A standard solution is a solution of known concentration used in analytical techniques like titrations and spectrophotometry to determine unknown concentrations and ensure accuracy.
1. 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% pure.
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.
Examples:
Anhydrous sodium carbonate (Na₂CO₃) for acid-base titrations
Potassium dichromate (K₂Cr₂O₇) for redox titrations
Anhydrous sodium chloride (NaCl) for argentometric titrations
2. 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.
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.