Sodium chloride deliquescence

Atmospheric aerosols are hygroscopic, taking up and releasing water as the RH changes (see also Section C.l) because some of the chemical components are themselves deliquescent in pure form. For example, sodium chloride, the major component of sea salt, deliquesces at 298 K at an RH of 75%, whereas ammonium sulfate, (NH4)2S04, and ammonium nitrate, NH4N03, deliquesce at 80 and 62% RH, respectively. (See Table 9.16 for the deliquescence points of some common constituents of atmospheric particles.) De-... 

Deliquescence is a property of substances very soluble in water. When such substances, potassium carbonate or calcium chloride for example, are exposed to the air, the water vapor forms with the substance a small quantity of a saturated solution. This saturated solution has a lower vapor pressure than that of the atmosphere, that is, the water is held by the substance, it does not tend to escape, hence more water vapor is added from the air, and finally the substance is entirely dissolved in this condensed vapor. Common salt or sodium chloride often appears to deliquesce, but the deliquescence is due to the very soluble magnesium and calcium chlorides which are usually mixed with commercial sodium chloride. Sodium nitrate is very soluble in water at the ordinary temperature, but potassium nitrate is only slightly soluble. Hence potassium nitrate, and not sodium nitrate, is used in the manufacture of gunpowder. 

Sodium Tungstocyanide, 2Na4[W(CN)s].5H20, may be obtained by neutralisation of the free acid with sodium hydroxide, or by double decomposition from the silver salt and sodium chloride. It yields small yellow deliquescent crystals. 

Sodium chlorate is deliquescent and is relatively tittle used. It may be tested in the same way as potassium chlorate. Roman gives the following specification sodium chlorate 99 5 per cent., sodium chloride o 2, bromide o i traces of heavy metals and alkaline earths no mechanical impurities no insoluble residue in water. 

Borax, Na2B407 IOH2O. Molecular weight 382.0. A commercial preparation may be recrystallized three times from water (below 50°) and dried to constant weight in a desiccator containing deliquescent sodium chloride. The purity of samples is easily tested by titrating against methyl red (pT = 5.0) or methyl yellow. 

CON,H, NaGl,H,O. It is obtained in prismatic ciystals when solutions of equal molecules of urea and sodium chloride are evaporated together. It is deliquescent and very soluble in water. Its solution, when mixed with solution of oxalio acid, only forms urea oxalate idter long standing, or on evaporation. 

Fig. 7-6. Variation of particle size with relative humidity (Kohler diagram) for sodium chloride particles having different dry radii. Deliquescence occurs at 75% r.h. Note the hysteresis effect when the humidity is raised or lowered beyond the critical value. Curves for relative humidities greater than 75% were calculated. The scale above 100% is expanded. The experimental points refer to observations of Tang et al. (1977) on submicrometer-sized monodis-perse sodium chloride particles.

Seawater contains sea salt to about 3.5% by weight, of which 85% is sodium chloride. It can be safely assumed that the sea-salt content of jet and film drops is similar. As the drops enter the atmosphere they experience lower relative humidities and dry up until their water content is in equilibrium with the environment. The particle radius then is about one-quarter of the parent drop s radius. Although parts of sodium chloride may crystallize, one should not expect the particles to dry up completely, because the deliquescence point of magnesium chloride, which is also present in sea salt, lies at 31% r.h. (see Table 7-6). Such low relative humidities are not reached in marine air. 

Here sodium chloride was used as an example. Starting at a relative humidity of 50%, the RH was stepped in increments of 2.5% to 70% RH. The resulting signal was seen to be adsorption type peaks. Above 12% RH the signal became stepped, indicating the onset of deliquescence... 

Chen, Y-Y and W.-M. G. Lee (2001) The effect of surfactants on the deliquescence of sodium chloride. Journal of Environmental Science and Health, Part A 36, 229-242 Chin, M. (1992) Atmospheric studies of carbonyl sulfide and carbon disulfide and their relationship to stratospheric background sulfur aerosol. Doctoral Thesis, Georgia Institute of Technology... 

Chile saltpeter is composed of sodium nitrate, NaNOj. It is found in South America, particularly in Chile. It occurs in beds that vary in thickness from 15 cm to 3.6 m. These beds, called caHche, are interspersed with deposits of gypsum, sodium chloride and other salts, and sand. The caliche is quarried and purified in Chile before the product is ejcported. It is used extensively as a fertiHzer but not in the manufacture of gunpowder because of its deliquescence. 

Bismuth trichloride also unites with quinoline and pyridine. Quino-lino-bismuth chloride, [Bi(C9H7N)]Cl3, is obtained as a white crystalline mass on mixing the chloride with the base. This derivative is only slowly decomposed by water and is not deliquescent.3 Bismuth chloride and pyridine form a white compound of composition BiCl3.l-5(CsH5N) or 2BiCl3.3(C5H5N) on adding pyridine to an ethereal solution of bismuth chloride,4 or it may be formed from an acetone solution of the chloride and excess of pyridine.3 It is a white crystalline powder, fairly stable towards water, and only attacked slowly by hydrochloric acid. Aqueous sodium hydroxide quickly decomposes the additive compound. 

M. Couleru recommends the use of a cone. soln. of sodium chromate for the successful production of perchlorates. The action is similar to that which is obtained by a little chromate in the electrolyte during the production of chlorates. The sodium perchlorate so produced is very soluble and deliquescent, and it is not usually worked up, but rather converted into the ammonium or potassium salt by the addition of ammonium or potassium chloride. It is advisable to ensure that all the chlorate has been converted into perchlorate before precipitating the perchlorate, otherwise the perchlorate will be contaminated by the chlorate in solid soln. which cannot be removed by washing. A. Angeli recommends electrolyzing... 

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