Analysis of an [OA]

Frage	Antworten
What titration to use	Iodine/thiosulfate
What can it be used to determine?	ClO- in bleach Cu2+ in copper(ii) compounds Cu content in copper alloys
Step 1.	Add a std. sol of Na2S2O3 to the burette
Step 2.	Prepare solution of OA to be analysed and pipette into a conical flask
Step 3.	Add an excess of KI. OA will react with I- to produce I2. Solution turns yellow-brown
Step 4.	Titrate this solution with the Na2S2O3 Iodine will be reduced back to I- and brown colour fades gradually, making end point difficult to see.
Step 5.	When end point is being approached, the iodine colour has faded enough to become a pale straw colour
Step 5.	As end point is being approached and iodine colour has become pale straw, add small amount starch indicator. Deep blue-black colour forms
Step 6	As more Na2S2O3 is added the blue-black colour fades, at end point all iodine will have reacted and blue-black colour disappears.
Why starch at end?	As iodine-starch complex very stable at high iodine conc., would require more titrant

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