Experiment 1 – Qualitative Analysis of Anions Page 1 of 1
Note: Protective eyewear must be worn AT ALL TIMES during this activity
1. Prepare a mixture of the four anions by adding ~1 mL each of the following 0.10 mol/L solutions to one clean test tube; Na2SO4 (aq), Na2CO3 (aq), NaCl (aq) and Nal (aq).
(Mix by pouring back and forth between 2 test tubes), (sodium is spectator ion).
Record detailed observations including the colours and relative amounts of solutions and precipitates as they are observed.
2. Pour ~2 mL of your anion solution into another clean test tube and add 20 drops of 0.20 mol/L Ba(NO3)2 (aq). Record your observations.
(The extra 2 mL of anion solution can be used to repeat the whole procedure later if time).
3. Centrifuge for 1 minute then test the solution for complete precipitation by adding 2 more drops of the Ba(NO3)2 solution. If more precipitate forms, centrifuge again then go into step 4.
If no more precipitate forms, go on to step 4 right away.
4. Decant the supernatant liquid and save it for step 7.
5. Wash the precipitate with 10 drops of distilled water then centrifuge, decant and discard the wash solution.
6. To the precipitate add 10 drops of 6.0 mol/L HNO3(aq)(Danger!). Stir with a glass rod and
observe. If a bubbling occurs (what gas is being released?) and some of the precipitate disappears, this confirms the presence of CO32-(aq) in BaCO3(s). If some of the precipitate
remains, this implies the presence of SO42-(aq) in BaSO4(s). If all of the precipitate disappears, or none of it disappears, then you likely had just one of the precipitates.
7. To the solution from step 4, add 20 drops of 0.10 mol/L AgNO3(aq) solution (interesting fact:
stains skin temporarily black - used to be put in babies eyes as antibiotic). Record your observations.
8. Centrifuge, decant and discard the supernatant liquid. Wash the precipitate with 10 drops of distilled water. Decant and discard the wash solution.
9. Add 20 drops of 6.0 mol/L NH4OH(aq)(Danger! Don’t smell the solution) to the precipitate and
stir with a glass rod to break up the precipitate.
10. Centrifuge and separate the solution from the remaining precipitate of AgI(s) and set it aside for
step 11. Note the colour of the precipitate. Finding a precipitate of this colour at this point in the procedure confirms the presence of I-.
11. To the solution from step 10, add 20 drops of 6.0 mol/L HNO3(aq). If a precipitate of AgCl(s)
forms (note the colour), this confirms the presence of Cl
Reminder: Protective eyewear must be worn at all times in the lab
1. Make up a test solution by adding 20 drops 1mL (burette) of each of the available 0.25 mol/L nitrate solutions of Fe3+
(aq); Cr3+(aq); Ni2+(aq) and Co2+ into one test tube.
Part A: The Iron Group – Fe3+
(aq) and Cr3+(aq)
2. Add 2.0 mL of 6.0 mol/L NH4OH(aq) to your test solution and stir thoroughly. The dark
precipitate that forms is a mixture of Fe(OH)3(s) and Cr(OH)3(s).
3. Centrifuge then test the solution for complete precipitation. When you are sure that all the Fe(OH)3(s) and Cr(OH)3(s) has been removed, decant the solution and set it aside for step 11.
Wash the precipitate with a few drops of distilled water, re-centrifuge and discard the wash solution.
4. Dissolve the precipitate from step 3 by adding 6.0mol/L HCl(aq)drop by drop and stirring with
a glass rod.
5. Make strongly basic to litmus paper by adding 6.0 mol/L NaOH(aq) until litmus turns blue then
add three more drops. (Test on the litmus paper by using a stirring rod to get a drop of solution and placing the drop on the litmus paper.) A rust coloured precipitate of Fe(OH)3(s)
should appear.
6. Using a test tube holder, carry the test tube to the fume hood and carefully add 20 drops 10-15 seconds apart (one or two at a time) of 30% H2O2(aq) (hydrogen peroxide) solution. (If too much
H2O2(aq) is added at one time, the solution may foam up and spill down the outside of the test
tube creating a very dangerous situation.) Do this in the Fume Hood. Caution: H2O2(aq) severely burns skin.
