Borax or sodium tetraborate is the compound commonly known as 'Suhaga'. It is used for the identification of coloured substances or those white substances which give coloured residue on heating. This test is used for the detection of cations like \(\mathrm{Cu}^{2+}, \mathrm{Fe}^{3+}, \mathrm{Cr}^{3+}, \mathrm{Co}^{2+}, \mathrm{Ni}^{2+}\) and \(\mathrm{Mn}^{2+}\).
In practice, borax is heated on the loop of a platinum wire. On heating, the compound loses water, swells and ultimately fuses to form a transparent colourless glassy solid, commonly known as borax glass or boarx bead.
The bead is moistened with water, dipped in the powdered substance to be tested and heated again when a characteristic colour appears in the flame.
Theory. Borax on heating leads to the formation of glassy bead of boric anhydride, \(\mathrm{B}_2 \mathrm{O}_3\). Boric anhydride is a non-volatile acidic compound. On heating with coloured salts, it combines with basic part of the salt (to be tested) forming metaborates having characteristic colours.
\(\underset {Borax}{ \mathrm{Na}_2 \mathrm{~B}_4 \mathrm{O}_7 .10 \mathrm{H}_2 \mathrm{O} }\xrightarrow[\left(-10 \mathrm{H}_2 \mathrm{O}\right)]{\text { heat }} \mathrm{Na}_2 \mathrm{~B}_4 \mathrm{O}_7 \xrightarrow{\text { heat }} \underset{(Glassy\,bead)} {2 \mathrm{NaBO}_2+\mathrm{B}_2 \mathrm{O}_3}\)
\(\mathrm{B}_2 \mathrm{O}_3+\mathrm{CuO}^{* *} \longrightarrow \underset{\text { Cupric metaborate (blue) }}{\mathrm{Cu}\left(\mathrm{BO}_2\right)_2} \text {; } \)
\(B_2 O_3 + Ni O \longrightarrow \underset {Nickel \,metaborate \,(brown)}{\mathrm{Ni}\left(\mathrm{BO}_2\right)_2}\)
Characteristic Colours Obtained in Borax Bead Test
|
Cr |
Co |
Mn |
Ni |
Fe |
| Colour in oxi. flame |
|
|
|
Pinkish violet |
- |
Brown |
| Cold |
|
|
Pinkish violet |
Brown |
Yellow |
| Colour in redu. flame |
Hot |
|
|
Colourless |
- |
Bottle green |
| Cold |
|
|
Colourless |
Grey |
Bottle green |
In some cases different colours are observed in oxidising and reducing flames. This is due to the formation of different metaborates under different oxidation states of the metal. For example, in case of copper following different reactions take place in oxidising and reducing flames.
Oxidising flame.
\(\mathrm{B}_2 \mathrm{O}_3+\mathrm{CuO} \longrightarrow \underset {Cupric\,metaborate \,(blue)}{\mathrm{Cu}\left(\mathrm{BO}_2\right)_2}\)
Reducing flame.
(i) \(\underset {Cuprous \,metaborate \\(colourless)}{2 \mathrm{Cu}\left(\mathrm{BO}_2\right)_2+}2 \mathrm{NaBO}_2+\mathrm{C} \longrightarrow 2 \mathrm{CuBO}_2+\mathrm{Na}_2 \mathrm{~B}_4 \mathrm{O}_7+\mathrm{CO}\)
(ii) \(2Cu (BO_2)_2 + 4NaBO_2 + 2C \longrightarrow \underset {red}{2Cu} + 2Na_2 B_4 O_7 + 2CO\)
Microcosmic salt, \(\mathrm{Na}\left(\mathrm{NH}_4\right) \mathrm{HPO}_4\), pre- pared by dissolving ammonium chloride and disodium hydrogen phosphate in equimoleular proportion in hot water is also used for performing bead test.
\( \mathrm{NH}_4 \mathrm{Cl}+\mathrm{Na}_2 \mathrm{HPO}_4 \xrightarrow{\mathrm{Na}} \mathrm{Na}\left(\mathrm{NH}_4\right) \mathrm{HPO}_4+\mathrm{NaCl} \)
It is a colourless crystalline solid, sparingly soluble in water. Like borax, it is used for detecting coloured ions \(\left(\mathrm{Cu}^{2+}, \mathrm{Co}^{2+}, \mathrm{Ni}^{2+}, \mathrm{Mn}^{2+}\right.,\) etc.) in qualitative inorganic analysis. The bead test is based on the fact that on heating it forms a transparent glassy bead of metaphosphate. The metaphosphate so formed gives coloured beads of orthophosphates when heated with coloured salts (microcosmic bead test).
\(\underset{Microcosmic\, salt}{Na(NH_4)HPO_4} \longrightarrow NH_3 + H_2O + \underset{Sod.\,metaphosphate}{NaPO_3} \)
\(CuSO_4 \longrightarrow CuO + SO_3\, ;\)
\(CuO + NaPO_3 \longrightarrow \underset {Sod.\,copper\,orthophosphate \,(blue \,bead)}{CuNaPO_4}\)
