Sodium thiosulphate is used in photography

All are correct

The correct answer is option 4. All are correct.

Let us delve into the detailed explanation of why sodium thiosulphate is used in photography and how each statement is true.

Sodium thiosulfate \((Na_2S_2O_3)\), often referred to as "hypo," is used in the photographic fixing process. This process is essential for making photographic images permanent after development. Here's how each statement about sodium thiosulphate's role in photography is correct:

1. Because of its complex forming behaviour

Sodium thiosulphate has the ability to form complexes with metal ions. In the context of photography, this property is utilized to dissolve the unreacted silver halide crystals that remain on the film or photographic paper after development. This complexation reaction helps to remove the unexposed silver halide, preventing it from being reduced to metallic silver, which would otherwise obscure the image.

2. Remove undecomposed \(AgBr\) as soluble complex

During the development process, not all silver bromide \((AgBr)\) crystals are exposed to light and reduced to metallic silver. The remaining undecomposed AgBr must be removed to prevent it from darkening upon further exposure to light. Sodium thiosulfate reacts with these undecomposed AgBr crystals to form a soluble complex, effectively removing them from the film or paper.

The chemical reaction is as follows:

\(\text{AgBr (s)} + 2\text{Na}_2\text{S}_2\text{O}_3 \rightarrow \text{Na}_3\text{[Ag(S}_2\text{O}_3\text{)}_2\text{]} + \text{NaBr}\)

This reaction makes the remaining silver halide soluble in water, allowing it to be washed away, thus "fixing" the image.

3. Forms sodium argento thiosulphate and \(NaBr\)

The specific reaction between sodium thiosulfate and silver bromide produces sodium argento thiosulphate \((Na_3[Ag(S_2O_3)_2])\) and sodium bromide \((NaBr)\). Sodium argento thiosulfate is a soluble complex that can be easily washed out from the film or paper. The formation of this soluble complex is critical for the fixing process as it ensures the complete removal of unexposed silver bromide, which stabilizes the image and prevents it from further exposure to light.

Summary

The use of sodium thiosulfate in photography is crucial due to its complex-forming ability, which allows it to remove undecomposed silver bromide as a soluble complex. The overall chemical reactions ensure that the image is stable and permanent by removing all unexposed silver halide.

Given these explanations, the correct answer is: All are correct.