Use Fume Hood/Air Vent
7. Return to the fume hood and gently heat the test tube in a hot water bath. Once again there is a
danger the solution may foam up and spill down the outside of the test tube. If this starts to happen, remove the tube from the water bath until the bubbling subsides. This warming decomposes the remaining hydrogen peroxide. It takes about 15 minutes. While you are waiting, you could go to step 11 (or read to learn the purpose of the peroxide)
8. Centrifuge and decant the solution into another test tube and set it aside for step 10. (The solution should be yellow, indicating the presence of the ion.) Treat the precipitate in step 9.
9. Dissolve the precipitate from step 8 by adding 6.0 mol/L HCl(aq) a few drops at a time and
stirring. Add 30 drops of 0.1 mol/L KSCN(aq) solution. The appearance of a deep blood red
coloured solution of Fe(SCN)2+
(aq) confirms the presence of the Fe3+(aq) ion.
10. Add 5 drops of 0.1 mol/L BaCl2(aq) solution to the solution from step 8. A yellow precipitate of
Use Fume Hood/Air Vent
Part B: The Nickel Group – Ni2+
(aq) and Co2+(aq)
11. Divide the solution from step 3 into two separate test tubes. Use one portion for step 12, and one for step 13.
12. To the first portion of the solution from step 3, add 3 to 10 drops of dimethyl glyoxime and warm in a water bath (perhaps try warming the solution before adding the drops?). A
strawberry-red precipitate of NiC8H14N4O4(s) confirms the presence of the Ni2+(aq) ion in the
original solution.
13. Acidify the second portion of the solution from step 3 by adding 6.0 mol/L HNO3(aq) one drop at
a time (add drops until all blue colour is gone). Add 1.0 mL of 6.0 mol/L KNO2(aq) solution then
warm and let stand for 15 minutes in a hot water bath. Beware of the brown fumes of NO2(g)
that are emitted fro the mouth of the test tube. They are dangerous to inhale. Touch to litmus paper using glass rod.
14. After heating the test tube for 15 minutes, place it in the centrifuge. A yellowish precipitate of K3Co(NO2)6(s) in the bottom of the test tube confirms the presence of Co2+(aq) in the original
Equations and Explanatory Notes
The following notes and equations should help students to understand the chemistry behind each step of the procedure. Students are not expected to memorize these equations but they should study this material in order to be able to explain orally what is happening
Step 2 - the addition of excess 6.0 mol/L NH4OH(aq) to precipitate the Iron Group. a) For Fe3+
(aq) and Cr3+(aq)
eg. Fe3+
(aq) + 3NH4OH(aq) Fe(OH)3(s) + 3 NH4+(aq)
Notes: 1. Colours – Fe(OH)3(s) is brownish-red
Cr(OH)3(s) is green
(together all three produce a dark brown colour)
2. Both hydroxides will precipitate in a weak base such as NH4OH but will
re-dissolve in strong base such as NaOH(aq).
b) For Ni2+
(aq) and Co2+(aq)
eg. Co2+
(aq) + 2NH4OH(aq) Co(OH)2(s)+2NH4+(aq)
However, in excess NH4OH(aq) the hydroxides of Ni2+(aq) and Co2+(aq)will re-dissolve.
For example:
Co(OH)2(s) + 6 NH4OH(aq) Co(NH3)62+(aq) + 6 H2O(1) + 2 OH(aq)
Note: Co(NH3)62+(aq) is a deep amber colour
Ni(NH3)62+(aq) is a pale violet-blue colour.
Therefore, step 2 allows the separation of Fe3+
(aq) and Cr3+(aq) from Ni2+(aq) and Co2+(aq).
Step 3 – the addition of 6.0 mol/L HCl(aq) to dissolve the hydroxide precipitates of Fe3+(aq) and Cr3(aq)+.
For example:
Equations and Explanatory Notes
Step 5 – the addition of a strong base such as NaOH(aq).
a) For Cr3+(aq) we get a precipitate which re-dissolves
Eg: Cr3+
(aq) +3NaOH(aq) Cr(OH)3(s) + 3 Na+(aq)
Then Cr(OH)3(s) + OH-(aq) Cr(OH)4-(aq)
b) However, for Fe3+
(aq), the precipitate does not re-dissolve
Fe3+
(aq) + 3NaOH(aq) Fe(OH)3(s) + 3 Na+(aq)
Therefore we have been provided with a way to separate Fe3+
(aq) from the other ion in this group.
