Testing for ions

​​In a nutshell

Hydrochloric acid can be used to test for carbonates. Testing for sulfates requires hydrochloric acid and barium chloride, a positive result will return a white precipitate. Litmus paper and sodium hydroxide can be used to test for ammonium ions.

Equations

Testing for: carbonates

Dilute acid reacted with carbonates $$CO_3^{2-}(aq)$$ will produce bubbles because $$CO_2$$ is given off.

$$CO_3^{2-}(aq)\ +\ 2H^+(aq)\ \rightarrow\ CO_2(g)\ +\ H_2O(l)$$​​

Dilute hydrochloric acid and hydrogen carbonates will also produce bubbles because carbon dioxide is given off. 

​$$HCO_3^-(s)\ +\ H^+(aq)\ \rightarrow\ CO_2(g)\ +\ H_2O(l)$$

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​$$CO_2$$ is then bubbled through limewater, saturated $$NaOH$$ solution, if $$CO_2$$ is present the solution will go from clear to cloudy.​

Testing for: sulfates

To identify the sulfate ion $$(SO_4^{2-})$$, add dilute $$(HCl)$$ followed by barium chloride $$(BaCl_2)$$. Dilute $$(HCl)$$ is added first to remove any trace carbonate ions since they would also produce a white precipitate.​

​$$Ba^{2+}(aq)\ +\ SO_4^{2-}(aq)\ \rightarrow\ BaSO_4(s)$$​​

If a white precipitate forms this is barium sulfate which means that the original compound contained sulfates. 

Testing for: ammonium ions

Ammonia gas is alkaline, and can be checked for using a damp piece of red litmus paper. If ammonia is present it will turn the red litmus paper blue. Ammonium ions $$(NH_4^+)$$ can be tested for by adding sodium hydroxide and gently heating the mixture. If ammonia is given off this means that there are ammonium ions present. This experiment should be carried out in a fume cupboard as it produces an irritant gas.

$$NH_4^+(aq)\ +\ OH^-(aq)\ \rightarrow\ NH_3(g)\ +\ H_2O(l) \\NH_4Cl(aq)\ +\ NaOH(aq)\ \rightarrow\ NH_3(g)\ +\ H_2O(l)\ +NaCl(aq)$$