QUALITATIVE CHEMICAL ANALYSIS

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QUALITATIVE CHEMICAL ANALYSIS
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Qualitative Analysis

• A qualitative characteristic is a description of
  something that does not involve numbers or units
  of measurement.
• We will try to identify a substances using
  characteristic such as colour and solubility.

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Ion Colour

• The colour of substances can sometimes be used to
  identify ions or compounds within the substance.
• See page 665

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CHEMISTRY CSI

• You can also use the solubility rules to
  determine the presence of certain ions in a
  solution .
• Mix your sample that contains the suspected ion
  with a solution that contains an ion that will
  form a precipitate with your suspect ion.
• Take advantage of the net ionic equation.

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EXAMPLE 1

• Lets say you suspect your water sample has
  acetate in it. What could you do?
• If you combine your water sample with a solution
  that contains silver ions (Ag) or mercury ions
  (Hg) a precipitate will form.
• silver acetate/mercury acetate will form which is
  insoluble meaning it will form a precipitate.
• Hg(aq) C2H3O2-(aq) ? HgC2H3O2 (s)
• Ag(aq) C2H3O2-(aq) ? AgC2H3O2 (s)

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THE BIG PROBLEM

• The major issue that has to be dealt with is that
  most solutions or samples contain more than one
  type of ion so chemists must design test
  procedures to identify and remove any suspected
  ions one at a time
• You need to find a solution that will react (form
  a precipitate) with only one of the ions at a
  time.

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EXAMPLE 2

• Lets say we suspect a solution has both iron
  (Fe2) and strontium (Sr2) ions in it.
• Solubility rules say that hydroxide ions (OH-)
  will react with the iron ions (Fe2) and form a
  precipitate but will not have the same reaction
  with strontium ions.
• We could also use a solution containing sulphate
  (SO42-)since it will react with the strontium and
  not the iron.
• Fe2(aq) OH-(aq) ? Fe(OH)2(s)
• Sr2(aq) SO4-2(aq) ? SrSO4(s)

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EXAMPLE 2Part 2

• To remove the compound from the solution we will
  filter the solution with the precipitate. The
  solid compound will be trapped by the filter
  paper.
• The solution passing through the filter paper is
  called the filtrate and will contain the
  unreacted ions. We keep this for further testing.

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EXAMPLE 2Part 3

• Now we have the filtrate with the unreacted ions
  in ithow do we test for the ions?
• As before use the appropriate ion to cause
  precipitation.

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Practice Problem

• How would you determine if a solution contains
  ions of silver Ag1 (aq) and/or ions of zinc,
  Zn2 (aq) ?
• Chloride Cl1- and sulphate SO42- ions precipitate
  silver but not zinc.
• Add a source of either ion, if a precipitate
  forms the solution contained silver ions. If no
  precipitate forms, silver ions were not present.
• Ag1(aq) Cl1-(aq)? AgCl(s)
• Filter out the solid if a positive test.

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Practice Problem Cont.

• Find an ion that will precipitate out Zn2(aq)
• Any ion from the bottom of the solubility table
  will precipitate out Zn2(aq) ie. hydroxide ions
  OH1-(aq)
• Add a source of the ion and check for
  precipitation. ppt ion present, no ppt ion
  not present.
• Zn2(aq) 2OH1-(aq) ? Zn(OH)2(s)

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Practice

• Devise a method to determine if a solution
  contains the following ions.
• strontium and/or manganese2
• copper1 and/or iron3
• hydroxide and/or acetate

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Home Work

• Read section 9.3
• Page 441 1-3, 5, 7, 8

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Flame Test For Cations

• The flame test is a procedure used in chemistry
  to detect the presence of certain metal ions,
  based on each element's characteristic emission
  spectrum. The color of flames in general also
  depends on temperature.
• The flame test is fast and easy to perform, and
  does not require special. However, the range of
  detected elements is small, and the test relies
  on the subjective experience of the experimenter
  rather than any objective measurements.

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Flame Tests

• The test involves introducing a sample of the
  element or compound to a hot, non-luminous (blue)
  bunsen flame, and observing the color that
  results.

Samples are usually held on a nichrome wire
cleaned with hydrochloric acid to remove traces
of previous analytes.
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Flame Tests
Potassium - Purple
Sodium - Yellow
Barium - Green
Sodium is a common component or contaminant in
many compounds and its spectrum tends to dominate
over others. Thus the color yellow overpowers
the true color. The test flame is often viewed
through cobalt blue glass to filter out the
yellow of sodium and allow for easier viewing of
other metal ions.
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Flame Colours

• As Arsenic - Blue
• B Boron - Bright Green
• Ba Barium - Apple Green
• Ca Calcium - Brick Red
• Cs Cesium - Pale Violet
• Cu(I) Copper(I) - Blue
• Cu(II) Copper(II) (non-halide) - Green
• Cu(II) Copper(II) (halide) -Blue-Green
• Fe Iron - Gold
• In Indium - Blue
• K Potassium - Lilac
• Li Lithium Carmine Red

Mg Magnesium - Brilliant white Mn(II)
Manganese(II) Yellowish green Mo Molybdenum
- Yellowish green Na Sodium - Intense Yellow P
Phosphorus - Pale bluish green Pb Lead - Pale
green Rb Rubidium - Pale violet Sb Antimony -
Pale green Se Selenium - Azure blue Sr
Strontium - Crimson Red Te Tellurium - Pale green
Tl Thallium - Pure green Zn Zinc - Bluish Green