Crystallisation hydrates

Among the numerous commercial derivatives of ethylenediaminetetracetic acid (EDTA), a mixed salt of Zn, Na containing one atom of zinc and two atoms of sodium per molecule of EDTA is found. This mixed salt presents itself in the form of a crystallised hydrate. An attempt is described to deduce the number of water molecules of this hydrate by measuring the presence of zinc by atomic absorption. The experimental approach was as follows ... 

The heating of the solid salt again leads to melting and dissolution in the water of crystallisation hydrated Mg ions and H3O ions are formed just as in the aqueous solution (eqn. 2), so that the red coloration typical of methyl red at pH<6 is observed. In the crystal lattice the indicator does not change from its normal yellow color. 

For this second reaction Kjgs = 181 x 10" and hence pK, for ammonia solution is 4.75. The entity NHj. H2O is often referred to as ammonium hydroxide, NH4OH, a formula which would imply that either nitrogen has a covalency of five, an impossible arrangement, or that NH4OH existed as the ions NH4 and OH". It is possible to crystallise two hydrates from concentrated ammonia solution but neither of these hydrates is ionic. Hence use of the term ammonium hydroxide is to be discouraged in favour of ammonia solution . 

The base strength of hydrazine is, however, lower than that of ammonia. As might be expected, hydrazine is readily soluble in water from which the hydrate N2H4.H2O can be crystallised. 

A somewhat similar reaction is the power of sulphur oxide dichloride to remove water of crystallisation from hydrated chlorides, the hydroxyl groups of the water molecule reacting as do those in the acid molecules in the above reaction. 

Sodium sulphate crystallises out in hydrated form (common ion effect) and is filtered off on concentration, sodium dichromate is obtained. For analytical purposes, the potassium salt. K2Cr20-. is preferred potassium chloride is added and the less soluble potassium dichromate obtained. 

Hydrated cobalt III) sulphate, Co2(S04)3. JSHjO is obtained when cobalt(II) sulphate is oxidised electrolytically in moderately concentrated sulphuric acid solution it is stable when dry but liberates oxygen from water. Some alums, for example KCo(S04)2.12H,0 can be obtained by crystallisation from sulphuric acid solutions. In these and the sulphate, the cation [CofHjO) ] may exist it is both acidic and strongly oxidising. 

Nickel forms yellow anhydrous halides NiXjlX = F. Cl. Br) and a black iodide Nil2 all these halides are made by direct combination of the elements, and the chloride by reaction of sulphur dichloride oxide with the hydrated salt. All dissolve in water to give green solutions from which the hydrates can be crystallised the solutions contain the ion [NifHjOls], and the chloride crystallises as NiCl2.6H2O, nickel(II) chloride hexahydrate. 

The anhydrous chloride is prepared by standard methods. It is readily soluble in water to give a blue-green solution from which the blue hydrated salt CuClj. 2H2O can be crystallised here, two water molecules replace two of the planar chlorine ligands in the structure given above. Addition of dilute hydrochloric acid to copper(II) hydroxide or carbonate also gives a blue-green solution of the chloride CuClj but addition of concentrated hydrochloric acid (or any source of chloride ion) produces a yellow solution due to formation of chloro-copper(ll) complexes (see below). 

Hg -F 8H-" -F 2N03" 3Hgi+ -F 2NO -F 4H2O From the acid solution white hydrated mercury(I) nitrate Hg2(N03)2.2H20 can be crystallised out this contains the ion... 

The salt is extremely soluble ia water (Table 4), crystallising from aqueous solution as the hydrates LiBr H20 [23303-71-17, LiBr 2H20 [13453-70-8] and LiBr 3H2O [76082-04-7]. The anhydrous salt is obtained by dryiag under vacuum at elevated temperatures. 

Selected physical properties of sodium thiosulfate pentahydrate are shown in Table 1. The crystals are relatively stable, efflorescing in warm, dry air and dehquescing slightly in moist air. They melt in their water of hydration at 48°C and can be completely dehydrated in a vacuum oven at this temperature, or at atmospheric pressure at 105°C. Anhydrous sodium thiosulfate can also be crystallised direcdy from a 72% solution above 75°C. It decomposes at 233°C ... 

Titanium Trichloride Hexahydrate. Titanium trichloride hexahydrate [19114-57-9] can be prepared by dissolving anhydrous titanium trichloride ia water or by reduciag a solutioa of titanium tetrachloride. Evaporation and crystallisation of the solution yield violet crystals of the hexahydrate. The hydrated salt has had some commercial appHcation as a stripping or bleaching agent ia the dyeiag iadustry, particularly where chlorine must be avoided. 

Calcium siHcate hydrate is not only variable ia composition, but is very poody crystallised, and is generally referred to as calcium siHcate hydrate gel or tobermorite gel because of the coUoidal sizes (<0.1 fiva) of the gel particles. The calcium siHcate hydrates ate layer minerals having many similarities to the limited swelling clay minerals found ia nature. The layers are bonded together by excess lime and iatedayer water to form iadividual gel particles only 2—3 layers thick. Surface forces, and excess lime on the particle surfaces, tend to bond these particles together iato aggregations or stacks of the iadividual particles to form the porous gel stmcture. 

Naphthionic acid (4-aminonaphthalene-l-sulfonic acid) [84-86-6] M 223.3, m > 300°(dec), pK 2.68. It crystallises from H2O as needles of the 0.5 hydrate. Salt solns fluoresce strongly blue. 

Lead (II) bromide [10031-22-8] M 367.0, m 373°. Crystallised from water containing a few drops of HBr (25mL of water per gram PbBr2) between 100° and 0°. A neutral solution was evaporated at 110° and the crystals that separated were collected by rapid filtration at 70°, and dried at 105° (to give the monohydrate). To prepare the anhydrous bromide, the hydrate is heated for several hours at 170° and then in a Pt boat at 200° in a stream of HBr and H2. Finally fused [Clayton et al. J Chem Soc, Faraday Trans 1 76 2362 1980]. 

Sodium dimethyidithiocarbamate hydrate [128-04-1] M 143.2, m 106-108", 120-122", pK 3.36 (diethyldithiocarbamic acid). Crystallise from a small volume of H2O, or dissolve in minimum volume of H2O and add cold Me2CO and dry in air. The solution in Me2CO is -50g/400mL. The dihydrate loses H2O on heating at 115° to give the hemi hydrate which decomposes on further heating [IR Can J Chem. 34 1096 7956]. 

Adenosine-3 -monophosphoric acid hydrate [3 -adenylic acid, 3 -AMP] [84-21-9] M 347.3, m 197°(dec, as 2H2O), 210°(dec), m 210°(dec), [a]s46 -50° (c 0.5, 0.5M Na2HP04), pK 3.65, pKz 6.05. It crystallises from large volumes of H2O in needles as the monohydrate, but is not very soluble in boiling H2O. Under acidic conditions it forms an equilibrium mixture of 2 and 3 adenylic acids via the 2, 3 -cyclic phosphate. When heated with 20% HCl it gives a quantitative yield of furfural after 3hours, unlike 5 -adenylic acid which only gives traces of furfural. The yellow monoacridine salt has m 175°(dec) and... 

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