Primary Standard Solutions (HSC SSCE Chemistry): Revision Notes

Primary Standard Solutions

What are primary standard solutions?

When conducting volumetric analysis, we need to determine the concentration of an unknown solution by comparing it with a solution that has a precisely known concentration. This reference solution is called a primary standard solution.

A primary standard is a chemical substance that can be prepared into a solution with an accurately known concentration. These solutions are essential because they serve as the reference point for all volumetric analysis procedures, such as titrations. If the concentration of the primary standard is inaccurate, all subsequent measurements will be unreliable.

Understanding accuracy

Before exploring primary standards further, it's important to understand what we mean by accuracy. In chemistry, accuracy refers to the number of significant figures to which a measurement is known. The accuracy of any measurement depends on the precision of the measuring instrument being used.

For example:

• A mass balance measuring to the nearest gram ($1 \text{ g}$) provides measurements accurate to $1 \text{ g}$
• A mass balance measuring to the nearest milligram ($0.001 \text{ g}$) provides more accurate measurements

The electronic balance that measures to the nearest milligram is the more accurate device because it provides more significant figures in the measurement.

Characteristics of primary standards

Not every chemical can be used as a primary standard. To ensure accurate and reliable results, a substance must meet several important criteria:

Essential criteria

Large molar mass: Primary standards should have a relatively large molar mass so that a reasonable mass needs to be weighed out. When measuring larger masses, the percentage error becomes smaller, improving the overall accuracy of the solution.

Affordability: The chemical must be inexpensive and readily available so that it can be used routinely in laboratory work without excessive cost.

High purity: Primary standards must be extremely pure, typically $99.9\%$ or higher. Impurities could participate in unwanted side reactions, affecting the accuracy of results.

Stability in air: The substance must remain stable when exposed to air. It should not react with oxygen, carbon dioxide, or moisture from the atmosphere, as these reactions would alter its composition over time.

Anhydrous nature: Primary standards should not contain water of hydration (water molecules bound within the crystal structure). Hydrated compounds can lose or gain water depending on atmospheric humidity, which would change their composition and make accurate concentration determination impossible.

High solubility: The substance must dissolve readily in the chosen solvent (usually water) to form a stable solution that doesn't decompose or precipitate over time.

Common primary standards in school laboratories

Two substances commonly used as primary standards in school chemistry laboratories are:

Anhydrous sodium carbonate ($\text{Na}_2\text{CO}_3$): This is the most frequently used primary standard for acid-base titrations. It is anhydrous (contains no water of hydration), stable, pure, and has a suitable molar mass.

Sodium hydrogen carbonate ($\text{NaHCO}_3$): Also known as sodium bicarbonate, this compound meets all the criteria for a primary standard and is commonly used in volumetric analysis.

Both substances are ideal because they fulfil all the requirements listed above and are readily available in school laboratories.

Why some substances cannot be primary standards

Sodium hydroxide ($\text{NaOH}$) cannot be used as a primary standard because:

• It absorbs water vapour from the air (hygroscopic)
• It reacts with carbon dioxide in the air to form sodium carbonate
• These changes alter its composition, making accurate concentration determination impossible

Hydrochloric acid ($\text{HCl}$) cannot be used as a primary standard because:

• It is supplied as a solution of unknown exact concentration
• The volatile nature of $\text{HCl}$ gas means the concentration changes over time
• It cannot be obtained in pure solid form that can be accurately weighed

Sodium carbonate decahydrate ($\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$) cannot be used because:

• Despite having a higher molar mass and being cheaper than anhydrous sodium carbonate
• It contains water of hydration which can vary depending on atmospheric humidity
• This violates the requirement that primary standards must be anhydrous

Preparing a primary standard solution

Making a primary standard solution requires careful technique and attention to detail. The following procedure demonstrates how to prepare a primary standard solution of sodium carbonate.

