Mechanism: Adding aqueous sodium hydroxide to a solution containing metal ions often results in the formation of an insoluble metal hydroxide precipitate. The color and solubility of this precipitate are used for identification.
Transition Metal Results: Copper(II) forms a blue precipitate, Iron(II) forms a sludgy green precipitate, and Iron(III) forms a reddish-brown precipitate.
White Precipitates: Three common ions—Aluminum (), Zinc (), and Calcium ()—all form white precipitates initially. To distinguish them, excess is added; the precipitates of and will redissolve to form colorless solutions, while remains as a solid.
Distinguishing Zinc and Aluminum: Since both and dissolve in excess , aqueous ammonia is used as a secondary test. Only the Zinc precipitate will redissolve in excess ammonia, while the Aluminum precipitate remains insoluble.
Copper(II) Confirmation: When ammonia is added to ions, a light blue precipitate forms initially. Upon adding excess ammonia, this precipitate redissolves to form a distinctive deep blue (royal blue) solution due to the formation of a complex ion.
Quantitative Capability: Unlike manual flame tests, flame photometry measures the intensity of the light emitted. This intensity is directly proportional to the concentration of the metal ion in the sample.
Multi-Ion Detection: Because every ion has a unique emission spectrum (like a barcode), flame photometry can identify multiple different metal ions present in a single mixture, which is difficult to do by eye.
Accuracy and Sensitivity: Instrumental methods are far more sensitive than human observation, allowing for the detection of trace amounts of metals and providing highly accurate numerical data.
| Test Method | Primary Use | Key Limitation |
|---|---|---|
| Flame Test | Rapid identification of Group 1 and 2 metals | Subjective; mixtures mask colors |
| NaOH Test | Identifying transition metals and white precipitates | Requires multiple steps for white precipitates |
| Ammonia Test | Distinguishing from | Specific to certain ions |
| Photometry | Precise concentration and mixture analysis | Requires expensive equipment |
Cleaning is Critical: Always state that the wire must be cleaned with concentrated between tests. Contamination from previous samples (especially sodium, which has a very intense yellow flame) can easily mask other results.
Precise Color Terminology: Use specific descriptors like 'lilac' for Potassium or 'reddish-brown' for Iron(III). Simply saying 'purple' or 'brown' may not meet the marking criteria in many chemistry specifications.
The 'Excess' Step: When describing precipitation tests, always specify the observation for both 'a few drops' and 'in excess.' Forgetting to mention that a precipitate redissolves is a frequent cause of lost marks.
