Water, regardless of its degree of purity, contains low concentrations of ions that can be detected through the use of conductivity measurements. These ions arise from the transfer of a proton from water molecule to another, a process known as autoionization:
In equation (13), the H3O+ species is the hydronium ion, and the OH- is the hydroxide ion. This reaction is reversible, and it is well established that the reactants proceed only slightly on to the products. Therefore, the approximation that the activity of the various species can be expressed by their concentrations is accurate, so the equilibrium constant for this reaction can be written as:
In aqueous solutions, the activity of water is a constant owing to its high concentration (55.55 M), so it is possible to simplify equation (14) to:
KW is the autoionization constant of water, and is sometimes identified as being the ion product of water. The magnitude of KW is very small, being equal to 1.007 x 10-14 at a temperature of 25°C (Dean 1979).
For the sake of convenience, S0rensen (1909) proposed the "p" scale, where numbers such as KW would be expressed as the negative of their base10 logarithms. The value of pKW would then be calculated as:
and has a value equal to 13.997 at 25°C. Defining pH as:
then equation (15) can then be expressed as:
The autoionization of water is an endothermic reaction, so Kw increases as the temperature is increased (Dean 1979). This temperature dependence is plotted in Figure 1.
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