Difference between pH and pKa
What Is pH
The term “pH” indicates how basic or acidic a given solution is; therefore, it indicates the amount of H+ ions found within the respective solution(s).
The following examples show how to classify various pH values and the amount of acidity in each scenario:
pH Value = Acidic (0-6)
pH Value = Neutral (7)
pH Value = Basic (8-14).
Note that on a logarithmic scaling of acidity, even a very small change to pH entails a very large change in an acidity level.
What Is pKa (Difference between pH and pKa)
pKa has to do with the acidity of a particular acid substance; it is not about the acidity of a solution. In simple terms, pKa expresses the strength of a given acid.
There are three basic methods to quantify the acidic strength of a given acid using pKa.
Low pKa = Strong Acid.
High pKa = Weak Acid.
pKa technically means: 50% of the acid is dissociated, while the other 50% remains undissociated.
The Relationship Between pH and pKa: Difference between pH and pKa
Knowledge of how the two concepts (pH and pKa) relate allows an understanding of whether an acid exists in primarily its ionized or unionized state.
If pH < pKa, the majority of the acid exists in its unionized form;
If pH = pKa, then 50% of the acid is ionized and 50% is unionized;
If pH > pKa then the majority of the acid exists in its ionized form.
This simple rule is a very powerful predictive tool for how potential reactions produced by the various acids will behave.
A Little Logic to Participate
Think of the concept of pKa like the “gate” of a tipping point:
In a solution with a pKa of < 5 and a pH of > 5, the respective acid wants to remain in solution, because it will not easily lose a proton; conversely, if the respective acid has a pKa of < 5, then it wants to lose a proton at that point.
Why the pH and pKa Relationship Is So Important
The pH level of weak acids affects their ionization state and, subsequently, their absorption and other properties, such as creating riboflavin when they dissolve in water. Conversely, weak bases have a similar but opposite relationship with pH and their ionization state.
Drug absorption is influenced by both unionized and ionized forms of drugs, so pharmaceutical formulation needs to take into consideration what the best pH will be according to the pKa value. Additionally, analytical chemistry and high-performance liquid chromatography (HPLC) are similarly affected by how pH compares to pKa with regards to retention time, peak shape, and resolution. By properly adjusting the mobile phase pH based on pKa, one can significantly increase the robustness of their method.
A Basic Comparison Chart of pH and pKa (Difference between pH and pKa)
Condition Outcome
pH < pKa Generally not ionized
pH = pKa 50% ionized
pH > pKa Mostly ionized
Low value of pKa Indicates strong acid
High value of pKa Indicates weak acid
Misunderstandings on pH vs pKa
pKa varies with solution pH – Answer: It doesn’t
pKa is a constant for a single compound – Answer: Yes
pH measures strength of acid – Answer: No
pH measures acidity of solution, and pKa measures strength of acid – Answer: Yes
All acids act the same way – Answer: No
Each acid has a specific pKa – Answer: Yes
Will be mainly not ionized (hypothetically) would have good transmembrane permeability, but poor solubility.
When formulated at pH 7, the drug becomes ionized, thus having poor transmembrane permeability, but increased solubility.
The Importance of Understanding pH and pKa
To summarize the importance of pH and pKa, their relationship is critical to acid base chemistry. This relationship explains ionization, solubility, stability, absorption, and reactivity in a single concept.