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Determining the k a of an acid-base indicator

Determining the k a of an acid-base indicator. Experiment 6. Indicator. HIn + H 2 O ↔ In - + H 3 O + Red blue Ka = [H 3 O + ] [In - ]/[HIn] [H 3 O + ] = Ka [HIn]/[In - ] (-log [H 3 O + ]) = (-log Ka) + (log [In - ]/[HIn]) pH = pKa + log [In - ]/[HIn]. Determining Ka.

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Determining the k a of an acid-base indicator

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  1. Determining the ka of an acid-base indicator Experiment 6

  2. Indicator HIn + H2O ↔ In- + H3O+ Redblue Ka = [H3O+] [In-]/[HIn] [H3O+] = Ka [HIn]/[In-] (-log [H3O+]) = (-log Ka) + (log [In-]/[HIn]) pH = pKa + log [In-]/[HIn]

  3. Determining Ka • Ka = [H3O+] [In-]/[HIn] • Adjust pH slightly and measure the concentration of all three species, independently • H3O+ (pH meter) • In- and HIn (spectroscopy)

  4. Ka = [H+][In-]/[HIn]

  5. Quantitative Spectroscopy • Beer’s Law Al1 = el1bc e is molar absorptivity (unique for a given compound at l1) b is path length c concentration

  6. Standard Practice • Prepare standards of known concentration • Measure absorbance at lmax • Plot A vs. concentration • Obtain slope • Use slope (and intercept) to determine the concentration of the analyte in the unknown

  7. Typical Beer’s Law Plot

  8. Spectroscopy • Indicators give fairly broad visible absorption spectra • Blue solution absorbs in the red region • Red solution absorbs in the blue region • But there is usually some significant overlap

  9. Absorption measurement Must make measurements at two wavelengths Al1 = el1HInbcHIn + el1InbcIn- Al2 = el2HInbcHIn + el2InbcIn- Must have the slopes of four Beer’s law plots. Then you have two equations with two unknowns.

  10. The four slopes • Measure standard solutions of Indicator adjusted to high pH (three pH units above pKa) [In-]/[HIn] = 1000 • Measure standard solutions of Indicator adjusted to low pH (three pH units below pKa) [HIn]/[In-] = 1000 • These solution will be prepared and measured to produce the four Beer’s Law Plots (each set at both wavelengths)

  11. In the lab • Prepare Indicator solution adjusted to the proper pH • Measure its pH and Absorbance at 430 nm and 565 nm • Adjust pH with dilute HCl or NaOH by .2 pH units and remeasure • Continue until you have five measurements within 1.0 pH units above and below the starting pH

  12. Spreadsheet • Use data to produce four Beer’s Law Plots • Determine slopes of Beer’s Law Plots • Calculate [H+] from the pH • Use slopes to solve the system of two equations for [HIn] and [In-] • Solve for Ka, Ka = [H+][In-]/[HIn] • Solve for pKa, pKa = -logKa • Calculate averages and 95 % CL

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