8.3 identification of ions by chemical and spectroscopic means

testing for cations (+ ions)

• testing for metal ions

• testing for ammonium ions (NH₄)

testing for anions (- ions)

• testing for carbonate ions

• testing for halide ions

• testing for sulfate ions

purpose of flame tests

to identify certain metal ions (cations ‘+’) which produce distinctive colours in flame test

how to carry out a flame test

• get a nichrome wire mounted with a handle

• clean it using dilute hydrochloric acid

• place a small amount of the chemical onto the wire

• place the end of this into a blue Bunsen burner flame

• observe the colour of the flame to identify the metal ion present

how to test for lithium (Li⁺) ions

• use a flame test

• produces a crimson flame

how to test for sodium (Na⁺) ions

• use a flame test

• produces a yellow flame

how to test for potassium (K⁺) ions

• use a flame test

• produces a lilac flame

how to test for calcium (Ca²⁺) ions

• use a flame test

• produces a brick-red (orange-red) flame

how to test for copper (Cu²⁺) ions

• use a flame test

• produces a blue-green flame

how to test for barium (Ba²⁺) ions

• use a flame test

• produces a green flame

flame test advantages

• basic

• cheap

• easy and simple to carry out

• quick

• repeatable

• colours can be easily compared side by side

flame test disadvantages

• if a sample contains a mixture of (metal) ions, some flame colours can mask the colours of other ions present

• the colours can be difficult to distinguish if there is a low concentration of the metal compound

• the test cannot differentiate between all elements

• the test is subjective

how to test for magnesium (Mg²⁺) ions

• add sodium hydroxide solution (NaOH)

• produces a white precipitate

how to test for calcium (Ca²⁺) ions

• add sodium hydroxide solution (NaOH)

• produces a white precipitate

• produces a brick-red flame in a flame test

how to test for aluminium (Al²⁺) ions

• add sodium hydroxide solution (NaOH)

• produces a white precipitate

• the precipitate redissolves in excess sodium hydroxide

how to test for copper II (Cu²⁺) ions

• add sodium hydroxide solution (NaOH)

• produces a pale blue precipitate

how to test for iron II (Fe²⁺) ions

• add sodium hydroxide solution (NaOH)

• produces a dark green precipitate

how to test for iron III (Fe³⁺) ions

• add sodium hydroxide solution (NaOH)

• produces a brown precipitate

what precipitates are formed when sodium hydroxide is added to metal?

insoluble metal hydroxides

how to test for ammonium (NH₄⁺) ions

• add sodium hydroxide solution (NaOH)

• warm the mixture with a Bunsen burner

• the gas given off turns damp red litmus paper blue

how to test for carbonate (CO₃^-) ions

• add dilute hydrochloric acid (HCl)

• reaction creates efferverscence or ‘fizzing’

• test that the gas produced from the reaction is carbon dioxide by bubbling through limewater

• limewater should turn cloudy/milky

how to test for chloride (Cl^-) ions

• halide test

• add dilute nitric acid (HNO₃)

• add silver nitrate solution (AgNO₃)

• produces a white precipitate

how to test for bromide (Br^-) ions

• halide test

• add dilute nitric acid (HNO₃)

• add silver nitrate solution (AgNO₃)

• produces a cream precipitate

how to test for iodide (I^-) ions

• halide test

• add dilute nitric acid (HNO₃)

• add silver nitrate solution (AgNO₃)

• produces a yellow precipitate

how to test for sulfate (SO₄^2-) ions

• add dilute hydrochloric acid (HCl)

• add barium chloride solution (BaCl₂)

• produces a white precipitate

instrumental methods

using machines to detect and identify elements and compounds

advantageous compared to chemical tests

examples of instrumental methods

• flame emission spectroscopy

• mass spectroscopy

• gas-liquid chromotagraphy

flame emission spectroscopy

an instrumental method used to identify metal ions in solutions, and also allows scientists to overcome issues with flame tests

what happens in flame emission spectroscopy

1. A sample of the metal ion is placed onto a flame.

2. As the ions heat up, they drop back to their original energy levels and transfer energy as light.

3. The light given out is then passed through a spectroscope.

4. The spectroscope measures the wavelength of the light and convert the light into a line spectrum.

5. The positions of the lines in the spectrum are specific for a given metal ion, which helps to identify it.

what else does flame emission spectroscopy tell you about metal ions?

the lines on the line spectrum become more intense at a higher concentration, so you can measure the concentration of the metal ion

the advantages of flame emission spectroscopy over to flame tests

• more accurate - wavelengths are specific to metal ions, you you’re more likely to identify them correctly

• more sensitive - will work even on very small amounts of a substance in a small amount of sample

• more rapid - quicker than flame tests

• versalite - can analyse a wide range of samples, and if a sample contains multiple different metal ions, the spectrum will show the lines of all of them

• easy/simple to use -

flame emission spectroscopy disadvantages

• anions and a number of cations (metal ions) cannot be analysed by this method

• reliant on a well-constructed flame

• sensitive to interference from other elements

• doesn’t provide information about the quantity of an element in a sample

another name for flame emission spectroscopy

flame photometry