Double sulfates

Opa.nte. There are two methods used at various plants in Russia for loparite concentrate processing (12). The chlorination technique is carried out using gaseous chlorine at 800°C in the presence of carbon. The volatile chlorides are then separated from the calcium—sodium—rare-earth fused chloride, and the resultant cake dissolved in water. Alternatively, sulfuric acid digestion may be carried out using 85% sulfuric acid at 150—200°C in the presence of ammonium sulfate. The ensuing product is leached with water, while the double sulfates of the rare earths remain in the residue. The titanium, tantalum, and niobium sulfates transfer into the solution. The residue is converted to rare-earth carbonate, and then dissolved into nitric acid. 

Fra.ctiona.1 Precipituition. A preliminary enrichment of certain lanthanides can be carried out by selective precipitation of the hydroxides or double salts. The lighter lanthanides (La, Ce, Pr, Nd, Sm) do not easily form soluble double sulfates, whereas those of the heavier lanthanides (Ho, Er, Tm, Yb, Lu) and yttrium are soluble. Generally, the use of this method has been confined to cmde separation of the rare-earth mixture into three groups light, medium, and heavy. 

Aluminum Sulfate (Alum). Alum, a double sulfate of potassium and aluminum having twelve waters of crystallization, KA1(S0 2 12H20, is the earliest referenced aluminum containing compound. It was mentioned by Herodotus in the fifth century BC. The Egyptians used alum as a mordant and as a medicine the Romans used it for fireproofing. Some alums contain sodium or ammonium ions in place of potassium. 

The word alum is derived from the Latin alumen, which was appHed to several astringent substances, most of which contained aluminum sulfate (20). Unfortunately, the term alum is now used for several different materials. Papermakers alum or simply alum refers to commercial aluminum sulfate. Common alum or ordinary alum usually refers to potash alum which can be written in the form K SO Al2(SO 24H20, or it can refer to ammonium alum, ammonium aluminum sulfate. The term is also appHed to a whole series of crystallised double sulfates [M(l)M (lII)(SO 2 12H20] having the same crystal stmcture as the common alums, in which sodium and other univalent metals may replace the potassium or ammonium, and other metals may replace the alurninum. Even the sulfate radical may be replaced, by selenate, for example. Some examples of alums are cesium alum [7784-17-OJ,... 

Pseudoalums are a series of double sulfates, such as iron(lI) aluminum sulfate [22429-82-9], FeSO Al2(SO 24H20, containing a bivalent metal ion in place of the univalent element of ordinary alums. These pseudoalums have different crystal stmctures from those of the ordinary alums. 

The chemical reaction of the lead-acid battery was explained as early as 1882 (11). The double sulfate theory has been confirmed by a number of methods (12—14) as the only reaction consistent with the thermodynamics of the system. The thermodynamics of the lead —acid battery has been reviewed in great detail (15). 

Because Pb, Pb02, and PbSO are all soHds having low solubiHties, the activities of these substances are unity. At 25 °C, the absolute temperature Tis 298.15 K. The value of R, the gas constant, used is 8.3144 J/(molK). E, the Earaday constant, is 96,485 C/mol. The standard ceU voltage for the double sulfate reaction must be known as weU as the activities of sulfuric acid and water at any given concentration or temperature. 

To calculate the open circuit voltage of the lead—acid battery, an accurate value for the standard cell potential, which is consistent with the activity coefficients of sulfuric acid, must also be known. The standard cell potential for the double sulfate reaction is 2.048 V at 25 °C. This value is calculated from the standard electrode potentials for the (Pt)H2 H2S04(yw) PbS04 Pb02(Pt) electrode 1.690 V (14), for the Pb(Hg) PbS04 H2S04(yw) H2(Pt) electrode 0.3526 V (19), and for the Pb Pb2+ Pb(Hg) 0.0057 V (21). 

Miscellaneous Compounds. Among simple ionic salts cerium(III) acetate [17829-82-2] as commercially prepared, has lV2 H2O, has a moderate (- 100 g/L) aqueous solubiUty that decreases with increased temperature, and is an attractive precursor to the oxide. Cerous sulfate [13454-94-9] can be made in a wide range of hydrated forms and has solubiUty behavior comparable to that of the acetate. Many double sulfates having alkaU metal and/or ammonium cations, and varying degrees of aqueous solubiUty are known. Cerium(III) phosphate [13454-71 -2] being equivalent to mona2ite, is very stable. 

Alum Astringent crystalline double sulfate of an alkali. K2SO4AL2 (S04)j 24H2O. Used in the processing of pickles and as a flocking agent. Excess aluminum in the environment can be hazardous. 

Sulfates and nitrates are known and in all cases they decompose to the oxides on heating. Double sulfates of the type M (S04)3.3Na2S04.12H20 can be prepared, and La (unlike Sc and Y) forms a double nitrate, La(N03)3,2NH4N03.4H20, which is of the type once used extensively in fractional crystallization procedures for separating individual lanthanides. 

The possibility of oxidation to Fe is a crucial theme in the chemistry of Fe and most of its salts are unstable with respect to aerial oxidation, though double sulfates are much less so (e.g. Mohr s salt above). However, the susceptibility of Fe to oxidation is dependent on the nature of the ligands attached to it and, in aqueous solution, on the pH. Thus the solid hydroxide and alkaline solutions are very readily oxidized whereas acid solutions are much more stable (see Panel opposite). 

Tutton salts are the double sulfates M2Cu(S04)2.6H20 which all contain [Cu(H20)e] + and belong to the more general class of double sulfates of M and M cations which are known as schbnites after the naturally occurring K /Mg compound. 

