This Demonstration shows the steps in the qualitative analysis of an unknown mixture of cations in an aqueous solution, without the formation of insoluble precipitates. This is a classic procedure, long since superseded by spectroscopic methods, but possibly still useful for teaching chemical laboratory techniques. The following anions are assumed to have been already eliminated: , , and . Other interfering anions are eliminated in the course of the procedures.
The analysis in this Demonstration is based upon separating the different cations into groups, following selective precipitations due to the common ion effect, which, in turn, is pH related. Each precipitate must be washed at each separation, according to the standard procedures of analytical chemistry. Our choice is to represent the solution path through the vertical flow line, while the precipitates are represented by horizontal flow arrows. The bifurcations show the division of the solution into different portions. A missing cation does not guarantee the absence of precipitate because of several possible problems (e.g. incomplete precipitation). To highlight this, the precipitate that should not be there but could show up is written in red.
Using the drop-down menu, it is possible to select the general scheme or a scheme limited to specific groups. When "general scheme" is selected, by checking the relevant checkbox it is possible to choose the cations and display the path followed in the analysis. The path should almost always end up through a special reaction uniquely identifying the cation; such unique identification is possible because of cation separation. The VIB Group procedure is not taken into consideration, as in standard lab practice these cations are more easily identified by means of a flame test.
Snapshot 1: In the general scheme, we can see that , because of incomplete precipitation, may even end up in the IIB Group, and therefore, the analysis would only consist of the IB Group, while the IIB Group research would only be used as a confirmation. On the other hand, should only be included in the IVB Group. However, because of a partial air oxidation, it could become and precipitate in the IIIB Group too, and therefore, either the confirmation or the research will depend upon the specific situation.
Snapshot 2: In the IB Group scheme, the black precipitate is already a confirmation that is there, so no specific identification reaction will be required (as in the case of ).
Snapshot 3: In the IIB Group scheme, the group is divided into two subgroups, where the copper group is a precipitate and the process with will dissolve it completely, and therefore, the process will continue on the solution so obtained, according to the rule as detailed above.