Step 6 – the addition of 30% H2O2(aq) to oxidize Cr3+(aq) to Cr6+(aq)
2 Cr(OH)4-(aq) + 3 H2O2(aq) + 2 OH-(aq) 2 CrO42-(aq)+ 8 H2O(l)
green yellow
Note: any excess H2O2(aq)must be decomposed by heat or it will interfere with later
reactions
2H2O2(aq) + heat 2H2O + O2(g)
then, as in step 2
Cr3+
(aq) + 3NH4OH(aq) Cr(OH)3(s) + 3 NH4+(aq)
green
Step 9 – the addition of HCl(aq) and KSCN(aq) to Fe(OH)3(s)
Fe(OH)3(s) + 3 HCl(aq) Fe3+(aq) + 3 H2O(l) + 3 Cl-(aq)
- now add KSCN(aq)
Fe3+
(aq) + 6SCN-(aq) Fe(SCN)63-(aq)
blood-red
Equations and Explanatory Notes
Step 10 - the addition of BaCl2(aq) to the yellow CrO42-(aq) solution
BaCl2(aq) + CrO42-(aq) BaCrO4(s) + 2 Cl-(aq)
yellow
- this is the confirmatory test for the presence of Cr3+ (aq)
Step 12 - the addition of dimethyl glyoxime to Ni(NH3)62+(aq)
Ni(NH3)62+(aq) + 2 (CH3)2C2(NOH)2(aq)
NiC8H14N4O4(s) + 2 NH4+(aq) + 4 NH3(g)
strawberry-red
- this is the confirmatory test for the presence of Ni2+ (aq)
Step 13 - the addition of HNO3(aq) and KNO2(aq) to Co(NH3)62+(aq)
Co(NH3)62+(aq) + 6 HNO3(aq) Co2+(aq)+ 6 NH4+(aq) + 6 NO3-(aq)
then 3 Co2+
(aq) + NO3-(aq) + 4H+(aq) 3 Co3+(aq) + NO(g) + 2 H2O(l)
finally Co3+
(aq) + 6 KNO2(aq) K3Co(NO2)6(s) + 3 K+(aq)
yellow
Experiment 3:
Analysis of an Unknown Solution Page 1 of 4Reminder: Protective eyewear must be worn at all times in the lab. Introduction:
At this point, you are ready to proceed with your independent laboratory work involving the analysis of one unknown solution. Each numbered test tube may contain any combination of the four anions and seven cations you have studied. There will be 4 ions in each test tube. Each student will have different solutions.
Each test tube contains approximately 8 mL of solution, which should be sufficient to complete and double check each analysis. If the tube is broken or lost or the contents are spilled or used up before you are finished, you may obtain more but penalty marks will be levied.
Procedure: Part A – Anion Analysis – SO42-(aq), CO32(aq)-, Cl-(aq) and l-(aq)
1. This may be done independently of the cation analysis. Measure out approximately 1.0 mL of your unknown solution into a clean test tube. Add 5 drops of 6.0 mol/L NH4OH(aq) and stir. A
precipitate of iron and nickel group ions should appear if your sample contains these metal ions. What could happen if you add too much NH4OH(aq)?
2. Centrifuge and decant the solution into a clean test tube. Discard the precipitate. Add 20 drops of 0.20 mol/L Ba(NO3)2(aq) and stir.
3. Centrifuge for 1 minute then test the solution for complete precipitation by adding 2 more drops of the Ba(NO3)2(aq) solution. If more precipitate forms, centrifuge again then go onto step
4. If no precipitate forms, go on to step 7 right away.
4. Decant the supernatant liquid and save it for step 7.
5. Wash the precipitate with 10 drops of distilled water then centrifuge. Decant and discard the wash solution.
6. To the precipitate from step 3 add 10 drops of 6.0 mol/L HNO3(aq)(Danger!). Stir with a glass
rod and observe.
If bubbles form and the coloured precipitate disappears, this confirms the presence of CO32-(aq)
in the original solution.
If none of the coloured precipitate disappears and no bubbles form, this confirms the presence of SO42-(aq) in the original solution.
