A = abc - Academic Computer Center

1 CHM 103 Sinex The process of light being absorbed by a solution concentration 2 with sample concentration 1. I < Io blank where Io = I. Spectrophotometry: light An Analytical Tool source detector Io I. As concentration b increased, less light was Cell with transmitted (more Pathlength, b, light absorbed). PGCC CHM 103 Sinex containing solution Some terminology Beer's Law I intensity where Io is initial intensity A = abc T transmission or %T = 100 x T where a molar absorptivity, b pathlength, (absorption: Abs = 1 T or %Abs = 100 - %T) and c molar concentration T = I/ Io See the Beer's Law Simulator A absorbance A = - log T = -log I/ Io PGCC CHM 103 Sinex PGCC CHM 103 Sinex Analyze at what wavelength? The BLANK. Scan visible wavelengths from 400 650 nm (detector range) to produce an absorption The blank contains all substances spectrum (A vs.)

2 Expect the analyte. Crystal Violet Absorption Spectrum Is used to set the absorbance to zero: 1. Ablank = 0. Absorbance This removes any absorption of light max due to these substances and the cell. 0. 200 250 300 350 400 450 500 550 600 650 700 750 All measured absorbance is due to wavelength, nm phototube detector range analyte. max - wavelength where maximum absorbance occurs PGCC CHM 103 Sinex PGCC CHM 103 Sinex 1. CHM 103 Sinex What does the absorbed light The components of a Spec-20D. (electromagnetic radiation). Light source - white light of constant intensity do to the molecule? IR UV. slits 700 nm visible 400 nm Energy increasing filter occluder high energy UV ionizes electrons Grating low energy UV and visible promotes electrons Phototube slits Separates white light to higher energy orbitals into various colors detects light & (absorption of visible light leads to a colored solution).

3 Measures intensity Rotating the grating Sample changes the wavelength When blank is the sample going through the sample IR causes molecules to vibrate (more later). Io is determined PGCC CHM 103 otherwise Sinex I is measured PGCC CHM 103 Sinex Uses of visible UV/visible light absorption spectrophotometry Valence electrons In organic molecules, electronic transitions to Analysis of unknowns using Beer's Law higher energy molecular orbitals double calibration curve (Been there, done that!). bonds: * Absorbance vs. time graphs for kinetics In transition metals, hydrated ions as Cu++ Single-point calibration for an equilibrium have splitting of d orbital energies and constant determination electronic transitions weak absorption Spectrophotometric titrations a way to follow In complexed transition metals, charge a reaction if at least one substance is colored.

4 Transfer of electrons from metal to ligand as sudden or sharp change in absorbance at Cu(NH3)4++ strong absorption equivalence point, a piece-wise function PGCC CHM 103 Sinex PGCC CHM 103 Sinex CV+ + OH- CV-OH. Kinetics of Crystal Violet Reaction purple colorless colorless This is tracking reaction progress over time. CV+ + OH- CV-OH. purple colorless colorless Since the absorbance is absorbance related to concentration, Follow concentration of crystal violet over rate or A/ time is the time as it reacts by measuring its absorbance. slope of a regression line. How will absorbance change with time? For a absorbance vs. time plot, how will you Short run times to get initial rates. determine the rate of the reaction? time PGCC CHM 103 Sinex Chime structures PGCC CHM 103 Sinex STELLA model 2. CHM 103 Sinex Single-point calibration Equilibrium Constant Determination Standard with measured absorbance Fe+3 + SCN- = Fe(SCN)++.

5 Colorless colorless orange Astd = abcstd Unknown with measured absorbance K = (Fe(SCN)++)/(Fe+3 )(SCN-). Aunk = abcunk Using the reactants, shift reaction based Ratio the two equations on Le Chatelier's principle. Aunk/ Astd = abcunk /abcstd Fe(SCN)++ + SCN- = Fe(SCN)2+. Aunk/ Astd = cunk /cstd We start with a high concentration of Fe+3. Solve for cunk and lower its value by dilution. PGCC CHM 103 Sinex PGCC CHM 103 Sinex Interactive Excel spreadsheet When calibration curves go bad! Spectrophotometric titration The linear Beer's Law relationship starts to Let's consider the analysis of hydrogen show curvature at high concentrations peroxide with potassium permanganate 1 Calibration Curve in an acidic solution. Non-linear Absorbance linear The potassium permanganate or MnO 4- is the only colored substance in the curved Linear (linear).

6 0. 0 1. reaction. (It can serve as its own concentration indicator.). Single-point calibration assumes a linear How would the absorbance change as calibration curve titrant was added? PGCC CHM 103 Sinex PGCC CHM 103 Sinex 5H2O 2 + 2 MnO 4- + 6H+ 5O 2 (g) + 2Mn+2 + 8H2O. purple Notice you do not need to have a data point at the equivalence point. Equivalence point located by absorbance extrapolation of the two lines. Equivalence point MnO 4- reacting, color disappears xs MnO 4- accumulates Volume of titrant (mL KMnO 4 ). PGCC CHM 103 Sinex 3.