Microcosmic salt bead test - halides

By "microcosmic salt bead test" we're referring to the use of ammonium sodium phosphate tetrahydrated, or NH4NaHPO4 · 4H2O, in order to create a glass - like bead necessary to recognize anionic (halides) or cationic species.

Let's see the most interesting case, that's halides recognition.

Recognition of halides

The assay is performed by making a small loop (just like we saw for borax bead test) on a platinum wire. The wire shall be immersed in concentrated HCl in order to remove impurities and deposits possibly remained on the wire because of previous uses. The hydrochloric acid converts the impurities into volatile chlorides that are actually volatilised (become gases) into the flame of a Bunsen burner. We have to continue wash as long as you do not get colorful emission in flame. Now we have to create our own glass - like bead. In order to do this we load a bit of NH4NaHPO4 · 4H2O on the loop previously practiced on the top of our wire. What happens for oxidizing flame heating (higher calorific value) is:

NH4NaHPO4 · 4H2O NH3   + NaPO3 + 5H2O ↑

The monohydrogen phosphate loses ammonia and crystallization water, and melts to give a clear and colorless bead (if on the wire are not present impurities), consisting in sodium metaphosphate (NaPO3). But in order to identify halides (Cl-, Br-, I-) it has to be further loaded with cupric oxide, CuO (other cupric salts would work as well)

In practice, we bring the bead to fusion point into the bunsen burner flame and then, with the melted bead, we touch the cupric oxide, and then bring in flame again. The reaction that takes place at this point is:

NaPO3 + CuO CuNaPO4

Our bead is then ready to be used in order to recognize the halide. Once again we heat the bead up to incandescence and subsequently we touch with the bead itself our unknown mixture. Putting now the bead back into flame we can observed the differen outcomes, then the colored emissions given from the formation of volatile halides. Let's pretend that you have loaded the bead with the most common salt, sodium chloride (NaCl):

CuNaPO4 + 2NaCl Na3PO4 + CuCl2

The copper halides are very volatile and in flame they give charachteristic colored emissions. Here are the possible outcomes, very fascinating to watch.

  • Chlorides blue color around the bead
  • Bromides blue color around the bead and green on top of the flame
  • Iodides green coloration on the entire flame

Summary of the reactions

  • We load the loop of the wire with NH4NaHPO4 · 4H2O (ammonium sodium phosphate). Bringing the substance in the flame it lost ammonia and crystallization water. We get a small pearl, a bead, that should look like glass (if it wasn't so, see above how to clean the bead):      

             NH4NaHPO4 · 4H2O NH3   + NaPO3 + 5H2O ↑

  • We heat up to fusion the bead and with it we touch a small amount of CuO (cupric oxide) dark powder. We get a sodium and copper orthophosphate:          

NaPO3 + CuO CuNaPO4

  • Again we bring to fusion the bead loaded with cupric oxide and with it we touch our unknown substance. For a double-exchange reaction we get the cupric halide, very volatile, which imparts to the flame a characteristic color depending on the halide:

      CuNaPO4 + 2NaX Na3PO4 + 2CuX