MCQ Detailed View

In traditional photography, capturing an image involves several chemical steps. After light exposure on the film, silver halide crystals (mainly AgBr) are partially reduced to metallic silver, forming a latent image. However, not all silver halide crystals are exposed to light during this process. If left untreated, these unreacted crystals would continue to react when exposed to light, eventually ruining the image.

To prevent this, a fixer solution is applied. The most common fixer used in photography is sodium thiosulfate (Na₂S₂O₃), historically known as "hypo." Its role is crucial because it selectively removes unexposed and undeveloped silver halide crystals from the emulsion, making the image permanent and stable.

The chemical reaction between sodium thiosulfate and silver halide (AgBr) can be represented as:

AgBr + 2Na₂S₂O₃ → Na₃[Ag(S₂O₃)₂] + NaBr

Here, the thiosulfate ions (S₂O₃²⁻) form a soluble complex with silver ions, effectively dissolving the unreacted AgBr crystals into solution. Once this step is completed, the photographic film can be safely exposed to light without undergoing further changes.

Sodium thiosulfate not only stabilizes the image but also enhances its sharpness and clarity. Before its discovery in the 1830s by Sir John Herschel, early photographs would darken completely over time because no reliable fixer existed. The introduction of sodium thiosulfate revolutionized photography by making images permanent.

Even in modern times, sodium thiosulfate continues to be used in specialized photographic papers, X-ray films, and lithographic processes. Besides photography, it is also applied in analytical chemistry (iodometry), medicine (as an antidote for cyanide poisoning), and in water treatment (neutralizing chlorine).

✅ Thus, the correct answer is Na₂S₂O₃ (Sodium thiosulfate), the chemical fixer that ensures photographic images remain stable and long-lasting.