Measurement of Indicator pKa - Goals • Re-enforce ideas...

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Unformatted text preview: 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? I f not, can we figure out why? • How to design an experiment – leadup to final “design your own” procedure 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 • I mprove (?) fit by including activity corrections • Report pK a of assigned indicator Understanding the Measurements • I ndicator 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) I ndicator is a Weak Acid...
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This note was uploaded on 01/06/2011 for the course CHEM 223 taught by Professor Scheeline during the Fall '08 term at University of Illinois at Urbana–Champaign.

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Measurement of Indicator pKa - Goals • Re-enforce ideas...

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