Topics
1.
2.
3.
4.
5.
6.
H+
Acids and Bases
Definition of pH
Reversible reactions, equilibrium, mas action
HendersonpHasselbalch equation
Buffers. Buffer capacity
H+
Suppose chloride acid dissolved in water
HCl
H+ + Cl-
The entity H+, hydrogen stripped from the electron, is simply a proton, without electronic
cloud, with dimensions at least 4 orders smaller than a real atom. Its strong electrical field
Impedes a free existence. What really happens, upon dissolution of HCl in water is:
HCl + H2O
H3O+ + ClH3O+
H2O + H+
HCl
H++ Cl-
[H+]
Rutherford-Thompson atom: Dimensions
10-15(Fermi)
10-10 (Å)
m
Acids and Bases
Brønsted-Lewy Concept (1923)
Acid
HA
H+ + A Base
B- + H+
BH
Arrhenius Concept (1890)
Acid
HA
Base
COH
H+ + A C+ + OH-
Acid + base
salt + water
2NaOH + H2SO4
Na2SO4 +
2H2O
Water has amphoteric character
2H2O
H3O- + H+
pH, reversible reaction, equilibrium, mass action
𝑝𝐻 = −𝑙𝑜𝑔10 𝐻+ = 𝑙𝑜𝑔10
𝐸 = 𝐸0 +
𝑅𝑇
ln
𝐹
𝐻+ = 𝐸 0 +
1
𝐻+
2.3𝑅𝑇
pH
𝐹
Reversible Reactions – Rate constants - Equilibrium
K1
BA
B+A
k-1
𝑘1 𝐵𝐴 = 𝑘−1 𝐵 𝐴
Henderson-Hasselbalch equation
K1
HCl
k-1
At equilibrium
H+ + Cl-
𝑘1 𝐾𝐶𝑙 = 𝑘−1 𝐻+ 𝐶𝑙−
𝑘1
[𝐶𝑙 − ]
+
= 𝐻
𝑘−1
𝐻𝐶𝑙
1
1 𝐶𝑙 −
=
𝑘1 𝐻𝐶𝑙
𝐻+
𝑘−1
−𝑙𝑜𝑔
𝐻+
𝑘1
=𝐾
𝑘−1
[𝐶𝑙 − ]
= −𝑙𝑜𝑔 𝐾 + log
𝐻𝐶𝑙
𝐶𝑙−
𝑝𝐻 = 𝑝𝐾 + 𝑙𝑜𝑔
𝐻𝐶𝑙
Buffers and Buffer capacity
𝑑𝑛𝑎
𝛽=
= 2.3
𝑑𝑝𝐻
𝐻+
𝑛𝑎 𝐾 𝐻+
+
+ 𝑂𝐻−
+
2
𝐾+ 𝐻
In a given pH, β is a function of pH
and buffer concentration
Bibliography
• Bockris, J.O’M and Reddy, A.K.N.: Modern Electrochemistry.
Plenum Press, 1970. Vol.1, 1970. Chap. 5. Protons in solution.
Questions
1.
2.
3.
For a [H+] of 10-10M to 10-1M, in steps fo 10-3M, draw a plot of
pH x [H+].
Consider 1 L of a solution of a buffer of pK=7.5 amd
concentration of 10 mM. Starting with a buffer base
concentration of 9,9 mM, add progressively a strong acid, in
amounts of 0.05 mmol. At equilibrium draw the curve relating
pH to the total amount of acid added. Where is the point of
maximal buffering power?
Suppose a buffer if pK=7.0 in concentration of 5 mM. What are
the concentrations of acid and base for buffering a solution at
a pH of 6,0.
Medidas de pH
I. Eletródios
II. Indicadores fluorescentes
Bibliografia
Koryta, J.: Ion-Selective Electrodes. 1974. Cambridge
University Press.
Vanysek, P.> The glass pH electrode.The Electrochemical
Society Interface. 2004
Lakowicz, J.R.: Principles of fluorescence spectroscopy.
2nd ed., 1999. Fluwer Academy/Plenum Press
Electrochemical potential of a solute in a phase – Macroscopic view
Thermal energy
C2
ø2
C1
ø1
T (K)
C: concentration,
mol/l
Ø: Electrical potential, V
M
𝜇𝑖 1 = 𝜇𝑖
𝜇𝑖 2 = 𝜇𝑖
∆𝜇𝑖 = 𝑅𝑇𝑙𝑛
𝑜
𝑜
1 + 𝑅𝑇𝑙𝑛𝑐𝑖 1 + 𝑧𝑖 𝐹∅(1)
2 + 𝑅𝑇𝑙𝑛𝑐𝑖 2 + 𝑧𝑖 𝐹∅(2)
𝑐𝑖 1
+ 𝑧𝑖 𝐹 ∅(1)− ∅(2)
𝑐𝑖 2 𝑖
R= 8.3 J mol-1 K-1
𝜇𝑖 = 𝐽 𝑚𝑜𝑙 −1
𝐹 = 𝑁𝐴 𝑒 − = 1,6022 × 104 × 6.03 × 1023
𝐹 = 9.6485 × 104 𝑐𝑜𝑢𝑙 𝑚𝑜𝑙 −1
Thermal energy – microscopic view
Thermal energy
Bezanilla simulation
Campos elétricos – forças elétricas
Força elétrica – lei de Coulomb
+
+
Carga do
e-
Constante de
Faraday
q1  q2
f k*
2
r
1,60*10-19 coul
F=NA*e-=
96484 coul/mol
Diferença de potencial elétrico
W
V 
q
Campo elétrico


f

q
dV

dx
Membrane (M) Properties
C2
ø2
C1
ø1
M
1. Impermeable membrane
2. Membrane permeable to solutes
𝑐 1
𝑖 2 𝑖
∆𝜇𝑖 = 𝑅𝑇𝑙𝑛 𝑐𝑖
+ 𝑧𝑖 𝐹 ∅(1)− ∅(2) =0
3. Membrane permeable to cations or to anions
∆𝜇𝑖 = 𝑅𝑇𝑙𝑛
𝑐𝑖 1
𝑐𝑖 2 𝑖
+ 𝑧𝑖 𝐹 ∅(1)− ∅(2) =0
𝑅𝑇 𝑐𝑖 1
∆∅ = −
ln
𝑧𝑖 𝐹 𝑐𝑖 2
Ion Exchangers – Glass Electrodes
H+
H+
+
H+ H
H+
H+
H+
-
-
H+
H+
H+
H+
H+
H+
H+
H+
H+
𝑉𝑤𝑎𝑙𝑙/𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 =
𝑅𝑇
2.303𝑙𝑜𝑔 𝐻3 𝑂+
𝐹
𝑉𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒 = 𝑉 ′ +
𝑅𝑇
2.303 𝑝𝐻
𝐹
BCECF