Test for sulfates, carbonates and ammonium / PAG4
Testing for Negative ions (anions) 3.1.4 Qualitative analysis Fizzing due to CO2 would be observed if a carbonate was present Testing for Presence of a carbonate Add any dilute acid and observe effervescence. Bubble gas through limewater to test for CO2 – will turn limewater cloudy 2HCl + Na2CO3 2NaCl + H2O + CO2 Testing for Presence of a sulphate Acidified BaCl2 solution is used as a reagent to test for sulfate ions If Barium Chloride is added to a solution that contains sulphate ions a white precipitate forms Ba2+ (aq) + SO4 2-(aq) BaSO4 (s). Other anions should give a negative result which is no precipitate forming The acid is needed to react with carbonate impurities that are often found in salts which would form a white Barium carbonate precipitate and so give a false result Sulphuric acid cannot be used to acidify the mixture because it contains sulphate ions which would form a precipitate. The sequence of tests required is carbonate, sulfate then halide. (This will prevent false results of as both BaCO3 and Ag2 SO4 are insoluble.) Testing for positive ions (cations) Test for ammonium ion NH4 + , by reaction with warm NaOH(aq) forming NH3 Ammonia gas can be identified by its pungent smell or by turning red litmus paper blue.
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3.1.4 Qualitative analysis Learning outcomes Additional guidance Learners should be able to demonstrate and apply their knowledge and understanding of: Tests for ions (a) qualitative analysis of ions on a test-tube scale; processes and techniques needed to identify the following ions in an unknown compound: (i) anions: • CO3 2–, by reaction with H+(aq) forming CO2(g) (see 2.1.4 c) • SO4 2–, by precipitation with Ba2+(aq) • Cl –, Br–, I– (see 3.1.3 g) (ii) cations: NH4 +, by reaction with warm NaOH(aq) forming NH3. Sequence of tests required is carbonate, sulfate then halide. (BaCO3 and Ag2SO4 are both insoluble.) PAG4 HSW4 Qualitative analysis.4 Qualitative analysis of
ions
• Use of apparatus for qualitative tests for ions
• Make and record qualitative observations
Identification of the anions and cations
present in a mixture of Group 2 salts
3.1.3(g), 3.1.4(a), 5.3.2(a)
ions
• Use of apparatus for qualitative tests for ions
• Make and record qualitative observations
Identification of the anions and cations
present in a mixture of Group 2 salts
3.1.3(g), 3.1.4(a), 5.3.2(a)
Test for halides / PAG4
Please refer to Halide ions with silver nitrate under Group 7
Credits: Neil Goalby