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In Tollens' test, ammonia is always added to the silver nitrate solution before it is used to oxidise glucose to gluconic acid. Regarding the role of ammonia, which of the following statements must be INCORRECT?

1. NH3 combines with Ag+ to form the complex [Ag(NH3)2]+.
2. [Ag(NH3)2]+ is a weaker oxidising agent than free Ag+.
3. In the absence of NH3, the silver salt of gluconic acid would be formed.
4. NH3 alters the standard reduction potential of the glucose|gluconic acid electrode.

Correct answer: In the absence of NH3, the silver salt of gluconic acid would be formed.

Solution

Ammonia complexes Ag+ to [Ag(NH3)2]+ (Tollens' reagent), keeping silver in solution and lowering its reduction potential so the oxidation is controlled. Statements A, B and D correctly describe this. Statement C is wrong: ammonia complexes the silver ion, it does not determine whether a silver salt of gluconic acid forms; in any case the product carboxylic acid exists as its ammonium salt in the basic medium, not as a silver salt.

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