25 - Qualitative Analysis: Tests for Ions Flashcards
Why is it important that the test for any ion is unique?
The results of a test must let you determine which ion is present, rather than being uncertain about which ion it is.
How are flame tests carried out?
1) Light a Bunsen burner (to blue flame)
2) Pick up small sample of test using a wire loop
3)Hold sample above Bunsen burner and observe flame colour
What substance is the wire loop cleaned in before testing each sample?
Hydrochloric acid
What colour does Lithium make?
Red
What colour does Sodium make?
Yellow
What colour does Potassium make?
Lilac
What colour does Calcium make?
Orange-red
What colour does Copper make?
Blue-green
What does the flame photometer measure?
The light intensity of the flame colours produces by metal ions
What can data from a flame photometer be used for?
- To identify metal ions in a sample
- Determine the concentration of metal ions in dilute solutions
How do we identify which metal ion is present in an emission spectrum?
Each metal ion produces a unique emission spectrum
What is an emission spectrum?
Light given off by a substance, split into its component colours or wavelengths
The metal present in a sample is identified by comparing its
spectrum with reference spectra
What is a calibration curve?
Graph with the readings from a machine plotted against known amounts of a substance
How do we determine concentration when using a flame photometer to test for an metal ion?
A reading is taken from the flame photometer for different concentrations of a metal ion in solution.
Readings are used to plot a calibration curve
Why might we use instrumental methods of analysis instead of simple laboratory tests?
They are faster, more accurate and more sensitive than simple chemical tests
How do you test for metals?
Dilute sodium hydroxide solution reacts with some metal ions in solution, forming metal hydroxides. Some of these metal hydroxides are insoluble (so they appear as
precipitates)
What colour does the metal ion Iron (II) make?
Green
What colour does the metal ion Iron (III) make?
Brown
What colour does the metal ion Copper make?
Blue
What colour does the metal ion Calcium make?
White
What colour does the metal ion Aluminium make?
White
What is the chemical test for Ammonia?
- Damp red litmus paper –> blue
- Hydrogen chloride gas (from hydrochloric acid) held near it –> forms white smoke of ammonium chloride
How do you distinguish between aluminium ions and calcium ions?
If you add excess sodium hydroxide solution:
- The aluminium hydroxide precipitate reacts to form a colourless solution
- The calcium hydroxide precipitate is unchanged
How can we identify negatively charged ions in a solution?
The formation of different precipitates allows us to identify different negatively charged ions in solution
How do we test for Carbonate Ions?
We add dilute acid (usually hydrochloric acid) to the carbonate ions. Bubbles are given off when an acid, usually dilute hydrochloric acid, is added to the test compound.
Bubbling is usually Co2
How do we identify the gas produced whilst testing for Carbonate Ions?
Limewater is used to confirm that the gas is carbon dioxide
What acid do we use to test for Carbonate Ions?
The test for carbonate ions works whatever acid is added
How do we test for Sulfate Ions in a solution?
Barium ions react with sulphate ions to form insoluble white barium sulphate
1) Add a few drops of dilute hydrochloric to the sample
2) Add a few drops of dilute barium chloride solution
A white precipitate forms if sulphate ions are present
Why do you add dilute hydrochloric acid to a solution when testing for sulphate ions?
This acidifies the solution and removes carbonate ions that might also give a precipitate in the test
How do we test for Halide ions?
Silver ions react with halide ions (Cl-, Br- or I- ions) to form insoluble precipitates
1) Add a few drops of dilute nitric acid to the sample
2) Add a few drops of dilute silver nitrate solution
Observe and record the colour of any precipitate that forms.
What precipitate colour does the halide ion Chloride make?
White
What precipitate colour does the halide ion Bromide make?
Cream
What precipitate colour does the halide ion Iodide make?
Yellow
Core practical
How can you identify the ions in an unknown salt?
You need to carry out:
* Flame tests
* Hydroxide precipitate tests
* Test for ammonium ions
* Test for carbonate ions
* Test for Sulfate ions
* Test for Halide ions
Core practical
Describe the flame tests for metal cations?
1) Light a Bunsen burner and open the air hole to give a hot blue flame
2) Pick up a small sample of a solid salt (using a clean wire loop or damp wooden splint)
3) Hold the sample in the edge of the flame
4) Observe and record the flame colour
Core practical
Describe the Hydroxide precipitate test for metal cations?
1) Dissolve a small amount of salt in a test tube using distilled water
2) Add a few drops of dilute sodium hydroxide solution (one drop at a time)
3) Record the colour of any precipitate formed
4) If a white precipitate forms, add excess dilute sodium hydroxide solution (to distinguish between aluminium and calcium ions)
Core practical
Describe how to test for Ammonium ions?
1) Dissolve a little solid salt in a test tube using distilled water
2) Add dilute sodium hydroxide solution & warm gently
3) Remove from the flame. Hold a piece of damp red litmus paper near the mouth of the test tube. Record what happens to its colour
Core practical
Describe how to test for Carbonate ions?
1) Put a little solid salt in a test tube + add a few drops of dilute acid
2) Record whether any effervescence occurs
3) Use limewater to check that any bubbles contain Co2
Core practical
Describe how to test for Sulfate ions?
1) Dissolve a little solid salt in a test tube using distilled water
2) Add a few drops of dilute hydrochloric acid, then a few drops of barium chloride solution. Record whether a white precipitate forms
Core practical
Describe how to test for Halide ions?
1) Dissolve a little solid salt in a test tube using distilled water
2) Add a few drops of dilute nitric acid, then a few drops of silver nitrate solution. Record the colour of any precipitate formed
