Activity Coefficients and Buffer Capacity Titrations

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Activity Coefficients and Buffer Capacity
Titrations

• Joanna Ma and Sylvia Garcia
• Faculty Advisor Dr. Grady Carney
• New York Institute of Technology, Life Sciences

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Introduction

• Conjugate Acid-Base Mixtures in the General
  Chemistry
• Laboratory by George R. Wiger and Ulrich de la
  Camp (1978)
• Objective To allow the student, using
  experimental data which he or she has gathered,
  to develop the Henderson-Hasselbach relationship
  and also understand the behavior of buffers
• The experiment is carried out before any mention
  has been made in lecture of buffers
• Follow Up
• Henderson-Hasselbatch Equation Why do the buffer
  solutions display a different pH change upon
  addition of a strong acid/ strong base?
• Addition of NaOH and HCl alters the ionic
  strength of a buffer solution

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Background

• Definitions
• Buffer Capacity the mmoles of NaOH or HCl per mL
  of buffer solution needed to produce a unit
  change in pH
• Activity Coefficient a factor used in chemistry
  that accounts for deviations from ideal behavior
  in a mixture of chemical substances
• Debye Huckel Theory
• Relates activity coefficient to ionic strength
  for electrolytes in dilute
• aqueous solutions
• Only valid for solutions with very small ionic
  strength

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Background

• Goals of Our Research
• Recreate Wiger and Camps comprehensive buffer
  experiment
• Compare experimental data to Wiger and Camps
• Determine whether experimental data and Debye
  Huckel Theory are in agreement in low ionic
  strength domain
• Purpose of This Experiment
• Finding activity coefficients
• Determining the composition of the buffer
• Custom-designed buffers

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Experimental

• Preparation of Acid, Base and Salt Solutions
• 0.50M HOAc (acetic acid)
• 0.50M NaOAc (sodium acetate)
• 0.10M NaOH (sodium hydroxide)
• 0.10M HCl (hydrochloric acid)
• Standardization of Acids and Bases
• Sodium Hydroxide 0.0903M NaOH
• Hydrochloric Acid 0.0962M HCl
• Acetic acid 0.4799M HOAc
• Sodium acetate 0.4990M NaOAc

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Preparation of Buffer Solutions

• What do buffer solutions consist of?
• Buffer solutions consist of a weak acid and its
  conjugate base or a weak base
• and its conjugate acid.
• a acid
• s salt of a conjugate base

Buffer 1 a 0.25M acetic acid s 0.25M sodium
acetate
Buffer 2 a 0.025M acetic acid s 0.025M
sodium acetate
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Buffer pH Titrations

• 1. Calibrate pH meter
• Used standard buffer solutions pH 4, 7, 10
• 2. Measure initial pH of buffer
• Initial pH should essentially be the same for
  each trial, since it is coming from the same
  container
• Initial pH range 4.50 4.64
• 3. Titration buffer solution with NaOH to
  increase pH by 1 unit
• 4. Titration buffer solution with HCl to decrease
  pH by 1 unit

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Results and Discussion

• Calculating Buffer Capacity

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Results and Discussion

• Comparison of Results

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Results and Discussion

• Calculated Activity Coefficients

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Results and Discussion
Debye Huckel Theory
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Conclusion

• Buffer 1
• Value of activity coefficient is proportional to
  salt concentration of buffer
• Salt concentration decreases activity
  coefficient also decreases
• Buffer 2
• There is very little ionic activity occurring in
  the solution
• Buffer capacity for NaOH and HCl should be
  relatively equal to each other
• Activity coefficient SHOULD remain close to 1

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Further Applications

• Testing the salt concentration of buffer
  solutions
• Custom-designed buffers

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Acknowledgements

• Dr. Grady Carney, Life Sciences
• New York Institute of Technology