Getting Started
Using Plastic Droppers
Separation of the Group I cations
HCl is used to separate silver and lead from the other Group II and Group III cations.
Isolating Group IThis precipitates out AgCl(s) and PbCl2(s) and leaves Cu2+, Bi3+, Ni2+, and Al3+ in solution. Decant the supernatant solution into a clean, dry test tube. Label the test tube containing the solid: Group I ppt and save it for later analysis.
Decanting Group II/III From Group ISeparating the Group II Cations From the Group III Cations
We will utilize the selective solubility of sulfides to separate the Group II and Group III cations. Before this separation can be performed, the initial solution must be placed in a casserole and heated to remove most of the water, resulting in a more concentrated solution of cations.
Placing Group II&III Cations in Casserole and EvaporatingThe sulfide source will be thioacetamide, which produces a saturated solution of H2S when heated.
The solution from the casserole should be placed in a test tube and heated for at least 5 minutes. A dark precipitate will occur indicating the Group II cations have precipitated out of the solution.
Precipitating Group IIDecant the solution containing the Group III cations and now you will have three test tubes. One containing the Group I ppt, another containing the Group II precipitate, and a third containing the Group III solution.
Isolating Groups I, II, & IIIIsolating and Identifying the Group I Cations
It is fairly simple to separate the Group I cations once they are isolated from the other Group II and Group III cations. The solubility of AgCl does not change appreciably with temperature, but PbCl2 is much more soluble at high temperatures. If water is added to the test tube containing the Group I ppt, and it is heated in a boiling water bath, the PbCl2 will dissolve and the AgCl will remain as a solid in the solution.
Isolating the Group I CationsBy placing the hot supernatant solution in a clean, dry test tube you have now isolated the Pb2+ from the AgCl(s).
To Confirm the presence of Pb2+ add a drop of potassium dichromate, which produces the CrO42- ion in solution. A yellow solid (PbCrO4) confirms the presence of lead in your sample.
The white solid can now be tested to see if it indeed contains silver. Adding concentrated NH3 to AgCl(s) will form a complex ion and dissolve the solid. [Note: If excess PbCl2 is present it will not dissolve in the presence of NH3 and some white solid might still be present.] To confirm silver is present add nitric acid until the solution is acidic and the white precipitate will re-form, confirming the presence of silver in your sample.
Isolating and Identifying the Group II Cations
The dark precipitate is made up of CuS and Bi2S3. The strategy here is to dissolve this solid using concentrated nitric acid to put both ions in solution.
Dissolving the Group II SolidIf you take a look at the formation constant table you will see that Cu2+ forms a complex ion with ammonia, while Bi3+ does not. This will allow us to separate the Cu2+ from Bi3+ by adding concentrated NH3. The copper ions will remain in solution as a complex ion and the bismuth ions will form a precipitate with hydoxide ions produced in the solution from the ammonia.
Separating the Group II CationsThe supernatant liquid can be decanted into a clean, dry test tube, which effectively separates the Cu2+ and the Bi3+. Cu(NH3)42+ is a distinct blue color and if your solution is blue, it confirms the presence of copper in your sample.
It's possible that if the Pb2+ was not properly removed in the Group I separation, that the white precipitate could be formed from Pb2+. To verify the white solid is Bi(OH)3 we will take advantage of some oxidation/reduction chemistry (we will discuss the details late in Chapter 20). Bismuth can be confirmed by adding NaOH and SnCl2. In the process, Bi3+ is reduced to Bi(s), which is a dark black color. The presence of this dark black solid confirms bismuth.
Confirming BismuthIsolating and Identifying the Group III Cations
The Group III solution set aside from the initial separations is acidic. The separation of Ni2+ and Al3+ is very similar to the separation of Ni2+ and Bi3+. Looking at the Kf table Ni2+ forms the Ni(NH3)6 complex ion with ammonia, where Al3+ does not form a complex ion with ammonia. There is one catch though. Al3+ is amphoteric. This means that when concentrated NH3 is added to the solution, producing OH- ions, the Al3+ can redissolve to form the Al(OH)4- complex ion. In order to prevent this complex ion from forming, a buffer must be added to keep the pH around 9. If these steps are properly executed, all of the Al3+ will precipitate out as Al(OH)3 and all of the Ni2+ will be present in solution as Ni(NH3)62+.
Group III SeparationThe Ni2+ and Al3+ can be separated by decanting the supernatant solution into a clean, dry test tube.
To confirm the presence of nickel in your sample, add the dimethylglyoxime anion, which will bind to nickel to produce a red gel-like solid.
In order to test for aluminum, the Al(OH)3 solid must be dissolved back into solution. This can be done by adding nitric acid.
An aluminon dye is added to the solution to confirm aluminum is present in your sample.
