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) Sodium thiosulphate is used in photography for its complex forming behaviour.

Step 1: Understanding the role of sodium thiosulphate in photography

In photography, sodium thiosulphate plays a specific role that is different from reacting with light or acting as an oxidizing or reducing agent. It is primarily used to fix photographs after they have been developed. This fixing process involves the removal of silver halides that have not been reduced to silver during the development stage.

Step 2: Analyzing the properties of sodium thiosulphate

Sodium thiosulphate acts as a complexing agent. It can form complexes with silver ions, effectively dissolving the unreacted silver halide crystals. This is key in the photographic fixing process as it prevents them from reacting with light and thus spoiling the photograph.

Step 3: Identifying the correct behavior of sodium thiosulphate

Based on the role of sodium thiosulphate in forming complexes with silver halides during the photographic fixing process, it is clear that the primary behavior of sodium thiosulphate used in this context is its complex forming behavior.

Key Concepts

Complex forming behavior

In photography, sodium thiosulphate is essential for its unique ability to form complexes. This complex-forming behavior is pivotal for processing film. Sodium thiosulphate acts by binding with silver ions, creating stable complexes.
This behavior helps dissolve and remove any excess silver halides on the film that did not get exposed or developed during the photograph development.
Without this process, any unreacted silver halide would interfere with the light sensitivity of the film.

• The complexation with silver ions prevents unwanted reactions.
• Makes the photograph stable and clear.

Silver halide complexes

Silver halides, compounds like silver bromide and silver chloride, are a critical part of the photographic process. They are light-sensitive, thus transforming when exposed to light to form an image. However, not all silver halide crystals are exposed during development.
This is why the formation of silver halide complexes with sodium thiosulphate is crucial.
Sodium thiosulphate creates a soluble, inert complex with unreacted silver halides:

• These complexes prevent further unwanted reactions with light.
• They ensure that the negatives are clear and the final photographic image doesn't get distorted.

Photographic fixing process

The photographic fixing process is a vital step in developing photographs. After the development phase, residual light-sensitive silver halide crystals remain on the film. If they are not removed, they can expose to light and distort the image.
This is where sodium thiosulphate steps in—it acts as a fixer.
During fixing:

• Sodium thiosulphate forms complexes with remaining silver halide crystals.
• Dissolves them, making them non-reactive to light.
• This ensures sharp and stable photographs that do not change over time.