Outline Final Comments on Titrations/Equilibria Titration of Base with a strong acid End-point detection Choice of indicators Titration Curve method Start.
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Outline Final Comments on Titrations/Equilibria Titration of Base with a strong acid End-point detection Choice of indicators Titration Curve method Start Chapter 18 Spectroscopy and Quantitative Analysis
Weak Base titrated with strong acid Consider a 100 ml of a 0.0100 M base with 0.0500 M HCl K b = 1 x 10 -5
Initial pH Buffer Region pH @ equivalence pH after equivalence Dominated by remaining [H + ]
The Quantitative Picture Transmittance: T = P/P 0 b(path through sample) P 0 (power in) P (power out) Absorbance: A = -log 10 T = log 10 P 0 /P The Beer-Lambert Law (a.k.a. Beer’s Law): A = bc Where the absorbance A has no units, since A = log 10 P 0 / P is the molar absorbtivity with units of L mol -1 cm -1 b is the path length of the sample in cm c is the concentration of the compound in solution, expressed in mol L -1 (or M, molarity) How do “we” select the wavelength to measure the absorbance? to measure the absorbance?
Absorbance vs. Wavelength A 420 440 460400380 Wavelength, nm Why? 1.Maximum Response for a given concentration 2.Small changes in Wavelength, result in small errors in Absorbance
Consider molecules With molecules, many energy levels. Interactions between other molecules and with the solvent result in an increase in the width of the spectra.
Electronic Spectrum Absorbance Wavelength,, generally in nanometers (nm) 0.0 400 800 1.0 200 UV Visible max with certain extinction Make solution of concentration low enough that A≤ 1 (Helps to Ensure Linear Beer’s law behavior) UV bands are much broader than the photonic transition event. This is because vibration levels are superimposed.
The UV Absorption process * transitions: high-energy, accessible in vacuum UV ( max <150 nm). Not usually observed in molecular UV-Vis. n * transitions: non-bonding electrons (lone pairs), wavelength ( max ) in the 150-250 nm region. n * and * transitions: most common transitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with max = 200-600 nm. Any of these require that incoming photons match in energy the gap corresponding to a transition from ground to excited state.
What are the nature of these absorptions? Example: * transitions responsible for ethylene UV absorption at ~170 nm calculated with semi-empirical excited-states methods (Gaussian 03W): bonding molecular orbital antibonding molecular orbital h 170nm photon
Experimental details What compounds show UV spectra? Generally think of any unsaturated compounds as good candidates. Conjugated double bonds are strong absorbers. The NIST databases have UV spectra for many compoundsYou will find molar absorbtivities in Lcm/mol, tabulated.The NIST databases have UV spectra for many compounds You will find molar absorbtivities in Lcm/mol, tabulated. Transition metal complexes, inorganics
Final notes on UV/Vis Qualitatively Not too useful Band broadening Quantitatively Quite Useful Beer’s Law is obeyed through long range of concentrations Thousands of methods Most commonly used Detection Limits ~ 10 -4 – 10 -6 M
Final notes on UV/Vis (cont’d) Quant (cont’d) Cheap, inexpensive, can be relatively fast Reasonably selective Can find colorimetric method or use color of solution Good accuracy ~1-5%
Chapter 5 – Calibration Methods Open Excel Find data sheet Input data table
Uncertainty in Concentration Where: x = determined concentration k = number of samples m = slope n = number of Standards (data points) D = ??