Chemistry Calculators

pH Calculator – Instantly Calculate Acidity, Alkalinity & Hydrogen Ion Concentration

3 days ago
2 mins
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Advanced pH Calculator

Strong Acid pH

Strong Base pH

Weak Acid pH

Weak Base pH

Buffer Solution pH (Henderson-Hasselbalch)

pH from [H⁺]

pH from [OH⁻]

Calculation Results:

pH is a scale used to specify the acidity or basicity of an aqueous solution. It is defined as the negative base-10 logarithm of the hydrogen ion activity (aH⁺). For dilute solutions, hydrogen ion activity is approximately equal to its concentration [H⁺].

pH = -log₁₀[H⁺]

  • pH < 7: Acidic solution
  • pH = 7: Neutral solution (at 25°C)
  • pH > 7: Basic (alkaline) solution

Similarly, pOH is defined as the negative base-10 logarithm of the hydroxide ion concentration [OH⁻].

pOH = -log₁₀[OH⁻]

The relationship between pH and pOH is derived from the autoionization of water (Kw = [H⁺][OH⁻] = 1.0 x 10⁻¹⁴ at 25°C):

pH + pOH = 14 (at 25°C)

General:

  • pH = -log₁₀[H⁺]
  • pOH = -log₁₀[OH⁻]
  • [H⁺] = 10-pH
  • [OH⁻] = 10-pOH
  • pH + pOH = pKw (pKw = 14.00 at 25°C)
  • Kw = [H⁺][OH⁻] = 1.0 x 10⁻¹⁴ (at 25°C)

Strong Acids (SA): (e.g., HCl, H₂SO₄ - first proton)

  • [H⁺] = Initial concentration of SA

Strong Bases (SB): (e.g., NaOH, Ba(OH)₂)

  • [OH⁻] = Initial concentration of SB * (number of OH⁻ per formula unit)

Weak Acids (WA): HA ⇌ H⁺ + A⁻

  • Ka = [H⁺][A⁻] / [HA]
  • pKa = -log₁₀(Ka)
  • If CHA/Ka > 400 (approx.), then [H⁺] ≈ √(Ka * CHA)
  • Otherwise, solve quadratic: [H⁺]² + Ka[H⁺] - KaCHA = 0
  • Percent Ionization = ([H⁺]eq / CHA) * 100%

Weak Bases (WB): B + H₂O ⇌ BH⁺ + OH⁻

  • Kb = [BH⁺][OH⁻] / [B]
  • pKb = -log₁₀(Kb)
  • If CB/Kb > 400 (approx.), then [OH⁻] ≈ √(Kb * CB)
  • Otherwise, solve quadratic: [OH⁻]² + Kb[OH⁻] - KbCB = 0
  • Ka * Kb = Kw (for a conjugate acid-base pair)
  • Percent Ionization = ([OH⁻]eq / CB) * 100%

Buffer Solutions (Henderson-Hasselbalch Equation):

  • For acidic buffer (HA/A⁻): pH = pKa + log₁₀([A⁻] / [HA])
  • For basic buffer (B/BH⁺): pOH = pKb + log₁₀([BH⁺] / [B]), then pH = 14 - pOH

1. Select Calculation Mode:

  • Choose the type of substance or calculation you want to perform from the radio buttons at the top (e.g., Strong Acid, Weak Base, Buffer, etc.).

2. Enter Input Values:

  • Based on the selected mode, specific input fields will appear.
  • Concentrations: Enter concentrations in Molarity (M). You can use the dropdown to specify if your input is in mM (millimolar) or µM (micromolar), and it will be converted to M for calculation.
    • Example: For 0.05 M, enter 0.05 and select M. For 50 mM, enter 50 and select mM.
  • Ka/Kb or pKa/pKb: For weak acids/bases, enter the dissociation constant. You can input it as Ka/Kb (e.g., 1.8e-5) or as pKa/pKb (e.g., 4.75) and select the correct type from the dropdown.
  • Buffer Inputs:
    • Select "Acidic Buffer" or "Basic Buffer".
    • For acidic buffers, provide concentrations of the weak acid (HA) and its conjugate base (A⁻), along with the pKa of the weak acid.
    • For basic buffers, provide concentrations of the weak base (B) and its conjugate acid (BH⁺), along with the pKb of the weak base.
  • [H⁺] or [OH⁻]: If calculating directly from ion concentrations, enter the value and select its unit (M, mM, µM, nM).

3. Calculate:

  • Click the "Calculate pH" button.

4. View Results:

  • The calculated pH will be prominently displayed.
  • Depending on the calculation, pOH, [H⁺], [OH⁻], and/or percent ionization may also be shown.
  • A detailed step-by-step solution will illustrate how the result was obtained.
  • If there are any errors in your input (e.g., non-numeric values, invalid Ka), an error message will appear.

5. Reset:

  • Click the "Reset" button to clear all input fields and results.
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