A Guide to Acid-Base Equilibrium Constants

In chemistry, several related scales and calculations are used to measure the acidity or basicity of a solution, as well as to understand the strength of acids and bases. While the pH scale is the most widely recognized and frequently used, there are other important measurements such as pKa, Ka, pKb, and Kb. Each of these offers insight into the behaviors and properties of acids and bases, and they are crucial in helping to understand acid-base reactions.

The pH scale is the most familiar to many because it directly measures the hydrogen ion concentration in a solution, indicating whether the solution is acidic, neutral, or basic. A pH value below seven signifies an acidic solution, a pH of seven indicates a solution is neutral, and a pH above seven indicates a basic solution.

Here's an explanation of the different terms and how they differ.

What Does the 'P' Mean?

Whenever you see a "p" in front of a value, like pH, pKa, and pKb, it means you're dealing with a negative log of the value following the "p". For example, pKa is the negative log of Ka. Because of the way the log function works, a smaller pKa means a larger Ka. Further, pH is the negative log of hydrogen ion concentration, and so on.

Formulas and Definitions for pH and Equilibrium Constant

pH and pOH are related, just as Ka, pKa, Kb, and pKb are. If you know pH, you can calculate pOH. If you know an equilibrium constant, you can calculate the others.

About pH

pH is a measure of hydrogen ion concentration, [H+], in an aqueous (water) solution. The pH scale ranges from 0 to 14. A low pH value indicates acidity, a pH of 7 is neutral, and a high pH value indicates alkalinity. The pH value can tell you whether you're dealing with an acid or a base, but it offers limited value indicating the true strength of the acid. The formulas to calculate pH and pOH are:

pH = - log [H+]

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Understanding Acid Strength through pKa

By Anne Marie Helmenstine, Ph.D.

pOH = - log [OH-]

At 25 degrees Celsius:

pH + pOH = 14

Understanding Ka and pKa

Ka, pKa, Kb, and pKb are most helpful when predicting whether a species will donate or accept protons at a specific pH value. They describe the degree of ionization of an acid or base and are true indicators of acid or base strength because adding water to a solution will not change the equilibrium constant. Ka and pKa relate to acids, while Kb and pKb deal with bases. Like pH and pOH, these values also account for hydrogen ion or proton concentration (for Ka and pKa) or hydroxide ion concentration (for Kb and pKb).

Ka and Kb are related to each other through the ion constant for water, Kw:

• Kw = Ka x Kb

Ka is the acid dissociation constant. pKa is simply the negative log of this constant. Similarly, Kb is the base dissociation constant, while pKb is the negative log of the constant. The acid and base dissociation constants are usually expressed in terms of moles per liter (mol/L). Acids and bases dissociate according to general equations:

• HA + H₂O ⇆ A^- + H₃O⁺
• HB + H₂O ⇆ B⁺ + OH^-

In the formulas, A stands for acid, and B is for base.

• Ka = [H+][A-]/ [HA]
• pKa = - log Ka
• At half the equivalence point, pH = pKa = -log Ka

A large Ka value indicates a strong acid because it means the acid is largely dissociated into its ions. A large Ka value also means the formation of products in the reaction is favored. A small Ka value means little of the acid dissociates, so you have a weak acid. The Ka value for most weak acids ranges from 10^-2 to 10^-14.

The pKa gives the same information, just in a different way. The smaller the value of pKa, the stronger the acid. Weak acids have a pKa ranging from 2–14.

Understanding Kb and pKb

Kb is the base dissociation constant. The base dissociation constant is a measure of how completely a base dissociates into its component ions in water.

• Kb = [B+][OH-]/[BOH]
• pKb = -log Kb

A large Kb value indicates the high level of dissociation of a strong base. A lower pKb value indicates a stronger base.

pKa and pKb are related by the simple relation:

• pKa + pKb = 14

What Is pI?

Another important point is pI. This is the isoelectric point. It is the pH at which a protein (or another molecule) is electrically neutral, or has no net electrical charge.