Question

\(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3} .5 \mathrm{H}_{2} \mathrm{O}\) Sodium thiosulphate is used in photography to: (1) remove reduced silver (2) remove undecomposed AgBr as soluble silver thiosulphate complex (3) convert the metallic silver to silver salt (4) reduce the silver bromide grains to metallic silver .

Short answer

Option (2)

Step-by-step solution

Understand the Problem

Identify what is being asked. The problem is about the use of sodium thiosulphate \((\text{Na}_2\text{S}_2\text{O}_3 \bullet 5 \text{H}_2\text{O})\) in photography.

Review the Options

Look at the provided options and understand what each option implies in the context of a photographic process.

Role of Sodium Thiosulphate in Photography

Sodium thiosulphate is used to remove undecomposed silver bromide (AgBr) from photographic film, where it forms a soluble complex with AgBr. The chemical reaction involved is: \[ \text{AgBr} + 2 \text{Na}_2\text{S}_2\text{O}_3 \rightarrow \text{Na}_3\text{[Ag}(\text{S}_2\text{O}_3)_2] + \text{NaBr} \] This helps in making the unexposed silver bromide soluble, which can then be washed away.

Eliminate Incorrect Choices

Review each option and eliminate the ones that do not match the role of sodium thiosulphate in the photographic process. Option (3) and (4) are incorrect because sodium thiosulphate does not convert metallic silver or reduce silver bromide to metallic silver. Option (1) is incorrect because sodium thiosulphate specifically removes undecomposed AgBr, not just reduced silver.

Identify the Correct Answer

The correct role of sodium thiosulphate in photography is to remove undecomposed AgBr by converting it into a soluble complex. Thus, the correct answer is Option (2).

Key concepts

Photographic Chemicals

In the world of photography, several chemicals are pivotal for the development of photos. Each substance has a unique role. Sodium thiosulphate, for instance, is vital in the process.
This chemical, also known as hypo or fixer, makes unreacted silver halide soluble so it can be washed away. Other essential chemicals include developers, which reduce exposed silver halide grains to metallic silver.
Moreover, stop baths are used to halt the development process by neutralizing the developer. Understanding these photographic chemicals is crucial for anyone delving into traditional photography techniques.

• Sodium thiosulphate: Used to remove undecomposed silver halide
• Developers: Reduce exposed silver halide to metallic silver
• Stop Baths: Neutralize the developer to stop further development

Silver Halide Chemistry

Silver halide chemistry is at the heart of traditional photographic processes. This involves compounds like silver bromide (AgBr), which are light-sensitive.
When photographic film is exposed to light, the silver halide crystals in the film emulsion undergo a chemical reaction. These exposed crystals are then reduced to metallic silver in the developer.
Sodium thiosulphate plays its role in removing the unreacted silver bromide by forming a soluble complex. The relevant reaction is:
\[ \text{AgBr} + 2 \text{Na}_2\text{S}_2\text{O}_3 \rightarrow \text{Na}_3\left[ \text{Ag}(\text{S}_2\text{O}_3)_2 \right] + \text{NaBr} \]

• Light exposure alters silver halide crystals
• Developers reduce exposed silver halide to metallic silver
• Sodium thiosulphate removes unreacted silver halide

Understanding silver halide chemistry is fundamental for mastering the photographic development process.

Complex Formation Reactions

Complex formation reactions are essential in many chemical applications, including photography. These reactions involve the formation of a complex compound, where a central metal ion bonds to surrounding molecules or ions.
In photography, sodium thiosulphate forms a complex with silver ions, making the unreacted silver halide water-soluble. This helps eliminate any undecomposed silver halide from the film.
For instance, the reaction where sodium thiosulphate reacts with silver bromide (AgBr) to form a soluble complex is:
\[ \text{AgBr} + 2 \text{Na}_2\text{S}_2\text{O}_3 \rightarrow \text{Na}_3\left[ \text{Ag}(\text{S}_2\text{O}_3)_2 \right] + \text{NaBr} \]
This reaction ensures that no unexposed silver halide remains, giving a clear and lasting photographic image.
Complex formation reactions are not only crucial in photography but also in various fields such as biochemistry and industrial processes.

• Form central metal ion complexes
• Enhance solubility of certain compounds
• Critical in creating clear photographic images