11 Test Cations Anions

TESTING FOR CATIONS AND ANIONS Introduction Many times a chemist is asked about the composition of a substance. Classically the sample is dissolved (sometimes not a very simple task) and then tested with some specific reactions. For instance, suppose some old bags in a depot are known to be either sodium sulphate or sodium chloride. To decide which the substance in the bag is, a sample is dissolved in water and the tests described in the chart for sulphate and for chloride are carried out in different test tubes. According to the results the samples can be easily identified. Aims To observe some specific tests for cations and anions. Apparatus Test tubes and test tube racks, teat pipettes. Nichrome wire loop for flame tests, Bunsen burner. Procedures 1- TESTING FOR CATIONS The cations and tests to be carried on them are listed at the end of this paper. The following are just general working procedures. The tests should be carried this way: a- A sample (1-2 ml and no more than that volume) of the solution to be tested is poured into a test tube. b- As you should carry out two or three tests prepare three test tubes with the sample: the first one is just to show how the solution originally looks like so that you can you can compare and see if there has been any change. The other two for the “normal” and the “excess” tests. c- The specific reactive you are testing with (sodium hydroxide solution or dilute ammonia) will be added drop wise (2 or 3 drops unless otherwise indicated). d- If you have to test with excess of reactive solution then add 1 ml. If there is no change, try a further 1ml addition and no more. e- Write down any changes you notice in the corresponding boxes of the tables. Maybe you have to smell carefully the vapours coming out from the test tube

2- FLAME TESTS a- The Bunsen burner’s flame should be blue (air vents open) and the metallic (platinum or more frequently nichrome alloy) loop placed on the tip of the blue inner cone of the flame. b- First, wash the loop dipping it into hydrochloric acid, rinsing with distilled water and checking the flame is no longer coloured, so you are sure the test is not “invalidated” by any previous experiments. c- Put some crystals of sample (in case it is a solid sample) or dip it into the solution to be tested (if it were a liquid one). d- Put the loop once again on the tip of the inner blue cone of the flame and observe. e- It is a matter of chemical politeness to burn away the remnants of sample so that the next one to use the loop finds it is clean. 3- TESTING FOR ANIONS The anions should be tested following a similar procedure as described in Part 1. a- In case some pH or acidity regulation is indicated in the specific test in the syllabus never use more than the amounts indicated in point 1. b- In the test for nitrates aluminium foil should be added using tongs and the ammonia vapours smelled carefully waving the vapours coming out from the test tube with your hand and to your nose. Never ever smell a test tube putting your nose into it.

TESTS FOR CATIONS: RESULTS Cation

Shade of Aqueous solution NaOH

Excess aqueous NaOH

Ammonium (NH4)+ Copper (Cu 2+) Calcium (Ca2+) Iron (II) (Fe2+) Iron (III) (Fe3+) Zinc (Zn2+)

Aqueous NH3

Excess aqueous NH3

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FLAME TESTS:RESULTS Cation Sodium

Colour of flame

Potassium Calcium Copper TESTS FOR ANIONS: RESULTS Anion

Test

Carbonate (CO32-) Chloride (Cl-) Iodide (I-) Nitrate (NO3-)

Add dilute HNO3

Sulphate (SO42-)

Add 1 drop HNO3 solution, then AgNO3 solution Add 1 drop HNO3 solution Then Pb(NO3)2 solution Add 10 drops NaOH, next Al foil ; then warm carefully Add 1 drop HNO3 solution, then BaCl2 solution

Results