Ex1: Spectrophotometric Analysis Breathalyzer Reaction

what does beers law describe

quantitative relationship between a substance’s concentration and light absorbency

A = εlc what does c represent and units

molar concentration, M

A = εlc what does ε represent and units

molary absoptivity in M^-1 cm^-1

A = εlc what does l represent and units

path length of light through the solution in cm

A = εlc what does A represent and units

absorbances/optical density, unitless

what is absorbance

how much light a solution absorbers

what does l (path length) correspond to in the experiment

inner diameter of the cuvette

what does ε (molar absorptivity) correspond to in the experiment

proportionality constant specific to the colored substance; varies with wavelength

what is a standard curve

graph of absorbance and concentrations of a series of standard solutions

how is the standard curve used to determine the unknown wine concentration

absorbance of the unknown is measured, then use the standard curve to find what concentration should’ve been

when does beers law fail

concentration gets big → from instrument limitations and chemical interactions

what is the nature of the relationship between concentration and absorbance

linear at low concentrations

what is lambda max

wavelength of light absorbed most strongly by a sample

there are 2 colours ions in this experiment, which lambda max was used and why

lambda max of Cr³⁺ was used because we are interested in the [ ] of alcohol → alcohol produces Cr³⁺

dialution series

repeatedly diluting a solution step-by-step → creates a set of lower concentrations

standard solution

solution of known concentration

wavelength

distance between 2 points on a wave

spectronic 200E spectrophotometer component and their roles: light source

ordinary light bulb that produces white light

spectronic 200E spectrophotometer component and their roles: entrance slit

white light from the light source enters her into the prism

spectronic 200E spectrophotometer component and their roles: prism

splits white light from entrance slit into its spectrum of wavelengths

spectronic 200E spectrophotometer component and their roles: wavelength selector

adjusting this causes light of desired wavelength to pass through the exit slit into the sample

spectronic 200E spectrophotometer component and their roles: sample

incident light (I_o) enters → sample absorbs some light → trasmitted light (l_t) exits

spectronic 200E spectrophotometer component and their roles: detector

photocell or phototube, produced voltage proportional to l_t that hits it

spectronic 200E spectrophotometer component and their roles:display

voltage gets displayed as either absorbance or transmittance → can convert mathematically between the 2

which chemical reaction does this experiment use

redox rxn. potassium dichromate reacts with ethanol in acidic pH → reduces Cr⁶⁺ to Cr³⁺

dichromate ion

Cr₂O₇^2-

chromium ion

Cr³⁺

what colour is the dichromate ion

orange

what colour is the chromium ion

blue-green

color of resulting solution: no alcohol present

remains orange

color of resulting solution: alcohol present, excess

blue green

ions in solution and color of resulting solution: alcohol present, limiting

golden yellow → both dichromate and chromium are in solution

what is a blank

contains solvent only (ie everything were NOT interested in measuring), sets instrument to A=0 (T=100%)

what is the purpose of using a blank

there are compounds in the sample that absorb light → negates their effects

what to do if unknown wine is coloured

use carbon to decolourize → ensures the colour won’t interfere with absorbance readings

general steps of this experiment

1. 5 ethanol standards made

2. dilution of unknown

3. addition of acidic potassium dichromate into each sample plus a blank

4. heating all samples to 80C, 10 min

5. measuring absorbance of all samples

why should cuvettes only be handled by the top corners

the bottom is where light passes, prints and residue strongly affect absorbance readings

whats used to wipe cuvettes

kimwipes

limitations of using beers law for the standard curve: absorbance fall just outside range of standard curve

if absorbance falls just outside the range of standards, reading may or may not be accurate → the curve might not be linear anymore

limitations of using beers law for the standard curve: absorbance is in non-linear range

slope of earlier trendline cant be used, no [ ] determines

limitations of using beers law for the standard curve: absorbance is the max possible display value

sample is too concentrated, no [ ] determined