Which of the following reagent can not be used for the distinction between Fe+2 & Fe+3.

To distinguish between Fe$^{2+}$ and Fe$^{3+}$, certain reagents are more effective than others. Here are some common reagents used for this distinction:

1. Potassium Ferricyanide (K$_3$ [Fe(CN)$_6$]): This reagent will form a blue precipitate with Fe$^{2+}$ (Turnbull's Blue) and no precipitate with Fe$^{3+}$.

2. Potassium Ferrocianide (K$_4$ [Fe(CN)$_6$]): This reagent will form a blue precipitate with Fe$^{3+}$ (Prussian Blue) but does not react with Fe$^{2+}$.

3. Tannins or certain phenolic compounds: These reagents may react differently, but they are not reliable for a clear distinction.

4. Hydrochloric Acid (HCl): While it can help dissolve iron compounds, it does not distinguish between the two oxidation states.

Conclusion

Hydrochloric Acid (HCl) can be considered the reagent that cannot be used for the distinction between Fe$^{2+}$ and Fe$^{3+}$, as it does not provide any specific reaction that differentiates between them.