Efflorescences

When the dissociation pressure of the hydrate is small, as with CuS04-5H20, the drying of the salt presents no difficulty. Even when there is some disposition toward efflorescence, the crystals may be dried on paper in the usual way if they are stirred frequently and watched closely. At the first sign of efflorescence they should be bottled up. In many cases, however, some crystals will begin to effloresce while others are still wet with mother liquor. In such an event it is best to proceed as follows  

Take a portion of the crystals (or some of the material in crude form) and allow it to effloresce freely in a warm place. Put it in a roomy desiccator in place of the usual drying agent (calcium chloride). Place the wet crystals in a suitable container and support them over the effloresced material in the desiccator (Fig. 2). There are 

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Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

Deliquescence and efflorescence. A substance is said to deliquesce (Latin to become liquid) when it forms a solution or liquid phase upon standing in the air. The essential condition is that the vapour pressure of the saturated solution of the highest hydrate at the ordinary temperature should be less than the partial pressure of the aqueous vapour in the atmosphere. Water will be absorbed by the substance, which gradually liquefies to a saturated solution water vapour will continue to be absorbed by the latter until an unsaturated solution, having the same vapour pressure as the partial pressure of water vapour in the air, is formed. In order that the vapour pressure of the saturated solution may be sufficiently low, the substance must be extremely soluble in water, and it is only such substances (e.g., calcium chloride, zinc chloride and potassium hydroxide) that deliquesce. 

It must be emphasised that deliquescence and efflorescence are relative properties, since they depend upon the actual presence of water vapour in the atmosphere, which varies considerably with place and time. 

Tetrasodium hexakiscyanoferrate decahydrate [14434-22-1], Na4[Fe(CN)g] IOH2O, or yellow pmssiate of soda, forms yellow monoclinic crystals that are soluble in water but insoluble in alcohol. It is slightly efflorescent at room temperature, but the anhydrous material, tetrasodium hexakiscyanoferrate [13601 -19-9], Na4[Fe(CN)J, is obtained at 100°C. The decahydrate is produced from calcium cyanide, iron(II) sulfate, and sodium carbonate in a process similar to that for the production of K4[Fe(CN)g] 3H2O. It is used in the manufacture of trisodium hexakiscyanoferrate, black and blue dyes, as a metal surface coating, and in photographic processing. 

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 ... 

Zirconium oxychloride is an important intermediate from which other zirconium chemicals are produced. It readily effloresces, and hydrates with 5—7 H2O are common. The salt caimot be dried to the anhydrous form, and decomposes to hydrogen chloride and zirconium oxide. 

Barium carbonate prevents formation of scum and efflorescence in brick, tile, masonry cement, terra cotta, and sewer pipe by insolubilizing the soluble sulfates contained in many of the otherwise unsuitable clays. At the same time, it aids other deflocculants by precipitating calcium and magnesium as the carbonates. This reaction is relatively slow and normally requites several days to mature even when very fine powder is used. Consequentiy, often a barium carbonate emulsion in water is prepared with carbonic acid to further increase the solubiUty and speed the reaction. 

Cadmium Fluoride. Elemental fluorine reacts with cadmium metal as well as the oxide, sulfide, and chloride to give CdF2 [7790-79-6]. Alternatively, treatment of CdCO with 40% HF yields a solution of CdF2, which may be evaporated to recover efflorescent crystals of the dihydrate. CdF2 has been used in phosphors, glass manufacture, nuclear reactor controls, and electric bmshes and in 1991 sold as a pure electronic grade (99.99%) at l/g. 

Sulfonamidates. Chloramine-T, A/-chloro-A/-sodiomethylbenzenesulfonamidate ttihydrate [127-65-1], CH3C3H4S02NClNa-3H20, a white to slightly yellow soHd, effloresces in air losing chlorine, becoming less soluble in water (71). It has a mp of 175°C, an av CI2 of 25%, and is moderately soluble... 

Cobalt(Il) sulfate heptahydrate [10026-24-17, CoSO is a reddish pink monoclinic crystalline material that effloresces in dry air to form the... 

Gopper(II) Sulfates. Copper(II) sulfate pentahydrate [7758-99-8] CuS04-5H20, occurs in nature as the blue triclinic crystalline mineral chalcanthite [13817-21 -5]. It is the most common commercial compound of copper. The pentahydrate slowly effloresces in low humidity or above 30.6°C. Above 88°C dehydration occurs rapidly. 

Cupric chloride [7447-39-4] M 134.4, m 498 , 630 (dec). Crystd from hot dilute aq HCl (0.6mL/g) by cooling in a CaCl2-ice bath. Dehydrated by heating on a steam-bath under vacuum. It is deliquescent in moist air but efflorescent in dry air. 

Potassium tetracyanopalladate (II) 3H2O [10025-98-6] M 377.4. All operations should be carried out in an efficient fume cupboard - Cyanide is very POISONOUS Dissolve the complex (ca 5g) in a solution of KCN (4g) in H2O (75mL) with warming and stirring and evaporate hot till crystals appear. Cool, filter off the crystals and wash with a few drops of cold H2O. Further concentration of the mother liquors provides more crystals. The complex is recrystallised from H2O as the colourless trihydrate. It effloresces in... 

Visible exudation or efflorescence on the surface of a plastic caused by plasticizer, lubricant, etc. Spacer or filler material in a mold. 

In recent years this simple picture has been completely transformed and it is now recognized that the alkali metals have a rich and extremely varied coordination chemistry which frequently transcends even that of the transition metals. The efflorescence is due to several factors such as the emerging molecular chemistry of lithium in particular, the imaginative use of bulky ligands, the burgeoning numbers of metal amides, alkoxides, enolates and organometallic compounds, and the exploitation of multidentate... 

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