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 (C2H2O4) 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% (w/w), with a density of ~1.18 g/mL.
Concentration of stock HCl: ~12 M
Preparation of 0.1 N Hydrochloric Acid:
Use a volumetric calculation to dilute the stock solution:
V1 = (N2 V2)/N1 = (0.1×1000)/12 ≈ 8.33 mL
Dilute 8.33 mL of concentrated HCl to 1 liter with distilled water.
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% (w/w) with a density of ~1.84 g/mL.
Concentration of stock H₂SO₄: ~18 M
Preparation of 0.1 N Sulfuric Acid:
Use the dilution formula:
V1 = (N2 V2)/N1 = (0.1×1000)/18 ≈ 5.56 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).
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.
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).
General Notes:
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.