Spontaneous evaporation

Sodium metaniob ate (1 1) [67211-31-8] Na20 Nb20 7H20 or Na2Nb20g-7H20, separates as colorless triclinic crystals as a result of concentrating the mother Hquor from the preparation of the 7 6 sodium niobate by spontaneous evaporation. It also can be obtained by fusion of the anhydrous pentoxide in sodium hydroxide or carbonate. 

An ethereal solution of some 100 g of the crude nitrile was allowed to spontaneously evaporate and crystallise. The crystalline slurry so produced exploded violently without warning. Previously such material had been found not to be shock-sensitive to hammer blows, but dry recrystallised material was very shock-sensitive. Traces of free hydrogen azide could have been present, and a metal spatula had been used to stir the slurry, so metal azides could have been formed. See Other CYANO COMPOUNDS, ORGANIC AZIDES... 

The type of vessel employed depends on volatility of the solvent obviously the conical flask already recommended for "crystallization by cooling" is not suitable for spontaneous evaporation, while a beaker or shallow dish is. When the latter type of vessel is used, "crusts" often form on the sides above the surface of the liquid. Such cmsts seldom consist of pure substance so they should be removed carefully with a spatula or spoon before attempting to filter off the crystals. 

J. J. Watts and W. A. Richards patented the preparation of this salt by removing the proper quantity of carbon dioxide from sodium hydrocarbonate by adding sodium hydroxide, sodium carbonate, or the hydroxides of the alkaline earths, and crystallizing the soln. at about 35° C. Winkler obtained it from carbonated liquor of the ammonia-soda process and T. M. Chatard by the spontaneous evaporation of soln. of normal sodium carbonate which had been exposed to the air some time and thereby absorbed carbon dioxide. There is a fairly general agreement that a temp, below 35° is not favourable to the formation of trona and that the crystals develop better in sodium chloride soln. and excess of the normal carbonate also favours the formation of trona. J. J. Watts and W. A. Richards say that if an excess of the hydrocarbonate be used, it crystallizes out unchanged. T. M. Chatard studied the influence of the composition of the soln. on the formation of trona, and obtained the results indicated in Table LIV with soln. containing a mol. of the normal carbonate. H. N. McCoy and C. D. Test have studied the conditions under which the sesquiearbonate is formed, and their results are summarized in Fig. 73. 

Sulphate of manganese is obtained in transparent, slightly ross-colored crystals, by dissolving the pure carbonate in dilute sulphuric acid, and concentrating by spontaneous evaporation. The crystals have the form of sulphate of copper, and a similar composition, Mu 0, SO + 5 HO but by evaporation and crystallization at various temperatures, crystals with greater and less proportions of water may be obtained. The crystals are remarkably soluble in water. 

Special Methods.—With some substances it is difficult to obtain good crystals by the methods already described. A method which frequently gives excellent results consists in dissolving the substance in some solvent, then adding a second solvent miscible with the first, but in which the substance is sparingly soluble. The first solvent is then gradually removed and the substance separates out—usually in the crystalline form. If the first solvent is the more volatile in air, spontaneous evaporation in air may diminish its concentration in the solution. The solution may be placed in a desiccator over some substance which absorbs the first solvent but not the second in this way water may be removed from a water-alcohol solution by solid caustic potash or quicklime. 

Another method—applicable when the substance is soluble in alcohol and in ether, but insoluble in water—consists in making a saturated solution in cold alcohol, adding water until considerable precipitation has taken place, then adding ether until the precipitate has redissolved, and finally allowing the ether to diminish by spontaneous evaporation. 

When a volatile liquid (taken as the thermodynamic system) (Frames 1, 7) (say ether or alcohol) is applied to the skin (taken to be the surroundings) it rapidly and spontaneously evaporates (evidenced by a cooling sensation). Heat, q > = AvapH° is absorbed from the skin by the ether during the evaporation process of the ether molecules ... 

This kind of chiral crystallization of an achiral molecule always gives both right- and left-handed crystals. However, under ordinary crystallization conditions, enantiomorphous control is not possible. We have reported that a cocrystal is formed by the crystallization of a solution of a 1 1 mixture of 3-indolepropionic acid and phenanthridine in acetonitrile [28]. When the acetonitrile solution was divided into six parts and spontaneously evaporated in six containers, P-crystals appeared in four containers and M-crystals in two. If such chiral crystallization is carried out using a much larger number of flasks, the ratio of the formation of both enantiomeric crystals will approach 1 1. 

Mr De Luc is here now, he is busy writing a dissertation on spontaneous evaporation, which he contends is not caused by a solution in air but by Latent heat, in the same manner as vapour is produced by boiling water. I am giving him all the assistance in this subject I can, and he makes frequent and honourable mention of me in the work which is of consequence as it is to be published at Paris.21... 

Spontaneous evaporation of the solution of the chloride yields dark, orange-yellow crystals of the dihydrate, AuCl3,2H20, which changes to the anhydrous salt on exposure to the air. The monohydrate, AuC13,H20, is supposed to exist in solution in the form of an acid, HgAuClgO.3 With silver carbonate this acid yields a yellowish silver salt, Ag2AuCl30. The constitution of the acid and of its salts is a matter of uncertainty. 

Chromotellurates, 2R 20.4Cr03.Te03, of sodium, potassium, and ammonium, have been prepared by spontaneous evaporation of an aqueous solution containing the eorresiDonding dichromate (1 molecule), chromium trioxide (2 molecules), and telluric acid (1 molecule). 

Triphenylmethylarsonium hydroxide. — The hydroxide is formed when the iodide is digested with silver oxide in aqueous or alcoholic solution. It melts at 125° to 126° C., and prolonged heating at 100° C. converts it into triphenylarsine and methyl alcohol. Spontaneous evaporation in air yields the hydrogen earbmiate, (C3Hj)3As. HCOg.HjO, which crystallises in transparent plates. 

---