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Identification of ions by chemical and spectroscopic means (chemistry only)

8.3.1 Flame tests

Positive Ion (Cation) Test Positive Result
Lithium, Li+ Flame Test

  • Clean wire loop by dripping it in HCl
  • Place loop in solid of dry sample A
  • Place loop in blue Bunsen burner
 Crimson Red
Sodium, Na+ Yellow
Potassium , K+ Lilac
Calcium, Ca2+ Orange-Red
Copper, Cu2+ Green

Why should wire be clean when used for a flame test? (1)

  • To prevent contamination that gives an accurate result

What are the properties that wire must have for a flame test? (2)

  • High melting point
  • Unreactive

Why is it not possible to tell from the flame test that both ions are present in low concentration? (1)

Colours mask each other

8.3.2 Metal hydroxides

Positive Ion (Cation) Test Positive Result
Zinc, Zn2+

Precipitation

  • NaOH is added dropwise at first

A precipitate will appear depending on the ion in the solution

 White
Calcium, Ca2+ White
Aluminium, Al3+ White, redissolves in excess NaOH
Copper (II), Cu2+ Blue
Iron (II), Fe2+ Green
Iron (III), Fe3+ Brown
Magnesium, Mg2+ White

8.3.3 Carbonates

8.3.4 Halides

8.3.5 Sulfates

Negative Ion (Anion) Test Positive Result
Carbonate, CO32-
  • Dissolve a fresh sample A in water to make a solution
  • Add dilute acid
 Bubbles of CO2 gas will produce → turn limewater milky
Chloride, Cl-

Precipitation

  • Dissolve a fresh sample A in water to make a solution
  • Add AgNO3 and dilute HNO3
 White precipitate
Bromide, Br- Cream precipitate
Iodide, I- Yellow precipitate
Sulfate, SO42-
  • Dissolve a fresh sample A in water to make a solution

Add BaCl2 and dilute HCl

 White precipitate

8.3.6 Instrumental methods

State the adv of using instrumental methods compared with chemical tests (1)

  • Accurate (machines can be more accurate than humans)
  • Sensitive (only a small amount of sample is needed)
  • Rapid (the information can be found very quickly)

8.3.7 Flame emission spectroscopy

Name one instrumental method used to identify elements or compounds (1)

  • Atomic absorption (/emission) spectroscopy

Why would use atomic absorption spectroscopy instead of flame test?

  • In flame test, 2 different colours mask each other (esp in low concentration)
  • AES separates out individual colours more accurately
  • Quicker
  • Use a small amount of sample
  • More sensitive / accurate / precise

Flame emission spectroscopy

  1. A sample of metal ion in solution is placed into a flame
  2. The light given out is then passed through a spectroscope
  3. Spectroscope converts light into a line spectrum
  4. Positions of lines in spectrum are specific for a given metal ion so can be analysed to identify metal ions in solution & measure their concentration coz lines become more intense at higher concentration

Higher con of ion, greater intensity of light given out