The fixing process of photographic film involves removal of unchanged silver bromide as: (a) \(\mathrm{Na}_{2}\left[\mathrm{AgBr}\left(\mathrm{SO}_{3}\right)\right]\) (b) \(\mathrm{Na}_{3}\left[\mathrm{AgBr}\left(\mathrm{S}_{2} \mathrm{O}_{3}\right)\right]\) (c) \(\mathrm{Na}_{3}\left[\mathrm{Ag}\left(\mathrm{SO}_{3}\right)_{2}\right]\) (d) \(\mathrm{Na}_{3}\left[\mathrm{Ag}\left(\mathrm{S}_{2} \mathrm{O}_{3}\right)_{2}\right]\)

The concept of silver halide removal is central to understanding the chemistry behind the photographic fixing process. Silver halides, primarily silver bromide (AgBr), are light-sensitive compounds used in photographic film to capture images. However, after exposure to light and subsequent development, the unexposed silver halides need to be removed to prevent the photograph from further darkening and to make the image permanent.

To achieve this, the fixing stage involves a chemical reaction where these unexposed silver halide grains are dissolved away. A fixer solution is used for this purpose, typically containing a compound known as sodium thiosulfate. The effectiveness of this removal process is crucial as any residual silver halides would react with light over time, leading to a deterioration of the captured image. Therefore, thorough removal ensures the longevity and stability of photographs.

Sodium thiosulfate (Na2S2O3) plays a pivotal role in the photographic fixing process due to its ability to dissolve unexposed silver halides. In photography, this chemical is often referred to as the 'hypo.' It is advantageously employed after the development stage to eliminate any remaining silver halide from the film or photographic paper.

Once in solution, sodium thiosulfate reacts with the silver halides to form a complex ion that is soluble in water. This property is essential for washing away the unreacted silver halides, which otherwise would affect the quality and durability of the image. It is an excellent example of how simple inorganic chemistry is harnessed in practical applications such as photography, where complex ion reactions are instrumental in image processing.

During the photographic fixing process, complex ion formation occurs as a chemical reaction between silver ions from the silver halides and thiosulfate ions from the sodium thiosulfate fixer. A complex ion is a structure composed of a metal ion at its center with several other molecules or ions bonded to it.

The specific ion formed in film fixing is a silver-thiosulfate complex. Each silver ion, from the dissociated silver bromide (AgBr), bonds with two thiosulfate ions, resulting in a soluble complex ion. As the thiosulfate ions bond with the silver ions, they effectively render the silver halides soluble, which allows them to be removed from the film. This solubilization is vital because it prevents any further exposure to light from affecting the photograph. The correct chemical formula for the silver-thiosulfate complex typically formed is Na3[Ag(S2O3)2], wherein this complex, silver ions are stabilized by the thiosulfate ligands, permitting their removal through a water wash.