acids, bases, buffers, titrations, polyprotic...

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

Melamine contaminated milk

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: