Iodometry vs Iodimetry
Iodometry and iodimetry are two common titration methods used in analytical chemistry. These two types of titrations are based on oxidation reduction, and can be used to determine redox species quantitatively. Titrations are based on a reaction between the analyte and a standard reagent known as the titrant. The quantity of the analyte can be determined if we know the reaction, stoichiometry and the volume/mass of the titrant needed to react completely with the analyte. Iodine can be used for this redox titrations due to its capability of reacting fast with many species. Reversibility of iodine/iodide, reaction is also an advantage when using them in iodometric reactions.
What is Iodometry?
In iodometry, iodides are allowed to react with another oxidizing agent in an acidic medium or neutral medium. When this reaction takes place, iodide (iodide will be added in the form of KI) will be oxidized to iodine and the other species will be reduced by iodide. The released iodine is then titrated with another species. This titrating species is a standard solution of a reducing agent, which is capable of reducing iodine back to iodide form. Usually, a standard thiosulphate solution is used for this. For example, if we want to quantify the amount of chlorine dissolved in a mixture, following is the method to carry out an iodometric titration.
First, a known amount of volume from the mixture (in which chlorine is dissolved) should be taken into a titration flask. This is titrated with a known solution of KI, and the volume consumed is found. Following redox reaction will take place in the reaction flask.
Cl2 + 2I- —> 2 Cl- + I2
Another titration should be then carried out with the same mixture to determine the released amount of iodine. For this, the mixture is titrated with a standard thiosulphate solution. Starch can be added as an indicator, to determine the end point of this reaction. With iodine and starch in the mixture, it will be observed in dark-blue color, but at the end point when all the iodine is finishes, the dark color will be disappeared.
I2 + 2 S2O32− → S4O62− + 2 I-
From the above two titrations, the amount of Cl2 can be determined.
What is Iodimetry?
In iodimetry, free iodine is used to titrate with a reducing agent. Therefore, iodine will be reduced to iodide, and the other species will be oxidized by iodine. Since free iodine solution cannot be easily prepared, iodine is mixed with potassium iodide and KI3 solution is prepared. And a standard solution of this is used for the iodimetric titrations.
Following reaction takes place when titrating. Starch can be used as an indicator in iodimetric titrations too.
I2 + reducing agent → 2 I-
What is the difference between Iodometry and Iodimetry?
• In iodimetry, a species is directly titrated with an iodine solution, but in iodometry a species is titrated with an iodide solution and then the released iodine is titrated with thiosulphate.
• Therefore, iodimetry is a direct method and iodometry is an indirect method.
• Iodometry can be used to quantify oxidizing agents, whereas iodimetry can be used to quantify reducing agents.