Equipment required

• $250 \text{ mL}$ volumetric flask with lid
• Electronic balance (measuring to $0.001 \text{ g}$)
• Clean, dry $150 \text{ mL}$ beaker
• Spatula
• Approximately $1.5 \text{ g}$ anhydrous sodium carbonate
• Distilled water ($300 \text{ mL}$)
• Wash bottle filled with distilled water
• Filter funnel
• Stirring rod
• Disposable droppers
• Safety glasses

Safety considerations

When working with sodium carbonate, always take appropriate safety precautions. Solid sodium carbonate can cause irritation if it comes into contact with skin or eyes. Always wear safety glasses throughout the experiment and wash your hands thoroughly at the end of the practical work.

Method

The following steps outline the careful procedure needed to prepare an accurate primary standard solution:

1. Prepare the volumetric flask: Rinse the $250 \text{ mL}$ volumetric flask with a small amount of distilled water. This removes any contamination that might affect the concentration of your solution.

2. Weigh the sodium carbonate: Place a clean, dry $150 \text{ mL}$ beaker on the electronic balance and press the tare button to zero the balance. Carefully measure approximately $1.4 \text{ g}$ of anhydrous sodium carbonate into the beaker, recording the mass to the nearest $0.001 \text{ g}$ (three decimal places).

3. Dissolve the sodium carbonate: Add about $80 \text{ mL}$ of distilled water to the beaker. Stir the mixture with a stirring rod until all the sodium carbonate has completely dissolved. This ensures a homogeneous solution.

4. Transfer to volumetric flask: Place the filter funnel in the neck of the volumetric flask. Carefully pour the sodium carbonate solution through the funnel into the flask.

5. Rinse the beaker: This is a crucial step! Pour a small volume (approximately $10-20 \text{ mL}$) of distilled water into the beaker, swirl it around to collect any remaining sodium carbonate solution, and pour this into the volumetric flask. Repeat this rinsing process three times. This ensures that all the weighed sodium carbonate is transferred to the flask.

6. Rinse the funnel: Remove the filter funnel and rinse it by pouring distilled water from the wash bottle through it into the volumetric flask. This ensures no solution is lost on the funnel.

7. Fill to the mark: Remove the filter funnel. Carefully add distilled water to the volumetric flask until the liquid level is close to the calibration mark. For the final adjustment, use a disposable dropper to add water drop by drop until the bottom of the meniscus just touches the calibration line on the flask. This must be done at eye level to avoid parallax error.

8. Mix the solution: Place the lid firmly on the volumetric flask. Hold the lid in place with your finger, then invert the flask and swirl it several times to ensure thorough mixing of the solution.

9. Label and store: Label your solution clearly with its contents, concentration, date, and your name. Store it appropriately for future use in titration experiments.

Calculating concentration

After preparing the solution, you need to calculate its exact concentration. The concentration should be expressed to four significant figures to reflect the accuracy of the electronic balance used.

The calculation follows these steps:

1. Record the accurate mass of sodium carbonate measured (e.g., $1.413 \text{ g}$)
2. Calculate moles using: $n = \frac{m}{M}$Where: $M(\text{Na}_2\text{CO}_3) = (2 \times 23.0) + 12.0 + (3 \times 16.0) = 106.0 \text{ g mol}^{-1}$
3. Calculate concentration using: $c = \frac{n}{V}$Where $V = 0.250 \text{ L}$ (the volume of the volumetric flask)

Why rinsing is essential

Practical tips for accuracy

To ensure your primary standard solution is as accurate as possible:

• Always use a clean, dry beaker for weighing to prevent contamination
• Record all masses to the maximum number of decimal places the balance provides
• Rinse the beaker and funnel thoroughly to transfer all the solute
• Add the final drops of water carefully using a dropper to reach the calibration mark precisely
• View the meniscus at eye level to avoid parallax error
• Mix the solution thoroughly by inverting and swirling multiple times
• Label the solution immediately to prevent confusion

Remember!