Beer's Law Flashcards
the irradiance before and after it goes through the sample is measured
TRUE
Basic design of a spectrophotometer
light source - wavelength selector - sample holder - detector
why avoid curved sample holders?
the curved sides will reflect and refract the light
why avoid frosted or colored sample holders?
they will absorb the light and limit their useful range of wavelengths
what range of light does electronic excitation occur at?
UV-Vis range
what does absorbance depend on?
concentration of absorbing analyte (c), how well the analyte absorbs the wavelength selected (ε), distance through the solution the light passes (b)
concentration (c) units
M
molar absorptivity (ε) units
1 / (M*cm)
path length (b) units
cm
Where does Beer’s Law fail?
multiple wavelengths present (polychromatic), concentration-dependent equilibrium, and highly concentrated solutions (c <= 0.01 M) the solute starts to look like the solvent
if the concentration is doubled, absorbance is?
doubled
if concentration is doubled, transmittance?
A = -logT
10^-A = T
A will be doubled, 10^-2 = 0.01 = T
compound have difference absorbances for different wavelengths
TRUE
if light source has polychromatic light, how does this affect measuring molar absorptivity?
you will get a light intensity that is a combination of multiple wavelengths and absorbance that reflects the absorbance across multiple wavelengths; cannot calculate the exact ε because it is unique to a single wavelength
some compounds may contain multiple components that can absorb light of a given wavelength, how does this affect measuring concentration of one analyte in solution?
if sample is not pure, it will have overlapping absorbance spectra and you won’t be able to identify what absorbance is from each species
