Inhibition of Nitrite Interference: Nitrite ions can react with liberated iodine (I$_2$) during the titration step of the Winkler method, leading to an underestimation of the dissolved oxygen concentration. Sodium azide effectively inhibits this interference by reacting with nitrous acid (HNO$_2$), which is formed from the reaction of nitrite with sulfuric acid (H$_2$SO$_4$), converting it into harmless nitrogen gas (N$_2$):
$$\text{2NaN}_3 \ + \ \text{2HNO}_2 \ \xrightarrow{} \ \text{3N}_2 \ + \ 2\text{NO} \ + \ 2\text{NaOH}+\ \text{H}_2\text{O}$$
By preventing the interference of nitrite, sodium azide ensures accurate determination of dissolved oxygen concentration in the sample.
Additionally, sodium azide also stabilizes the iodine-azide complex formed during the titration, which improves the precision and accuracy of the analysis.
It's worth noting that while sodium azide is effective in eliminating nitrite interference, it can be toxic and explosive when handled improperly. Appropriate safety precautions, such as wearing gloves, eye protection, and working in a well-ventilated area, should be followed when using sodium azide in the dissolved oxygen experiment.
