Minggu, 11 Oktober 2009

Qualitative Analysis - Bead Tests

What is the Bead Test
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.

Performing 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 wire in a Bunsen burner flame. Sodium carbonate (Na2CO3) is used sometimes for the bead test, too. After the bead has been formed, coat it with a dry sample of the material to be tested and reintroduce the bead into the burner flame. The inner cone of the flame is the reducing flame; the outer portion is the oxidizing flame. Observe the color and match it to the corresponding bead type and flame portion. 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 colors indicate?

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

Color Oxidizing Reducing
Colorless hc: Al, Si, Sn, Bi, Cd, Mo, Pb, Sb, Ti, V, W
ns: Ag, Al, Ba, Ca, Mg, Sr
Al, Si, Sn, alk. earths, earths
h: Cu
hc: Ce, Mn
Gray/Opaque sprs: Al, Si, Sn Ag, Bi, Cd, Ni, Pb, Sb, Zn
s: Al, Si, Sn
sprs: Cu
Blue c: Cu
hc: Co
hc: Co
Green c: Cr, Cu
h: Cu, Fe+Co
Cr
hc: U
sprs: Fe
c: Mo, V
Red c: Ni
h: Ce, Fe
c: Cu
Yellow/Brown h, ns: Fe, U, V
h, sprs: Bi, Pb, Sb
W
h: Mo, Ti, V
Violet h: Ni+Co
hc: Mn
c: Ti

MICROCOSMIC SALT BEADS

Color Oxidizing Reducing
Colorless Si (undissolved)
Al, Ba, Ca, Mg, Sn, Sr
ns: Bi, Cd, Mo, Pb, Sb, Ti, Zn
Si (undissolved)
Ce, Mn, Sn, Al, Ba, Ca, Mg
Sr (sprs, not clear)
Gray/Opaque s: Al, Ba, Ca, Mg, Sn, Sr Ag, Bi, Cd, Ni, Pb, Sb, Zn
Blue c: Cu
hc: Co
c: W
hc: Co
Green U
c: Cr
h: Cu, Mo, Fe+(Co or Cu)
c: Cr
h: Mo, U
Red h, s: Ce, Cr, Fe, Ni c: Cu
h: Ni, Ti+Fe
Yellow/Brown c: Ni
h, s: Co, Fe, U
c: Ni
h: Fe, Ti
Violet hc: Mn c: Ti

Primary Reference: Lange's Handbook of Chemistry, 8th Edition, Handbook Publishers Inc., 1952.

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