Experiment # 8
Qualitative Analysis of Cations
Part I: Separate a mixture of known cations: Pb2+, Fe3+, Al3+, Cu2+, and Ca2+ into individual ions. Perform a series of tests to become familiar with reactions characteristic of each cation.
Part II: Separate and identify individual cations in an unknown solution containing a mixture of up to five cations. Use information gathered in part I to identify the cations. Introduction
Qualitative analysis is a general name for the process of determining the identity rather than the amount of chemical species. The qualitative process utilizes reaction(s) characteristic of a given chemical species and interprets the obtained results using a deductive thought process. Qualitative analysis of cations requires an extensive knowledge of various aspects of chemistry including acid-base equilibria, complex ion equilibria, solubility, etc. However, in the deductive process, common sense and logic can be as helpful as a knowledge of the chemistry involved.
In this lab you will be working with a solution containing a mixture of cations. Before individual components of the mixture can be successfully identified, they have to be separated. The individual cations, once separated, can then be identified in ensuing confirmatory reactions (see the flowchart below). Knowing the actual results of the reactions characteristic for a given cation (or a group of cations) makes it easier to identify the ion in an unknown sample. Therefore, it is useful to perform all of the characteristic reactions first on a mixture of ions of known identity, and then proceed to the unknown sample.
Reactions Characteristic for Pb+2
Separation of Pb+2 from Fe+3, Al+3, Cu+2, and Ca+2
Out the five cations in the mixture, only lead ion will precipitate, as PbCl2, when hydrochloric acid is added to the solution.
Pb2+ (aq) + 2Cl- (aq)
The solid PbCl2 is separated from the solution containing the other four ions by centrifuging the reaction mixture.
Confirmation for Pb+2
If the separated solid dissolves to form a colorless solution of a lead(II) complex when treated with NaOH, the presence of Pb+2 in the mixture is confirmed: PbCl2 (s, white) + 4OH- (aq)
[Pb(OH)4]2- (aq, colorless) + 2Cl- (aq)
Reactions Characteristic for Fe+3 and Al+3
Separation of Fe+3 and Al+3 from Cu+2 and Ca+2
The supernatant solution from the previous test now contains only four ions. Iron(III) ions and aluminum ions are precipitated together as hydroxides upon addition of aqueous ammonia:
Fe+3 (aq, yellow) + 3OH- (aq)
Al+3(aq, colorless) + 3OH- (aq)
Fe(OH)3 (s, orange-brown)
Al(OH)3 (s, gelatinous white)
Remember that aqueous ammonia is a weak base that is only partially ionized by water to form OH- ions and NH4+ ions. A strong hydroxide such as NaOH would precipitate copper(II) hydroxide while keeping aluminum ions in solution as a complex ion (equation 5).
Separation of Fe+3 and Al+3
Before each ion can be identified, the Fe+3 and Al+3 must be separated from each other. This is achieved by adding a strong base, NaOH. Because aluminum ion is amphoteric, Al(OH)3 will dissolve in a strong base while Fe(OH)3 will not: Al(OH)3 (s, gelatinous white) + OH- (aq)
[Al(OH)4]- (aq, colorless)
The orange solid, Fe(OH)3, is separated by centrifugation from the colorless supernatant solution containing [Al(OH)4]- ions and the pellet is washed with water to ensure that any adhering Al(OH)3 is washed away.
Confirmation of Fe+3
Since Fe+3 ion is in a solid at this point it has to be dissolved by hydrochloric acid before it can undergo a confirmation reaction. The resulting solution contains aqueous iron (III) chloride.
Fe(OH)3 (s, orange-brown) + 3 H +(aq)
Fe3+ (aq, yellow) + 3H2O(l)
The confirmation reaction for Fe+3 ions involves addition of aqueous potassium thiocyanate, KSCN, to form dark red FeSCN+2.
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