Borax bead test

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

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 lower, oxidising, 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 jerking it into a vessel of water.

Reducing flame	Oxidising flame	Metal
Colourless when hot; opaque red when cold	Green when hot; blue when cold	Cu
Green, hot and cold	Yellow, hot and cold	Fe
Green, hot and cold	Dark yellow when hot; green when cold	Cr
Colourless, hot and cold	Violet, hot and cold	Mn
Blue, hot and cold	Blue, hot and cold	Co
Grey when cold	Reddish-brown when cold	Ni