An optical sensor of acidity in fuels based on a porphyrin derivative

The measure of the acid concentration in an organic media is a common issue in chemistry. With the aim to overcome this problem in a facile and inexpensive way, the spectroscopic properties of a PEGylate porphyrin derivative, the 5,10,15,20-p-(ω-methoxypolyethyleneoxyphenyl) porphyrin (P), as an acidity sensor were tested. Its behaviour was analysed in toluene and several fuels through acid titration experiments. In each case, examining UV–vis and fluorescence spectroscopic data, the disappearance of the free-base porphyrin derivative Uv–vis signals (B- and Q-bands) and the increasing of new bands, as a consequence of the added acid, was monitored. On the basis of these data it was possible to evaluate the presence of acid in several organic media as toluene, gasoline, diesel, and jet fuel, using our porphyrin derivative in very low amount (about 3 ppm). Finally, in order to exclude the risk of a negative contribution of P in ash formation, the complete oxidation of the porphyrin compound under common engine's combustion conditions was also ascertained.