acids, bases, buffers, titrations, polyprotic...
TRANSCRIPT
Topic 7Acids, Bases,
Buffers, Titrations,
Polyprotic acids
Conjugate acids & bases
Strengths of acids & bases
• A strong acid or strong base is completely dissociated in aqueous solution.
•• Weak Acids and Weak BasesWeak Acids and Weak Bases
• Ask yourself
Carboxylic Acids Are Weak Acids and Amines Are Weak Bases
BuffersBuffered solution resists changes in pH when small
amounts of acids or base are added or when dilution occurs.
pH dependence of the rate of a particular enzyme-catalyzed reaction. The rate near pH 8 is twice as the rate at pH 7 or 9
The Henderson-Hasselbalch eqn
HAAn whepKpH acidbaselogpKpH
HAAlogpHpK
HAAlogH loglogK
HA
A HK AHHA
-a
a
a
a
a(aq)(aq)(aq)
If pH = pKa, [HA] = [A-] If pH < pKa, [HA] > [A-] If pH > pKa, [HA] < [A-]
[A-] / [HA] pH
100:1 pKa
+ 2
10:1 pKa
+ 1
1:1 pKa
1:10 pKa
- 1
1:100 pKa
- 2
• Suppose you wish to prepare 1.00 L of buffer containing 0.100 M tris at pH 7.60. When we say 0.100 M tris, we mean that the total concentration of tris plus tris H+ will be 0.100M.
1. Weigh out 0.100 mol tris hydrochloride and dissolve it in a beaker containing about 800 mL water and a stirring bar.
2. Place a pH electrode in the solution and monitor the pH.3. Add NaOH solution until the pH is exactly 7.60. The electrode
does not respond instantly.4. Transfer the solution to a volumetric flask and wash the
beaker and stirring bar a few times. Add the washings to the volumetric flask.
5. Dilute to the mark and mix.
Preparing a Buffer in Real Life
Buffer capacity
The amount of H+ or OH- that buffered solution can absorb without a significant change in pH
Buffer capacity measures how well a solution resists changes in pH when acid or base is added. The greater the buffer capacity, the less the pH changes.
Buffer capacity
0.670.060.04
HAA1
0.050.05
HAA
:B
0.9965.014.99
HAA1
5.005.00
HAA
:A
H mol 0.01
H mol 0.01
2) Magnitudes of [HA] and [A-] the capacity of a buffered soln.Ex : soln A : 5.00 M HOAc + 5.00 M NaOAc
soln B : 0.05 M HOAc + 0.05 M NaOAcpH change when 0.01 mol of HCl(g) is added
Buffer capacity
3) [A-] / [HA] ratio the pH of a buffered soln.
soln new 50.5% soln original
49.5HAA100
0.011.00
HAA
0.98HAA1.00
1.001.00
HAA
2% :C
H mol 0.01
H mol 0.01
How indicators work
1) Usually a weak organic acid or base that has distinctly different colors in its nonionized & ionized forms.
HIn(aq)
H+(aq) + In-
(aq) pKHIn
nonionized ionizedform form
How indicators work
2)
-
HIn
-
HIn
In & HIn of color the of ncombinatio InHIn
1pKpH In of color theshow
10HInIn
1-pKpH HIn of color theshow
10InHIn
How indicators work
3) The useful pH range for indicator ispKHIn ±
1
encompass the pH at equivalence point of titration curve
4) Not all indicators change color at the same pH.
1) Strong base + Strong acid
3 regions : a) excess base b) equivalence point
(pH = 7.00)c) excess acid
Acid-Base Titrations
2) Weak acid + Strong base
4 regions: a) excess weak
acidb) buffer regionc) equivalence
point (pH > 7.0)d) excess base
3) Weak base + Strong acid4 regions: a) excess weak baseb) buffer regionc) equivalence point (pH < 7.00)d) excess acid
b) Using a pH Electrode to Find the End Point
using titration curve
Practical Notes
• Primary standards: Acids and bases can be purchased in forms pure enough.
• NaOH, KOH must be standardized against a primary standard
• Alkaline solutions: (ex: NaOH) OH- + CO2
HCO3
-
• Strong base attacks glass.
Practical Notes
Typical applications of neutralization titrations : Kjeldahl Nitrogen Analysis
Developed in 1883, the analysis remains one of the most widely used methods for determining nitrogen in organic substances such as protein, cereal, and flour. The solid is digested (decomposed and dissolved) in boiling sulfuric acid to convert nitrogen into ammonium ion, NH4
+, in a long-neck Kjeldahl flask
Kjeldahl Nitrogen Analysis: Chemistry Behind the Health
• Melamine (66.6wt% nitrogen)
• Cyanuric acid (32.6 wt% N)
Protein source Weight% nitrogen
MeatBlood plasmaMilkFlourEgg
16.015.315.617.514.9
Polyprotic Acids and Bases
Amino acids are polyproticAmino acids from which proteins are built have an
acidic carboxylic acid group, a basic amino group, and a variable substituent designated R:
The resulting structure, with positive and negative sites, is called a zwitterion.
Amino acids are polyprotic
Finding the pH1) Titration of polyprotic acids
(ex) H3 PO4 (H3 A)
][HA]A[H 22
]A[H2
]A[HA][H 23
AH3
pH
NAOH ofV
2pKapH
2pKapKapH 21
1pKapH
50% 50%
100%
50% 50%100%
Finding the pH
2) solutions containing Amphoteric Anions (H2 A-) as the only A-B major species
-23
a
a2
2
23
(aq)2
(aq)3(aq)2(aq)2
HAAH
KK
AHHA AHK
HAAHAHAH
1
2
Finding the pH
.
.
AHAH
KH
AHAH HK
AHAH HK
AHHAH from
KK
AHAHK
2
3
a3
-2
a
3
-2
a
2(aq)3
a
a2
2
23
1
1
1
1
2
Finding the pH
&
2pKpK
pH
KKH
KH
KK
21
21
11
2
aa
aa
2
aa
a
Which is the principle species ?
1) .
HAA pKpH
HAA pKpH
1HAA 4.20pH at
HAAlogpKapH
a
a
2) For H2 A
HAApKpH
AHHApKpH
2
a
2a
2
1
22
21
AAH
)pK(pK21pH
Which is the principle species ?
• 3) For H3 A
ExWhat is the principal form of arginine at pH 10.0? Approximately what fraction is in this form? What is the second most abundant form at this pH?
Proteins are polyprotic acids and bases
myoglobin
Isoelectric pH (isoionic pH)The pH at which the average charge of the
polyprotic acid is zero
Titrations in Polyprotic Systems
(ex)
21
2
pKpK21pH where
AAH &HA as formmost :pH cisoelectri
Titrations in Polyprotic Systems
H3+N CHCOOH
CH3
H3+N C
H
CH3
COO- + H+
H3+N CHCO2
-
CH3
H3+N CHCO2
-
CH3
+ H+
alanine cation H2A+
A- alanine antion
HA
HA
pK1=2.35
pK2=9.87
Isoelectric focusing : A technique of protein separation
Titrations in Polyprotic Systems
pH gradient designProtein stop migrating in an electric field at isoelectric pH
Homework1. Draw the structures of the predominant forms of
glutamine acid and tyrosine at pH 9.0 and pH 10.0. What is the second most abundant species at each pH?
2. Calculate the pH of 0.10M solution of each of the following amino acids in the form drawn: