Buffer solutionsBuffer solutions

Conjugate Acid and BaseConjugate Acid and Base

Conjugate acid and baseConjugate acid and base, HA/A, HA/A--, differ by , differ by one proton.one proton.

The The conjugate acidconjugate acid of a base, is the base of a base, is the base plus the attached proton.plus the attached proton.

And the And the conjugate baseconjugate base of an acid is the of an acid is the acid minus the proton. acid minus the proton.

Example:Example:BaseAcidAcidBase

NH4+ + OH-NH3 + H2O

BaseAcidAcidBaseNH4

+ + OH-NH3 + H2O

Acid And Base Ionization ConstantsAcid And Base Ionization Constants

weak acid:weak acid: CHCH33COOH + HCOOH + H22O O ↔↔ H H33OO++ + CH + CH33COOCOO--

[H[H33OO++][CH][CH33COOCOO--]]

Acid ionization constantAcid ionization constant: K: Kaa==

[CH[CH33COOH]COOH]

weak base:weak base: NHNH33 + H + H22O O ↔↔ NH NH44++ + OH + OH--

[NH[NH44++][OH][OH--]]

Base ionization constantBase ionization constant: K: Kbb= =

[NH[NH33]]

Acid and base ionization constants are the measure of the Acid and base ionization constants are the measure of the strengths of acids and bases.strengths of acids and bases.

pKpKaa and pK and pKbb

pKa = -logKa pKb = -logKb

larger Ka → smaller pKa → stronger acid

larger Kb → smaller pKb → stronger base

Henderson-Hasselbalch EquationHenderson-Hasselbalch Equation

[conjugate base][conjugate base]

pH = pKpH = pKaa + log + log

[weak acid][weak acid]

If: [weak acid] = [conjugate base],If: [weak acid] = [conjugate base],

then: then: pH = pKpH = pKaa

BuffersBuffers

•A buffer is a solution characterised by the ability to resist changes in pH when limited amounts of acid or base are added to it.

•Buffers contain either a weak acid and its conjugate base or a weak base and its conjugate acid.

•Thus, a buffer solution contains both an acid species and a base species in equilibrium.

Example of weak acid/conjugate base as Example of weak acid/conjugate base as buffer systembuffer system::

•HC2H3O2 + C2H3O2-

•How does this work?•What happens when H+ is added?•What happens when OH- is added?

How A Buffer Solution WorksHow A Buffer Solution Works

The acid component of the buffer can The acid component of the buffer can neutralize small added amounts of OHneutralize small added amounts of OH--, , and the basic component can neutralize and the basic component can neutralize small added amounts of Hsmall added amounts of H33OO++..

The result is just more weak acid or more The result is just more weak acid or more weak base in the system.weak base in the system.

Acetic Acid/Acetate BufferAcetic Acid/Acetate Buffer

Buffer resist change in pHBuffer resist change in pH

Buffer Capacity And Buffer RangeBuffer Capacity And Buffer Range

There is a limit to the capacity of a buffer solution There is a limit to the capacity of a buffer solution to neutralize added acid or base, and this limit is to neutralize added acid or base, and this limit is reached before all of one of the buffer reached before all of one of the buffer components has been consumed.components has been consumed.

In general, the more concentrated the buffer In general, the more concentrated the buffer components in a solution, the more added acid or components in a solution, the more added acid or base the solution can neutralize.base the solution can neutralize.

As a rule, a buffer is most effective if the As a rule, a buffer is most effective if the concentrations of the buffer acid and its conjugate concentrations of the buffer acid and its conjugate base are base are equalequal..

Buffers are effective around pH values that are equal to Buffers are effective around pH values that are equal to the pKa ±1.the pKa ±1.

Power of Buffering ActionPower of Buffering Action

•summary: •Buffers

•consist of

•Weak acid and its salt

•Weak base and its salt

•or

•and

•resist changes in pH

•to

•Maintain pH balance

•preventing

•Acidosis •and •Alkalosis

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