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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 tutorial, see the sample Beer's Law experiment. Classroom ready experiments are also available in the PASCO Experiment Library.

Procedure

  1. Prepare the samples for analysis.

    1. Fill the cuvettes ¾ full with the sample solutions and cap.

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

      Note

      Handle the cuvettes by the lined sides to avoid getting fingerprints on the smooth sides.

  2. Calibrate the spectrometer.

    1. Select the Analyze Solution page .
    2. Cover the spectrometer to block ambient light and click Calibrate Dark.
    3. Insert the reference solution into the spectrometer, then click Calibrate Reference.

  3. Select an analysis wavelength.

    1. Place the most concentrated solution to be analyzed into the spectrometer, then click Record .
    2. Use the tools to adjust the smoothing and number of scans to average as needed.
    3. Click Stop when you are satisfied with the results.
    4. Drag the coordinate tool on the graph to the wavelength you want to analyze. This is usually a high point on the curve.
    5. Once you have found a desirable wavelength, click Accept.

  4. Collect data.

    1. Select the Concentration page from the menu at the top of the screen. A table will appear on the left side of the screen that has columns for Concentration and Absorbance.
    2. In the Enter Concentration Values table, select the concentration cells in the column titled Concentration and type over the given values with the concentrations of the samples prepared.
    3. Select Record again to start analyzing the solution.

    4. Place the highest-concentration sample into the spectrometer and click the cell in the Absorbance column that corresponds to the respective concentration. Once the absorbance stabilizes click Accept next to the absorbance to record the value.

      Note

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

    5. Repeat the previous step for the samples of lesser concentration, recording each of their absorbance values.

    6. Click Stop again to stop data collection.

  5. Analyze the data.

    1. Click Show Live Scan Display to remove the Live Scan Display from the graph.
    2. Click Scale To Fit to rescale your data.
    3. Click Linear Fit Tool to create a best fit line and display the equation for the line.
    4. Place the cuvette containing a solution of unknown concentration into the spectrometer.
    5. Select the Absorbance cell in the Determine Unknown Concentration table at the bottom of the screen.
    6. Click Record to start analyzing the solution.
    7. Once the absorbance stabilizes, click Accept next to the absorbance to record the value.
    8. Click Stop to stop 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.

Classroom-ready experiments

Kinetics of Crystal Violet Fading
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 determined by the student. Crystal violet fading is investigated using different concentrations of sodium hydroxide.
Light, Color, and Concentration
In this lab, students use a spectrometer to learn how to use visible light to determine the concentration of colored ion species in a solution.
Empirical Formula
In this lab, students use a spectrometer and stoichiometric calculations to obtain the chemical formula of a compound.