February, 2002, Chemistry Experiment:

Salt Solution Conductivity

- Purpose
- Background Information
- Equipment
- Software Set-up
- Experimental Procedure
- Data Analysis
- Conclusions and Extensions

 

Conductivity Sensor (PS-2116)

Purpose:

Students will measure the ability of a salt solution (sodium chloride, NaCl) to conduct electrical current. As students increase the amount of salt added to the solution, they will predict how the solution’s conductivity will change.

Background Information:

Conductivity is a measure of the ability of a substance to conduct an electrical current. In liquids, the presence of free ions (positive and negative charges) is necessary for conductivity. When two electrodes, one positively charged and the other negatively charged, are introduced into a liquid containing free ions, the positive and negative ions will move in opposite directions (toward oppositely charged electrodes). This movement of charged particles constitutes an electric current through the liquid.

Solutions that conduct electricity due to the presence of ions are called electrolytes. The strength of an electrolytic solution depends on the degree of ionization in solution: for example, strong acids such as nitric acid or phosphoric acid dissociate nearly 100% and are called stong electrolytes. Weak acids and bases ionize to varying lesser degrees and are called weak electrolytes. Covalent compounds which are not acidic or basic do not ionize, and are called nonelectrolytes. A solution’s conductivity is therefore affected by the kind of solute, the concentration of solute, and ion mobility in the solution. Since temperature is a measure of the average kinetic energy of an atom, molecule, or ion, an increase in temperature will increase particle movement and will therefore increase conductivity.

For aqueous solutions, the most commonly used units of measurement for conductivity are microsiemens/centimeter (µS/cm) and millisiemens/centimeter (mS/cm). See "Teacher’s Hints" for more information about these standard units.

Back to top

Equipment:

For each lab group:

Back to top

Software Setup:

  1. Click on one of the links below to download a pre-configured DataStudio file for this conductivity experiment, and then open the file.

    PASPORT users: Windows (.zip file) or Macintosh (.sit file)

    ScienceWorkshop 500 users: Windows (.zip file) or Macintosh (.sit file)

    When the file is opened, you should see a graph display of Conductivity vs. Time, as well as a digits display of conductivity.

  2. Connect the Conductivity Sensor to an Xplorer or USB link (PASPORT users), or plug the sensor into the 500 interface (ScienceWorkshop 500 users).
    If you are using the ScienceWorkshop 500 interface, be sure the sensors are associated correctly in the Experiment Setup window when you open the DataStudio file.

Back to top

Experimental Procedure:

Sensor calibration:

  1. PASPORT users: Refer to the "Setup and Calibration" instructions on the Quick Start card for calibration procedures.

  2. ScienceWorkshop 500 interface users: Refer to the Conductivity Sensor Instruction Manual and Experiment Guide (pages 7-8).
Data Recording:
  1. Pour 400 mL of distilled water into the 500-mL beaker.

  2. Place the Conductivity Sensor into the beaker of distilled water.

  3. After 30 seconds, click the Start button ( ) to begin collecting data.

  4. Click the Scale to Fit button ( ) in the Graph toolbar to resize the axes.

  5. After 2 minutes, add the 0.1 grams of salt to the beaker. Stir the solution for 30 seconds.
    (Note: While stirring, you’ll notice the conductivity level rapidly change. Once you stop stirring, the conductivity level will stabilize.)

  6. Every 2 minutes, add an additional 0.1 grams of salt until a total of 0.5 grams have been added.

  7. After the last 0.1 grams have been added, wait 2 minutes then click the Stop ( ) button to end the experiment.

Back to top

Data Analysis:

  1. Examine the graph display to view your data, using the Scale to Fit button ( ) in the Graph toolbar to resize the axes.

  2. Determine the minimum and maximum values of conductivity: click the Statistics button ( ) in the Graph toolbar and look for the minimum and maximum values to appear in the graph legend.

  3. Use the Slope tool ( ) to determine when during the data run did the level of conductivity changed most quickly.

Conclusions and Extensions:

  1. What was the relationship between amount of electrical current passing through the solution and solute concentration?

  2. Explain why ionic compounds (i.e., salts and bases) in the solid phase do not conduct an electric current, but in the liquid state and in aqueous solution, these same compounds act as electrolytes.

  3. How well can two different salts be compared by measuring current, if their molecular sizes are very similar?

  4. Why is it important to keep the temperature constant during the experiment described above?

  5. How do different salts compare in their ability to carry an electric current? Design an experiment to find out.