The Properties of Buffers

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The Properties of Buffers

• School Chem. Chem. Tech.

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OBJECTIVES

• Develop an understanding of the pH scale.
• Examine the differences between strong and weak
  acids.
• Investigate how buffers work.
• Prepare a buffer solution with a target
  concentration and pH.
• Determine the buffer capacity of a solution.

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BACKGROUND

• Calculating the pH
• Preparing a buffer
• The properties of buffers

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Calculating the pH
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The properties of buffers (1)--- Depicting Buffer
Action
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The properties of buffers (2)--- Common Ion
Effect Illustrated

• Composition of a buffer solution
• a weak acid /conjugate base or
• a weak base /conjugate acid
• CH3COOH/CH3COONa NH3H2O/NH4Cl

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Preparation of a buffer (1)---Henderson-Hasselbalc
h Equation

• If weak acid conjugate base, pH pKa

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Preparation of a buffer (2)---Henderson-Hasselbalc
h Equation

• Requirements
• The ratio of conjugate base to weak acid is
  between 0.10 and 10
• pHpKa1

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Preparation of a buffer (3)
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Preparation of a buffer (4)---Related Calculation

• e.g. How to prepare 200mL pH4.0 buffer solution
  with a total concentration of 0.1M?

Analysis pKa(HAc)4.75, therefore, HAcNaAc
can be chosen.
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Preparation of a buffer (5)---Related Calculation

• e.g. How to prepare 200mL pH4.0 buffer solution
  with a total concentration of 0.1M?

HAc0.015M Ac-0.085M
HAcAc-0.1M
3MHAc?mL mNaAc?g
3MHAc?mL 0.0150.23V, V1mL mNaAc
0.20.085821.4g
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PROCEDURE-- Part A.1

• Strong and Weak Acids
• Prepare 50.00mL 0.50M CH3COOH solution with 5.0M
  CH3COOH solution.

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Scheme 1
Attention! Add drop-wise with a pipet!

• Invert
• gt20 times

Pipet 5.00mL 5.0M HAc
Dilute the solution to 50.00mL
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PROCEDURE-- Part A.2

• Strong and Weak Acids
• Use the pH meter to record the pH of the 0.50 M
  HCl solution, the 0.50 M HAc solution and one of
  the unknown HAc solutions.

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PROCEDURE-- Part A.3

• Strong and Weak Acids
• Abrade a short Mg strip with a piece of abrasive
  paper and put it in the bottom of the empty wells
  of a porcelain spot plate, add 5-10 drops 0.50M
  HCl solution and HAc solution to just cover the
  Mg, record your observations.

Mg
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PROCEDURE--- Part B.1

• Designing a Buffer
• Prepare 0.100L of a buffer solution (0.100M) in a
  100-mL volumetric flask with x mL 5.00M CH3COOH
  and y mL 2.00M CH3COONa solutions.

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Scheme 2
Attention! Add drop-wise with a pipet!
Pipet x mL 5.00M HAc and y mL 2.00M NaAc

• Invert
• gt20 times

Dilute the solution to 100.00mL
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PROCEDURE--- Part C.

• Properties of a Buffer

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PROCEDURE--- Part D.

• Determination of Buffer Capacity

!
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Until the pH increases sharply over 7.0
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After lab

• Exchange data with the others in your team who
  had a different buffer.
• Mark the solution you prepared and measured.

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pH meter

• Calibrating a Digital pH Meter
• Switch on the pH meter.
• Press the ?? button to begin the new
  calibration.
• Remove the electrode from the bottle of storage
  solution. Rinse with de-ionized water.

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Calibrating a Digital pH Meter

• Immerse the electrode in pH 4.00 buffer. Swirl
  the solution to fully saturate the electrode with
  buffer.

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Calibrating a Digital pH Meter

• Press the ?? button again. After the reading is
  stable, the meter will return to the Measure
  screen.

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Calibrating a Digital pH Meter

• Remove the solution from the pH 4.00 buffer and
  rinse the electrode with de-ionized water. Dry
  the electrode tip carefully with a tissue.

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pH measurement

• Now the meter is ready to use for measuring the
  pH of any solution.
• Immerse the electrode in the pH unknown solution
  and swirl.
• Press ?? button,after the reading is stable,
  record pH.