Base constant

The base constant ( K _B ) is a material constant and indicates the extent to which a base reacts alkaline in water. It is rarely used because it can be calculated directly from the acid constant K _{S of} the corresponding acid. The following applies: the smaller the p K _B value, the stronger the tendency of the base to take up protons. For acids and bases dissolved in water , with the dissociation equilibrium of water at 25 ° C, neglecting the fugacities, the following approximation applies :

${\displaystyle K_{\text{S}}\cdot K_{\text{B}}=10^{-14}}$

Acid-base reaction

Between a base B and its conjugate acid BH ⁺ is in aqueous solution, the following equilibrium reaction steps:

${\displaystyle \mathrm {B+H_{2}O} \leftrightharpoons \mathrm {OH^{-}+BH^{+}} }$

According to the law of mass action , the position of the equilibrium is described by the equilibrium constant K :

${\displaystyle {\frac {c(\mathrm {OH} ^{-})\cdot c(\mathrm {BH} ^{+})}{c(\mathrm {B} )\cdot c(\mathrm {H} _{2}\mathrm {O} )}}=K}$

Since the concentration of water remains practically constant during the reaction, it can be included in the constant . This finally gives the base constant : ${\displaystyle \mathrm {c(H_{2}O)} }$ ${\displaystyle \mathrm {c(H_{2}O)} }$ ${\displaystyle K}$ ${\displaystyle K_{\text{B}}}$

${\displaystyle K_{\text{B}}={\frac {c(\mathrm {B} \mathrm {H} ^{+})\cdot c(\mathrm {OH} ^{-})}{c(\mathrm {B} )}}}$

The negative decadic logarithm of , the so-called -value, is often given: ${\displaystyle K_{\text{B}}}$ ${\displaystyle \mathrm {p} K_{\text{B}}}$

${\displaystyle \mathrm {p} K_{\text{B}}=-\lg \left(K_{\text{B}}\cdot 1\mathrm {\frac {l}{mol}} \right)=-\lg \left({\frac {c(\mathrm {B} \mathrm {H} ^{+})\cdot c(\mathrm {OH} ^{-})}{c(\mathrm {B} )}}\cdot 1\mathrm {\frac {l}{mol}} \right)}$

For the value and the analogously defined value, the following applies at room temperature: ${\displaystyle \mathrm {p} K_{\text{B}}}$ ${\displaystyle \mathrm {p} K_{\text{S}}}$

${\displaystyle \mathrm {p} K_{\text{B}}+\mathrm {p} K_{\text{S}}=14}$

A table with many values ​​and values ​​can be found in the acid constant . ${\displaystyle \mathrm {p} K_{\text{B}}}$ ${\displaystyle \mathrm {p} K_{\text{S}}}$

Individual evidence

1. ↑ Gerhard Schulze, Jürgen Simon: Jander year measurement analysis , 17th edition, de Gruyter, Berlin, 2009, p. 79 f.
2. ^ Karl-Heinz Lautenschläger, Werner Schröter, Joachim Teschner, Hildegard Bibrack: Taschenbuch der Chemie , 18th edition, Harri Deutsch, Frankfurt (Main), 2001, p. 186 f.

Web links

• Comprehensive tables with acids and bases ( MS Excel ; 152 kB)