4.7 - An Iodine and Sodium Thiosulfate Titration (Leaving Cert Chemistry): Flashcards

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- An Iodine and Sodium Thiosulfate Titration
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Indicator in I2I_2/thiosulfate titration

Starch

Colour of starch-I2I_2 complex

Blue-black

Endpoint colour in I2I_2/thiosulfate titration

Colourless

Molar ratio: Na2S2O3Na_2S_2O_3 to I2I_2

2:1

Why add starch near endpoint, not early?

Early addition causes slow/unclear endpoint

Why is Na2S2O3Na_2S_2O_3 not a primary standard?

Efflorescent, variable water content

Role of KI in this titration

Keeps I2I_2 in solution as I3−I_3^- ions

Why must starch be freshly prepared?

Deteriorates quickly, gives false endpoints

Colour when I2I_2 is liberated

Golden-brown

When to add starch indicator?

When solution fades to pale yellow

Na2S2O3Na_2S_2O_3 reduces I2I_2 to what?

I−I^- (iodide)

Titres must agree within what range?

0.1 cm30.1 \text{ cm}^3

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