Acid-base titrations can be classified into different categories based on the nature of the acid and base involved, as well as the type of reaction taking place.
The theory behind titrations involving strong, weak, and very weak acids and bases is based on the concepts of chemical equilibrium, ionization, and the pH scale.
The main classifications include:
1. Strong Acid - Strong Base Titration:
Theory:
In a titration involving a strong acid and a strong base, both the acid and the base completely ionize in water, producing a high concentration of H⁺ and OH⁻ ions, respectively.
The reaction results in the formation of water and a neutral salt.
The pH at the equivalence point is close to 7.0.
Example:
Reaction:
HCl (strong acid) + NaOH (strong base) → NaCl (salt) + H₂O (water)
Hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH) to form sodium chloride (NaCl) and water.
Procedure:
A burette is used to carefully add the NaOH solution to the HCl solution until the equivalence point is reached, where the moles of H⁺ ions equal the moles of OH⁻ ions.
The pH at the equivalence point is near 7.0, indicating neutrality.
Indicator: Phenolphthalein, which is colorless in acidic solutions and turns pink in basic solutions.
As NaOH is added, the solution remains colorless until the endpoint is reached, where the pink color indicates neutralization.
Calculation:
By measuring the volume of NaOH required to reach the endpoint, the concentration of the HCl solution can be calculated using stoichiometry and the known concentration of NaOH.
2. Weak Acid - Strong Base Titration:
Theory:
In this case, the weak acid only partially ionizes, producing a relatively low concentration of H⁺ ions, while the strong base completely ionizes, producing a high concentration of OH⁻ ions.
The reaction forms a basic salt, and the pH at the equivalence point will be greater than 7.0 due to the basic nature of the salt.
Example:
Reaction:
CH₃COOH (weak acid) + NaOH (strong base) → CH₃COONa (basic salt) + H₂O (water)
Acetic acid (CH₃COOH) reacts with sodium hydroxide (NaOH) to form sodium acetate (CH₃COONa) and water.
Procedure:
A burette is used to add NaOH to the acetic acid solution until the equivalence point is reached, where the moles of H⁺ ions from the acid equal the moles of OH⁻ ions from the base.
The pH at the equivalence point is greater than 7.0, indicating a slightly basic solution due to sodium acetate.
Indicator: Phenolphthalein is used, turning from colorless to pink at the endpoint.
Calculation:
By measuring the volume of NaOH needed to reach the endpoint, the concentration of the acetic acid solution can be calculated.
3. Strong Acid - Weak Base Titration:
Theory:
The strong acid completely ionizes in water, producing a high concentration of H⁺ ions, while the weak base only partially ionizes, resulting in a lower concentration of OH⁻ ions.
The reaction forms an acidic salt, and the pH at the equivalence point will be less than 7.0.
Example:
Reaction:
HCl (strong acid) + NH₃ (weak base) → NH₄Cl (acidic salt)
Hydrochloric acid (HCl) reacts with ammonia (NH₃) to form ammonium chloride (NH₄Cl).
Procedure:
A burette is used to add ammonia solution to the HCl solution until the equivalence point, where the moles of H⁺ ions equal the moles of NH₃.
The pH at the equivalence point is less than 7.0, indicating a slightly acidic solution due to ammonium chloride.
Indicator: Methyl orange, which is red in acidic solutions and turns yellow in basic solutions, can be used to signal the endpoint.
Calculation:
By measuring the volume of ammonia required to reach the endpoint, the concentration of the HCl solution can be calculated using the stoichiometry of the reaction.
4. Weak Acid - Weak Base Titration:
Theory:
In this case, both the weak acid and weak base only partially ionize, producing low concentrations of H⁺ and OH⁻ ions.
The pH at the equivalence point can vary depending on the relative strengths of the acid and base and may be close to, less than, or greater than 7.0.
The titration endpoint is less distinct, making precise pH measurement techniques important.
Example:
Reaction:
CH₃COOH (weak acid) + NH₃ (weak base) → CH₃COONH₄ (salt with variable pH)
Acetic acid (CH₃COOH) reacts with ammonia (NH₃) to produce ammonium acetate (CH₃COONH₄).
Procedure:
A burette is used to add the ammonia solution to the acetic acid solution until the equivalence point is reached.
The pH at the equivalence point can be close to, less than, or greater than 7.0, depending on the strengths of the acid and base.
Indicator: Due to the less distinct endpoints, a potentiometric titration using a pH meter is often used.
Calculation:
By measuring the volume of ammonia required to reach the endpoint, the concentration of the acetic acid can be calculated.
However, the titration of weak acid and weak base is less precise and accurate compared to other titrations.