The Photocatalysed Deposition of Silver

When anatase pigment eg P25, suspended in silver nitrate solution, is irradiated by a uv source, the electrons are promoted from the valence band of the semiconductor to the conduction band. On reaching the surface, these electrons reduce Ag to Ag. This can be used as a method of removing silver from aqueous solution. 

Since titanium dioxide is not affected by acids even when they are hot, whereas silver dissolves in hot dilute nitric acid, it is possible to recover silver from the catalyst. 

While keeping the crucible connected to a clean Buchner flask, add small volumes of hot dilute nitric acid, allow it to react and then apply suction. Repeat until all the dark silver particles have dissolved. Wash on the filter with small volumes of acid. Collect the filtrate and washings into a high frequency cell and titrate against standard 0.01 M NaCl. Locate the end-point by the point of the intersection of the two linear parts (Sec.2.5.2). 

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