8.3 Identification of ions by chemical and spectroscopic means
8.3.1 Flame Tests
- Flame tests can be used to identify some metal ions (cations).
- If a sample containing a mixture of ions is used, some flame colours
can be masked.
| Metal ion | Flame colour |
|---|---|
| Calcium | Orange-red |
| Potassium | Lilac |
| Sodium | Yellow |
| Copper | Green |
| Lithium | Crimson |
8.3.2 Metal Hydroxides
- Sodium hydroxide solution can be used to identify some cations.
| Metal Cation | Effect of adding NaOH | Ionic equation |
|---|---|---|
| Al3+ | White precipitate formed, but when adding more NaOH it dissolves (we found this out the hard way lmao) | Al3+(aq) + 3OH-(aq) -> Al(OH)3(s) |
| Ca2+ | White precipitate formed | Ca2+(aq) + 2OH-(aq) -> Ca(OH)2(s) |
| Mg2+ | White precipitate formed | Mg2+(aq) + 2OH-(aq) -> Mg(OH)2(s) |
| Cu2+ | Light blue precipitate formed | Cu2+(aq) + 2OH-(aq) -> Cu(OH)2(s) |
| Fe2+ | Green precipitate formed | Fe2+(aq) + 2OH-(aq) -> Fe(OH)2(s) |
| Fe3+ | Red-brown precipitate formed | Fe3+(aq) + 3OH-(aq) -> Fe(OH)3(s) |
8.3.3 Carbonates & 8.3.4 Halides & 8.3.5 Sulfates
| Test | Anion | Observations |
|---|---|---|
| Add acid e.g. HCl, HNO3, or H2SO4 | Carbonate (CO32-) | Fizzing, gas turns limewater cloudy |
| Add HNO3, then AgNO3 | Chloride (Cl-) | White precipitate formed |
| Bromide (Br-) | Cream precipitate formed | |
| Iodide (I-) | Yellow precipitate formed | |
| Add Hcl then BaCl2 | Sulfate (SO42-) | White precipitate formed |
Required Practical 7 Analysis of Ionic Compounds
- To use chemical tests to identify the ions in unknown single ionic compounds.
- The salt can be identified by analysis of the ions, which can be tested for using
the chemical tests above.
Required Equipment/Materials
For Flame Tests
- Nichrome wire loop
- Bunsen burner
- Hydrochloric acid (dilute)
- Test compounds (e.g. lithium, sodium, potassium, calcium, copper salts)
For Sodium Hydroxide Tests (Cations)
- Sodium hydroxide solution
- Test tubes
- Pipettes
- Solutions of:
- Copper(II) sulfate
- Iron(II) sulfate
- Iron(III) chloride
- Calcium nitrate
- Aluminium nitrate
- Magnesium nitrate
For Anion Tests
- Carbonates::
Dilute hydrochloric acid
Limewater - Halides:
Nitric acid (dilute)
Silver nitrate solution - Sulfates:
Hydrochloric acid (dilute)
Barium chloride solution
General Equipment
- Test tube rack
- Beakers
- Stirring rod
- Safety goggles
- Lab coat
- Dropper bottles
Methods
Flame Tests (for metal ions)
- Clean a nichrome wire loop by dipping it in HCl and holding it in a flame until no colour shows.
- Dip the loop into the sample and place it in the edge of a blue Bunsen flame.
- Identify the colour of the flame.
- See table above to identify the ion.
Sodium Hydroxide Precipitation Tests (for cations)
- Add a few drops of metal ion solution to a test tube.
- Add sodium hydroxide solution dropwise.
- Once precipitate starts to form, check the colour against this table.
Carbonate Test
- Add dilute HCl to the sample.
- If fizzing occurs, bubble the gas through limewater.
- If limewater turns cloudy, CO₂ is present → carbonate confirmed.
Halide Test
- Add dilute nitric acid to the sample.
- Add silver nitrate solution.
- Observe precipitate:
- Chloride: white
- Bromide: cream
- Iodide: yellow
Sulfate Test
- Add dilute HCl to the sample.
- Add barium chloride solution.
- A white precipitate indicates a sulfate ion.
Identifying the Unknown Salts
- To identify an unknown ionic salt, follow these steps in order:
- Perform a Flame Test: If the salt contains a metal cation that gives a flame colour, identify it using the flame test first. This provides a quick clue about possible metal ions (e.g., lithium, sodium, potassium, calcium, copper).
- Carry Out Sodium Hydroxide Precipitation Tests: If no clear flame colour is observed or to confirm the metal ion, add sodium hydroxide dropwise to a solution of the salt. Note the colour of any precipitate formed and compare with known results.
- Test for Carbonates: Add dilute HCl to the salt. Effervescence (fizzing) indicates the presence of carbonate ions. Confirm by bubbling the gas into limewater; cloudiness confirms CO₂.
- Test for Halides: If no carbonate is detected, add dilute nitric acid followed by silver nitrate solution. Observe the colour of any precipitate to identify chloride, bromide, or iodide ions.
- Test for Sulfates: If halide tests are negative, add dilute HCl followed by barium chloride solution. A white precipitate indicates the presence of sulfate ions.
8.3.6 Instrumental Methods
- Elements and compounds can be identified by using instrumental methods as well
as chemical ones.
- Instrumental methods are accurate, sensitive, and rapid.
8.3.7 Flame emission spectroscopy
- Flame emission spectroscopy is an instrumental method used to analyse metal ions
in solutions.
- The sample is put into a flame and the light given out is passed
through a spectroscope.
- The output is a line spectrum that can be analysed to identify the metal ions
in the solution and measure their concentration.
source