LC CHEMISTRY- REDOX TITRATION: To determine the percentage (w/v) of sodium hypochlorite (NaClO) in bleach.

Oxidising agent

ClO- or I₂

Reducing agent

S₂O₃²⁻ (thiosulfate)

Indicator

Starch

when do we add the starch

straw yellow stage (when most of the iodine has reacted with thiosulphate)

what happens if we add the starch too early

can cause iodine to form a strong complex with starch which won't decolourise

colour change

1. red brown

2. straw yellow

3. blue black

4. colourless

ratio

2 S2O32- : 1 CLO-

describe how the 25.0cm3 of original bleach solution was provided in a beaker and was diluted to exactly 500cm3

6 points

• use funnel to transfer og bleach contents to 500cm³ volumetric flask

• rinse beaker funnel and glass rod into volumetric flask with deionised water

• make up flask to 500cm3 on mark with deionsed water

• bottom of meniscus

• stopper

• invert several times

describe clearly the procedure for using the pipette to measure exactly 25cm3 portions of the diluted bleach into the titration flask

3 points

• rinse pipete with deionsed water then bleach respectively

• fill with pipette filler

• have bottom of meniscus on mark

two reasons why why must excess Kl be added to the diluted bleach

• to ensure all the hypochlorite reacts to form iodine

• to keep iodine in solution

what's the purpose of the white tile

colour change at end point is seen clearly

Why is potassium iodine used instead of iodine itself

potassium iodine is soluble in water

name the reagent that is used as a primary standard in redox titrations

potassium iron II sulfate