7. To the solution from step 4, add 20 drops of 0.10 mol/L AgNO3(aq). If a precipitate forms,
proceed with step 8; if no precipitate forms… what does that tell you?
8. Centrifuge, decant and discard the supernatant liquid. Wash the precipitate with 10 drops of distilled water. Decant and discard the wash solution.
9. Add 20 drops of 6.0 mol/L NH4OH(aq) solution to the precipitate and stir with a glass rod.
10. Centrifuge and separate the solution from the remaining precipitate (if there is one) and set it aside for step 11. A precipitate remaining at this point in the procedure implies the presence of l
-(aq) in the original solution.
11. To the solution from step 10, add 10 drops of 6.0 mol/L HNO3(aq). The formation of a
precipitate at this point, implies confirms the presence of Cl
-(aq) in the original solution.
12. Discard the remaining solution and precipitates from this part of the procedure.
13. Repeat Part A – Anion Analysis , but only after you have finished Part B –Cation Analysis
on your unknown sample.
Procedure: Part B – Cation Analysis Use Air Vent/Fume Hood I) The Iron Group – Fe3+
(aq) and Cr3+(aq)
1. To a clean test tube, add approximately 1.0 mL of your unknown solution. To this, add 2.0 mL of 6.0 mol/L NH4OH(aq) to your test solution and stir thoroughly. If a precipitate forms, what does this tell you? What does the colour of the precipitate tell you? If no precipitate forms,
what does this tell you?
2. Centrifuge, then test the solution for complete precipitation. When you are sure that all the Iron Group ions have been removed, decant the solution and set it aside for step 11.
3. Wash the precipitate with a few drops of distilled water, re-centrifuge and discard the wash solution. Treat the precipitate by step 4.
4. Dissolve the precipitate from step 3 by adding 6.0 mol/L HCl(aq)drop by drop and stirring with
a glass rod.
5. Make strongly basic to litmus paper by adding 6.0 mol/L NaOH(aq) until litmus paper turns blue
then add three more drops.
6. Using a test tube holder, carry the test tube to the fume hood and carefully add 20 drops (one or two at a time) of 30% H2O2(aq) solution. (Remember, if too much H2O2(aq) is added at one time,
the solution may foam up and spill down the outside of the test tube creating a very dangerous
situation).
7. Return to your lab station and gently heat the test tube in a hot water bath. Once again there is a danger that the solution may foam up and spill down the outside of the test tube. If this starts to happen, remove the tube from the water bath until the bubbling subsides. While you are waiting, you could go on to step 11.
8. Centrifuge and decant the solution into another test tube and set it aside for step 10. If there is a precipitate, treat it by step 9. If there is no precipitate, what does this tell you?
9. Dissolve the precipitate from step 8 by adding 6.0 mol/L HCl(aq) a few drops at a time and
stirring. Add 30 drops of 0.1 mol/L KSCN(aq) solution. The appearance of a dark red solution confirms the presence of Fe3+
(aq) in the original solution.
10. Add 5 drops of 0.1 mol/L BaCl2(aq) to the solution from step 8. A precipitate of BaCrO4(s) confirms the presence of Cr3+
ll) The Nickel Group – Ni (aq) and Co (aq)
11. Divide the solution from step 2 into two separate test tubes. Use one portion for step 12 and one for step 13.
12. To the first portion of the solution from step 2, add 3 to 10 drops of dimethyl glyoxime and warm in a water bath. A strawberry-red precipitate of NiC8H14N4O4(s) confirms the presence of
the Ni2+
(aq) ion in the original solution.
13. Acidify the second portion of the solution from step 2 by adding 6.0 mol/L HNO3(aq) one drop at
a time. Add 1.0 mL of 6.0 mol/L KNO2(aq) solution then warm and let stand for 15 minutes in a
hot water bath. Beware of the brown fumes of NO2(g) that are emitted from the mouth of the
test tube. They are dangerous to inhale.
14. After heating the test tube for 15 minutes, place it in the centrifuge. A yellowish precipitate of K3Co(NO2)6(s) in the bottom of the test tube confirms the presence of Co2+(aq) in the original
solution.
You have now completed Part B – Cation Analysis of your unknown.
