Answers

2015-03-10T19:10:10+05:30
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.
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