Using the absorbance of light to measure concentration Beer’s Law Lab Using the absorbance of light to measure concentration

Absorbance and Concentration This lab is based on the principle that the “darker” a solution, the more concentrated it is A concentrated solution is “darker” because the solute absorbs light

Absorbance and Concentration You can QUANTITATIVELY measure the concentration of solution by measuring how much light is absorbed

Colorimeter Measures numerically how much light is absorbed by a sample “Absorbance”

Colorimeter A colour of light is used that is absorbed well by the solution The colorimeter compares the intensity of light entering and leaving the sample.

Cuvette Used to hold the sample Handle by the ridges Light enters and leaves on the smooth sides

Cuvette make a reference mark on one smooth side to line up the cuvette consistently Wipe fingerprints and dust away from smooth surface with a tissue

Calibration When the cuvette contains only water, it should read 0.00 absorbance. Calibration corrects the absorbance reading to take into account any light absorbed by the plastic cuvette or the water.

Making Standard Solutions A Series of dilutions make a set of solutions of known (calculated ) concentrations Absorbance of each solution is measured

Creating a calibration curve Absorbance vs. Concentration is plotted

Determining the concentration of an unknown sample Measure Absorbance Use graph to determine Concentration of unknown sample

Determining the concentration of an unknown sample Use the equation for the line: Abs = (slope) Conc and the slope value from the graph to calculate Conc. Of the unknown

Example If your unknown is measured to have an absorbance of 0.250, what is its concentration? Use the graph Use the equation: y = 1.2109 x or Abs = 1.2109 (Conc) Rearrange: x = y/1.2109 or Conc = Abs/ 1.209 x = Conc. = 0.250 /1.209 x = Conc. = 0.206 mol/L

Expectations You will be evaluated on the following: A pre-lab quiz testing your knowledge of the theory and the procedure /8 Your dilution calculations for your standard solutions /3 Your graph /5 The accuracy of your unknown concentration /3 Total marks = /19