PURIFICATION OF SALT. 13 



SODIC CHLORIDE. 



In most cases a perfectly pure salt can not be made by precipi- 

 tating the impurities, as many are prone to assume, but must itself be 

 precipitated or crystallized from the less pure solution. Sodic chlo- 

 ride is nearly insoluble in a concentrated solution of hydrochloric acid ; 

 therefore the usual method of precipitation by means of hydrochloric 

 gas seemed the best possible method of obtaining pure sodic chlo- 

 ride, since any included hydrochloric acid could be completely expelled 

 by fusion. This expectation was confirmed by trial, and when it had 

 been found that this means of purification was unusually suitable, most 

 of the specimens of salt were treated in this way. This and other 

 purifications were greatly assisted by centrifugal draining of the crys- 

 tals. For this purpose salt was collected in a platinum funnel, and 

 this was covered and supported on a stout frame capable of being 

 whirled by means of strong cord about a radius of a meter. A test-tube, 

 firmly fixed in the frame, collected the separated mother liquor. Cen- 

 trifugal draining, so important in technical work, has hardly received 

 the attention which it deserves in the scientific laboratory. Experiments 

 have shown that a good centrifugal apparatus is capable of separating 

 nine-tenths or more , according to the habit of the crystals , of the mother 

 liquor which would otherwise contaminate the solid, hence making the 

 purification at least ten times as effective as it would otherwise be.* 

 The whole treatment of precipitation and draining'^was carried on in 

 platinum vessels which had been effectively freed from superficial iron. 



Besides this chief mode of purification, others were used in special 

 cases, and the original material came from a variety of sources, as 

 recorded below. One of these was Merck's purest sodic chloride, 

 which this firm stated had been prepared from German rock salt. 

 Another was the very pure sodic sulphate prepared for the experiments 

 on the transition temperature of this substance, and known to be very 

 pure, because of the constancy and accuracy of its transition point 

 (32.383).! Another was a fine specimen of Stassfurt halite, perfectly 

 clear and colorless. A fourth specimen was made from the purest 

 sodic bicarbonate manufactured by the Solvay Process Company, of 

 Syracuse, New York, and very kindly furnished in large quantity by 

 that company. A fifth sample was made from pure sodic carbonate 

 from Merck. These preparations, differing widely in the steps of 

 manufacture, and in geographical source, all yielded essentially the 

 same atomic weight. 



*Richards, Journ. Am. Chem. Soc.,27, 104 (1905). 



fRiohards and Wells, Proc, Am. Acad., 38, 431 (1902) ; also Zeitschr. phys. 

 Chem., 43, 465 (1903). 



