Problems in sodium sulfite analysis

Add hydrochloric acid first; sodium sulfite will react with it in advance, and a small amount of sulfur dioxide will be released before you add the iodine solution. Although the amount is small, it can still affect the analysis results. Therefore, iodine must be added first, followed by acidification.

OP, do you mean sodium sulfite solution, sodium sulfite in saltwater, or sodium sulfite in hydrochloric acid?

The issue you raised can be explained from two perspectives: 1. Under acidic conditions, adding water first and then iodine solution will cause a small amount of SO2 to escape. 2. Adding iodine solution first and then water results in a higher initial reaction concentration, which facilitates a more complete reaction. This is because the rate of redox reactions is relatively slow compared to other reactions.

I think everyone hasn’t understood what the original poster meant. What the poster meant was: Step 1: First, weigh about 0.2 g of sodium sulfite in an iodometry flask, then add iodine and water, followed by acid, and then conduct the analysis； Method 2: First, add iodine and water to an iodometric flask, then weigh in about 0.2 g of sodium sulfite using the gravimetric method, add acid, and proceed with the analysis. The results of these two analysis methods differ significantly. Method 2 is a national standard method that requires rapid sample weighing. The analysis result is high, while the result from Method 1 is much lower. Especially when the iodine flask in Method 1 is not dried, the analysis results are even more underestimated. Personally, I think that in addition to the opinions of the teachers above, it is possible that when the water in the iodometric flask comes into contact with sodium sulfite, it reacts more easily with oxygen in the air, thereby affecting the analysis results. I hope all the teachers can give me some feedback!