Identification of Ions by Chemical and Spectroscopic Means
(chemistry only)

Chemical Analysis

Flame Tests

Flame tests can be used to identify metal ions (cations) in compounds. Steps to produce flame colours include:

  1. wearing eye protection, clean a wire loop by dipping in hydrochloric acid, and then holding in a blue flame for a few seconds
  2. dip the clean wire loop into a solid sample of the compound being tested, and then hold in a blue flame
  3. observe and record the flame colour produced

You need to know the colour of :​

  • lithium ions, Li+ (crimson red)
  • sodium ions, Na+ (yellow)
  • potassium ion, K+ (lilac)
  • calcium ions, Ca2+ (orange-red)
  • copper ions, Cu2+ (green)

If a sample containing a mixture of ions is used, some flame colours can be masked.

Flame Tests

Testing for Cations

Compounds that contain transition metals are often coloured. If we react two chemicals together and a new solid is made (one that doesn't dissolve in the solution) we call it a precipitation reaction.

Using sodium hydroxide

Dilute sodium hydroxide solution (NaOH) is used to test for some metal ions and can also be used to identify ammonium ions.

ion colour of precipitate
aluminium, Al3+
white
adding excess NaOH will cause the precipitate to dissolve
calcium, Ca2+
white
adding excess NaOH does not change the precipitate
magnesium, Mg2+
white
adding excess NaOH does not change the precipitate
copper(II), Cu2+
blue
iron(II), Fe2+
green
iron(III), Fe3+
brown
ammonium, NH4+
when warmed will release ammonia gas

Testing for Cations

Testing for ammonia

Holding a piece of damp, red litmus paper over ammonia gas will turn the litmus paper blue.

Testing for Anions

Testing for carbonate ions

Carbonate ions, CO32- are detected using any dilute acid (e.g. hydrochloric acid). Bubbles of carbon dioxide gas will be given off when an acid is added.

We can pass the gas through limewater and if it turns the limewater cloudy it will confirm the gas is carbon dioxide.

Testing for sulfate ions

Sulfate ions (SO42-) can be tested for using barium chloride. Barium sulfate will form, which is an insoluble white precipitate.

To test for sulfate ions in solution:

  1. add a few drops of dilute hydrochloric acid to the sample
  2. add a few drops of dilute barium chloride solution

We add acid first, because carbonate ions will also produce a white precipitate with barium chloride solution. The acid is added so it can react with any carbonate ions that may be present, and so removing them, so they can't give a false result for sulfate ions.

Testing for halide ions

Silver ions will react with halide ions (Cl⁻, Br⁻ or I⁻) to form insoluble precipitates.

To test for halide ions in solution:

  1. add a few drops of dilute nitric acid to the sample
  2. add a few drops of dilute silver nitrate solution
  3. observe and record the colour of any precipitate that forms

We add acid first, because carbonate ions will also produce a white precipitate with silver nitrate solution. The acid is added so it can react with any carbonate ions that may be present, and so removing them, so they can't give a false result for chloride ions.

Testing for Anions

Silver halide precipitates

ion colour of precipitate
chloride, Cl⁻
white
bromide, Br⁻
cream
iodide, I⁻
yellow

Instrumental Methods of Analysis

New technology, developed over the last few decades, have considerably advanced analytical chemistry. These new instrumental methods have three main advantages, they are:

  • more rapid (produce results faster)
  • more accurate (they are more reliable at identifying elements/compounds)
  • more sensitive (they can detect very small amounts of substances)

Each method works in a different way, but they essentially have most features in common:

  1. a stimulus enters the sample (light, heat, current, voltage etc)
  2. this is then read by a detector, producing a signal
  3. the signal is amplified/digitalised
  4. the output is plotted (usually) on a graph

Instrumental Analysis

Flame Emission Spectroscopy

Flame photometers (often referred to as a flame emission spectroscope) is an instrumental method used to analyse metal ions. 

  1. a sample is heated in a flame, and the light emitted from the flame is passed through a spectroscope
  2. the resulting pattern is known as a spectrum

Every element has its own specific set of wavelengths, so every pattern is unique (can be thought of as an element's 'fingerprint').​

If there is a mixture of ions, one colour may be covered up by another in a flame test. In an emission spectrum, the intensity of the light is actually a measure of concentration of the different elements. This means that the ratio of ions present can be determined.

Determining concentrations

A reading can be taken from the flame photometer for different concentrations of a metal ion in solution. These readings are then used to plot a calibration curve.

Having a calibration curve of known samples, allows scientists to be able to identify the concentration of a sample just by looking at the graph!

For example, if a solution of sodium ions gave a reading of 5 units on the flame photometer, then the calibration curve allows us to read off that the sample had a concentration of 0.025 g/dm3.

Flame Photometer

Calibration Curve