Extraction of Ag from sulphide ore and removal of unreacted silver from photographic plate involve complexes
Answer & explanation
Correct answer: option 4
The correct answer is option 4. \([Ag(CN)_2]^- \), \([Ag(S_2O_3)_2]^{3-}\)
I. Extraction of Ag from sulphide ore: In the extraction process of silver from its sulphide ores, such as argentite \((Ag_2S)\), the ore is first roasted in the presence of air to convert the silver sulphide into silver oxide \((Ag_2O)\). The silver oxide is then treated with a sodium thiosulphate solution \((Na_2S_2O_3)\) to form the complex ion \([Ag(S_2O_3)_2]^{3-}\). This complex is water-soluble and can be easily separated from the impurities present in the ore.
II. Removal of unreacted silver from the photographic plate: In the process of developing a photographic plate, unreacted silver bromide \((AgBr)\) is removed to obtain the final image. This is done by treating the photographic plate with a solution of sodium thiosulphate \((Na_2S_2O_3)\), commonly known as "fixing" or "hypo." The thiosulphate ion \((S2O3^2-)\) forms a complex with the unreacted silver bromide, converting it into the water-soluble complex \([Ag(S_2O_3)_2]^{3-}\), which can be easily washed away, leaving the developed image on the plate.
In both processes, the complexes \([Ag(CN)_2]^- \) and \([Ag(S_2O_3)_2]^{3-}\) are involved. Therefore, option 4 is the correct choice. The complex \([Ag(CN)_2]^- \) is relevant in the context of silver plating using potassium silver cyanide \((K[Ag(CN)_2])\), while \([Ag(S_2O_3)_2]^{3-}\) is involved in the extraction and removal of unreacted silver as explained above.