Applied Qualitative Analysis Lab

Tips for Success in the Qualitative Analysis Lab

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 I

This 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 I

Separating 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 Evaporating

The 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 II

Decant 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, & III

Isolating 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 Cations

By 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 CrO₄^2- 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 Solid

If 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 Cations


The 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 Bismuth

Isolating 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 K_f 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 Separation

The 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.