chemistry - separate chemistry 2: qualitative analysis: test for ions (9.1 - 9.9)

9.1 why must test for any ion be unique?

test for ion should only detect that ion

couldn’t know which ion is present if more than 1 ion gave same result

identifying cations - carrying out flame tests

1. light Bunsen burner & open air hole to give hot blue flame

2. pick up small sample of test substance using wire loop

3. hold sample in edge of flame

4. observe flame colour

9.2 flame test to identify lithium ion (Li⁺) in solid

red

9.2 flame test to identify sodium ion (Na⁺) in solid

yellow

9.2 flame test to identify potassium ion (K⁺) in solid

lilac

9.2 flame test to identify calcium ion (Ca²⁺) in solid

orange-red

9.2 flame test to identify copper ion (Cu²⁺) in solid

blue-green

identifying cations - adding sodium hydroxide

add few drops at a time to test solution

9.3 test to identify aluminium ion (Al³⁺) in solid/solution

add sodium hydroxide solution

white precipitate

to distinguish between calcium ions & aluminium ions (both produce white precipitates): add excess sodium hydroxide solution - turns colourless

9.3 test to identify calcium ion (Ca²⁺) in solid/solution

add sodium hydroxide solution

white precipitate

to distinguish between calcium ions & aluminium ions (both produce white precipitates): add excess sodium hydroxide solution - stays white

9.3 test to identify copper ion (Cu²⁺) in solid/solution

add sodium hydroxide solution

blue precipitate

9.3 test to identify iron (II) ion (Fe²⁺) in solid/solution

add sodium hydroxide solution

green precipitate

9.3 test to identify iron (III) ion (Fe³⁺) in solid/solution

add sodium hydroxide solution

brown precipitate

9.3 test to identify ammonium ion (NH₄⁺) in solid/solution

add sodium hydroxide solution

warm mixture - ammonia gas produced

ammonia turns damp red litmus paper → blue

9.4 chemical test for ammonia

place damp red litmus paper in ammonia gas

turns blue

9.5 test to identify carbonate ion (CO₃^2-) in solid/solution

1. add dilute hydrochloric acid

2. bubble gas produced through limewater - turns cloudy if CO₂ present

positive result = lime water cloudy

9.5 test to identify sulfate ion (SO₄^2-) in solid/solution

1. add dilute hydrochloric acid - removes carbonate ions that might give precipitate in test

2. add barium chloride solution

positive result = white precipitate forms

9.5 test to identify chloride ion (Cl^-), bromide ion (Br^-), iodide ion (I^-) in solid/ solution

1. add nitric acid - removes carbonate ions that might give precipitate in test

2. add silver nitrate solution

chloride positive result = white precipitate forms

bromide positive result = cream precipitate forms

iodide positive result = yellow precipitate forms

9.6 core practical: identify ions in unknown salts using tests for specified cations & anions - flame tests for metal cations

1. light Bunsen burner & open air hole to give hot blue flame

2. pick up small sample of solid salt using clean wire loop/damp wooden splint

3. hold sample in edge over flame

4. observe & record colour

9.6 core practical: identify ions in unknown salts using tests for specified cations & anions - hydroxide precipitate tests for metal cations

1. dissolve a little salt in test tube using distilled water

2. add few drops dilute sodium hydroxide solution one drop at a time

3. record colour of any precipitate formed

4. if white precipitate forms, add excess dilute sodium hydroxide solution to see if it will disappear to leave clear solution

9.6 core practical: identify ions in unknown salts using tests for specified cations & anions - testing for ammonium ions

1. dissolve a little solid salt in test tube using distilled water

2. add dilute sodium hydroxide solution

3. warm gently

4. remove from flame, hold damp red litmus paper near mouth of test tube

5. record what happens to its colour

9.6 core practical: identify ions in unknown salts using tests for specified cations & anions - testing for carbonate ions

1. put a little salt in test tube & add few drops of dilute acid

2. record whether any effervescence occurs

3. use limewater to check if bubbles contain carbon dioxide

9.6 core practical: identify ions in unknown salts using tests for specified cations & anions - testing for sulfate ions

1. dissolve a little solid salt in test tube using distilled water

2. add a few drops of dilute hydrochloric acid

3. add a few drops of barium chloride solution

4. record whether white precipitate forms

9.6 core practical: identify ions in unknown salts using tests for specified cations & anions - testing for halide ions

1. dissolve a little solid salt in test tube using distilled water

2. add a few drops of dilute nitric acid

3. add a few drops of silver nitrate solution

4. record colour of any precipitate formed

9.8 other methods of analysis

instrumental methods of analysis (machines) available

9.8 what do instrumental methods of analysis improve?

sensitivity - detect much smaller amounts

accuracy - give values closer to true values

speed of tests

what does a flame photometer do?

measures light intensity of flame colours metal ions produce

determines concentration of metal ion in dilute solution

9.9 evaluate data from flame photometer - determine concentration of ions in dilute solution using calibration curve

9.9 evaluate data from flame photometer - identify metal ions by comparing data with reference data

colour of light we see is flame test is usually mixture of diff. colours

flame photometer separates out these colours - produces spectrum of light emitted by each metal ion

diff. metal ions produce diff. emission spectra

identify metal ion in unknown solution - match its spectrum to spectrum from known metal ion