Spectrophotometry and the Beer-Lambert Law

Section 3.13

Many chemical analysis methods are more indirect than simply weighing a component.

Spectroscopy - using color intensity to determine the concentration of a solution

  • we can use an instrument to analyze the intensity of the color in the solution

  • we use that information to determine the concentration of the solution

Instrument used: Spectrophotometer

  • take a light source

  • allow the light to pass through a color filter (can adjust the frequency of that color)

  • different color light comes out the other side

  • colored light is directed and interacts with the solution sample in a cuvette

  • light that passes through the solution is detected by a light detector

  • compare the intensity of the original light to the final light

Common name: “Spec-20”

Many solutions contain different substances, all of which absorb light over a range of different wavelengths.

  • when looking for the concentration of only one substance, look for a wavelength at which there is little/no interference from another substance

How do we use this method to determine the concentration of a solution?

Beer-Lambert Law: A= ϵbC

In which:

  • A= absorbance (a number between 0 and 1 read directly from the instrument)

  • ϵ=molar absorptivity (a constant measuring how well the substance absorbs light)

  • b=path length in cm (width of the cuvette)

  • c=concentration (in moles per liter)

The molar absorptivity for a substance at a specific wavelength will not change.

The cuvette size (path length) in an experiment should not change.

Meaning: Absorbance is directly proportional to concentration.

→ the important part of Beer’s Law in AP Chemistry

Absorbance at 0 M should always be 0.

Prepare other solutions of known concentration. Plot their absorbances.

Prepare a best-fit line (calibration curve)

By comparing the absorbance of the unknown concentration with the calibration curve, the concentration of the sample can be found.

Co3+, Cu2+, and Fe3+ are the focus of most of the problems; these ions have visible colors that can be detected by spectrophotometers.

Spectrophotometers usually have a range that extends a little bit beyond the visible light spectrum. (More expensive equipment will usually have a somewhat higher range of available wavelengths)

Most spectrophotometers cover from about 330-1000nm.

Which ions have visible colors?

  • Solutions containing copper ions are blue

  • Solutions containing nickel ions are green

  • Solutions containing iron ions are yellow/orange

  • Solutions containing chromium ions are yellow/orange

  • Solutions containing cobalt ions are pink

In summary…

When looking at a spectrophotometry graph, double-check to make sure there is a blank point at zero.

Line up absorbance with the calibration curve to find concentration.

If asked to find the optimal wavelength for an analysis, choose the maximum absorbance for the ion having the least interference.

Lower concentrations (water contamination) drop absorbance while longer cuvette lengths raise absorbance.