Use this powerful pKa Calculator based on the **Henderson-Hasselbalch equation** to determine the pH, pKa, or the concentration ratio of the conjugate base ($[A^-]$) to the weak acid ($[HA]$) in a buffer solution.
pKa Calculator
Enter any three values to solve for the missing one. All concentrations must be greater than zero.
pKa Calculator Formula: The Henderson-Hasselbalch Equation
The core of this calculator is the Henderson-Hasselbalch (H-H) equation, which relates the pH of a solution, the pKa of the acid, and the ratio of the concentrations of the conjugate base and the weak acid.
pH = pKa + log ( [A-] / [HA] )
Formula Source: LibreTexts Chemistry – H-H Equation, NCBI Bookshelf – Acid-Base Physiology
Variables Explained
Understanding the components is essential for accurate calculations:
- pH: The measure of hydrogen ion concentration, indicating the acidity or basicity of the solution.
- pKa: The negative logarithm of the acid dissociation constant ($K_a$). It quantifies the strength of an acid; a lower pKa indicates a stronger acid.
- Conjugate Base Concentration [A-]: The molar concentration (M) of the base form that remains after the weak acid has donated a proton.
- Weak Acid Concentration [HA]: The molar concentration (M) of the undissociated weak acid in the solution.
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What is pKa Calculator?
The pKa calculator is a tool designed primarily for chemists, biochemists, and medical professionals to quickly solve problems related to acid-base equilibrium and buffer preparation. It is based on the Henderson-Hasselbalch equation, which is fundamental to understanding how buffer solutions resist changes in pH.
Using this calculator, you can determine any of the four variables ($pH, pK_a, [A^-], [HA]$) given the other three. This is especially useful in laboratories when designing experiments that require specific buffer conditions, such as cell culture media or protein purification processes where maintaining a stable pH is critical for molecular integrity and function.
The pKa value itself is a characteristic property of a molecule that dictates the pH at which it is 50% protonated and 50% deprotonated. This knowledge is crucial for predicting the charge state of an amino acid or a drug molecule at a given physiological pH.
How to Calculate pKa (Example)
Consider a buffer made from acetic acid ($HA$) and sodium acetate ($A^-$). We want to find the pKa if the solution pH is 5.0, the concentration of acetate is 0.15 M, and the concentration of acetic acid is 0.10 M.
- Identify known variables: $pH = 5.0$, $[A^-] = 0.15$ M, $[HA] = 0.10$ M.
- Determine the ratio: Calculate the concentration ratio, $[A^-]/[HA] = 0.15 / 0.10 = 1.5$.
- Calculate the log of the ratio: $\log(1.5) \approx 0.176$.
- Apply the H-H equation: $pH = pK_a + \log ([A^-]/[HA])$, which rearranges to $pK_a = pH – \log ([A^-]/[HA])$.
- Solve for pKa: $pK_a = 5.0 – 0.176$.
- Final Result: $pK_a = 4.824$.
Frequently Asked Questions (FAQ)
Is the pKa value temperature dependent?
Yes, the pKa value is indeed temperature-dependent. Since $K_a$ (the acid dissociation constant) is related to the equilibrium of the reaction, and equilibrium constants change with temperature (according to the van’t Hoff equation), the pKa will also shift. Most reported pKa values are standardized to $25^\circ C$.
What does it mean if the pH equals the pKa?
If the $pH = pK_a$, then the ratio $\log ([A^-]/[HA])$ must be equal to zero. This means that $[A^-]/[HA] = 1$, or $[A^-] = [HA]$. In simpler terms, the concentration of the conjugate base is equal to the concentration of the weak acid. This point represents the maximum buffering capacity of the solution.
What are the limitations of the Henderson-Hasselbalch equation?
The H-H equation relies on several approximations. It assumes that the concentrations used are equal to the activities (which is only true in very dilute solutions), and it ignores the dissociation of water. It is highly accurate for well-behaved buffers far from extremes of pH but less accurate for very concentrated or very dilute solutions.
How many variables must I enter to get a result?
You must enter exactly three out of the four variables (pH, pKa, $[A^-]$, $[HA]$). The calculator is designed to solve for the single missing variable. If you enter all four, it will check for mathematical consistency.