Borax Bead Test
This test is performed only for coloured salts.
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 coloured metallic salt, a characte ¬ristic coloured bead of metal metaborate is formed.
In the cases where different coloured beads are obtained in the oxidising and reducing flames, metaborates in various oxidation states of metals are formed. For example, in oxidising flame, copper forms blue copper metaborate.
In reducing flame cupric metaborate is reduced to metallic copper, which is red and opaque.
Borax, Na2B4O7.10H2O is heated in the loop of platinum wire, it swells and forms transparent colourless glassy bead. When this hot bead is touched with small amount of coloured salt and is heated again, it acquires a characteristic colour. The colour of bead gives indication of the type of the cation present. The colour of the bead is noted separately in oxidising and in reducing flame (Fig. 9.6).
Fig. 9.6. Borax bead test (a) Heating in reducing flame, (b) Heating in oxidising flame.
Table 9.6. Borax Bead Test
|Color of the bead||Inference|
|In Oxidising flame||In Reducing flame|
|1. Green when hot, light blue when cold.||Colourless when hot, opaque red when cold.||Copper (Cu)|
|2. Yellowish brown when hot, pale yellow when cold||Green, hot and cold.
|3. Amethyst (pinkish violet) in both hot and cold.||Colourless, hot and cold.
|4. Deep blue in both hot and cold||Deep blue in both hot and cold.||Cobalt (Co)|
To remove the bead from platinum wire, heat the bead to redness. Tap the rod with finger stroke, till the bead jumps off (Fig. 9.7).