Introduction
In every chemistry and pharmaceutical analysis laboratory, the preparation of accurate solutions is one of the most important tasks. Whether we are working on titration, drug analysis, water testing, or industrial quality control, the results always depend on how well the solutions are prepared and standardized.
A standard solution is simply a solution of known concentration. There are two important categories:
Primary Standard Solution: A solution prepared by directly dissolving a pure, stable, non-hygroscopic substance in a measured volume of solvent. For example, oxalic acid and sodium carbonate are often used as primary standards.
Secondary Standard Solution: A solution whose concentration cannot be prepared accurately just by weighing because the substance is impure, unstable, or reacts easily with moisture or air. These solutions must be standardized (checked and adjusted) using a primary standard. Examples include NaOH, HCl, H₂SO₄, KMnO₄, sodium thiosulphate, and ceric ammonium sulphate.
1. Oxalic Acid (H₂C₂O₄·2H₂O)
Why is it important?
Oxalic acid is a primary standard.
It is pure, crystalline, stable in air, and can be weighed accurately.
Commonly used to standardize potassium permanganate solution.
Preparation of 0.1 N Oxalic Acid
Weigh 6.3 g of oxalic acid dihydrate accurately.
Transfer into a beaker, dissolve in distilled water.
Transfer the solution into a 1-liter volumetric flask.
Make up the volume with distilled water to the mark.
This gives 0.1 N oxalic acid solution.
Standardization
Since oxalic acid is a primary standard, no further standardization is necessary.
It is often used to standardize KMnO₄.
Reaction with KMnO₄ (in acidic medium):
2. Sodium Hydroxide (NaOH)
Why is it important?
Widely used for acid-base titrations.
But NaOH is not a primary standard because it is hygroscopic (absorbs moisture from air) and reacts with CO₂ forming sodium carbonate.
Preparation of Approx. 0.1 N NaOH
Weigh about 4 g NaOH pellets.
Dissolve in freshly boiled and cooled distilled water (to remove CO₂).
Transfer into a 1-liter volumetric flask.
Make up to the mark with distilled water.
Store in a tightly closed plastic bottle to prevent CO₂ absorption.
Standardization
Usually done against a primary standard oxalic acid or potassium hydrogen phthalate (KHP).
Procedure (with oxalic acid):
- Pipette 25 ml of 0.1 N oxalic acid solution into a conical flask.
- Add 2–3 drops of phenolphthalein indicator.
- Titrate with NaOH until a faint pink color appears (end point).
Reaction:
With KHP:
3. Hydrochloric Acid (HCl)
Why is it important?
Strong acid, widely used in acid-base titrations.
Not a primary standard because concentrated HCl is volatile, fuming, and cannot be weighed accurately.
Preparation of Approx. 0.1 N HCl
Measure about 8.3 ml conc. HCl (sp. gr. 1.18, ~36%).
Dilute carefully in distilled water and make up to 1 liter.
Always add acid to water, never water to acid.
Standardization
Done against sodium carbonate (Na₂CO₃), a primary standard.
Procedure:
Prepare 0.1 N Na₂CO₃ solution (by dissolving 1.325 g in 250 ml water).
Pipette 25 ml Na₂CO₃ into a flask.
Add methyl orange indicator.
Titrate with HCl until color changes from yellow to orange.
Reaction:
4. Sodium Thiosulphate (Na₂S₂O₃·5H₂O)
Why is it important?
Used in iodometric titrations.
Not a primary standard because it slowly decomposes in solution.
Must be kept in a dark bottle to prevent photodecomposition.
Preparation of Approx. 0.1 N Sodium Thiosulphate
Weigh 25 g sodium thiosulphate pentahydrate.
Dissolve in freshly boiled and cooled distilled water.
Transfer to a 1-liter volumetric flask and dilute to the mark.
Store in an amber bottle.
Standardization
Commonly standardized against potassium dichromate (K₂Cr₂O₇).
