Borax bead

The borax bead is used as a preliminary test for some transition metals in the separation process in analytical chemistry .

outermost tip of the inner flame cone has a reducing effect, the entire outer flame cone is an oxidation zone

For this purpose, borax (Na ₂ B ₄ O ₇ 10 H ₂ O) is taken up with a previously annealed magnesia stick and melted in the Bunsen burner to a crystal-clear pearl. The water of crystallization contained in it escapes . A small amount of the powdery sample is then taken up with this bead and held in the burner flame until the bead is completely melted again and the sample is evenly distributed in it. The color of the pearl is then observed both in the heat and in the cooled state. ${\displaystyle \cdot }$

Depending on the flame zone used (bright yellow or edge zone = reduction flame , non-luminous = oxidation flame), the pearl is colored differently. These are largely identical to the colors of the phosphorus salt pearl .

Reactions in the pearl

From a chemical point of view, heavy metal borates are formed in the heat. These have different compositions.

Step 1: annealing the hydrous borax:

${\displaystyle \mathrm {Na_{2}[B_{4}O_{5}(OH)_{4}]\cdot 8\ H_{2}O\ {\xrightarrow {\triangle }}\ Na_{2}B_{4}O_{7}+10\ H_{2}O} }$

Step 2: Formation of the heavy metal borate using the example of cobalt (II) sulfate :

${\displaystyle \mathrm {Na_{2}B_{4}O_{7}+CoSO_{4}\ {\xrightarrow {\triangle }}\ 2\ NaBO_{2}+Co(BO_{2})_{2}+SO_{3}\uparrow } }$

Risk assessment

Borax (sodium tetraborate) has meanwhile been classified as being fertile and potentially damaging to fertility. Since the phosphorus salt pearl is a less dangerous alternative that is equivalent from an analytical point of view, borax should not be used for the purpose of the corresponding preliminary test.

Colors in the borax bead

The color depends on the cation contained and can be a clear indication of the corresponding metal. In the case of mixed samples, however, it is often only possible to infer the presence or absence of certain strongly coloring ions (e.g. Co ²⁺ , Cr ³⁺ ), since mixed coloring or covering of less intensive coloring can also occur.

The borax pearl is therefore not sufficient as evidence. The information received from the borax bead must be confirmed by suitable detection reactions.

Examples of staining

cation	Oxidizing flame	Reduction flame
Fe 2+	colorless to yellow-red	green
Mn 2+	violet	colorless
Co 2+	blue	blue
Ni 2+	yellow to brown	brown
Cr 3+	green	emerald green
Cu 2+	blue green	red-brown

Individual evidence

1. ^ Brockhaus ABC chemistry. FA Brockhaus Verlag, Leipzig 1965, p. 196.
2. ↑ ^{a b} Jander-Blasius: Introduction to the inorganic-chemical practical course. 13th edition. S. Hirzel Verlag, Stuttgart 1990, p. 182.
3. ^ Entry on sodium tetraborate in the GESTIS substance database of the IFA , accessed on February 11, 2012(JavaScript required) .
4. ↑ Jander-Blasius: Introduction to the inorganic-chemical practical course. 13th edition. S. Hirzel, Verlag Stuttgart 1990, p. 282.

Web links

Wikibooks: Practical Inorganic Chemistry / Borax and Phosphorus Salt Pearl  - Learning and teaching materials