Weak Acids, Weak Bases

And Buffers

Ionization/Dissociation of water

H2O + H2O H3O+ + OH-

H2O H+ + OH-

K=[H+] [OH-]

[H2O]K=1.8 x 10-16 M

[H+] [OH-] = 55.5 M x (1.8 x 10-16 M)[H+] [OH-] = 1 x 10-14 M2 (Kw)[OH-] = [H+] = 1 x 10-7 M

pH= -log10 [H+] = log

Log [H+] + log [OH-] = log 10-14

pH + pOH = 14

1H+

Dissotiation of Weak Acids

CH3COOH CH3COO- + H+

(HA) (A-)

Henderson-Hasselbach Equation

[H+] = Ka[HA]

[A-]

pH = pKa + log[A-] Conjugated base

[HA] Acid

Ka = [H+] [A-]

[HA]

-log [H+] = -log Ka - log[HA]

[A-]

CH3COOH CH3COO- + H+ Ka = 1.74 x 10-5

X2 = 1.74 x 10-5 [1-CH3COO-}

X = 1.74 x 10-5 x 1

pH = - logX = 2.38

Ka = 1.74 x 10-5 =[CH3COO-] [H+]

[CH3COOH]

[CH3COO-] [H+] = (1.74 x 10-5) [CH3COOH]X X 1-X

Calculating the pH of Weak Acid Solutions

d % MW

NH3 0.91 25 15HCl 1.184 85 35.5H3PO4 1.7 85 98H2SO4 1.84 98 98CH3COOH 1.049 99.9 60

After Calculation:V1 N1 = V2 N2 or V1 C1 = V2 C2

can be used to prepare desired solution.

M =d x percentage x 1000

Molecular weight

! !

The Mechanism of Buffer ActionKısmen dissosiye olmaları nedeniyle zayıf asitler ve onların konjuge baz formları proton verici ve proton alıcı olarak davranırlar:

CH3COOH CH3COO- + H+

NaOH Na+ + OH- H2O

By the same mechanism, weak bases can act as buffers.

CH3COOH CH3COO- + H+

HCl Cl- + H*

0.5 1.0Ekivalent baz

pH

pH of Body Fluids

Fluid pH

Blood 7.4Milk 6.6 – 6.9Urine (normal) 6.0Gastric juice 0.88Pancreatic secretions 8.0Intestinal secretions 7.7Erebrospinal fluid 7.4Saliva 7.2Tears 7.4

Some weak acids or basesacid Conjugated base pK

H-COOH HCOO- + H+ 3.75

CH3-COOH CH3COO- + 4.76

CH3CHCOOH CH3CHCOO- + H+ 3.86

OH OH

H3PO4 H++H2PO4- H++HPO4= H+ PO4

3-

2.34 6.86 12.4

H2CO3 H+ + HCO3- CO3

2- + H+

3.8 10.2

C6H5OH C6H5O + H+ 9.89

N H4 NH3 + H+ 9.25 +

Major Buffers

Acid form pKaCacodylic acid 6.2

BISTRIS 6.5

PIPES 6.8

İmidazole 7.0

HEPES 7.6

Tris 8.3

Physiological Buffer Systems

1. Hemoglobin

2. Proteins

3. Phosphate buffer system

4. Carbonic acid / Bicarbonate system

Non-bicarbonatebuffers

Carbonic acid-Bicarbonate Buffer system

H2CO3 H+ + HCO3- pK1 = 3.8

HCO3- H+ + CO3

2- pK2 = 10.2

CO2 + H2O H2CO3 H+ + HCO3-

CA

HCO3- / H2CO3 ratio is 20/1

pK’(apperent) = [HCO3

-] [H+]

H2CO3

= 6.1

7.4 = 6.1 + log [HCO3

-]

[H2CO3]

Phosphate Buffer System

H2PO4- H+ + HPO4

2-

HPO42- / H2PO4 = 4/1

* Major intracellular inorganic buffer.

*H2PO4 excretion in urine is important for the regulation of blood pH.

pK = 6.8

7.4 = 6.8 + log HPO42-

H2PO4-

Plasma Red blood cells

CO2

H2O

H2CO3

H+ + HCO3-

HCO3-

O2 release

Cl- Tis

su

e m

eta

bo

lism

CO2

O2

CO2 + H2O H2CO3

HCO3- + H+

Cl-

HHb

Carbamino Hb

H2PO4-

HPO4=

HbO2

O2

CA

Plasma Red blood cells

CO2

H2O

H2CO3

H+ + HCO3-

HCO3-

O2 diffusion

Cl- Alv

eols

CO2

O2

CO2 + H2O H2CO3

HCO3- + H+

Cl-

HHb

Carbamino Hb

H2PO4-

HPO4=

HbO2

O2

CA