Question

how does the addition of aqueous NH₃ to a solution separate and identify Cu²⁺ and Bi³⁺ ions? why wouldn't the addition of a strong base, such as KOH, work just as well as the addition of aqueous NH₃?

Answer 1

Keep in mind two things:

1. Adding ammonia to this solution introduces ammonia molecules
2. In addition, because ammonia is basic, addition of ammonia to the solution also increases the concentration of OH- ions.


When ammonia is added to a mixture of copper and bismuth ions, two reactions take place:

1. Cu(2+) (aq) + 4 NH3(aq) -------------> [Cu(NH3)4]2+ (aq)

2. Bi(3+) (aq) + 3 OH-(aq) ----------------> Bi(OH)3(s)


Thus, the copper remains in solution as its ammonia complex, while the bismuth is precipitated from the solution as its insoluble hydroxide salt.

This works because the reaction to form the copper complex is so favorable, and bismuth doesn't form an analogous compound.


If, however, KOH was added (and no ammonia), then the Cu(2+) ions would also precipitate out as Cu(OH)2. Since in this case both ions are removed from solution as insoluble salts, no separation of the metals occurs.

Answer 2

According to the lab manual, "Although Cu²⁺ ion forms a soluble, bright blue ammonia complex in solutions of aqueous ammonia, Bi³⁺ ion does not form a stable ammonia complex. Instead, the very insoluble, white bismuth hydroxide, Bi(OH)₃, precipitates from this alkaline solution. Use of a base other than NH₃ would cause both cations to precipitate as hydroxides, instead of separating them."