Laboratory Safety, Qualitative Analysis and Experimental Technique
Laboratory Safety, Qualitative Analysis and Experimental Technique covers safe handling of reagents, identification of common ions, and the practical skills that exam questions test through described procedures.
Safety essentials
- Never add water to concentrated acid; add acid slowly to water with stirring.
- Waft vapours toward the nose; never point a heated test tube at anyone; use a holder and heat the upper part of the liquid.
- Organic solvents are flammable - no naked flames nearby.
- Spills on skin: rinse with plenty of water and report the incident.
Qualitative tests
- Flame tests: Na yellow, K lilac (view through cobalt glass), Ca brick-red, Ba apple-green, Cu blue-green.
- Halides: acidify with dilute HNO₃, add AgNO₃. AgCl is white and dissolves in dilute ammonia; AgBr is cream and dissolves only in concentrated ammonia; AgI is yellow and insoluble.
- Sulfate: BaCl₂ gives a white precipitate insoluble in dilute HCl. Carbonate: acid gives CO₂, which turns limewater milky.
- NH₄⁺: warm with NaOH; ammonia turns damp red litmus blue. Fe³⁺ with KSCN gives blood-red; Cu²⁺ with excess NH₃ gives a deep blue solution.
Worked example
An unknown solution gives a white precipitate with AgNO₃ after acidification with dilute HNO₃, and the precipitate dissolves completely in dilute ammonia. Silver chloride, bromide and iodide can all appear white to pale yellow in dilute solution, but only AgCl dissolves in dilute NH₃, so the anion is Cl⁻. Acidification with HNO₃ rather than HCl is essential - HCl would itself precipitate silver chloride and give a false positive.
Traps
- Acidifying with HCl or H₂SO₄ before adding AgNO₃.
- Adding BaCl₂ without acidifying, so carbonates and sulfites also precipitate.
- Judging a flame colour with a dirty nichrome wire or the wrong part of the flame.