Experiment 1: Spectrometric Analysis - The Breathalyzer Reaction Flashcards
What is the reaction that occurs?
Oxidation-reduction in which potassium dichromate reacts with ethanol under acidic conditions:
3C2H5OH+2K2Cr2O7 +8H2SO4 -> 2Cr2(SO4)3 + 3CH3COOH +2K2SO4 +11H2O
ethanol+ dichromate ions (Cr6+ orange) -> chromium ions (Cr 3+ blue-green)
When do you have orange vs. blue green?
- detection of alcohol relies on the reduction of the orange coloured dichromate ion, Cr2O7^2- (Cr^+6) to the blue green chromium ion (Cr^3+)
- aka the dichromate ions in potassium dichromate are an orange color but become reduced (gain 3 electrons) from ethanol to become chromium ions which are a blue green color
When do you have orange vs. blue green? part 2
By observing the colour of the resulting solution, the amount of alcohol present can be determined
- if there is no alcohol present in the solution the colour remains orange
- if there is enough alcohol to completely react with the dichromate present the resulting solution will be blue green
- in most cases the amount of alcohol lies somewhere in between and the resulting solution is a mixture of dichromate and chromium ions, giving a golden yellow color
What is beers law
- Beer’s Law (also known as the Beer-Lambert Law) is the quantitative relationship between the concentration of a substance in solution and the extent to which it absorbs light
A=elc
A=elc - what does each variable represent
A - is the absorbance (sometimes optical density) and is a measure of how much light the solution absorbs
c - is the molar concentration of solute in (M) (in E1 it was [Cr^3+]
I - is the path length of light through the solution (cm) ie. the inner diameter of the sample cuvette (1 cm)
e - is the molar absorptivity (in M^-1 cm^-1) and is a proportionality constant that is characteristic of the coloured substance and varies with wavelength
note: some wavelengths of light are absorbed more strongly than others, readings are taken at the wavelength of maximum absorption to give maximum sensitivity
Describe a Beers laws plot
- if both the molar absorptivity and path length are known, you could just take an absorbance reading and then use Beer law equation to isolate and solve for the unknown concentration but this is not the best technique
-the better way is to determine the concentration by comparing the absorbance readings of unknowns to those of solutions with known concentrations - this is done by preparing a standard curve by plotting absorbance and concentration for a series of standard solutions
- you then measure the absorbance of the unknown solution and then plot it and you can then find it’s corresponding concentration from the standard curve
Describe a Beers Law plot pt 2
- at low concentration solution the relationship between concentration and absorbance is linear but as concentration increases Beer’s law fails, for a variety of reasons including both instrument limitations and chemical interactions
What is a spectrophotometer?
- it’s the instrument used to select the desired wavelength of light and to measure the absorbance at that wavelength
- the spectrophotometer is set to measure absorbance readings at the wavelength that will give the best readings for a particular sample
- this is usually the max wavelength which is the wavelength of light that is absorbed most strongly by that sample
- the dichromate and chromium ions each have their own wavelength but we measure the amount of chromium ions
Explain the schematic layout of the optical system in the spectronic 200E
- a light source (white light from a light bulb) that passes through the entrance slit and hits a prism or diffraction grating that spreads out the light into its spectrum of wavelengths and by adjusting the wavelength selector, it causes light of the desired wavelength to pass through the exit slit and into the sample tube and represent the incident light (Io), the amount of light entering the sample
-the solution in the sample tube then absorbs some of the light and some of the light exits, the amount that exits is the transmitted light (It) which will then hit the detector (a photocell or phototube) which produces a voltage that is proportional to the light intensity
- the voltage can be displayed in terms of either absorbance (A=-logT) or transmittance (T=It/Io)
blank definition
- a blank which contains the solvent only (everything except the substance of interest) is first used to set the instrument to read A=0 (100% transmittance)
- this way if there is any other compounds in the sample other than the substance of interest that absorb at the measuring wavelength, zeroing the blank will remove the absorbance contribution of these compounds
What is the relationship between concentration and absorbance?
Linear, but as concentration increases, Beer’s law fails.
What does λmax represent?
The wavelength of light that is absorbed most strongly by the sample.
What is the purpose of the water bath in this experiment?
To help the dichromate reaction to occur faster.
Why do we work from lowest to highest concentration in this experiment?
You do not need to clean instruments used between uses.
defintion of wavelength
wavelength typically refers to the distance between successive peaks or troughs of a wave associated with particles or electromagnetic radiation
