Textbook Notes (368,986)
Canada (162,320)
Chemistry (275)
CHMB16H3 (16)
Chapter 17

CHMB16 Chapter 17

4 Pages
153 Views
Unlock Document

Department
Chemistry
Course
CHMB16H3
Professor
Kagan Kerman
Semester
Fall

Description
Chapter 17: Fundamentals of Spectrophotometry Properties of Light • Spectrophotometry: any technique that uses light to measure chemical concentrations. • Colorimetry: a procedure based on absorption of visible light. • Wavelength (λ): the crest-to-crest distance between waves. • Frequency (v): the number of complete oscillations that the wave makes each second. • Relation between frequency and wavelength: λv = c, where c is the speed of light at 2.998 x 10 m/s. o In a medium other than a vacuum, the speed of light is c/n, where n is the refractive index. o For visible wavelengths in most substances, n > 1, so visible light travels more slowly through matter than through vacuum. • Photons: particles of light with regard to energy. • Relation between energy and frequency: E = hv, where h is Planck’s constant of 6.626 x 10 -34Js. o E = hc/v = hcṽ, where ṽ is 1/ λ or the wavenumber. o Energy is inversely proportional to wavelength and directly proportional to wavenumber. o Electromagnetic Spectrum: the visible spectrum spans the wavelength range of 380-780 nm. Absorption of Light • Excited State: when a molecule absorbs a photon, the energy of the molecule increases and the molecule is promoted to an excited state. • Ground State: if a molecule emits a photon, the energy of the molecule is lowered and this is the lowest energy state of a molecule. • Irradiance/Intensity/Radiant (P): the energy per unit time per unit area in the light 2 beam (W/m ). • Monochromator: a wavelength selector in which light passes through and it selects one wavelength. • Transmittance (T): the fraction of original light that passes through the sample (T = P/Po) and has a range of 0 to 1. • Percent Transmittance: 100T and ranges between 0 to 100%. • Absorbance: directly proportional to the concentration of the light-absorbing species in the sample. o A = log(P oP) = -logT -1 - o Beer’s Law: A = Ԑbc, where Ԑ is the molar absorptivity and has units of M cm . o The greater the molar absorptivity, the greater the absorbance. o Absorption Spectrum: a graph showing how A or Ԑ varies with wavelength. o Chromophore: the part of a molecule responsible for light absorption. • Any substance that absorbs visible light appears colored when white light is transmitted through it or reflected from it; the substance absorbs certain wavelengths of the white light and our eyes detect the wavelengths that are not absorbed. • When Beer’s Law Fails o At very high concentration, the solute becomes the solvent so the properties of a molecule are not exactly the same in different solvents. o Nonabsorbing solutes in a solution can also interact with the absorbing species and alter the absorptivity. o Beer’s Law works for monochromatic radiation passing through a dilute solution in which the absorbing species is not participating in a concentration- dependent equilibrium. Measuring Absorbance • Cuvet: where liquid samples are usually contained and has flat, fused-silica faces. • Do not touch the clear faces of a cuvet because fingerprints scatter and absorb light. • Gases are more dilute than liquids and require cells with longer pathlengths (10cm to many meters). • For spectrophotometric analysis, we choose the wavelength of maximum absorbance because: o The sensitivity of the analysis is greatest at maximum absorbance; we get the maximum response for a given concentration of analyte. o The curve is relatively flat at the maximum, so there is little variation in absorbance if the monochromator drifts a little or if the width of the transmitted band changes slightly. • Modern spectrophotometers are most precise at intermediate levels of absorbance, around 0.3 to 2. Beer’s Law in Chemical Analysis • Proteins are normally assayed in the ultraviolet region at 280 nm because aromatic groups present in virtually every protein have an absorbance maximum at 280 nm. • Reagent Blank: contains all reagents but with analyte replaced by distilled water. • Subtract the blank absorbance from the absorbance of samples and standards before doing any calculations. • Supernate: the liquid layer above the solid that collects at the bottom of a tube during centrifugation. Spectrophotometric Titrations • Spectrophotometric Titration: we monitor changes in absorbance during a titration to tell when the equivalence point has
More Less

Related notes for CHMB16H3

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit