Preparation and standardization of solutions

• Preparation and standardization of solutions is essential for accurate chemical analysis.
• Below are step-by-step guides for preparing and standardizing the given molar and normal solutions.

1. Oxalic Acid (H₂C₂O₄) Solution

• Molecular weight: 126.07 g/mol
• Normality: For reactions with KMnO₄ (5e⁻), Oxalic acid is 2N (since it donates two electrons in redox reactions).

• Preparation of 0.1 M Oxalic Acid:

  • Weigh 6.3035 g of oxalic acid dihydrate (H₂C₂O₄·2H₂O).
  • Dissolve in distilled water and make up the volume to 1 liter.
  • Mix thoroughly.

• Standardization:

  • Titrate against a standard solution of sodium hydroxide using phenolphthalein as an indicator (end point: pink color).

2. Sodium Hydroxide (NaOH) Solution

• Molecular weight: 40.00 g/mol
• Normality: For NaOH, M = N since it is monoprotic (1 OH⁻ ion per molecule).

• Preparation of 0.1 N Sodium Hydroxide:

  • Weigh 4.0 g of sodium hydroxide.
  • Dissolve in 1 liter of distilled water.
  • Store in a sealed container to avoid absorption of CO₂ from the air.

• Standardization:

  • Titrate against a standard solution of oxalic acid using phenolphthalein as an indicator. The end point is the appearance of a pink color.

3. Hydrochloric Acid (HCl) Solution

• Molecular weight: 36.46 g/mol
• Commercial HCl is usually around 37
• Concentration of stock HCl: ~12 M

• Preparation of 0.1 N Hydrochloric Acid:

  • Use a volumetric calculation to dilute the stock solution:
  • $V_1 = \frac{N_2 V_2}{N_1} = \frac{0.1 \times 1000}{12} \approx 8.33\,\text{mL}$
  • Dilute 8.33 mL of concentrated HCl to 1 liter with distilled water.
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• Standardization:

• • Titrate against a standard solution of sodium carbonate (Na₂CO₃) using methyl orange as an indicator (end point: orange).

4. Sodium Thiosulphate (Na₂S₂O₃) Solution

• Molecular weight: 248.18 g/mol
• Sodium thiosulphate is typically used in redox titrations (Iodometric titration).

• Preparation of 0.1 N Sodium Thiosulphate:

  • Weigh 24.818 g of Na₂S₂O₃·5H₂O.
  • Dissolve in distilled water and dilute to 1 liter.
  • Store in a cool, dark place to prevent decomposition.

• Standardization:

  • Titrate against a standard solution of potassium dichromate (K₂Cr₂O₇) or potassium iodate (KIO₃) in the presence of iodine and starch as an indicator (end point: disappearance of blue color).

5. Sulfuric Acid (H₂SO₄) Solution

• Molecular weight: 98.08 g/mol
  • Concentrated H₂SO₄ is around 98
  • Concentration of stock H₂SO₄: ~18 M

• Preparation of 0.1 N Sulfuric Acid:

  • Use the dilution formula:
  • $V_1 = \frac{N_2 V_2}{N_1} = \frac{0.1 \times 1000}{18} \approx 5.56\,\text{mL}$
  • Carefully dilute 5.56 mL of concentrated H₂SO₄ to 1 liter with distilled water (add acid to water, not the reverse).

• Standardization:

  • Titrate against a standard solution of sodium carbonate (Na₂CO₃) using methyl orange as an indicator (end point: orange).
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6. Potassium Permanganate (KMnO₄) Solution

• Molecular weight: 158.03 g/mol
• Potassium permanganate is a strong oxidizing agent and is used in redox titrations.

• Preparation of 0.1 N Potassium Permanganate:

  • Weigh 3.1606 g of KMnO₄.
  • Dissolve in distilled water and dilute to 1 liter.
  • The solution needs to be standardized before use as it may decompose over time.

• Standardization:

  • Titrate against a standard oxalic acid solution in acidic medium (with H₂SO₄). Heat the solution to about 70°C to accelerate the reaction. The end point is the disappearance of the purple color of KMnO₄.
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7. Ceric Ammonium Sulphate [(NH₄)₄Ce(SO₄)₄] Solution

• Molecular weight: 632.57 g/mol
• Ceric ammonium sulfate is used as an oxidizing agent in redox titrations.

• Preparation of 0.1 N Ceric Ammonium Sulphate:

  • Weigh 63.257 g of ceric ammonium sulfate.
  • Dissolve in 1 liter of distilled water containing a small amount of sulfuric acid (to prevent hydrolysis).

• Standardization:

  • Titrate against a primary standard such as arsenious oxide (As₂O₃) in acidic conditions with ferroin as an indicator (end point: red color to pale blue).
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General Notes for Preparation and standardization of solutions:

• Glassware: Always use clean and calibrated volumetric flasks, pipettes, and burettes for accurate results.
• Indicators: Choose an appropriate indicator based on the nature of the reaction (acid-base or redox).
• Safety: Always handle concentrated acids and bases with care. Use proper personal protective equipment (PPE) like gloves and goggles.

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