Introduction
Sodium hydroxide (NaOH), commonly known as caustic soda, is a strong base widely used in laboratories, industries, and chemical processes. Preparing and standardizing NaOH solutions accurately is crucial for various applications, including titrations, pH adjustments, and chemical synthesis. This article provides a detailed guide on the preparation and standardization of molar and normal solutions of NaOH, ensuring accuracy and reliability in laboratory settings.
Understanding Molarity and Normality of NaOH
- Molarity (M): The molarity of a solution refers to the number of moles of solute per liter of solution (mol/L). A 1 M NaOH solution contains 1 mole (40 g) of NaOH in 1 liter of solution.
- Normality (N): Normality represents the number of gram-equivalents of solute per liter of solution. Since NaOH has one replaceable hydroxide ion (OH⁻), its normality is equal to its molarity (N = M) in acid-base reactions.
Preparation of NaOH Molar and Normal Solutions
1. Required Materials
- Analytical balance
- Sodium hydroxide (NaOH) pellets
- Distilled water
- Volumetric flask (1000 mL, 500 mL, or 250 mL)
- Beaker
- Glass rod
- Measuring cylinder
- Protective gear (gloves, goggles)
2. Preparing 1 M NaOH Solution
- Calculate the required NaOH mass:
- Molecular weight of NaOH = 40 g/mol
- To prepare 1 M solution, weigh 40 g of NaOH pellets for 1 liter of solution.
- Dissolve NaOH in water:
- Add 500 mL of distilled water into a beaker.
- Slowly add 40 g of NaOH pellets, stirring continuously to dissolve completely.
- Caution: The dissolution of NaOH is exothermic, generating heat. Always add NaOH to water, not vice versa.
- Transfer to a volumetric flask:
- After dissolution, transfer the solution into a 1000-mL volumetric flask.
- Add more distilled water to make the final volume exactly 1 liter.
- Mix and store:
- Seal the flask and shake gently to ensure uniformity.
- Store in a tightly closed, labeled bottle to prevent CO₂ absorption, which can alter concentration.
3. Preparing 1 N NaOH Solution
Since NaOH has one hydroxide ion (OH⁻), its normality is equal to molarity. Therefore, a 1 N NaOH solution is identical to a 1 M NaOH solution.
To prepare other normal solutions:
- 0.1 N NaOH: Dissolve 4 g of NaOH in 1 liter of water.
- 0.5 N NaOH: Dissolve 20 g of NaOH in 1 liter of water.
Standardization of NaOH Solution
Since NaOH absorbs CO₂ from the air, its concentration may change over time. Standardization ensures accuracy. Potassium hydrogen phthalate (KHP) is commonly used as a primary standard for NaOH.
1. Required Materials
- NaOH solution
- Potassium hydrogen phthalate (KHP) (C₈H₅KO₄) (primary standard)
- Analytical balance
- Distilled water
- Burette
- Conical flask
- Phenolphthalein indicator
2. Standardization Procedure
- Prepare KHP solution:
- Weigh 0.204 g of KHP (equivalent to 10 mL of 0.1 N NaOH).
- Dissolve in 50 mL of distilled water in a conical flask.
- Titrate with NaOH:
- Fill a burette with NaOH solution and record the initial reading.
- Add 2-3 drops of phenolphthalein indicator to the KHP solution.
- Slowly add NaOH from the burette while swirling the flask.
- Stop when the solution turns faint pink, indicating the endpoint.
- Calculate the exact concentration of NaOH:
- Use the formula:
Adjust calculations accordingly if using a different normality or volume of NaOH.
Precautions for Accuracy
- Always use freshly boiled, cooled distilled water to minimize CO₂ absorption.
- Use airtight bottles for storing NaOH solutions.
- Standardize NaOH solutions regularly for precise experimental results.
- Wear protective gear, as NaOH is caustic and can cause burns.
Conclusion
Preparing and standardizing sodium hydroxide (NaOH) solutions are essential steps in laboratory practices. Since NaOH is hygroscopic and absorbs CO₂, accurate weighing, dissolving, and storage are crucial. Standardization with KHP ensures precision for analytical applications. By following these guidelines, you can ensure reliable molar and normal NaOH solutions for scientific and industrial use.
