6 – Preparing Sodium Carbonate (LC 2027) (Leaving Cert Chemistry): Revision Notes

6 – Preparing Sodium Carbonate

Introduction and purpose

This experiment involves creating a standard solution of sodium carbonate with a precisely known concentration. The process is fundamental to quantitative analysis and forms the basis for many analytical chemistry techniques, particularly acid-base titrations.

A standard solution is essential because it allows us to determine unknown concentrations of other substances through careful measurement and calculation. The accuracy of this preparation directly affects the reliability of any subsequent analysis.

Understanding primary standards

Anhydrous sodium carbonate is classified as a primary standard. This means it possesses several important characteristics:

• High purity - The substance is essentially 100% pure with minimal impurities
• Stable composition - It doesn't change composition when exposed to air
• Known formula - We can calculate its exact molecular mass
• Non-hygroscopic - It doesn't absorb moisture from the atmosphere

These properties make sodium carbonate ideal for preparing solutions with precisely known concentrations, which is why we use it rather than other sodium compounds.

Equipment and materials required

The following apparatus is essential for accurate solution preparation:

• Electronic balance (capable of weighing to 0.001g)
• Clock glass (for holding the solid during weighing)
• 250 cm³ volumetric flask (for final solution volume)
• Glass funnel and glass rod
• Plastic wash bottle containing deionised water
• Dropper for fine adjustment
• 250 cm³ beaker for initial dissolution
• Stirring rod for mixing

Using deionised water throughout is crucial because tap water contains dissolved ions that would affect the final concentration.

Step-by-step experimental procedure

Weighing and initial dissolution

The process begins with precise measurement of the sodium carbonate:

1. Accurate weighing - Use an electronic balance to weigh the required mass of anhydrous sodium carbonate on a clock glass. The mass needed depends on the desired final concentration.
2. Initial dissolution - Transfer the weighed sodium carbonate to a beaker containing deionised water. The amount of water used here isn't critical as this is just for dissolution.
3. Washing the clock glass - Use deionised water from a wash bottle to rinse any remaining traces of sodium carbonate from the clock glass into the beaker. This ensures no material is lost.
4. Complete dissolution - Stir the mixture with a glass rod until all the solid has completely dissolved, creating a clear solution.

Transfer and washing procedures

Once dissolved, the solution must be transferred to the volumetric flask:

1. Transfer using funnel - Pour the dissolved solution through a funnel into a clean 250 cm³ volumetric flask. The funnel prevents spillage and ensures all solution enters the flask.
2. Washing apparatus - Rinse the beaker, stirring rod, and funnel with deionised water, allowing all washings to flow into the volumetric flask. This step is critical because it ensures that every trace of the sodium carbonate reaches the final solution.

Final dilution and mixing

The final steps create the precisely measured volume:

1. Initial dilution - Add deionised water to the volumetric flask until the solution level is near, but below, the graduation mark.
2. Final adjustment - Use a dropper to add water drop by drop until the bottom of the meniscus aligns exactly with the graduation mark when viewed at eye level.
3. Thorough mixing - Replace the stopper and invert the flask approximately 20 times to ensure the solution is completely homogeneous.

Key laboratory techniques

Accurate measurement techniques

Several techniques are crucial for preparing an accurate standard solution:

• Quantitative transfer - Every particle of the weighed sodium carbonate must reach the final solution
• Multiple washing - Rinse all apparatus that contacted the sodium carbonate
• Proper mixing - Ensure uniform distribution throughout the solution

Meniscus reading

Reading the meniscus correctly is essential for accurate volume measurement:

• Eye-level viewing - Your eye must be level with the graduation mark
• Bottom of meniscus - Read from the lowest point of the curved liquid surface
• Good lighting - Ensure adequate illumination to see the meniscus clearly

Volumetric flask handling

Proper use of volumetric flasks ensures accurate final volumes:

• Temperature consideration - Use at room temperature for calibrated volume
• Gentle mixing - Invert rather than shake to avoid breaking
• Single-use preparation - Don't attempt to adjust volume once mixed

Calculations and final solution

After completing the preparation, you can calculate the exact concentration of your standard solution using:

$\text{Concentration (mol/L)} = \frac{\text{Number of moles}}{\text{Volume (L)}}$

Where the number of moles equals the mass weighed divided by the molecular mass of sodium carbonate ($\text{Na}_2\text{CO}_3 = 106 \text{ g/mol}$).

This standard solution now has a precisely known concentration and can be used in quantitative analysis experiments such as titrations with acids.