Another New Estimate of the Hydrogen Ion Concentration in Pure Water (With Some Results for Methanol and Ethanol)

1980 ◽  
Vol 84 (7) ◽  
pp. 629-634 ◽  
Author(s):  
H. G. Hertz
Keyword(s):  
Pure Water ◽  
Hydrogen Ion ◽  
Ion Concentration ◽  
Hydrogen Ion Concentration

Ionic Equilibria and pH

2009 ◽  
Author(s):  
Christopher O. Oriakhi
Keyword(s):  
Pure Water ◽  
Weak Acid ◽  
Hydrogen Ion ◽  
Ion Concentration ◽  
Hydrogen Ions ◽  
Hydrogen Ion Concentration ◽  
M 10 ◽  
Ionic Equilibria ◽  
Hydrated Protons ◽  
Dilute Aqueous Solution

Water is a weak acid. At 25°C, pure water ionizes to form a hydrogen ion and a hydroxide ion: H2O ⇋ H+ + OH− Hydration of the proton (hydrogen ion) to form hydroxonium ion is ignored here for simplicity. This equilibrium lies mainly to the left; that is, the ionization happens only to a slight extent. We know that 1 L of pure water contains 55.6 mol. Of this, only 10−7 mol actually ionizes into equal amounts of [H+] and [OH−], i.e., [H+] = [OH−] = 10−7M Because these concentrations are equal, pure water is neither acidic nor basic. A solution is acidic if it contains more hydrogen ions than hydroxide ions. Similarly, a solution is basic if it contains more hydroxide ions than hydrogen ions. Acidity is defined as the concentration of hydrated protons (hydrogen ions); basicity is the concentration of hydroxide ions. Pure water ionizes at 25°C to produce 10−7 M of [H+] and 10−7 M of [OH−]. The product Kw = [H+]×[OH−] = 10−7 M×10−7 M= 10−14 M is known as the ionic product of water. Note that this is simply the equilibrium expression for the dissociation of water. This equation holds for any dilute aqueous solution of acid, base, and salt. The pH of a solution is defined as the negative logarithm of the molar concentration of hydrogen ions. The lower the pH, the greater the acidity of the solution. Mathematically: pH=−log10[ H+] or −log10[H3O+] This can also be written as: pH = log10 1/[H+] or log10 1/[H3O+] Taking the antilogarithm of both sides and rearranging gives: [H+] = 10−pH This equation can be used to calculate the hydrogen ion concentration when the pH of the solution is known.

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