Ex1: Spectrophotometric Analysis The Breathalyzer Reaction

• what is the reaction occuring inside a breathalyzer?

  oxidation-reduction (redox) where potassium dichromate reacts with ethanol under acidic conditions
• dichromate ion colour

  orange
• chromium ion colour

  blue-green
• no alcohol = orange, lots = green
• Alcohol detection is the reduction of dichromate Cr2O72- from Cr6+ to Cr3+
• Beer's law is the relationship between concentration and light absorbtion
• A = Elc

  A - absorbance (unitless)

  E - molar absorptivity (1/M cm)

  I = path length of light through the sol. (cm)

  c = molar concentration of solute (in M)
• what are the axes on a standard curve?

  absorbance and concentration
• what ion was absorbance measured for?

  Cr 3+
• spectrophotometer was used to select the desired wavelength of light and measure absorbance at that wavelength
• what is lamba max?

  the wavelength of light that is absorbed most strongly by the sample
• what are the components of the spectrophotometer?

  light source, monochromator (prism), sample, detector, display
• Light source passes through entrance slit and hits prism that spreads light into wavelength spectrum. Adjusted wavelength selector causes light of desired wavelength to pass through exit slit and into sample tube. Light exiting sample hits detector to produce voltage

  
• Io, incident light


  amount of light entering the sample
• It, transmitted light


  light that exits the sample
• what is in a blank? what is its purpose?

  solvent only, remove absorbance contribution from a wavelength
• how did absorbance readings change with concentration of alcohol?

  higher alcohol concentration = higher absorbance reading
• what was the max wavelength used in the alcohol experiment

  585 nm
• % ethanol = [(volume ethanol in tube) x ( % ethanol soln.) / total volume in tube]
• % ethanol in diluted wine sample = use slope of of graph and solve for x to get percent ethanol
• % ethanol in undiluted wine sample = [(% ethanol in diluted wine sample) / (vol. undiluted sample)] x100
• absorbance and concentration are proportional
• Ethanol (C2H5OH) is oxidized to acetic acid (CH3COOH)
• potassium dichromate is reduced to chromium sulfate
• standard curve

  graphs of light absorbance vs solution concentration, used to identify solute concentrations in unknown samples
• dilution series

  a step wise of dilutions, dilution factor stays same for each step , used to estimate c of samples