5. NON AQUEOUS TITRATIONS

Non Aqueous Titrations – Introduction

Non-aqueous titration refers to a titration in which the analyte (the substance being tested) is dissolved in a solvent that does not contain water. This type of titration is mainly used when the substance is either insoluble or unstable in water, or when it is too weak to react properly in an aqueous medium.

Why Non-Aqueous Titrations Are Used

  • When the compound is insoluble in water: Some compounds do not dissolve in water, so an alternative solvent is needed.
  • When the compound reacts with water: Certain substances decompose or react chemically in water, making non-aqueous solvents necessary.
  • When the substance is a weak acid or weak base: Weak acids or bases do not dissociate well in water because of water’s amphiprotic nature (acting as both acid and base), so they are titrated in non-aqueous solvents that can enhance their reactivity.

By replacing water with another solvent, we make the reaction sharper, faster, and more accurate.

Types of Solvents in Non-Aqueous Titrations

1. Protogenic Solvents

  • These are acidic solvents that increase the basicity of weak bases.
  • Example: Glacial acetic acid (pure anhydrous acetic acid, solidifies at 16.7°C).

2. Protophilic Solvents

  • These are basic solvents that enhance the acidity of weak acids.
  • Examples: Pyridine, Ethylenediamine, and Dimethylformamide (DMF).

3. Amphoteric Solvents

  • These solvents can act as both acid and base depending on the substance dissolved in them.
  • Examples: Alcohols like ethanol and methanol.

4. Aprotic Solvents

  • These solvents neither donate nor accept protons. They are useful for dissolving non-polar substances that are insoluble in water.
  • Examples: Benzene and carbon tetrachloride.

Selection of Solvent

The choice of solvent in non-aqueous titration depends on several factors:

  • Solubility of the drug: The drug must dissolve completely in the solvent.
  • Nature of the drug: Whether it is a weak acid or weak base decides the type of solvent.
  • Unreactivity: The solvent should not react chemically with the drug or titrant.

Commonly Used Non-Aqueous Solvents

1. Glacial Acetic Acid (CH3COOH)

Most commonly used solvent. Water content must be checked and should be within 0.1–1.0%. It enhances the basicity of weak bases.

2. Acetonitrile (CH3CN)

Used along with chloroform or acetic acid. It provides sharp end points in titrations of metal salts using perchloric acid.

3. Alcohols (Methanol, Ethanol, Propanol, Butanol, Ethylene Glycol)

Useful for both polar and non-polar reactants. They must be water-free for accurate results.

4. Dioxane

Used as an alternative to glacial acetic acid. It does not show levelling effects and gives precise results.

5. Dimethylformamide (DMF)

A basic (protophilic) solvent. Commonly used for titration of weak acids such as benzoic acid or amides.

Levelling Effect

The levelling effect refers to the phenomenon where the strength of weak acids or bases increases in the presence of a suitable solvent.

  • In a basic solvent (like ammonia), weak acids become stronger because the solvent accepts protons easily.
  • In an acidic solvent (like acetic acid), weak bases become stronger because the solvent donates protons.

By choosing the right solvent, we can “level up” the strength of weak acids or bases, making titration possible and accurate.

Example: Acetic acid behaves like a strong acid in ammonia solution due to this effect.

Differentiating Solvents

Unlike levelling solvents, differentiating solvents allow acids or bases to show different strengths. The degree of dissociation varies depending on the solvent used.

Example: In glacial acetic acid, HI acts as the strongest acid, HBr is moderate, and HCl is weakest. This is called the differential effect.

Acidimetry and Alkalimetry in Non-Aqueous Titrations

  • Acidimetry: Used for quantitative determination of weak bases in non-aqueous solvents.
  • Alkalimetry: Used for quantitative determination of weak acids in non-aqueous solvents.

Indicators Used in Non-Aqueous Titrations

Both ionized and non-ionized indicators are used. The color change at the end point depends on the titrant and solvent. The end point can also be confirmed using potentiometric titration for greater accuracy.

Common Indicators:

  1. Crystal Violet (0.5% in glacial acetic acid):
    Color changes from violet → blue → green → greenish-yellow when titrating bases like pyridine with perchloric acid.
  2. Methyl Red (0.2% in dioxane):
    Color changes from yellow (alkaline medium) to red (acidic medium).
  3. Naphthol Benzein (0.2% in glacial acetic acid):
    Color changes from yellow (alkaline) to green (acidic). Used in acetic anhydride with nitromethane for titration of weak bases.
  4. Quinoline Red (0.1% in ethanol):
    Color changes from purple-red (acidic) to pale green (alkaline).
  5. Thymol Blue (0.2% in methanol):
    Color changes from pink (acidic) to blue (alkaline) at the end point.

Precautions in Non-Aqueous Titrations

  • All solvents must be free from moisture because water can interfere with the titration and affect accuracy.
  • Temperature should be constant during titration, as organic solvents expand with heat.
  • Proper selection of solvent and indicator ensures sharp end points.

Applications

  • Estimation of Sodium Benzoate – performed in non-aqueous medium using perchloric acid as titrant.
  • Estimation of Ephedrine Hydrochloride – titrated in glacial acetic acid using crystal violet as indicator.

Detailed Notes:

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