Acid-Base Titrations. Introduction 1.) Experimental Measurements of p K a p K a of amino acids in an active-site of a protein are related to its function Protein structure and environment significantly perturb p K a values
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Acid-Base Titrations
Acid-Base Titrations
The pKa of His-240 in the G6PD apoenzyme is found to be 6.4, which corresponds to an unidentified pKa value of 6.3 that was previously derived from the dependence of kcat on pH. These results suggest that the pKa of His-240 is unperturbed by Asp.
Biochemistry, Vol. 41, No. 22, 2002 6945
Acid-Base Titrations
proton release from PAA decreases with
increase in the degree of dissociation for the highest polymer concentration
conformational change of the PAA from rod-like conformation to a random coil form,
J. Phys. Org. Chem. 2006; 19: 129–135
Acid-Base Titrations
Acid-Base Titrations
Titrant
Analyte
Acid-Base Titrations
mmol of OH-
being titrated
mmol of HBr
at equivalence point
When 10.00 mL of HBr has been added, the titration is complete.
Prior to this point, there is excess OH- present.
After this point there is excess H+ present.
Acid-Base Titrations
Initial volume of OH-
Calculate Remaining [OH-]:
Total volume
Fraction of OH-
Remaining
Initial concentration
of OH-
Dilution Factor
Calculate [H+] and pH:
Acid-Base Titrations
Kw
Kw= 1x10-14
x x
Acid-Base Titrations
Calculate volume of excess H+:
Calculate excess [H+]:
Volume of excess H+
Initial
concentration
of H+
Dilution factor
Total volume
Calculate pH:
Acid-Base Titrations
Acid-Base Titrations
Ka
Kb
K
Acid-Base Titrations
2.)Illustrated Example:
Acid-Base Titrations
3.) Volume Needed to Reach the Equivalence Point
mmol of HA
mmol of base
Acid-Base Titrations
4.) Region 1: Before Base is Added
Ka
Ka= 10-6.27
Calculate [H+]:
F - x x x
Calculate pH:
Acid-Base Titrations
5.) Region 2: Before the Equivalence Point
Simply the difference
of initial quantities
Calculate [A-]/HA]:
Simply ratio of volumes
Amount of added NaOH is 3 mL with equivalence point is 10 mL
Calculate pH:
Acid-Base Titrations
5.) Region 2: Before the Equivalence Point
Acid-Base Titrations
5.) Region 3: At the Equivalence Point
Kb
F - x x x
Acid-Base Titrations
5.) Region 3: At the Equivalence Point
Calculate Formal concentration of [A-]:
A- is no longer 0.02000 M, diluted by the addition of NaOH
Initial volume of HA
Initial
concentration
of HA
Dilution factor
Total volume
Acid-Base Titrations
5.) Region 3: At the Equivalence Point
Calculate [OH-]:
Calculate pH:
pH at equivalence point is not 7.00
pH will always be above 7.00 for titration of a weak acid
because acid is converted into conjugate base at the equivalence point
Volume of excess OH-
Initial
concentration
of OH-
Total volume
Dilution factor
Acid-Base Titrations
5.) Region 4: After the Equivalence Point
Calculate volume of excess OH-:
Amount of added NaOH is 10.10 mL with equivalence point is 10 mL
Calculate excess [OH-]:
Calculate pH:
Acid-Base Titrations
5.) Titration Curve
Equivalence point: [OH-] = [HA]
Steepest part of curve
Maximum slope
pH=pKa
Vb = ½Ve
Minimum slope
Maximum Buffer Capacity
Acid-Base Titrations
5.) Titration Curve
weak acid small slope change in
titration curve
Difficult to detect equivalence point
Strong acid large slope change in
titration curve
Easy to detect equivalence point
Acid-Base Titrations
5.) Titration Curve
High concentration large slope change in
titration curve
Easy to detect equivalence point
Low concentration small slope change in
titration curve
Difficult to detect equivalence point
At low enough concentration, can not detect change
Acid-Base Titrations
1.) Simply the Reverse of the Titration of a Weak Acid with a Strong Acid
Kb
F - x x x
Acid-Base Titrations
1.) Principals for Monoprotic Systems Apply to Diprotic Systems
Two equivalence points
Kb1
Kb2
Acid-Base Titrations
2.) A Typical Case
mmol of B
mmol of HCl
Acid-Base Titrations
2.) A Typical Case
Kb1
0.100 - x x x
Acid-Base Titrations
2.) A Typical Case
Point (1.5 mL) is before first equivalence point (10 mL)
Acid-Base Titrations
2.) A Typical Case
Point B (5 mL) is halfway to first equivalence point (10 mL)
pH = pKa2=10.00
Acid-Base Titrations
2.) A Typical Case
Account for dilution for formal concentration (F) of BH+
Solve for pH using intermediate form equation
Initial volume of B
Initial
concentration
of B
Total volume
Dilution factor
Acid-Base Titrations
2.) A Typical Case
Point D (15 mL) is halfway to second equivalence point (2x10 mL). First, subtract Ve (10 mL)
pH = pKa1=5.00
Acid-Base Titrations
2.) A Typical Case
Account for dilution for formal concentration (F) of BH2+2
Initial volume of B
Initial
concentration
of B
Total volume
Dilution factor
pH determined by acid dissociation of BH2+2
Kb2
Acid-Base Titrations
2.) A Typical Case
Ka1
0.0333 - x x x
Acid-Base Titrations
2.) A Typical Case
pH from volume of strong acid added. Addition of 25.00 mL:
Excess acid:
Concentration of H+:
pH:
Acid-Base Titrations
3.) Blurred End Points
Only one Equivalence point is clearly evident
Second Ka is too strong and is not a weak acid relative to titrant
Acid-Base Titrations
4.) Using Derivatives to Find End Point
End point: 2nd derivative is zero
End point: 1st derivative is maximum
Dph = 4.400-4.245=0.155
Acid-Base Titrations
5.) Using Gran Plot to Find End Point
where: Vb = volume of strong base added
Ve = volume of base needed to reach equivalence point
gA-, gHA = activity coefficients ≈ 1
Acid-Base Titrations
5.) Using Gran Plot to Find End Point
Slope Gives Ka
x-intercept gives Ve
Never Goes to Zero, approximation that every mole of OH- generates one mole of A- is not true as Vb approaches Ve
Acid-Base Titrations
1.) Indicators: compound added in an acid-base titration to allow end point detection
Acid-Base Titrations
1.) Indicators: compound added in an acid-base titration to allow end point detection
pK = 8.9
pK = 1.7
Acid-Base Titrations
2.) Choosing an Indicator
Bromocresol purple color change brackets the equivalence point and is a good indicator choice
Bromocresol green will change color
Significantly past the equivalence point resulting in an error.
Acid-Base Titrations
2.) Choosing an Indicator
The difference between the end point (point of detected color change) and the true equivalence point is the indicator error
Amount of indicator added should be negligible
Indicators cover a range of pHs
Acid-Base Titrations
3.) Example:
a) What is the pH at the equivalence point when 0.100 M hydroxyacetic acid is titrated with 0.0500 M KOH?
b) What indicator would be a good choice to monitor the endpoint?