NaOH Test for Metal Cations
- Metal cations in aqueous solution can be identified by the colour of the precipitate they form on addition of sodium hydroxide.
- If only a small amount of NaOH is used then normally the metal hydroxide precipitates.
- In excess NaOH some of the precipitates may dissolve.
- For this reason just a few drops of NaOH are added at first and very slowly.
- If if is added too quickly and the precipitate is soluble in excess, then you run the risk of missing the formation of the initial precipitate which dissolves as quickly as it forms if excess solution is added.
- A small amount is thus added, very gradually and any colour changes or precipitates formed are noted.
- Then the NaOH is added in excess and the reaction is observed again.
- If a precipitate is formed from reaction with NaOH then the hydroxide is insoluble in water.
- Ca2+ and Mg2+ ions can be distinguished from Al3+ as calcium hydroxide and magnesium hydroxide precipitates do not dissolve in excess NaOH but aluminium hydroxide does.
- Another test could be used to distinguish between the Ca2+ and Mg2+ ions, for example a flame test.
- Most transition metals produce hydroxides with distinctive colours.
- The table below contains the results for each of the cations.
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Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.