Potassium Iodide

Summary: Potassium iodide (KI) is a white ionic solid whose aqueous solution acts as a reducing agent in redox reactions. The iodide ion (I⁻) is oxidised to iodine (I₂), producing a characteristic colour change from colourless to brown that serves as a visual indicator for the presence of oxidising agents. Tags: igcse chemistry potassium-iodide reducing-agent redox iodine halogens oxidation-number electrochemistry Created: 2026-07-18

Potassium iodide is an ionic compound with the formula KI, composed of potassium cations (K⁺) and iodide anions (I⁻), in which iodine carries an oxidation number of -1 — the lowest oxidation state available to iodine and the one that makes the iodide ion a competent reducing agent. When aqueous potassium iodide encounters an oxidising agent — a substance that accepts electrons — the iodide ions lose electrons and are oxidised to molecular iodine (I₂) according to the half-equation 2I⁻ → I₂ + 2e⁻, causing the solution to change from colourless to brown as elemental iodine is formed. This visually striking transformation is exploited in the IGCSE laboratory as a simple qualitative test for the presence of oxidising agents: if a few drops of potassium iodide solution are added to an unknown substance and the mixture turns brown, an oxidising agent is present and a redox reaction has occurred. Common oxidising agents that bring about this colour change include Potassium Manganate(VII) (which itself turns from purple to colourless), hydrogen peroxide (H₂O₂), chlorine water (Cl₂(aq)), and acidified potassium dichromate(VI) (which turns from orange to green). Beyond its role as a redox indicator, potassium iodide also participates in halogen displacement reactions: because iodide is the most easily oxidised halide ion, more reactive halogens such as chlorine and bromine will displace iodine from potassium iodide solution, providing a direct experimental demonstration of the trend in oxidising power down Group 7.

Chemical Formula and Structure

Potassium iodide is a binary ionic compound consisting of K⁺ and I⁻ ions held together in a giant ionic lattice by strong electrostatic forces of attraction.

The iodide ion (I⁻) contains iodine in its minimum oxidation number of -1. This is the most reduced form of iodine, meaning the iodide ion can only undergo oxidation (loss of electrons) — it cannot be reduced further. This is why KI acts specifically as a reducing agent: the I⁻ ion is the species that donates electrons to another substance.

Potassium Iodide as a Reducing Agent

What Is a Reducing Agent?

A reducing agent is a substance that reduces another substance by donating electrons to it, while the reducing agent itself is oxidised (it loses electrons). Potassium iodide functions as a reducing agent because the iodide ion (I⁻) readily gives up electrons to suitable oxidising agents.

The key half-equation for the oxidation of iodide ions is:

2I⁻(aq) → I₂(aq) + 2e⁻

Each iodide ion loses one electron. Two iodide ions are required to form one molecule of iodine (I₂), and two electrons are released in total. This half-equation is an oxidation process because electrons are lost.

The Colour Change: A Visible Redox Indicator

The oxidation of iodide ions produces a dramatic and easily observable colour change that makes potassium iodide an excellent qualitative test reagent:

When iodide ions are oxidised, the oxidation number of iodine increases from -1 to 0, and the solution turns from colourless to brown. This colour change is the experimental evidence that a redox reaction has taken place.

Common Oxidising Agents That React with KI

The following oxidising agents are commonly tested with potassium iodide solution in the IGCSE laboratory. In each case, the brown colour of iodine (I₂) confirms that oxidation of I⁻ has occurred.

Worked Example: KI + Acidified Potassium Manganate(VII)

This is the most commonly examined reaction pairing at IGCSE.

Reagents: Aqueous potassium iodide (colourless) + acidified potassium manganate(VII) (purple)

Overall redox equation:

2MnO₄⁻(aq) + 16H⁺(aq) + 10I⁻(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 5I₂(aq)

Half-equations:

• Oxidation (KI acts as the reducing agent): 2I⁻(aq) → I₂(aq) + 2e⁻
• Reduction (MnO₄⁻ acts as the oxidising agent): MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ → Mn²⁺(aq) + 4H₂O(l)

Observations:

• The purple colour of potassium manganate(VII) fades and the solution turns colourless (Mn²⁺ ions are nearly colourless in dilute solution).
• Simultaneously, the mixture turns brown from the iodine (I₂) produced.

