Iodine/ Thiosulfate Redox Titrations

Iodine/ Thiosulfate Titrations: Oxidising agent must first oxidise I- ions to I2. Then thiosulfate
ions S2O3
2- are oxidised and iodine reduced. Amount of iodine can be determined. As well iodine,
other oxidising agents can be determined such as chlorate ions ClO- in bleach, Cu2+ and Cu. Brass
contains copper. I-, OR, I-.
 Method: Same method as before.
– But add excess of potassium iodide. Excess ensures all I2 is formed for titration. Oxidising
agent used reacts with iodine ions to produce iodine. It turns solution yellow- brown colour.
Add HCl to provide H+.
ClO- is oxidising agent here.
– Then titrate with sodium thiosulfate and I2 will be reduced back to I-. Brown colour fades to
pale straw colour.
– Towards end, add starch indicator, so as sodium thiosulfate added, from blue- black goes
colourless at end point- when all iodine reacted and reduced to I-.
This happens during all titrations, no matter the
oxidising agent. Learn these two.
 Analysis:
– Calculate moles of S2O3
2- reacted using CV/ 1000 and mean titre.
– Use molar ratio to find amount of ClO- that reacted with I2- use both equations above to
work out. Scale up x10 to find moles of ClO- in 250cm3
. Since 250cm3
made of up of ClOand water in volumetric flask like iron before.
– Can then work of concentration of ClO- using CV/1000.
 Titration of Copper:
– Cu2+ produced by dissolving compound in water. If insoluble react with acid. For copper
alloys, reacted with nitric acid and then neutralised to form Cu2+
.
– Cu2+ reacts with I- (from potassium iodide) to form I2 and white ppt of copper iodide. Forms
brown colour.
– Iodine then titrated with sodium thiosulfate like above.
– Can analyse copper to work out % composition of copper in copper alloy. Work out mass of
copper from molar ratio and divide by mass of whole compound x100.