Ion Tests Flashcards
What are the different tests for metal cations?
- flame test
- (flame photometry/ flame emission spectroscopy)
- precipitation reactions
- ammonium ion test
What cations can the flame test indicate?
- Lithium (Li+)
- Sodium (Na+)
- Potassium (K+)
- Calcium (Ca2+)
- Copper (Cu2+)
What colour will lithium cations show in a flame test?
Red (crimson)
What colour will sodium cations show in a flame test?
Yellow
What colour will potassium cations show in a flame test?
Lilac
What colour will calcium cations show in a flame test?
Orange-red
What colour will copper cations show in a flame test?
Blue-green
Why might the instrumental method - a flame photometer - be used instead of the flame test to recognise the cations?
- speed
- sensitivity (detect tiny amounts)
- accuracy
(It overcomes problems of comparing similar ones, when low concentration + when samples contain multiple metal cations)
What does flame photometry involve?
A spectroscope - detects individual wavelengths emitted (which are unique to each) which are emitted when the metal cations are heated
What does the intensity (dull/bright) of lines indicate in flame photometry?
The concentration of the metal cation
What can be used (with flame photometry) to identify cations + their concentrations in unknown samples?
Known metal cation spectra
What does a calibration curve allow us to determine?
The concentration of a substance in an unknown sample by comparing the unknown to a set of standard samples
What are precipitation reactions used to identify?
Dissolved metal cations - they produce precipitates
What does a precipitation reaction involve?
Sodium hydroxide solution is added to test solutions to produce a hydroxide precipitate
What cations can a precipitation reaction be used to identify?
- Iron (II) (Fe2+)
- Iron (III) (Fe3+)
- Copper (Cu2+)
- Calcium (Ca2+)
- Aluminium (Al3+)
What colour is the hydroxide precipitate for iron (II) (iron hydroxide) cations in the precipitation reaction?
Green
What colour is the hydroxide precipitate for iron (III) (iron hydroxide) cations in the precipitation reaction?
Brown
What colour is the hydroxide precipitate for copper (copper hydroxide) cations in the precipitation reaction?
Blue
What colour is the hydroxide precipitate for calcium (calcium hydroxide) cations in the precipitation reaction?
White
What colour is the hydroxide precipitate for aluminium (aluminium hydroxide) cations in the precipitation reaction?
White
The precipitate for both calcium and aluminium in the precipitation reactions test is white. How can they be distinguished from one another?
- add excess sodium hydroxide solution
- aluminium - precipitate dissolves to form colourless solution
- calcium - precipitate uncharged (remains in excess)
What is the symbol for ammonium ions?
NH4^+
How are ammonium ions tested for? (Ammonia has characteristic sharp smell, but confirmatory gas is used to identify gas)
- add dilute sodium hydroxide solution
- warm mixture -> ammonia gas (NH3) is formed
- the gas will change damp red litmus paper blue
What is another test for ammonium ions?
- a glass rod dipped in concentrated hydrochloric acid
- held near mouth of test tube containing mixture
- ammonium chloride formed as white smoke
