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 Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.