Topics

1. H+

2. Acids and Bases3. Definition of pH4. Reversible reactions, equilibrium, mas action5. HendersonpHasselbalch equation6. 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+ + Cl-

H3O+ H2O + H+

HCl H++ Cl-

[H+]

Rutherford-Thompson atom: Dimensions

m10-10 (Å)10-15(Fermi)

Acids and Bases

Brønsted-Lewy Concept (1923)AcidHA H+ + A-

BaseB- + H+ BH

Arrhenius Concept (1890) AcidHA H+ + A-

BaseCOH C+ + OH-

Acid + base salt + water 2NaOH + H2SO4 Na2SO4 + 2H2O

Water has amphoteric character2H2O H3O- + H+

pH, reversible reaction, equilibrium, mass action

𝑝𝐻=− 𝑙𝑜𝑔10¿pH

Reversible Reactions – Rate constants - Equilibrium

BA B + AK1

k-1

𝑘1 [𝐵𝐴 ]=𝑘− 1 ( [𝐵 ] [ 𝐴 ] )

Henderson-Hasselbalch equation

HCl H+ + Cl-K1

k-1

At equilibrium

𝑘1𝑘− 1

=¿

−𝑙𝑜𝑔¿

𝑘1𝑘− 1

=𝐾

𝑝𝐻=𝑝𝐾 +𝑙𝑜𝑔[𝐶𝑙−]

[ 𝐻𝐶𝑙 ]

Buffers and Buffer capacity

𝛽=𝑑𝑛𝑎

𝑑𝑝𝐻=2.3¿

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. For a [H+] of 10-10M to 10-1M, in steps fo 10-3M, draw a plot of pH x [H+].

2. 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?

3. 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. EletródiosII. Indicadores fluorescentes

BibliografiaKoryta, J.: Ion-Selective Electrodes. 1974. Cambridge University Press.Vanysek, P.> The glass pH electrode.The Electrochemical Society Interface. 2004Lakowicz, 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 T (K)

C1

1

C2

ø2

M

C: concentration, mol/lØ: Electrical potential, V

~𝜇𝑖 (1 )=𝜇𝑖❑

❑𝑜 (1 )+𝑅𝑇𝑙𝑛𝑐𝑖 (1 )+𝑧 𝑖𝐹 ∅ (1)

R= 8.3 J mol-1 K-1

⌊~𝜇𝑖 ⌋= 𝐽 𝑚𝑜𝑙−1

𝐹=𝑁 𝐴𝑒−=1,6022×104×6.03×1023

𝐹=9.6485×104𝑐𝑜𝑢𝑙𝑚𝑜𝑙− 1

~𝜇𝑖 (2 )=𝜇𝑖❑

❑𝑜 (2 )+𝑅𝑇𝑙𝑛𝑐 𝑖 (2 )+𝑧𝑖 𝐹∅ (2)

∆~𝜇𝑖=𝑅𝑇𝑙𝑛𝑐 𝑖 (1 )𝑐 𝑖 (2 ) 𝑖

+𝑧𝑖 𝐹 (∅ (1)−  ∅ (2))

Thermal energy – microscopic view

Thermal energyBezanilla simulation

Campos elétricos – forças elétricas

Força elétrica – lei de Coulomb

q

WV

dx

dV

q

f

Campo elétrico

Diferença de potencial elétrico

+-

+

-

221*

r

qqkf

Carga do e- 1,60*10-19 coul

Constante de Faraday

F=NA*e-=

96484 coul/mol

C1

1

C2

ø2

M

Membrane (M) Properties

1. Impermeable membrane

2. Membrane permeable to solutes

=0

3. Membrane permeable to cations or to anions

=0

∆∅=− 𝑅𝑇𝑧𝑖 𝐹

ln𝑐𝑖 (1 )𝑐𝑖 (2 )

Ion Exchangers – Glass Electrodes

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H+

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

𝑉𝑤𝑎𝑙𝑙 /𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛=𝑅𝑇𝐹2.303 𝑙𝑜𝑔¿

𝑉 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒=𝑉 ′+𝑅𝑇𝐹2.303 (𝑝𝐻 )

BCECF