Overview
This lecture covers the equilibrium of bromocresol green, using indicators, spectrophotometry, and pH measurements to determine the equilibrium constant (K), including experimental setup and calculations.
Introduction to Indicators and Equilibrium
- Indicators are weak acids that partially dissociate and exist in equilibrium, changing color based on the dominant form.
- Bromocresol green’s conjugate base is blue/green; the acid form is yellow.
- In solution, the sodium salt dissolves to form Na+ and indicator ion (IN–), which equilibrates with H+ to form HIN.
Experimental Methods and Calculating K
- K (equilibrium constant) is calculated as [IN–][H+]/[HIN].
- Use spectrophotometry at two wavelengths to measure the concentrations of both forms based on their absorbance.
- pH meters determine [H+]; convert pH to [H+] using [H+] = 10^(-pH).
- Use the Beer-Lambert law (A = εlc) to relate absorbance to concentration; path length (l) is 1 cm.
- Determine four molar absorptivities (ε) for both forms at two wavelengths by preparing acid-only and base-only standards.
Lab Procedure and Data Collection
- Prepare eight solutions: two standards (full acid, full base), five mixtures for a standard curve, and one unknown sample.
- Measure absorbance for all samples at both chosen wavelengths and record pH.
- Use measured absorbances and molar absorptivities to set up two equations for two unknown concentrations ([HIN] and [IN–]) per sample.
Data Analysis and Calculations
- Plug absorbance data and corresponding ε values into the two equations to solve for [HIN] and [IN–] using systems of equations.
- Perform this calculation for each sample, showing one example calculation; use Excel to handle repetitive calculations.
- Calculate K for each condition to test for constancy and consider temperature effects.
Key Terms & Definitions
- Indicator — a weak acid that changes color depending on pH due to equilibrium between acid/base forms.
- Equilibrium Constant (K) — ratio representing the concentrations of products to reactants at equilibrium.
- Beer-Lambert Law — relates absorbance (A) to molar absorptivity (ε), path length (l), and concentration (c).
- Molar Absorptivity (ε) — measure of how strongly a substance absorbs light at a specific wavelength.
- pH — measure of hydrogen ion concentration, calculated as –log[H+].
Action Items / Next Steps
- Set up an Excel spreadsheet for calculations before beginning lab work.
- Prepare and label all eight solutions as described.
- Measure absorbance and pH for all samples at specified wavelengths.
- Show one sample calculation; perform remaining calculations using Excel.
- Review standard solution preparation and unit conversions as needed.