Here are some common color reactions used in qualitative analysis to identify different ions or compounds:
- Flame Tests (for Metal Ions):
- Sodium (Na⁺): Yellow flame
- Potassium (K⁺): Lilac flame
- Calcium (Ca²⁺): Brick red flame
- Copper (Cu²⁺): Blue-green flame
- Barium (Ba²⁺): Apple green flame
These help identify metal ions based on flame color.
- Hydroxide Precipitation Reactions:
- Fe²⁺ + NaOH → Green precipitate of Fe(OH)₂
- Fe³⁺ + NaOH → Reddish-brown precipitate of Fe(OH)₃
- Cu²⁺ + NaOH → Blue precipitate of Cu(OH)₂
- Zn²⁺ + NaOH → White precipitate (dissolves in excess NaOH)
- Complex Formation Reactions:
- Cu²⁺ + NH₃ (aq) → Deep blue solution (tetraammine copper complex)
- Ni²⁺ + dimethylglyoxime → Red precipitate
- Fe³⁺ + thiocyanate (SCN⁻) → Blood-red complex
- Oxidation-Reduction Color Changes:
- Fe²⁺ → Fe³⁺ changes from pale green to yellow/brown.
- MnO₄⁻ (purple) gets reduced to Mn²⁺ (colorless or pale pink).
- Cr₂O₇²⁻ (orange) changes to Cr³⁺ (green) on reduction.
- Anion Color Tests:
- Cl⁻ + AgNO₃ → White precipitate (AgCl)
- Br⁻ + AgNO₃ → Cream precipitate (AgBr)
- I⁻ + AgNO₃ → Yellow precipitate (AgI)
- CO₃²⁻ + Acid → Effervescence due to CO₂ gas
In short:
Color reactions in qualitative analysis help detect specific ions or compounds by showing distinct color changes, precipitates, or flame colors, which act as visual indicators of the substances present.