Litmus "Lecanora tartarea" (Norway, Sweden), "Variolaria dealbata", "Ochrolechia parella", "Parmotrema tinctorum", and "Parmelia". Currently, the main sources are "Roccella montagnei" (Mozambique) and "Dendrographa leucophoea" (California). The main use of litmus is to test whether a solution is acidic or basic. Wet litmus paper can also be used to test for water-soluble gases that affect acidity or alkalinity; the gas dissolves in the water and the resulting solution colors the litmus paper. For instance, ammonia gas, which is alkaline, turn the red litmus paper blue. Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic or
Hammett acidity function is a much stronger acid. The value H = -12 for pure sulfuric acid must not be interpreted as pH = -12 (which would imply an impossibly high HO concentration of 10 mol/L in ideal solution). Instead it means that the acid species present (HSO) has a protonating ability equivalent to HO at a fictitious (ideal) concentration of 10 mol/L, as measured by its ability to protonate weak bases. Although the Hammett acidity function is the best known acidity function, other acidity functions have been developed by authors such as Arnett, Cox, Katrizky, Yates, and Stevens. On this scale, pure