QUALITATIVE ANALYSIS OF CATIONS
I. INTRODUCTION
The detection and identification of ions in solution is called "Qualitative Analysis". This analysis process is
based on the differing solubilities of specific groups of ions when combined with particular anions. The
analysis process relies on the formation of distinctive and characteristic precipitates and colours in solution.
2
In this experiment you will be asked to analyze a solution containing one or more of the ions: Ag+, Hg 2 , Fe3+,
Al3+, Zn2+ and Ni2+. In order to correctly identify the ions present in your solution, it is necessary that you work
EXTREMELY carefully, making sure that you FOLLOW INSTRUCTIONS EXACTLY and that you don't mix up,
contaminate or spill test tubes. It is very important that your equipment be CLEAN (but not necessarily DRY)
so as to avoid accidental contamination and false results.
II. SPECIAL NOTES
1. The following terms and abbreviations will be used in this lab:
tt = test tube, d = drop, ppt = precipitate
2. As a rule of thumb, 1 mL = 1/8 of a test tube volume. See outline at side of page 2 for a more exact
volume; you can hold the test tube up to the paper to make sure that you have 1 mL, for example.
3. Use the dropping bottles provided when asked to add a certain number of drops of a solution; don't use a
dropping pipet.
4. After adding any chemical you must thoroughly stir the solution by either
i) tapping the tube vigorously (as demonstrated by your teacher), or
ii) using a CLEAN stirring rod.
5. If a procedure produces a transparent solution, no precipitate has been produced. If an obvious solid has
formed or the solution has a milky appearance, a precipitate has been formed. "Gelatinous" precipitates
have an almost–clear, flaky appearance to the solid particles; the centrifuged precipitate will have a
translucent appearance (i.e. some light can pass through the precipitate).
6. When using a centrifuge, you MUST counterbalance your test tube with a second test tube filled with
water to roughly the same depth as your first test tube. The second test tube is placed opposite the first
one. When centrifuging a solution, leave the centrifuge "ON" for 30 s after the centrifuge has reached
top speed. (At top speed the sound emitted by the centrifuge reaches a steady pitch.)
7. When using a micropipet to remove a solution from a ppt, tip the test tube as shown below.
Keep the pipet tip just below the surface of the liquid and slowly remove the liquid. This will help to avoid
"stirring up" the ppt. It isn't necessary to save all the liquid; as long as you get at least 90% of the liquid,
uncontaminated by ppt, you will have enough to continue with subsequent tests. Once you have at least
90% of the liquid saved, simply remove the last bit of liquid and discard it without worrying about "sucking
up" and discarding small bits of ppt.
8. When you are asked to "wash" a precipitate, separate off the liquid present by means of a pipet, then add
the liquid to be used to wash the precipitate, stir the precipitate, centrifuge the mixture to force the
precipitate to the bottom, and draw off and discard the wash solution, leaving the precipitate behind.
–2–
9. If a metal ion is not completely removed from a mixture in some step, it is possible
that the “left over” ions can form a bit of unwanted precipitate at a later procedure
step. Hence, if you only see a few specks of precipitate in some step you can safely
ignore them. Such contamination can also cause white precipitates to have a
slightly grey or tan colour, and is normal. However, if a precipitate’s colour is very
different from the expected colour, the metal ion you are looking for should be
considered to be absent.
8
10. TESTING A SOLUTION WITH LITMUS: Tear a piece of neutral litmus into small
squares and arrange on a watch glass. Take a stirring rod out of the solution to be
tested and touch the rod to a litmus square. If the litmus turns PINK, the solution is
ACIDIC. If the litmus turns PURPLE-BLUE, the solution is BASIC.
11. TESTING A SOLUTION WITH UNIVERSAL INDICATOR PAPER: Use the same
procedure as when testing with LITMUS paper, but the colours to watch out for are:
Strongly acidic =
Weakly acidic =
Neutral
Weakly basic =
Strongly basic =
4
red-pink to orange-pink
orange to yellow3
= green
blue-green to grey-blue
2
purple
1
12. Use a 400 mL beaker to hold “dirty” (used) stirring rods, pipets and test tubes so as
to avoid getting your work place contaminated.
13. Use a grease pencil to label test tubes you are setting aside for later tests. Label the test tube with the
NUMBER OF THE NEXT STEP TO BE PERFORMED ON THE CONTENTS. Note: the outside of the
test tube must be wiped dry and clean of finger oil with a paper towel in order to allow the grease pencil to
write properly on glass.
III. PROCEDURE
OBTAIN AN UNKNOWN SOLUTION AND RECORD THE SAMPLE NUMBER.
Set up your lab report sheet as shown below, and record your results as you perform the procedures. You
MUST accurately record the COLOUR of all precipitates and all solutions at every step, INCLUDING
THE STARTING COLOUR. The marking scheme for this lab will be:
Finding 6 ions to be present or absent
(2 marks each) = 12 marks
Quality of observations = 3 marks
—————————————————————————————
Maximum mark for lab
= 15 marks
(Penalty for NOT submitting your sample number
Procedure
SAMPLE # = ...
