Determination of Acidity Constants of Methylene Blue and Alizarin with Luminescence Spectrometry and Investigation of Some Metal Sensitivities

Organic coloring materials are important raw materials which can be used as indicators and / or sensors other than being used for coloring purposes. Many of them gain indicator properties when they can be sensitive to metal and proton ions. Also, many dyes that are active against metal ions also exhibit sensor capability. In this study, the working pH ranges and acidity constants (pKa) of aqueous solutions of methylene blue and Alizarin dyes and also their metal sensitivities were investigated. For this purpose, excitation and emission spectra of freshly prepared 10^-3M aqueous solutions of both dye materials were taken separately with Luminescence Spectrometer. Using the emission spectra, it was determined that the working pH ranges of Methylene blue and Alizarin are 2-6 and 2-9, respectively. The pKa value was calculated as 4,97 for Methylene blue and 3,70 for Alizarin. In addition, Cu2+ and Zn2+ ions, which are ions of IB and IIB groups-d block transition elements, are observed to be able to form complex with Methylene blue. Similarly, Al3+, Cu2+, Co2+, Ni2+ which are ions of the 3A group and IB and VIIIB groups-d block transition elements, are also observed to be able to form complex with Alizarin dyes. It has been shown that Methylene blue and Alizarin have been capable of working as pH sensors depending on the working pH ranges, and can form complexes with selected metals. Moreover, Alizarin is able to capture more metal ions than Methylene blue, has a lower pKa value, and consequently has been more easily complexed with Cu2+ ions.