When is the Henderson-Hasselbalch Equation used?

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Answer 1 · 2016-06-25T13:26:25

The Henderson-Hasselbalch equation is used when you are doing buffer calculations.

An acid buffer is a solution of a weak acid and its salt.

The equilibrium reaction is

#"HA + H"_2"O" ⇌ "H"_3"O"^+ + "A"^"-"#

#K_a = (["H"_3"O"^+]["A"^"-"])/(["HA"]]#

The Henderson-Hasselbalch equation is

#"pH" = "p"K_a + log((["A"^"-"])/"[HA]")#

There are four variables in this equation: #"pH", "p"K_a, ["A"^"-"]"#, and #["HA"]#.

You can calculate any one of them if you know the values of the other three.

Similarly, a basic buffer is a solution of a weak base and its salt.

The equilibrium reaction is

#"B + H"_2"O" ⇌ "BH"^+ + "OH"^"-"#

#K_b = (["BH"^+]["OH"^"-"])/(["B"]]#

The Henderson-Hasselbalch equation is

#"pOH" = "p"K_b + log ((["BH"^+])/(["B"]))#

There are four variables in this equation: #"pOH", "p"K_b, ["BH"^+]"#, and #["B"]##.

You can calculate any one of them if you know the values of the other three.

You can calculate #"[pH](https://socratic.org/chemistry/acids-and-bases/the-ph-concept)"# from the relationship

#"pH" = "14.00 - pOH"#