Identification of Cations

Cations are positively charged ions (for example, $\mathrm{Na^+}$ and $\mathrm{Cu^{2+}}$). In the lab, we identify unknown cations using simple wet tests that form coloured solids and using flame colours.

Key reagents and ideas

Aqueous sodium hydroxide (NaOH, aq) and aqueous ammonia (NH₃, aq) are added to solutions of the unknown. A solid that appears is called a precipitate. Adding a lot more reagent is called adding it in excess. Some precipitates dissolve in excess; this helps us tell ions apart.

Observations for common aqueous cations

• Aluminium $\mathrm{Al^{3+}}$: with NaOH → white ppt, soluble in excess to give a colourless solution; with NH₃ → white ppt, insoluble in excess.
• Ammonium \(\mathrm{NH_4^+}}\): with NaOH and warm gently → ammonia gas made (pungent) that turns damp red litmus paper blue; no ppt forms. NH₃ reagent is not used for this ion.
• Calcium $\mathrm{Ca^{2+}}$: with NaOH → white ppt, insoluble in excess; with NH₃ → no ppt or only a very slight white ppt.
• Chromium(III) $\mathrm{Cr^{3+}}$: with NaOH → green ppt, soluble in excess to a dark green solution; with NH₃ → grey‑green ppt, insoluble in excess.
• Copper(II) $\mathrm{Cu^{2+}}$: with NaOH → light blue ppt, insoluble in excess; with NH₃ → light blue ppt, soluble in excess to a deep blue solution.
• Iron(II) $\mathrm{Fe^{2+}}$: with either reagent → green ppt, insoluble in excess; turns brown on standing (it oxidises to iron(III)).
• Iron(III) $\mathrm{Fe^{3+}}$: with either reagent → brown (reddish‑brown) ppt, insoluble in excess.
• Zinc $\mathrm{Zn^{2+}}$: with NaOH → white ppt, soluble in excess; with NH₃ → white ppt, soluble in excess.

Flame tests (solid or solution on a clean wire)

Dip a clean nichrome wire in the sample, hold in a hot, non‑luminous flame, and observe the colour:

• $\mathrm{Li^+}$: crimson red
• $\mathrm{Na^+}$: yellow
• $\mathrm{K^+}$: lilac
• $\mathrm{Ca^{2+}}$: orange‑red (brick red)
• $\mathrm{Ba^{2+}}$: apple green
• $\mathrm{Cu^{2+}}$: blue‑green

Common misconceptions

• White ppt does not always mean the same ion: use solubility in excess to tell $\mathrm{Al^{3+}}$, $\mathrm{Ca^{2+}}$, and $\mathrm{Zn^{2+}}$ apart.
• A green ppt that turns brown over time suggests $\mathrm{Fe^{2+}}$, not $\mathrm{Fe^{3+}}$.
• Ammonium is identified by the gas made with warm NaOH, not by a precipitate.