December 2001, Biology Experiment:

Acids and Bases

- Equipment
- Equipment Set-up
- Software Set-up
- Data Collection Procedure
- Data Analysis
- Data Table
- Conclusions and Extensions

Purpose:

In this experiment, students will measure the pH of various solutions and then use their data to classify those substances as acids, bases, or neutral solutions. The Danish chemist Soren Sorenson is credited with introducing the pH scale in 1909, giving scientists a means of characterizing a substance’s acidity on a scale of 0 to 14.

Equipment:

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Equipment Setup:

  1. Label each of the 10 test tubes with the names of the various solutions.

  2. Pour 10 ml of each solution into the respective, labeled test tube.

  3. Set wash bottle and waste beaker close by so that the pH Sensor can be easily rinsed between readings.

Software Setup:

  1. Click on one of the links below to download a pre-configured DataStudio file for this pH 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)

  2. Connect the pH sensor to the USB link or Xplorer (PASPORT users), or to the 500 interface (ScienceWorkshop 500 users).

    Additionally, for ScienceWorkshop 500 users – associate the pH sensor with the interface in the Experiment Setup window (double-click or drag).

  3. The PASPORT pH sensor typically does not need to be calibrated; its accuracy is approximately ± 0.5 pH units. If better accuracy is needed, or to calibrate the CI-6507A pH Sensor, refer to DataStudio’s online help menu for specific calibration instructions.

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Data Collection Procedure:

  1. Place the pH Sensor into the first test tube (solution #1) so that the solution just covers the tip of the probe.

  2. Click the Start button ( ) to begin collecting data. Because the reconfigured DataStudio file has been prepared for manual sampling, the Start button will change to a Keep ( ) button.

  3. Watch the digits display for the pH reading to stabilize, then click the left side of the Keep button.

  4. A dialog box like the one shown below will appear and allow you to enter a value for solution #. Enter "1" and click OK. Notice the graph and table will update automatically.


  5. Remove the pH Sensor from the test tube and thoroughly rinse it with distilled water before measuring the next solution.

  6. Place the pH Sensor into the second test tube (solution #2) and repeat the steps described above to record the second pH measurement. Continue in this manner until all ten solutions have been measured, then click the right side ("Stop" ) of the Keep button.

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Data Analysis:

  1. Scale the axes to fit the data using the Scale to Fit button ( ) in the Graph toolbar.

  2. Observe how many substances registered below 7 on the pH scale, at or near 7, or above 7. Use the data to classify the substances as acidic, basic, or neutral. Record these observations in a data table like the one shown below.

Data Table:

Name of Substance
Solution Color
pH Level Measured
Substance classification:
Acid, Base, or Neutral?
1.
 
 
 
2.
 
 
 
3.      
4.      
5.      
6.      
7.      
8.      
9.      
10.      

Conclusions and Extensions:

  1. Were more of the test substances acidic or basic? How can you tell?

  2. Does the color of the solution determine its pH level?

  3. What was the pH of distilled water? Why is this value significant?

  4. A local pond had a pH reading of 6 but then it measured 4 after a recent rain storm. By what factor did its acidity increase? What likely caused the pH level change?

  5. In order to increase the pH of a solution, what kind of substance (acidic, basice, neutral) must be added?

  6. Why is pH important to all living things?

  7. Explain the role of a buffer in a living system.

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