Deliquescence

Tellurium tetrafluoride, Tep4 m.p. 130 C. Colourless deliquescent solid prepared SeF4 plus TeOj. Hydrolysed by water. 

Vanadium oxide tribromide, VOBr3. Dark red deliquescent liquid formed by heating V2O3 plus Brj. 

It is a white, deliquescent solid, very powdery, which exhibits polymorphism on heating, several different crystalline forms appear over definite ranges of temperature -ultimately, the P4O10 unit in the crystal disappears and a polymerised glass is obtained, which melts to a clear liquid. 

Arsenic(V) oxide is a white deliquescent solid, which liberates oxygen only on very strong heating, leaving the (III) oxide ... 

Hydrated chromium Ill) nitrate is a dark green, very deliquescent solid, very soluble in water. The anhydrous nitrate is covalent. 

Sodium dichromate is used instead of the potassium salt because it is far more soluble in water, and is not precipitated from its aqueous solution by addition of the ethanol. It is also cheaper than the potassium salt, but has the disadvantage of being deliquescent. 

The crystalline sodium sulphide (NajS,9H20) used to prepare the disulphide is very deliquescent, and only a sample which has been kept in a well-stoppered bottle and therefore reasonably dry should be used. A sample from a badly-stoppered bottle may contain, in addition to the crystals, a certain amount of aqueous solution, in which hydrolysis and partial decomposition will have occurred such a sample should therefore be rejected. Add 4 2 g. of finely powdered sulphur to a solution of 16 g. of the crystalline sodium sulphide in 60 ml. of water, and boil the mixture gently for a few minutes until a clear solution of the disulphide is obtained. 

Toluene however sulphonates readily, and the following preparation illustrates the rapid formation of toluene-p ulphonic acid mixed with a small proportion of the deliquescent o-sulphonic acid, and the isolation of the pure crystalline /lara-isomer. 

A further crystallisation from 15 ml. of hydrochloric acid gives the sulphonic acid, 5 5 g., m.p. 105°, almost devoid of deliquescent properties. 

Note. Dimethylamine hydrochloride is a deliquescent substance the sample if damp must be dried in a vacuum desiccator and then rapidly weighed before use. 

B) SULPHONIC ACIDS. Aliphatic sulphonic acids are rarely encountered, for they are very soluble in water, and many are deliquescent. The aromatic sulphonic acids, although less uncommon, are encountered most frequently as their derivatives e.g.t the sulphonamides, pp. 247, 251), or as nuclear-substituted acids (e.g., sulphanilic acid, p. 384). 

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. 

Zinc chloride is extremely deliquescent and it must therefore be introduced into the flask as rapidly as possible. Place a small stick of zinc chloride in a glass mortar, powder rapidly, and weigh out the required quantity. 

Crystallised sodium sulphide NajS,9HjO is very deliquescent, and only a sample which has been kept in a tightly-stoppered bottle should be used crystals as dry as possible should be selected. Altemativelj, an equivalent amount of analysed fused sodium sulphide may be employed this dis.solves somewhat more slowly in alcohol. 

The deliquescent solid must be stored in a dry, tightly-stoppered container. If exposed to the air it deteriorates rapidly, developing an unpleasant odour. Samples that have been kept for some time are best recrystallised from absolute ethanol before use. 

If the crystals are opaque white and do not deliquesce quickly in air of average humidity (65% rh), they may be contaminated with some Hex amine. Washing 100g of the crude product with lOOmL of Chloroform by stirring in a beaker then filtering, repeated as many times as necessary, will remove Hex amine. Methylamine HCI is insoluble in Chloroform whereas Hexamine Is at the rate of 1g to 10mL. 

The life persistency of a smoke cloud is deterrnined chiefly by wind and convection currents in the air. Ambient temperature also plays a part in the continuance or disappearance of fog oil smokes. Water vapor in the air has an important role in the formation of most chemically generated smokes, and high relative humidity improves the performance of these smokes. The water vapor not only exerts effects through hydrolysis, but it also assists the growth of hygroscopic (deliquescent) smoke particles to an effective size by a process of hydration. Smoke may be generated by mechanical, thermal, or chemical means, or by a combination of these processes (7). 

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