Final answer:
The agent used to remove unexposed/underdeveloped silver halide from photographic film is sodium thiosulfate or hypo. It forms a soluble complex with unexposed silver halides, which increases the efficiency of their removal and prevents the need for thousands of liters of water that would normally be required.
Step-by-step explanation:
The agent that removes all unexposed/underdeveloped silver halide from photographic film is sodium thiosulfate (Na2S2O3), commonly referred to as hypo. This chemical reacts with the unexposed silver halides to form a soluble complex ion, Ag(S₂O₃)₂³⁻, which is then washed away. The use of sodium thiosulfate is crucial because it reacts with silver halide to form a soluble complex based on the principles of multiple equilibria and Le Chatelier's principle. When excess AgBr needs to be removed, a concentrated solution of sodium thiosulfate is employed to dissolve it efficiently, which would be impractical with pure water due to the very low equilibrium concentrations of Ag* and Br⁻ ions. The process of developing photographic images involves the reduction of silver from radiation-sensitive silver compounds in the film's emulsion, and hypo is almost universally used to ensure only the exposed silver halide remains, due to its low cost, low toxicity, and high efficiency in increasing the solubility of AgBr.
Comparing the complex formation constant (Kf) with the solubility product constant (Ksp) shows a significant increase in solubility for AgBr in the presence of sodium thiosulfate, making the removal process effective and environmentally friendly since large volumes of water are not required.