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Pharmaceutical Analytical Chemistry
213 PHC
Salt of Weak Acids and Bases
&Buffers
By the end of this lecture you will be able to:
Calculate the pH of salt of weak acids. Calculate the pH of salt of weak bases. Define buffer, buffer ratio, and buffer
capacity. Calculate the pH of weak acid and
weak base buffers.
Salts of WeakAcids and Bases
Salts of weak acids
e.g. NaOAc (sodium acetate) It is completely ionizes It is a Bronsted base It is hydrolyzes in water
OAc- + H2O HOAc + OH-
Kb = [HOAc] [OH-]\ [OAc-]
Cont.
A- + H2O HA + OH-
salt weak acid
Kb
= Kw
\ Ka = [HA] [OH-] \ [A-]
pH Calculation
Calculated as weak bases
Kb = Kw \ Ka = [x] [x] \ CA-
Salts of weak bases
e.g. NH4Cl (ammonium chloride) It is completely ionizes It is a Bronsted acid It is hydrolyzes in water
NH4+ + H2O NH4OH + H+
Ka = [NH4OH] [H+] \ [NH4+]
Cont.
BH+ + H2O B + H3O+
salt weak base
Ka = Kw \ Kb = [B] [H3O+] \ [BH+]
pH Calculation
Calculated as weak acids
Ka = Kw \ Kb = [x] [x] \ CBH+
Questions
Buffers
Definition A solution that resists change in pH
when a small amount of an acid or base is added or when the solution is diluted
It consists of a weak acid and its conjugate base or a weak base and its conjugate acid
A mixture of a weak acid and its salt or a weak base and its salt
Buffer of weak acids
HA H+ + A-
pH = pKa + log [A-] / [HA](Henderson-Hasselbalch equation)
Problem 1:
Calculate the pH of a buffer solution containing 0.1M acetic acid and 0.1M sodium acetate (Ka = 1.75 x 10-5).
Buffer of weak bases
BH+ B + H+
pH = pKa + log [B] / [BH+]
pH = (pKw – pKb) + log [B] / [BH+]
pOH = pKb + log [BH+]/ [B]
Problem 2:
Calculate the pH of a solution containing 0.07M NH3 and 0.08M NH4Cl (pKb = 4.76)
Buffer ratio
The ratio of salt / acid
[salt] / [acid] = 1pH = pKa
By changing the buffer ratio, buffers
of different pH values can be prepared
Buffer capacity It is the number of moles /liter of strong
acid or base needed to change the pH of buffer solution
ß = dCacid or base/dpH The buffering capacity is maximum at:
pH = pKa pOH = pKb
It is satisfactory over a pH range of pKa ± 1
The larger it is, the more resistant the solution to pH change
Questions
Summary
pH Calculation of salts of weak acids and bases.
Buffer Definition, buffer ratio, and buffer capacity.
pH Calculation of weak acid and weak base buffers.
Thank you