Acid Base Titrations

Acid-base titration is a quantitative analytical chemistry technique used to determine the concentration of an unknown acid or base solution by neutralizing it with a standard solution of known concentration. The process involves the gradual addition of the titrant (the solution of known concentration) to the analyte (the solution of unknown concentration) until the reaction reaches the equivalence point, where the amount of titrant is stoichiometrically equal to the amount of analyte. The equivalence point is typically detected using an indicator, which changes color at or near the equivalence point. This technique is widely used in laboratories to determine the amount of acid or base in a variety of samples.

Table of Contents

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Theories of acid base indicators

There are two primary theories that explain how acid-base indicators work: the Ostwald Theory and the Quinonoid Theory. These theories help us understand how acid-base indicators function during titrations, changing color to signal the endpoint of the reaction.

Ostwald Theory

According to this theory, acid-base indicators are weak organic acids (HIn) or weak organic bases (InOH). When these indicators undergo ionization, the ionized form (In-) and unionized form (HIn) exhibit different colors in an aqueous solution. The greater the extent of ionization, the more visible the color will be. For example, Methyl orange is a weak acid that undergoes ionization in an aqueous solution. The unionized form of methyl orange is red in color and the ionized form is yellow.

Quinonoid Theory

This theory, also known as the benzenoid quinonoid theory, tries to explain the cause of color change in the indicator. According to this theory, the indicator’s unionized form (HIn) and ionized form (In-) are tautomeric forms. One tautomeric form has a quinoid structural unit (Quinonoid form), and another tautomeric form has benzene rings (Benzenoid form). The Quinonoid form exhibits a deep color, and the Benzenoid form exhibits no color or a very light color.

Classification of Acid Base Titrations

Strong Acid – Strong Base

In this type of titration, both the acid and base are strong, meaning they completely dissociate in water. An example is the titration between Hydrochloric acid (HCl) and Sodium hydroxide (NaOH). The reaction can be represented as,

HCl + NaOH → NaCl + H2O

At the equivalence point, equal amounts of H+ and OH- ions will combine to form H2O, resulting in a pH of 7.

Weak Acid – Strong Base

Here, the acid is weak while the base is strong. An example is the titration between Acetic acid (CH3COOH) and Sodium hydroxide (NaOH). The reaction can be represented as,

CH3COOH + OH- → CH3COO- + H2O

At the equivalence point, all of the acid (HA) has been converted to its conjugate base (A-) by the addition of NaOH and now the equilibrium moves backwards towards HA and produces hydroxide, that is,

+ H2O ↔ AH + OH-3

This results in a pH greater than 7.

Strong Acid – Weak Base

In this case, the acid is strong and the base is weak. An example is the titration of Hydrochloric acid (strong acid) into Ammonia (weak base), which forms the conjugate acid Ammonium and produces an acidic solution. At the equivalence point, a conjugate acid will be produced during the titration, which then reacts with water to form hydronium ions. This results in a solution with a pH lower than 7.

Weak Acid – Weak Base

Both the acid and base are weak in this type of titration. An example is the reaction of Hydrofluoric acid with Ammonia to form the Ammonium ion and the Fluoride anion. There is no sharp rise in the neutralization curve and, generally, no simple indicator can be used. The titration should therefore be avoided, if possible. The approximate pH at the equivalence point can be computed from the equation,

pH=pKw+pKa−pKb

Type of Titration	Example	Reaction	pH at Equivalence Point
Strong Acid – Strong Base	HCl + NaOH	HCl + NaOH → NaCl + H2O	7
Weak Acid – Strong Base	CH3COOH + NaOH	CH3COOH + OH- → CH3COO- + H2O	Greater than 7
Strong Acid – Weak Base	HCl + NH3	HCl + NH3 → NH4+ + Cl-	Less than 7
Weak Acid – Weak Base	HF + NH3	HF + NH3 → NH4+ + F-	Computed from: pH = pKw +pKa – pKbpH=pKw+pKa−pKb

Neutralization curves

Neutralization curves are graphical representations that depict the pH changes during an acid-base titration. They are used as fundamental tools by analytical chemists to visualize the progression of a neutralization reaction. A neutralization curve is a plot of the pH of a solution as a function of the volume of a titrant added during an acid-base titration. The shape of the curve depends on the strength of the acid and base involved, as well as the concentration and volume of the solutions being titrated.

Here’s a brief explanation of how these curves are generated for different types of titrations.

Strong Acid – Strong Base: The pH rapidly decreases until it reaches a minimum at the pH of the strong acid. Conversely, when a strong base is added to the solution, the pH rapidly increases until it reaches a maximum at the pH of the strong base.
Weak Acid – Strong Base: The pH increases more gradually, eventually approaching a limit of the pH of the strong base.
Strong Acid – Weak Base: The pH decreases more gradually, eventually approaching a limit of the pH of the strong acid.
Weak Acid – Weak Base: The curve is less defined because both reactants are weak, and there is no sharp rise in the curve.

Summary

Acid-base titrations are a common method in analytical chemistry used to determine the concentration of an unknown acid or base solution by neutralizing it with a solution of known concentration. They are classified into four types: Strong Acid – Strong Base, Weak Acid – Strong Base, Strong Acid – Weak Base, and Weak Acid – Weak Base. The titration process involves the gradual addition of the titrant (the solution of known concentration) to the analyte (the solution of unknown concentration) until the reaction reaches the equivalence point, where the amount of titrant is stoichiometrically equal to the amount of analyte. The pH at the equivalence point varies depending on the strength of the acid and base involved. The progress of the titration is often monitored using an indicator or a pH meter, and the results are graphically represented in a neutralization curve. The shape of this curve provides valuable information about the acid-base reaction and the pH at the equivalence point.