Denoting Buffer Solutions
A buffer solution denoted, "100 mM phosphoric acid (sodium) buffer solution pH = 2.1," for example, contains
phosphoric acid as the acid, sodium as the counterion, 100 mM total concentration of the phosphoric acid group, and
a guaranteed buffer solution pH of 2.1.
Maximum Buffer Action Close to the Acid (or Alkali) pKa
When an acetic acid (sodium) buffer solution is prepared from 1:1 acetic acid and sodium acetate, for example, the
buffer solution pH is approximately 4.7 (near the acetic acid pKa), and this is where the maximum buffer action can
be obtained.
Buffer Capacity Increases as Concentration Increases
The buffer capacity of an acetic acid (sodium) buffer solution is larger at 100 mM concentration than at 10 mM, for
example. However, precipitation occurs more readily at higher concentrations.
Beware of Salt Solubility and Precipitation
The salt solubility depends on the type of salt, such as potassium salt or sodium salt. Salts precipitate out more
readily when an organic solvent is mixed in.
In addition, avoid using buffer solutions based on organic acids (carboxylic acid) as much as possible for highly
sensitive analysis at short UV wavelengths. Consider the various analytical conditions and use an appropriate buffer
solution, such as an organic acid with a hydroxyl group at the α position (see Supplement) to restrict the effects of
metal impurity ions. (J.Ma,Y.Eg)
Reference: 1) LCtalk Vol. 29, 8 pKa and Dissociation Equilibrium, 2) LCtalk Vol. 26, 11, 3) LCtalk Vol. 40, 4 Preparing
the Mobile Phase.
Methods to Prepare Buffer Solutions
100 mM phosphoric acid (sodium) buffer solution (pH=2.1)
Sodium dihydrogen phosphate dihydrate (M.W.=156.01)..50 mmol (7.8 g)
Phosphoric acid (85 %, 14.7 mol/L).........................50 mmol (3.4 mL)
Add water to make up to 1 L.
10 mM phosphoric acid (sodium) buffer solution (pH=2.6)
Sodium dihydrogen phosphate dihydrate (M.W.=156.01)..5 mmol (0.78 g)
Phosphoric acid (85 %, 14.7 mol/L).........................5 mmol (0.34 mL)
Add water to make up to 1 L.
(Alternatively, dilute 100 mM phosphoric acid (sodium) buffer solution (pH=2.1) ten times.)
50 mM phosphoric acid (sodium) buffer solution (pH=2.8)
Sodium dihydrogen phosphate dihydrate (M.W.=156.01)..40 mmol (6.24 g)
Phosphoric acid (85 %, 14.7 mol/L).........................10 mmol (0.68 mL)
Add water to make up to 1 L.
100 mM phosphoric acid (sodium) buffer solution (pH=6.8)
Sodium dihydrogen phosphate dihydrate (M.W.=156.01)..50 mmol (7.8 g)
Sodium dihydrogen phosphate 12-hydrate (M.W.=358.14)..50 mmol (17.9 g)
Add water to make up to 1 L.
