Problem 100 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

(d) complex forming behaviour

Step 1: Understanding the Use of Sodium Thiosulphate

Sodium thiosulphate is commonly used in photography, particularly in the fixing stage of photographic processing. The purpose of a fixer is to dissolve unexposed silver halide crystals from the film, making the image stable and removing any unwanted compounds that might affect its clarity.

Step 2: Active Mechanism of Sodium Thiosulphate

Sodium thiosulphate works in photography because it forms a soluble complex with silver bromide, which is one of the unresolved chemicals present after film exposure. The exact reaction that occurs is:\[ ext{AgBr} + 2 ext{Na}_2 ext{S}_2 ext{O}_3 ightarrow ext{Na}_3[ ext{Ag}( ext{S}_2 ext{O}_3)_2] + ext{NaBr} \]This reaction shows the complex formation, where silver bromide is converted into a soluble complex, making it easier to wash away.

Step 3: Conclusion about the Chemical Behavior

Based on the reaction of sodium thiosulphate with silver bromide described above, it shows the formation of a complex compound. Therefore, the key aspect of sodium thiosulphate's behavior in photographic fixing is its capability to form complex compounds.

Key Concepts

Sodium Thiosulphate

Sodium thiosulphate, with its chemical formula \( ext{Na}_2 ext{S}_2 ext{O}_3 \), is a versatile compound used in various applications, but it is perhaps best known for its role in the world of photography. This compound serves as a fixer in photographic development. During this fixing process, sodium thiosulphate is crucial in making exposed images stable by dissolving the unexposed silver halide crystals on photographic film or paper.

• It does not react with light directly or involve any oxidizing or reducing behaviors in the context of photography.
• Its primary role is to form a complex with unreacted silver halide, ensuring the photographic image remains intact over time.

This characteristic chemical behavior is essential for producing clear and stable photographs. Without sodium thiosulphate, images would be inconsistent and exhibit rapid degradation.

Complex Formation

Complex formation is a key chemical process where atoms or molecules combine to form a stable compound through coordination bonds. Sodium thiosulphate exemplifies this phenomenon when it interacts with silver bromide during the photographic fixing stage.
The following reaction captures this transformation:\[ \text{AgBr} + 2 \text{Na}_2\text{S}_2\text{O}_3 \rightarrow \text{Na}_3[\text{Ag}(\text{S}_2\text{O}_3)_2] + \text{NaBr} \]

• Here, silver bromide (\( \text{AgBr} \)) reacts with sodium thiosulphate to form a soluble complex \( \text{Na}_3[\text{Ag}(\text{S}_2\text{O}_3)_2] \).
• This complexing reaction is crucial as it allows the previously insoluble silver bromide to dissolve and be washed away, leaving behind a stable image on the film.

The ability of sodium thiosulphate to form such complexes is central to its functionality in photographic processing.

Silver Bromide Reaction

In the realm of photographic chemistry, silver bromide \( (\text{AgBr}) \) plays a critical role, particularly during the development of photographic films and papers. It's one of the light-sensitive compounds that capture and hold the image during exposure.
However, not all silver bromide exposed to light turns into metallic silver, and this is where sodium thiosulphate comes into play.

• Unexposed \( \text{AgBr} \) needs to be removed to prevent the image from further exposure and degradation.
• The reaction between silver bromide and sodium thiosulphate results in the formation of a soluble complex, which ensures its easy removal.

This reaction not only stabilizes the image but also enhances its clarity by eliminating the potential for further reactions with light. Thus, the silver bromide reaction is integral in producing enduring photographic prints.