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Topic 8 Chemical Analysis By Bethan Poole (8.3 Identification of Ions by…
Topic 8 Chemical Analysis
By Bethan Poole
8.1 Purity, Formulation and Chromatography
8.1.2 Formulations
Mixtures that have been designed to have specific properties
Many are complex mixtures in which each chemical has a particular purpose
Components of a formulation are carefully controlled
e.g
- Paint
A Binder
A Solvent
Pigments
8.1.3 Chromatography
Used to separate and identify a mixture of liquids
Paper chromatography used to separate dyes, inks or food colourings
2 stages
A stationary phase
in paper chromatography this is the paper
The thing that doesn't move
A mobile phase
usually the solvent
The thing that moves
Method
The solvent dissolves the samples and carries them up the paper
Each component moves a different distance up the paper depending on its attraction for the paper and for the solvent
Can be used to identify artificial colours by comparing them to the results obtained from known substances
Rf values can be used to identify the components in a mixture
Rf= distance moved by the substance/ distance moved by solvent
Different components have different Rf values
Providing the same temperature and solvent are used the Rf value is constant for different components and can be used to identify the component
8.1.1 Pure Substances
You can use
melting
and
boiling
points to identify a pure substance
Melting and boiling point of an element or compound are called it's sharp points
A pure substance is a single element or compound, not mixed with any other compound
Pure substances melt and boil at a
specific temperature
Impure substances are mixtures
Melt and boil over a range of temperatures
Impurities lower melting pot and increase boiling point
8.2 Identification of common gases
8.2.2 Test for
Oxygen
To make
electrolysis of a substance without oxygen
e.g.
electrolysis of water
To test
if you blow out a lit splint and place it in the test-tube, it will relight if oxygen is present
8.2.3 Test for
Carbon Dioxide
To make
metal carbonate + acid -> salt + water + carbon dioxide
e.g.
copper carbonate + hydrochloric acid -> copper chloride + water + carbon dioxide
To test
Lime water test
- when a delivery tube is put in the boiling tube the lime water will go cloudy if Carbon Dioxide is present
8.2.1 Test for
Hydrogen
To make
e.g.
magnesium + hydrochloric acid -> magnesium chloride + hydrogen
metal + acid -> salt + hydrogen
To test
Squeaky pop test
- if hydrogen is present, when a lit splint is held over the test tube a squeaky pop will be produced
8.2.4 Test for
Chlorine
To make
Electrolysis of salt water- chlorine will form at the anode
To test
Litmus Paper
- Damp blue litmus paper will turn red and then bleach in the presence of chlorine
8.3 Identification of Ions by chemical and spectroscopic means (chemistry only)
8.3.4 Halides
Solutions of halide ions react with silver nitrate solution in the presence of dilute nitric acid to produce silver halide precipitates
Silver chloride is
WHITE
Silver iodide is
YELLOW
Siver bromide is
CREAM
8.3.5 Sulfates
If dilute hydrochloric acid and barium chloride solution are added to a solution containing sulphate ions, a white precipitate of barium sulphate is formed
8.3.3 Carbonates
React with dilute acids to form carbon dioxide gas. Carbon dioxide can be identified with limewater
8.3.6 Instrumental methods
Development
Aided by the rapid progress in technologies like electronics and computing
Modern machines will do the analysis and interpret the results
Application
Detection of alcohol, drugs or metal ions in blood
Detect impurities products that need to be pure, i.e. drugs molecules and alloys
Detect pollutants in soil water and air
Advantages
Can detect extremely small quantities; such as one part in a billion
Very accurate and quick
Disadvantages
Usually need a spectra of known compounds to compare unknown results
Machines can be expensive and only suitable for particular tests
8.3.2 Metal hydroxides
Can be used to identify some metal ions (Cations)
Some of these for precipitates when sodium hydroxide is added to them
Precipitates formed
Metal Ion
- Magnesium , Mg2+ (aq),
Precipitate formed
-Magnesium hydroxide,
Precipitate Colour
- White
Metal Ion
- copper(II), Cu2+ (aq),
Precipitate formed
-Copper(II) hydroxide,
Precipitate Colour
- Blue
Metal Ion
- Iron(II), Fe2+ (aq),
Precipitate formed
-Iron(II) hydroxide,
Precipitate Colour
- Green
Metal Ion
- Iron(III), Fe3+ (aq),
Precipitate formed
-Iron(III) hydroxide,
Precipitate Colour
- Brown
Metal Ion
- Calcium, Ca2+ (aq),
Precipitate formed
-Calcium hydroxide,
Precipitate Colour
- White
Metal Ion
- Aluminium Al3+ (aq),
Precipitate formed
-Aluminium hydroxide,
Precipitate Colour
- White
8.3.7 Flame emission spectrosopy
Example of an instrumental method- used to analyse solutions containing metal ions
Method
Sample of a metal solution is placed in a flame and the light emitted is passed through a spectroscope
Produces a line spectrum which can be used to:
identify the metal ions in the solution
measure the concentration of the metal ions
8.3.1 Flame tests
Can be used to identify some metal ions (cations)
Colours
Potassium
-
Lilac
Calcium
-
Brick Red
Sodium
-
Yellow
Copper
-
Green
Litium
-
Crimson Red
If a sample contains a mixture of ions some flame colours can be masked