Determining the Concentration of a Solution: Beer’s Law

Chemistry 117

http://en.wikipedia.org/wiki/File:Beer–Lambert_law_in_solution.JPG

Spectophotometry

• Measures absorbance spectrum of a substance• Must first measure the absorbance spectra VS

wavelength. Maximum wavelength is used to calibrate and to be able to use beer’s law most effectively

http://www.sarracenia.com/astronomy/remotesensing/physics050.html

Absorbance

• Compounds absorb UV or Visible light• 100% transmittance means no light was

absorbed• 0% transmittance means all light was

absorbed• Relationship between absorbance and

transmittance is beer’s law

Beer-Lambert Law

A= ε b CWhere A = absorbance, ε = molar extinction coefficient, b =

path length (1cm), and C = concentration. A = – Log (T) (Note 0<T<1)

T = I1/I0

http://en.wikipedia.org/wiki/File:Beer_lambert.png http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/beers1.htm

Concentration and Transmittance

• Transmittance is exponentially related to the concentration of the solution

• We will plot Concentration Vs Absorbance with our spectrometer

• Remember-Dilution Equation: M1V1 = M2V2