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Wize University Organic Chemistry Textbook > Acid/Base Chemistry & Intro to Organic Reactions

Relationship Between pH and pKa

Factors Influencing Acid/Base StrengthPrevious Section
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Relationship Between pH and pKa



Watch Out!
It is really common for students to mix up pH and pKa. pH is a measurement made for how acidic a solution is, pKa is a measurement made for how acidic a hydrogen atom on a molecule is!


Important pKas You Should Know

Knowing a series of pKas (below) will help you a great deal in organic chemistry. If you are asked to determine which side of an equilibrium is favoured, you pick the side with the more stable conjugate base!









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Relationship Between pH and pKa


The pH of a solution will have an effect on the predominant structure of an acid. Take for example a generic acid HA shown below:


The ratio of the protonated acid (HA) to its conjugate base (A-) will be directly related to the pH (which is a measure of H+ ions in solution). If pH = pKa, the concentration of HA will equal A-. In higher (more basic) pH, the quantity of A- will increase. In lower (more acidic) pH, the quantity of HA will increase.




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Amino Acids at Different pHs


Amino acids, the building blocks of proteins and enzymes, are important biological molecules. They also serve as great tool to explore the relationship of pH to pKa. Amino acids are similar in that they all feature an amine and a carboxylic acid functionality. They differ in the structure of their side chain (labelled R, see below), which is a key feature of their reactivity. We can predict the predominant structure of an amino acid in acidic (pH = 1-3), neutral (pH = 6-8), and basic (pH = 11-13) solutions:




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Practice: Amino Acids in Solution


What are the predominant structures in solution of L-glutamic acid (shown below) at pH = 2, 7, and 12?


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Mark Yourself Question
  1. Grab a piece of paper and try this problem yourself.
  2. When you're done, check the "I have answered this question" box below.
  3. View the solution and report whether you got it right or wrong.

Practice: Amino Acids pKa


The pKa of a phenol functionality (Ph-OH) is approximately 10. Draw the predominant structures of L-Tyrosine (shown below) at pH = 2, 7 and 12.


Summary Sheet: Amino Acids in Solution of Various pH



Amino acids only have a few different functional groups (carboxylic acids, alcohols, thiols, thioethers, amines, amides, (and aromatic imidazole for histidine)). Remember, if the pH is below than the pKa, the functional group is protonated, and if the pH is above the pKa, the functional group is deprotonated. Alcohols, amides, and amines will never be deprotonated in these aqueous solutions (except for alcohols with aromatic stabilization (see below)). So you just need to concern yourself with the groups shown below:


Summary Sheet: pKa's of Common Molecules

Below are the pKa values of some of the most important functional groups in organic chemistry. Memorizing an approximate value is more than adequate to predict the appropriate bases to deprotonate any of these acids.