Determination of Acid Dissociation Constant of Bromocresol Green, Phenolphtalein and Methyl Orange by a Novel Program in MATLAB Software and Changing Color of pH Paper

Document Type : Full research article


Department of Chemistry, Payame Noor University 19395-4697 Tehran, Iran


In this study, the acid dissociation constants (pKa) of three indicators, bromocresol green, phenolphthalein and methyl orange, were determined by scanning the solution of indicators and deposited pH paper in these solutions (in each step of color-changing solution) and then chemometrics method. These methods are simple, fast, and inexpensive. For this reason, first, the vessels containing the indicator solution and pH paper (in each step of color-changing solution) were scanned by the scanner, and then the images of sample solutions and pH papers were transferred to a computer using Microsoft Photo Editor (Microsoft XP). RGB values were measured, in each pixel, with the image processing tool box of MATLAB. In MATLAB (2013) software, a novel program was written based on RGB values, for calculating pKa indicators. The agreement between obtained pKa by this method and values reported in the literature demonstrates the utility of the method here used.


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