| MadSci Network: Chemistry |
A borax bead test is a rather old-fashioned qualitative test for the
presence of first-row transition metals in solid samples. It is a test for
the presence of chromium, manganese, iron, cobalt, nickel, or copper. It
uses only a very small amount of sample.
The idea is to put some powdered borax onto a platinum wire (possibly with
a small loop on the end), and heat it vigorously until it forms a small
liquid drop which cools to a glassy bead. If the bead is moistened, a
little of a powdery sample will adhere to it. The bead is then returned to
the flame for a while, and taken out and allowed to cool. Two different
tests are done -- one where a bead is held in the oxidizing part of the
flame, and another in the reducing part of the flame. But in both cases,
the very hottest part of the flame is to be avoided. The bead takes on a
characteristic colour according to the metal present.
The chemical basis of the test is that the original bead consists of sodium
metaborate and boric oxide, formed from the thermal decomposition of borax:
Na2B4O7.10H2O ---> 2 NaBO2 + B2O3 + 10 H2O (lost as gas)
The sodium metaborate and boric oxide then react with the ions of
transition metals to produce borates of the metals, mixed metal & sodium
borates, metal oxides, or metal powder, which colour the glassy bead. When
a different result is obtained in oxidizing and reducing flames, you see
the different colours characteristic of the different oxidation states of
the metal.
Metal Oxidizing flame Reducing Flame
Cr Green {Cr(III)} Green {Cr(III)}
Mn Amethyst {Mn(III)? or MnO2} colourless/pale pink {Mn(II)}
Fe Brown hot, yellow cold {Fe(III)} pale green {Fe(II)}
Co Blue {Co(II)} Blue {Co(II)}
Ni Brown {Ni(II)} Grey {Ni metal}
Cu Green {Cu(II)} colourless/red {Cu(I) or Cu metal}
The standard book in this area is Arthur Vogel's 'Textbook of Qualitative
Chemical Analysis', first published in 1937, and published in many
different editions under slightly different titles since then.
(I have been cribbing from a 1955 edition entitled 'A Textbook of Macro and
Semi-Micro Qualitative Inorganic Analysis').
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