Testing for cations group by group

Sorting cations into groups

When many cations are present, every confirmatory test would be confused by the others. So we first separate them in a fixed order. Each group reagent precipitates one set of cations and leaves the rest in solution. We filter, and then use the next reagent on the filtrate.

• Zero group: ammonium, $\text{NH}_{4}^{+}$. It is tested on the original salt with sodium hydroxide, because ammonium chloride is added later as a reagent.
• Group I: $\text{Pb}^{2+}$. Dilute hydrochloric acid gives white lead chloride.
• Group II: $\text{Cu}^{2+}$ and the rest of the $\text{Pb}^{2+}$. Hydrogen sulphide gas in the presence of dilute hydrochloric acid gives black sulphides.
• Group III: $\text{Fe}^{3+}$ and $\text{Al}^{3+}$. Ammonium chloride, then ammonium hydroxide in excess, gives the hydroxides.
• Group IV: $\text{Zn}^{2+}$, $\text{Ni}^{2+}$, $\text{Co}^{2+}$ and $\text{Mn}^{2+}$. Hydrogen sulphide in the presence of ammonium hydroxide gives the sulphides.
• Group V: $\text{Ca}^{2+}$ and $\text{Ba}^{2+}$. Ammonium carbonate in the presence of ammonium chloride and ammonium hydroxide gives white carbonates.
• Group VI: $\text{Mg}^{2+}$. It stays in solution after group V and is found by its own test.

Before group III, boil off the hydrogen sulphide and add a few drops of concentrated nitric acid. This oxidises any iron(II) back to iron(III).

The tests for each cation

Ammonium, $\text{NH}_{4}^{+}$. Warm the salt with sodium hydroxide solution. Ammonia comes off with a sharp smell. A rod dipped in concentrated hydrochloric acid gives dense white fumes near the mouth of the tube. Nessler's reagent gives a brown precipitate or colour. $\text{NH}_{4}^{+}\ +\ \text{OH}^{-}\ \rightarrow \ \text{NH}_{3}\ +\ \text{H}_{2}\text{O}$

Lead, $\text{Pb}^{2+}$. Dilute hydrochloric acid gives a white precipitate of lead chloride that dissolves in hot water. Then:

• potassium iodide gives a yellow precipitate of lead iodide. $\text{Pb}^{2+}\ +\ 2\text{I}^{-}\ \rightarrow \ \text{PbI}_{2}$
• potassium chromate gives a yellow precipitate of lead chromate.
• dilute sulphuric acid gives a white precipitate of lead sulphate.
• sodium hydroxide gives a white precipitate that dissolves in excess, as plumbite ions, $[\text{Pb}(\text{OH})_{4}]^{2-}$.

Copper, $\text{Cu}^{2+}$. The solution is blue.

• Hydrogen sulphide in dilute acid gives black copper sulphide.
• Ammonium hydroxide gives a bluish-white precipitate that dissolves in excess to a deep blue solution. $\text{Cu}^{2+}\ +\ 4\text{NH}_{3}\ \rightarrow \ [\text{Cu}(\text{NH}_{3})_{4}]^{2+}$
• Potassium hexacyanoferrate(II) gives a chocolate brown precipitate.
• Potassium iodide gives a white precipitate of copper(I) iodide in a brown solution of iodine.
• The flame is bluish green.

Iron(III), $\text{Fe}^{3+}$. The solution is yellow-brown.

• Ammonium hydroxide, or sodium hydroxide, gives a reddish brown precipitate of iron(III) hydroxide.
• Potassium hexacyanoferrate(II) gives Prussian blue.
• Potassium thiocyanate gives a blood red colour.

Aluminium, $\text{Al}^{3+}$. The solution is colourless.

• Sodium hydroxide gives a white, jelly-like precipitate that dissolves in excess. $\text{Al}(\text{OH})_{3}\ +\ \text{OH}^{-}\ \rightarrow \ [\text{Al}(\text{OH})_{4}]^{-}$
• Ammonium hydroxide gives the same precipitate, but it does not dissolve in excess.
• The cobalt nitrate test: heat the precipitate on a charcoal block with a drop of cobalt nitrate. A blue ash (Thenard's blue) forms.

Zinc, $\text{Zn}^{2+}$. Colourless.

