Problem 119 - A Complete Resource Book in Chemistry for JEE Main

Question

The fixing process of photographic film involves removal of unchanged silver bromide as (a) \(\mathrm{Na}_{2}\left[\mathrm{AgBr}\left(\mathrm{SO}_{3}\right)\right]\) (b) \(\mathrm{Na}_{3}\left[\mathrm{AgBr}\left(\mathrm{S}_{2} \mathrm{O}_{3}\right)\right]\) (c) \(\mathrm{Na}_{3}\left[\mathrm{Ag}\left(\mathrm{SO}_{3}\right)_{2}\right]\) (d) \(\mathrm{Na}_{3}\left[\mathrm{Ag}\left(\mathrm{S}_{2} \mathrm{O}_{3}\right)_{2}\right]\)

Step-by-Step Solution

Answer

The correct answer is (d) \(\mathrm{Na}_{3}\left[\mathrm{Ag}\left(\mathrm{S}_{2}\mathrm{O}_{3}\right)_{2}\right]\).

Step 1: Understanding the Fixing Process

The fixing process in photography is aimed at dissolving and removing unreacted silver bromide from the film. This involves using a chemical that can form a stable complex with silver ions.

Step 2: Identifying the Complex Forming Agent

The common fixing agent used in photography is sodium thiosulfate, \(\text{Na}_2\text{S}_2\text{O}_3\). It acts with unreacted silver bromide \(\text{AgBr}\) on the film to form a soluble complex.

Step 3: Writing the Reaction Equation

The reaction between silver bromide and sodium thiosulfate 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 shows the formation of a soluble complex that removes silver bromide from the film.

Step 4: Identifying the Correct Option

Based on the reaction, the soluble complex formed is \[\text{Na}_3[\text{Ag}(\text{S}_2\text{O}_3)_2]\] which matches option (d).

Key Concepts

Silver Bromide

Silver bromide (1073AgBr) is an essential component in traditional black and white photographic films. It belongs to a class of compounds known as halides, where silver is bonded to a halogen, bromine in this case. The unique property of silver bromide is its sensitivity to light. When exposed to light, it undergoes a photochemical reaction, allowing it to capture images. This reaction produces metallic silver on the photographic film, creating a latent image.

When you take a photograph using traditional photographic film, light hits the silver bromide crystals suspended within the film emulsion. The part of the film exposed to the most light undergoes more extensive reaction, forming more silver particles. This creates a 'negative' image where the dark parts of the scene appear light on the film and vice versa.

However, not all silver bromide crystals react during exposure. Therefore, the fixing process is necessary to remove the unreacted silver bromide from the film, ensuring that the latent image becomes a stable, visible photograph.

Sodium Thiosulfate

Sodium thiosulfate (1073Na_2S_2O_3) is the chemical compound often utilized in photography as a fixing agent. Its role is crucial, as it interacts with the unreacted silver bromide crystals left in the photographic film after exposure and development.

Role in the Fixing Process

During the fixing stage, sodium thiosulfate dissolves the unreacted silver bromide by forming a soluble complex. The chemical equation for this reaction is as follows:
\[\text{AgBr} + 2\ \text{Na}_2\text{S}_2\text{O}_3 \rightarrow \text{Na}_3[\text{Ag}(\text{S}_2\text{O}_3)_2] + \text{NaBr}\]

This reaction forms a complex called sodium argentothiosulfate \(\text{Na}_3[\text{Ag}(\text{S}_2\text{O}_3)_2]\), which is both soluble and stable, allowing the unwanted silver bromide to be washed away safely. This step ensures that no further chemical changes occur on the film, stabilizing the image, and allowing it to withstand exposure to light without altering the picture.

Properties and Applications

Sodium thiosulfate is also known outside of photography for its use in neutralizing chlorine, treating wastewater, and in various medical treatments. Its versatility and effectiveness make it a significant compound in chemistry.

Photographic Film

Photographic film is at the core of traditional photography, serving as the medium to capture images. It is a strip or sheet made from a transparent plastic base coated with a gelatin emulsion containing silver halide crystals.

The core function of photographic film is to record an image when light reflections from a scene hit the light-sensitive silver bromide crystals embedded in the emulsion. The image captured on the film is initially a latent image, totally invisible to the naked eye. It requires chemical development to reveal the final photograph.

Developing the Film

This involves immersing the film in a developer solution that reduces the exposed silver bromide to black metallic silver, forming the visible part of the image. After development,

photo film undergoes the fixing process to remove the remaining unreacted silver bromide, ensuring the developed image doesn’t continue to react further or deteriorate.

Traditional photographic film has been largely replaced by digital photography today. However, it remains widely appreciated for its unique texture and aesthetic, fostering interest among hobbyists and professional photographers who value the traditional methods of capturing and developing images.