Bead Test in Chemical Analysis

The bead test, sometimes called the borax bead or blister test, is an analytical method used to test for the presence of certain metals. The premise of the test is that oxides of these metals produce characteristic colors when exposed to a burner flame. The test is sometimes used to identify the metals in minerals. In this case, a mineral-coated bead is heated in a flame and cooled to observe its characteristic color.

The bead test may be used on its own in chemical analysis but it's more common to use it in conjunction with the flame test to better identify the composition of the sample.

How to Perform a Bead Test

First, make a clear bead by fusing a small quantity of borax (sodium tetraborate: Na₂B₄O₇ • 10H₂O) or microcosmic salt (NaNH₄HPO₄) onto a loop of platinum or Nichrome wire in the hottest part of a Bunsen burner flame. Sodium carbonate (Na₂CO₃) is used sometimes for the bead test, too. Whichever salt you use, heat the loop until it glows red-hot. Initially the salt will swell as the water of crystallization is lost. The result is a transparent, glassy bead. For the borax bead test, the bead consists of a mix of sodium metaborate and boric anhydride.

After the bead has been formed, moisten it and coat it with a dry sample of the material to be tested. You only need a tiny amount of sample, as too much will make the bead too dark to see the result.

Reintroduce the bead into the burner flame. The inner cone of the flame is the reducing flame; the outer portion is the oxidizing flame. Remove the bead from the flame and let it cool. Observe the color and match it to the corresponding bead type and flame portion.

Once you have recorded a result, you can remove the bead from the wire loop by heating it once again and dipping it into water.

The bead test is not a definitive method for identifying an unknown metal, but may be used to quickly eliminate or to narrow possibilities.

What Metals Do Bead Test Colors Indicate?

It's a good idea to test a sample in both the oxidizing and reducing flame to help narrow down the possibilities. Some materials don't change the color of the bead, plus the color may change depending on whether the bead is observed when it is still hot or after it has cooled. To further complicate matters, the results depend on whether you have a dilute solution or a small amount of chemical, versus a saturated solution or a large amount of compound.

The following abbreviations are used in the tables:

• h: hot
• c: cold
• hc: hot or cold
• ns: not saturated
• s: saturated
• sprs: supersaturated

Borax Beads

Microcosmic Salt Beads

Key Points

• The bead test or blister test is used in analytical chemistry to help identify the elements in a sample, based on the color the bead turns after exposure to a flame.
• The bead test is similar to the flame test.
• Neither the bead test nor the flame test can positively identify a sample's identity on their own, but they can help narrow down possibilities.

Sources

• Pratt, J.H. "Determinative Mineralogy and Blowpipe Analysis." Vol. 4, Issue 103, Science, American Association for the Advancement of Science, December 18, 1896.
• Speight, James. "Lange's Handbook of Chemistry." Hardcover, 17th Edition, McGraw-Hill Education, October 5, 2016.