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CHM12 Experiment 6 Determination of the Equilibrium Constant for Bromocresol Green

CHM12 Experiment 6 Determination of the Equilibrium Constant for Bromocresol Green

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Department of Physical Sciences Kingsborough Community College The City University of New York Spring 2012 Experiment 5: Determination of the Equilibrium Constant for Bromocresol Green 1 Experiment 6 Determination of the Equilibrium Constant for Bromocresol Green Reading assignment: Chang, Chemistry 10 th edition, Chapter 15: Acids and Bases, sections 1-5. Goals To determine the acid dissociation constant (K a ) for bromocresol green (BCG), an acid-base indicator. Discussion Acid-base indicators are often used to demonstrate the end-point of an acid-base reaction. Examples include phenolphthalein and the mixture of indicators used in universal indicator solution. Acid-base indicators are weak acids that dissociate into a hydronium ion (H 3 O + ) and a conjugate base anion (In ). This dissociation can be represented through the following equation and equilibrium expression: + - + - 3 2 3 weakacid conjugatebase hydronium indicator indicator ion [H O ][In ] HIn (aq) + H O (l) H O (aq) + In (aq) K = [HIn] a In order for a compound to be a useful indicator, the acidic form (HIn) and the basic form (In ) of the indicator should differ in color. Since equilibrium in acidic solution favors the formation of HIn, this species is called the acidic form of the indicator. Likewise, the In form is called the basic form since it is favored in basic solutions. An equilibrium mixture of the indicator will be colored according to the relative concentration of each form of the indicator. The position of the equilibrium and, therefore, the relative concentration of the two forms of the indicator will depend on the H 3 O + concentration, [H 3 O + ] or in shorthand notation [H + ]. The absorption curve of an indicator at different pH values can be studied to determine the equilibrium constant of the indicator. In this experiment, we will determine the equilibrium constant of bromocresol green (BCG). BCG is an indicator that is yellow in acidic solutions blue in basic solutions. When dissolved in water the conjugate pair (acidic and basic forms) display different absorption spectra since they possess different colors. Structure of Bromocresol Green (C 21 H 14 Br 4 O 5 S, MM = 698 g/mol) Acidic form (yellow in solution) Basic form- two resonance forms (blue in solution)
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