March, 2003, Chemistry Experiment:

High-Tech Titration

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

PASPORT pH Sensor
(PS-2102)

PASPORT pH Sensor

ScienceWorkshop pH Sensor
(CI-6507A)

ScienceWorkshop pH Sensor

Purpose:

Students will use titration to discover the concentration of a solution of hydrochloric acid.
 

Background Information:

When acids and bases react in typical neutralization reactions, each chemical species supplies either hydroxide ions (OH-) or protons (H+) to the solution. Water molecules accept the protons to form hydronium ions (H3O+), which react in a one-to-one ratio with the hydroxide ions supplied by the base. One mole of hydroxide ions is therefore chemically equivalent to one mole of hydronium ions. As neutralization occurs, the pH of the solution changes. In order to determine when chemically equivalent amounts of acid and base are present, chemists use the process of titration. A solution of known concentration (the titrant) is carefully measured and added gradually to a solution of unknown concentration, and the resulting pH is monitored. The equivalence point of the titration is the point at which chemically equivalent amounts of acid and base are present. A pH meter or sensor will show a large voltage change at the equivalence point. Alternatively, a well-chosen indicator will change color at the equivalence point, as long as it changes color over the same range as the equivalence point. Knowing the volume of titrant at the equivalence point, along with knowledge of the stoichiometry of the reaction, allows the chemist to calculate the concentration of the unknown solution.

Hypothesize: Since the pH of the solution will change most dramatically at the equivalence point, predict the shape of the graph that will result when pH is plotted vs. volume of titrant for the reaction between a strong acid and a strong base. What pH might the equivalence point be close to?

Back to top

Equipment & Supplies:

For each lab group:

Back to top

Software & Probeware Set-up:

  1. Ensure that your USB Link, Xplorer, PowerLink, or ScienceWorkshop 500 interface is connected to the computer.

  2. Click on one of the links below to download a pre-configured DataStudio file for this 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 pH vs. Volume of Titrant, as well as a Table Display of pH and Volume and a Digits Display of pH.

  3. Connect the pH Sensor to the Xplorer, USB Link, or PowerLink (PASPORT users), or plug the pH Sensor into the 500 Interface (ScienceWorkshop 500 users).
    If you are using the ScienceWorkshop 500 Interface, be sure the interface is turned on and the pH Sensor is associated correctly in the Experiment Setup window.

  4. Resize and arrange the displays as needed so that you can see them all.

  5. For the most accurate results, calibrate the pH Sensor. Refer to DataStudio’s Help menu or to the manual or Quick-Start Card that came with your sensor.

Back to top

Experimental Procedure:

Equipment Setup:
  1. Measure 50 mL of distilled water into the 250-mL beaker.

  2. Use the pipette to add 10 mL of hydrochloric acid solution to the beaker containing the distilled water.

  3. If using a magnetic stirrer, carefully add the magnetic stir bar (don't splash!) to the beaker. Place the beaker on the magnetic stirrer.

  4. Use one of the buret clamps to position the pH Sensor so that the end of the electrode is in the acid solution but is not interfering with the spin bar.

  5. Rinse the buret with a few milliliters of the 0.1 M NaOH solution. Dispose of the rinse solution as directed by your instructor.

  6. Be sure the buret valve is closed, then use the second clamp to support the buret in position above the beaker.

  7. Fill the buret with 0.1 M NaOH solution exactly to the 0.00 mL mark.

  8. Turn on the magnetic stirrer. (If a magnetic stirrer is not available, use a stirring rod.)

Data Collection & Recording:

  1. Click the Start button ( ) to begin collecting data. The Start button will change to a Keep button ( ).

  2. The first row in the Table shows the starting pH. Do not add any NaOH titrant for the first reading. When the pH stabilizes, click Keep and then click the Tab key to move to the next row in the Table. The Graph Display will automatically be updated.

  3. Open the buret valve and add enough NaOH solution to raise the pH by approximately 0.15 units, and then close the valve. After the pH reading stabilizes, type the total volume of NaOH added into the next row in the Table and then click Keep.

  4. Repeat step #11 until approximately pH 3.5 is reached. Then change to adding NaOH in 2-drop increments, each time typing in the total volume of NaOH added after each increment.

  5. Continue in this manner until approximately pH 10.0 is reached. Then return to adding NaOH in larger increments as before, still typing in the total volume of NaOH added after each increment.

  6. Continue the titration until the pH stops changing. Click the Stop ( ) button to end data collection.

  7. Turn off the magnetic stirrer and remove the pH Sensor from the solution. Rinse the electrode thoroughly with distilled water and dry gently. Dispose of the solutions as directed.

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 as needed.

  2. Use the Smart Tool ( ) to pinpoint the equivalence point of the titration.

  3. From the volume of titrant that corresponds to the equivalence point, calculate the number of moles of NaOH used. Use the stoichiometry of the neutralization reaction to first calculate the number of moles of HCl used, and then to calculate the molarity of the HCl solution.

Back to top

Conclusions and Extensions:

  1. Compare your calculated value for the molarity of the HCl solution with the solution’s actual concentration.

  2. Why is it important to know the exact concentration of the titrant solution (in this case, NaOH) in order to calculate the concentration of the unknown acid solution?

  3. How might the titration graph be different if NaOH was titrated with an acid solution such as sulfuric acid (H2SO4), where the acid contains two protons instead of one?


Back to top