Estimation of Copper Iodometrically Using Standard Sodium Thiosulphate Solution

Procedure:

• Pipette 10.0 mL of Cu(II) solution into a 250 mL conical flask.
• Add a few drops of dilute sodium carbonate solution until a faint permanent precipitate forms.
• Remove the precipitate by adding 1–2 drops of acetic acid.
• Add 0.25 g KI (or 2 mL of a 10% KI solution).
• Titrate the liberated iodine with a standard sodium thiosulphate solution until the brown iodine color fades.
• Add 0.4 mL of starch solution – the mixture will turn blue.
• Continue titrating with thiosulphate until the blue color starts to fade.
• Add ~0.25 g potassium thiocyanate (or ammonium thiocyanate) (preferably as a 10% aqueous solution) – the blue color will intensify.
• Complete the titration quickly – the precipitate will have a milky white, giving a sharp, permanent endpoint.

Discussion

• In order to neutralize acidic nature of Cu²⁺ solution, sodium carbonate (Na₂CO₃) is used. When excess Na₂CO₃ is added to copper(II) solution, it reacts to form CuCO₃ precipitate.
• CuSO₄(aq)+Na₂CO₃(aq)→CuCO₃(s)+Na₂SO₄(aq)
• CuCO₃ is decomposed by reacting it with acetic acid (a weak acid)
• On adding excess KI, the liberated iodine (I₂) forms a more soluble triiodide complex (I₃⁻), which helps keep iodine in solution
• Cu₂I₂ adsorbs the released I₂. When NH₄SCN or KSCN is added, Cu₂(SCN)₂ is formed, and the adsorbed iodine (I₂) is released into the solution.         
• Cu₂I₂+2KSCN→Cu₂(SCN)₂+2KI

 Important Terms & Definitions

1. Iodometry

An indirect redox titration method where an oxidizing agent (like Cu²⁺) reacts with excess iodide (I⁻) to liberate iodine (I₂), which is then titrated with a standard solution of sodium thiosulphate (Na₂S₂O₃).

2. Sodium Thiosulphate

Definition:
A reducing agent used as the titrant in iodometric titrations. It reacts with iodine to form iodide and tetrathionate.

Reaction:

I₂+2S₂O₃²⁻→2I⁻+S₄O₆^2–

3. Copper(II) Sulphate (CuSO₄)

Definition:
The analyte containing Cu²⁺ ions, which act as an oxidizing agent. It reacts with iodide to liberate iodine.

Reaction:

2Cu²⁺ + 4I⁻→2CuI(s)+I₂

4. Potassium Iodide (KI)

Definition:
Added in excess to reduce Cu²⁺ to Cu⁺ and liberate iodine (I₂). Also helps form soluble I₃⁻ complex to keep iodine in solution.

5. Starch Indicator

Definition:
A visual indicator used in iodometric titrations. It forms a blue complex with iodine, which disappears at the endpoint.

�� Note: Starch is added near the end point to avoid formation of stable complexes that may delay the reaction.

6. Cuprous Iodide (CuI)

Definition:
A white precipitate formed when Cu²⁺ is reduced to Cu⁺ in presence of iodide (I⁻). It adsorbs iodine on its surface.

7. Potassium Thiocyanate (KSCN)

Definition:
Added near the end point to form Cu₂(SCN)₂, which helps release iodine adsorbed on CuI, giving a sharp endpoint.

8. Triiodide Ion (I₃⁻)

Definition:
A more soluble form of iodine formed when I₂ reacts with excess I⁻. This keeps iodine in solution and improves titration accuracy.

