What is "20% Free SO3"

Nov 2013
2
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We are using fuming sulfuric acid in our lab and I don't understand what "20% Free SO3" means. The manufacturer/supplier (Sigma Aldrich) lists this as the assay, whereas the fuming nitric acid we use from them is listed as 99.5%. My understanding is that fuming indicates an assay greater than 90%. Can someone explain?

Just did a bit of research, and apparently sulfuric acid purity is expressed by the amount of SO3 dissolved in the mixture. So that would mean 20% free SO3 means our acid has 80% H2SO4. I'm curious as to how much H2SO4 (or how little SO3) constitutes fuming, and exactly what is meant by "free" SO3.
 
Last edited:
Nov 2013
2
0
Ok. Did a little research and this is what I found. The assay of H2SO4 (sulfuric acid) is apparently expressed in the amount of SO3 (sulfur trioxide) it contains. So H2SO4 with 20% SO3 has 80% H2SO4.