Experiment 3 - pKa Indicator

Experiment 3 - pKa Indicator - Chem 257 The pKa of an...

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Chem 257 Experiment 3 The pK a of an Indicator This experiment has several purposes. First, it will demonstrate the use of spectrophotometry as an analytical tool; secondly, it will demonstrate the determination of an equilibrium constant; and finally, since the equilibrium constant to be determined is the dissociation constant of a weak acid, the experiment will also illustrate some of the basic principles of aqueous acid/base chemistry. The dissociation constants of weak acids and bases are important because much of aqueous solution chemistry depends on the degree of protonation of the reactants, and aqueous solution chemistry itself is of interest since much of the chemistry of most immediate concern to us (i.e. biochemistry) occurs in aqueous solution. The weak acid whose dissociation constant is to be determined is bromothymol blue, the well-known acid/base titration indicator, which dissociates as follows: + - In aq + H aq K a HIn aq where the subscripts signify that the reaction occurs in dilute aqueous solution. The dissociation constant is then given by: K a = [H aq + ][In aq - ] [HIn aq ] log K a = log H + + log In - /HIn multiply by minus one: -log K a = -log H + - log In - /HIn pK a = pH – log(In - /HIn) and pH = pK a + log(base/acid) Henderson-Hasselbalch K a can be determined if measurements of [H aq + ], [In aq - ] and [HIn aq ] are made for a particular solution. For greater precision measurements are made in several solutions and the data is fit in a least squares manner to determine K a . In the case of bromothymol blue, HIn aq is colored yellow and In aq blue. This difference is, of course, the basis for the use of this compound as an end point indicator for acid/base titrations. We can also make use of this color difference to quantitatively estimate the amounts of HIn aq and In aq present in a given solution and thus obtain the dissociation constant K a .
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Light interacts with matter in a number of ways giving rise to the phenomena of reflection, refraction and absorption and so on. All of these in the limit involve the interaction of light with molecules. As you learned in first year chemistry the molecules of a compound can exist only in certain discrete energy states (molecular energy is quantized) and in order that absorption of light (or any other electromagnetic radiation) might occur, the energy of the light photons h υ must correspond to the energy difference E between two of the available molecular energy levels, i.e. on absorption of a photon of light a molecule is excited from a lower energy level to a higher one where E = h υ (Fig. 1). Fig. 1 ground energy state excited state h υ ² E The total energy E tot of a molecule (apart from its transitional energy) can be represented as the sum of the energies of three primary intramolecular motions, rotation E rot , vibration E vib , and electronic E el . E
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Experiment 3 - pKa Indicator - Chem 257 The pKa of an...

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