Standard Solutions (Leaving Cert Chemistry): Revision Notes
Standard Solutions
What are standard solutions?
When you're working in the chemistry laboratory, you often need solutions with precisely known concentrations. These are called standard solutions - essentially solutions where you know exactly how much dissolved substance is present in a given volume.
A standard solution contains a precisely measured amount of solute dissolved in a suitable solvent (usually deionised water) to create a specific, accurately known volume. The key word here is "accurately" - we need to know the concentration to a high degree of precision for analytical work.
Why do we need standard solutions?
Standard solutions are essential for volumetric analysis, particularly in acid-base titrations. They allow us to:
- Determine unknown concentrations of other solutions
- Carry out quantitative chemical analysis
- Ensure accuracy and reliability in experimental results
The concentration can be determined in two main ways:
- Preparing the solution directly by dissolving a known mass of pure substance
- Finding the concentration through analysis after preparation
Primary standards
Not all substances can be used to make reliable standard solutions directly. Some chemicals absorb moisture from air, others lose water over time, and some aren't pure enough. This is where primary standards come in.
A primary standard is a substance that can be obtained in a stable, pure and soluble solid form, allowing it to be weighed accurately and dissolved in water to give a solution of precisely known concentration.
The Six Essential Criteria for Primary Standards
A substance must meet ALL of these criteria to be classified as a primary standard:
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High purity: The substance must be available in a highly pure state to ensure accurate calculations.
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Stability in air: It shouldn't lose water of crystallisation to the air (efflorescence) or absorb water from the air (be deliquescent or hygroscopic).
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Water solubility: It must dissolve easily in water to create the standard solution.
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High molecular mass: A fairly high relative molecular mass ensures that weighing errors have minimal impact on precision.
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Complete reaction: When used in volumetric analysis, it should undergo complete and rapid reactions.
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Anhydrous nature: Preferably, it should contain no water of crystallisation, making calculations more straightforward.
Substances that cannot be primary standards
Many common laboratory chemicals fail to meet the strict criteria for primary standards. Here's why some familiar substances aren't suitable:
Sulphuric acid: This is hygroscopic, meaning it absorbs water vapour from the air, making its concentration unpredictable.
Hydrochloric acid: As a gas at room temperature, it's difficult to weigh accurately and handle safely.
Sodium hydroxide: This substance is deliquescent - it absorbs so much water from air that it forms a solution, making accurate weighing impossible.
Hydrated sodium carbonate: It's efflorescent, losing water of crystallisation to the air and changing its mass over time.
Iodine: This sublimes at close to room temperature, meaning it changes directly from solid to gas.
Potassium permanganate: It's not available in a pure state and isn't stable, with solutions deteriorating when exposed to light.
Making standard solutions in practice
Only a small number of substances actually satisfy all the criteria for primary standards. Some examples include anhydrous sodium carbonate, sodium chloride, and potassium hydrogen phthalate. These substances are widely used in industrial and research settings because they allow chemists to prepare solutions with precisely known concentrations.
Standard Solution Preparation Process
When preparing a standard solution, you must follow these essential steps:
- Use a highly pure primary standard
- Weigh the substance accurately using an analytical balance
- Dissolve it completely in deionised water
- Transfer to a volumetric flask and make up to the exact volume
- Mix thoroughly to ensure uniform concentration
Key Points to Remember:
- Standard solutions have accurately known concentrations and are essential for quantitative analysis
- Primary standards must meet six strict criteria including high purity, stability in air, and complete water solubility
- Many common chemicals like sulphuric acid and sodium hydroxide cannot be used as primary standards due to properties like hygroscopic or deliquescent behaviour
- Only substances that are stable, pure, and reliably weighed can serve as primary standards
- Primary standards enable accurate preparation of solutions for volumetric analysis and titrations