Colorimetric visualization of acid-base equilibria in non-polar solvent.

Visualization of an acid-base equilibrium in a non-polar solvent (dichloromethane), which may be extended to other solvents, is reported. It is based on an oxoporphyrinogen as a multichromic indicator of prevailing acidity in solution and presents up to six distinct hues depending on degree of protonation, tautomeric state or presence of a basic guest.