Henderson–Hasselbalch equation

The Henderson-Hasselbalch equation, also called the buffer equation describes the relationship between the pH value and the position of the equilibrium of an acid -base reaction between a moderately strong acid, and their corresponding moderate base in dilute ( ≤ 1 mol / l), aqueous solutions.

It goes back to Lawrence Joseph Henderson and Karl Albert Hasselbalch. Henderson developed his eponymous equation 1908. Hasselbalch was the Henderson equation confirm experimentally in the human blood and wrote the equation in 1916 in order to place the hydrogen ion concentration to calculate the pH. Mistakenly, the equation, often in literature, known as Henderson -Hasselbach equation.

This equation is used in particular in the pH - value calculation of buffer solutions, and describes a portion of the curve of the acid-base titration curves of medium medium- strong acids or strong bases.

This equation is derived from a general acid -base reaction:

Here, HA is a general acid and its conjugate base A-.

The acidity constant of HA arises from the law of mass action. After simple transformations and taking logarithms, one obtains the Henderson -Hasselbalch equation. Of this there are two equivalent versions that result from the calculation rules of the Logarithmierens:

The definition of value is used

In the buffer area of the acid -base titration, the ratio corresponds to the ratio, so that one can write:

τ ( the Titrationsgrad ) is the ratio of the amount (or concentration) of the added standard solution to the amount of substance (or concentration) of the analyte.

In the range from τ = 0 and τ = 1 or with higher dilution ( 0.01 M), this formula is no longer valid because the small proteolysis of HA and A- with the solvent or the autoionization of water to adjust the pH of 7 (eg phosphate buffer) would have to be taken into account even for the calculation of concentration and it may well be differences of 0.4 from the calculated pH. For an exact calculation of the pH to the equation for the mass action law derives itself.