Sinhon thiourea | The difference between thiourea in fixing solutions and developing solutions.

In the fixer solution

Stabilizing Fixing Solution Components: The main component of the fixing solution, sodium thiosulfate, easily decomposes under acidic conditions. Thiourea has a certain alkalinity, which can neutralize some of the acidic substances in the fixing solution, adjusting and stabilizing the pH value of the fixing solution, inhibiting the decomposition of sodium thiosulfate, thereby extending the lifespan of the fixing solution.Complexing

Residual Silver Salts: During the fixing process, unexposed silver halides on the film or photographic paper dissolve slightly and ionize to release silver ions. The nitrogen and sulfur atoms in the thiourea molecules contain lone pairs of electrons, which can form stable water-soluble complexes with silver ions through coordinate bonds, causing residual silver salts to dissolve and be removed from the film or photographic paper, preventing image discoloration or fading, thus stabilizing the image.

In the developer solution

Suppressing fog: During the developing process, various reasons may lead to fog on the film or photo paper, affecting image quality. Thiourea can suppress the excessive reduction action of the developer, reducing fog formation and improving the contrast and clarity of the image.

Adjusting developing speed: Thiourea can make certain adjustments to the developing speed. It can interact with the developer or silver halides to control the rate of the developing reaction to a certain extent, making the developing process more uniform and stable, which helps achieve better image results.

Overall, thiourea primarily stabilizes the fixing solution and removes residual silver salts to stabilize the image in the fixing solution; in the developing solution, it mainly suppresses fog and adjusts the development speed. The mechanisms and purposes of the two are significantly different, but both aim to improve the quality and stability of the image.

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