In the preparation of sulfuric acid through contact process,

Hot sulfur trioxide passes through the heat exchanger and is dissolved in concentrated $\ce{H2SO4}$ in the absorption tower to form oleum:

$$\ce{H2SO4(l) + SO3(g) → H2S2O7(l)}$$

Oleum is reacted with water to form concentrated$\ce{H2SO4}$.

$$\ce{H2S2O7(l) + H2O(l) → 2 H2SO4(l)}$$

The sulfuric acid obtained is $\ce{96-98}$% pure. That means $\ce{2-4}$% is impure and contains byproducts and other oxoacids of sulfur including trisulfuric acid. If the sulfur trioxide to be adsorbed on $\ce{H2SO4}$ is in excess, it can form trisulfuric acid.

$$\ce{H2SO4(l) + 2SO3(g) → H2S3O10(l)}$$

But the amount of trisulfuric acid is negligible and can be omitted. But is it an actual compound? Can it be isolated? Can it exist in room temperature?

Pubchem identify it as a compound and also has a unique ID. It also has an unique CAS number.

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It has $\ce{S=O}$ bonds and $\ce{S-OH}$ bonds (characteristics of oxoacid of sulfur) but still it is not mentioned in that list (link above). Why?

  • $\begingroup$ It is mentioned on this list, you should read carefully. $\endgroup$ – Mithoron Feb 3 '16 at 15:46
  • $\begingroup$ @Mithoron Are you saying H2SO4.2SO3 is trisulfuric acid? If it is so, there are no proof of its existence. "It has not been characterized yet" as per the answer. $\endgroup$ – Nilay Ghosh Feb 3 '16 at 15:55

It seems that this compound has not yet been characterized, although traces of it could be found in mixtures of $\ce{H2SO4}$ and $\ce{SO3}$. The most recent paper I could find is this one from 2012, showing that salts of the respective anion exist and theoretical calculations imply that the acid is stable with respect to decomposition into $\ce{H2S2O7}$ and $\ce{SO3}$. The conditions under which it may be stable are however unknown.

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    $\begingroup$ I’m pretty sure that it’ll be very unstable to hydrolysis and possibly even long-term unstable to sulphuric acid solvolysis ($\ce{H2S3O10 + H2SO4 <=> 2 H2S2O7}$). $\endgroup$ – Jan Feb 4 '16 at 21:07

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