Double salt

Many of the minerals in the Earth s crust are not simple ionic compounds consisting of one type of cation such as Cu and one type of anion such as carbonate, COs . These more complex compounds can be classified as double (or triple or quadruple) salts. (These salts are also referred to as ternary or quaternary.) That is, they contain more than one type of cation and/or more than one type of anion. For example, one of the conunon ores of copper is malachite, which has the formula Cu2C03(0H)2. This is an ionic compound that contains the Cu ion and both the carbonate and hydroxide anions. Double salts can be thought of as combinations (but not mixtmes) of two or more simple salts. Malachite can be visualized as CuCOs -1- Cu(OH)2. Malachite is frequently found with azurite, which has the formula Cu3(C03)2(0H)2. Azurite simply has a different ratio of the two simple salts 2 CuCOs + Cu(OH)2. 

Another copper mineral, brochantite, has the formula Cu4804(0H)6. Into what simple salts can you mentally dissect this compound  

Brochantite is particularly interesting from the standpoint of its solubility. Copper sulfate is certainly soluble in water, but copper hydroxide is not. Of course, brochantite is not a mixture of CU8O4 and Cu(OH)2 the Cu, 864, and OH have very different positions in the structure of brochantite than they do in the individual simple salts. Nevertheless, it is useful to predict the water solubility of double salts from the solubilities of the simple salt components. A good general rule is that the double salt will have a solubility similar 

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Vanadium IV) forms blue VOSO4, (0, 3 and 5H2O), vanadyl sulphate, which forms ranges of double salts, Prepared by SO, reduction of V2O5 in H2SO4. 

Vanadium II) forms green V2(S04)3 (electrolytic or Mg reduction of V2O5 in H2SO4). Alums and other double salts are formed. 

Vanadium II) forms violet VS04,7H20 (electrolytic or Na/Hg reduction). Forms double salts. 

These are double salts which have the general formula... 

Double salts of general formula M2SO4 -FeS04.6H2O (M = alkali metal or ammonium) can be obtained by crystallisation of solutions containing the appropriate proportions of the two simple salts ... 

All the early literature concerning thiazoles mentions numerous metallic complex-salts formed by addition to the thiazole of the aqueous solution of the metal salt and that could be used for identification purposes. The most usual complexes so obtained are platinum double salts, for example, (4-methylthiazole HC1)2 PtCU (m.p. deep 204°C) (25), or mercuric chloride derivatives, for example, 2,4-dimethyl-thiazole 2 HgCl (m.p. deep 176-177°C) (458). 

Potassium manganate(VI), manganate(VII) double salt [123d2-73-l]... 

Anhydrous aluminum triduotide, A1F., is a white crystalline soHd. Physical properties are Hsted ia Table 2. Aluminum duotide is spatingly soluble ia water (0.4%) and iasoluble ia dilute mineral acids as well as organic acids at ambient temperatures, but when heated with concentrated sulfuric acid, HF is hberated, and with strong alkah solutions, aluminates are formed. A1F. is slowly attacked by fused alkahes with the formation of soluble metal duotides and aluminate. A series of double salts with the duotides of many metals and with ammonium ion can be made by precipitation or by soHd-state reactions. 

Magnesium sulfate heptahydrate may be prepared by neutralization of sulfuric acid with magnesium carbonate or oxide, or it can be obtained directly from natural sources. It occurs abundantly as a double salt and can also be obtained from the magnesium salts that occur in brines used for the extraction of bromine (qv). The brine is treated with calcium hydroxide to precipitate magnesium hydroxide. Sulfur dioxide and air are passed through the suspension to yield magnesium sulfate (see Chemicals frombrine). Magnesium sulfate is a saline cathartic. 

Halides. Indium trichloride [10025-83-8] InCl, can be made by heating indium in excess chlorine or by chlorinating lower chlorides. It is a white crystalline soHd, deUquescent, soluble in water, and has a high vapor pressure. InCl forms chloroindates, double salts with chlorides of alkaLi metals, and organic bases. 

