5.8 pH of Protonation and Deprotonation

The pH scale is a logarithmic (exponential) measurement of the activity of ions in a solution, and refers to how strongly acidic or basic something is. A pH of 7 is neutral, neither acidic nor basic.

Some common acidic substances and their pH ranges include milk, 6.5-6.8; black coffee, about 5; orange juice, about 3.5-4; stomach acid, about 1 to 1.5; battery acid, less than 1. Common basic substances and their pH ranges include sea water, 7.5 to 8.5; egg white, about 7.5 to 9; ammonia, about 11; bleach, 12.5; lye solution, 14.

The related measurement pK represents the pH at which half of the molecules of a substance will be protonated and half will be deprotonated. Below the pK, the majority of molecules will be protonated; above the pK, the majority will be deprotonated.

The first pK of a molecule, i.e. the pH to balance the most easily protonated or deprotonated atom, is the pKa. The second pK is the pKb, and so on.

Acids are in their uncharged state at low pH. At higher pH, they deprotonate and become negative ions. Bases are in their positively charged ionic state at low pH. At higher pH, they deprotonate and become uncharged molecules.

An example of a molecule with a pKb is purine:

Its pKa is 2.5; at this acidic pH, half of all molecules are protonated on one of the double-bonded nitrogens with a positive charge. Its pKb for deprotonation of its NH is 8.9; at this pH, half of all molecules are deprotonated at that site with a negative charge.

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