Interpretation: Iodide ions (I⁻) have been oxidised to iodine (I₂), so KI has acted as a reducing agent. Manganate(VII) ions (MnO₄⁻) have been reduced to Mn²⁺, so KMnO₄ has acted as an oxidising agent. A redox reaction has occurred.

Halogen Displacement Reactions and KI

Potassium iodide is central to demonstrating the trend in halogen reactivity. Halogens become less powerful oxidising agents down Group 7 (F₂ > Cl₂ > Br₂ > I₂). A more reactive halogen will displace a less reactive halogen from an aqueous solution of its halide salt. Because iodide (I⁻) is the most easily oxidised halide ion, iodine is displaced from KI by both chlorine and bromine.

Displacement Reactions with KI(aq)

In each displacement, the halide ion (I⁻) is oxidised and the halogen molecule (Cl₂ or Br₂) is reduced. Potassium iodide acts as the reducing agent — it provides the electrons that reduce the halogen.

This demonstrates the reactivity trend: the oxidising ability of halogens decreases down the group (Cl₂ > Br₂ > I₂), while the reducing ability of halide ions increases down the group (I⁻ > Br⁻ > Cl⁻). Iodide is the strongest halide reducing agent.

Distinguishing KI from Other Colourless Halide Solutions

Potassium iodide solution can be distinguished from potassium chloride (KCl) and potassium bromide (KBr) solutions — all of which are colourless — using the following chemical tests taught in the IGCSE syllabus:

The silver nitrate test (producing a yellow precipitate of silver iodide, AgI) is a specific confirmatory test for iodide ions that does not rely on redox chemistry.

Oxidation Numbers in KI Reactions

Tracking oxidation numbers across the reaction confirms that redox has occurred, and this is a core IGCSE skill.

The oxidation number of iodine increases from -1 to 0 in every reaction where KI acts as a reducing agent. This is the quantitative proof that KI has been oxidised.

Confirming the Presence of Iodine (I₂)

When the brown colour appears after adding an oxidising agent to KI, the presence of iodine can be confirmed using:

1. Starch solution: Add a few drops of starch solution. Iodine forms a blue-black complex with starch — this is the definitive test for I₂.
2. Organic solvent extraction: Shake the brown solution with a small volume of hexane or cyclohexane. Iodine dissolves preferentially in the organic layer, turning it purple/violet — a characteristic colour distinct from bromine (orange) or chlorine (pale green).

Summary of Key Points

• Formula: KI — an ionic compound of K⁺ and I⁻.
• Oxidation number of iodine in I⁻: -1 (the minimum, most reduced form).
• Role: KI acts as a reducing agent — the iodide ion (I⁻) donates electrons.
• Half-equation: 2I⁻(aq) → I₂(aq) + 2e⁻.
• Colour change: Colourless (I⁻) → brown (I₂) — this is the key experimental observation.
• Used to test for: Oxidising agents (substances that accept electrons).
• Halogen displacement: Cl₂ and Br₂ displace I₂ from KI(aq), confirming the Group 7 reactivity trend.
• Confirmatory test for iodine: Starch solution turns blue-black.

Related Pages

• Reducing Agent — definition and examples of reducing agents
• Redox Reactions — the underlying theory of oxidation and reduction
• Potassium Manganate(VII) — a key oxidising agent used with KI in redox test reactions
• Oxidising Agent — the complementary concept to reducing agents
• Oxidation Number — tracking electron transfer quantitatively
• Group 7 Halogens — halogen displacement and trends in oxidising power
• Potassium Dichromate(VI) — another oxidising agent that reacts with KI
• Silver Nitrate Test — confirmatory test for halide ions including iodide
• Starch — indicator for iodine
• Displacement Reactions — general principle of halogen/halide displacement

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023–2025), Section 6.4: “Redox” — Supplement content on aqueous potassium iodide as a reducing agent.
• Harwood, R. & Lodge, I., Cambridge IGCSE Chemistry Coursebook, 5th Edition, Cambridge University Press, 2021.
• Gallagher, R. & Ingram, P., Complete Chemistry for Cambridge IGCSE, 3rd Edition, Oxford University Press, 2016.
• Clegg, A. et al., Cambridge IGCSE Chemistry Study and Revision Guide, Hodder Education, 2017.
• Earl, B. & Wilford, L. D. R., IGCSE Chemistry, Hodder Education, 2015.
• Royal Society of Chemistry, “Halogen Displacement Reactions,” rsc.org.

Common Misconceptions