Solution C was prepared by dissolving 172 mg (1 mmol) of sulfanilamide and 130 mg (0.5 mmol) of N-(1-naphthyl)ethylenediamine dihydrochloride in 10 mL of 37% HCl and, afterwards, diluting the mixture to 200 mL with H2O. Subsequently, 470 mg (3 mmol) of VCl3 was added to the mixture. Solution C is stored in the fridge and can be used for up to 6 weeks. However, better results are obtained when the same colour solution is used for the calibration and the experiments.