Buffers

Description

Indicators are added to dishes of water and buffer. These are "titrated" on an overhead projector. The effect of the buffer in resisting change to pH on the addition of either acid or base is demonstrated.

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A buffer tends to keep the pH of a system within a narrow pH range, even though acids and bases are added. The demonstration shows that, to achieve a dramatic change in pH, a buffer solution will require many more drops of a strong acid or strong base than will unbuffered water.

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Acids and bases may cause blindness.

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Wear eye protection during the demonstration.

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1. Prepare an acetate sheet with the letter W placed at the left on the top and bottom, and the letter B placed at the right on the top and bottom. Place the sheet on an overhead projector. Place an empty Petri dish over each letter.
2. Place 10 mL of water in each dish over the letter W. Place 10 mL of buffer in each dish over the letter B.
3. Add several drops of phenolphthalein indicator to each of the dishes at the top. Add several drops of methyl orange indicator to each of the dishes at the bottom. Mix these with a rubber policeman or other stirring device.
4. Use a thin stem plastic pipet to add 1 M NaOH to each of the top dishes, dropwise, with stirring, until a persisting color change is noticed. Count the drops.
5. Use a thin stem pipet to add 1 M HCl, to each of the bottom dishes, dropwise, with stirring, until a persisting color change is noticed. Count the drops.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 105 Buffers

Watch the movie.

Does the amount of indicator affect the number of drops?

Closure Questions:

1. Describe how a buffer works, using appropriate chemical equations to illustrate this effect.
2. How can one solution neutralize both acids and bases?

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Purpose

To illustrate the action of a buffer.

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• 250 mL buffer solution (15.0 g of glacial acetic acid and 20.5 g of sodium acetate mixed with enough water to make 250 mL of solution.
• methyl orange indicator (0.1 g/100 mL; filter if necessary) in suitable dropping device
• phenolphthalein indicator (1 g in 50 mL ethanol; add 50 mL water) in suitable dropping device
• 100 mL 1.0 M HCl solution (8.6 mL 11.6 M acid (36%)/100 mL)
• 100 mL 1.0 M NaOH solution (4.0 g/ 100 mL)
• 2 thin stem plastic transfer pipets
• 100 mL of distilled water in 250-mL wash bottle
• 4 Petri dishes
• 1 overhead projector and screen
• stirring rods (rubber policemen)

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Set up the overhead projector. Label the bottoms of four Petri dishes, 2 with a large W, and 2 with a large B. (As an alternative, use two pieces of acetate. Label the bottom piece, and cover with a blank piece.)

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Teacher preparation: 30 minutes

Presentation: 10 minutes

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Acids and bases may cause blindness.

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Wear eye protection during the demonstration.

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All of the chemicals recommended for this experiment may be disposed of safely at the sink.

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Presentation Question:

• Does the amount of indicator affect the number of drops?
  • In general, the amount of indicator does not affect the number of drops very greatly.

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Closure Questions:

1. Describe how a buffer works, using appropriate chemical equations to illustrate this effect.
2. How can one solution neutralize both acids and bases?

Answers to Closure Questions:

1. A buffer (HA, NaA) must contain both an acid and a base in order to neutralize both added acid and base:

   A^- + H⁺ HA
   (resists effects of adding H⁺)

   HA + OH⁼ H₂O + A^-
   (resists effects of adding OH^-)

   So long as neither the acid nor the base is completely used up, the ratio of the concentration of the acid to that of the base does not change much, so the [H⁺] does not change much.

2. The equilibrium operating in this system involves the dissociation of acetic acid:

   HC₂H₃O₂ H⁺ + C₂H₃O₂^-

   The solution contains substantial concentration of both HC₂H₃O₂ and C₂H₃O₂^-. Added strong acid (H⁺) reacts with C₂H₃O₂^- and shifts the equilibrium to left; added strong base (OH^-) reacts with HC₂H₃O₂ and shifts the equilibrium to the right.

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A buffer solution resists changes in pH. A buffer solution contains both an acid component and a base component. In the buffer used here, HC₂H₃O₂ is the acid component and C₂H₃O₂^- is the base component. The acid component neutralizes added OH^-:

The base component neutralizes added H⁺:

When the buffer has enough capacity, until enough acid is added to use up all of the base species in the buffer, or until enough base is added to react with all of the acid component, the ratio of the concentration of these species will not have changed much. Therefore, the [H⁺] will not have changed much and only relatively small changes in pH are observed.

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• buffer
• pH
• titration
• acid
• base
• pH indicator
• concentration neutralization

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