How do you test for Fe³⁺ ions?

Add a few drops of potassium thiocyanate. A blood-red colour shows Fe³⁺. NaOH also gives a red-brown solid.

How do you tell Fe²⁺ from Fe³⁺ with NaOH?

Fe²⁺ gives a white-green solid that turns brown in air. Fe³⁺ gives a red-brown solid at once.

How can Fe²⁺ be converted to Fe³⁺ in the lab?

Add an oxidising agent such as hydrogen peroxide or chlorine water. Then KSCN turns blood red to prove Fe³⁺ has formed.

Lab: Test for Fe²⁺ and Fe³⁺ Ions (Iron Compounds)

In this practical you identify Fe²⁺ and Fe³⁺ in solution. Sodium hydroxide gives a white-green solid with Fe²⁺ (turning brown in air) and a red-brown solid with Fe³⁺. Potassium thiocyanate (KSCN) turns Fe³⁺ blood red but leaves Fe²⁺ unchanged. Hydrogen peroxide oxidises Fe²⁺ to Fe³⁺ (then KSCN turns red), and iron powder reduces Fe³⁺ to Fe²⁺ (then KSCN gives no red). The lab shows that iron(II) is reducing, iron(III) is oxidising, and each can change into the other.

Aim, materials and safety

Aim: to identify Fe²⁺ and Fe³⁺ ions by their reactions, and to show that one can change into the other.

You need: a solution of iron(II) sulfate (Fe²⁺), a solution of iron(III) chloride (Fe³⁺), dilute sodium hydroxide (NaOH), potassium thiocyanate (KSCN) solution, dilute hydrogen peroxide (H₂O₂), a little iron powder, clean test tubes, a rack and droppers.

Safety: wear goggles; NaOH can burn skin and eyes. Use only a few drops. Never taste anything. Wash hands after the practical. Use fresh iron(II) solution, because old solution already has some Fe³⁺ in it.

Test 1: sodium hydroxide

Put about 2 cm of each iron solution in separate tubes. Add NaOH drop by drop.

Sample	You see	Equation
Fe²⁺	white-green solid, which slowly turns grey-green then red-brown in air	Fe²⁺ + 2OH⁻ → Fe(OH)₂
Fe³⁺	red-brown solid at once	Fe³⁺ + 3OH⁻ → Fe(OH)₃

The solids are precipitates: they do not dissolve in extra NaOH. The brown colour on Fe(OH)₂ tells you that oxygen is turning Fe²⁺ into Fe³⁺: 4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃.

Test 2: potassium thiocyanate (KSCN)

Put a fresh 2 cm of each iron solution in clean tubes. Add 2 to 3 drops of KSCN solution.

Fe³⁺: blood-red colour, because Fe³⁺ + SCN⁻ → [Fe(SCN)]²⁺ (a complex ion).
Fe²⁺: no red. The solution stays pale green (or very faint pink if a little Fe³⁺ is already there).

This is the most sensitive test for Fe³⁺: even a trace gives a clear red. It is the best way to prove that Fe³⁺ is absent or present.

Test 3: change Fe²⁺ into Fe³⁺ (oxidation)

Take 2 cm of fresh Fe²⁺ solution. Add KSCN: no red. Now add a few drops of hydrogen peroxide (acidify with a drop of dilute sulfuric acid if your teacher says so). The solution turns yellow-brown. Add KSCN again: blood red.

Meaning: H₂O₂ is an oxidising agent. It took one electron from each Fe²⁺: 2Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O. So iron(II) is a reducing substance, because it gives up electrons easily.

Test 4: change Fe³⁺ into Fe²⁺ (reduction)

Take 2 cm of Fe³⁺ solution. Add KSCN: red. In a new tube of Fe³⁺, add a pinch of iron powder and shake for a minute, then filter off the extra iron. The yellow-brown colour fades to pale green. Add KSCN to the filtrate: no red.

Meaning: iron powder is a reducing agent. It gave electrons to Fe³⁺: 2Fe³⁺ + Fe → 3Fe²⁺. So iron(III) is an oxidising substance, because it takes electrons easily.

Summary table and conclusion

Test	Fe²⁺	Fe³⁺
Colour of solution	pale green	yellow-brown
NaOH	white-green solid, turns brown	red-brown solid
KSCN	no change	blood red
Effect of oxidiser (H₂O₂)	becomes Fe³⁺	no change
Effect of reducer (Fe)	no change	becomes Fe²⁺

Error analysis and good practice

Old Fe²⁺ solution may already contain Fe³⁺, giving a faint red with KSCN. Use fresh solution.
Too much NaOH or air can hide the white-green colour. Add a few drops and look quickly.
Dirty tubes can give false colours. Rinse with distilled water.
Add KSCN only a few drops; too much can make a very dark colour that is hard to compare.

Try it: predict, then check

Use the free-play step: pick a sample and a test, and write your prediction before tapping. Can you explain why the oxidised sample (Fe²⁺ + H₂O₂) behaves like Fe³⁺? At home, with an adult: cut a piece of a rusty nail, dip it in vinegar overnight, and ask your teacher how a KSCN test could show Fe³⁺ in the liquid.

Key formulas and definitions

Fe²⁺ + 2OH⁻ → Fe(OH)₂ (white-green); Fe³⁺ + 3OH⁻ → Fe(OH)₃ (red-brown)
4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃
Fe³⁺ + SCN⁻ → [Fe(SCN)]²⁺ (blood red)
2Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O (oxidation)
2Fe³⁺ + Fe → 3Fe²⁺ (reduction)

Worked examples

1. A solution gives a red-brown solid with NaOH and blood-red colour with KSCN. Which ion is present?

Fe³⁺. Both the red-brown Fe(OH)₃ and the blood-red thiocyanate complex point to iron(III).

2. A pale green solution gives a white-green solid with NaOH and no colour with KSCN. After adding H₂O₂, KSCN gives red. What do you conclude?

The original solution has Fe²⁺ (white-green solid, no red with KSCN). H₂O₂ oxidised it to Fe³⁺, which then gave the red colour with KSCN.

3. Why do we filter the solution before adding KSCN in Test 4?

Leftover iron powder could keep reducing Fe³⁺ and may also make the mixture cloudy. Filtering gives a clear solution, so the colour result is easy to read.

Common mistakes

Saying KSCN turns Fe²⁺ red. Only Fe³⁺ turns blood red.
Not using fresh Fe²⁺ solution. Old solution already has Fe³⁺ and gives a faint red.
Forgetting that white-green Fe(OH)₂ turns brown in air, then calling it Fe(OH)₃ at the start.
Mixing up the agents: H₂O₂ oxidises Fe²⁺ (up), iron powder reduces Fe³⁺ (down).

Practice quiz

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Lab: Properties of Iron Compounds (Testing Fe²⁺ and Fe³⁺)

🎬 Step-by-step story

🤔 Common doubts, cleared

Why is goggle use stressed so much?

Why does the white-green solid turn brown if left?

Why is KSCN the best test for Fe³⁺?

How does H₂O₂ change the iron?

Why does the red colour vanish after iron powder is added?

Can I use these tests for other metals?