The metaborates of many transition metals have characteristic colours, and, therefore, the borax bead test can be used to identify them in the laboratory.
Borax Bead Test
- Borax (sodium pyroborate), Na2B4O7.10H2O, on heating, gets fused and loses water of crystallisation.
- It swells up into a fluffy white porous mass which then melts into a colourless liquid which later forms a clear transparent glassy bead consisting of boric anhydride and sodium metaborate. Boric anhydride is non-volatile.
- When it is reacted with a coloured metallic salt, a characteristic coloured bead of metal metaborate is formed.
In the loop of platinum wire Borax, Na2B4O7.10H2O is heated, and it swells and forms a transparent colourless glassy bead. When this hot bead is touched with a small amount of coloured salt and is heated again, it acquires a characteristic colour. The colour of the bead indicates the type of cation present. The colour of the bead is noted separately in oxidising and in reducing flame.