why we not use HNO3 and HCl in iodometry
In iodometry, HNO3 (nitric acid) and HCl (hydrochloric acid) are not commonly used as titrants because they can interfere with the reaction between iodine and thiosulfate, which is the basis of iodometric titrations.
Iodometry is a technique used to determine the concentration of an oxidizing agent by titrating it with a reducing agent, usually sodium thiosulfate (Na2S2O3). The reaction between iodine (I2) and thiosulfate (S2O32-) is as follows:
I2 + 2S2O32- → 2I- + S4O62-
It is important to have a clear and well-defined endpoint for the titration, which is typically the appearance of a pale yellow color, indicating that all the iodine has been reacted. However, HNO3 and HCl can interfere with this endpoint due to their ability to oxidize iodide ions (I-) to iodine (I2):
2I- + 2H+ + HNO3 → I2 + NO + H2O
2I- + 2H+ + 2HCl → I2 + Cl2 + 2H2O
These reactions can lead to an overestimation of the concentration of the oxidizing agent because the iodine produced will consume additional thiosulfate, resulting in a higher titration value. Therefore, it is essential to use a non-oxidizing acid, such as sulfuric acid (H2SO4), as the medium for iodometry to prevent the interference from HNO3 and HCl.
So, in iodometry, HNO3 and HCl are not used as titrants to maintain the accuracy of the analysis and ensure reliable results.