There are many types of these compounds, of which the potassium salt, K₂ZrF₆, is the most important. The solubility of this salt increases very rapidly with the temperature; 100 parts of water dissolve, at 15°, 1·41 parts, at 100°, 25 parts of the compound. It has been frequently used for the purification of zirconium compounds, for the preparation of the element, and for analytical determinations. Other potassium salts, K₃ZrF₇ and KZrF₅,H₂O, are obtained by using a large excess of potassium fluoride and zirconium fluoride respectively. The ammonium compounds are analogous in composition to the potassium salts, but the sodium salt, Na₅ZrF₉, is obtained from mixtures of the components in all proportions; on account of its very low solubility, it can be obtained by double decomposition of the potassium salt with sodium chloride. Of the salts with divalent metals, the types R´´ZrF₆,_x_H₂O and R´´₂ZrF₈,_x_H₂O, are the most common. The _chloride_, ZrCl₄, is known, on account of the ease with which it hydrolyses, in the anhydrous state only. It can be obtained by all the usual methods, of which perhaps the action of chlorine on the carbide, and of carbon tetrachloride, or a mixture of chlorine and sulphur monochloride, on the oxide, are the most convenient; an interesting method consists in heating the oxide with phosphorus pentachloride in a closed tube at 190°. It forms a volatile white sublimate, which fumes strongly in air, and reacts vigorously with water; it is soluble in ether. It forms a series of addition compounds with ammonia and organic bases, as well as with the chlorides of non-metallic elements; warmed with phosphorus pentachloride, it forms a stable solid, 2ZrCl₄,PCl₅, which melts at 240°, and boils at 345°. With organic compounds, especially with esters, acids, and phenols, it forms a long series of addition and condensation products, of which the compounds ZrCl₄(C₆H₅·COOC₂H₅)₂ and ZrCl₂[O·C₆H₅·CHO]₂ may be taken as examples. By addition of organic bases to a solution of the chloride in alcoholic hydrogen chloride, double chlorides of the type (C₅H₅NH)₂ZrCl₆ are obtained. The _oxychloride_, ZrOCl₂,8H₂O, separates in characteristic tetragonal prisms when the tetrachloride is dissolved in water or hydrochloric acid of any concentration. It is readily soluble in water and alcohol, but sparingly soluble in hydrochloric acid, from which therefore it is generally recrystallised. According to Chauvenet,[466] it effloresces in dry air, forming the hexahydrate, ZrOCl₂,6H₂O; when dried in a vacuum, it forms the hydrate, ZrOCl₂,3¹⁄₂H₂O, whilst the dihydrate, ZrOCl₂,2H₂O, is obtained by heating at 100°-105° in hydrogen chloride. When the dihydrate is heated to 230°, it forms another basic chloride, ZrOCl₂ZrO₂,[467] which is stable up to 600°; above this temperature, it breaks up, forming the volatile tetrachloride, and leaving a residue of the dioxide. [466] _Compt. rend._ 1912, ~154~, 821. [467] _Ibid._ 1234.