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Measurement of Indicator pKa - Goals Re-enfor ce ideas on...

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Goals Re-enforce ideas on pH, activity, spectrophotometry, equilibrium How to make and use a buffer Successive approximations in solving a problem Using multiple tools to solve one problem This lab failed in the past; did we figure out why? If not, can we figure out why? How to design an experiment – leadup to final “design your own” procedure
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Procedure Outline Find approximate pK a visually Collect several spectrophotometric measurements near pK a Measure pH of buffer solutions (don’t trust the labels!) Fit absorbance data to theoretical functional form Improve (?) fit by including activity corrections Report pK a of assigned indicator
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Understanding the Measurements Indicator is a weak acidHA H + + A - Buffer has high enough ionic strength that Henderson-Hasselbalch equation is only an approximation Spectrum of HA differs from spectrum of A - Conditions to consider: Only HA absorbs Only A - absorbs Both absorb Both absorb with same molar absorptivity (isosbestic point)
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