Sodium thiosulphate is used in photography because of its (a) Reaction with light (b) Oxidizing behaviour (c) Reducing behaviour (d) Complex forming behaviour

Sodium thiosulphate (Na₂S₂O₃), is a compound of significant importance in various applications, including photography. One of its notable chemical properties is its strong reducing ability. In aqueous solutions, sodium thiosulphate can donate electrons to chemical species that accept them, which makes it a useful reducing agent.

Another key property is its solubility in water, enabling it to interact easily with other substances in a solution. When it reacts with acids, sodium thiosulphate undergoes decomposition producing sulfur dioxide, sulfur, and water as products. This reaction with acids is also used to standardize iodine solutions in iodometry. Its ability to form complexes with silver ions is particularly important in photographic processes, facilitating the essential step of 'fixing' images on photographic films or papers.

A noteworthy characteristic of sodium thiosulphate is its relative stability, allowing it to be stored and handled safely under proper conditions. This stability combined with its solubility characteristics and reducing power make it a uniquely valuable chemical for the photographic industry.

Reducing agents play a pivotal role in the art and science of photography. A reducing agent is a substance that loses electrons in a chemical reaction and, by doing so, reduces another substance. In the context of photography, the reducing agent is vital for the 'fixing' stage of photographic development.

In traditional film photography, silver halides sensitive to light are used to capture images. When exposed to light, they are reduced to elemental silver, which forms the image. However, unexposed silver halides remain on the film and must be removed to prevent a fogged or completely blackened photograph. This is where sodium thiosulphate comes into play.

As a potent reducing agent, sodium thiosulphate reacts with these residual light-sensitive materials, converting them into soluble compounds that can be washed away, thus stabilizing the image. This action prevents any further reaction of the unexposed silver halide with light, ensuring the longevity of the photograph and clearly distinguishing the image areas from the unexposed parts.

Photo processing, specifically the step known as 'fixing', involves a critical chemical reaction where sodium thiosulphate acts as an agent to remove unexposed silver halides from the film or photographic paper. This step is essential for producing clear and stable images.

During development, exposed silver halides are reduced to metallic silver, creating the latent image. Post-exposure, the fixing solution—containing sodium thiosulphate—dissolves the remaining unexposed silver halides. The reaction can be described by a simplified equation: \[ AgX(s) + 2Na₂S₂O₃(aq) \rightarrow Na₃[Ag(S₂O₃)₂](aq) + NaX(aq) \] where AgX represents silver halides and NaX is a sodium halide.

Through this reaction, a complex called sodium argentothiosulphate is formed, which is water-soluble. The chemicals facilitate the removal of unexposed and undeveloped silver halide crystals, which, if left in place, would darken and ruin the photographic image over time due to further exposure to light. Proper washing of the photograph after fixing is critical to remove all residues, ensuring high-quality, and long-lasting images.