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CHM12 Experiment 6 Determination of the Equilibrium Constant for Bromocresol Green

A spectrophotometer is an instrument used for

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Unformatted text preview: ation in this experiment. A spectrophotometer is an instrument used for measuring the amount of absorption at different wavelengths. Many such instruments are now commercially available. Some are designed to operate in the region of visible light, some in infra-red regions, some in ultra-violet and still others in several of these energy bands. You will use either a Spectronic 301 Spectrophotometer or a Genesys 20 Spectrophotometer to measured absorbance in this experiment. Both instruments are designed for use in the visible portion of the spectrum (400 to 700 nanometers, nm). Determining the Equilibrium Constant If the acidic form (HB–) and basic form (B2–) of an indicator both [B2- ] absorb light, then the ratio [HB- ] can be determined by measuring the absorption of light at the wavelength at which one of the forms absorbs a lot and the other form absorbs a little. Plots of the absorption of light of various wavelengths (λ) versus wavelength for bromocresol green in its basic form and in its acidic form are shown in the chart to the right. Such graphs are called absorption spectra. λmax: Wavelength of maximum absorbance for base form The graph shows that the basic form has maximum absorption (AII) at λmax. The absorbance of the acidic form (AI) is small at λmax (almost zero). If we start with a solution of the pure basic form (B2–) and add an acid to the solution (for example, acetic acid), then some of the basic form will be converted to the acid form (HB–). Then the absorbance at λmax will drop because the acidic form absorbs almost no light at λmax. This change may be visible in that the solution will change from blue to yellow. Department of Physical Sciences Kingsborough Community College The City University of New York Spring 2012 Experiment 6: Determination of the Equilibrium Constant for Bromocresol Green 3 We will call an absorbance measurement with a mixture of the acidic and basic forms of the indicator Ax. In fact, all absorbance values along the vertical line drawn at λmax in the graph are Ax values. These points all lie between AII and AI. Referring back to the equation, log [B2- ] = pH + log K , we see that in order to calculate the equilibrium [HB- ] [...
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