Bead test

Bead test

The bead test is a traditional part of qualitative inorganic analysis to test for the presence of certain metals. The oldest one is the borax bead test or blister test. It was introduced by Berzelius in 1812.[1] Since then other salts were used as fluxing agents, such as sodium carbonate or sodium fluoride. The most important one after borax is microcosmic salt,[1] which is the basis of the microcosmic salt bead test.[2]

Borax bead test

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, oxidizing, part of the flame, allowed to cool, and the colour observed again.[2]

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 plunging it into a vessel of water.

Metal[3] Oxidising flame Reducing flame
Aluminum colorless (hot and cold), opaque colorless, opaque
Antimony colorless, yellow or brown (hot) gray and opaque
Barium colorless
Bismuth colorless, yellow or brownish (hot) gray and opaque
Cadmium colorless gray and opaque
Calcium colorless
Cerium red (hot) colorless (hot and cold)
Chromium Dark yellow (hot), green (cold) green (hot and cold)
Cobalt blue (hot and cold) blue (hot and cold)
Copper green (hot), blue (cold) red, opaque (cold), colorless (hot)
Iron yellow or brownish red (hot and cold) green (hot and cold)
Lead colorless, yellow or brownish (hot) gray and opaque
Magnesium colorless
Manganese violet (hot and cold) colorless (hot and cold)
Molybdenum colorless yellow or brown (hot)
Nickel brown, red (cold) gray and opaque (cold)
Silicon colorless (hot and cold), opaque colorless, opaque
Silver colorless gray and opaque
Strontium colorless
Tin colorless (hot and cold), opaque colorless, opaque
Titanium colorless yellow (hot), violet (cold)
Tungsten colorless brown
Uranium Yellow or brownish (hot) green
Vanadium colorless green

References

  1. ^ a b Materials Handbook: A Concise Desktop Reference, by François Cardarelli [1]
  2. ^ a b Vogel, Arthur I.; Svehla, G. (1979), Vogel's Textbook of Macro and Semimicro Qualitative Inorganic Analysis (5th ed.), London: Longman, ISBN 0-582-44367-9 
  3. ^ CRC Handbook of Chemistry and Physics. CRC Press. 1985. ISBN 0849304660. 

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