Problem 48 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Sodium thiosulphate is used in photography because of its (a) reducing behaviour (b) oxidising behaviour (c) complex forming behaviour (d) reaction with light

Step-by-Step Solution

Answer

(c) complex forming behaviour

Step 1: Understand the Role of Sodium Thiosulphate

In photography, sodium thiosulphate is used during the fixing stage where it helps to remove unreacted silver halide from the film or paper after it has been exposed to light and developed.

Step 2: Analyze Sodium Thiosulphate's Behavior

Sodium thiosulphate forms a stable, soluble complex with silver halide on the photographic film or paper. This complex formation is crucial for the 'fixing' process in photography.

Step 3: Determine the Correct Option

Given its role of forming a complex to remove unreacted silver halide, the correct behavior of sodium thiosulphate utilized in photography is its complex forming behaviour.

Key Concepts

Photographic Processing

Photography involves multiple chemical and mechanical steps to produce a visible image. It starts with capturing light on a sensitive surface, typically a film or a photographic paper that contains light-sensitive compounds. After exposure, the film undergoes a series of chemical treatments.

• **Developing:** This process involves reducing exposed silver halides to metallic silver, forming a visible image.
• **Stopping:** A bath that halts the development process.
• **Fixing:** This stage makes the image permanent and removes any unexposed silver halide, preventing further reaction to light.

Sodium thiosulphate plays a crucial role in the fixing stage, which ensures the image is stable and will not change over time with further light exposure.

Silver Halide

Silver halides are light-sensitive materials used in photographic films. They include compounds such as silver chloride, silver bromide, and silver iodide. These are crucial to capturing images. When exposed to light:

• Silver halide crystals in the film undergo a chemical reaction.
• Light exposure reduces some of these crystals to metallic silver.
• The unreacted crystals remain sensitive to further light unless removed.

The fixing solution, such as sodium thiosulphate, helps eliminate the remaining silver halide so that it does not affect the final image. This is critical to stabilize and make the visual representation permanent.

Complex Formation

Sodium thiosulphate is known for its ability to form complexes with silver halides. This is a key aspect of its function in photography. When included in the fixing bath, sodium thiosulphate reacts with silver halide to form a soluble complex:

• The complex is non-reactive to further light exposure.
• This soluble complex easily washes out from the photographic material.

Consider the reaction:\[ ext{AgBr (s) + 2} ext{Na}_2 ext{S}_2 ext{O}_3 ightarrow ext{Na}_3 ext{[Ag(S}_2 ext{O}_3)_2] + ext{NaBr} \] This reaction highlights the conversion of insoluble silver bromide into a soluble thiosulphate complex, illustrating the role of complex formation in photographic processing.

Fixing Process

The 'fixing process' in photography is the final chemical step to make the image permanent. Fixers like sodium thiosulphate ensure:

• All unreacted silver halide is converted into soluble complexes.
• The final image will not darken or change over time due to exposure.

This action preserves the image's integrity, making it safe to handle under normal lighting conditions. Once the fixing process is complete, the photograph can be rinsed and dried, revealing a stable and long-lasting image.