Determination of acid concentration using iodate(V) and iodide.
Iodate(V) ions, IO3-, oxidise iodide ions in acidic solution, both iodate(V) and iodide becoming iodine:
IO3- + 6H+ + 5I- à 3I2 + 3H2O
The liberated iodine is titrated with standard sodium thiosulphate solution. Any of the three ionic species present can be the limiting factor in the amount of iodine formed; in this case it is the hydrogen ions.
1 Pipette 25.0 cm3 of the approximately 0.1 mol dm-3 hydrochloric acid into a 250 cm3 conical flask.
2 Add 1 1.5 g of solid potassium iodide and swirl to dissolve.
3 Add 10 cm3 of 5% potassium iodate(V) solution.
4 Titrate the liberated iodine immediately with standard 0.1 mol dm-3 sodium thiosulphate solution; add starch solution when the mixture is a pale straw colour, the endpoint being from dark blue to colourless.
5 Repeat to obtain three consistent titres.
Final volume/cm3 Initial volume/cm3 Titre/cm3
Mean titre: cm3.
Find the concentration of the hydrochloric acid solution.
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