The classical methods used to separate the lanthanides from aqueous solutions depended on (i) differences in basicity, the less-basic hydroxides of the heavy lanthanides precipitating before those of the lighter ones on gradual addition of alkali (ii) differences in solubility of salts such as oxalates, double sulfates, and double nitrates and (iii) conversion, if possible, to an oxidation state other than -1-3, e g. Ce(IV), Eu(II). This latter process provided the cleanest method but was only occasionally applicable. Methods (i) and (ii) required much repetition to be effective, and fractional recrystallizations were sometimes repeated thousands of times. (In 1911 the American C. James performed 15 000 recrystallizations in order to obtain pure thulium bromate). 

However, solubility, depending as it does on the rather small difference between solvation energy and lattice energy (both large quantities which themselves increase as cation size decreases) and on entropy effects, cannot be simply related to cation radius. No consistent trends are apparent in aqueous, or for that matter nonaqueous, solutions but an empirical distinction can often be made between the lighter cerium lanthanides and the heavier yttrium lanthanides. Thus oxalates, double sulfates and double nitrates of the former are rather less soluble and basic nitrates more soluble than those of the latter. The differences are by no means sharp, but classical separation procedures depended on them. 

Doppel-stuck, n. duplicate, -sulfat, n. double sulfate bisulfate, -sulfitt n. double sulfite bisulfite. 

Montemartini, C. Losana, L. (1929). Equilibria between double sulfates and aqueous solutions of sulfuric acids of various concentrations. Industria Chimica (Rome), 4, 199-205. 

Rubidium also has the ability to form what are called double sulfates. 

Rubidium cobalt sulfate (Rb SO CoSO 6HjO) is an example of several double sulfates that rubidium has the ability to form. Rubidium cobalt sulfate is a combined rubidium-cobalt compound in the form of ruby-red crystals. Other rubidium sulfate crystal compounds and their colors are rubidium + copper = white rubidium + iron = dark green and rubidium + magnesium = colorless. 

Mercury(ll) sulfate hydrolyzes in water forming a basic sulfate HgS04 2Hg0. It forms double sulfates with alkali metal sulfates, such as K2S04-3HgS04-2H20. 

Nickel sulfate forms double salts with ammonium or alkali metal sulfates. For example, blue-green hydrated ammonium nickel sulfate, (NH4)2S04 NiS04 6H20, crystallizes from a mixed solution of nickel sulfate and ammonium sulfate. Such double sulfates are isomorphous to corresponding alkali metal or ammonium double sulfates of iron, cobalt, magnesium, zinc, and other bivalent metals. 

Potassium and sodium sulfates and their double sulfates with calcium and magnesium occur naturally in various salt lakes. Potassium sulfate also occurs in certain volcanic lava. Its double salt with magnesium occurs in nature, as the mineral langbeinite. 

The monazite sand is heated with sulfuric acid at about 120 to 170°C. An exothermic reaction ensues raising the temperature to above 200°C. Samarium and other rare earths are converted to their water-soluble sulfates. The residue is extracted with water and the solution is treated with sodium pyrophosphate to precipitate thorium. After removing thorium, the solution is treated with sodium sulfate to precipitate rare earths as their double sulfates, that is, rare earth sulfates-sodium sulfate. The double sulfates are heated with sodium hydroxide to convert them into rare earth hydroxides. The hydroxides are treated with hydrochloric or nitric acid to solubihze all rare earths except cerium. The insoluble cerium(IV) hydroxide is filtered. Lanthanum and other rare earths are then separated by fractional crystallization after converting them to double salts with ammonium or magnesium nitrate. The samarium—europium fraction is converted to acetates and reduced with sodium amalgam to low valence states. The reduced metals are extracted with dilute acid. As mentioned above, this fractional crystallization process is very tedious, time-consuming, and currently rare earths are separated by relatively easier methods based on ion exchange and solvent extraction. 

Arfwedson prepared lithium acetate, ignited it, and noted the insolubility of the resulting lithium carbonate in water and its action on platinum. He also prepared and studied the bicarbonate, sulfate, nitrate, chloride, tartrate, borate, hydroxide, and a double sulfate which he reported as lithium alum. He mentioned that lithium hydroxide is much less soluble than the other caustic alkalies and that it has a greater saturation capacity [lower equivalent weight] than they. Because of its ability to form deliquescent salts with nitric and hydrochloric acids, Arfwedson recognized the close relation between the new alkali and the alkaline earths, especially magnesia. 

In 1821 Arfwedson published a supplementary note to his lithium research (11), in which he stated that the salt which he had previously reported as lithium acid sulfate must be the normal sulfate and that the double sulfate he had at first taken for lithium alum was really potassium alum resulting from a trace of potassium in his alumina. 

Alum is the generic name given to an import ant group of double sulfates of the general formula M aS04. Ma (S04)s, 24HaO, which is sometimes written MlMJ(S04)3. 12H30, where... 

Chromium(II) double sulfates can be crystallized by the addition of ethanol to concentrated aqueous solutions containing equimolar quantities of the components. The hexahydrates A2S04>CrS04-6H20 (A = NH4, Rb or Cs) are high-spin and have reflectance spectra compatible with the presence of tetragonally distorted [Cr(H20)6]2+ ions (Table 24). 4 The potassium and sodium salts crystallize as pale blue dihydrates with similar properties, and from the splittings of the SO2- absorption bands in their IR spectra it seems that coordinated sulfate anions are present. 

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