Laboratory Techniques, Qualitative and Quantitative Analysis
What this domain tests
Practical technique, qualitative identification of ions and gases, purification, and interpreting spectra.
Core rules
- Titration: rinse the burette with the solution it will contain and the pipette with the solution being measured; the conical flask is rinsed with distilled water. Read the bottom of the meniscus at eye level and use concordant titres within 0.10 cm3.
- Flame tests: Li crimson, Na yellow-orange, K lilac (through cobalt glass), Ca brick-red, Cu blue-green.
- Cations with NaOH: Cu2+ blue precipitate, Fe2+ green, Fe3+ red-brown, Zn2+ and Al3+ white, but only Zn2+ dissolves in excess NaOH.
- Anions: carbonate gives CO2 with acid; sulfate gives a white precipitate with BaCl2 that is insoluble in acid; halides give precipitates with AgNO3 (AgCl white, AgBr cream, AgI yellow); nitrate gives ammonia with NaOH and aluminium.
- IR identifies functional groups; mass spectrometry gives Mr from the molecular ion and structural clues from fragmentation.
Worked example
A white solid gives a yellow-orange flame and effervesces with dilute hydrochloric acid, the gas turning limewater milky. The flame identifies Na+ and the gas identifies CO32−, so a likely compound is Na2CO3. Confirm by adding BaCl2 solution: a white precipitate of BaCO3 forms.
Traps
Averaging non-concordant titres or including the rough titre in the mean; rinsing the pipette with distilled water rather than the solution; confusing the AgBr and AgI colours; reading a meniscus from above; assuming a white precipitate with BaCl2 must be sulfate without checking its insolubility in acid.