2.3 pH

To treat these aspects of chemical reaction more precisely，a quantitative system of expressing acidity or basicity （alkalinity） is needed. This need could be met by using the value of [H⁺]，where [H⁺] is expressed in moles/liter，as a measure of acidity. But，in most cases，[H⁺] is in the range of 10^-14 to 10^-1 moles/liter. Because numbers of this magnitude are inconvenient to work with，an alternate system for expressing the acidity of dilute solutions has been devised. This system is based on a quantity called pH which is defined as the negative logarithm of the hydrogen concentration，represented as [H⁺] in moles/liters.

pH=-lg [H⁺]　　　　　　（2-6）

[H⁺]=10^-pH　　　　　　（2-7）

Because the logarithm of any number less than 1 is negative，multiplication by -1 causes the values of pH to be positive over the range in which we are interested. （The term pH was first defined by a Danish chemist and is derived from p for the Danish word power and H for hydrogen.） Because pH is simply a way to express hydrogen ion concentration，acidic and basic solution at 25℃ can be identified by their pH values as:

Acidic solutions: [H⁺]>1.0×10^-7 moles/liter，pH<7

Basic solutions: [H⁺]<1.0×10^-7 moles/liter，pH>7

Neutral solutions: [H⁺]=1.0×10^-7 moles/liter，pH=7

Notice that pH increases as [H⁺] decreases.

It is also important to understand the relationship between the [H⁺] and the [OH^-] concentrations. The pOH of a solution is defined as the negative logarithm of the hydroxyl concentration，represented as [OH^-] in moles/liter.

pOH=-lg [OH^-]　　　　　　（2-8）

[OH^-]=10^-pOH　　　　　　（2-9）

For water solutions，the product of the hydrogen ion concentration and the hydroxyl concentration is always 1×10^-14 at 25℃. This means that the sum of pH and pOH is equal to 14 under these conditions.

[H⁺]×[OH^-]=1×10^-14　　　　　　（2-10）

pH+pOH=14　　　　　　（2-11）