60

APPENDIX TO CHAPTER II.

FLAME REACTIONS.

66. Bunsen1 has shown that almost all reactions which can be performed by means of the blowpipe can be accomplished with far greater ease and precision in the non-luminous flame of the gas-burner invented by himself. This burner, the upper part of which is shown in Fig. 14, is furnished with a hoop near the bottom for closing and opening the air-holes there situated. The conical chimney, dddd, is made of such a size that the flame may burn quite steadily. This flame, also shown in the figure, consists of the dark cone, aaaa', the flame mantle, aeab, and the luminous point aba, not seen when the airholes are fully open, but obtained on closing these holes up to a certain point. The following six points in the flame are used in the reactions :

d

d

d

FIG. 14.

d

(1) The base of the flame at a, where the lowest temperature exists, is specially employed to heat mixtures so as to obtain the flame-colouration due to the most volatile only. (2) The zone of fusion at ẞ constitutes the hottest part 1 Ann. Chem. Pharm., cxxxviii. 257. Phil. Mag., [4] xxxii. 81.

of the flame, and hence it is used in testing the fusibility and volatility, &c. of substances.

(3) The lower oxidizing flame at y, in the outer margin of the zone of fusion, is especially suitable for the oxidation of substances dissolved in beads of fused salts.

(4) The upper oxidizing flame ate is available for the roasting and oxidation of all substances which do not require an excessively high temperature. This acts most powerfully when the air-holes are quite open.

(5) The lower reducing flame at 8, on the inner edge of the mantle, next to the dark central cone. The reducing action of this part is not of the strongest; but it is specially available for reductions on charcoal, and in beads of fused salts.

(6) The upper reducing flame at ʼn is formed above the dark cone when the admission of air is lessened by the gradual closing of the air-holes. If the luminous point is made too large soot is deposited on the objects placed within it; this ought never to occur. This part is especially available for reducing metals when it is desired to collect them in the form of films.

67. The following materials are used as flame supports: (1) Platinum wire; this must scarcely exceed the thickness of a horse-hair, and one decimeter in length of the wire should not weigh more than o'034 grm. It serves for investigations on fusibility, volatility, and flamecolouration, and also for examinations with borax, microcosmic salt and sodium carbonate.

(2) Asbestos fibre is used in place of platinum wire when the latter would be attacked by the substance under examination ; the threads must not be more than one quarter the thickness of a lucifer match. In taking up the substance the asbestos is slightly moistened.

(3) Charcoal splinter, which serves as a substitute for

charcoal, is prepared by breaking off the head of a lucifer match and smearing over the rest of the match with a crystal of sodium carbonate partially fused by holding it near to the flame; the match is then held in the flame and slowly rotated. A charcoal splinter is thus obtained which is partially protected from burning by its coating of fused sodium carbonate.

(4) Glass tubes, about 3 mm. wide, and 30 mm. long, and closed at one end.

When substances have to be held a long time in the reducing flame, it is convenient to use a Bunsen's stand, which is provided with horizontal clips and arms, moveable, horizontally and vertically, on the vertical support. The arms carry small glass tubes supporting platinum wire or asbestos fibre, and the clips are used for holding test-tubes.

68. In addition to the reagents described in par. 5, the following are employed in the Bunsen flame reactions :

Stannous chloride solution, which should be kept in a well-stoppered bottle, in contact with a little metallic tin, to prevent the formation of stannic chloride which is useless for the purpose required. Stannous chloride is a strong reducing agent and serves for distinguishing films and to detect gold, molybdenum, tungsten, &c.

Caustic soda solution also serves for distinguishing films, and is further employed in testing for cobalt, nickel, tin, &c. Silver nitrate, in perfectly neutral solution, serves for distinguishing films and in the detection of chromium and vanadium.

Fuming hydriodic acid, which, together with phosphorous acid, is formed by the action of moist air on phosphorus triiodide. For the preparation of the phosphorous iodide eight parts of iodine are added to one part of phosphorus in a test tube filled with carbon dioxide, and after the ignition

has ceased the mass is heated to fusion. The tube is then carefully closed, and when the mass has solidified, it is placed in a shallow, wide-necked, well-stoppered glass bottle. To use the reagent a film produced on the outside of a porcelain dish is held over the open mouth of the bottle, when, by the action of the gaseous hydriodic acid, iodine compounds are formed. If the reagent ceases to fume, some dry phosphoric acid is added.

The iodine compounds can also be produced by burning iodine tincture (solution of iodine in alcohol) on asbestos and holding the film over the flame. By this treatment the film sometimes receives a brown colouration, which, however, can be easily removed by warming gently. Ammonia and Ammonium sulphide are employed in various tests, especially in the examination of films.

Bromine, preserved in a wide-necked, well-stoppered bottle, is applied by holding substances in the vapour evolved from it. Bromine vapour in the presence of water acts as an oxidizing agent.

Potassium ferrocyanide solution serves for the detection of iron, copper, and molybdenum.

Lead acetate (sugar of lead) is used for the detection of chromium.

Bismuth nitrate serves for the detection of tin.

Acetic acid is used in testing for chromium, vanadium, manganese, and uranium.

Mercuric cyanide serves for the detection of palladium, and is used but rarely.

Aqua-regia, a mixture of hydrochloric and nitric acids, is employed in testing for gold, platinum, &c.

Only very small quantities of the substance to be examined are employed. Decrepitating substances are ground to the finest powder and taken up on a small piece

of moistened filter paper about a square centimeter in size. When this paper is carefully burnt between two rings of fine platinum wire, the assay remains behind as a compact crust which can then be treated in the flame without any difficulty.

METHOD OF EXAMINATION.

A. Behaviour on Heating.

69. On heating the substance the following appearances may be looked for :

(1) Whether the substance becomes luminous when brought into the hottest part of the flame.

(2) Whether the substance fuses, and whether at a higher or a lower temperature. It should be here noted whether the assay intumesces, changes colour, or decreases in volume, or becomes transparent after cooling. (3) Whether the substance volatilizes, and emits an odour.

(4) Whether the substance colours the flame. Flamecolouring substances when brought into the upper reducing flame produce a colouration in the upper oxidizing flame. Mixtures of flame-colouring substances are first held in the lowest and coldest part of the flame so that the more easily volatile constituents are first vaporized.

B. Oxidation and Reduction.

70. (1) Beads. Oxidation and reduction in beads are effected by holding the bead, supported on platinum wire, in the lower oxidizing or lower reducing flame respectively.

(2) Reduction in small glass tubes. The perfectly dry substance is heated with sodium carbonate and carbon (soot from oil of turpentine) or with metallic sodium or magnesium in a thin tube closed at one end, about 3 mm. wide and 3 cm. long. The sodium is wiped free from