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) \(\mathrm{Na}_{2}\left[\operatorname{AgBr}\left(\mathrm{SO}_{3}\right)\right]\) (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 Context

The given problem is about the fixing process in photographic film development. This step involves dissolving the unreacted silver bromide (AgBr) crystals that are left on the film after it has been exposed and developed.

Step 2: Analyzing the Fixation Reaction

In the fixing process, sodium thiosulfate ( ext{Na}_2 ext{S}_2 ext{O}_3 ) is commonly used as a fixer. The reaction involves complex formation with silver bromide (( ext{AgBr} )), converting insoluble silver bromide into a soluble complex.

Step 3: Examining Available Options

Look for the option that forms a complex with silver bromide including thiosulfate, ext{S}_2 ext{O}_3^{2-} , which indicates the role of the thiosulfate ion. Check the options if they contain thiosulfate in coordination with silver.

Step 4: Eliminating Incorrect Answers

Options (a) and (c) contain sulfate ( ext{SO}_3^{2-} ), which does not participate in the fixation process involving thiosulfate. Therefore, they can be eliminated immediately as incorrect options.

Step 5: Identifying the Correct Option

Option (d) contains thiosulfate alongside silver in the complex, ext{Na}_3 ext{[Ag(S}_2 ext{O}_3)_2] , reflecting the actual chemical reaction in the fixing process. Thus, this is the correct representation of the complex.

Key Concepts

Silver Bromide

One of the core components in photographic film is silver bromide, which is a compound composed of silver and bromine, represented chemically as AgBr.
When exposed to light, silver bromide molecules break down, producing elemental silver, which is crucial for capturing images on photographic film.
However, any silver bromide that has not been exposed remains unchanged on the film.
This unreacted silver bromide can cloud and obscure the image if not properly treated, so it is important to remove it during the photographic film development process.

• Silver bromide is sensitive to light, which makes it perfect for capturing images.
• Post-exposure, unaltered silver bromide makes up the cloudy parts of undeveloped photographic film.

Understanding this compound's role helps in comprehending why its removal is necessary through fixation.

Sodium Thiosulfate

Sodium thiosulfate is a crucial chemical in the photographic film development process, particularly during fixation.
It plays a central role by converting the unreacted silver bromide into a soluble complex. This transformation is essential, as it allows for the removal of unreacted silver bromide from the film.

• Commonly known as the fixer in this context.
• Sodium thiosulfate solution dissolves unwanted silver compounds.

The reaction between sodium thiosulfate and silver bromide prevents clouding and enhances the clarity of the final image. By dissolving the remaining silver bromide, sodium thiosulfate ensures that only the image created by developed silver remains, making it an indispensable part of the photographic process.

Complex Formation

When the unexposed silver bromide reacts with sodium thiosulfate, a complex formation occurs. This involves the transformation of the insoluble silver bromide into a soluble thiosulfate complex.
Here, the thiosulfate ion ( S_2O_3^{2-} ) acts as a ligand, forming strong covalent bonds with the silver ion, facilitating this change.

• This soluble complex allows the silver bromide to be washed away easily.
• The fixation process prevents leftover silver bromide from affecting image quality.

Without this complex formation, silver bromide would remain on the film, which could degrade and disrupt the captured image clarity.

Fixation Reaction

The fixation reaction is a chemical process essential for photographic film development.
It involves transforming unexposed silver bromide on the film into a water-soluble complex through its reaction with sodium thiosulfate.
This reaction ensures the image captured is clean and clear, removing surplus silver particles.

• The chemical equation for the reaction is represented as: \[ 2 ext{AgBr} + 3 ext{Na}_2 ext{S}_2 ext{O}_3 ightarrow ext{Na}_3[ ext{Ag}( ext{S}_2 ext{O}_3)_2] + 2 ext{NaBr} \]
• This process removes unexposed silver while preserving developed silver, which holds the image.

Through efficient fixation, the film preserves its clarity and detail by eliminating residual silver bromide, critical for high-quality photographic development.