The sulphates, selenates, and tellurates of barium are nearly insoluble in water; those of strontium and lead are very sparingly soluble, and those of calcium are still sparingly soluble, though more easily soluble than the salts previously mentioned. All these salts, therefore, are most conveniently prepared by the addition of a soluble sulphate, selenate, or tellurate to a soluble salt of calcium, strontium, barium, or lead, thus : CaCl2.Aq + Na2SO4.Aq = CaSO4.2H2O + 2NaCl.Aq ; Pb(NO3)2.Aq + K2SO4Aq = PbSO4 + 2KNOs.Aq. These salts are still less soluble in alcohol than in water, hence addition of alcohol to their solutions produces a turbidity. While barium sulphate is not attacked by boiling hydrochloric acid, barium selenate evolves chlorine, and is changed to barium chloride and selenious acid, thus : BaSeO4 + 4HCI.Aq = BaCL. Aq + H2SeO3.Aq + Cl2 + H2O.
All other sulphates are soluble in water, and can therefore be prepared by one of the usual methods, such as treatment of the oxide, carbonate, or metal with the acid. Dilute sulphuric acid dissolves magnesium, zinc, cadmium, aluminium, chromium, iron, manganese, nickel, and cobalt; other metals resist its attack, because their electro-affinity is less than that of hydrogen. The order is: Cs, Rb, K, Na, Li, Ba, Sr, Ca, Mg, Al, Mn, Zn, Cd, Cr, Fe, Co, Ni, Pb:-H:-Cu, Hg, Ag, Pt etc, Au. All the metals to the left of hydrogen in the table are attacked, because they receive their ionic charge from the hydrogen of the dilute acid: Zn" + H2|=SO4.Aq = Zn|=SO4.Aq + H-0 ; the zinc is ionised, giving up its two electrons to the hydrogen ions, which escape in the molecular condition. But this transference of charge appears to require the contact of some metal which has a greater affinity for electrons than hydrogen, for pure zinc is not attacked by pure dilute acid; in fact, the arrangement must be analogous to that of a battery. It is possible that this is due to the protection of the zinc by a film of condensed hydrogen-in other words, to polarisation; contact with another metal affords a means whereby the hydrogen receives electrons, and is evolved, not from the surface of the zinc, but from the surface of the less electro-positive metal. (See Vol. I. p. 47.)
With concentrated acid, these metals, as before remarked, are dissolved as sulphates, with evolution of sulphur dioxide.
The sulphates form an important group of salts. Among the best known are: Sodium sulphate, Na2SO4.ioH2O, " Glauber's salt," contained in sea-water and in many mineral springs; K2SO4, and (NH4)2SO4, hard rhombic prisms ; the double salts, NaHSO4, and KHSO4, obtained by mixture ; when heated, these salts lose water and are converted into pyrosulphates : 2KHSO4 = H2O + K2S.2O7. CaSO4 occurs native, as anhydrite, and CaSO4.2H2O, as gypsum and alabaster. Gypsum, when gently heated, loses its water, and is then known as " plaster of Paris ; 99 on mixing it to a paste with water, combination takes place slowly, and the plaster " sets ; 99 and in this way casts may be taken. SrSO4 is found native as celestine ; BaSO4, as heavy-spar or barytes. It is the commonest mineral containing barium ; from it barium salts are prepared, by heating it with ground coke, which reduces it to the sulphide : BaSO4 + 4C = 4CO + BaS. The barium sulphide is then dissolved in the appropriate acid, and the required salt is made. Precipitated barium sulphate is known as " permanent white;9 owing to its low price, it is much used as a paint, although its covering power is small.
MgSO4.7H2O, ZnSO4.7H2O, and CdSO4.7H2O, as well as FeSO4-7H2O, MnSO4.7H2O, and the corresponding cobalt and nickel salts, are " isomorphous," that is, they crystallise in the same form-rhombic prisms. Magnesium sulphate, or " Epsom salts," is present in sea-water and in many mineral waters; it also occurs in the salt deposits at Stassfurth, in S. Germany, and is termed kieserite. It is used as a purgative. Zinc sulphate is known as " white vitriol," and ferrous sulphate as "green vitriol" or " copperas." A large number of double salts exists, of the formulas of which MgSO4.K2SO4.6H9O may serve as a type ; they are all soluble, and they are ionised in solution into the same ions as the simple salts would furnish ; thus, the ions of the salt mentioned above are Mg, 2K, and 2=SO4. They differ in this respect from such salts as K2SiF6, of which the ions are 2K and =SiF6. The alums form a similar series of double salts, in which monad metals, such as sodium, potassium, and ammonium, and triad metals, aluminium, chromium, iron, manganese and others, are combined together as sulphates with water of crystallisation: K2SO4.AI2(SO4)3.24H2O,orKAl(SO4)2.12H2O. The molecular weight of the compound is unknown ; hence, as usual, the simpler formula is preferable. These compounds are named from their analogy with the original " alum," of which the formula is given above; they all crystallise in regular octahedra, and, like all true isomorphous salts, they are able to crystallise together; so that if a crystal of KAl(SO4)2.12H2O is placed, for example, in a solution of (NH4)Cr(SO4)9.12H2O, the latter will form a dark-red layer on the surface of the former.
"Alum " finds use as a mordant; when textile fabrics are boiled in its solution, the fibre becomes incrusted wi:h a layer of aluminium hydroxide, and when subsequently dyed the colouring matter is retained in combination with the alumina and with the fibre, so that it cannot be removed by washing. This phenomenon depends on the fact that aluminium sulphate is partially hydrolysed by water into 2A1(0H)3 and 3H2SO4; the adhesion of the alumina to the fibre is attributed to " adsorption," a term applied to the adhesion of gases, liquids, or of substances in solution, to the surface of solids. A solution of alum also gives coloured precipitates with many dye-stuffs, which arc known as "lakes." Selenic acid also yields alums.
Bismuth sulphate, Bi2(SO4)3, obtained by evaporating a solution of bismuth oxide, Bi2Og, in sulphuric acid, forms acicular crystals, which, on addition of water, like all other bismuth salts, yield a basic salt, in which the group 0=Bi-, bismuthyl, plays the part of a monad metal; hence the formula of the basic sulphate is (O-Bi)9SO4; it is an insoluble powder. Copper sulphate, or " blue vitriol," CuSO4.5H2O, forms blue soluble crystals; silver, mercurous, and mercuric sulphates, Ag2SO4, Hg2SO4, and HgSO4, are sparingly soluble, white crystalline powders. As the ion =SO4 is colourless, all these salts possess the colour of the metallic ion which they contain; thus, Fe11 is green, Mn11 pink, Crm green, Fem yellow, Ni11 green, Co11 red, Cuu blue, and the others colourless.1
Sulphates of the alkali- and alkaline-earth metals are stable at all temperatures lower than that of the electric arc ; but all other sulphates decompose, the primary product being the oxide of the metal and sulphuric anhydride; the latter, however, being unstable at a red-heat, decomposes partly into sulphur dioxide and free oxygen. This decomposition is made use of in the preparation of "Nordhausen sulphuric acid," a fuming liquid, consisting chiefly of H2S2O^; it is made by distilling partially dried ferrous sulphate from fireclay retorts: 2FeSO4 = Fe2O3 + SO2 + SO3; thepyrosulphuric acid is produced by the union of the anhydride with water : 2SO3 + H2O = H2S2Or. The iron oxide has a fine red colour, and is sold as a paint under the name " Venetian red."