Activity Activity Coefficients and Coefficients and Buffer Capacity Buffer Capacity

TitrationsTitrationsJoanna Ma and Sylvia GarciaJoanna Ma and Sylvia Garcia

Faculty Advisor: Dr. Grady CarneyFaculty Advisor: Dr. Grady CarneyNew York Institute of Technology, New York Institute of Technology,

Life Sciences Life Sciences

IntroductionIntroduction

““Conjugate Acid-Base Mixtures in the General Chemistry Conjugate Acid-Base Mixtures in the General Chemistry

Laboratory” by George R. Wiger and Ulrich de la Camp Laboratory” by George R. Wiger and Ulrich de la Camp (1978)(1978)

ObjectiveObjective: To allow the student, using experimental data which he : To allow the student, using experimental data which he or she has gathered, to develop the Henderson-Hasselbach or she has gathered, to develop the Henderson-Hasselbach relationship and also understand the behavior of buffers relationship and also understand the behavior of buffers

The experiment is carried out before any mention has been made The experiment is carried out before any mention has been made in lecture of buffers in lecture of buffers

Follow UpFollow Up Henderson-Hasselbatch Equation: Why do the buffer solutions Henderson-Hasselbatch Equation: Why do the buffer solutions

display a different pH change upon addition of a strong acid/ display a different pH change upon addition of a strong acid/ strong base? strong base?

Addition of NaOH and HCl alters the ionic strength of a buffer Addition of NaOH and HCl alters the ionic strength of a buffer solutionsolution

BackgroundBackground

DefinitionsDefinitions Buffer Capacity: Buffer Capacity: the mmoles of NaOH or HCl per mL of the mmoles of NaOH or HCl per mL of

buffer solution needed to produce a unit change in pHbuffer solution needed to produce a unit change in pH Activity Coefficient: Activity Coefficient: a factor used in chemistry that a factor used in chemistry that

accounts for deviations from ideal behavior in a mixture of accounts for deviations from ideal behavior in a mixture of chemical substanceschemical substances

Debye Huckel TheoryDebye Huckel TheoryRelates activity coefficient to ionic strength for electrolytes in Relates activity coefficient to ionic strength for electrolytes in

dilute dilute aqueous solutionsaqueous solutions

Only valid for solutions with very small ionic strengthOnly valid for solutions with very small ionic strength

s0.50910γ

BackgroundBackground

Goals of Our ResearchGoals of Our Research Recreate Wiger and Camp’s comprehensive buffer Recreate Wiger and Camp’s comprehensive buffer

experimentexperiment Compare experimental data to Wiger and Camp’sCompare experimental data to Wiger and Camp’s Determine whether experimental data and Debye Determine whether experimental data and Debye

Huckel Theory are in agreement in low ionic strength Huckel Theory are in agreement in low ionic strength domaindomain

Purpose of This ExperimentPurpose of This Experiment Finding activity coefficientsFinding activity coefficients Determining the composition of the bufferDetermining the composition of the buffer Custom-designed buffersCustom-designed buffers

ExperimentalExperimentalPreparation of Acid, Base and Salt SolutionsPreparation 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 BasesStandardization of Acids and Bases Sodium Hydroxide: 0.0903M NaOHSodium Hydroxide: 0.0903M NaOH Hydrochloric Acid: 0.0962M HClHydrochloric Acid: 0.0962M HCl Acetic acid: 0.4799M HOAcAcetic acid: 0.4799M HOAc Sodium acetate: 0.4990M NaOAcSodium acetate: 0.4990M NaOAc

Preparation of Buffer Preparation of Buffer SolutionsSolutions

Buffer # mL 0.5M NaOAc

mL 0.5M HOAc

a (mmoles/ mL)

s (mmoles/ mL)

1 130.24mL 125.25mL 0.2446 0.2446

2 13.02mL 12.53mL 0.25 0.25

What do buffer solutions consist of? What do buffer solutions consist of? Buffer solutions consist of a weak acid and its conjugate Buffer solutions consist of a weak acid and its conjugate

base or a weak base base or a weak base

and its conjugate acid. and its conjugate acid. a = acida = acid s = salt of a conjugate bases = salt of a conjugate base

Buffer #1a = 0.25M acetic acids = 0.25M sodium acetate

Buffer #2a = 0.025M acetic acids = 0.025M sodium acetate

Buffer pH TitrationsBuffer pH Titrations

1. Calibrate pH meter1. Calibrate pH meter Used standard buffer solutions – pH 4, 7, 10Used standard buffer solutions – pH 4, 7, 10

2. Measure initial pH of buffer2. Measure initial pH of buffer Initial pH should essentially be the same for each Initial pH should essentially be the same for each

trial, since it is coming from the same containertrial, since it is coming from the same container Initial pH range: 4.50 – 4.64Initial pH range: 4.50 – 4.64

3. Titration buffer solution with NaOH to increase 3. Titration buffer solution with NaOH to increase pH by 1 unitpH by 1 unit

4. Titration buffer solution with HCl to decrease pH 4. Titration buffer solution with HCl to decrease pH by 1 unitby 1 unit

Results and DiscussionResults and Discussion Calculating Buffer CapacityCalculating Buffer Capacity

s119

10a s9as

BC a;119

10s a9as

BC

CapacityBuffer lTheoretica

ΔpHVxVC

BC ;ΔpHV

xVCBC

CapacityBuffer alExperiment

NaOHHCl

Buffer

NaOHNaOHNaOH

Buffer

HClHClHCl

Results and DiscussionResults and Discussion

BufferNumb

er

Composition BCNaOH BCHCl

a sTitratio

n9as/

(a+10s)Titratio

n9as/

(s+10a)

1 0.2446 0.2446

0.2191 0.200 0.1790 0.200

2 0.025 0.025 0.0167 0.0205 0.0167 0.0205

Comparison of ResultsComparison of Results

Results and DiscussionResults and Discussion Calculated Activity CoefficientsCalculated Activity Coefficients

BufferNumb

er

CompositionActivity

Coefficients

a s γa γs

1 0.2446 0.2446

1.0955 0.895

2 0.025 0.025 0.8146 0.8146

10sa9as

BCγ NaOH

a

10as9as

BCγ HCl

s

Results and Discussions0.50910γ Debye Huckel Theory:

Activity Coefficient vs. Composition

0.5

0.6

0.7

0.8

0.9

1

1.1

0 0.05 0.1 0.15 0.2 0.25 0.3

Composition

Act

ivit

y C

oef

fici

ent

s (HCl)

a (NaOH)

ConclusionConclusion

Buffer #1Buffer #1 Value of activity coefficient is proportional to salt Value of activity coefficient is proportional to salt

concentration of bufferconcentration of buffer Salt concentration decreases = activity coefficient also Salt concentration decreases = activity coefficient also

decreasesdecreases

Buffer #2Buffer #2 There is very little ionic activity occurring in the solutionThere is very little ionic activity occurring in the solution Buffer capacity for NaOH and HCl should be relatively Buffer capacity for NaOH and HCl should be relatively

equal to each otherequal to each other Activity coefficient SHOULD remain close to 1Activity coefficient SHOULD remain close to 1

0.8146γ

Further ApplicationsFurther Applications Testing the salt concentration of buffer solutionsTesting the salt concentration of buffer solutions Custom-designed buffersCustom-designed buffers

AcknowledgementsAcknowledgements Dr. Grady Carney, Life SciencesDr. Grady Carney, Life Sciences New York Institute of TechnologyNew York Institute of Technology