Crystallization by evaporation

Lead nitrate [10099-74-8] Pb(N02)2, mol wt 331.23, sp gr 4.53, forms cubic or monoclinic colorless crystals. Above 205°C, oxygen and nitrogen dioxide are driven off, and basic lead nitrates are formed. Above 470°C, lead nitrate is decomposed to lead monoxide and Pb O. Lead nitrate is highly soluble in water (56.5 g/100 mL at 20°C 127 g/100 mL at 100°C), soluble in alkalies and ammonia, and fairly soluble in alcohol (8.77 g/100 mL of 43% aqueous ethanol at 22°C). Lead nitrate is readily obtained by dissolving metallic lead, lead monoxide, or lead carbonate in dilute nitric acid. Excess acid prevents the formation of basic nitrates, and the desired lead nitrate can be crystallized by evaporation. 

Potassium chloride is crystallized from sea bitterns containing chlorides of potassium, sodium and calcium by ammoniation (Jagadesh etai, 1992). This process is less energy intensive and more efficient than by fractional crystallization by evaporation, as the ammonia used is recovered by distillation. Crystallization produces a better quality product in terms of both size and purity than by other methods. 

TII3 is an intriguing compound which is isomorphous with NH4I3 and Csly (p. 836) it therefore contains the linear I3 ion and is a compound of Tl rather than Tl . It is obtained as black crystals by evaporating an equimolar solution of Til and I2 in concentrated aqueous HI. The formulation Tl (l3 ) rather than Tl (I )y is consistent with the standard reduction potentials °(T1"VT1 )1.26 V and °(il2/I )-(-0.54 V,... 

Barium acetate [Ba(CjHj02)2 H20], a white crystal, is used as a dryer for paints and varnishes. It is produced by adding acetic acid to barium sulfate and recovering the crystals by evaporation. It is also used as a textile mordant and catalyst. 

Crystallization by Evaporation. This method is employed when the substance is so easily soluble in all solvents (hot or cold), that it will only crystallize after, partial or complete evaporation. If complete evaporation must be employed, impurities will remain. So, if possible, filter off the mother liquor (solvent), as this is where the dissolved impurities will be. If you should need to heat the product with an effective solvent until thoroughly dissolved, pour through filter paper to remove solid impurities. 

Little attention was paid to these reactions until 1921 when Hoffman 174-176) prepared tetrakis(hydroxymethyl)phosphonium chloride by passing phosphine into a warm, aqueous hydrochloric acid solution of formaldehyde. The product was obtained in the form of colourless crystals by evaporation of the reaction solution. 

Magnesium chloride is prepared by treating magnesium carbonate, hydroxide or oxide with hydrochloric acid followed by crystallization by evaporation. The hexahydrate of the salt MgCb 6H2O is obtained upon crystallization. 

Prepare 10 g. of potassium metavanadate, KVO 3, either by heating a solution of vanadium pentoxide, V2O5, with the necessary amount of a solution of potassium hydroxide or by boiling the necessary amount of ammonium metavanadate (difficultly soluble) with the required quantity of potassium hydroxide. In either case, obtain the white, needle-shaped crystals by evaporation. 

The dihydrate, As205.2H20, or pyroarsenic acid, H4As207, was described by Kopp10 as a hard mass formed when aqueous arsenic acid was evaporated at a temperature between 140° and 180° C., but the compound could not be obtained by Auger11 and other workers.12 Rosenheim and Antelmann,13 however, maintain that pyroarsenic acid does exist and is obtained in the form of hard microscopic prismatic crystals by evaporating a concentrated aqueous solution of pure arsenic... 

The name, pentaerythrite, indicates that the substance contains five carbon atoms and (like erythrite) four hydroxyl groups. In commercial practice108 the reaction is carried out at 65-70°. After 2 hours at this temperature, the calcium is precipitated by means of sulfuric acid, the mixture is filtered, and the filtrate is concentrated and crystallized by evaporation in vacuum. Penta-... 

The name potash derives from an early production method in which potassium carbonate, leached from wood ashes, was crystallized by evaporating the leachate in large iron pots. The salt potassium chloride (muriate of potash or KC1) is now the major source of the element (95%) other important salts are potassium sulfate (sulfate of potash, K2S04), potassium magnesium sulfates of varying K/Mg ratios, and potassium nitrate (KN03). 

Hydrated Cobaltous Fluoride, CoF2.2H20, is easily prepared as rose-red crystals by evaporation of a solution of the oxide or carbonate in aqueous hydrogen fluoride. Boiling water decomposes the salt., yielding an insoluble oxy-fluoride of light red colour. 

The tetrahydrate, Co(I03)2.4H20, is obtained as red crystals by evaporating a 5 per cent, solution containing equivalent proportions of cobalt nitrate and iodic acid at 0° to 20° C. When agitated for a prolonged period with water at 30° C. the tetrahydrate yields the dihydrate. 

The synthesis is normally carried out by refluxing an aqueous mixture of polyaminopolycarboxylic acid and a selected rare earth oxide in a molar ratio of RE L = 1 1. The resulting solution can be neutralized to pH 5-6 by adding a dilute solution of NaOH, KOH, NH3 H2O, NaHCOs, or NaHC03. After filtration, the complex can be crystallized by evaporation or with the addition of EtOH. The rare earth oxide can be replaced by a rare earth carbonate or other soluble salts, such as chlorides, nitrates, perchlorates, or acetates [104], The synthesis can also be done under hydrothermal conditions [105], which leads to complexes of polymeric structure, while most of the complexes obtained under ambient conditions are monomeric (see Section 3.3.2 for examples). 

Ferric chloride, FeClg fiHoO, is obtained as yellow deliquescent crystals by evaporation of a solution made by oxidation of ferrous chloride with chlorine. Solutions of ferric ion containing chloride ion are more intensely colored, yellow or brown, than nitrate or sulfate solutions because of the formation of ferric chloride < omplexes. Anhydrous ferric chloride, FCoClg, can be made by passing chlorine over heated iron. 

Double selenates, of composition R 2Se04.U02Se04.2H20 (R =K, NH4), have been obtained in crusts of yellow crystals by evaporation of solutions of the alkali uranate in selenic acid. 

Rapid or shock cooling and high agitation favor the formation of thin, small crystals and should be avoided. Slow crystallization by evaporation yields compact crystals. 

Figure 8-1 Concentration profiles for crystallization by evaporation as a function of time of distillation or amount of solvent removed. A-B-C-E is the preferred pathway for favoring growth.

Figure 8-2 Concentration profiles for crystallization by evaporation when the solubility of the substrate is increasing because of increasing temperature and/or impurity concentration. The solubility profile is no longer curve H-I but becomes H-E.

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