Chapter 19 Laboratory: Qualitative Analysis 337POCEDURER
1.ou have not already done so, put on your splash If y
goggles, gloves, and protective clothing.
- Heat the loop portion of a platinum or Nichrome
inoculating loop in a gas burner flame until it is red hot. - Dip the hot loop into borax powder (sodium tetraborate
decahydrate, Na 2 B 4 O 7 · 10H 2 O), a small amount of
which adheres to the hot loop. Heat the adhering borax
in the hottest part of the gas burner flame. At first, the
borax swells as it loses its water of crystallization. As you
continue to heat the anhydrous sodium tetraborate, it
melts and shrinks, forming a colorless, transparent,
glassy bead. - Allow the bead to cool for a moment, dip it in distilled
water to moisten it, and then dip it into the powdered
sample, a few grains of which will adhere to the bead. - Reheat the bead in the reducing (inner, blue) part of the
gas burner flame until the bead remelts. Observe the
color of the bead, if any, while it is hot. The bead should
dISpoSAL:
dispose of all waste materials by flushing them down the
drain with plenty of water. (The tiny amounts of metals
contained in the beads are not hazardous.)remain transparent. If it becomes cloudy or opaque,
you’ve used too much of the sample. Only a grain or two
is needed.- Allow the bead to cool and again observe the color.
- Reheat the bead in the oxidizing (outer, colorless) part of
the gas burner flame until the bead remelts. Observe the
color of the bead, if any, while it is hot. - Allow it to cool and again observe the color.
- Record your observations from steps 5 through 8 in
Table 19-4.
When you finish the test, reheat the bead and plunge the
loop into cold water to remove the bead.
TABLE 19-4: Using borax bead tests to discriminate metal ions—observed data
ElementReducing flame
(hot bead)Reducing flame
(cold bead)Oxidizing flame
(hot bead)Oxidizing flame
(cold bead)A. ChromiumB. CobaltC. CopperD. IronE. ManganeseF. Nickel