Iodometry is a volumetric analysis technique commonly used for quantitative analysis of the concentration of oxidants. In an iodometric analysis the oxidant reacts with excess iodide ion in the presence of acid to generate an amount of iodine that can be related to the amount of oxidant used through the balanced equation for the reaction.
The amount of iodine liberated is determined by titration with a standard solution of the reductant, sodium thiosulfate.(Na2S2O3).
Sodium thiosulfate is not a primary standard because the composition of a solid sample of this substance cannot be known with certainty. Therefore a solution of the approximate concentration is made by dissolving the solid. The exact concentration is determined by titration with a solution containing a known quantity of iodine.
This solution is produced by reaction of a solution containing a known a primary standard oxidant with excess iodide in the presence of acid. The primary standard oxidant used is potassium iodate.
Note that in this reaction I is present in the two reactants in different oxidation states. Transfer of electrons occurs between these to give I2 which has oxidation state 0 (intermediate between the oxidation states of I in the two reactants.
The objective of this activity is to demonstrate the principle of iodometry for an experiment where the HOCl concentration in a bleach sample is being determined. This includes
strong oxidant + 2I– 
reductant + I2
reductant + I2The amount of iodine liberated is determined by titration with a standard solution of the reductant, sodium thiosulfate.(Na2S2O3).
| I2 | + | S2O32– | | 2I– | + | S4O62– (not balanced) |
| oxidant | reductant |
Sodium thiosulfate is not a primary standard because the composition of a solid sample of this substance cannot be known with certainty. Therefore a solution of the approximate concentration is made by dissolving the solid. The exact concentration is determined by titration with a solution containing a known quantity of iodine.
This solution is produced by reaction of a solution containing a known a primary standard oxidant with excess iodide in the presence of acid. The primary standard oxidant used is potassium iodate.
| IO3–(aq) | + | H+(aq) | + | I–(aq) | | I2(aq) | + | H2O (not balanced) |
| oxidant | reductant |
Note that in this reaction I is present in the two reactants in different oxidation states. Transfer of electrons occurs between these to give I2 which has oxidation state 0 (intermediate between the oxidation states of I in the two reactants.
The objective of this activity is to demonstrate the principle of iodometry for an experiment where the HOCl concentration in a bleach sample is being determined. This includes
- preparing the standard solution of the potassium iodate
- using the iodate to standardise the sodium thiosulfate
- using the standard sodium thiosulfate to determine the concentration of the HOCl
- balancing equations for all of the reactions involved
- carrying out appropriate calculations