated and the experimental titration curve is quite good, confirming the reliability of our computational strategy.