How to Accurately Determine Iodine Content in Edible Salt: A Comprehensive Guide

R&D Scientist

Theory

Iodometric titration is based on the reaction between iodine and sodium thiosulfate. Iodine present in iodized salt is released and reacts with a titrant, allowing for the quantitative measurement of iodine content.

KIO₃ + 5KI + 3H₂SO₄ → 3K₂SO₄ + 3I₂ + 3H₂O

KI + I₂ → KI₃

2Na₂S₂O₃ + I₂ → 2NaI + Na₂S₄O₆

Equipment and Chemicals

Distilled water
Potassium iodide (KI)
Standardized sodium
thiosulfate solution
Starch solution
Burette
Pipette and pipette
filter
Erlenmeyer flask
Beaker

Preparation of Standard solutions and reagents

Sodium thiosulphate (0.005 N)

A precise mass of 1.24 g of sodium thiosulfate was measured and transferred to a 1000 mL volumetric flask using a funnel. A small volume of distilled water was added to the flask, and the mixture was agitated until complete dissolution of the solute occurred. The flask was subsequently filled with distilled water to the 1000 mL mark to achieve the desired concentration.

Sulfuric acid (2M H₂SO₄)

In a 100 mL volumetric flask, 11.222 mL of concentrated sulfuric acid was accurately measured using a pipette and gradually added to distilled water. The solution was then diluted to the 100 mL mark with additional distilled water to achieve the desired concentration.

Preparation of 10% potassium iodide

A mass of 10 g of potassium iodide was accurately weighed and dissolved in 100 mL of distilled water within a volumetric flask to prepare a 10% potassium iodide solution.

Preparation of 1% starch indicator solution

A mass of 1 g of soluble starch was accurately weighed and placed in a 100 mL conical flask. The starch was then dissolved by heating the mixture to 80 ⁰C while continuously stirring for 5 minutes in 100 mL of distilled water. After the dissolution was complete, the solution was allowed to cool to room temperature.

Procedure

Dissolve 10 g of sodium chloride (NaCl) in 50 mL of distilled water.
Subsequently, add 2 mL of 2 M sulfuric acid (H₂SO₄) and 5 mL of a 10% potassium iodide (KI).
Seal the flask with a stopper and incubate in a dark environment for approximately 10 minutes to facilitate the reaction.
After incubation, remove the flask from the dark and perform a titration using a sodium thiosulfate (Na₂S₂O₃) solution as the titrant. Continue titration until the solution exhibits a pale-yellow color.
At this point, introduce a few drops of 1% starch solution to the mixture.
The solution will transition to a deep purple color due to the formation of the starch-iodine complex.
Continue the titration by adding sodium thiosulfate dropwise from the burette until the solution becomes colorless, and record the final volume of titrant used.
Add thiosulphate drop by drop from the burette until the solution becomes colorless and note the final reading.