Coloured Borax Beads

Heat a small amount of borax in the eye of a platinum wire in the flame of a burner. After water stops evolving (how can this be established ), slightly cool the melt and wet it with a chromium(III) or cobalt(n) nitrate solution. Again heat the substance up to melting (in the flame of a burner). What colour does the substance acquire after cooling Write the equation of the reaction. What does the colour of borax beads depend on  

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The coloured borax beads are due to the formation of coloured borates in those cases where different coloured beads are obtained in the oxidizing and the reducing flames, borates corresponding to varying stages of oxidation of the metal are produced. Thus with copper salts in the oxidizing flame, one has ... 

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. 

Make a borax bead and dip it into a solution of glycerol and bring it into the flame. A gieen colouration due to boric acid IS produced... 

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. 

Detection.—Dry Tests.—Nickel salts are usually green in colour when hydrated, but yellow when anhydrous. They impart a reddish brown colour to the "borax bead when heated in the oxidising flame of the blowpipe, but dark and opaque in the reducing flame. 

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. 

In general, it may be stated that the borax beads are more viscous than the phosphate beads. They accordingly adhere better to the platinum wire loop. The colours of the phosphates, which are generally similar to those of the borax beads, are usually more pronounced. The various colours of the phosphate beads are collected in the following table. 

Borax bead reactions (T able V.7.) Prepare a borax bead in a loop of platinum wire by dipping the hot wire into borax and heating until colourless and transparent. Bring a minute quantity of the substance into contact with the hot bead and heat in the outer or oxidizing flame. Observe the colour when the bead is hot and also when it is cold. Heat the bead in the inner or reducing flame and observe the colours in the hot and cold states. Coloured beads are obtained with compounds of copper, iron, chromium, manganese, cobalt, and nickel (see however, Section VII.25, Table VII.8). ... 

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. 

In qualitative analysis copper is detected by precipitation as cupric sulphide from hydrochloric-acid solutions of its salts. To prevent the formation of a colloidal precipitate, the solution should be hot, and should contain excess of the acid. The sulphide is soluble in hot, dilute nitric acid, and in potassium-cyanide solution, but almost insoluble in solutions of alkali-metal sulphides. It dissolves to some extent in ammonium-sulphide solution. Other aids in the detection of copper are the blue colour of solutions of cupric-ammonia salts the reddish-brown precipitate of cupric ferrocyanide, produced by addition of potassium ferro-cyanide to cupric solutions the formation of an intense purple coloration by the interaction of hydrogen bromide and cupric salts, a very delicate reaction2 the formation of a bluish-green borax bead and the ready isolation of the metal from its compounds by the action of reducers. 

Detection in the Dry Way.—When a uranium compound is heated in a borax bead, or in a bead of microeosmic salt, in an oxidising flame, an orange-yellow coloured bead results which may become greenish on cooling. In the reducing flame the bead assumes a green colour. 

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). 

If a trace of chromate or diehromate be heated in an opaque bead of fusion mixture on a platinum wire, a yellow bead results a chromium salt will give the same result if the bead is touched with a minute quantity of sodium peroxide. When heated with borax on platinum wire in an oxidising flame, chromium compounds impart to the bead a yellow or dark red colour, which on cooling becomes yellowish green if heated in the reducing flame an emerald green bead is obtained. 

The usual method of blowpipe analysis was to direct the flame on to a small portion of the material on a charcoal block. The substance under investigation was often mixed with sodium carbonate, borax, or microcosmic salt (sodium ammonium phosphate). When the material was heated alone or with sodium carbonate, it often yielded decomposition products with a characteristic appearance and borax or microcosmic salt fused to a glass to which the unknown material might impart a characteristic colour. After Wollaston had introduced a method of producing malleable platinum in 1800, a platinum wire was frequently used to support the material in blowpipe analysis, particularly in the production of glassy beads with borax and microcosmic salt. 

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