Diluting Sulfuric Acid: Fuming

[Subystud]

When diluting 98% sulfuric acid it fumes. What is this fume? H2SO4 as a vapor? SO3? H2O?

 

[moltenmetal]

Why are you diluting fuming sulphuric acid, that someone has put all that extra effort into concentrating for you, and saturating with SO3? Why not use the 93% commercial material, which doesn't fume?

 

[Subystud]

The 98.7% acid is not fuming. I have 500 mL of DI water in an ice bath on a stir plate. I am adding 450 mL of 98.7%. I slowly add via a 1L graduated cylinder in to the DI water. When even small portions of 98.7% are added, it fumes.

What is this fume?

 

[moltenmetal]

Sorry for the error- it's been a while since I've used it, but the tech grade acid around here is typically 95-98% too rather than the 93% that I recall. I do recall seeing some fume generation during open pour dilution, particularly during humid days. Could it have to do with the hydration of the dry acid? Just guessing- don't really know.

 

[Subystud]

Thanks for the response. For reference, it was in Florida. It was 50F ambient, and 100% humidity.