- 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 of acid base titrations include:
1. Strong Acid – Strong Base Titration:
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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.
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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.
- Reaction:
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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⁻
- 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.
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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:
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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⁻
- 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.
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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.
- Reaction:
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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.
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3. Strong Acid – Weak Base Titration:
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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⁻
- The reaction forms an acidic salt, and the pH at the equivalence point will be less than 7.0.
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Example:
- Reaction:
HCl (strong acid) + NH₃ (weak base) → NH₄Cl (acidic salt) - Hydrochloric acid (HCl) reacts with ammonia (NH₃) to form ammonium chloride (NH₄Cl).
- Reaction:
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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.
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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:
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Theory:
- In this case, both the weak acid and weak base only partially ionize, producing low concentrations of H⁺ and OH⁻
- 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.
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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).
- Reaction:
-
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.
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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.