Formation of which complex, among the following, is not a confirmatory test of Pb2+ ions

Step 1: Evaluate confirmatory tests for Lead ($\ce{Pb^{2+}}$)

Confirmatory tests in qualitative analysis overwhelmingly rely upon creating visually striking, insoluble precipitates or deeply colored complexes. Let's analyze the options representing precipitates formed by $\ce{Pb^{2+}}$:

• Lead chromate ($\ce{PbCrO4}$): A vibrant yellow precipitate. Very commonly used confirming test.
• Lead iodide ($\ce{PbI2}$): A bright yellow, spangle-like crystalline precipitate. Distinctively confirms $\ce{Pb^{2+}}$.
• Lead sulphate ($\ce{PbSO4}$): A white precipitate distinctively soluble in ammonium acetate. This forms a confirmatory test step.

Step 2: Understand the outlier

• Lead nitrate ($\ce{Pb(NO3)2}$): This is NOT a precipitate. Essentially all inorganic nitrates are highly soluble in water. Since no characteristic precipitate or colour change eventuates, adding nitrate ions to a solution of lead proves nothing and does not function as a confirmatory test.

Step 3: Conclusion

The formation of lead nitrate cannot serve as a confirmatory test.

Key Points to Remember:

• All common nitrate salts predictably dissolve entirely in water, preventing them from being utilized visually as confirmatory precipitates.
• Tests for $\ce{Pb^{2+}}$ typically utilize $\ce{CrO4^{2-}}$ (yellow ppt), $\ce{I-}$ (yellow ppt), or $\ce{SO4^{2-}}$ (white ppt).