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Lets look at the acidic form first in highly

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Unformatted text preview: ncentrations of the acidic and basic forms requires the use of techniques ! studied in Experiment 2A. Because both of the species are colored, spectrophotometric techniques can be used to determine the concentrations of both the acidic and basic forms of the indicator in each solution with a different pH. Let’s look at the acidic form first. In highly acidic solution, where there is an abundance of H3O+ ions, the equilibrium lies to the left and the indicator is primarily in the acidic (HIn) form: HIn( aq ) + H 2O( ) ⇔ H 3O(+aq ) + In(−aq ) € (3) Similarly, in a highly basic solution, the equilibrium lies to the right and the indicator is predominantly in its basic form (In- ). A plot of absorbance vs. wavelength will show a peak that corresponds to the acidic form of the indicator in solutions with low pH; this peak will decrease in intensity as the pH increases and, at very high pH values, will disappear completely. A peak that corresponds to the basic form of the indicator, will gain intensity as the pH is raised. Because the acidic and basic forms of the indicator have different colors, the peaks will be at different wavelengths. At pH values intermediate between these extremes, both forms (HIn and In- ) will contribute peaks to the spectrum. By monitoring the absorbance of the solution at two wavelengths (one that corresponds to HIn, λ1, the other to In- , λ2) as a function of pH, the relative concentrations of the two species can be determined. If the absorbance spectra of both the a...
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