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Theory If Br- ions are added to any solution containing Ag+ ions, AgBr cannot precipitate until the product of the ion concentrations, (Ag+) (Br-), exceeds the solubility product constant, Ksp, for AgBr. A solution that contains NH3 molecules in high concentration can only contain Ag+ ions in very low concentration because NH3 molecules and Ag+ ions unite to form a complex with a large formation constant. If we represent the formation of the complex by the equation, Ag+ + nNH3 = Ag(NH3)n+ (1) Then the expression for the formation constant (equilibrium constant with the concentration of the complex in the numerator) is Kf = [Ag(NH3)n+] (2) [Ag+] [NH3]n Since the formation constant of the aminesilver complex has a large numerical value, a high concentration of Br- ions must be established in order to produce a AgBr precipitate from a solution of “ammoniacal silver nitrate.” At the point at which the precipitate first forms we can calculate the concentration of the umcomplexed Ag+ ions if we nkw the concentration of the Br- ions.
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