I₂+I⁻ = I₃⁻

Viva Questions

• What is iodometric titration? How does it differ from iodimetric titration?Iodometric titration involves indirect use of iodine, where iodine is liberated by an oxidizing agent and then titrated with a reducing agent (like Na₂S₂O₃).Iodimetric titration involves direct use of iodine as a titrant.
• Why is sodium thiosulphate used in this experiment?It is a mild reducing agent that quantitatively reduces iodine (I₂) to iodide (I⁻) and is used to determine the amount of iodine released.
• What is the principle of the iodometric estimation of copper(II)?Cu²⁺ reacts with excess I⁻ to form CuI (ppt) and liberate I₂, which is titrated with Na₂S₂O₃.
• Write the balanced chemical equation between Cu²⁺ and KI.
• 2Cu²⁺+4I⁻→ 2CuI(s) + I₂
• What happens when Cu²⁺ reacts with KI? Name the precipitate formed. Iodine is liberated and cuprous iodide (CuI) precipitates as a white solid.
• Why do we add sodium carbonate and then acetic acid before titration?
• Purpose of Adding Sodium Carbonate:
• Neutralization of Excess Acid:
• The copper(II) solution (usually prepared using nitric acid or sulfuric acid) may contain free mineral acid, which can:
• Interfere with the iodometric reaction.
• Promote unwanted oxidation of iodide (I⁻) by air or acid.
• Formation of Basic Precipitate:
• Sodium carbonate reacts with Cu²⁺ to form a basic copper carbonate (CuCO₃) precipitate:
• Cu²⁺+ CO₃²⁻  →  CuCO₃(s)
• Why is KI added to the copper sulfate solution?
• To reduce Cu²⁺ to Cu⁺ and simultaneously liberate iodine (I₂) for titration.
• What is the function of starch in this titration?
• It acts as an indicator, forming a dark blue complex with iodine near the endpoint.
• Why is starch added near the end point, not at the beginning?
• Because starch binds iodine strongly. If added early, it can trap iodine, causing a diffuse or incorrect endpoint.
• What colour change indicates the end point of the titration?
• The solution turns from blue to colourless as the iodine-starch complex disappears.
• Why is potassium thiocyanate (KSCN) added during the titration?
• To release iodine adsorbed on the CuI precipitate, which sharpens the end point.
• At what point in the titration should KSCN be added and why?
• Near the end point, after most of the free iodine is consumed, because KSCN is slowly oxidized by iodine.
• What will happen if KSCN is added too early in the titration?
• It will be oxidized by iodine, reducing iodine concentration and introducing errors in the result.
• What is the role of CuSCN in improving the endpoint detection?
• CuSCN forms a coating that displaces iodine from CuI, making it available to react quickly with thiosulphate.
• Write the reaction that occurs between CuI and SCN⁻.
• CuI+SCN⁻→CuSCN  + I⁻
• Why should the pH be maintained around 3 in this titration?
• At pH ~3, side reactions are minimized. Too high pH leads to Cu(OH)₂ precipitation; too low pH promotes air oxidation of iodide.
• What will happen if the pH is too high or too low?
• High pH: Cu(OH)₂ forms → Cu²⁺ removed from solution.
• Low pH: I⁻ is oxidized by air → excess I₂ forms → overestimation.
• Why should starch solution be freshly prepared?
• Starch degrades over time and may form reducing sugars like glucose, which interfere in redox titrations.
• Why is starch not suitable for use in strong acidic or hot solutions?
• It hydrolyzes in acid and loses sensitivity at high temperature, weakening the indicator response.
• What causes a “diffuse” or “drift” end point in this titration?
• Slow reaction of adsorbed iodine or early starch addition, which traps iodine and delays colour change.
• What is the stoichiometry between iodine and sodium thiosulphate?
• (Try yourself)
• How many moles of iodine are liberated per mole of Cu²⁺? (Try yourself)

• Why can’t sodium thiosulphate be used as a primary standard?
• It is efflorescent and contains variable amounts of water, making its exact concentration uncertain.
• What are other examples of iodometric titrations?
• Estimation of K₂Cr₂O₇, KMnO₄, Fe³⁺, H₂O₂, Cl₂, and Br₂.
• What are some limitations of using starch as an indicator?
• Not stable in acid, hydrolyzes, and binds iodine strongly. Not usable at high temperatures or early in titration.
• What is the chemical formula of the blue complex formed with starch?
• It’s not a fixed formula but typically referred to as Starch-I₃⁻ complex.
• Why should the titration be done quickly after KSCN addition?
• To prevent oxidation of SCN⁻ by I₂, which would alter iodine concentration.
• What happens if air oxidizes iodide before titration?
• Extra I₂ is formed, leading to overestimation of copper content.

Important Links

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