Qualitative analysis – the identification of the specific substances present

Quantitative analysis – the measurement of the quantity of a substance present

Qualitative Analysis

• Many substances can be identified by their unique physical or chemical properties:

1. Observation of colour of solution

2. Reactions with other substances to produce a new colour and/or precipitate

3. For cations that are usually colourless in solution and highly soluble, a flame test may be used (only works when a single cation is present)

1. Qualitative Analysis – Colour of Solution

2. Qualitative Analysis – Reactions in Solution

• This is a type of test where we set up a double displacement reaction using one unknown solution and one known solution

• In this test, we would predict that if a precipitate forms then a certain ion must have been present in the unknown solution

Indicate the test being performed

Positive Test

Observations

Analysis

Conclusion

Negative Test

Observations

Analysis

Conclusion

To Complete A Sequential Analysis

1. Locate the possible cations on the solubility table.

2. Determine which anions would precipitate the possible cations.

3. Plan a sequence of precipitation reactions that would use anions to precipitate a single cation at a time.

4. Use filtration between the steps to remove cation precipitates that might interfere with subsequent additions of anions.

5. Draw a flow chart to assist testing & communication.

Example 1

A solution known to contain Pb2+

Positive test is a white precipitate

Solution contained Lead(II) ions, precipitated as PbCl2

Negative test is no precipitate

No Lead (II) ions present

Add NaCl

Example 2

A solution known to contain Sr2+

Positive test is a white precipitate

Solution contained Sr2+ ions, precipitated as SrSO4

Negative test is no precipitate

No Sr2+ ions present

Add Na2SO4

Example 3

A solution known to contain Cu2+

Positive test is a white precipitate

Solution contained Copper (II) ions, precipitated as Cu(OH)2

Negative test is no precipitate

No Copper (II) ions present

Add NaOH

You try!

• Design a sequential analysis for each of the ions below, to determine if it is present in the solution.

a. Ag+

b. Ca2+

c. Mg2+

Example with Two Ions• A solution is thought to contain lead (II) and

magnesium. Design a sequential qualitative analysis to determine whether the solution contains either or both of these ions

Solution known to contain Pb2+ (aq) or Mg2+ (aq)

If Solution contains Pb2+ ions, a ppt will form of PbCl2

Solution contained Pb2+ ions, precipitated as PbCl2

If Solution does not contain Pb2+ ions then no ppt will

form

No Lead (II) ions present

Add NaCl – the Na+ is always soluble, the Cl- will ppt with

Pb2+, the Cl- is soluble with Mg2+

• If a positive test occurred, filter off the ppt and run a second test for the second cation

Add Na2CO3 – again, Na+ is always soluble, but the CO3

2- will ppt with Mg2+

If a white ppt forms, the Mg2+ ions

present in form if MgCO3

If no ppt forms then no Mg2+ was present

Another Example with Two Ions• A solution is thought to contain calcium and

mercury. Design a sequential qualitative analysis to determine whether the solution contains either or both of these ions

Solution known to contain Ca2+ (aq) or Hg2+ (aq)

If Solution contains Hg2+ ions, a ppt will form of HgCl2

Solution contained Hg2+ ions, precipitated as HgCl2

If Solution does not contain Hg2+ ions then no ppt will

form

No Hg2+ ions present

Add NaCl – the Na+ is always soluble, the Cl- will ppt with

Hg2+, the Cl- is soluble with Ca2+

• If a positive test occurred, filter off the ppt and run a second test for the second cation

Add Na2SO4 – again, Na+ is always soluble, but the SO4

2- will ppt with Ca2+

If a white ppt forms, the Ca2+ ions present

If no ppt forms then no Ca2+ was present

You Try!!

• Design a sequential analysis for each of the pairs of ions below, to determine if one or both are present in the solution.

a. Ba2+ and Pb2+

b. Ag+ and Zn2+

c. Sr2+ and Mn2+

d. Cu+ and Fe3+

Solution known to contain Ag+1aq or

Cu2+aq or Ba2+ ion

Chose an anion that will produce a ppt for one of the

cations

Add NaCl – the Na+ is always soluble, but the Cl- will ppt with Ag1+

If Solution contains Ag1+ ions, a ppt will form of AgCl

If Solution does not contain Ag1+ ions then no

ppt will form

Example containing three ions!

• If a positive test occurred, filter off the ppt and run a second test for the second cation

Add NaOH – This causes basic

conditions and OH- ion will ppt with Cu2+ (Ba2+

is soluble with OH-)

If a ppt forms, the Cu2+ ions present in the

form of Cu(OH)2

If no ppt forms then no Cu2+ was present

• If a positive test occurred, filter off the ppt and run a third test for the third cation

Add Na2SO4 –Na+ is always soluble and Ba2+ ion will

ppt with SO42- ion

If a ppt forms, the Ba2+ ion

present

If no ppt forms then no Ba2+ was

present

3. Qualitative Analysis – Flame Test• The majority of solids are white powders and difficult to

distinguish between just by looking.

• Flame tests can give a good indication of what metal ion it contains.

• You should perform the colour of solution test and the PPT tests to narrow down which ions you think are present.

• You should also determine the colours that each ion would produce in a flame test before performing the flame test

• The flame test is not used to confirm or eliminate the presence of a particular ion.

4. Qualitative Analysis – Gas Test

• Most gases are clear, colourless and odourless.

• So it's not easy to distinguish between them with a simple chemical test.

Test for Hydrogen

• Hydrogen is less dense than air and explosive when mixed with Oxygen.

• A test-tube filled with hydrogen can be safely held up-turned since no Hydrogen can escape by sinking into denser air.

Oxygen Test• Oxygen has a similar density to air, so needs to be contained in a

"right-side up" container

• So, if we light a splint and blow out the flame so that the end is still glowing (the reason the flame does not reappear is that 79% of the air around it is not Oxygen)

• Placing the glowing splint in pure Oxygen means that every gas molecule that collides with the glowing ember is Oxygen.

• This increases the rate of reaction, providing enough heat to re-light the flame.

Carbon Dioxide Test

• Carbon Dioxide puts out fires. But so do most other gases.

• So the test for Carbon Dioxide must be something that only

Carbon Dioxide does.

• Only Carbon Dioxide turns Lime-water milky white • The white colour is a precipitate of Calcium Carbonate ( like

chalk).