120

PROPERTIES OF BROMINE.

stench. It boils at about 60°, but is very volatile even at the ordinary temperature of the air.

Exp. 66.—By means of a small pipette, throw into a flask or bottle, of the capacity of 1 or 2 litres, 3 or 4 drops of bromine. Cover the bottle loosely, and leave it standing. In a short time it will be filled with a red vapor, which is bromine gas. This vapor is very heavy, more than five times as heavy as air, and eighty times as heavy as hydrogen.

At about 7° bromine crystallizes in brittle plates. It dissolves sparingly in water, but is soluble in alcohol, and in all proportions in ether.

In its chemical behavior, as well as in many of its physical properties, bromine closely resembles chlorine. Its affinity for hydrogen, though weaker than that of chlorine, is still powerful. Like chlorine, it is an energetic bleaching and disinfecting agent, and it decomposes the vapor of water when passed with it through a tube heated to bright redness, bromhydric acid and oxygen being the products of the reaction. A lighted taper burns for an instant in bromine vapor and is then extinguished. Phosphorus, antimony, potassium, and the like, take fire on being thrown into bromine, in the same way as in chlorine, a bromide of the other element being produced.

Exp. 67.-Fit a thin cork to a large, wide-mouthed bottle; perforate the cork, and through the hole pass a tube of thin glass (No. 2) closed at one end. The tube should reach nearly to the bottom of the bottle, and should project two or three inches above the cork. Within the tube place a few drops of bromine; throw in upon this a very small quantity of finely powdered antimony, and instantly cover the mouth of the tube with an inverted crucible or wide-mouthed phial, in order that nothing may be thrown out of the tube by the violent action which attends the combination. If the tube be broken, its fragments will be retained within the bottle.

Bromine is used to a certain extent in medicine, and largely in photography. In the chemical laboratory it is often employed, not only for its own sake, but as a substitute for chlorine; for, though less energetic, it is more manageable than the latter, especially in those cases where a liquid is desirable.

128. Bromhydric Acid (HBr).-In spite of the strong affinity of bromine for hydrogen, these elements cannot readily be made

to unite directly. A mixture of equal volumes of hydrogen and bromine vapor cannot be made to combine with explosion by exposure to the sun's rays, or by the introduction of a burning lamp, though a certain amount of combination occurs in the immediate neighborhood of the flame. But by immersing in the mixture a platinum wire, kept red-hot by a galvanic current, the two elements may be made to unite slowly; and a similar result is obtained by passing the mixed gases through a red-hot tube. Bromhydric acid gas can, however, be readily prepared by decomposing bromide of potassium with sulphuric acid, or, better, with a concentrated solution of phosphoric acid. The reaction is analogous to that in which chlorhydric acid is obtained from chloride of sodium:

2KBr+H,O,SO,

=

K,O,SO, + 2HBr.

If sulphuric acid be employed, a secondary reaction occurs; a small part of the bromhydric acid suffers decomposition, and the product is slightly contaminated with free bromine and with sulphurous acid:

2HBr H,O,SO, = 2H2O + 2Br + SO,.

Since phosphoric acid does not thus decompose bromhydric acid, the latter can be obtained in a state of purity by distilling a mixture of bromide of potassium and phosphoric acid.

129. Bromhydric acid is a colorless, irritating gas, which, on coming in contact with the moisture of the air, fumes even more strongly than chlorhydric acid. By powerful pressure it can be reduced to the liquid condition, and upon being exposed to intense cold it may be obtained in the form of a crystalline solid.

It is readily soluble in water, forming a strongly acid solution which resembles chlorhydric acid in many respects, and, like it, fumes in the air. A ready method of preparing the solution is to decompose a strong solution of bromide of barium with sulphuric acid diluted with its own weight of water. The solution of the free acid may then be separated from the insoluble sulphate of barium by filtration or by distillation.

When the solution of this acid is mixed with nitric acid, there is obtained another aqua regia capable of dissolving gold and pla

tinum, like the mixture of chlorhydric and nitric acids, though less readily.

130. The gas undergoes no change when passed through a red-hot tube; but it is readily decomposed by metals like potassium at the ordinary temperature, and by tin gently heated. A bromide of the metal is formed in either case, and there remains a volume of hydrogen equal to half that of the original gas. Observation has shown that the specific gravity of the gas is very nearly 40-5, or half the sum of the specific gravities of bromine vapor and hydrogen. From this fact and the above decomposition of the gas by metals, it follows that bromhydric acid is composed of equal volumes of bromine and hydrogen united without condensation.

2

131. Bromic Acid, HO,Br,O,.-Only one oxide of bromine has been studied, and even this has never been obtained free from water. It is bromic acid, a substance corresponding to chloric acid in composition and properties. Its compounds, also, known as bromates, generally resemble very closely the corresponding chlorates.

