In this experiment, you will use analyze the characteristic reactions that a variety of cations undergo. You will then devise a qualitative analysis scheme to analyze a solution that contains a variety of these cations in one solution. You will then use this analysis to determine the identity of an unknown cation solution.
Introduction
When a chemist conducts a qualitative analysis, the primary goal is to identify the substances that are present in a mixture. It is important to separate substances from each other in a mixture, so that a confirming test can be as definitive as possible. After a thorough qualitative analysis investigation, it would be nice to say, for example, that potassium is present instead of saying merely that an alkali metal is present.
Conducting a qualitative analysis, or “qual scheme” as it is sometimes nicknamed, is a methodical process that requires great care. A qual scheme can be used to identify a wide variety of substances; in this experiment, you will focus on ions. In this unit you have been learning about solubility equilibria and complex-ion equilibria, both of which play crucial roles in the separations and identifications that are part of a qual scheme. You will see some very interesting things during this experiment, things that will help you understand more about the nature of cations and anions.
Reaction with HCl
A common step in qualitative analysis schemes is to react with chlorides. Usually only a few cations, Pb2+, Hg22+, and Ag+, will react to form precipitates with chloride ions. It is useful to use reagants that only react with a few cations to more easily narrow down the identity of some cations.
In this lab you will be testing Pb2+ and Ag+. The way you can determine the difference between the two ions is by adding NH3 to the precipitate. AgCl will dissolve in a basic solution but PbCl2 will not.
Reaction with NH3
Ammonia is a unique reagent because it will show a variety of products. Sometimes a precipitate occurs with a characteristic color, but in other cases a complex ion is formed with a very vibrant color change. Therefore it is useful to record all changes that you observe.