23.3 - Iodine/thiosulfate redox titrations Flashcards Preview

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Flashcards in 23.3 - Iodine/thiosulfate redox titrations Deck (19):

What happens to S2O3 2- ions in iodine-thiosulfate titrations?

Become oxidised


What happens to iodine in iodine-thiosulfate titrations?

Becomes reduced


What can iodine/thiosulfate titrations be used to determine?

ClO- content in household bleach
Cu2+ content in copper (ii) compounds
Cu content in copper alloys


Outline how you would set up an iodine/thiosulfate redox titration

1) Prepare a standard solution of sodium thiosulfate and add to burette
2) Add solution to be analysed to a conical flask
3) Add an excess of KI


Why is KI added?

Reacts with the oxidising agent to produce iodine, turning the solution a yellow-brown colour


What is the colour change observed in iodine-thiosulfate reactions?

Yellow-brown --> Pale straw colour


What colour indicates the end point of an iodine-thiosulfate titration?

Pale straw colour


Why is a starch indicator added towards the end of the titration?

Turn solution from blue-black --> colourless
Reduces the iodine to I-


Outline the procedure used to analyse household bleach

1) Pipette 10cm3 bleach into vol. flask and add water up to 250cm3
2) Measure 25cm3 of this solution and add to conical flask
3) Add 10cm3 KI + sufficient HCl to acidify the solution
4) Titrate this solution with sodium thiosulfate
5) Analyse results to determine conc. Cl+ ions in the bleach


Why is HCl added in the procedure to analyse household bleach?

Provides the H+ ions needed for the reaction


What is an active ingredient of bleach?

ClO- ions
Often supplied by NaClO


Write equation for ClO- + I- + H+

Write equation for the titration reaction between I2 + S2O3 2- ions

ClO- + 2I- + 2H+ --> Cl- + I2 + H2O

I2 + 2S2O3 2- --> 2I- + S4O6 2-


How would you prepare a practical for analysis of brass?

1) React brass with HNO3
2) Neutralise the solution
3) Add excess KI
4) Titrate iodine with sodium thiosulfate


Give the equation for a reaction in which brass is reacted with conc. HNO3 and is then neutralised to form Cu2+ ions

Cu --> Cu 2+


Write the equation for the reaction that takes place when excess KI is added to Cu 2+ ions (balance by oxidation numbers)

2Cu 2+ + 4 I- --> 2 CuI + I2


What colour is the CuI precipitate formed?



What colour is the solution formed when excess KI has been added?



Write the equation for the reaction between iodine and thiosulfate ions that occurs during titration

2 S2O3 2- + I2 --> 2 I- + S4O6 2-


Why is an excess of KI added when analysing oxidising agents in iodine-thiosulfate reactions?

To reduce I- to iodine
Iodine (I2) is then used in the titration