Borax bead test

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

Detection.—Dry Tests.—Salts of cobalt are usually of a rose-red colour when hydrated, and yield a beautiful blue colour in the borax bead test provided they are not present in excessive amount, otherwise the bead becomes opaque and appears black. When heated on charcoal with reduction mixture in the blowpipe test, metallic cobalt separates out in the form of small magnetic beads. When strongly heated with alumina a blue colour is produced, known as Thenard s blue. 

Characteristic coloured beads are produced with salts of copper, iron, chromium, manganese, cobalt, and nickel. The student should carry out borax bead tests with salts of these metals and compare his result with those given in Chapter III. 

Dry tests a. Borax bead test The bead produced in the oxidizing flame by small amounts of manganese salts is violet whilst hot and amethyst-red... 

A useful reaction which may be carried out at this stage is the microcosmic bead test (Section II.2, test 6). This test is carried out in a loop of platinum wire exactly as for the borax bead test. The presence of a white skeleton (of silica) in the coloured glass indicates silicate. Tin(IV) oxide, Sn02, dissolves slowly in the bead may be mistaken for silica. 

Sodium tetraborate (Borax), Na2B407.10H2O. The anhydrous (fused) sodium tetraborate, Na2B407, is even more suitable for the borax bead tests Sodium thiosulphate, Na2S203.5H20 Starch (potato-starch or soluble starch)... 

Experiment 186. — a) Subject a minute quantity of manganese dioxide to the borax bead test, and note the color of the bead after heating in each flame. 

A crystalline form (p. 274), m.p. 450°, appears after dehydrating HBOg by keeping it under reduced pressure and slowly raising the temperature to 400° over a period of some weeks. Boric oxide, though more acidic than AI2O3, is amphoteric it combines with metal oxides to give metaborates as in the borax-bead test ... 

Results obtainable with the borax-bead test are summarized in Table 2.4. In certain circumstances the borax-bead test is inconclusive, but it can be supplemented with the phosphate (or microcosmic salt) bead test (see below). 

The borax-bead test gives characteristic colors with certain cations. 

The color of the borax is due to copper metaborate, Cu(B02)2/ that has been formed. The color of many metallic borates is characteristic of the particular metal that is present. Thus, the presence of certain metals is sometimes determined by what are called borax bead tests. Sodium metaborate, NaB02, was the other product in the borax bead reaction. A different borate of sodium will be prepared below. 

The formation of coloured glass beads as in Equations 14.13 and 14.14 constitutes a simple method of detecting certain metal elements (this test is useful when done in parallel with a borax bead test). 

B4O72- + 7H2O = 20H- + 4H3BO3 See also borax-bead test. 

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