While performing test for basic radical in analyzing a salt qualitatively. I found out that the salt gave the blue lake test and since we were taught that only Al 3+ gives blue lake test, I thought it was Aluminium ion and wrote it the same. Later, my teacher told me I was wrong and it was Zinc ion. But then how did it give the blue lake test. How do I tell if the ion is Zinc or Aluminium?

If I could check the ion by any other reagent?

  • $\begingroup$ What’s the Blue Lake Test? $\endgroup$
    – Jan
    Jan 30, 2016 at 16:14
  • $\begingroup$ @Jan You add H2S in presence of acid to the salt solution. Then if the ppt it white ( which shows it maybe Aluminium ) you add dil HCl to the ppt and then add blue litmus solution (which is turned red ) and then NH4OH is added dropwise. So that precipitate of Al(OH)3 is formed (which floats) and the blue litmus formed from red ( formed by excess OH- ions maybe) appears like it is floating on the Al(OH)3. $\endgroup$
    – Quark
    Jan 30, 2016 at 16:59
  • $\begingroup$ I thought aluminium does not precipitate under $\ce{H2S}$ conditions; it belonged to the $\ce{NH4S}$ group in my qualitative inorganic analysis course (the ion lottery), meaning that it precipitated as $\ce{Al(OH)3}$ under basic conditions … $\endgroup$
    – Jan
    Jan 30, 2016 at 17:11
  • $\begingroup$ Chemists usually take a course "Qualitative inorganic analysis" which gives a decision tree of tests to decide which cations and anions are present. There are few "specific" tests that will react with only one cation (Ni and DMG is the only one that comes to mind). $\endgroup$
    – MaxW
    Jan 30, 2016 at 17:12
  • 1
    $\begingroup$ You can use Thénard’s Blue (or Rinman’s Green in the zinc case; they are called Cobalt blue/green in English, apparantly). Also, aluminium displays characteristic fluorescence with morin (make sure there are no sodium ions present) and I used a different test which I forgot. $\endgroup$
    – Jan
    Jan 30, 2016 at 19:01


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