Determination of Titratable Acidity in Wine Using Potentiometric, Conductometric, and Photometric Methods

This experiment describes a simple protocol for teaching acid-base titrations using potentiometry, conductivity, and/or photometry to determine end points without an added indicator. The chosen example examines the titratable acidity of a red wine with NaOH. Wines contain anthocyanins, the colors of which change with pH. Importantly, at the equivalence point, anthocyanins maintain their color, and this effect can be captured optically using a desktop scanner. RGB-based color values are obtained from the digitized images using a dedicated application and employed to generate plots of |v| versus titrant volume. The end point is the point at which the slope (d|v|/Delta/dV) of the titration curve is at a maximum; (d|v| is a vector expression of the color change). The photometric titrations were carried out using a plastic 96-well immunology plate and a flat-bed scanner, and RGB values were extracted simultaneously from all 96 wells in less than 5 min from the images using the ImageJ plugin "ReadPlate". The wine was also titrated using conventional potentiometric and conductometric techniques, and methods were compared using F-test, t-test, and one-way ANOVA. The potentiometric titration yields pH versus titrant volume curves, and the equivalence point is where the slope, or the slope of the derivative curve (dpH/dV), is greatest. The conductometric titration yields conductivity versus titrant volume curves, and the end point is determined as the volume where the slope exhibits a marked change. All titration and derivative curves were plotted and analyzed using spreadsheet software.