pKa and pH

Main Difference – pKa vs pH

The difference is a logarithmic scale. Taking the logarithmic value of very big or minimal numbers is the most straightforward approach to explaining and comparing them. The logarithm produces a specific value that is easy to manipulate. pKa and pH are two concepts in physical chemistry that refer to a system’s acidity. The fundamental distinction between pKa and pH is that pKa denotes an acid’s dissociation, whereas pH denotes a system’s acidity or alkalinity.

Table of Contents

Difference Between pKa and pH

The symbol Ka denotes the acid dissociation constant. pKa is represented by the negative logarithm of Ka.

Some acids are robust, while others are weak. In aqueous solutions, strong acids break down into their ions. On the other hand, weak acids partially dissociate, resulting in a balance between the acid and its conjugate base. For example, the dissociation of a weak acid termed HA can be written as follows.

\(\begin{array}{l}HA \rightleftharpoons A^{-}+H^{+}\end{array} \)

Further Reading:

What is pKa?

The logarithm of the inverse of H+ concentration is pH. The pH value of a system shows its acidity or alkalinity. When a system’s pH value is high, it’s called analkaline or basic system. When a system’s pH value is low, it’s considered acidic. pH levels range from 1 to 14. The pH value of 7 is known as the neutral pH, indicating no acidity or alkalinity present.

pH = -log10 [H+]

pKa Versus pH
The negative logarithmic of Ka is denoted by pKa. The logarithm of the inverse of H+ concentration is pH.
Indication of Acidity
The pKa value determines whether an acid is strong or weak. The pH of a system determines whether it is acidic or alkaline.
Details Given
The dissociation of acid in an aqueous solution is indicated by pKa. The concentration of H+ ions in a system is determined by pH.
If an acid’s pKa is high, it is a weak acid, and if it is low, it is a strong acid. When a system’s pH is high, it is alkaline; when it is low, it is acidic.
Effect of Other Parameters
The concentration of acid, conjugate base, and H+ affects the pKa. The concentration of H+ determines the pH.

Further Reading:

What is pH?

pH and pKa Relationship: The Henderson-Hasselbalch Equation

The pH value at which a chemical species will take or donate a proton is known as the pKa.

The Henderson-Hasselbalch equation is used to calculate the relationship between pKa and pH. It is, however, simply an approximation and should not be used for concentrated solutions of acids or bases with excessively low or high pH.

We can solve for the other value using an approximation known as the Henderson-Hasselbalch equation if you know either pH or pKa:

pH = pKa + log ([conjugate base]/[weak acid])

pH = pKa+log ([A]/[HA])

pH is equal to the sum of the pKa value and the log of the conjugate base concentration divided by the weak acid concentration.

Halfway through the equivalence point:

pH = pKa

It is worth mentioning that this equation is occasionally written for the Ka value instead of the pKa value, so you should be familiar with the relationship:

pKa = -logKa

The Henderson-Hasselbalch Equation’s Assumptions

Because it excludes water chemistry from the equation, the Henderson-Hasselbalch equation is an approximation. This works when the solvent is water and is present in a large proportion to the [H+] and acid/conjugate base. For concentrated solutions, the approximation should not be used.

How to Determine pH From pKa

Each acid has its unique ionisation constant,

Let us consider a weak acid HA

HA+H2O ⇆ A + H3O+

\(\begin{array}{l}K_{a} = [A^{-}][H_{3}O^{+}]/[HA]\end{array} \)

The equilibrium concentrations of ionised acid, protons, and unionised (i.e., “intact”) acid are represented by [A], [H3O+], and [HA], respectively. The higher the numerator in regard to the denominator in this equation, the higher the Ka; the more highly dissociated the acid at equilibrium, the higher the numerator in relation to the denominator in this equation, and thus the higher the Ka.

Given the pKa of the acid and the concentrations above (excluding the donated protons), you may compute the pH of a solution. Calculating pKa from Ka entails the same steps as calculating pH: Take Ka’s negative logarithm, and there you have it.

Although there is a derivation involved, the Henderson-Hasselbalch equation links these quantities as follows:

\(\begin{array}{l}pH = pKa + log_{10}[A^{-}]/[HA]\end{array} \)

Frequently Asked Questions on pKa and pH


How are pKA and pH related?

The Henderson-Hasselbalch equation is used to calculate the relationship between pKa and pH
pH is equal to the sum of the pKa value and the log of the conjugate base concentration divided by the weak acid concentration.


What happens if the pH falls below pKa?

The group is in the conjugate acid form if the pH of the solution is less than the pKa (protonated).


What happens if the pKa value is higher than the pH value?

We can predict whether a molecule will be protonated or deprotonated based on the connection between its pKa and the pH of its solution. The molecule will be deprotonated if the pH is higher than the pKa.


Is pKa the same as pH?

It is important to remember that when the pH equals the pKa value, the proportions of conjugate base and conjugate acid are identical.


What is the significance of pKa?

Many fundamental chemistry reactions require an understanding of acid dissociation constants or pKa values. These numbers reflect a molecule’s deprotonation state in a specific solvent. The use of theoretical approaches to derive pKa values for a variety of compounds has sparked a lot of interest.


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