Transcript titrations presentation - University of Massachusetts Boston

Titrations
Strong Acid with Strong Base
• Starting pH
pH = -log[FAcid]
• Just before the Equivalence Point
[H+] = (Vacid·Facid-Vbase·Fbase)/(Vsol+Vbase)
• Very Sharp Equivalence Point
pH = 7.00
• Excess base
pH = 14 – (-log[OH-])
[OH-] = Fbase·{(Vbase-V*)/(Vbase+Vsol)]}
25.00 mL of 0.10 M HCl titrated
with 0.10 M NaOH
14
12
pH
10
8
6
4
2
0
0
10
20
Vol NaOH (mL)
30
40
25.00 mL of ? M HCl titrated with
0.10 M NaOH
14
12
pH
10
8
6
4
2
0
0
5
10
Vol NaOH (mL)
15
20
V* = 12.50 mL of NaOH
[HCl] = (12.50 mL)(0.1000 M)/(25.00 mL)
= 0.0500 M
Strong Base with Strong Acid
• Starting pH
pH = 14 – {-log[Base]
• Very Sharp Equivalence Point
pH = 7.00
• Excess acid
pH = -log[H+])
[H+] = Facid·{(Vacid-V*)/(Vacid+Vsol)]}
25.00 mL of 0.10 M NaOH titrated
with 0.10 M HCl
14
12
pH
10
8
6
4
2
0
0
5
10
15
20
Vol HCl (mL)
25
30
35
25.00 mL of ? NaOH titrated with
0.20 M HCl
14
12
pH
10
8
6
4
2
0
0
5
10
15
20
Vol HCl (mL)
25
30
35
V* = 12.50 mL of HCl
[NaOH] = (12.50 mL)(0.2000 M)/(25.00 mL)
= 0.1000 M
Titrating Monoprotic Weak Acids
with Strong Base
• Starting pH: Use weak acid dissociation
Ka = [H+]2/(FAcid – [H+])
pH = -log[H+]
HA + OH- → A- + H2O
• Before the Equivalence Point use H-H eq
pH = pKa +
log [(FNaOH·VNaOH)/(FHA·VHA- FNaOH·VNaOH)]
½ way to Equivalence point, pH = pKa
• At Equivalence point: use weak base
dissociation
Kb = [OH-]2/(Fbase – [OH-]),
where Fbase = FHA·VHA/(Vsol+Veq)
• Excess base
[OH-] = Fbase·{(Vbase-V*)/(Vbase+Vsol)]}
pH = 14 – (-log[OH-])
Ka 1.0·10-5
1.2
1
pH
0.8
HA
0.6
A-
0.4
0.2
0
0
2
4
6
8
alpha fraction
10
12
14
Weak acid with 0.10 M NaOH
14
12
pH
10
8
6
4
2
0
0
5
10 15 20 25 30 35 40 45 50 55
V NaOH (mL)
Example for Ka = 10-5
• Starting pH
[H+] = ((1∙10-5∙0.1)1/2 = 0.0010 M
pH = 3.00
• ½ way – pH = pKa = 5.00
• At Equiv. pt
[OH-] = ((1∙10-9∙0.05)1/2 = 7.1·10-6 M
pH = 14.00 – [-log(7.1·10-6)] = 8.85
Titrating Monoprotic Weak Bases
with Strong Acid
• Starting pH: Use weak base dissociation
Kb = [OH-]2/(Fbase – [OH-])
pH = 14.00 – (-log[OH-])
A- + H+ → HA
• Before the Equivalence Point use H-H eq
pH = pKa +
log [(FA-·VA-- FHCl·VHCl)/ (FHCl·VHCl)]
½ way to Equivalence point, pH = pKa
• At Equivalence point: use weak acid
dissociation
Kb = [H+]2/(Facid – [H+]),
where FACID = FA-·VA-/(Vsol+Veq)
• Excess HCl
[H+] = Facid·{(Vacid-V*)/(Vacid+Vsol)]}
pH = log[H+])
Titrate with 0.10 M HCl
12
10
pH
8
6
4
2
0
0
10
20
30
40
Vol NaOH (mL)
50
60
Example for Ka = 10-7
• Starting pH
[OH-] = ((1∙10-7∙0.1)1/2 = 0.00010 M
pH = 10.00
• ½ way – pH = pKa = 7.00
• At Equiv. pt
[H-] = ((1∙10-7∙0.05)1/2 = 7.1·10-5 M
pH = 4.15
Titration of H2A with strong base
• Starting pH – weak acid equilibrium
• ½ way to first eq.: pH = pKa1
• At first equivalence pt:
pH = (pKa1 + pKa2)/2
• ½ way to 2nd eq.: pH = pKa2
• At 2nd equiv. pt.: weak base equilibrium
alpha fraction plot
alpha fraction
1.2
1
0.8
H2A
0.6
HA-
0.4
A2-
0.2
0
0
2
4
6
8
pH
10
12
14
Titrating w eak acid w ith strong base
12
10
pH
8
6
4
2
0
0
10
20
30
40
Volum e base
50
60
Example
• Starting pH – [H+] = (10-3*0.1)1/2 = 0.0100
M
• ½ way to first eq.: pH = 3.00
• At first equivalence pt:
pH = (pKa1 + pKa2)/2 = 6.00
• ½ way to 2nd eq.: pH = 9.00
• At 2nd equiv. pt.: [OH-] = (10-5*0.05)1/2 =
7.07∙10-4 M, pH = 10.85
Amino acids
• Amphoteric
– Can act as an acid and a base
• Glycine (GH2+, GH, G-)
pKa1 = 2.350, pKa2 = 9.778
Alpha Fraction Plot for Glycine
alpha fraction plot
alpha fraction
1.2
1
0.8
H2A
0.6
HA-
0.4
A2-
0.2
0
0
2
4
6
8
pH
10
12
14
Titration of Glycine
14
12
10
pH
8
6
4
2
0
-30
-20
-10
0
10
Vol
20
30
40
Triprotic system
• Example: H3PO4
alpha fraction plot
alpha fraction
1.20E+00
1.00E+00
8.00E-01
H3A
6.00E-01
H2A-
4.00E-01
HA2-
2.00E-01
a3-
0.00E+00
0
2
4
6
pH
8
10
12
Titration of a triprotic acid
with strong base
• Starting pH – weak acid equilibrium
• ½ way to first eq.: pH = pKa1
• At first equivalence pt:
pH = (pKa1 + pKa2)/2
• ½ way to 2nd eq.: pH = pKa2
• At 2nd equiv. pt.:
pH = (pKa2 + pKa3)/2
• ½ way to third equiv. pt.: pH = pKa3
• At third equiv pt: weak base equilibrium
pKa’s are 4, 7, and 10
Titrating w eak acid w ith strong base
12
10
pH
8
6
4
2
0
0
10
20 30
40 50 60
Volum e base
70 80
90 100
H3PO4 (pKa’s 2.2, 7.2, 12.2)
Titrating w eak acid w ith strong base
14
12
pH
10
8
6
4
2
0
0
10
20 30
40 50 60
Volum e base
70 80
90 100