I'd have to say if the acid got close enough to you to **** up your watch, that would be the least of your worries. Also, a lot of the time when H2SO4 "burns" organic compounds, carbon is a reaction product. That might be some of what causes the dark (sometimes black) color of drain cleaner. Perhaps boiling the drain cleaner would decompose most organics in the mix and make them harder (hopefully easier) to seperate.
You could always distill but in my experience H2SO4 is one of the hardest substances to distill because of the extreme reactivity of the boiling acid and the low volatility/high temps involved.
About the density. Maybe this will help answer your question.
Addition of water to stoichiometric 100% sulfuric acid increases the density until a maximum results near 87 mole% H_2SO_4. The density and conductivity maxima and viscosity minimum, the latter two near 75 mole%, are direct macroscopic responses to microscopic quantum mechanical properties of H_3O^+ and of nearly symmetric H-bond double-well potentials, as follows: (1) lack of H bonding to the O atom of H_3O^+; (2) short, 2.4–2.6 A, O—O distances of nearly symmetric H bonds; and, (3) increased mobility of protons in such short H bonds, give rise to the density maximum via (1) and (2); (1) produces the viscosity minimum; and the conductivity maximum results from (2) and (3). A pronounced minimum near 1030 cm^−1 in the symmetric SO_3 stretching Raman frequency of HSO_4^−, observed near 45 mole% also results from double-well effects involving the short H bonds of direct hydronium ion—bisulfate ion pair interactions. Estimates of the concentrations of the (H_3O^+)(HSO_4^−) and (H_2SO_4)(HSO_4^−) pair interactions were determined from Raman intensity data and are given for compositions between 42–100 mole%
That's from ingentaconnect.com