1.
Conclusion
White ppt
Ag+ or Hg 2 present
SAMPLE COLOUR = ...
Added HCl
2, 3. Centrifuged and washed ppt
4.
Observations
Added 10 d 5 M NH3 & heated
2
—
Ppt dissolves
–3–
2
Hg 2 absent
= 12 marks)
2
A. Separation of the Silver Group: Ag+ and Hg 2
1. To 1 mL of your unknown solution add 5 d of 5 M HCl. Stir.
2
If a white ppt forms, one or more of Ag+ and Hg 2 may be present and you must go on to
step 2. The ppt is AgCl and/or Hg2Cl2.
2
If no precipitate forms, Ag+ and Hg 2 are both absent, so that you can immediately skip to
step 6 to test for the ions in Part B.
2. Centrifuge the solution from step 1. Use a pipet to remove the liquid and save it for step 6 to test for
the ions in Part B. (If a few flecks of ppt float on the surface of the liquid, put the tip of the pipet below
the surface of the liquid to draw up the liquid and avoid any contamination by any persistently–floating
particles.)
3. Wash the ppt from step 2 by adding 1 mL of distilled water, stirring and centrifuging the mixture. Use
a pipet to draw off and discard the wash liquid.
4. Add 10 d of 5 M NH3 to the ppt, heat in a boiling–water bath for 2 min with STIRRING (even if the ppt
turns black immediately, this heating is necessary to complete the reaction). Centrifuge the mixture.
Use a pipet to remove the liquid and test it in step 5.
If the ppt is a tightly–collected black or dark–grey ppt which sits solidly on the bottom
2
of the tt, you have confirmed the presence of Hg 2 . Use a pipet to remove the liquid and test
it in step 5. The ppt can be discarded.
If you find that there is
• no ppt, or
• a dark brown ppt which floats around, OR
• only a few flecks of black material are present, OR
• the ppt is not black and much of the ppt dissolves.
2
then Hg 2 Is absent. Use a pipet to remove the liquid and test it in step 5. The ppt can be
discarded.
5. To the solution remaining from step 4, add 1 d of phenolphthalein (the solution should be pink at this
point) and then add 5 M HCl drop by drop, with stirring, until the solution loses its pink coloration
(becomes acidic). A considerable amount of curdy white ppt (AgCl) at this point confirms the
presence of Ag+ (a few specks should not be considered to be a positive test). Regardless of the
outcome, discard the contents of the tt and proceed to step 6 to test for the ions in Part B.
B. Separation of the Iron Group: Fe3+ and Al3+
6. To the solution set aside from step 1 (or 2), add 6 d of 20 % NH4Cl, then add 5 M NH3 drop by drop,
with stirring, until the solution becomes basic. (Test with litmus after each 2–3 drops; you may need
several drops. Gelatinous flakes of ppt or a transparent, thin "slush" may form as the solution is
becoming basic.) Then add another 10 d of 5 M NH3. STIR VERY WELL, TOP TO BOTTOM, and
heat for 4 min in a boiling water bath. Centrifuge.
If a gelatinous layer forms on the bottom of the tt, Fe3+ may be present as orange–brown
Fe(OH)3 and/or Al3+ as white to colourless Al(OH)3. Go to step 7 to test for Fe3+ and Al3+.
If NO gelatinous layer forms on the bottom of the tt, both Fe3+ and Al3+ are absent, so that
you can skip to step 12 to test the solution obtained in step 6 for the ions in Part C.
7. Use a pipet to remove and save the liquid for step 12 to test for ions in Part C. Wash the ppt with 1
mL of 2 % NH4NO3. Stir, re–centrifuge, and discard the wash liquid.
–4–
8. Add 12 d of 5 M NaOH and 1 mL of 3 % H2O2 to the ppt and stir. Heat in a boiling water bath for
8 minutes while stirring occasionally, leaving the stirring rod in the tt between stirring. (CARE: The
solution may foam up. If it does, remove the tt from the heat and stir before returning the tt to the
water bath.) At the end of this time no more tiny bubbles should be forming in the solution.
If a red, brown or black ppt is present, Fe3+ may be present. Centrifuge, remove the liquid
with a pipet and save it for step 11 to test for Al3+. (If the ppt is any other colour, Fe3+ is
absent and you should skip to step 11 to test for Al3+.) If Fe3+ may be present, go to step 9 to
test the ppt for Fe3+.
If no ppt is present, Fe3+ is absent. Skip to step 11 to test this solution for Al3+.
9. To the ppt from step 8 add 1 mL of 5 M HNO3 and 4 d of 3 % H2O2, and put into a boiling water bath
for 5 minutes (or less, if the ppt dissolves sooner).
If the ppt dissolves, proceed to step 10.
If the ppt won't dissolve, continue to boil gently for another 4 minutes, centrifuge the
mixture, remove the solution and proceed to step 10. Any ppt left can be discarded.
