What is pKa or Hammett's function of 2,4,6-trinitrobenzenesulfonic acid? PubChem gives pKa=0.13, but doesn't specify the solvent. Even if it's real value I doubt it's a value for water. Why? Because even aliphatic sulfonic acids are strong, and with electron withdrawing groups, even stronger.
For example this article gives -4.0 for p-nitrosulfonic acid. Even if resonance of o-nitro groups is blocked, it doesn't negate inductive effects. Reasoning with solvation effects also seems dubious - nitro groups aren't hydrofobic at all. Is there some reliable data for this, rather obscure, acid?