Iodine Clock Reaction

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Required Training	Required PPE
UC Lab Safety Fundamentals	Lab coat, safety glasses/goggles, nitrile gloves

Equipment	Chemicals
230 mL Deionized Water	20 mL 0.25 M NaHSO₃, _{MW = 104} (2.6 g/100 mL) Sodium Bisulfite
400 mL Beaker	100 mL 0.1 M KIO₃, _{MW = 214} (2.14 g/100 mL) Potassium Iodate
600 mL Beaker	5 mL 1% Starch Solution (no more than 3 months old)

Procedure:

In a 400 mL beaker, add 100 mL 0.1M KIO₃, 5 mL 1% starch, and 100 mL H₂O
In a 600 mL beaker, put in 20 mL 0.25M NaHSO₃, and 130 mL H₂O
Mix the two solutions and after a short delay, the clear solution will instantly turn a dark blue/black (_~10 seconds)

Discussion:

Bisulfite anions (HSO₃^-) from NaHSO3 reduce KIO3 to form iodide anions (I-), which further react with KIO3 to form iodine (I2). In solution I2 reacts with I to form triiodide anions (I3-). I3- is immediately reduced back to I- by any remaining HSO3-. Once the supply of HSO3- is exhausted, I3- persists in solution and reacts with starch molecules to form a dark blue starch-iodine complex. Excess I3- is a brown color in solution, and together this produces the dark blue/black color. As the concentration of I3- rises extremely quickly, the color change is almost instantaneous. The volume of NaHSO3 solution added to the reaction will change the time required for the color change – larger volumes will increase the delay, and smaller volumes will decrease it.

Hazards:

KIO₃ is an oxidizer and should be kept away from flammable materials and reducing agents. NaHSO₃ is a strong reducer and should be kept away from acids and oxidizing agents.

SOP

Strong Oxidizing Agent – Potassium Iodate

Strong Reducing Agent – Sodium bisulfite

Disposal (by Storeroom)

While stirring, slowly add solid sodium thiosulfate (Na₂S₂O₃^•5H₂O) until the mixture is no longer blue. Flush this mixture down the drain with large quantities of water.