bead test

bead test

bead test, test used in the identification of certain metals. Some metallic ions that cannot be identified by a flame test are identified by a bead test. The test can also be used to confirm the results of a flame test. The borax bead test is the most common. A small loop is formed at the end of a platinum wire. The loop is cleaned with concentrated hydrochloric acid and dipped in powdered borax, then heated in the flame of a Bunsen burner until the borax melts, forming a bead. The bead is dipped into a tiny amount of the compound to be tested and is reheated in the flame. The metal borate that is formed colors the bead. Some metals and the colors they produce in an oxidizing flame are: chromium, green; cobalt, blue; copper, blue-green; iron, yellow to brown; manganese, violet; nickel, reddish-brown. If too much of the unknown compound is used, the bead may be opaque and the color difficult to determine. A different color is often obtained in a reducing flame. Several metals may give the same color. Some metals give only colorless or gray beads. A test similar to the borax bead test is often made using microcosmic salt. Results of the bead test may be confirmed by other methods of chemical analysis.
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 upper, 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 Copper (Cu)
Green, hot and cold Yellow, hot and cold Iron (Fe)
Green, hot and cold Dark yellow when hot; green when cold Chromium (Cr)
Colourless, hot and cold Violet, hot and cold Manganese (Mn)
Blue, hot and cold Blue, hot and cold Cobalt (Co)
Grey when cold Reddish-brown when cold Nickel (Ni)

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