Bead test

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español: ensayo en perla. Ensayo de análisis, consiste en impregnar un hilo de platino en borax, llevándolo a la formación de una perla en la llama oxidante, a continuación se impregna esta bola en la sustancia que queremos analizar, llevándola al fuego, dependiendo del color obtenido tanto en oxidación como en llama reductora conoceremos el óxido presente.

The bead test is a traditional part of qualitative inorganic analysis to test for the presence of certain metals.

The oldest one is the borax bead test or blister test. It was introduced by Berzelius in 1812.

Since then other salts were used as fluxing agents, such as sodium carbonate or sodium fluoride. The most important one after borax is microcosmic salt, which is the basis of the microcosmic salt bead test.

Borax bead test

Plantilla:Unreferenced-section A small loop is made in the end of a platinum wire (as used in the flame test) and heated in a Bunsen flame until red hot. It is then dipped into powdered borax, and the adhering solid is held in the hottest part of the flame where it swells up as it loses its water of crystallization and then shrinks, forming a colourless, transparent glass-like bead (a mixture of sodium metaborate and boric anhydride),

The bead is moistened (traditionally with the tongue) and dipped into the sample to be tested such that only a tiny amount of the substance adheres to the bead. If too much substance is used, the bead will become dark and opaque. The bead and adhering substance is then heated in the lower, reducing, part of the flame, allowed to cool, and the colour observed. It is then heated in the upper, oxidizing, part of the flame, allowed to cool, and the colour observed again.

Characteristic coloured beads are produced with salts of copper, iron, chromium, manganese, cobalt and nickel. After the test, the bead is removed by heating it to fusion point, and plunging it into a vessel of water.

Reducing flame	Oxidising flame	Metal
Colourless when hot; opaque red when cold	Green when hot; blue when cold	Copper (Cu)
Green, hot and cold	Yellow, hot and cold	Iron (Fe)
Green, hot and cold	Dark yellow when hot; green when cold	Chromium (Cr)
Colourless, hot and cold	Violet, hot and cold	Manganese (Mn)
Blue, hot and cold	Blue, hot and cold	Cobalt (Co)
Grey when cold	Reddish-brown when cold	Nickel (Ni)

References

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