Salt efflorescences

The hexammino-salts are yellow or brownish compounds, easily crystallised and fairly stable. They withstand the action of boiling water for some time, and can therefore be crystallised from aqueous solution if crystallisation be carried out quickly. Acid solutions of the salts are very stable, but alkaline and neutral solutions are decomposed on heating. The salts are more soluble than the corresponding aquo-pentammino-salts, and their solutions are brownish yellow in colour. The hydrated salts effloresce in air and become opaque and reddish brown in colour. On heating they lose ammonia, leaving eobaltic oxide and a cobaltous salt. In cold solution they give no precipitate with sodium hydroxide, ammonium hydroxide, or alkali... 

The crystals of this salt prepared by T. Thomson s process were formerly stated to be sodium octohydrated carbonate, Na2C03.8H20, but H. Lowel proved that this is a mistake the product is Bodium heptahydrated carbonate. The /8-hepta-hydrated carbonate is less soluble than the a-salt, and more soluble than the decahydrated carbonate, so that a soln. Bat. with the a-salt is supersaturated with respect to the j8-salt, and the decahydrated salt. The transformation of the a- into the /8-variety occurs while the salt is in contact with its mother liquid at very variable temp.—sometimes at 23°, more often below 10°. The /3-salt effloresces in dry air, and passes into the monohydrated carbonate at about 32°. An appreciable amount of heat is developed during the crystallization of a supersaturated soln., and this iB greater for the -heptahydrated carbonate than for the a-Balt but less than for the decahydrated carbonate. The temp, rose from 20 5° to 22 5° during the crystallization of the /3-salt from a sat. soln. 

The salt effloresces, losing one molecule of water. It gradually loses the remainder of its water on heating, and at red heat decomposes, leaving a residue of urano-uranic oxide. 

The various playa environments include a central salt crust or saline pan, composed of dry salt, commonly halite (Figure 10.3D, E), but in some cases trona (Eugster, 1970), gypsum (Stoertz and Ericksen, 1974) or other sulphates such as mirabilite, epsomite or bloedite. The saline mudflat is typically moist clay to silt with surface salt efflorescences and intrasediment (displacive) evaporite minerals. In some systems, these may be zoned on a broad scale, with more-soluble minerals towards the lowest central portion of the mudflat, caused by groundwater evaporation gradients (e.g. Saline Valley, California Hardie, 1968). 

Zehnder, K. Arnold, A. (1989) Crystal Growth in Salt Efflorescence. Journal of Crystal Growth 97, 513-521. 

Figure 16 Salt efflorescence on ceramic body (shown by optical microscopy of the surface)...

Figure 5 SEM of subsurface layer shows microbial growth and salt efflorescences...

Is the salt efflorescent in ordinary air (3) What is the evidence (4) What is the cause of its change in the tube containing the calcium chloride ... 

The salt effloresces in air and is simultaneously decomposed by CO 3. Therefore it must be stored in airti t containers. Readily soluble in water. 

Well-crystallized salts the water content varies somewhat depending on the method of preparation. The large, transparent crystals of the K salt effloresce when stored under sharply desiccating conditions they then become cloudy, but retain their good solubility in water. 

Figure 4. Field illustrations of salt efflorescences on building materials (a) efflorescences on the surface of a granitic ashlars, (b) efflorescences on mortars.

It is also necessary to consider the distribution of the alteration agents in the affected materials. If the alteration features are limited to the surface then it is only necessary to do a surface cleaning. However, in some cases surface features are evidence of a deeper contamination as in the case of salt efflorescences that marks salt contamination of the porous substrate. In this case it is necessary to implement processes that remove the pollutants from insider the substrate. The usefulness of a protective treatment in an already affected material must be questioned as is illustrated by the study of Polder et al. (2001) who concluded that under wet conditions. 

Shepherd, K., Subramanian, S. P., and Rajamanickam, G. V. (1994). Saline and Alkaline Soil Formation Due to Salt Efflorescence Around Ennur Coastal Tract, Madras, Tamil Nadu. J. Appl. Hydro 7(1 -4), 13-23. 

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