Qualitive Analysis Flashcards
What are flame tests used for?
To identify metal ions in substances
How to carry out a flame test?
- Light Bunsen Burner and open air hole to give hot blue flame
- Pick up small smaple using wire loop
- Hold sample in edge of flame and observe and record flame colour
What is used to clean the wire loop?
Hydrochloric acid
Why is platinum used for the wire loop?
Has a high melting point, is unreactive and gives no colour to the flame
What is normally used instead of platinum for the wire loop?
Nichrome alloy because although it produces faint orange colour in flame, it is much cheaper
What are the different colours produced by these different cations?…
- Lithium
- Sodium
- Potassium
- Calcium
- Copper
- Iron (II)
- Iron (III)
Li+ = red Na+ = yellow K+ = lilac Ca2+ = orange-red Cu2+ = blue-green
What can photometry do better than simple flame tests?
- Better sensitivity (detect much smaller amounts)
- Accuracy (give values closer to true values)
- Speed (gives results faster)
What does a flame photometer measure?
Light intensity of the flame colours produced by metal ions
What does flame photometry produce?
A calibration curve produced using different standard solutions, each containing known concentrations of the metal ion dissolved in distilled water
What does a spectrum produced by a flame photometer show?
Since the colours produced are usually a mixture, the spectrum separates them out to detect which metal ion is is in the solution as different metal ions produce different emission spectra
How is a hydroxide precipitate produced?
- Sample solution placed in test tube
- Few drops of dilute sodium hydroxide solution are added
What hydroxide precipitates are produced by copper, iron (II) and iron (III)?
Cu2+ = blue Fe2+ = green Fe3+ = brown
How is a precipiate formed since sodium hydroxide is soluble in water?
Most hydroxides are insoluble and so form precipitates in these tests
How do you treat water that flows through mines and contaminates rivers?
React the metals with sodium hydroxide which will produce a precipitate which will settle out and can be removed
What colour precipitate does aluminium and calcium produce?
White
How can you distinguish between aluminium and calcium?
Do a further test - aluminium hydroxide disappears to form a colourless solution when excess sodium hydroxide is added, but calcium hydroxide does not
How do you test for ammonium ions?
- Add dilute sodium hydroxide solution to ammonium ions solution
- Ammonia gas is produced when ammonium ions mixture is warmed
- Ammonia has characteristic sharp smell but confirmatory test is used to identify it
What is the confirmatory test for ammonia?
Damp red litmus paper changes to blue when ammonia is present
Describe the test to identify metal ions in a solution using dilute sodium hydroxidde solution
- Using a dropping pipette, fill a test tube to depth of 2cm with solution
- Using different dropping pipette, add few drops of dilute sodium hydroxide solution to the tube
- Gently shake tube
- Observe and record colour
- If white do further test to distinguish between aluminium hydroxide and calcium hydroxide
How do you test for carbonate ions?
- Add dilute hydrochloric acid to the test substance
- Look for effervescence of CO2
- Carry out CO2 confirmatory test (bubble gas through limewater and should turn milky)
Why should you use dilute hydrochloric acid as opposed to concentrated?
- Less expensive
- Less damaging to the environment
How do you test for sulfate ions?
- Add few drops of dilute hydrochloric acid which acidifies the solution and removes carbonate ions that might give a precipitate in the test
- Add a few drops of barium chloride solution
- White precipitate of barium sulfate forms if the sample contains sulfate ions
Where do halide ions come from?
Group 7 halogens form compounds containing halide ions
Which silver halide is soluble unlike all others while are insoluble?
Silver fluoride
