Buffers

1
Buffers

• DEFINITIONa solution containing both a weak
  acid/base and its salt which resists change in pH
  due to
• temperature
• dilution
• and addition of SMALL amounts of strong acid or
  base

2
Examples

• HC2H3O2 and NaC2H3O2
• HCOOH and HCOONa
• NaH2PO4 and Na2HPO4
• pyridine and pyridinium chloride
• ammonia and ammonium chloride

3
Problem

• Solutions are made by combining equal volumes of
  the following. Which is/are a buffer(s)?a) 0.1
  M NH4Cl 0.1 M NH4b) 0.1 M HF 0.05 M NaOHc)
  0.05 M HF 0.1 M NaOHd) 0.1 M NaF 0.05 M
  HCle) 0.1 M NaF 0.05 M Na

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Henderson Hasselbach Equation

• pH pKa log conj. base/conj. acidorpH
  pKa log salt/acidfor a weak acid and its
  salt
• Recall pKa -log Ka

5
Buffers - Effect of Dilution

• Consider pH of phosphate buffer containing 100 mL
  0.1 M NaH2PO4 and 100 mL 0.1 M Na2HPO4. What is
  the pH if the solution is diluted by a factor of
  2? 5? Generalize your findings.

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Buffers - Effect of Temperature
Buffers Calbiochem Corp., Doc. No. CB0052-591
Perrin Dempsey Buffers for pH an dMetal Ion
Control Chapman Hall London, 1979.
7
Problem

• Calculate the pH of a solution that is 0.25 M
  sodium acetate and 0.30 M acetic acid given Ka
  1.8 x 10-5 for acetic acid.(hint what is the
  pKa?)
• ANSpH pKa log salt/acidpH 4.74
  log(0.25/0.30) 4.74 - 0.08pH 4.66

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Problem

• Calculate the pH that results when 200 mL of
  0.300 M ammonia are mixed with 250 mL of 0.150 M
  ammonium chloride.
• ANSpH pKa log (conj. base/conj. acid)
  9.25 log ammonia/ammonium 9.25
  log (0.06/0.0375) 9.25 0.20 9.45

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Comparison - Effect of Addition of SMALL Amount
of Strong Acid or Base

• Buffer itself (100 mL 0.1 M NaH2PO4 and 100 mL
  0.1 M Na2HPO4) pH 7.20vs. 200 mL water pH 7.00
• Upon addition 0.005 moles strong acid, buffer pH
  6.72water pH 1.60
• Upon addition 0.005 moles strong base, buffer pH
  7.68water pH 10.40

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Preparing Buffers - Useful References

• Perrin Dempsey Buffers for pH and Metal Ion
  Control New York Wiley, 1974.
• Chemical Company booklets.Example Calbiochem
  Doc. No. CB0052-591

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Practical Preparation

• Practical
• Identify reagent based on pKa
• Prepare appropriate molarity and add NaOH/HCl to
  adjust pH

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Special Types of Buffers

• GOOD Buffers - temperature resistent
• Volatile Buffers - can be removed by freeze
  drying
• Universal Buffers - wide effective pH range

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GOOD Buffers

• zwitterionic - have both amino and sulfonyl
  groups
• EXAMPLESMES pK 6.15HEPES pK 7.55

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MES and HEPES
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GOOD Buffers

• pK typically 6-8 (physiological pH)
• No complexation with metal ions (no inhibition of
  enzymes)
• High aqueous solubility
• Minimal salt effects
• No UV-vis absorption (240-280 nm)

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Desirable Characteristics of GOOD Buffers

• pH independent of temperature
• Compare with TRIS

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Volatile Buffers

• Can be removed by simple evaporation or
  lyophilization
• good for electrophoresis or preparative ion
  exchange chromatography

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Volatile Buffers

• EXAMPLES
• ammonium acetate pH 4-6
• pyridinium formate pH 3-6
• ammonium carbonate pH 8-10

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Universal Buffers

• Mixture of two or more buffers
• effects of buffers are additive
• greater buffering capacity
• wider effective pH range
• EXAMPLEcitric acid 3.13, 4.76, 6.40phosphoric
  acid 2.15, 7.20, 12.15boric acid 9.24, 12.74,
  13.80