Qualitative Analysis: Tests for Ions - Topic 9 Flashcards
Why must the test for any ion be unique?
- so the test only detects that ion -> so no confusion with other ions
- if same result is given by more than one ion -> so don’t know which ion is present
What are flame tests used to test for?
test for positive ions
How do you carry out a flame test?
- Dip platinum loop into concentrated HCl
- place wire in the hottest part of the flame
- Repeat steps 1 and 2 until no colour change is produced
- Dip the wire into concentrated HCl
- Dip the wire into the chemical to be tested
- Place into the hottest part of the flame and note the colour
Colour of lithium ions (Li+) in flame test:
crimson, red
Colour of sodium ions (Na+) in flame test:
yellow/orange
Colour of potassium ions (K+) in flame test:
pale lilac
Colour of calcium ions (Ca+) in flame test:
brick red, orange-red
Colour of barium ions (Ba2+) in flame test:
apple green
Colour of copper ions (Cu2+) in flame test:
blue-green
How do you test for positive ions with sodium hydroxide?
- Add 1cm3 of chosen ion solution to test tube
- Observe colour of solution carefully
- Add a few drops NaOH solution
- Write down observations
- To test for ammonium ions heat the solution gently after NaOH is added and insert a piece of damp red litmus paper
- To test for Al3+ and Ca2+ ions add excess NaOH solution to the resulting solution to distinguish between them
Tests for copper (II) (Cu2+) ions + observation when sodium hydroxide added:
blue precipitate
Test for iron (II) (Fe2+) ions + observation when sodium hydroxide added:
green precipitate which turns brown on standing
Test for iron (III) (Fe3+) ions + observation when sodium hydroxide added:
brown precipitate
Test for ammonium (NH4+) ions + observation when sodium hydroxide added:
- to test for ammonium ions heat the solution gently after NaOH is added and insert a piece of damp red litmus paper
- NH3 gas produced - pungent smelling gas produced + this gas turns damp red litmus paper blue
Test for aluminium (Al3+) ions + observation when sodium hydroxide added:
- to test for Al3+ and Ca2+ ions add excess NaOH solution to the resulting solution to distinguish between them
- Al3+ has white precipitate which redissolves in excess NaOH
Test for calcium (Ca2+) ions + observation when sodium hydroxide added:
- to test for Al3+ and Ca2+ ions add excess NaOH solution to the resulting solution to distinguish between them
- white precipitate which doesn’t re-dissolve in excess NaOH
What is the chemical test for ammonia?
- NH3 - pungent smelling gas
- turns damp red litmus paper blue
- NH3 also forms a white smoke of ammonium chloride when hydrogen chloride gas from concentrated HCl acid is held near it
How do you test for carbonates?
- Take 1cm3 of carbonate of 1 spatula and add to test tube
- Add HCl acid (can use any other acids - sulphuric acid, nitric acid) to test tube
- Gas produced bubbled through limewater - if limewater goes cloudy, gas produced is CO2 (carbonates react with dilute acid to produce CO2)
What happens when the carbonate is added to the HCl acid?
- effervescence
- CO2 gas produced
What happens when the product is passed through limewater (testing for carbonates)?
cloudy white precipitate formed
How do you test for sulphate ions?
- Add few drops of HCl acid to a solution of BaCl₂ - this is now acidified BaCl₂ - acid is added to ensure no carbonate ions are present and no unwanted precipitates form
- Add few drops of the “sulphate” to the acidified BaCl₂ - if sulphate present cloudy white precipitate formed
Observations when testing for sulphate ions:
after “sulphate” added to acidified barium chloride sulphate if cloudy white precipitate formed then sulphate ions are present in the solution
How do you test for halide ions?
- Take small amount of AgNO₃ in a test tube and add few drops of HNO₃ - this is now acidified silver nitrate
- nitric acid is added to silver nitrate solution to remove any carbonates so preventing any unwanted precipitates from forming
- Add 1cm3 of the “halide” to a test tube
- Add few drops of the acidified AgNO₃ solution to your halide
Observations when testing for chloride ions (Cl⁻) using acidified silver nitrate:
white precipitate
Observations when testing for bromide ions (Br⁻) using acidified silver nitrate:
cream precipitate
Observations when testing for iodide ions (I⁻) using acidified silver nitrate:
yellow precipitate
Positives of instrumental methods of analysis:
- sensitive - can detect v. small amounts of substance
- rapid, v. fast - many hundreds of samples in a day
- v. accurate - can cope with mixtures, objective
Negatives of instrumental methods of analysis:
- expensive
- require expertise to operate
- can be larger machines
- cannot cope with new chemicals - only report on things they have seen + require databases to work
What does flame photometry allow?
this method of analysis - allows us to be able to identify ions in dilute solutions
How does flame photometry work?
- high-tech flame test
- measures wavelength of light given out in flame
- can identify each ion in the mixture
Uses of flame photometry:
- can be used to find concentration of ions in solution e.g. for kidney dialysis
- measure the intensity of light for solutions of known concentrations to create graph
- measure intensity of solution being tested and use graph to measure concentration
