A standard solution is a solution of known concentration used in analytical techniques like titrations and spectrophotometry to determine unknown concentrations and ensure accuracy.
Primary Standards:
- Primary standards are high-purity substances used to prepare standard solutions with accurate concentrations.
- Key characteristics of primary standards include:
- High purity: Typically at least 99.9
- Stability: Chemically stable and not reactive with air or moisture.
- Absence of hydrates: Should not form hydrates or lose water.
- Solubility: Must be soluble in the solvent used for analysis.
- Availability: Readily available and affordable.
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Examples:
- Anhydrous sodium carbonate (Na₂CO₃) for acid-base titrations
- Potassium dichromate (K₂Cr₂O₇) for redox titrations
- Anhydrous sodium chloride (NaCl) for argentometric titrations
Secondary Standards:
- Secondary standards are substances whose concentration is determined by comparison with primary standards.
- They are typically less pure and stable than primary standards but are more cost-effective and easier to handle.
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- Preparation: A known mass of secondary standard is dissolved in a solvent and standardized against a primary standard.
Examples:
- Hydrochloric acid (HCl)
- Sulfuric acid (H₂SO₄)
These are secondary standards for acid-base titrations but must be standardized using a primary standard like sodium carbonate.
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