EXPT 13. The Secret of 13 Test Tubes
[Key Contents]
- qualitative analysis of cations
- precipitation equiliria, complex equilibria
[References]
Principles of Modern Chemistry, 6th Ed. (Oxtoby et al.)
Ch 16. Solubility and Precipitation Equilibria
Chemistry for Life, Chemistry for Better Life (Kim et al.)
Ch 9. Equilibrium Reactions
[Goal]
Most chemical elements in nature are metals. Different metals exhibit
different chemical properties such as precipitation reactions. The goal of
this experiment is to learn to identify cations in 13 unknown samples
based on the precipitation and complex formation reactions as well as
the color and pH of the solution
[Background]
Synthesis of a new element with atomic number 116 has been
announced.
In
the
mid-1940,
Seaborg
and
McMillan
first
made
transuranic elements heavier than uranium and received the 1951
Nobel Chemistry Prize. Since then elements up to 114 had been made
without skipping any. This time atomic number 115 was skipped.
In Mendeleev's periodic table, elements were arranged in the order
of increasing atomic weight. Two years after Rutherford discovered
the atomic nucleus in 1911, Mosley showed that the nuclear charge is
a whole number. Rutherford identified a nuclear particle with a unit
positive charge and called it proton in 1919. It became evident that
Moseley discovered the number of protons in the nucleus.
Now elements in the periodic table are arranged in the order of
increasing atomic number (number of protons in the nucleus). So
identifying an element is equivalent to counting the number of protons
in the nucleus.
Majority of elements are metals. Most of them are found in
relatively minute amounts in nature. Some of them play key roles in
life; others are harmful and need to be monitored in environmental
analyses. Metallic elements exhibit different color in the flame test,
and
this
principle
is
utilized
in
atomic
absorption
or
emission
spectroscopy. Cations of metallic elements often make salts with very
low solubility or make complex ions with intense color.
Whatever method you use, identification of an element in an
unknown sample is equivalent to counting the number of protons
buried deep in the atomic nucleus. In this experiment, you will
witness
how
19th
century
chemists
identified
metallic
elements
without the knowledge of the nucleus or the electrons around it. That
needs a systemic approach illustrated in the diagram below. You must
remember that the group number referred to here is different from
the group number in the periodic table.
Interestingly enough, the analytical scheme and the counter anions
used like chloride, sulfide, ammonium hydroxide, and carbonate are
similar to what Curies used in the discovery of radium and polinium.
Thorough
knowledge
of
these
reactions
enabled
Curies
isolate
milligram quantities of then unknown elements from kilograms of
pitchblend.
Obviously,
experience.
the
Now
differences
we
can
in
solubility
calculate
were
solubility
discovery
from
product
from
thermodynamic data. And we can predict the strength of ionic bonds.
The insolubility of calcium phosphate is not surprising. Sosium and
potassium ions seldom make insoluble salts.
[Apparatus and Chemiclas]
mixing well, dropper, beaker for waste disposal
silver nitrate (AgNO3) solution, distilled water, pH paper
list of 13 unknown sample solutions
0.1 M H2SO4
6 M NH3
0.1 M K2CrO4
0.1 M Fe(NO3)3
0.1 M K2C2O4
0.05 M Na2S
0.1 M Cu(NO3)2
0.1 M KNO3
0.1 M KSCN
0.1 M NiSO4
0.05 M SnCl2
0.1 M NaCl
0.1 M Ba(NO3)2
[Procedure]
As the title of the experiment implies, the identity of the samples in 13
test tubes is hidden. First of all, number the test tubes provided. Your job
is to match the test tube number with the candidate cations on the list.
You may not be able to break the secret unless you carefully study the
chemicals on the list and prepare a game plan in advance.
1. First, see if you could identify a few based on color alone.
Obviously, your job would be easier if you looked up the color of the
candidates in advance.
2. You could identify some based on smell. You are not supposed to
smell chemicals by directly placing it under your nose. Wave your hand
gently over the chemical to deliver a small amount of the vapor to your
nose.
3. Next, check the pH of the solutions. Place a piece of the pH paper
on a white sheet of paper, and deliver a drop of the test solution using a
dropper. Compare the color of the pH paper against the standard color
sheet and estimate the pH of the solution. Wash the dropper before the
next test to avoid cross contamination.
4. Next is the most critical test of all. You will be identifying cations
based on precipitation reactions. Into each well, deliver several drops of
the solution you identified above based on color and pH.
5. Using a washed dropper, deliver several drops of each of the
remaining unknown solution into each well and note color change.
6. Compare observed color against the known color of the precipitates.
The identificationshould be straightforward. It might be a good idea to
plan the combinations ahead and be prepared for the anticipated color
change. Cross-contamination should be avoided by washing the dropper
after each use.
[Additional Material]
Clues for Breaking the Secret
AgNO3+ NaCl → white ppt
AgNO3 +K2C2O4 → white ppt
AgNO3 +KSCN → white ppt
AgNO3 +K2CrO4 → bloody ppt
or bloody soln
Ba(NO3)2 + K2CrO4
→ bright yellow ppt
Ba(NO3)2 + K2C2O4
→ bright white ppt
Ba(NO3)2 + H2SO4
→ white ppt
Cu(NO3)2 + Na2S
→ dark ppt
Cu(NO3)2 + NH3
Cu(NO3)2 + KSCN
→ deep blue soln + ppt
→ dark ppt
Fe(NO3)3 + Na2S
Fe(NO3)3 + NH3
→ bloody soln
→ dark ppt
→ dark ppt
NiSO4 + Na2S
NiSO4 + NH3
SnCl2 + Na2S
Fe(NO3)3 + KSCN
→ dark ppt
→ blue soln
→ dark brown ppt