pH, pKa and Buffering

Most organic compounds in cell are in charged and hydrated state---ion (anions, cations & ampholytes). Formation of ions through dissociation of a substance is ionization (Svante Arrhenius, 19th C). Ionization helps classify acids and bases. Upon ionization, acids yield H+

(protons), and bases yield OH-.

Based on the extent to which ionization takes place ... strong acids and strong bases ionize completely (100%), whereas weak acids and weak bases ionize partially (<100%).

H2O

Acids: HA (Strong) ------- H+ + A- (in aqueous) HCl, H2SO4, HNO3

H2O

HA (Weak) ←--------- H+ + A- (in aqueous) CH3COOH →

H2O

Bases: MOH (Strong) -------- M+ + OH- (in aqueous)KOH, NaOH

H2O

MOH (Strong) ←-------- M+ + OH- (in aqueous) NH4OH →

Ionization of weak acids and bases results in a dynamic equilibrium between forward and reverse processes ... leading to a coexistence of an undissociated acid/base in solution with the dissociated ions. Majority of the macromolecules in living cells are in fact organic compounds of weak acids and bases.

The Arrhenius’ acids and bases Acids are substances which produce hydrogen ions in solution. Bases are substances which produce hydroxide ions in solution.

The Lewis Theory of acids and bases An acid is an electron pair acceptor A base is an electron pair donor.

The Bronsted-Lowry acids donate protons, and bases accept protons

HA ←------ A+ + H3O- (Hydronium ion) →

Here HA is Bronsted acid, and H2O is Bronsted base. Conversely, H3O- is an acid and A- a base. Due to the paired relationship, HA and A- are conjugate acid – base pair.

A conjugate acid ←------ conjugate base + H+

→

Strength of weak acid depends on its tendency to ionize. An acid that ionizes 50% yields more conjugate base than another acid that ionizes 5%. The extent of ionization of weak acid is constant at a given temperature, characteristic of the acid. Since weak acids maintain an equilibrium, the law of mass action yields an equilibrium constant K. K refers to the [] of products and reactants present. Since we deal with the ionization of weak acids K is also called acid ionization constant, Ka.

For a conjugate acid ←------ conjugate base + H+

→

Greater the Ka stronger is the acid; lesser the Ka weaker is the acid.Ka can be defined as pKa.

Hence, smaller the pKa stronger is the acid; and the vice versa.

Since Ka is constant, and

Equation 1 can be written as

It denotes, at any given temperature:

1. The ratio of equi-molar concentrations the conjugate acid-base pair is solely dependent on the [H+].2. Since the living cell is an equilibrium mixture consisting of weak acids and their conjugate bases, any change in the physiological pH will have significant consequences.3. The pH of an equi-molar (50 – 50) solution of a conjugate acid-base pair is equal to its pKa. 4. That is, when the pKa is = pH the solution buffers the best.

............................Self StudyMonoprotic acids & Polyprotic acids: definition and examples and their pKa values

Physiological pH and Buffers

Blood is buffered by: Inorganic phosphates, Carbonic anhydrase and

Haemoglobin (oxygenated and deoxygenated)

ExerciseAcid-base Titration: 1. Weak acid vs strong base: CH3COOH vs NaOH

2. Phosphoric acid vs NaOH

ThemeBiological molecules are charged ions; their structural stability and functional integrity depends on the ionic state. If the physiological pH changes drastically, the molecular configuration will be disoriented resulting in the loss of structure and function ------- cell death.

Ref: Rawn D 2008 Biochemistry WMH Boston USA____________________

[conjugate base] [H+]K = ------------------------- = Ka equation 1 [conjugate acid]

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