Cation Analysis

Separation and Identification of Group II Cations

• Group II cations consists of ions that forms acid-insoluble sulfides.

• Cations in the 2nd group include: Cd2+, Bi3+, Cu2+, As3+, As5+, Sb3+, Sb5+, Sn2+, Sn4+ and Hg2+. Pb2+ is usually also included here in addition to the first group.

• The reagent can be any substance that gives S2- ions in such solutions. H2S

CH3CSNH2 (thioacetamide)

• The test with the sulfide ion must be conducted in the presence of dilute HCl

to keep the sulfide ion concentration at a required minimum, so as to allow the precipitation of 2nd group cations alone

• Precipitates are almost indistinguishable, except for CdS which is yellow.

• All the precipitates, except for HgS, are soluble in dilute mineral acids. HgS is soluble only in aqua regia, which can be used to separate it from the rest.

• The action of ammonia is also useful in differentiating the cations. CuS dissolves in ammonia forming an intense blue solution, while CdS dissolves forming a colourless solution.

IONCOLOR OF SOLUTION

OF IONPRECIPITATED FORM

OF IONCOLOR OF THE PRECIPITATE

Hg2+ Colorless HgS Black

Pb2+ Colorless PbS Black

Bi3+ Colorless Bi2S3 Brown to Black

Cu2+ Blue CuS Black

Cd2+ Colorless CdS Yellow

As3+ Colorless As2S3 Yellow

Sb3+ Colorless Sb2S3 Orange

Sn4+ Colorless SnS2 Yellow

Yellow ppt- Pb+2Yellow ppt- Pb+2

Red ppt- Cu+2Red ppt- Cu+2

Yellow ppt- Cd+2Yellow ppt- Cd+2

Cation Analysis

Separation and Identification of Group V Cations

• Cations which are left after carefully separating previous groups are considered to be in the fifth analytical group.

• The most important ones are Mg2+, Li+, Na+, K+ and NH4

+.

Yellow ppt- Mg+2Yellow ppt- Mg+2

White ppt- Mg+2

Flame test in 10 drops of original sample

Compare the intensity and duration of flame with distilled water and of 0.1 M NaCl solution

Pour 10 drops of the fresh sample into a 50 mL beaker.

Pour 10 drops of the fresh sample into a 50 mL beaker

Moisten a piece of red litmus paper and put at the bottom of a small watch glass.

+10 drops of 6 M NaOH; swirl to stir.

Cover the beaker with watch glass; heat the solution.

Do not boil and be careful that no liquid solution comes in contact with the litmus paper.

Observe the color of the litmus paper.

Sources

• Group II Cation– bnaddy.columbiastate.edu/HANDOUTS/

CATIONS/2CATION.DOC

• Group V Cation– bnaddy.columbiastate.edu/HANDOUTS/

CATIONS/5CATION.DOC

• Introduction to Qualitative Analysis– http://chemistry.about.com/library/weekly/

aa091001a.htm