THEORY OF DEPOSITION OF POTASH SALTS 



C/ f< 



Fig. 44. Diagram Showing I,aw of Crystal- 

 lization of Potash Salts. 



the same result will be secured but in an inverse order. A solu- 

 tion which has been shaken for a long time with an excess of both 

 salts so as to become thoroughly saturated will give the follow- 

 ing composition: iooH 2 O4-8o,NaCl-f-39KCl. (C Fig. 44). 

 Thus solutions which contain 

 a greater ratio of sodium 

 chlorid to potassium chlorid 

 than 89x58.5:39x74.5 will 

 first deposit chlorid of sodium. 

 In the opposite condition of 

 the ratio potassium chlorid 

 will first appear. 



A consideration of this prob- 

 lem leads to the formation of 

 the law which even in the most 

 complicated cases governs 

 the process of crystallization. 

 This law, as formulated by 

 van't Hoff, shows that in depositing its contents, the solution 

 gradually varies its composition away from that of a solution 

 which is saturated with the substance being deposited at the 

 moment and contains nothing but this substance. The principle 

 becomes quite clear if the process which takes place during crys- 

 tallization from an evaporating solution is reversed and water be 

 continually added together with the salt which is being deposited. 



Under these circumstances obviously the solution tends to be- 

 come more and more a saturated solution of this salt alone, since 

 the other constituents, whatever they may be, must gradually be- 

 come relatively negligible in quantity. 



Thus, if a solution which is practically that of sea water be 

 considered corresponding to the formula for the salt in sea water, 

 already given, it is evident that other salts could be added in 

 order to change the degree of concentration. The principle, how- 

 ever, may be more briefly defined if the case of potassium chlorid, 

 magnesium chlorid, and magnesium sulfate be first considered and 

 only at the very end the chlorid of sodium which is always pres- 

 ent in excess in sea water be taken into consideration. 



