491.
The brown colour of an acidified dilute solution of iodine in aqueous potassium iodide is intensified by the addition of a nitrite but is discharged by the addition of sulphite.
Nitrites oxidise iodide ions to iodine, while sulphites get oxidised by iodine to colourless sulphates.
2KI + HNO3 ----> 2KNO2 +2H2O +2NO +I2
Thus more iodine is produced from KI and colour is intensified.
The sulphrous acid H2SO3 is a reducing acid
H2SO3 ---> H2O + SO2
SO2+ 2H2O ------> SO42- +4H+ +2e-
I2 +2e ----> 2I-
Due to conversion of iodine iodine (brown ) in iodine I- colourless.
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