Sinhon thiourea | The principle of thiourea used in developers

1.Inhibition of Sodium Thiosulfate Decomposition: Sodium thiosulfate (Hypo) in the fixing solution is unstable under acidic conditions and is prone to decomposition reactions. Thiourea has a certain alkalinity, which can neutralize some of the acidic substances in the fixing solution, keeping the pH value of the fixing solution within a relatively stable range, thereby inhibiting the decomposition of sodium thiosulfate. Additionally, the amino (-NH₂) and mercapto (-SH) groups in the thiourea molecule can react with the acidic substances produced by the decomposition of sodium thiosulfate, preventing the further progression of the decomposition reaction, enhancing the stability of the fixing solution, and extending its lifespan.

2. Complexing Residual Silver Salts: During the photographic process, some unexposed silver halide remains on the film or photographic paper after development. Thiourea can undergo a complexation reaction with these silver halides, where the mercapto (-SH) and amino (-NH₂) groups in its molecules can form stable complexes with silver ions (Ag⁺). These complexes are soluble in water, allowing the residual silver halides to be dissolved from the film or photographic paper, achieving the goal of removing residual silver salts, ensuring that the image can be stably preserved, and preventing discoloration or fading of the image due to continuing reactions from the residual silver salts.

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