pKa Calculator

Category: Chemistry
- March 29, 2025
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Henderson-Hasselbalch Equation:

pH = pKa + log([A-]/[HA])
Ratio [A-]/[HA]
398.11
% Ionized
99.75%
% Unionized
0.25%

Buffer Preparation

Acid Form
0.03 g
CH₃COOH
Base Form
6.47 g
CH₃COONa

1. Dissolve the indicated amounts in approximately 80% of the final volume.

2. Adjust pH using a pH meter if necessary.

3. Add water to reach the final volume.

Buffer Capacity

Effective pH Range: 3.8 - 5.8

Buffer Capacity: 0.058 mol/L/pH

Enter Multiple pKa Values

to

Species Distribution vs. pH

Predominant Species at pH 7.0

Titration Curve

Equivalence Point
50.0 mL
pH at Equivalence
7.0
Buffer Region
25-75 mL

Recommended Indicators

About pKa and Acid-Base Chemistry

Basic Concepts

The pKa value is the negative logarithm of the acid dissociation constant (Ka) and represents the pH at which exactly half of an acid is dissociated.

pKa = -log₁₀(Ka)

For acids, a lower pKa indicates a stronger acid, while for bases, a higher pKa of the conjugate acid indicates a stronger base.

Key Points:

  • At pH = pKa, an acid is 50% ionized
  • At pH = pKa + 1, an acid is approximately 91% ionized
  • At pH = pKa - 1, an acid is approximately 9% ionized
  • Buffer solutions work best when pH ≈ pKa (±1 pH unit)

Common pKa Values

Compound pKa Useful pH Range
Phosphoric acid (1st) 2.15 1.15 - 3.15
Citric acid (1st) 3.13 2.13 - 4.13
Acetic acid 4.76 3.76 - 5.76
MES 6.15 5.15 - 7.15
Phosphoric acid (2nd) 7.20 6.20 - 8.20
TRIS 8.06 7.06 - 9.06
Carbonic acid (2nd) 10.33 9.33 - 11.33
Phosphoric acid (3rd) 12.32 11.32 - 13.32

What Is the pKa Calculator?

The pKa Calculator is an interactive tool that helps you explore and understand acid-base behavior, buffer solutions, and titration processes in chemistry. Whether you're preparing a buffer, analyzing ionization at various pH levels, or studying titration curves, this tool makes the process faster and clearer.

It’s especially useful for students, lab technicians, and professionals working with chemical solutions, offering support similar to tools like a molarity calculator, titration calculator, or a molecular weight tool.

Key Features of the Calculator

  • Henderson-Hasselbalch Calculations: Understand the ionization state of acids or bases at a given pH.
  • Buffer Preparation: Calculate how much acid and base to mix to make a buffer solution.
  • Multiple pKa Analysis: Visualize species distribution for molecules with more than one pKa value.
  • Titration Curves: Generate pH vs volume plots for different acid-base titration types.
  • Built-in Reference: Access common pKa values and useful pH ranges for popular buffer systems.

Core Formula

Henderson-Hasselbalch Equation:

pH = pKa + log([A⁻]/[HA])

How to Use the pKa Calculator

1. Henderson-Hasselbalch Tab

  • Enter the known pKa and pH values.
  • Select whether you're analyzing an acid or base system.
  • Click “Calculate” to see the ratio of ionized to unionized species, along with percent ionization.

2. Buffer Preparation Tab

  • Select a common buffer or input a custom pKa value.
  • Enter target pH, concentration (mM), and final volume (mL).
  • The tool will show how much of each chemical form is needed, including useful preparation tips.

3. Multiple pKa Analysis Tab

  • Input up to three pKa values and a pH range.
  • Click "Calculate" to view species distribution curves.
  • You can also check which species dominates at any chosen pH.

4. Titration Curve Tab

  • Select titration type (e.g., weak acid + strong base).
  • Provide concentrations and volumes of the analyte and titrant.
  • The calculator plots the titration curve, highlights key points, and recommends indicators.

Why This Calculator Is Useful

Understanding the behavior of acids and bases is fundamental in chemistry. This calculator brings concepts to life through real-time results and visualizations, helping you:

  • Predict ionization levels in biological and chemical systems.
  • Plan accurate buffer solutions using solution concentration data.
  • Visualize titration behavior with a titration formula tool.
  • Quickly switch between common buffers without manual math.
  • Work confidently with molar concentration tools, mole calculation tools, and acid-base titration guides.

Frequently Asked Questions (FAQ)

What does pKa mean?

pKa is the pH at which a chemical species is 50% ionized. It helps predict the state of acids or bases in a given solution.

Can I use this for weak bases too?

Yes. The calculator allows you to analyze both acids and bases, adapting the formula accordingly.

How accurate are the buffer preparation results?

The results are based on the Henderson-Hasselbalch equation and assume ideal conditions. For best results, use a pH meter to confirm and adjust as needed.

Is this the same as a Molarity Calculator?

No, but it's related. While a molarity calculator focuses on determining solution concentration, the pKa Calculator applies that information in the context of buffers and ionization.

What other tools might I need?

Depending on your needs, you may also benefit from using tools such as:

Final Thoughts

The pKa Calculator brings clarity and speed to acid-base chemistry. Whether you’re adjusting buffer concentrations or plotting a titration curve, it gives you reliable, understandable insights in just a few clicks.

Use it alongside other chemistry tools like a molarity calculator or solution dilution tool to complete your analysis with confidence.