Because the concentrations of H⁺ and OH^– ions in aqueous solutions are usually very small, to specify the acidity of a solution with more convenient numbers, a quantity called pH is used.

 

The pH is defined as the negative logarithm of proton/hydronium ion concentration:

 

 

For example, if [H⁺] = 10^-4 mol/l, the pH will be:

 

pH = -log 10^-4 = 4 (acidic)

 

How do we know it is an acidic solution?

Remember, from the dissociation of water that K_w = [H⁺][OH^–] = 10^-14 and if [H⁺] > 10^-7 M, then the solution is acidic, and if [H⁺] < 10^-7 M, we have a basic solution.

So, let’s say we have a basic solution with [OH^–] = 10^-5. The [H⁺] can be calculated from K_w:

 

\[{\rm{[}}{{\rm{H}}^{\rm{ + }}}{\rm{]}}\;\;{\rm{ = }}\;\frac{{\sqrt {{K_{\rm{w}}}} \;}}{{{\rm{[O}}{{\rm{H}}^{\rm{ – }}}{\rm{]}}}}\; = \;\frac{{{{10}^{ – 14}}\;}}{{{{10}^{ – 5}}\;}}\, = 1.0\, \times \,{10^{ – 9}}\,M\]

 

And the pH, therefore, is:

 

pH = -log 10^-9 = 9 (basic)

 

Now, for a neutral solution, the pH is 7 because a solution is neutral when [H⁺] = [OH^–] = 10^-7 M, and the pH in this case is:

 

pH = -log 10^-7 = 7 (neutral)

 

So, the pattern is; if pH < 7, the solution is acidic, if pH = 7, the solution is neutral, and if pH > 7, we have a basic solution.

The pH scale goes from 0 to 14. The smaller the pH, the more acidic the solution.

 

 

The pOH and Basicity

Although, not as commonly used, the pOH values also describe the acidity/basicity of the solution. It is again the negative log which is a convenient method for expressing small numbers.

 

pOH = – log [OH^–]

 

Example:

Calculate the pOH of a solution with [OH^–] = 10^-2 (basic).

 

pOH = – log [OH^–] = -log 10^-2 = 2

 

Like in the pH scale, a solution is neutral if pOH = 7.  A pOH less than 7 is basic and a pOH greater than 7 is acidic.

In the next few post, we will discuss the relationship between pH and pOH and how it is used to calculate the pH and pOH from one another.

 

Check Also

• Definitions of Acids and Bases
• Acid-Base Reactions
• Acid-Base Titrations
• Conjugate Acid and Conjugate Base
• Autoionization of Water and K_w
• The pH and Acidity
• Acid Strength, K_a, and pK_a
• Base Strength, K_b and pK_b
• K_a, pK_a, K_b, and pK_b Relationship
• The pH of a Strong Acid and Base
• pH + pOH = 14
• The pH of a Weak Acid
• The pH of a Weak Base
• ThepH of Polyprotic Acids
• The acidity of a Salt Solution
• The pH of a Salt Solution
• The pH of Salts With Acidic Cations and Basic Anions
• Acids and Bases Practice Problems