Mono-, di-, and trivalent bromides and iodides may be made by methods similar to the chlorides. The lower valence salts also disproportionate in water. Indium trifluoride [7783-52-0] InF., is sparingly soluble in water. It forms an ammonium double salt, SNH F TnF. [15273-84-4] which decomposes on heating to indium nitride [25617-98-5] InN. 

Sulfates. Indium metal and its oxides dissolve in warm sulfuric acid to give a solution of the trisulfate [13464-82-9], In2(S0 2- It is a white, crystalline, deUquescent soHd, readily soluble in water that forms double salts with alkaLi sulfates and some organic substituted ammonium bases. Concentration of the acidified trisulfate solution produces indium acid sulfate crystal [57344-73-7], In(HS0 2> other reaction conditions give basic sulfates. 

Iron(II) sulfate forms double salts of formula M2SO4 FeSO 6H20 with aLkaU sulfates. Iron(II) ammonium sulfate [7783-85-9] (Mohr s salt),... 

The lanthanides readily form double salts, eg, Lri2(SO NajS04 2H2O Lu2(SO MgSO 24H20, which were used historically in the fractionation process. 

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. 

Fra.ctiona.1 Crystallization. Fractional crystallization, used until the early part of the twentieth century, is uneconomical for processing large quantities of lanthanides. Many recrystallization steps are required to recover high purity products. Several salts and double salts have been used ... 

Normal, hydrated, and the basic magnesium carbonates react with acids to yield salts which can be recovered by crystallization (43). Magnesium carbonate forms many double salts, some of which are Hsted ia Table 5. 

Magnesium sulfate [7487-88-9] MgSO, is found widely in nature as either a double salt or as a hydrate. The more important mineral forms are Hsted in Table 23. 

Mercury salts exist ia two oxidation states mercurous, Hg", and mercuric, The former exist as double salts for example, mercurous... 

Nickel Double Salts. Nickel ammonium chloride [16122-03-5], NiCl2 NH Cl -6H20, nickel ammonium sulfate [15699-18-0],... 

Group 13 (IIIA) Perchlorates. Perchlorate compounds of boron and aluminum are known. Boron perchlorates occur as double salts with alkah metal perchlorates, eg, cesium boron tetraperchlorate [33152-95-3] Cs(B(C104)4) (51). Aluminum perchlorate [14452-95-3] A1(C104)2, forms a series of hydrates having 3, 6, 9, or 15 moles of water per mole of compound. The anhydrous salt is prepared from the trihydrate by drying under reduced pressure at 145—155°C over P2O5 (32). 

Uses. The principal use of monosodium phosphate is as a water-soluble soHd acid and pH buffer, primarily in acid-type cleaners. The double salt, NaH2P04 H PO, referred to as hemisodium orthophosphate or sodium hemiphosphate, is often generated in situ from monosodium phosphate and phosphoric acid in these types of formulations. Mixtures of mono- and disodium phosphates are used in textile processing, food manufacture, and other industries to control pH at 4—9. Monosodium phosphate is also used in boiler-water treatment, as a precipitant for polyvalent metal ions, and as an animal-feed supplement. 

Potassium Phosphates. The K2O—P20 —H2O system parallels the sodium system in many respects. In addition to the three simple phosphate salts obtained by successive replacement of the protons of phosphoric acid by potassium ions, the system contains a number of crystalline hydrates and double salts (Table 7). Monopotassium phosphate (MKP), known only as the anhydrous salt, is the least soluble of the potassium orthophosphates. Monopotassium phosphate has been studied extensively owing to its piezoelectric and ferroelectric properties (see Ferroelectrics). At ordinary temperatures, KH2PO4 is so far above its Curie point as to give piezoelectric effects in which the emf is proportional to the distorting force. There is virtually no hysteresis. 

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