• Sodium hydroxide gives a white precipitate that dissolves in excess, as zincate ions.
• Potassium hexacyanoferrate(II) gives a bluish-white precipitate.
• Hydrogen sulphide in ammonium hydroxide gives white zinc sulphide.
• The cobalt nitrate test gives a green ash (Rinmann's green).

Nickel, $\text{Ni}^{2+}$. The solution is green.

• Sodium hydroxide gives a green precipitate that does not dissolve in excess.
• Ammonium hydroxide in excess gives a deep blue solution.
• Dimethylglyoxime (DMG) in ammoniacal solution gives a rosy red precipitate.

Cobalt, $\text{Co}^{2+}$. The solution is pink.

• Sodium hydroxide gives a blue precipitate that turns pink on warming.
• Potassium nitrite and acetic acid give a yellow precipitate of potassium hexanitritocobaltate(III).
• The borax bead is blue.

Manganese, $\text{Mn}^{2+}$. The solution is very pale pink.

• Sodium hydroxide gives a white precipitate that turns brown in air.
• Boil with lead(IV) oxide and concentrated nitric acid. A purple solution of permanganate forms.
• The borax bead is violet in the oxidising flame.
• Hydrogen sulphide in ammonium hydroxide gives a buff or flesh-coloured precipitate.

Calcium, $\text{Ca}^{2+}$. Ammonium oxalate gives a white precipitate of calcium oxalate that does not dissolve in acetic acid. The flame is brick red.

Barium, $\text{Ba}^{2+}$. Potassium chromate gives a yellow precipitate that does not dissolve in acetic acid. Dilute sulphuric acid gives a white precipitate. The flame is apple green.

Magnesium, $\text{Mg}^{2+}$. Disodium hydrogenphosphate with ammonium chloride and ammonium hydroxide gives a white crystalline precipitate of magnesium ammonium phosphate, $\text{MgNH}_{4}\text{PO}_{4}$. The magneson reagent gives a blue lake in alkaline solution.

The flame test

Dip a clean platinum wire (or a nichrome wire) in concentrated hydrochloric acid, then in the salt. Hold it at the edge of a non-luminous flame. Calcium gives brick red, barium apple green, copper bluish green and sodium golden yellow. Potassium gives lilac. A trace of sodium hides other colours, so use a blue cobalt glass to cut the yellow when you look for potassium.

Set 0.020 mol/L and 50 mL. Then double the volume. The mass doubles, since the line is straight. DMG is added in excess because it also gives a precipitate with other metals.

Practise with new numbers

Try it in Python

The program holds the group scheme for the twelve cations. It follows a mixture through each reagent.

Common mistakes

• Testing for ammonium after adding ammonium chloride. The reagent contains it.
• Adding too little excess of sodium hydroxide. A precipitate of aluminium, zinc or lead hydroxide seems to stay, but excess dissolves it.
• Confusing aluminium and zinc. Both dissolve in excess sodium hydroxide. Only zinc dissolves in excess ammonium hydroxide in the form of an ammine, and aluminium does not. Aluminium gives the blue ash and zinc the green.
• Reading a flame without cleaning the wire. Sodium is everywhere and gives yellow.
• Expecting all the lead in group I. Lead chloride is only partly removed, so some lead is left for group II.

Remember these

• Zero $\text{NH}_{4}^{+}$ (NaOH), I $\text{Pb}^{2+}$ (HCl), II $\text{Cu}^{2+}$ (H2S, acid), III $\text{Fe}^{3+}$ and $\text{Al}^{3+}$ (NH4OH), IV Zn, Ni, Co, Mn (H2S, ammonia), V Ca and Ba (carbonate), VI Mg.
• Iron(III): Prussian blue and blood red. Copper: deep blue with excess ammonia. Nickel: rosy red with DMG.
• Zinc and aluminium dissolve in excess sodium hydroxide. Calcium is brick red and barium apple green in the flame.

References

• NCERT Chemistry Lab Manual for Class 11 and 12, the experiments on qualitative analysis.
• Vogel's Qualitative Inorganic Analysis, the chapters on the reactions of cations.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How do you separate aluminium hydroxide from zinc hydroxide using ammonium hydroxide?

2. Which cation gives a rosy red precipitate with dimethylglyoxime?

3. Why is the solution boiled and treated with concentrated nitric acid before group III?

4. A salt gives a brick red flame and a white precipitate with ammonium oxalate. Which cation is present?