Bromate of potassium can be obtained by the action of bromine upon potash-lye, in the same way that chlorate of potassium is obtained by the action of chlorine :

6(K2O,H2O) + 12Br = K2O,Br2O, + 10KBr + 6H20.

The bromate, which is less soluble than the bromide, can subsequently be separated by crystallization. In order to obtain the hydrated acid, bromate of barium may be decomposed with dilute sulphuric acid :

:

BaO,Br20, + H2O,SO, = BaO,SO, + H2O,Br2O,.

5

The insoluble sulphate of barium is separated by filtration. The acid solution can be concentrated to a certain extent by evaporation at a gentle heat, but cannot readily be brought to a syrupy consistency without decomposition. It decomposes, also, on being heated to 100°, and in general gives up oxygen on being brought into contact with substances which readily combine with that element.

2

132. Hypobromous Acid, H,O,Br,O.-There can be no doubt of the existence of an oxide of bromine corresponding to hypochlorous acid. When bromine is added to an excess of a solution of nitrate of silver, a liquid of strong bleaching-properties is

obtained, from which a yellowish, acid fluid may be distilled. This distillate bleaches strongly, and yields, on analysis, numbers corresponding with the above formula. When cold dilute alkaline solutions are mixed with bromine they acquire a power of bleaching, and in general behave like the alkaline hypochlorites, which are formed under similar conditions. So too when bromine-water is treated with red oxide of mercury; a sparingly soluble oxybromide of mercury is formed, together with a bleaching liquor supposed to contain hypobromous acid.

The analogies which subsist between chlorine and bromine are, however, everywhere so clearly defined that there is good reason to believe that other oxides of bromine, corresponding to those of chlorine, will be sooner or later discovered.

133. Chloride of Bromine.-Liquid bromine absorbs a large quantity of chlorine-gas when the two elements are brought together, and there is formed a very volatile liquid called chloride of bromine. It exhales a pungent, irritating odor, and is soluble in water; the solution possesses considerable bleaching-power.

134. Bromide of Nitrogen is an explosive compound analogous to chloride of nitrogen, from which it may be prepared by means of bromide of potassium.

CHAPTER X.

IODINE.

135. In its chemical properties iodine bears a striking resemblance to bromine, and consequently to chlorine also. It exists in sea-water and in the water of many saline and mineral springs. The proportion of iodine in sea-water is exceedingly small, being even smaller than that of bromine. But iodine is obtained more readily than bromine; for iodine is absorbed from sea-water by various marine plants, which, during their growth, collect and concentrate the minute quantities of iodine which the sea-water

124

EXTRACTION OF IODINE.

contains, to such an extent that it can be extracted from them with profit.

Upon the coasts of Scotland, Ireland, and France, where labor is cheap, the sea-weeds which contain iodine are collected, dried, and burned, at a low heat, in shallow pits. The half-fused ashes thus obtained, called kelp or varec, contains, among other things, iodine in the form of iodide of sodium and iodide of potassium. It is lixiviated with boiling water, and the solution is then evaporated and set aside to crystallize. The iodides, being much more soluble than the other constituents of the ash, remain dissolved in the mother-liquor after most of the other salts have crystallized out. If this mother-liquor, or iodine-lye, be now mixed with a small quantity of sulphuric acid and left to itself for a day or two, it may be freed from a further portion of impurity; it is then transferred to a leaden retort, mixed with a suitable quantity of powdered black oxide of manganese, and gently heated. Iodine is set free, just as chlorine would be from chloride of sodium under similar circumstances, and may be collected in appropriate receivers :—

2NaI + 2(H2O,SO3) + MnO, = Na,O,SO, + MnO,SO ̧+ 211,0 + 21.

136. At the ordinary temperature iodine is a soft, heavy, crystalline solid of bluish-black color and metallic lustre. Its specific gravity is 4.948. It melts at a temperature (107°) a little above that at which water boils, and boils at a somewhat higher temperature (178°-180°); but in spite of this high boiling-point it evaporates rather freely at the ordinary temperature of the air, and the more rapidly when it is in a moist condition. It may be entirely volatilized by heating it upon writing-paper. Its odor is peculiar, somewhat resembling that of chlorine, but weaker, and easily distinguished from it. Its atomic weight is 127.

The vapor of iodine is of a magnificent purple color, whence the name iodine, derived from a Greek word signifying violetcolored; it is very heavy, indeed the heaviest of all known gases; it is nearly nine times as heavy as air. The specific gravity of the vapor is 127.

Exp. 68.-Hold a dry test-tube in the gas-lamp by means of the wooden nippers, and warm it along its entire length, in so far as this is practicable. Drop into the hot tube a small fragment of iodine, and observe the vapor as it rises in the tube. If only a small portion of