The redox systems of thiourea (Tu, RS), thiosemicarbazide (Tsc), N-phenylthiourea (Phtu), N-acetylthiourea (Atu) and their oxidized form (formamidine disulfide derivatives, RSSR2+) were studied in HCl at t = 25.0 +/- 0.1 degrees C. To determine the reversibility of the processes in the system (RSSR2+ + RS + 1 M HCl) and to form the corresponding disulfides, iodine solution (I-2) and hydrogen peroxide were used as oxidants. When RS is oxidized, equilibrium is established in the system according to the equations: 2 RS - 2 e = RSSR2+. The witnesses of the equations are the slope of the electrode function theta (theta = 2.3 RT/nF), which corresponds to n = 2, that is, each R=S molecule donates one electron. The oxidation reaction with hydrogen peroxide is slow. The values of the conventional standard potential of the redox system "RSSR2+ / RS - 1 mol/l HCl" have been determined. It was found that in the oxidized form thiourea and thiosemicarbazide exhibit a stable reproducible potential, while formamidine disulfide derivatives of N-phenylthiourea and N-acetylthiourea are unstable and after 30-60 min decompose with the formation of elemental sulfur and other products. Therefore, for these systems, only solution I-2 was used as an oxidizing agent. The oxidized form of thiosemicarbazide (Tsc) (diamminoformamidine disulfide, H(2)Damfds(2+)) remains in solution for up to 4 h, after which the appearance of colloidal sulfur is observed. Note that the decomposition of the oxidized form of Phtu and Atu depends on their concentrations: an increase in the concentration of the oxidized form in solution leads to a decrease in its stability.