Test for Anions in Aqueous Solutions

TESTED ANIONS

• carbonate ion     
• chloride ion
• sulphate ion
• nitrate ion

A) CARBONATE IONS

Method:

CARBONATE solution + dilute HCl (hydrochloric acid)  + linked with delivery tube to limewater

Observation:

Limewater turns milky/cloudy/chalky

Inference:

1. The acid (HCl) react with CARBONATE ION to produce CO2 (carbon dioxide) gas. 
2. CO2 is released, forming a white precipitate (calcium carbonate). 
3. Thus, CARBONATE ION is present.

B) SULPHATE IONS

Method:

SULPHATE solution + dilute HCl + a few drops of BaCl2 (barium chloride) solution

Note that HCl can be replace with HNO3 (nitric acid), but not H2SO4 (sulphuric acid). BaCl can also be replaced with BaNO3 (barium nitrate).

Observation:

White precipitate formed.

Inference:

1. Barium ion [from barium chloride] and SULPHATE ION combines and forms BaSO4 (barium sulphate), which is an insoluble salt and forms a white precipitate.
2.  HCl is added to prevent the precipitation of BaCO3 (barium carbonate).
3. Thus, SULPHATE ION is present

C) CHLORIDE IONS

Method:

CHLORIDE solution + dilute HNO3 (nitric acid) + a few drops of AgNO3 (silver nitrate)

Observation:

White precipitate formed.

Inference:

1. The combination of silver ion [from silver nitrate] and CHLORIDE ION results in an insobule salt, AgCl (silver chloride), which is the white precipitate formed.
2. HNO3 is added to prevent the precipitation of both sulphate and carbonate ions. 
3. Thus, CHLORIDE ION is present.

D) NITRATE IONS

Method:

NITRATE solution + dilute HSO4 (sulphuric acid) + iron(II) sulphate + *shake well* + concentrated HSO4 [Known as the brown ring test]

Observation:

Brown ring is formed.

Inference:

1. Concentrated HSO4 reacts with NITRATE ION and form NO (nitrogen monoxide) molecule. 
2. NO and iron(II) sulphate combines and forms a brown ring complex. 
3. Thus, NITRATE ION is present.

Sources and References

Eng Nguan Hong, Lim Eng Wah and Lim Yean Ching. Focus Chemistry SPM Form 4,5. Bangi: Penerbitan Pelangi Sdn. Bhd.

Page: 255-256

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