December 2001, Chemistry Experiment: Acids and Bases	- Equipment - Equipment Set-up - Software Set-up - Data Collection Procedure - Data Analysis - Data Table - Conclusions and Extensions

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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:

• pH Sensor
    PASPORT (PS-2102) | ScienceWorkshop (CI-6507A)
  
  
• Computer Interface:
     PASPORT: Xplorer GLX (PS-2002)  |  Xplorer (PS-2000)  |  USB Link (PS-2100A)  |  PowerLink (PS-2001)
     ScienceWorkshop: 500 Interface (CI-6400)   |   750 Interface - SCSI (CI-6450)   |   750 Interface - USB (CI-7650)
  
  
• 10 Test Tubes (small beakers or plastic cups may also be used) and a Test Tube Rack.
  
  
• Wash bottle filled with distilled water and large beaker (or use a sink), to rinse off the pH sensor between measurements.
  
  
• Solutions: Windex, Cola, Lemon Juice, Coffee, Vinegar, Salt Water, Orange Juice, Ammonia, Milk, and Distilled Water.

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.

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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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 preconfigured 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.
   
   

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.

Conclusions and Extensions:

1. Which solutions were acidic? Which were basic? Which were neutral?
   
   
2. Which test solution had the highest concentration of hydrogen ions [H⁺]?
   Which test solution had the highest concentration of hydroxide ions [OH^-]?
   
   
3. Does the color of the solution determine its pH level?
   
   
4. Compare and contrast the key physical and chemical properties of acids and bases. Which of your test solutions exhibited characteristics most like acids? Which are most like bases?
   
   
5. To increase the pH of a solution, what substance (acid, base, neutral) must be added to allow the increase to occur?
   
   
6. How does the concentration of hydrogen ions [H⁺] relate to the concentration of hydroxide ions [OH^-] in a solution that is neutral?
   
   
7. When an acid and a base combine in solution, what types of products typically result?