Calcium efflorescence

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. 

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. 

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. 

Salpeter-sauresalz. n. salt of nitric acid, nitrate, -schaum, m. wall saltpeter (calcium nitrate efflorescence). 

Efflorescence. The solvent properties of water also causes efflorescence, a phenomenon whereby soluble or slightly soluble substances migrate from the interior of porous solids to the surface, where they precipitate. Efflorescence is an important factor in the decay and disintegration of many rocks, and of human-made porous materials such as ceramics, and even of some types of glass. On archaeological objects, efflorescence generally occurs mostly as a white, powdery, but sometimes consolidated accretion on the surface of the objects. Calcite, a form of calcium carbonate, is one of the most common substances to effloresce on archaeological ceramics. 

C. W. Scheele (1773) noted that a damp mixture of common salt and calcium hydroxide effloresces after some weeks exposure to air, the efflorescent salt is sodium carbonate. It is probable that calcium bicarbonate is formed, which precipitates soluble sodium bicarbonate, and this explains the formation of the efflorescence of soda from salty soils. The observation made by C. W. Scheele... 

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... 

The normal selenates of potassium, sodium, calcium and magnesium are very similar in properties to the corresponding sulphates, except that the potassium salt is much more soluble in water than potassium sulphate.3 Sodium selenate gives a decahydrate which effloresces, and the transition-point between this and the anhydrous salt is 31-8° C., above which point the solubility falls as the temperature rises. Calcium selenate gives a hemihydrate resembling plaster of Paris, and also a hydrate of composition CaSe04.l-5H2O.4... 

Substances that are ordinarily deliquescent are sulfuric add (concentrated), glycerol, calcium chloride crystals, sodium hydroxide (solid), and 100% ethyl alcohol. In an enclosed space, these substances deplete the water vapor present to a definite degree. Other substances are used to accomplish this end by chemical reaction, e.g.. phosphorus pentoxide (forming phosphoric acid), and boron trioxide (forming boric acid). Water is absorbed from nonmiscible liquids by addition of such substances as anhydrous sodium sulfate, potassium carbonate, anhydrous calcium chloride. and solid sodium hydroxide. The converse phenomenon is known as efflorescence. 

With some substances, not necessarily salt hydrates, the reverse of efflorescence occurs. For example, if anhydrous calcium chloride is left in the air, it absorbs water vapour and eventually forms a very concentrated solution. This process is called deliquescence, and substances which behave like this are said to be deliquescent. Solid sodium hydroxide will deliquesce. 

Calcium Lactate occurs as a white to cream-colored, crystalline powder or granules. It contains up to five molecules of water of crystallization. The pentahydrate is somewhat efflorescent and at 120° becomes anhydrous. It is soluble in water and practically insoluble in alcohol. 

It is believed that the water in the crystals called efflorescent tends to pass off into the atmosphere. This tendency resembles the tendency of water to evaporate. And like water, this water of crystallization passes off only when the vapor tension of the crystal exceeds the tension of the water vapor in the atmosphere. Thus sodium sulphate at ordinary temperatures has a greater vapor tension than the atmospheric water vapor, but gypsum (crystallized calcium sulphate) has a lower vapor tension than the atmospheric water vapor. Hence sodium sulphate effloresces, but calcium sulphate does not. This is an easy way to distinguish the two compounds. 

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