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. 

Perlen-glanz, m. pearly (or nacreous) luster, -probe, /. bead test, -schnur, /. string of beads or pearls. 

Quartz glass (silica), pyrex (borosilicates) and other household and laboratory glasses (boroaluminosilicates) owe their high chemical resistance to the (tetrahedral) network forming properties of Si, B, P and A1 (cf., the borax and phosphate bead tests in qualitative dry reactions on the salts of numerous metals). 

Fig. 28. The cumulative COj/CO ratio for coke bum-off on spherical catalyst beads versus combustion temperature in air (O) white amorphous silica-alumina ( ) green Cr02-containing amorphous silica-alumina (M) macroporous white catalyst. The weight (mg) of the bead tested is denoted by the numerals adjacent to the respective symbol. Dashed line represents intrinsic ratios from carbon combustion research. From Weisz (1966).

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. 

Flame tests In order to understand the operations involved in the flame colour tests and the various bead tests to be described subsequently, it is necessary to have some knowledge of the structure of the non-luminous Bunsen flame (Fig. II.2). 

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. 

Phosphate (or microcosmic salt) bead tests The bead is produced similarly to the borax head except that microcosmic salt, sodium ammonium hydrogen phosphate tetrahydrate Na(NH4)HP04.4H20, is used. The colourless, transparent bead contains sodium metaphosphate ... 

Sodium carbonate bead tests The sodium carbonate bead is prepared by fusing a small quantity of sodium carbonate on a platinum wire loop in the Bunsen flame a white, opaque head is produced. If this is moistened, dipped into a little potassium nitrate and then into a small quantity of a manganese compound, and the whole heated in the oxidizing flame, a green bead of sodium manganate is formed ... 

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. 

Apply the microcosmic bead test If a skeleton bead is obtained, silica or a silicate is indicated. A negative result does not definitely prove that silica or a silicate is absent, as a skeleton is not always formed. The silicon tetrafluoride test should then be employed (Section IV.26, reaction 6). 

May contain Si02.f Confirm by microcosmic bead test or by the SiF4 test in lead capsule. 

Unless silicates are removed here, they are likely to be confused with Al(OH)3 in the group separation. Repeated evaporation with concentrated HC1 converts silicates into a granular form of hydrated silica, which is readily filtered, particularly after a final digestion with dilute HC1. The precipitate should be subjected to the microcosmic salt bead test or to the silicon tetrafluoride test (Section IV.26). 

Silicate, (i) Microcosmic bead test (IV.26, 5) and silicon tetrafluoride test (IV.26,6). (ii) Ammonium molybdate solution and SnCl2 solution test (IV.26,8). 

Microcosmic salt bead test reducing flame, cold - blue blood-red when fused with a trace of iron(II) sulphate. 

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)... 

Differently colored hot and cold beads are characteristic of the compounds of different elements, and the bead test is often used to confirm other observations or to suggest a further special examination. 

The Blowpipe and its Use. — Sometimes it is not convenient to apply the bead test, nor to hold a substance directly in the flame. It is then customary to study the action of a substance when heated on charcoal in a small flame, much like the Bunsen flame and produced by... 

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 ... 

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