Maria Fe GutierrezStanislov B.Cierra C.Jessica Patrick Berquist February 28, 2017Experiment #4 – What is in my sample? “Let’s find the cations” Introduction:Qualitative analysis is the process by which species in mixtures are separated and identified. Quantitative analysis determines in which amount it is present in the sample. The qualitative analysis of a mixture containing all the common cations involves first separating them into five major groups based on solubilities. Each group is then treated further to separate and identify th eindividual ions. Group I are the insoluble chlorides. When dilute aqueous HCl is added to a solution containing a mixture of the common cations, only Ag+, Pb2+, and Hg22+will precipitate out as insoluble chlorides. All other chlorides are soluble and remain in solution. The Group I precipitate is removed, leaving the other ions in solution for treatment with sulfide ion. Group II are the sulfide that are insoluble in acid solution. After the insoluble chlorides are removed, the solution is still acidic, since HCl was added. If H2S is added to this solution, only the most insoluble sulfides (those of Hg2+, Cd2+, Bi3+, Cu2+, and Sn4+) will precipitate, since [S2-] is relatively low because of the high concentration of H+. The more soluble sulfides will remain dissolved under these conditions, and the precipitate of the insoluble salt is removed. Group III are sulfides insoluble in basic solution. The solution is made basic at this stage, and more H2S is added. As we saw earlier, a basic solution produces a higher [S2-], which leads to precipitation of the more soluble sulfides. The cations precipitated as sulfides at this stage are Co2+, Zn2+, Mn2+, Ni2+, and Fe2+. The precipitate is separated from the solution containing the rest of the ions. Group IV are the insoluble carbonates. At this point, all the cations have been precipitated exceptthose from Groups 1A and 2A of the periodic table. The Group 2A cations form insoluble carbonates and can be precipitated by the addition of CO32-. For example, Ba2+, Ca2+, and Mg2+form solid carbonates and can be removed from the solution. Lastly, Group V are the alkali metals and ammonium ions. This experiment has two parts to it. The first part was completed during one lab class session andthe second part was completed during the following lab class session. During the first part, the goal is to use qualitative analysis to identify cations of group I (Ag+, Pb2+, Hg22+), group II (Cu2+, Bi3+), and group III (Fe3+). During the second part, the goal is to follow the same procedure as thefirst part, but this time we will have to identify the cations from an unknown solution sample given to us. We will accomplish this by using a centrifuge and then decantation.
Materials:Test tubes/test tube rack: These were used to hold the solutions after separating by centrifuging and decanting the cations.Wax pencil: Used to label the test tubes to avoid confusion.10 mL graduated cylinder: Used to dispense 4 mL of the known cation mixture in part 1.
You've reached the end of your free preview.
Want to read all 11 pages?