9.1 Qualitative analysis Flashcards
Flame tests
Metal ions produce a colour if heated strongly in a flame.
Ions from different metals produce different colours.
The flame test is thus used to identify metal ions by the colour of the flame they produce.
Flame test for Lithium, sodium, potassium, calcium and copper
lithium ion, Li+ (red)
sodium ion, Na+ (yellow)
potassium ion, K+ (lilac)
calcium ion, Ca2+ (orange-red)
copper ion, Cu2+ (blue-green)
Test for cations
Metal cations in aqueous solution can be identified by the colour of the precipitate they form on addition of sodium hydroxide.
If only a small amount of NaOH is used then normally the metal hydroxide precipitates.
In excess NaOH some of the precipitates may dissolve.
For this reason just a few drops of NaOH are added at first and very slowly
If it is added too quickly and the precipitate is soluble in excess, then you run the risk of missing the formation of the initial precipitate which dissolves as quickly as it forms if excess solution is added.
A small amount is thus added, very gradually and any colour changes or precipitates formed are noted.
Most transition metals produce hydroxides with distinctive colours.
Flame test procedure
Dip the loop of an unreactive metal wire such as nichrome or platinum in dilute acid, and then hold it in the blue flame of a Bunsen burner until there is no colour change.
This cleans the wire loop and avoids contamination.
This is an important step as the test will only work if there is just one type of ion present.
Two or more ions means the colours will mix, making identification erroneous
Dip the loop into the solid sample and place it in the edge of the blue Bunsen flame.
Avoid letting the wire get so hot that it glows red otherwise this can be confused with a flame colour.
Test for aluminium, calcium, copper, iron
Aluminium - White precipitate, dissolves in excess NaOH to form a colourless solution.
Calcium - White precipitate, insoluble so remains in excess NaOH.
Copper (Il) - Light blue precipitate, insoluble in excess NaOH
Iron (Il) - Green precipitate, insoluble in excess NaOH
Iron (III) - Red-brown precipitate, insoluble in excess NaOH
Test for ammonium
Ammonium ions are also tested with sodium hydroxide, but not by a precipitation reaction.
To test for the ammonium ion, gentle heating is required after adding the NaOH solution.
If the ammonium ion is present, ammonia gas is produced which can be tested with damp red litmus paper turning blue.
Test for carbonate
Add dilute acid and test the gas released.
Effervescence should be seen and the gas produced is CO2 which forms a white precipitate of calcium carbonate when bubbled through limewater.
Test for sulphate
Acidify with dilute hydrochloric acid and add aqueous barium chloride.
A white precipitate of barium sulphate is formed.
Test for halide
Acidify with dilute nitric acid (HNO3) followed by the addition of silver nitrate solution (AgNO3).
This forms a silver halide precipitate.
Depending on the halide present, a different coloured precipitate is formed, allowing for identification of the halide ion.
Silver chloride is white, silver bromide is cream and silver iodide is yellow.
Advantages of instrumental technology
Advancements in technology and computing have allowed for the development of instruments designed to analyse chemical substances.
Methods of analysis include X-ray, Infra-Red and Mass Spectroscopy, Gas Chromatography and Flame Photometry.
These analytical techniques require modern day instruments which are a vital part of chemistry laboratories.
The advantage of using these instruments over more traditional methods include:
They provide greater accuracy.
They are faster and easier to use.
They are automated and can perform multiple simultaneous sampling and testing.
Modern instruments are very sensitive and can work with very small sample sizes.
Flame photometry
This technique is used to analyse metal ions in solution.
When substances are heated they often emit energy in the form of light
This is due to electrons falling back to their original energy levels after becoming energised which causes them to jump up one or more energy levels.
Flame emission spectroscopy works by exposing the sample to a very hot flame and then measuring the intensity and wavelength of the light emitted.
The output is an emission spectrum in which different elements produce lines in different parts of the spectrum.
The emission spectrum consists of brightly coloured thin lines on a dark background and each element ion produces a unique spectrum.
Flame emission spectroscopy also works for mixtures of ions.
This is a major advantage over flame testing which can only analyze one ion at a time.
The intensity of the light produced is proportional to the number of ions vaporised, so the technique can be used to determine the concentration of metal ions in a solution by reference to a standard solution of known concentration.
Reference data
Ions in unknown samples can be identified by comparing the sample spectrum to reference spectra.
This is particularly useful if the sample contains a number of different ions.
Practical 8 (identifying ions)
Aim: To use chemical tests to identify the ions in unknown binary ionic compounds.
Procedure - There are a number of strategies you could choose in order to identify the ions in unknown salts.
Common analysis strategies include flame tests, and tests for sulphate, carbonate and halide ions.
They can be carried out in any particular order, and you will probably not need to carry them all out on any one sample
Only small amounts of each sample and reagent are needed.
You may need to dissolve a sample of salt in a little distilled water if the salt you are given is in the solid state.
Record your observations carefully in a table of results as you work through the tests.
Repeat any tests that do not provide a clear result i.e. a colour change that was difficult to identify.
