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 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 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? 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 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 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 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 Only valid for solutions with very small ionic strength

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

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

6. Preparation of Buffer Solutions Buffer #mL 0.5M NaOAc mL 0.5M HOAc a (mmoles/ mL) s (mmoles/ mL) 1130.24mL125.25mL0.2446 213.02mL12.53mL0.25 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 a = acid s = salt of a conjugate base 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

7. Buffer pH Titrations 1. Calibrate pH meter Used standard buffer solutions – pH 4, 7, 10 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 should essentially be the same for each trial, since it is coming from the same container Initial pH range: 4.50 – 4.64 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 Calculating Buffer Capacity

9. Results and Discussion Buffer Number CompositionBC NaOH BC HCl asTitration9as/(a+10s)Titration9as/(s+10a) 10.2446 0.21910.2000.17900.200 20.025 0.01670.02050.01670.0205 Comparison of Results Comparison of Results

10. Results and Discussion Calculated Activity Coefficients Calculated Activity Coefficients Buffer Number CompositionActivity Coefficients as γaγa γsγs 10.2446 1.09550.895 20.025 0.8146

11. Results and Discussion Debye Huckel Theory:

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

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

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