Analysis of inorganic compounds (Topic 4C)
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Halide ions can be identified with simple test tube reactions.
The unknown solution is first acidified with nitric acid.
Silver nitrate is then added dropwise to produce a silver halide precipitate.
The reaction follows the following general ionic equation:
If a solution contains halide ions it will form a silver halide precipitate, .
The colour of the precipitate formed depends on the halide ion present:
Silver chloride : white
Silver bromide : cream
Silver iodide : yellow
Silver carbonates also precipitate in neutral solutions, but the nitric acid added reacts with any carbonate ions that might be present, preventing a false positive result.
Hydrochloric acid should never be used to acidify the silver nitrate in this test as it is a source of chloride ions. These would react with silver ions, giving a false positive result.
Further testing of precipitates of silver halides, formed by the addition of acidified silver nitrate to a halide ion solution, can be carried out by adding dilute ammonia, followed by concentrated ammonia to confirm the silver halide present.
The table shows the observations of reactions of halide ions with silver nitrate, and the subsequent addition of ammonia solutions:
When testing for anions, the order in which the tests are completed is critical.
Both carbonate and sulfate ions will give a white precipitate when reacted with barium chloride, but only carbonate ions will cause effervescence with dilute acid; exclude the possibility of carbonate ions before testing for sulfates.
Both sulfate and chloride ions for a white precipitate when reacted with acidified silver nitrate, but only sulfate ions form a precipitate with barium chloride; exclude the possibility of sulfate ions before testing for halides.
The qualitative tests for anions included in this specification are summarised below.
The ammonium cation, , is tested for by:
1. Dissolving the sample in water.
2. Adding sodium hydroxide and warming the solution.
3. Holding damp red litmus paper in the fumes.
If ammonia is generated, it will turn damp red litmus paper blue.
Group 2 cations can be identified by their reactions with an excess of sodium hydroxide and sulfuric acid solutions.
All results are white precipitates or colourless solutions. The results vary according to the solubility of the various products.
Ions from Groups 1 and 2 metals are distinguishable by their characteristic flame colours.
Note: recall of these colours is required.
Flame colours in compounds of Group 1 and 2 elements can be observed in the lab.
Procedure:
1. Set the Bunsen burner to a blue flame with the air hole fully open.
2. Clean a nichrome or platinum wire by dipping it in concentrated hydrochloric acid and heating it in the Bunsen flame until no colour is visible in the flame.
3. Dip the wire back into the acid, then into a small amount of the powdered sample.
4. Place the sample in the hottest part of the Bunsen flame and observe the colour produced.