Acidimetry and alkalimetry titration are two branches of volumetric analysis used to determine the strength or concentration of acidic or basic solutions. These titrations involve a neutralization reaction between acids and bases in the presence of an appropriate indicator.
Definitions
Acidimetry: The quantitative analysis of acids using a standard base solution.
Alkalimetry: The quantitative analysis of bases using a standard acid solution.
Principle
Both acidimetry and alkalimetry titrations are based on neutralization reactions, where an acid reacts with a base to form salt and water: Acid+Base→Salt+WaterAcid + Base \rightarrow Salt + Water
For example, in the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH):
HCl+NaOH→NaCl+H2OHCl + NaOH → NaCl + H_2O
The endpoint is detected using pH-sensitive indicators, which change color at the equivalence point.
4. Types of Acid-Base Titrations
Acid-base titrations are classified based on the strength of acids and bases involved:
Strong Acid vs. Strong Base
- Example: HCl vs. NaOH
- Indicator: Phenolphthalein or Methyl orange
- Equivalence point: pH 7
Strong Acid vs. Weak Base
- Example: HCl vs. NH₄OH
- Indicator: Methyl orange
- Equivalence point: pH < 7
Weak Acid vs. Strong Base
- Example: CH₃COOH vs. NaOH
- Indicator: Phenolphthalein
- Equivalence point: pH > 7
Weak Acid vs. Weak Base
- Example: CH₃COOH vs. NH₄OH
- Indicator: No suitable indicator, potentiometric method is used.
- Equivalence point: Near neutral
Standard Solutions Used in Acid-Base Titrations
For Acidimetry (Standard Bases):
- Sodium hydroxide (NaOH)
- Potassium hydroxide (KOH)
- Sodium carbonate (Na₂CO₃)
For Alkalimetry (Standard Acids):
- Hydrochloric acid (HCl)
- Sulfuric acid (H₂SO₄)
- Oxalic acid (C₂H₂O₄)
Preparation of Standard Solutions
Primary Standards: A primary standard is a highly pure, stable, non-hygroscopic, and easily weighed substance used to prepare standard solutions. Examples:
- Oxalic acid for standardizing NaOH
- Sodium carbonate for standardizing HCl
Secondary Standards: A secondary standard is a solution whose concentration is determined by titration against a primary standard. Examples:
- NaOH standardized using oxalic acid
- HCl standardized using sodium carbonate
Indicators Used in Acid-Base Titrations
Indicators are substances that change color at a particular pH. The selection of an indicator depends on the type of titration:
| Titration Type | Indicator | Color Change |
|---|---|---|
| Strong Acid – Strong Base | Phenolphthalein | Colorless to pink |
| Strong Acid – Strong Base | Methyl orange | Red to yellow |
| Strong Acid – Weak Base | Methyl orange | Red to yellow |
| Weak Acid – Strong Base | Phenolphthalein | Colorless to pink |
8. pH Curve for Acid-Base Titrations
A pH curve shows how pH changes during the titration:
Strong Acid vs. Strong Base: Rapid pH change near equivalence point (~pH 7)
Strong Acid vs. Weak Base: Equivalence point below pH 7
Weak Acid vs. Strong Base: Equivalence point above pH 7
Weak Acid vs. Weak Base: Gradual pH change, making indicator selection difficult
Procedure for Acidimetry and Alkalimetry
General Steps
- Preparation of the solution: Prepare a known concentration of acid or base.
- Filling the burette: The standard titrant (acid or base) is filled into the burette.
- Pipetting the analyte: A specific volume of the unknown solution is taken in a conical flask using a pipette.
- Adding the indicator: A few drops of the appropriate indicator are added.
- Performing the titration: The titrant is added dropwise until the endpoint is reached (color change).
- Recording the burette reading: The volume of titrant used is noted.
- Calculation: The concentration of the unknown solution is determined using the titration formula.
10. Calculations in Acid-Base Titrations
The strength of an unknown solution is calculated using the formula: N1V1=N2V2N_1V_1 = N_2V_2
Where:
- N1N_1 = Normality of acid
- V1V_1 = Volume of acid used
- N2N_2 = Normality of base
- V2V_2 = Volume of base used
For molarity: M1V1=M2V2M_1V_1 = M_2V_2
Where:
- M1M_1 = Molarity of acid
- M2M_2 = Molarity of base
Example Calculation:
If 25 mL of NaOH (0.1N) is required to neutralize 50 mL of HCl, the normality of HCl is:
11. Applications of Acidimetry and Alkalimetry
- Pharmaceutical Industry: Standardization of acids and bases used in drug formulations.
- Chemical Industry: Quality control of acids and bases.
- Food Industry: Determining acidity in food and beverages.
- Water Analysis: Checking alkalinity/acidity of water.
12. Precautions in Acid-Base Titrations
- Use freshly prepared standard solutions.
- Rinse the burette and pipette properly before use.
- Add titrant dropwise near the endpoint.
- Read the burette reading at eye level to avoid parallax error.
Conclusion
Acidimetry and alkalimetry are essential quantitative techniques in volumetric analysis. Proper selection of indicators and accurate titration procedures are crucial for precise results. These titrations are widely used in pharmaceuticals, chemistry, and environmental sciences for analytical determinations.