Half-equivalence point

The half-equivalence point in an acid-base titration is the point at which half of a certain amount of acid has been neutralized with the corresponding amount of base that would have led to complete neutralization.

In the case of weak acids, the pH value at the half-equivalence point is equal to the p K _s value of the acid. With weak bases, the pH value is the same as the p K _s value of the conjugate acid.

Explanation

Since a weak acid is almost completely unprotolyzed in a solution , the following equilibrium is achieved by adding half the base that would lead to complete neutralization:

${\displaystyle c(\mathrm {A} ^{-})=c(\mathrm {HA} )}$

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

With the help of the Henderson-Hasselbalch equation , the following can be derived:

${\displaystyle \mathrm {pH} =\mathrm {p} K_{\text{S}}+\log _{10}{\frac {c\left(\mathrm {A^{-}} \right)}{c\left(\mathrm {HA} \right)}}}$

The equilibrium then gives:

${\displaystyle \mathrm {pH} =\mathrm {p} K_{\text{S}}+\log _{10}(1)}$

Since is the same , one comes to the conclusion: ${\displaystyle \log _{10}(1)}$ ${\displaystyle 0}$

${\displaystyle \mathrm {pH} =\mathrm {p} K_{\text{S}}}$ (for weak acids)

See also

• Equivalence point

Web links

Wiktionary: Half-equivalence point  - explanations of meanings, word origins, synonyms, translations

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• Jander, year: measurement analysis . 16th edition, de Gruyter, Berlin 2003, ISBN 3-11-017098-1 .