In photography, hypo is used in the reduction of: (a) \(\mathrm{Ag}\) from \(\mathrm{AgBr}\) (b) \(\mathrm{AgBr}\) into \(\mathrm{Ag}_{2} \mathrm{SO}_{4}\) (c) AgBr into soluble thiosulphate complex (d) both (b) and (c)

Sodium thiosulfate, often referred to as 'hypo' in the context of photography, plays a crucial role in the development of photographic films. After a film is exposed to light, it undergoes a development process where the exposed silver halide crystals are reduced to metallic silver, which creates the image. However, not all silver halides react during development. This is where sodium thiosulfate comes in.

Sodium thiosulfate is used in the photographic fixing process to dissolve the unreacted silver halides that remain on the film. If these halides were not removed, they would continue to react to light and darken the image, ruining the photograph. The 'fixing' stage ensures that the captured image stays unchanged after exposure to light and the development process. Sodium thiosulfate works by converting these unreacted silver halides into soluble complexes that can then be washed away, leaving behind a stable and clear image on the film.

Silver halide reduction is a chemical reaction that takes place during both the development and fixing stages of the photographic process. Initially, the reactive silver halides—silver bromide (AgBr), silver chloride (AgCl), and silver iodide (AgI)—create the latent image when they're exposed to light. This exposure allows the silver ions in the halides to reduce to metallic silver, which becomes visible upon development.

In fixing, the unexposed and unreduced silver halides must be removed to prevent further darkening of the image. The reduction we refer to in this stage is actually the chemical conversion facilitated by sodium thiosulfate, which turns these insoluble halides into soluble thiosulfate complexes. This 'fixing' prevents the image from changing after the process is complete. It is essential for creating a photograph that accurately represents the captured scene without unintended alterations due to residual reactivity.

The chemical 'hypo,' or sodium thiosulfate (\byear{\(\text{Na}_2\text{S}_2\text{O}_3\)}), possesses unique properties making it ideal for the photographic fixing process. One key property is its ability to dissolve silver halides. When hypo comes into contact with silver halides, it cleaves the bonds between the silver and halogen atoms by a redox reaction, transforming them into a complex anion, namely \byear{\(\text{Ag}(\text{S}_2\text{O}_3)_{2}^{3-}\)}, and soluble in water.

The chemical robustness of sodium thiosulfate allows it to work effectively in a range of conditions, which is crucial for the consistency required in photographic processing. Missteps in chemical formulation may not only hinder the clearing of photographs but also potentially damage the desired image. Hence, the hypo must be pure and of a precise concentration to ensure the optimum reduction of silver halides without affecting the metallic silver that forms the image.