272_Exp2B

# In order to correctly figure out the in hin ratio

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Unformatted text preview: cidic and basic forms overlap, then the absorbance measurement at a single wavelength will be the sum of the absorbances of each species that exists at that wavelength. Consider the absorbance spectra of the acidic and basic forms of unknown indicator A, as shown below: The absorbance at 443 nm, where the basic (In- ) form of the indicator maximally absorbs, will include a contribution from the acidic (HIn) form of the indicator (blue line), which has non- zero absorbance at this wavelength. Likewise, the absorbance at 523, where the acidic (HIn) form of the indicator absorbs, will include a contribution from the basic (In- ) form of the indicator (red line). In order to correctly figure out the [In- /[HIn] ratio, we need to determine the absorbance contribution from only the acidic and basic forms at the apporpriate wavelength, and not the contribution from the overlapping spectrum. As this is complicated, the simpler case will be consider first in which the peaks for the acidic and basic forms do not overlap at all. The Case of Zero Overlap Between Indicator Spectra At λ1, the absorbance at can be written as (4) A"1 = # HIn b[ HIn] where Aλ1 is the absorbance at wavelength 1, εHIn is the molar absorptivity constant for HIn, b is the cell path length (assumed to be constant), and [HIn] is the concentration of the acidic form of the indicator. ! The total concentration, at any time, Ctotal is given by: Ctotal = [ HIn ] + [ In " ] (5) However, at low pH values, the indicator is primarily in its acidic form, HIn, so the total concentration is given by: (6) Ctotal = [ HIn ] ! If we substitute Ctotal into the Beer’s Law expression (eqn 4), we get A"1,acid = # HIn bCtotal (7) ! ! where Aλ1...
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