Acid-Base Titration pH Curve (Strong Acid – Strong Base) – Tutorial

Strong Acid – Strong Base Titration (pH–Volume Curve) — Lesson Note

Titration is the process of determining the concentration of an unknown acid or base by neutralizing it in a controlled manner with a standard solution of known concentration. This calculator covers the strong acid–strong base system: e.g. HCl – NaOH.

"Strong" means they dissociate almost completely in water:

• HCl → H⁺ + Cl⁻ (complete dissociation)
• NaOH → Na⁺ + OH⁻ (complete dissociation)

Because of this, there are no additional complications like equilibrium constants (Ka, Kb). The pH calculation is entirely based on the excess H⁺ or OH⁻ moles remaining.

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Core Concept: "Mole Comparison"

The titration curve is essentially the graph of this question: "As we add base, is there more excess H⁺ or OH⁻ in the solution?"

The fundamental reaction:

H⁺ + OH⁻ → H₂O

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Step 1 — Mole Calculation (n = M·V)

Everything starts with the mole calculation:

n = M · V

• n: moles (mol)
• M: concentration (mol/L)
• V: volume (L)

For the acid

n_acid = C_a · V_a

For the base (depends on added volume)

n_base = C_b · V_b

Note: If you use mL in the calculation, convert to liters:

V(L) = V(mL) / 1000

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Step 2 — Equivalence Point and Equivalence Volume

The equivalence point is the moment when the moles of added base equal the moles of acid at the start:

n_acid = n_base

From this, the equivalence volume is derived:

C_a·V_a = C_b·V_eq
V_eq = (C_a·V_a) / C_b

The "Equivalence volume (mL)" shown in this tool is exactly this value.

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Step 3 — Total Volume (Dilution Effect)

In titration, pH is determined not only by the "remaining moles" but also by the total volume:

V_total = V_a + V_b

Because we need concentration to calculate pH:

[H⁺] = (remaining H⁺ moles) / V_total

[OH⁻] = (remaining OH⁻ moles) / V_total

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4 Regions of the Titration Curve (How does pH change?)

A strong acid–strong base titration is typically read in 4 main regions. As the volume (V_b) increases, pH is determined by whichever ion is in excess.

1) Initial (Vb = 0)

Only acid is present in the solution.

[H⁺] = C_a

pH = −log[H⁺]

2) Before Equivalence (Vb < Veq) — Excess Acid

The added base consumes some of the acid, but acid is still in excess.

remaining H⁺ moles = n_acid − n_base

[H⁺] = (n_acid − n_base) / V_total

pH = −log[H⁺]

3) Equivalence Point (Vb = Veq)

When n_acid = n_base, both H⁺ and OH⁻ are completely consumed. In a strong acid–strong base system, no species capable of hydrolysis remains (the salt is considered neutral), so at 25°C:

pH = 7

4) After Equivalence (Vb > Veq) — Excess Base

The base has consumed all the acid, and the solution now carries excess OH⁻.

remaining OH⁻ moles = n_base − n_acid

[OH⁻] = (n_base − n_acid) / V_total

pOH = −log[OH⁻]

pH = 14 − pOH

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Where Does pH + pOH = 14 Come From?

The ion product of water at 25°C:

K_w = [H⁺][OH⁻] = 1.0×10⁻¹⁴

Taking the log:

pH + pOH = 14

This relationship is the cornerstone of pH calculations in strong acid–strong base titrations.

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How to Read the Graph (pH–Volume Curve)?

• pH starts low (acid dominant).
• pH rises as we approach the equivalence point.
• pH jumps very rapidly near the equivalence point (the steepest part of the curve).
• After equivalence, pH continues to rise slowly in the basic region.

This "sudden jump" is very pronounced in strong–strong systems and makes it easy to visually identify the endpoint of the titration.

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What Does This Calculator Do?

The tool starts from 0 mL and adds base in increments of the volume step you set. At each step it:

• Calculates n_base
• Determines which species is in excess (H⁺ or OH⁻)
• Finds the concentration using total volume
• Calculates pH and adds it to the table/chart

This produces a complete pH–volume curve and clearly shows the equivalence volume.

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Common Mistakes (Important!)

• Forgetting to convert mL to liters (the most common mistake)
• Incorrectly applying the pH = 7 equivalence rule to weak acid–base systems
• Forgetting the total volume (dilution effect)
• Choosing too large a step and missing the jump region of the curve

Note: This lesson content is prepared for strong acid–strong base (fully dissociating) systems. For weak acid/base titrations, buffer regions, Ka/Kb, and hydrolysis effects come into play.