Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/507

Rh COMPOUNDS.] barium sulphate by filtration, and concentrated by evapo ration in vacuo until it forms a syrupy liquid : Ba(C10 3 ). 2 + H 2 SO 4 = 2HC10 3 + BaSO 4. Chloric acid has a faint chlorous odour, and a powerfully acid reaction ; it is instantly decomposed by contact with organic matter, with charring, and frequently even with ignition. In diffused daylight it gradually undergoes spontaneous decomposition, and when heated to a little above 100 C. it is rapidly converted into perchloric acid, water, chlorine, and oxygen 8HC1O 3 = 4HC10 4 + 2H 2 O + 30 2 + 2C1 2. Chloric acid. Perchloric acid. Water. Oxygen. Chlorine. It is a powerful oxidizing and bleaching agent. Chloric acid serves for the preparation of the remaining oxides and oxy-acids of chlorine. To prepare chlorous anhydride from it, 10 parts of pure benzene are dissolved in 100 parts of concentrated sulphuric acid, and when the mixture has cooled 12 parts of pulverized potassium chlorate are added. The chloric acid liberated from the chlorate under these circumstances is deprived of a por tion of its oxygen by the benzene, which is oxidized, and at the same time, by the dehydrating influence exercised by the sulphuric acid, the elements of a molecule of water are removed and chlorous anhydride is liberated ; thus 2HC10 3 - 0. 2 - OH 2 = C1 2 O 8. The mixture is heated to about 50 C. in a flask having a delivery tube fitted to its neck by grinding, and blown out in several places into small bulbs ; the gas, after passing through water contained in these bulbs, is con ducted into a glass tube surrounded by a mixture of ice and salt. The condensed liquid deposits crystals of a hydrate of chlorous acid, and the supernatant liquid decanted immediately into a small cooled glass cylinder forms the nearly anhydrous oxide C1. 2 O 3. Liquid chlorous oxide or anhydride thus obtained is very mobile, is of a deep red colour, and has a specific gravity of 1-330 to 1 387 at C. ; it boils a little above C., and it explodes some what readily at a few degrees above C. The specific gravity of the gaseous oxide accords with that required by the formula C1 2 O 3. Chlorous anhydride is not very soluble in water, 100 grammes of water at 23 C. dissolv ing 5 - G5 grammes of the oxide (Brandau). The solution of chlorous acid, HC10. 2, thus obtained, has powerful bleaching and oxidizing properties ; when it is heated, chloric and hydrochloric acids are formed and chlorine evolved. By neutralizing the solution with hydroxides of the alkali or alkaline earth metals corresponding chlorites are obtained, which are soluble ; and from these the insoluble chlorites of lead and silver are obtained by double decomposition. Chlorites are decomposed by the feeblest acids, even by carbonic acid. When fused potassium chlorate is carefully treated with concentrated sulphuric acid, chlorine dioxide or peroxide is evolved ; the reaction by which it is produced, it is stated, is represented by the following equation 3KC1O 3 + 2H 2 SO 4 = 2C1O 2 + H,0 + KC10 4 + 2KHS0 4. Potassium Sulphuric Chlorine&quot; ., t Potassium Potassium chlorate. acid. peroxide. perchlorate. hydrogen sulphate. The so-called euchlorine gas, which is obtained when a mixture of a chlorate and hydrochloric acid is gently heated, appears to be a mixture of chlorine peroxide with chlorine. Chlorine peroxide is gaseous at ordinaiy tem peratures, but by means of a mixture of ice and salt it may be condensed to a red liquid. It has a deep greenish- yellow colour, and peculiar sweet chlorous odour ; a slight degree of heat and mere contact with organic matters at once determine its explosion. Like all the oxides of chlorine, it acts rapidly upon mercury and most metals. From Pebal s recent experiments there appears to be no 495 doubt that chlorine peroxide is correctly represented by the formula C1O 2, and it is, therefore, to be included with nitric oxide and the few other compounds in which we must suppose an odd number of affinities are free (see p. 473). It is absorbed by alkaline solutions, forming a mixture of a chlorite and a chlorate 2C1O., + 2KHO = KC10., Chlorine peroxide. Potassium hydroxide. Potassium chloride. KC10 3 Potassium chlorate. OH 2. v a ter In this respect especially it differs from the other oxides of chlorine. The only remaining chlorine compound to be considered is perchloric acid, HC1O 4. It may be formed from chloric acid by oxidation, but, according to Roscoe, the best method of preparing it consists in boiling down a solution of chloric acid ; lower oxides of chlorine escape, and a solution of perchloric acid is left. It may also be ob tained from potassium perchlorate, farmed by heating potassium chlorate, by distilling it with sulphuric acid. Aqueous perchloric acid may be concentrated by boiling till it attains a temperature of 203 C., after which it passes over in the form of an oily liquid which approximately has the composition HC1O 4 -f 2H 2 0. By distilling this hydrate with twice its volume of sulphuric acid, nearly pure perchloric acid is obtained. The pure acid is a colourless very soluble liquid, of specific gravity T782 at 15 5 C. In this state it is one of the most powerful oxidizing agents known ; a single drop brought in contact with charcoal, wood, or almost any organic substance, immediately causes an explosive combustion, which in violence almost equals the sud den decomposition of the so-called chloride of nitrogen. It produces frightful burns if allowed to fall upon the skin. It undergoes spontaneous decomposition at ordinary tem peratures, and cannot be distilled unchanged. It unites very energetically with water, and when mixed with it in suitable proportions forms the hydrate HC10 4 + H 2 O ; this is a white crystalline substance, which melts at 50 C-, and undergoes decomposition when heated to 110 C., splitting up into the pure acid and the oily hydrate above men tioned. Aqueous perchloric acid has a sour taste, and reddens litmus strongly, but does not bleach ; it dissolves iron and zinc with evolution of hydrogen, and when dilute it is unaffected by hydrogen sulphide and sulphurous acid, which reduce all other oxy-acids of chlorine. Perchloric acid, in fact, is distinguished from all other oxidized chlo rine compounds by its superior stability, oxygen being withdrawn from it with comparative difficulty. The results of Thomsen s thermochemical examination of various reactions which are involved in the formation of some of the oxidized chlorine compounds are exhibited in the following table. The symbols in the first column indi cate the nature of the reaction, and the numbers in the second column the units of heat developed or absorbed, the reaction being supposed to take place between the substances of which the symbols are separated by commas. Thus, the C1 2, O - 18,040 Gaseous oxide is to be read as meaning that, in the formation of a mole cule (in grammes) of gaseous chlorine oxide from chlorine and oxygen, 18,040 units of heat would be absorbed Similarly, the reaction C1 2 O, Aq, that is to say, the absorp tion of gaseous chlorine oxide by water is attended with the development of 9440 units of heat ; hence the reaction C1 2, O , Aq , or the formation of a solution of bypochlor- ous acid (two molecules) from chlorine, oxygen, and water would involve the absorption of 8600 units of heat. For these, as in all Thomsen s experiments, the determinations were made at about 18 C.