SAFETY NOTES: Mono- and di-basic phosphate salts are caustic and can cause skin irritation and burns, take all due precaution when handling. GENERAL INSTRUCTIONS: Each student will prepare and test their own buffer. If interested, you can perform an Acid and Alkaline Self Test by collecting a small piece of litmus paper and applying some saliva to the paper, this will indicate whether your body fluids are too acidic or too alkaline. Litmus paper will change color to indicate pH levels of body fluids. These tests should be performed before eating or one hour after eating. |
| Part 1. Comparing the behavior of the buffer with distilled water | |
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Obtain an Orion® pH meter from the stock room. Calibrate the Orion® pH meter for data collection. |
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Place 25 mL of distilled, deionized water in a clean dry 100 mL beaker. |
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Using a disposable pipet add 1 drop (~0.05 mL) of 0.50 M HCl to the beaker. Record the change in pH. Discard the solution down the sink with lots of water and place a fresh 25 mL of distilled, deionized water in a clean dry 100 mL beaker. Using a disposable pipet add 1 drop (~0.05 mL) of 0.50 M NaOH to the beaker. Record the change in pH. |
| Part 2. Preparing and testing of the buffer solution for the assigned pH | |
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Using the calculations in your pre-lab, prepare 100 mL of your buffer at the assigned pH. Once complete, measure out a 25 mL aliquot of the prepared buffer and place it in a clean, dry 100 mL beaker. Measure and record the pH. |
| Part 3. Testing the buffer “capacity” with HCl and NaOH | |
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Using the 25 mL of buffer above, add 1 mL of 0.5 M HCl solution to the beaker and mix well. Record the new pH. Repeat the addition of 1 mL of 0.5 M HCl and continue to record the pH until the pH has dropped by 0.5 pH units. |
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Discard the used buffer solution down the sink with lots of water and place a fresh 25 mL of your prepared buffer in a clean dry 100 mL beaker. |
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Using the 25 mL of buffer above, add 1 mL of 0.5 M NaOH solution to the beaker and mix well. Record the new pH. Repeat the addition of 1 mL of 0.5 M NaOH and continue to record the pH until the pH has risen by 0.5 pH units. |
| Part 4. Determining the effect on the pH of diluting the buffer | |
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Discard the used buffer solution down the sink with lots of water and place a fresh 12.5 mL of your prepared buffer in a clean dry 100 mL beaker. Add 12.5 mL of distilled water to the beaker to dilute the buffer |
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Repeat all of Part 3 to determine the buffer capacity of the diluted buffer. |
