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Unformatted text preview: ,acid is the absorbance of the most acidic sample at λ1. The ratio of these two equation (4) and (7) yields Aλ1
Aλ1,acid = ε HIn b[ HIn] [ HIn]
= ε HIn bCTotal
CTotal (8) (9) Similarly, for the basic form of the indicator, In : Aλ 2 € Aλ 2,basic [In ] =
− CTotal Here λ2 is the wavelength where the In absorbs maximally, Aλ2 is the absorbance of a sample at intermediate pH values, Aλ2,basic is the absorbance of the most basic solution, and [In is the concentration of the conjugate €
base. Each of these equations assumes that the total concentration of indicator in the solutions studied is constant. By taking the reciprocol of equation (9) and multiplying both sides by both sides of equation (8), the ratio of the concentrations of [In to [HIn] in any sample can be determined: [ In ] = A
− • Aλ1,acid
Aλ 2,basic • Aλ1 λ2 [HIn] (10) This equation can be used to determine the relative concentrations of the conjugate base form of the indicator to the acidic form of the indicator, which will ultimately assist us in determining the pKa of an unknown indicator. € The Real Case Where Indicator Spectra Overlap (with thanks to Dr. Kahn) Where spectra overlap, we need to subtract the contribution from the species that we are not interested in to find the contribution of the species that we are interested in to the absorbance value at that point. This is complicated, but relies on the fact that, for a single species, the ratio of two absorbances at two wavelengths is equal to...
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 Summer '08
 Dr.Brooks
 pH

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