Water of constitution

One of the major uses of DTA has been to follow solid-state reactions as they occur. All decomposition reactions (loss of hydrates, water of constitution, decomposition of inorganic anions, e.g.- carbonate to carbon dioxide gas, etc.) are endothermic and irreversible. Likewise are the synthesis reactions such as... 

The somewhat remarkable fact that many sulphates when heated retain one molecule of water of crystallisation (called water of constitution ) very persistently, whilst the other molecules are eliminated with relative ease, this phenomenon being observable with copper, manganese, ferrous, nickel, cobalt, magnesium and zinc sulphates, has led to a... 

Millot, J. O., The Mineral Matter in Coal. I—The Water of Constitution... 

By the action of vanadium pentoxide on selenous acid solutions, or by reduction of a solution of vanadium pentoxide in selenic acid, red crystals of a free vanado-selenous acid have been obtained, the composition of which is SV206.4Se02.4H20.j aq. It contains four molecules of water of constitution, and, according to Prandtl, should be formulated H4Ve017.4H2Se03.(aj—2) aq. 

Sodium Tantalales.—4 3 Sodium tantalate, 4Na20.8Ta206.25H20, is prepared by fusing tantalum pentoxide with caustic soda in a silver crucible excess of caustic soda is removed by washing with water, and the residue is crystallised from hot water. Another method consists in adding a concentrated solution of sodium chloride to the aqueous extract from a fused mixture of tantalum pentoxide and potassium carbonate, when the salt separates as a dense, micro-crystalline powder. It forms six-sided plates, a c=l 1-0167. It loses most of its water of crystallisation between 105° and 110° C., but the remainder (apparently five molecules) appears to be water of constitution, as its removal takes place only at much higher temperatures, with decom-... 

Of the many phosphotungstic acids, the so-called 12-phos-photungstic acid is the most common and important. According to the Rosenheim-Miolati theory, this acid is heptabasic, and the heptaguanidine salt has been reported.3 Pauling1 and others have criticized their theory, however, and have suggested other formulae. The large amount of water contained in the acid is evidently partly water of constitution and partly water of hydration. 

The application of Bragg s X-ray spectrum analysis to hydrated crystals is throwing further hght upon the manner of attachment of combined water,3 and, in many cases, it would appear that no distinction can be made between water of crystallisation and water of constitution. 

This expression ignores the water of constitution of clays which is usually of negligible magnitude for Victorian brown coals. 

Acid hydrolysis of 5-amino-5-deoxy-l,2-0-isopropylidene-o -D-xy-lofuranose (15) might be expected to afiFord 5-amino-5-deoxy-D-xylose, but instead, at 70 , 3-pyridinol (21) is the main product. If the acid hydrolysis of compound 15 is conducted at room temperature, there is obtained, besides 3-pyridinol (21), the crystalline hydrochloride of l-amino-l,5-anhydro-l-deoxy-D-fhreo-pentulose hydrate (22). The crystalline hydrate exhibits no carbonyl band in its infrared and ultraviolet spectra. The water content cannot be removed without decomposition of the compound, and is, therefore, water of constitution. The nuclear magnetic resonance spectrum of 22 lacks the signal characteristic of an anomeric proton. The free ketone group is, however, detectable by the preparation of a (2,4-dinitrophenyl)-hydrazone. 

Salt hydrates hold water molecules as (i) co-ordinated water, (ii) anion water, (iii) lattice water, or (iv) zeohte water. The term water of constitution is a misnomer often applied to compounds wrongly formulated as FegOg.HgO and MgO.HgO. These are the true hydroxides FeO(OH) and Mg(OH)2 and most of the class have a layer lattice consisting of sheets of OH and Onions with cations between them. 

Solid hydrate research covers various classes of chemical compounds, which possess different importance and practical use. There are stoichiometric hydrates and those with varying water content as zeolitic hydrates. There are true hydrates and pseudohy-drates The latter contain water as hydroxide or hydroxonium ions or as -OH and -H groups ( water of constitution ). The true hydrates with separable H2O molecules ( water of crystallization ) include inorganic salts, i.e., the so-called salt hydrates, hydrates of organic compounds, and the clathrates, as, e.g., the novel gas clathrates. This article is mainly concerned with the salt hydrates. 

Essential water forms an integral part of the molecular or crystalline structure of a compound in its solid state. Thus, the water of crystallization in a stable. solid hydrate (for example, CaC204 2H2O and BaCE 2H2O) qualifies as a type of essential water. Water of constitution is a second type of essential water it is found in compounds that yield stoichiometric amounts of water when heated or... 

Water of constitution is water that is formed when a pure solid is decomposed by heat or other chemical treatment. 

Essential water Water in a solid that exists in a fixed amount, either within the molecular structure (water of constitution) or within the crystalline structure (water of crystalli-atioii). Ethylenediaminetetraacetic acid Probably the most versatile reagent for complex formation titrations forms chelates with most cations. See EDTA. 

Water of constitution Essential water that is derived from the molecular composition of the species. 

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