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Lecture 12

sbc 200 Lecture Notes - Lecture 12: Buffer Solution, Titration, Conjugate AcidPremium

2 pages45 viewsFall 2016

Department
biotechnology
Course Code
sbc 200
Professor
idk
Lecture
12

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Buffers and the Common-ion Effect
A buffer works through the common-ion effect.
Acetic acid in water dissociates slightly to produce some acetate ion:
CH3COOH(aq) + H2O(l) → CH3COO- (aq) + H3O+(aq)
The addition of CH3COO- reduces the % dissociation of the acid.
How a Buffer Works
The buffer components (HA and A- ) are able to consume small amounts of added OH-
or H3O+ by a shift in equilibrium position
Relative Concentrations of Buffer Components
Since Ka is constant, the [H3O+] of the solution depends on the ratio of buffer
component concentrations.
If the [HA]/[A-] ratio increases, [H3O+] increases
If the [HA]/[A-] ratio decreases, [H3O+] decreases.
The Henderson-Hasselbalch Equation
pH = pKa + log [base]/[acid]
Buffer Capacity
The buffer capacity is a measure of the “strength” of the buffer, its ability to maintain the
pH following addition of strong acid or base.
The greater the concentrations of the buffer components, the greater its capacity to
resist pH changes
The closer the component concentrations are to each other, the greater the buffer
capacity.
Buffer Range
The buffer range is the pH range over which the buffer is effective.
Buffer range is related to the ratio of buffer component concentrations.
[HA]/ [A- ] The closer is to 1, the more effective the buffer.
If one component is more than 10 times the other, buffering action is poor. Since log10 =
1, buffers have a usable range within ± 1 pH unit of the pKa of the acid component.
Acid-Base Indicators
An acid-base indicator is a weak organic acid (HIn) whose color differs from that of its
conjugate base (In- ).
The color of an indicator changes over a specific, narrow pH range, a range of about 2
pH units.
The ratio [HIn]/[In- ] is governed by the [H3O+] of the solution. Indicators can therefore
be used to monitor the pH change during an acid-base reaction. Indicators can therefore
be used to monitor the pH change during an acid-base reaction.
Acid-Base Titrations
In an acid-base titration, the concentration of an acid (or a base) is determined by
neutralizing the acid (or base) with a solution of base (or acid) of known concentration.
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