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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
