Phosphorus salt bead test - cations

More commonly, use is made ​​of the phosphorus salt bead test" as emission wise for the search of halides (bromides, iodides, chlorides) , but this can also be used to search for the right cations as we would do with the borax. Usually you prefer to use borax Pearl because less fluid and therefore more convenient and easy to use.

Even in this case, with a practiced eyelet on the end of a platinum wire (previously cleaned with HCl), we try to obtain a glassy pearl in which to recognize a wide range of cations for formation of colored compounds.

The starting material is always monohydrogen sodium phosphate tetrahydrate, ie → Na 2 HPO 4 · 4H 2 O

These is brought into bunsen burner flame to melt to obtain a glassy clear pearl:

NH 4 NaHPO 4 · 4H 2 O NH3   + NaPO 3 + 5H 2 O ↑

It consists of metaphosphate (PO 3 -) sodium. As you can see, in addition to the loss of water of crystallization is ammonia development. The "Pop Corn" effect is not as pronounced as we saw for the borax bead.

At this point, with the bead to the state semi-fluid (let cool slightly), you touch the unknown substance. And 'well that only a minimal amount of unknown substance should adhere to the bead. If the unknown substance is taken in excess, it may have no appreciable staining of colored compounds that we talked about, because covered with dark color of unreacted substance.

However, after loading the bead with the unknown substance, this must be brought back into flame. The flame zone can be selected depending on the compound which we want to observe. Depending on where we heat we can observe the changes in coloring or for termocromismi or for real chemical reactions (redox). In our case, however, only we observe in the oxidizing flame behavior, and in particular in the casting area, which is the part of the flame with the highest temperature.

Let's take an example of the chemical reactions that could face a compound may be present in the unknown.

The compound or salt of the cation inside of the unknown mixture is transformed, for prolonged heating, in oxide:

Co (NO 3) → CoO + 2NO 2 + \ Frac {1} {2} 12

O 2


Co (CO 3) → CoO + CO 2

At this point, the oxide reacts with metaphosphate:

CoO + NaPO 3 CONAPO 4 (double-sodium orthophosphate and cobalt blue color)

To be considered a well-executed essay should get a clear and colored bead. Considering the positive test for a cation when using the coloring compounds that impart to the pearl you can be undoubtedly related to the cation charge of coloration.

To download the bead brings this fusion in flame and while it is still hot you give patted the arm holding the wand, by depositing the glass lingo on the work surface.

To see the possible results of the essay → outcome of the essay to the pearl to sal phosphorus (cations)