Illustrated Guide to Home Chemistry Experiments

344 DIY Science: Illustrated Guide to Home Chemistry Experiments

SBSTITUTIU oNS ANd modIfICATIoNS

• You may substitute a reaction plate for the test tubes and rack. We recommend using the test tubes, despite the larger quantities
  of solutions required, because the reactions are much easier to observe if you work with larger quantities of the solutions.


• You may use bench solutions of any of the reagents or analytes rather than making them up specifically for this session.


• Make up approximately 0.1 M cation sample solutions using the nitrate salts of as many of the following metals as possible:
  aluminum, barium, calcium, chromium, cobalt, copper, iron(II) (ferrous sulfate), iron(III) (ferric), lead, manganese (as
  sulfate), nickel, silver, strontium, and zinc.


• Make up primary reagent solutions, including 3 M sulfuric acid and 6 M solutions of hydrochloric acid, sodium hydroxide,
  aqueous ammonia, and nitric acid. (Do not store the 6 M sodium hydroxide in a glass bottle; it dissolves glass, literally. Use
  a Nalgene or similar plastic bottle.)


• Make up secondary reagents: 0.25 M solutions of potassium ferricyanide, potassium ferrocyanide, and potassium thiocyanate.


• How much of the reagents and cation sample solutions you need depends on whether you observe the reactions with a
  reaction plate or with test tubes. Using a reaction plate, you’ll need 5 mL to 10 mL of each reagent (3 M sulfuric acid and
  6 M solutions of hydrochloric acid, sodium hydroxide, ammonia, and nitric acid) and of each cation sample solution. Using
  test tubes, you’ll need 25 mL to 50 mL of each reagent and of each cation sample solution.

The next step is to separate Group II ions, which form extremely
insoluble sulfide salts. The supernatant liquid from the first
step is made strongly acid, and thioacetamide is added to
the solution. In acidic solution, thioacetamide produces a
low concentration of sulfide ions, which causes the Group II
ions to precipitate. Although we don’t do so in this book, that
precipitate can be further analyzed to isolate and identify
the specific Group II ion or ions present in the sample. The
supernatant liquid can contain only Group III, Group IV, and
Group V cations.

That solution is treated with aqueous ammonia until it is
strongly basic, which causes the thioacetamide to dissociate
further, providing a higher concentration of sulfide ions. The
higher sulfide concentration causes Group III cations—whose
sulfides are more soluble than Group II sulfides—to precipitate,
leaving only Group IV and Group V cations in solution. Again,
the Group III precipitate can be further analyzed to isolate and
identify the specific Group III ion or ions present in the sample,
though that is not covered in this book.

Finally, the supernatant liquid, which can contain only Group
IV and Group V cations, is treated with carbonate ion, which
causes Group IV cations to precipitate and leaves only Group V
cations in solution. The Group IV precipitate is further analyzed
to isolate and identify the specific Group IV ion or ions present
in the sample. The solution, which can now contain only Group
V cations, can be further analyzed to isolate and identify the
specific Group V ion or ions present in the sample.

Order is critical when you separate cations. For example,
the carbonate ions used to precipitate Group IV cations also
precipitate all of the Group I, Group II, and Group III cations.
If you treat an unknown solution with carbonate ions first, you

precipitate all Group I through Group IV ions without achieving

any separation.

In a real qualitative inorganic analysis, any or all of these

cations might be present in the unknown, and the separation

and analysis would be done using microscale or semi-micro

procedures. For simplicity and to avoid using expensive

reagents, in this lab we’ll instead observe the reactions of

individual cations with various reagents and build a matrix of

the observable changes that occur with various combinations of

cation and reagent.

We’ll use only five primary reagents (six, counting distilled

water): 3 M sulfuric acid, and 6 M solutions of hydrochloric acid,

sodium hydroxide, aqueous ammonia, and nitric acid. We’ll also

use three secondary reagents to confirm the presence of some

cations: 0.25 M solutions of potassium ferricyanide, potassium

ferrocyanide, and potassium thiocyanate.

POCEDURER

1.ou have not already done so, put on your splash If y

goggles, gloves, and protective clothing.

2. Use the graduated cylinder to transfer about 5 mL of 0.1 M
   aluminum nitrate solution to each of seven test tubes in
   a rack.


3. Add a few drops of 3 M sulfuric acid to the first test tube
   and swirl or stir the tube gently. If no observable change
   occurs, note that fact in Table 19-5 and continue to
   the next step. If adding sulfuric acid produces a visible
   change such as a precipitate, color change, bubbling, or
   some other action, continue adding sulfuric acid until
   no further change occurs. If a precipitate occurs, add
   aqueous ammonia to neutralize the sulfuric acid (until a
   drop of the solution in the test tube turns litmus paper