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2.5: Buffer Titration/buffering Capacity

  • Page ID
    114280
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    RELATED READINGS: Pages 5-7, 13, 34-35, 278-281.

    OBJECTIVES

    Upon completion of this exercise, appropriate discussions, and related readings, the student will be able to:

    1. Prepare standard dilutions of a stock buffer.
    2. Select a pH Indicator based on reaction range and the pH of the solution to be titrated.
    3. Perform a titration.

    PRINCIPLE

    The capacity of a buffer solution to maintain a constant pH depends on the specific components of the buffer, it’s concentration and the amounts of acid or base challenging the buffer. Changing any of these variables will have an impact on the buffering capacity of a solution.

    GLOSSARY

    Buffer capacity: the ability of a buffered solution to accept added acid or alkali and maintain the initial pH of the solution.

    Titration: the process by which known amounts of dilute acid or alkali are added to a solution to achieve a neutral pH and determine the amount of acid or base in the solution.

    deionized water
    0.2M NaH2PO4 24.0 grams NaH2PO4 q.s. to 1L with
    deionized water

    Comments: In order to prevent mishandling of your pH meter, carefully go over and demonstrate proper use of the instrument with the students. You may find it useful to show how easy it is to contaminate materials by not properly cleaning the electrode between samples.

    Note that these phosphate solutions employ anhydrous salts. Weight will need to be adjusted if hydrated salts are used.

    MATERIALS

    • Flasks
    • pH indicators
    • Burettes
    • Volumetric pipets
    • 0.01 N NaOH solution
    • 100 mL volumetric flasks
    • Distilled water
    • Buffer Solutions from Exercise # 4

    PROCEDURE

    1. Prepare a 1:10 and a 1:100 dilution from each of the stock buffers prepared in the previous exercise as indicated by your Instructor.
    2. Measure the pH of the original buffer and each dilution, and record the values on the data sheet.
    3. Pipet 5.0 mL of the 1:100 dilution of buffer into a small Erlenmeyer flask.
    4. Add 3 drops of the *appropriate pH indicator to the flask; mix gently.
    5. Using a buret or pipet, slowly titrate the 5 mL aliquot of buffer with 0.01 M NaOH solution. Mix continuously by swirling the flask during the titration.
    6. Continue titrating until the pH indicator changes to the expected color.
    7. Record the volume of NaOH solution required for the titration on the data sheet.
    8. Repeat steps 3 through 7 for the 1:10 dilution and the undiluted (stock) buffer solutions. You may find It desirable to adjust the amount of stock buffer used for the titration based on the results of the 1:100 and 1:10 titrations.
    9. Correct the volume of NaOH used for the 1:10 and undiluted buffers to compensate for sample size and record these values on the data sheet.

    * Choose a pH indicator from the list shown below. When performing this type of procedure, it is desirable to use an indicator that, at the initial pH of the solution being titrated, shows one of it’s characteristic colors and has not begun to change color. For example, if the buffer you are titrating has an initial pH of 4.6, you would use an indicator that shows one color at a pH below 5.5-6 and a different color at 7—8. The specific range is not critical in this exercise, but you must be able to visualize an end point.

    Indicator Acid Color pH Range Basic Color
    Methyl Violet Yellow 0.15-3.2 Violet
    Congo Red Blue 3.0-5.0 Red
    Methyl Orange Red 3.2-4.5 Yellow
    Methyl Red Red 4.4-6.2 Yellow
    Phenol Red Yellow 6.4-8.2 Red
    Phenolphthalein Colorless 8.2-10 Pink
    DATA SHEET, EXERCISE #5

    NAME: ____________

    DATE: ____________

    Buffers Being Titrated

    Undiluted 1:10 1:100
    Initial pH
    Titration Sample Volume (mL)
    Volume of NaOH used (mL)
    Corrected NaOH Volume (mL)

    CALCULATIONS

    \[\text{Corrected NaOH volume} = \frac{\text{Volume of NaOH used}}{\frac{x}{5}}\]

    Where x is the volume of buffer being titrated.

    Discussion Questions

    1. What observation can be made regarding the pH of a buffer as it is diluted? How can this be explained on the basis of the Henderson-Hasselbach equation?
    2. How did you expect the volumes of NaOH needed for the titrations to compare? Did your results support this expectation?
    3. What observation can be made regarding the buffering capacity of a buffer as it is diluted?

    This page titled 2.5: Buffer Titration/buffering Capacity is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Lawrence Kaplan & Amadeo Pesce.

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