Acid-Base Titration Study Notes

Acid-Base Titrations

  • Definition: Acid-base titrations, also known as neutralisation titrations, are widely used to determine the concentration of acids and bases.

    • Procedure: The titration involves the neutralisation of an analyte of unknown concentration with a standard solution of known concentration.

    • Types of Standard Solutions:

    • Strong acids (e.g., HCl, HClO4, H2SO4)

    • Strong bases (e.g., NaOH, KOH, Ba(OH)2)

Strong vs Weak Acids and Bases

  • Reactivity:

    • Strong acids/bases react more completely with analytes than weak acids/bases, resulting in sharper end points.

  • Preparation of Standard Solutions:

    • Standard solutions of acids are prepared by diluting concentrated acids and standardised with primary standards.

    • Nitric acid is rarely used due to its oxidising properties, which can cause side reactions affecting titration results.

    • Standard solutions of bases are prepared by dissolving solid NaOH, KOH in water and standardised with primary standards.

Titration Curves

  • Titration Curve Definition: A graphical representation of the pH value plotted against the volume of titrant added.

    • Purpose: Helps identify the equivalence point of a titration, where the stoichiometric amounts of acid and base have been mixed.

    • End Point: Determined by experimental observation, often indicated by a colour change.

Titration of Strong Acids with Strong Bases

  • pH Change Characteristics:

    • Gradual increase of pH before and after the equivalence point.

    • Sharp rise in pH near the equivalence point.

    • Equivalence point pH = 7.

  • Process Explanation:

    • Initially, the neutralisation process results in a slow pH change.

    • Once most H+ ions have been neutralised, pH spikes rapidly at the equivalence point (where pH = 7).

    • Addition of more base after the equivalence point results in a slower pH change, approaching that of the base.

  • Mathematical Relationships:

    • pH = - ext{log} [H^+] = - ext{log} [H_3O^+]

    • pOH = - ext{log} [OH^-]

    • pH + pOH = 14

Practical Calculations

Example Question 1: Titration Calculation

  • Given: 50.00 mL of 0.05 M HCl with 0.10 M NaOH.

  • Required: Calculate pH at various points:

    • (a) Before addition of base

    • (b) After 10.00 mL of base added

    • (c) After 25.00 mL of base added

    • (d) After 25.10 mL of base added.

Indicators in Acid-Base Titration

  • Role of Indicators: Indicators are weak organic acids or bases that change colour based on the pH.

    • Chemical Equation:

    • HIn + H2O ⇌ In^- + H3O+

    • Example: Phenolphthalein changes from colourless in acid to pink in alkali.

  • Selection: Chosen based on where they change colour, typically at the steep part of the titration curve.

Choice of Indicators

  • Indicators and Their pH Ranges:

    • Methyl orange: 3.1 - 4.4 (Red to yellow)

    • Methyl red: 4.2 - 6.3 (Red to yellow)

    • Bromothymol blue: 6.2 - 7.6 (Yellow to blue)

    • Phenol red: 6.8 - 8.4 (Yellow to red)

    • Phenolphthalein: 8.3 - 10.0 (Colourless to pink)

Specific Examples of Indicator Use

  • Phenolphthalein:

    • Pink in alkali; colourless in acid; used in strong alkali titrations.

  • Methyl Orange:

    • Red in acid; yellow in alkali; orange at endpoint; suitable for strong acid titrations.

Titration Characteristics

Strong Acid with Strong Base

  • Equivalence Point: pH = 7.

  • Suitable Indicators: Phenolphthalein and methyl red due to their suitable pH ranges.

    • Methyl red (4.2 - 6.3) changes colour within the vertical portion of the titration curve.

Weak Acid with Strong Base

  • Equivalence point: pH = 8.72. Suitable indicator: Phenolphthalein.

  • Unsuitable: Methyl red changes colour at pH 4.2 - 6.3, which is not applicable at this equivalence point.

Strong Acid with Weak Base

  • Equivalence point: pH = 5.28.

  • Suitable Indicator: Methyl red.

  • Unsuitable: Phenolphthalein changes colour at pH 8.3 - 10.0.

Weak Acid with Weak Base

  • Equivalence point: Cannot be determined clearly due to no distinct vertical portion in the titration curve.

Graphical Analysis of Titration Curves

  • Methods:

    • Graphical plots of pH during titration to find the equivalence point (shown at a specific volume of NaOH).

    • First derivative method for better approximation of the equivalence point.

Equivalence Point Calculation Example

From graphical data: Volume at equivalence point identified at 24.90 mL.

Example Questions

  • Question 2: Determine acid/base types, titrant direction, volume to equivalence, and pH at equivalence from a provided graph.

  • Question 3: Analyze a monoprotic acid titration with NaOH: volume used for neutralisation,

    • Calculate concentration of acid.

    • Suggest suitable indicator.

  • Question 4: Concentration of HF based on titrank with NaOH.

Basicity of Acids and Bases

  • Definition of Basicity: Related to the number of protons (H⁺) an acid can donate:

    • Monoprotic: 1 H⁺ per molecule.

    • Diprotic: 2 H⁺ per molecule.

    • Triprotic: 3 H⁺ per molecule.

  • Basicities of Bases:

    • Monobasic: 1 proton accepted.

    • Dibasic: 2 protons accepted.

    • Tribasic: 3 protons accepted.

Classification Examples

  • Hydrochloric acid (HCl): Monoprotic acid

  • Sodium hydroxide (NaOH): Monobasic base

  • Sulphuric acid (H2SO4): Diprotic acid

  • Barium hydroxide (Ba(OH)2): Dibasic base

Titration of Polyprotic Acids

  • Titration Curve Characteristics: Show multiple equivalence points due to multiple acidic protons that react in sequence.

  • Example of H2CO3 Titration:

    • 2 equivalence points detected when titrated with NaOH:

    • First: Occurs after adding 25.00 mL of NaOH.

    • Second: At 50.00 mL of NaOH.

  • Phosphoric Acid (H3PO4): Titrates in three steps; equivalence points observed at pH values of 4.67, 9.46, and 11.93.

Conclusion and Questions

  • Question 5: Concentration of NaOH determined based on titration with sulphuric acid.

Thank You

  • Gratitude expressed for learning and engagement.