Determination of Pka of an Unknown Acid-Base Indicator

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DETERMINATION OF THE pKa OF AN UNKNOWN ACID-BASE INDICATOR. NAME: Nana Ekua Entsiwa Adenu-Mensah DATE: 10/11/2013 TA: ERIC FUEMMELER I. INTRODUCTION The aim of this experiment was to measure the pKa of an unknown Acid-Base indicator both qualitatively by measuring the visual change in color of the solutions aided by pH meter readings and quantitatively by the use of a visible spectrophotometer and a pH meter. The unknown solution was red in acidic conditions and yellow in basic solution. The absorbance measured was used to calculate the various pKaat different pH values, and hence the average pKa. The reaction that occurs with indicators is this: HInaq+ H2Ol⇌H3O+aq+In-(aq) (color 1) (color 2) I. METHODOLOGY Equal volumes of acid and base solutions were prepared by adding solutions of acid and base to equal volumes of 200ppm indicator solution. 50mL of the base solution was measured and small amounts of the acid solution were added until a color perceived to be halfway between the acid and base solutions was obtained (solution 3). The pH of this solution measured with the pH meter is equivalent to the pKa of the solution. This was termed ‘Qualitative pKa’. For the quantitative aspect, the absorbance of the acid solution, AHIn and that of the absorbance of the base solution, AIn- were measured via the visible spectrophotometer. Afterwards, small amounts of the acid and base solutions were added to the solution 3 to obtain four other pH values above and below the ‘qualitative pKa’ and their respective absorbance values were measured. Using the equation below, the various pKa values were calculated. pKa=pH+ log10A-AIn-AHIn-A Graphically, the pKa value was determined as the x-intercept of the graph of log10A-AIn-AHIn-A versus pH. II.

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