Activity Coefficients and Buffer Capacity Titrations

1. Activity Coefficients and Buffer Capacity Titrations Joanna Ma and Sylvia Garcia Faculty Advisor: Dr. Grady Carney New York Institute of Technology, Life Sciences

2. 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

3. 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

4. Background Goals of Our Research Recreate Wiger and Camp’s comprehensive buffer experiment Compare experimental data to Wiger and Camp’s 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

5. 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

6. Preparation of Buffer Solutions Buffer #1 a = 0.25M acetic acid s = 0.25M sodium acetate Buffer #2 a = 0.025M acetic acid s = 0.025M sodium acetate 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

7. 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

8. Results and Discussion Calculating Buffer Capacity

9. Results and Discussion • Comparison of Results

10. Results and Discussion Calculated Activity Coefficients

11. Results and Discussion Debye Huckel Theory:

12. 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

13. Further Applications Testing the salt concentration of buffer solutions Custom-designed buffers

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