10. You will perform TWO tests for Fe3+ on the solution set aside from step 9. If EITHER of the steps is
found to be positive, you can claim Fe3+ to be present and don't need to do the other test.
i)
Put 1 d of the solution from step 9 onto a clean white spot plate and add 1 d of 0.1 M
KSCN. A blood–red solution confirms the presence of Fe3+.
ii)
To the remainder of the solution from step 9, add 1 d of 0.125 M K4[Fe(CN)6], and
stir. A DARK BLUE, almost opaque, solution confirms the presence of Fe3+. If the
solution is any other colour, including GREEN, Fe3+ is ABSENT! You can discard this
solution.
11. To the liquid from step 8, add 5 M HCl dropwise until the solution is acidic (check after each drop with
a piece of universal indicator paper torn into small squares, obtainable from your teacher), then add 5
M NH3 until the solution is basic to litmus, and finally add an extra 5 d of 5 M NH3. Centrifuge the
mixture. The presence of a white or colourless gelatinous layer at the bottom of the tt confirms the
presence of Al3+. (If ONLY a dark ppt is present, some contaminant is present and Al3+ is absent.)
Discard this mixture and proceed to step 12.
C. Separation of the Zinc Group: Zn2+ and Ni2+
12. To the solution from step 6 (or 7) add 10 d of 5 M NH3 and 10 d of saturated thioacetamide. Heat for
7 min in a water bath in a fume hood (Stench! Poisonous fumes!), then take out the test tube and
allow to cool in the fume hood for a minute.
If a ppt is present, one or both of Zn2+ (white ppt of ZnS) and Ni2+ (black ppt of NiS) may be
present: centrifuge, discard the solution and proceed to step 13 to test the precipitate.
If no ppt is present, Zn2+ and Ni2+ are both absent and you have now finished the
analysis.
13. Wash the ppt from step 12 with 1 mL of 1 % NH4Cl, centrifuge and discard the wash liquid. In the
fumehood, add 1 mL of distilled water and 12 d of 5 M HCl, and stir the ppt for 1 min.
If all the ppt dissolves, Ni2+ is absent; go immediately to step 15 to test for Zn2+.
If a ppt is present, centrifuge for 2 min, save the liquid for step 15 and proceed to step 14 to
test the ppt. The presence of Ni2+ is strongly suggested if the ppt is black.
–5–
14. To the ppt from step 13, add 10 d of 5 M HCl and 5 d of 5 % NaOCl. In a fume hood, heat the
solution in a boiling water bath and stir until the ppt dissolves.
[If the ppt doesn't dissolve completely in 3–4 min, add another 3 d of 5 % NaOCl, leave the
stirring rod in the tt and boil gently for a further 3 min. Pour off the solution and discard any
ppt; a sufficient amount of the ppt should have dissolved to enable you to test the solution.]
Next, add 5 M NH3 until the solution is just basic (test with litmus paper). Then add about 10 d of
dimethylglyoxime solution and stir. Finally, add 5 d of 5 M NH3, WITHOUT STIRRING. A fluffy pink–
red ppt confirms the presence of Ni2+. [If the test turns out negative BUT you notice that on adding the
NH3 a pink ppt INITIALLY formed but quickly disappeared, add several more drops of NH3 to see if
you now get a positive test.] Discard this solution and proceed to step 15.
15. Put the solution from step 13 onto a clean evaporating dish and heat almost to dryness (that is, until
only a few drops of liquid remain.) with a bunsen burner. [Note: it's OK if all the liquid does evaporate,
just don't "bake" the solution to dryness.] Add 1 mL distilled water, stir well to dissolve any solid
present, and transfer the resulting solution to a small test tube with a dropper. This solution will be
tested for the presence of Zn2+ using the "Zinc Test Kit" at the back of the room. (IMPORTANT: all
equipment used in this test must be very clean – the test is so sensitive that it can detect minute
amounts of zinc ion contaminants from a previous user of the equipment.)
Using a clean pipet and a clean, dry, white spot plate, add 2 drops of the solution to be tested
to one depression of the spot plate. To the solution in the depression add 1 d of 1 M H2SO4,
followed by 1 d of 0.02 % CoSO4, and finally 2 d of ammonium mercuri–thiocyanate reagent.
Use a clean stirring rod to gently stir the mixture in the spot plate depression, then let the
mixture settle.
•
If a sky–blue ppt is seen within 15 seconds (as a granular spot about 0.5 cm
across at the bottom of the depression) Zn2+ is present.
• If the solution remains colourless, or if the ppt is any other colour than blue, Zn2+ is
absent.
IV. CONCLUSION
Submit your experimental notes giving the details of the tests you performed, the observations you made, and the
conclusions you made. A set of scrawled comments on the hand–out procedure sheets is not acceptable. At the
end of the sheets, summarize your results by stating the ions you found, your sample number and the initial colour
of the solution you were given.
–6–
FLOW CHARTS FOR SEPARATION STEPS
2
Part A: Separation of the Silver Group: Ag+and Hg 2
–7–
Part B: Separation of the Iron Group: Fe3+, Al3+
–8–
Part C: Separation of the Zinc Group: Zn2+, Ni2+