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Cation qualitative analysis

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Experimental procedures of cation QA. Coloured pictures fully available.

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Cation qualitative analysis

  1. 1. IDENTIFYING CATIONS: TECHNIQUES & OBSERVATIONS Tan Wan Jun Please view in ‘Slide Show’ mode.
  2. 2. Using NaOH(aq) Pour unknown salt solution into test tube until a height of about 1cm (~1cm3). Add 3 drops of NaOH(aq) to solution. Shake the mixture. Record observation(s): Any ppt? Colour of ppt formed? If there is ppt, add excess NaOH(aq) to about half test tube full. Shake the mixture. Record observation: Is ppt soluble in excess NaOH(aq)? How to Perform Qualitative Cation Analysis Note: 1. If unknown salt is given in solid form, dissolve a small amount in a test tube with deionised water. 2. Separate the resulting solution into 2 equal portions (~1cm3 each) for testing using NaOH(aq) and NH3(aq) respectively. Repeat analysis using NH3(aq) b dc a unknown
  3. 3. Cation NaOH(aq) NH3(aq) Al3+ White ppt Soluble in excess to give colourless Solution White ppt Insoluble in excess Pb2+ White ppt Soluble in excess to give colourless solution White ppt Insoluble in excess Zn2+ White ppt Soluble in excess to give colourless solution White ppt Soluble in excess to give colourless solution Ca2+ White ppt Insoluble in excess No ppt (solution remained colourless) Note: How do you distinguish between Pb2+ ions and Al3+ ions?
  4. 4. Cation NaOH(aq) NH3(aq) Cu2+ Light blue ppt Insoluble in excess Light blue ppt Soluble in excess to give dark blue solution Fe2+ Dirty-green ppt Insoluble in excess Dirty-green ppt Insoluble in excess *Fe3+ Reddish- brown ppt Insoluble in excess Reddish- brown ppt Insoluble in excess Note: On standing in air (oxygen), the dirty-green ppt of Fe(OH)2 turns reddish-brown due to oxidation of Fe2+ ions. *Colour of solution containing Fe3+ may range from colourless to pale yellow depending on concentration of Fe3+. Excess reagent: NaOH(aq)/NH3(aq) Dirty green ppt with reddish-brown tinge Stand in air
  5. 5. Using Sodium Hydroxide Solution, NaOH(aq) Salt Solution No ppt Colourless, pungent gas (NH3) evolved on heating. Turns moist red litmus blue. NH4 + White ppt ppt soluble in excess NaOH(aq) to form a colourless solution Zn2+ Al3+ Pb2+ ppt insoluble in excess NaOH(aq) Ca2+ Coloured ppt Light blue ppt insoluble in excess NaOH(aq) Cu2+ Dirty-green ppt insoluble in excess NaOH(aq) Fe2+ Reddish-brown ppt insoluble in excess NaOH(aq) Fe3+ Add NaOH(aq) If you cannot decide whether the ppt is soluble in excess reagent or not, pour away some of the ppt, leaving just a little bit at the bottom of the test tube, before adding more reagent to about half the test tube. Hold the test tube against the light. If you can see particles of the ppt, the ppt is not soluble in excess reagent.
  6. 6. Salt Solution No ppt Ca2+, NH4 + White ppt ppt soluble in excess NH3(aq) to form a colourless solution Zn2+ ppt insoluble in excess NH3(aq) Al3+, Pb2+ Coloured ppt Light blue ppt soluble in excess NH3(aq) to form a dark blue solution Cu2+ Dirty-green ppt insoluble in excess NH3(aq) Fe2+ Reddish-brown ppt insoluble in excess NH3(aq) Fe3+ Using Aqueous Ammonia, NH3(aq) Add NH3(aq) If you cannot decide whether the ppt is soluble in excess reagent or not, pour away some of the ppt, leaving just a little bit at the bottom of the test tube, before adding more reagent to about half the test tube. Hold the test tube against the light. If you can see particles of the ppt, the ppt is not soluble in excess reagent.
  7. 7.  The precipitate in each of the reactions with NaOH(aq)/NH3(aq) is the insoluble hydroxide of the metal ion.  E.g. Copper(II) ion + hydroxide ion  copper(II) hydroxide; Cu2+(aq) + 2OH-(aq)  Cu(OH)2(s)  Some of the precipitates dissolve in excess NaOH(aq)/NH3(aq) due to the formation of compounds that are soluble in water (complexes).  Pb2+ ions can be distinguished from Al3+ ions by adding hydrochloric acid, HCl, to the unknown solution.  If a white ppt forms, Pb2+ is present – due to the insolubility of lead(II) chloride, PbCl2.  If NH4 + ion is present in the unknown, addition of NaOH(aq) followed by warming will produce ammonia gas, NH3(g).  Refer to the video on how testing for NH3(g) is done.  When a gas is evolved, look out for the following: Any effervescence? Colour and odour of gas? Action of the gas on moist litmus paper? Review
  8. 8. Test for Ammonium Ion Things to note: 1)Remember to wet litmus paper before use. Can you recall the reason? [A moist litmus paper allows the gas to dissolve in water, forming H+(aq) or OH-(aq) ions.] 2)Observe how warming is done: Remove test tube from Bunsen flame periodically to prevent contents from spurting.
  9. 9.  Tan, Y. T., Chen, L. K., Sadler, J. & Clare, E. (2007). Salts. In GCE ‘O’ Level Chemistry Matters (pp.194-211). Singapore: Marshall Cavendish International Pte. Ltd.  Video “Test for ammonium ion” obtained and modified from: Yue-Chang, T. H. (n.d.). Ammonium ion test. Retrieved November 26, 2008, from http://www.youtube.com/watch?v=GkWAamtZnAc References & Acknowledgements

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