In an iodometric titration, excess potassium iodide solution (KI) is added to the sample.Analyte such as chlorine liberates iodine from KI under acidic condition. Liberated iodine is titrated against standard sodium thiosulphate.
The concentration of analyte in the sample is given as
mg analyte/L
= (mL sodium thiosulphate x normality of sodium thiosulphate x milliequivalent weight of analyte) / mL of sample
Why do we need to multiply by normality of sodium thiosulphate?
mL Na2S2O3 x N Na2S2O3 = # milliequivalents.
Then #milliequivalents x milliequivalent weight = grams.
In an iodometric titration, the liberated iodine from the analyte is titrated against standard sodium thiosulphate solution. Sodium thiosulphate is a reducing agent that reacts with iodine, converting it into a soluble iodide ion - thiosulphate ion reaction. The reaction can be represented as follows:
I2 + 2S2O3^2- → 2I^- + S4O6^2-
The iodine reacts with the thiosulphate in a 1:2 stoichiometry. This means that for every mole of iodine, two moles of thiosulphate are required to completely react with it.
Now, the basic principle of titration is to have a known concentration of a reagent (in this case, sodium thiosulphate) and determine the unknown quantity or concentration of the analyte. The concentration of the sodium thiosulphate solution is usually expressed in terms of its normality.
Normality (N) is a unit of concentration that represents the number of equivalent weights (or milliequivalents) of solute per liter of solution. The equivalent weight is the molecular weight of the substance divided by its valence.
In the equation you provided:
mg analyte/L = (mL sodium thiosulphate x normality of sodium thiosulphate x milliequivalent weight of analyte) / mL of sample
Multiplying by the normality of sodium thiosulphate is necessary to convert the volume (in mL) of sodium thiosulphate used in the titration to the number of milliequivalents of sodium thiosulphate present in the solution. This adjustment helps to accurately calculate the concentration of the analyte in terms of milligrams per liter (mg/L).
In simple terms, multiplying by the normality of sodium thiosulphate allows us to take into account the stoichiometry of the reaction between iodine and thiosulphate, ensuring that our calculation considers the correct amount of thiosulphate needed to react with the liberated iodine.
In an iodometric titration, the liberated iodine is titrated against standard sodium thiosulphate. Sodium thiosulphate is used as the titrant because it reacts directly with iodine, converting it to iodide ions. The reaction between sodium thiosulphate and iodine is as follows:
2Na2S2O3 + I2 → Na2S4O6 + 2NaI
In this equation, the ratio between sodium thiosulphate and iodine is 2:1. This means that for every two moles of sodium thiosulphate, one mole of iodine is consumed.
The normality (N) of a solution is defined as the number of equivalents of the solute present in one liter of the solution. In the case of sodium thiosulphate, it contains two equivalents of S2O3 ions for every mole of the compound.
By multiplying the volume of sodium thiosulphate used (in mL) by its normality (N), you are converting the volume of sodium thiosulphate into the number of milliequivalents of sodium thiosulphate used in the titration. This is important because the equation requires a 2:1 ratio between sodium thiosulphate and iodine.
Therefore, multiplying by the normality of sodium thiosulphate helps to ensure that the correct stoichiometric ratio is used in the calculation of the concentration of the analyte in the sample.