What is pH?
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 referred to as alkaline 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 that there is no acidity or alkalinity present.
What is pKa?
The pH value at which a chemical species will take or donate a proton is known as the pKa. The negative base-10 logarithm of a solution’s acid dissociation constant (Ka) is pKa.
Table of Contents
- pKa Definition
- Difference Between pKa and pH
- Calculating pH from pKa
- Frequently Asked Questions – FAQs
pKa Definition
The negative logarithm of Ka is denoted by pKa. The acid dissociation constant is denoted by the symbol Ka. Some acids are powerful, while others are weak. In aqueous solutions, strong acids totally break down into their ions. Weak acids, on the other hand, partially dissociate, resulting in a balance between the acid and its conjugate base.
Let us consider a weak acid HA
HA ⇆ A– + H+
The acid dissociation constant of this equilibrium is,
Ka = [A–][H+] / [HA]
Where
[A–] = Concentration of the conjugate base [H+] = Concentration of H+ ions [HA] = Concentration of Acid presentThen the pKa value of the acid dissociation constant can be indicated as below.
pKa = – log10[Ka]
We can determine whether an acid is a strong acid or a weak acid by looking at its pKa value. The acid is weak if the pKa value is high. Because a greater pKa number suggests that Ka is low, this is the case. The value of [A–][H+] should be lower than the value of [HA] in order for Ka to be low. This indicates that the acid has been partially dissociated. However, if the value of [A–][H+] is more than [HA], the Ka will be large and the pKa will be low. This shows that the acid is potent.
Difference Between pKa and pH
| Based on | pKa | pH |
|---|---|---|
| Definition | pKa is the negative value of the logarithm of Ka. | pH is the logarithmic value of the inverse of H+ concentration. |
| Indication of Acidity | pKa indicates whether an acid is a strong acid or a weak acid. | pH indicates whether a system is acidic or alkaline. |
| Values | If the pKa of an acid is high, it is a weak acid, and if the pKa of an acid is low, it is a strong acid. | If the pH of a system is high, the system is alkaline, but if the pH is low, that system is acidic. |
| Effect of Other Parameters | pKa is dependent on the concentration of acid, conjugate base and H+. | pH depends on the H+ concentration. |
| Henderson-Hasselbalch equation | Both pH and pKa are related by the Henderson-Hasselbalch equation.
pH = pKa + log ([A–] / [HA]) |
|
Calculating pH from pKa – Henderson-Hasselbalch Equation
Given the pKa of the acid and the concentrations above (excluding the donated protons), we may compute the pH of a solution. Calculating pKa from Ka entails the same steps as calculating pH: Consider Ka’s negative logarithm.
We can solve for the other value using an approximation known as the Henderson-Hasselbalch equation if you know either pH or pKa. The Henderson-Hasselbalch equation relates pKa and pH. However, it is only an approximation and should not be used for concentrated solutions or for extremely low pH acids or high pH bases.
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.
Half through the equivalence point:
pH = pKa
It’s worth mentioning that this equation is occasionally written for the Ka value instead of the pKa value, so it is familiar with the relationship.
pKa = – logKa
The concentration of the conjugate base and the conjugate acid are identical when the pH is equal to the pKa of an acid, implying that there is a 50 percent proportion of conjugate base and a 50 percent proportion of conjugate acid. By simply inserting the amounts of conjugate base and conjugate acid into the Henderson-Hasselbach equation, their ratio is 1:1, implying that the log of this ratio is zero, regardless of the concentrations.
Frequently Asked Questions on pKa to pH
Does pKa change with pH?
pKa is an equilibrium constant. pH is an indication of hydrogen ion content in a solution. Any changes to pH will therefore affect one of the factors in the pKa equation. The amount and direction of change in the pKa value will depend on whether the H+ ions are part of the reactant or product side of the equation.
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.
How does pKa relate to pH?
The pKa is the pH value at which a chemical species will accept or donate a proton. The lower the pKa, the stronger the acid and the greater the ability to donate a proton in an aqueous solution. The Henderson-Hasselbalch equation relates pKa and pH.
What is pKa called?
pKa is the negative log of the acid dissociation constant or Ka value. A lower pKa value indicates a stronger acid. That is, the lower value indicates the acid more fully dissociates in water.
What do pKa and ka mean?
Ka is the acid dissociation constant. pKa is simply the -log of this constant. Similarly, Kb is the base dissociation constant, while pKb is the -log of the constant. The acid and base dissociation constants are usually expressed in terms of moles per litre (mol/L).
Comments