Double salts, crystallisation from solution

Several tetrammino-derivatives of gold salts have been prepared. When dilute ehloraurie acid saturated with ammonium nitrate is added to a cold saturated solution of ammonium nitrate and the mixture treated with ammonia gas at ordinary temperature, a precipitate of fetrammino-auric nitrate, [Au(NH3)4](N03)3, is obtained. Tctrammino-aurie nitrate is soluble in water and may be crystallised from warm water. It may be precipitated from solution by the addition of any soluble nitrate, but with potassium, sodium, or ammonium nitrate it forms double salts. Thus, potassium nitrate if added to a concentrated solution of tetrammino-auric nitrate forms the compound [Au (NH 3).,] (NO 3)j.KNO. j, which crystallises from solution in needles. 

Sodium biphosphate was known as a dimorphous salt .. . of the 4 atoms of water which the crystals contain, they lose, I find, 2 atoms at the temperature of 212° (F.), and not a particle more till heated up to about 875°. After heating to 212° it contains 3 atoms base, namely, one atom soda and 2 atoms water united to a double atom of phosphoric acid. The salt cannot sustain the loss of any portion of this water without assuming a new train of properties. Several other forms were obtained by heating to higher temperatures, and at a low red heat a glass was obtained which was deliquescent, not crystallisable from solution, and which gave the reactions of phosphoric acid ignited per se. In modern symbols—... 

Ferrous potassium carbonate, FeK2(C03)2.4H20, may be prepared by mixing, out of contact with air, a concentrated solution of ferrous chloride with excess of potassium carbonate. At first a white precipitate of ferrous carbonate is formed, which soon dissolves to a greenish solution from which the double salt crystallises in nearly white scales.4... 

The amine (Imol) is added to a solution of anhydrous zinc chloride (Imol) in concentrated hydrochloric acid (42mL) in ethanol (200mL, or less depending on the solubility of the double salt). The solution is stirred for Ih and the precipitated salt is filtered off and recrystallised from ethanol. The free base is recovered by adding excess of 5-ION NaOH (to dissolve the zinc hydroxide that separates) and is steam distilled. Mercuric chloride in hot water can be used instead of zinc chloride and the salt is crystallised from 1% hydrochloric acid. Other double salts have been used, e.g. cuprous salts, but are not as convenient as the above salts. 

Di-iodo-diethylenediamino -chromic Salts are derived from di-iodo-diethylenediamino-ehromiciodide mercuric iodide, [Cr en2I2]I.HgI2, which is prepared by decomposing bromo-bisaquo-salts with hydriodic acid and mercuric iodide. It crystallises in very green leaflets, and is almost insoluble in water. The double salt is decomposed by hydrogen sulphide. The iodide is unstable, and in solution very readily decomposes.3... 

Ammonium Tetranitrito - diammino - cobaltate, [Co(XH3)2 (N02)4]NH4, is prepared from cobaltous chloride by mixing an aqueous solution of the salt with aqueous ammonium-chloride solution, sodium nitrite, and ammonia, and oxidising the mixture by passing air through it for several hours. The solution is allowed to stand in air for some days, when crystals gradually separate. These are collected, washed, and reerystallised from water.3 The substance crystallises in brown rhombic prisms. It is decomposed on treatment with potassium hydroxide with evolution of ammonia, and a cold solution reacts with ammonia in presence of ammonium salts, with formation of flavo-dinitro double salts, of which [Co(NH3)4(N02)2][Co(NH3)2(N02)4] is typical. Oxalic acid transforms it into the oxalato-dinitrito-diammino-salt, [Co(NH3)2(N02)2(C204)]NH4. 

Chloro - pentammino - iridium Chloride, [Ir(NH3)sCl]Cl2, is formed by the action of ammonia on iridium trichloride, iridium tetrachloride, or the ehloro-double salts. It may also be prepared from chloro-pentammino-iridium sulphate by treating it with barium chloride. Prepared by the first method it separates in wine-coloured crystals, whilst by the second method it is yellow. The red colour of the first product is due to a small quantity of iridium trichloride, which separates with the chloro-ehloride and may be removed by heating the hot aqueous solution with hydrogen sulphide. It crystallises in 1 Jorgensen, J. prakt. Ghem., 1888, 34, 394 Palmaer, Ber., 1891, 24, 2090. 

Nickel Hypophosphite, NiH4(P02). 6H20, may be obtained by double decomposition of nickel sulphate and barium hypophosphite solutions, or by dissolving nickel hydroxide in hypophosphorous acid. From the solution thus obtained the hexahydrated salt crystallises out in green regular octahedra. 

Potassium chlor-rhodite is decomposed by water into potassium chloride and the double salt RhCl3.2KCl, which is anhydrous and crystallises readily from aqueous solution. Owing to this ready decomposition, potassium chlor-rhodite cannot be prepared by simply mixing potassium and rhodium chlorides in solution. 

Trisodium dithiophosphate, Na3(PS202).llH20, has been prepared from P2S6 and a rather concentrated solution of sodium hydroxide. The solution was heated to 50° to 55° C. until the trithio-salt was decomposed. The salt was precipitated by alcohol, and when re-crystallised from water appeared as colourless six-sided prisms.5 The ammonium salt, (NH4)3(PS202).2H2O, was prepared similarly from aqueous ammonia and P2S5. From these soluble thiophosphates those of the heavy metals may be obtained by double decomposition.5... 

The double salt, 2K2SO4.UO2SO4, is obtained by dissolving uranyl sulphate and excess of potassium sulphate in hot water, and crystallising from the hot solution. At temperatures below 80° C. the salt is decomjDosed by water thus ... 

TJranyl nitrate forms double salts of the type E U02(N03)3, where R =K, NH4, Rb, Cs, or Tl. They may be prepared by crystallising a solution of the mixed nitrates in concentrated nitric acid or by crystallisation from a solution of the alkali uranate in excess of nitric acid. The crystals are all anhydrous, and in the case of the potassium salt are orthorhombic, whilst the rubidium and caesium salts are rhombo-hedral and isomorphous with each other. The ammonium salt yields both orthorhombic and rhombohedral crystals. All exhibit a yellowish-green fluorescence they are liygroscopic and readily decomposed by water into their components. The thallium salt is particularly unstable, being decomposed in moist air. On the other hand, the rubidium salt dissolves in water at 80° C. without decomposition. The corresponding salts of sodium, lithium, or of divalent metals have not been prepared. 

Crystallisation of Double Salt from Solutions containing... 

Although carnallite is decomposed by pure water, it will be possible to crystallise it from a solution having a composition represented by any point in the carnallite area. Since during the separation of the double salt the relative amount of magnesium chloride increases, it is most advantageous to commence with a solution the composition of which is represented by a point lying just above the curve EM [cf, P- 247). 

The System Sodium Chloride-Potassium Chloride— Water.— This may be taken as an illustration of a simple system in which there is no double-salt formation, and in which, at all temperatures above 0 , the salts crystallise anhydrous from solution. 

IS best crystallised from alcohol it melts at 84° C. It is very sparingly soluble in hot water. Its alcohol solution with an excess of sodium hydroxide yields the sodium salt as a thick, white precipitate. The potassium, ammonium, calcium, lead and barium salts are know n, the last-named containing three molecules of water. The double salt with phenylhydrazine is sparingly soluble m water and melts at 171° C. 

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