Arginine is an important amino acid involved in the production of NO, which is a vasodilatator. Its property as ligand towards cations was little studied. Also its protonation constants, even if subject of several studies, were not accurately determined. In this paper electromotive force measurements are performed by using a hydrogen electrode to accurately determine the three protonation constant of arginine. At 25 °C and in 1.00 mol dm-3 NaClO4, as ionic medium, they are log k1 = 12.53 ± 0.05; log k1 k2 = 21.36 ± 0.03 log k3 = 2.20 ± 0.07, while in 1.00 mol dm-3 NaCl, the following values were obtained: log k1 = 12.35 ± 0.05; log k1 k2 = 21.18 ± 0.03; log k3 = 2.08 ± 0.07.

Galvanic cells involving silver and glass electrodes were employed to study the behaviour of L-arginine as ligand towards silver (I). The experimental data, obtained at 25 °C and in 1.00 mol dm-3 NaClO4, were explained by assuming the formation of the species Ag(HL) (log β1,1,1 = 3.79 ± 0.04) and Ag(HL)2 (log β1,2,2 = 7.36 ± 0.03) The results of this paper will be used for future investigation.