Reactions:
Dichromate liberates iodine from KI:
Liberated iodine is titrated with Na₂S₂O₃:
Indicator: Starch solution (blue → colorless end point).
5. Sulphuric Acid (H₂SO₄)
Why is it important?
Strong acid, used in many titrations.
Not a primary standard (hygroscopic and viscous).
Preparation of Approx. 0.1 N Sulphuric Acid
Take 2.8 ml conc. H₂SO₄ (sp. gr. 1.84, 98%).
Slowly add to about 500 ml water in a beaker.
Cool and make up to 1 liter in a volumetric flask.
Always add acid to water carefully (never water to acid).
Standardization
Done against sodium carbonate using methyl orange indicator.
Reaction:
6. Potassium Permanganate (KMnO₄)
Why is it important?
Powerful oxidizing agent used in redox titrations.
Not a primary standard because:
It decomposes slowly on standing.
May contain MnO₂ impurities.
Preparation of Approx. 0.1 N KMnO₄
Weigh 3.2 g KMnO₄ crystals.
Dissolve in distilled water.
Allow to stand overnight.
Filter through glass wool to remove MnO₂.
Make up to 1 liter.
Standardization
Done against oxalic acid or sodium oxalate in acidic medium.
Procedure:
Pipette 25 ml oxalic acid solution into a flask.
Add 10 ml dilute H₂SO₄.
Heat to 60–70 °C.
Titrate with KMnO₄ until a permanent pink color appears.
Reaction:
7. Ceric Ammonium Sulphate (NH₄)₄Ce(SO₄)₄·2H₂O
Why is it important?
Strong oxidizing agent, used in redox titrations.
Stable in solid form but not considered a primary standard solution.
Preparation of Approx. 0.1 N Solution
Dissolve 66 g ceric ammonium sulphate in 500 ml distilled water.
Add 15 ml conc. H₂SO₄ to stabilize.
Dilute to 1 liter.
Standardization
Done against ferrous ammonium sulphate (FAS), a primary standard.
Procedure:
Pipette 25 ml FAS solution into a conical flask.
Add dilute H₂SO₄.
Use ferroin or ferroin-sulphate indicator.
Titrate with ceric ammonium sulphate until end point.
Reaction:
General Tips for Solution Preparation
Always use clean, dry glassware.
For acids, add acid to water (not water to acid).
For hygroscopic substances (NaOH, H₂SO₄), weigh quickly and minimize exposure to air.
Store light-sensitive solutions (Na₂S₂O₃, KMnO₄) in amber bottles.
Standardize secondary solutions frequently to ensure accuracy.
Use freshly boiled and cooled water for NaOH and Na₂S₂O₃ to avoid CO₂ interference.
Conclusion
Preparation and standardization of solutions form the backbone of laboratory analysis. The concept is simple:
Primary standards like oxalic acid, sodium carbonate, and potassium hydrogen phthalate are used to prepare solutions of exact concentration.
Secondary standards like NaOH, HCl, H₂SO₄, KMnO₄, sodium thiosulphate, and ceric ammonium sulphate must be standardized against primary standards before use.
By following correct preparation methods, careful dilution, and proper standardization steps, we can achieve solutions with reliable concentrations. This ensures accuracy, reproducibility, and confidence in analytical chemistry, pharmaceutical testing, and industrial quality control.
1 Comment
[…] Preparation and standardization of solutions is a fundamental skill in chemistry and pharmaceutical analysis. This guide explains step-by-step how to prepare and standardize various molar and normal solutions, including commonly used reagents like oxalic acid, sodium hydroxide, hydrochloric acid, sodium thiosulphate, sulphuric acid, potassium permanganate, and ceric ammonium sulphate. It covers the importance of accurate measurements, selecting primary and secondary standards, and performing titrations to achieve precise concentrations. Written in simple and easy-to-understand language, this resource helps students, researchers, and laboratory professionals gain practical knowledge for accurate experimental results and reliable chemical analysis. […]