Testing For Ions (Plan) Test for Potassium ions To test for positive ions (cations) we conduct a flame test and potassium ion is a cation. Ever ion has a specific colour which they liberate when exposed to a flame. The ions liberates a lilac colour when exposed to a flame while copper ions liberate a blue flame. The test it conducted as follows: You will need; Beaker, Hydrochloric acid, Cleaned Nichrome wire loop. (inoculating loop) A bunsen burner To clean the Nichrome wire, dip the wire in the beaker of HCl and place at the edge of the flame until no colour is produced. Once no colour has been liberated from the Nichrome wire place into the HCl once more and scoop the solid to be tested and place at the edge of the flame. If the flame produced is of a lilac colour then it consists of potassium ions. The colours produced are due to the movement of the electrons from one shell to another. As the electrons gain energy they move to a higher shell and when they return to their original shell they release energy in the form of a specific wavelength of light which we see. (Anon, 2020) Tests for halides Prepare 10mL of silver nitrate (AgNO3) solution at a concentration of 1 mole per litre (mol/L) by dissolving 1.7g of dry silver nitrate in distilled water and diluting to a final volume of 10mL. Dissolve a pea-size amount of the sample to be tested (if in solid form) in about 20 drops of water. Once dissolved (or if already in liquid form), place about 15 to 20 drops of the sample solution in a test tube and add 8 to 10 drops of the 1 mol/L silver nitrate solution. The formation of a yellow precipitate represents a positive test for iodide. (Brubaker, 2020) Using silver nitrate solution Carrying out the test This test has to be done in solution. If you start from a solid, it must first be dissolved in pure water. The solution is acidified by adding dilute nitric acid. (Remember: silver nitrate + dilute nitric acid.) The nitric acid reacts with, and removes, other ions that might also give a confusing precipitate with silver nitrate. Silver nitrate solution is then added to give: ion present observation F- no precipitate Cl- white precipitate Br- pale cream precipitate I- pale yellow precipitate Testing for carbonate ions Carbonate ions, CO32- can be detected whether in a solid compound or in solution. An acid, such as dilute hydrochloric acid, is added to the test compound. Carbon dioxide gas bubbles if carbonate ions are present. Limewater is used to confirm that the gas is carbon dioxide. It turns from clear to milky when carbon dioxide is bubbled through. Testing for sulfate ions Sulfate ions in solution, SO42-, are detected using barium chloride solution. The test solution is acidified using a few drops of dilute hydrochloric acid, and then a few drops of barium chloride solution are added. A white precipitate of barium sulfate forms if sulfate ions are present. For example: barium chloride + sodium sulfate → sodium chloride + barium sulfate The hydrochloric acid is added first to remove any carbonate ions that might be present - they would also produce a white precipitate, giving a false positive result. Barium nitrate solution can be used instead of barium chloride solution. However, nitric acid is added first to acidify the test solution. Sulfuric acid cannot be used because it contains sulfate ions these would interfere with the second part of the test. (BBC Bitesize, 2020) Test for ammonium ions Add a few drops of dilute sodium hydroxide solution, then warm gently. Ammonia gas is produced if ammonium ions are present. Confirm that the gas is ammonia - damp red litmus paper turns blue. Test for Magnesium Dilute sodium hydroxide solution reacts with some metal ions in solution, forming metal hydroxides. Some of these metal hydroxides are insoluble, so they appear as precipitates If a white precipitate is formed then it could be either calcium magnesium of aluminium. To distinguish between these three we can add dilute sodium hydroxide in excess and if it is calcium or magnesium there will be no change, if not it is aluminium. Then to find out whether it is magnesium, we have to perform a flame test on the solution, we do this by dissolving the solution in HCL and then using it on an inoculating loop and putting it in a flame. If the flame goes brick red/orange, it is calcium, if not then its is magnesium ions. (BBC Bitesize, 2020) Testing for hydroxides If sodium hydroxide solution is then added, a transition metal hydroxide is formed. Transition metal hydroxides are insoluble so they form solid precipitates. These precipitates often appear as small particles suspended in a solution. (Anon, 2019) Test for barium ions Mix in concentrated HCL and then perform a flame test as shown above, if barium ions are present then a Light green (slight) flame will show Testing For Ions (Plan)(References) Brubaker, J. (2020). How to Test for Potassium Iodide. [online] Sciencing.com. Available at: https://sciencing.com/test-potassium-iodide-6426504.html [Accessed 27 Feb. 2020]. Anon, (2020). How do I test for a potassium ion?. [online] Available at: https://www.quora.com/How-do-I-test-for-a-potassium-ion [Accessed 27 Feb. 2020]. BBC Bitesize. (2020). Testing for positively charged ions - Tests for ions - Edexcel - GCSE Chemistry (Single Science) Revision - Edexcel - BBC Bitesize. [online] Available at: https://www.bbc.co.uk/bitesize/guides/z9nr6yc/revision/2 [Accessed 27 Feb. 2020].
