- An acid-base titration curve plots pH against titrant volume.
- The half-equivalence point reveals the acid's pKa value.
- Strong and weak acid curves have distinctly different shapes.
An acid-base titration curve is a graph that plots pH against the volume of titrant added during a neutralization reaction. The curve reveals the equivalence point, buffer regions, and acid strength of the analyte, making it one of the most informative tools in analytical chemistry.
Why It Matters
Key figure
1791
Year Descroizilles invented the first burette
Titration curves do more than locate a single endpoint. They encode the identity and concentration of unknown acids and bases in a shape that trained chemists read the way musicians read sheet music.
The technique traces back to 1791, when French chemist François-Antoine-Henri Descroizilles built the first burette to measure bleach strength. Joseph-Louis Gay-Lussac, the physicist and chemist at the École Polytechnique, later refined the glassware and coined the term "titrate" in his publications. By the mid-19th century, titration had become a standard method in European analytical laboratories.
Today, acid-base titration curves appear in pharmaceutical quality control, environmental water testing, and clinical blood-gas analysis. A single curve can confirm whether a drug formulation meets its pH specification or whether a wastewater discharge falls within regulatory limits.
How the Curve Takes Shape
In a typical acid-base titration, a solution of known concentration (the titrant) is added drop by drop to an unknown solution (the analyte). A pH meter or indicator tracks the response.
The curve starts flat. Each drop of titrant neutralizes some analyte, but the pH shifts only slightly. This plateau is the buffer region, where the solution resists pH change because both the weak acid and its conjugate base are present in comparable amounts.
Key figure
pH = pKa
At the half-equivalence point, pH equals the acid dissociation constant
At the midpoint of the buffer region sits the half-equivalence point. Here, exactly half the analyte has been neutralized, and the concentrations of acid and conjugate base are equal. The pH at this point equals the pKa of the acid, a relationship derived from the Henderson-Hasselbalch equation.
Near the equivalence point, the curve steepens sharply. A single drop of titrant can swing the pH by several units. For a strong acid titrated with a strong base (hydrochloric acid with sodium hydroxide, for example), the equivalence point falls at pH 7. For a weak acid titrated with a strong base (acetic acid with sodium hydroxide), the equivalence point sits above pH 7 because the conjugate base of the weak acid is itself mildly basic.
After the equivalence point, excess titrant dominates and the curve flattens again, now at a high pH for base titrants or low pH for acid titrants.
Key Context
Strong vs. weak curves tell different stories. A strong acid-strong base titration produces a nearly symmetrical S-curve with a vertical jump at pH 7. A weak acid-strong base titration produces an asymmetric curve: the initial pH is higher, the buffer region is more pronounced, and the equivalence point shifts above 7. Polyprotic acids like phosphoric acid produce multiple inflection points, one for each ionizable proton.
Indicators vs. instruments. Classical titrations used color-change indicators such as phenolphthalein (which turns pink above pH 8.2) to estimate the endpoint visually. Modern laboratories typically use automated pH meters that record hundreds of data points per titration, generating smooth digital curves with precise equivalence-point detection.
FAQ
What is the difference between the equivalence point and the endpoint?
The equivalence point is the theoretical moment when moles of acid exactly equal moles of base. The endpoint is the experimentally observed moment, usually marked by a color change in an indicator like phenolphthalein. The two are close but rarely identical, because indicators change color over a pH range rather than at a single value.
Why does a weak acid titration curve look different from a strong acid curve?
A weak acid starts at a higher pH because it only partially dissociates in water. Its curve also shows a pronounced buffer region before the equivalence point, where the weak acid and its conjugate base resist pH change. The equivalence point sits above pH 7 because the conjugate base produced is itself mildly basic.
How can you determine pKa from a titration curve?
Find the half-equivalence point, which is the volume of titrant that is exactly half the volume needed to reach the equivalence point. At this point the pH of the solution equals the pKa of the acid. This relationship comes directly from the Henderson-Hasselbalch equation, because the concentrations of acid and conjugate base are equal at the halfway mark.
Can titration curves identify polyprotic acids?
Yes. Polyprotic acids like phosphoric acid produce multiple inflection points on the curve, one for each ionizable proton. Each inflection corresponds to a separate equivalence point. The spacing and shape of these inflections reveal the individual pKa values of the acid.
Related Reading
Sources
- Primary References:
- Acid-Base Titrations (UCF Chemistry Fundamentals)
- Acid-Base Titrations (Chemistry LibreTexts)
- Historical Context:
- Who Invented Titration? (Reagent.co.uk)
- Joseph-Louis Gay-Lussac (Encyclopaedia Britannica)
- Technical Detail:
- Titration Curves and Equivalence Point (Khan Academy)
Fact Check: Claim-by-Claim Verification Verified
All core claims verified against authoritative sources. Historical dates, chemical relationships, and technical explanations confirmed accurate.
Sources used for verification
- Acid-Base Titrations - UCF Chemistry Fundamentals
- Who Invented Titration? - Reagent.co.uk
- Joseph-Louis Gay-Lussac - Encyclopaedia Britannica
- Acid-Base Titrations - Chemistry LibreTexts
- Titration Curves and Equivalence Point - Khan Academy
