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Determine the concentration of a solution

Determine the concentration of a solution by plotting absorbance versus concentration.

Note

For a more detailed experiment procedure, see the Beer's law sample experiment. Classroom-ready experiments are also available in the PASCO Experiment Library.

Procedure

Part 1: Prepare the samples for analysis

  1. Fill the cuvettes ¾ full with the sample solutions, then cap the cuvettes.

  2. Prepare a reference solution by filling one cuvette ¾ full with the solvent used to make the sample solutions, then cap the cuvette.

    Note

    Handle the cuvettes only by the lined sides to avoid getting fingerprints on the smooth sides, which could interfere with results.

Part 2: Connect and calibrate the spectrometer

  1. Start Chemvue, then connect your spectrometer to the program. The software should automatically open to the solution spectra page of the Spectrometry Module.
  2. Ensure the spectrometer's sample chamber is covered to block ambient light, then select Calibrate Dark .
  3. Insert the reference solution cuvette into the spectrometer's sample chamber, then select Calibrate Reference .

Part 3: Select an analysis wavelength

  1. Place the cuvette containing the most concentrated solution to be analyzed into the spectrometer, then select Start .
  2. In the Tools panel, adjust the Smoothing and Number of Scans to Average sliders as needed.
  3. Once you are satisfied with the results, select Stop .
  4. Click the Select Wavelength tool, then drag the data target to the wavelength you want to analyze. For most experiments, this will be a high point on the curve.
  5. Once the data target is at the desired wavelength, select the target again and click the floating Select Wavelength button.

Part 4: Collect data

  1. Switch to the solution concentration page using the navigation bar, then select the Known Concentration icon.
  2. In the Enter Concentration Values table, select the concentration cells in the column titled Concentration and type over the given values with the known concentration values of the samples prepared.
  3. With the highest-concentration sample cuvette still in the sample chamber, select Start again to begin recording data.

  4. Select the cell in the Absorbance column that corresponds to the highest concentration value. Once the absorbance reading stabilizes, select Keep next to the absorbance to record the value.

    Note

    A value of three indicates that the solution is too concentrated to be measured for the selected wavelength.

  5. Remove the cuvette and replace it with a cuvette containing the sample with the next-highest concentration, then repeat Step 4 for the appropriate cell in the Absorbance column. Repeat this process for all samples, recording each of their absorbance values.

  6. Once you have recorded the absorbance values for all samples, select Stop to end data collection.

Part 5: Analyze the data

  1. Select Toggle Live Scan Display to remove the Live Scan Display from the graph.
  2. Select Scale to Fit to rescale your data as needed.
  3. Select Linear Fit to create a best fit line and display the equation for the line.
  4. Place a cuvette containing a solution of unknown concentration into the spectrometer.
  5. Select the Unknown Concentration icon to switch to the Determine Unknown Concentration table.
  6. Select Start to begin analyzing the solution.
  7. Select the first Absorbance cell in the table. Once the absorbance stabilizes, select Keep next to the absorbance to record the value.
  8. Select Stop to end data collection.
  9. Use either the graph or the equation of the line to determine the unknown concentration. In the Determine Unknown Concentration table, select the concentration cell and enter the concentration value.

Classroom-ready experiments

Kinetics of Crystal Violet Fading (Spectrometer)

Students determine the reaction order of crystal violet fading in the presence of sodium hydroxide. A calibration curve of crystal violet is generated and optimal absorbance wavelength is determined by the student. Crystal violet fading is investigated using different concentrations of sodium hydroxide.

Light, Color, and Concentration

Using a spectrometer, students learn how to use visible light to determine the concentration of colored ion species in a solution.

Empirical Formula

Students use a spectrometer and stoichiometric calculations to obtain the chemical formula of a compound.