Solution scanometry, a new method for determination of acidity constants of indicators

Abstract

Solution scanometry, a new, simple, fast and inexpensive method, was applied for the determination of the acidity constant, pK_a, of indicators, for the first time. Three azo indicators including methyl orange (MO), methyl yellow (MY) and methyl red (MR) were selected as appropriate templates of indicators for this purpose. The method is based on scanning of the cells containing the indicator solution with a scanner and then the color of each cell is analyzed with software written in visual basic (VB 6) media into red, green and blue values. The cells were built by creating holes in the Plexiglas® sheet. Also the acidity constants of the cited three azo indicators were determined spectrophotometrically. The pK_a of the indicators was evaluated using spectrophotometric data by the Hypspec program. The corresponding distribution diagrams were depicted through the Hyss2009 program. The comparison between the current and traditional UV-Vis spectrophotometric methods was performed and the results revealed similar trends in both methods. The results obtained from the two methods show good agreement with the literature.