Strength of K₂Cr₂O₇ by Iodometric Titration

To determine the strength of a given K₂Cr₂O₇ (potassium dichromate) solution using provided N/10 Na₂S₂O₃ and N/10 K₂Cr₂O₇ by iodometric titration.

Potassium dichromate in acidic solution oxidizes KI to liberate iodine:

K₂Cr₂O₇ + 6KI + 7H₂SO₄ → Cr₂(SO₄)₃ + 3I₂ + 4K₂SO₄ + 7H₂O

The liberated iodine is titrated against sodium thiosulphate:

I₂ + 2Na₂S₂O₃ → Na₂S₄O₆ + 2NaI

Starch is used as indicator — the blue starch-iodine complex disappears at the endpoint.

K₂Cr₂O₇ is a primary standard (n-factor = 6, M = 294.18 g/mol).

1. Pipette 20 mL of the given K₂Cr₂O₇ solution into an iodine flask.
2. Add 10 mL of dilute H₂SO₄ (1:5) and 1 g of KI crystals. Mix well.
3. Stopper the flask and keep in dark for 5 minutes for complete reaction of Cr₂O₇²⁻ with I⁻.
4. Add 100 mL of distilled water to dilute the solution.
5. Titrate the liberated iodine against N/10 Na₂S₂O₃.
6. Add 1 mL starch indicator when the solution turns pale yellow.
7. Continue titrating until the blue colour disappears (endpoint = green solution). Volume = V mL.
8. Strength of K₂Cr₂O₇ (N) = N_thiosulphate × V / 20.
9. Strength (g/L) = Normality × Equivalent weight = Normality × 49.03 g/L.

Normality of K₂Cr₂O₇ = ______ N. Strength = ______ g/L. The solution endpoint is characterised by a green colour (Cr³⁺ ions).

• Keep the flask in the dark for 5 minutes — iodine is light-sensitive.
• Dilute before titrating to reduce iodine losses by volatilization.
• Add starch indicator only near the endpoint (pale yellow).
• The endpoint is permanent green due to Cr³⁺ — do not confuse with residual blue.
• KI must be added in excess to ensure complete reduction of Cr₂O₇²⁻.

• Vogel's Textbook of Quantitative Chemical Analysis
• Day, R.A. – Quantitative Analytical Chemistry