Autoionization of Water and the pH Scale

Before we discuss the next major definition of acid-base behavior, let s examine a crucial property of water that enables us to quantify [H30 J in any aqueous system water is an extremely weak electrolyte. The electrical conductivity of tap water is due almost entirely to dissolved ions, but even water that has been repeatedly distilled and deionized exhibits a tiny conductance. The reason is that water itself dissociates into ions very slightly in an equilibrium process known as autoionization (or self-ionization)  

Like any equilibrium process, the autoionization of water is described quantitatively by an equilibrium constant  

Because the concentration of H2O is essentially constant here, we simplify this equilibrium expression by including the constant [H20] term with the value of to obtain a new equililDrium constant, the ion-product constant for water, A ,  

Notice that one HjO ion and one OH ion appear for each H2O molecule that dissociates. Therefore, in pure water, we find that 

Autoionization of water has two major consequences for aqueous acid-base chemistry  

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A Review of Strong Electroiytes 18-2 The Autoionization of Water 18-3 The pH and pOH Scales 18-4 Ionization Constants for Weak Monoprotic Adds and Bases 18-5 Poly protic Adds 18-6 Solvolysis... 

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