TITLE: Qualitative Analysis Of Cations: Ca2+, Ba2+, Mg2+, Zn2+ and Al3+ OBJECTIVE:
To identify the cations in known and unknown samples.
To construct a logical flow chart for identifying the components of a mixture of unknown cations. INTRODUCTION:
In this experiment we will study about qualitative analysis of cations: Ca2+, Ba2+, Mg2+, Zn2+ and Al3+. Qualitative analysis is an analytical procedure in which the question ‘what is present?’ is answered. In a systematic qualitative analysis scheme, each substance present is separated from the other substances. Then a confirmatory test is used to prove that the isolated substance is expected one. As the name indicates, qualitative analysis is not concerned with the quantity of substance present, but rather is simply used to confirm its existence. It is mainly focused on detecting ions in an aqueous solution. The solution is treated with reagents to test for reaction characteristics of certain ions, which may cause color change, solid forming and other obvious visible changes. The cations to be studied include some common alkaline earth metals, group 13 and Zn2+. These ions are not colored in solution, and most of their compounds are white. It is impossible therefore to use colors of solutions or precipitates to indicate which of these cations is present in solution. Instead, in this experiment, you will use differences in solubility to separate from each other.
Small test tubes, test tube rack, litmus paper, test-tube holders, hot plate, centrifuge, three beakers (250 mL or 400 mL), distilled water bottle, glass rod, dropping pipette, vial containing unknown solution. CHEMICALS:
Aqueous solutions of Ca2+, Ba2+, Mg2+, Zn2+ and Al3+
1 M Na2SO4 (aq)
6 M NaOH (aq)
6 M HCl (aq)
6 M NH3 (aq)
1 M HCl (aq)
0.25 M Ammonium oxalate, (NH4)2C204 (aq)
0.1 M NaH2PO4 (aq)
0.1 M Na2S (aq)
Part A: Preliminary Observation
Doing the Experiment
2 mL samples of 0.1 M aqueous salt of Ca2+, Ba2+, Mg2+, Zn2+ and Al3+ were placed into small test tubes. A drop of aqueous solution was transferred to some red and blue litmus papers with a stirring rod to test each solution to see if it is acidic, basic or neutral. The observations were recorded in the report. 2.
1 drop of 1 M Na2SO4 (aq) was added to each test tube. The formula of the precipitate was written in the report. 3.
1 mL of 6 M NaOH (aq) was added to each of the four remaining test tube and it was stirred well. The formula of the products was written in the report (two precipitates and two complex ions). The two test tubes containing complex ions were saved for (6). 4.
The two test tubes containing precipitates were centrifuged, and the solution in each of these tubes was discarding. Each test tube was dissolved with 1/2mL 6 M HCl (aq). If the precipitates doesn’t dissolved completely, more HCl was added, a drop at a time with stirring, until it does. Then 6 M NH3 (aq) was added, a drop at a time until the solution is slightly basic. If a precipitate formed 1 M HCl (aq) was added dropwise until it just dissolved. 1 drop of 0.25 M (NH4)2C204 (aq) was added to each test tube. The formula of the precipitate was written in the report. 5.
To the test tube above (4) where no precipitate formed with ammonium oxalate, 2 drops of 6 M NH3 (aq) was added and a few of 0.1 M NaH2PO4 (aq). The precipitates, which may form slowly, had the formula MNaH2PO4, where M is the metal ion. The formula of the precipitate was written in the report. 6.
1.5 mL of 6 M HCl (aq) was added to the two test tubes from (3) containing complex ions. It was tested with litmus paper. If any precipitate remained, or if the solutions are not distinct acidic, more HCl was added until any precipitate present is dissolved and the solutions are acidic. Then, 6 M NH3 (aq) (at least 1 mL) was added until the solutions are distinctly basic and have a strong odor of ammonia. It was stirred well. The...
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