Iodine thiosulfate titration

Question 1

Thiosulfate ions formula

Answer 1

S₂O₃²⁻

Question 2

Equation

Answer 2

2S₂O₃²⁻ + I₂ —-> S₄O₆²⁻ + 2I⁻

Question 3

idea for this titration

Answer 3

Find out the concentration of iodine in a sample by titrating against a standard solution of thiosulfate ions (S₂O₃²⁻)

Question 4

Indicator used to clarify end point

Answer 4

Starch

Question 5

End point of this reaction

Answer 5

All the iodine in conical flask has reacted with thiosulfate and reduced to I-
So when starch indicator is added, no blue black colour remains

Question 6

Colour change for end point

Answer 6

Straw colour in conical flask (iodine) decolourises to colourless

Question 7

Why can starch indicator clarify end point?

Answer 7

A drop to conical flask turns blue black to show iodine is still present so continue the titration

Question 8

When is the starch indicator added?

Answer 8

When the light brown becomes more pale to show little iodine is remaining:most has been reduced to iodide
So we clarify end point with starch

Question 9

Why does starch turn blue black in presence of iodine?

Answer 9

Any starch will form a complex with iodine to form blue black colour