APPLICATION OP EQUILIBRIUM LAW TO SEPARATION OF CRYSTALS. 275 



On the other hand — and the case is somewhat more complicated — at a 

 point on B F, a little to the left of B, the solution contains a small amount 

 of Magnesium sulphate, the reciprocal of the Potassium chloride con- 

 sidered in the previous case. On evaporating such a solution, change 

 proceeds along the line b f, KgSO., separating as before until the point F 

 is reached. The character of the subsequent change will be determined 

 by the presence or absence of Potassium sulphate : if it be removed, 

 crystallisation proceeds along f g ; but if it be left in contact with the 

 sokition Schonite is continually deposited, the composition of the liquid 

 remaining unchanged until the whole of the Potassium sulphate originally 

 deposited is redissolved by the excess of the Magnesium sulphate in the 

 solution. Only then will crystallisation proceed along f g, and when g is 

 reached the liquid will dry up without further change in composition. 



Starting within the diagram, again in the K2SO4 field — say from a 

 point Y, a little to the left of b and a little above B F — the track followed 

 will be along the line b y produced until f m is reached at a point /jvz. If 

 the Potassium sulphate be then removed, the Schonite field is entered. 

 To determine the course followed aci'oss this, it is to be noted that the 

 point at which Schonite alone is present in a saturated solution must be 

 taken as the origin. To deduce this we have to bear in mind that the 

 line GF represents the manner in which the solubility of Schonite varies 

 as the proportions of Magnesium and Potassium sulphates vary ; there- 

 fore the theoretical solubility of Schonite alone — i.e., when there is no 

 excess of either of the single salts present — is at a point f' on f produced 

 equidistant from the two axes on which the separate salts are plotted — 

 i.e., on the line bisecting the angle b c. 



The track followed across the Schonite field will therefore be in the 

 direction p'/m produced. When M N is reached Potassium chloride will 

 separate. It will be obvious that to reach the MgS04.7H20 field it would 

 be necessary to have but little chloride present. 



Beyond N Schonite gives way to Magnesium sulphate heptahydrate, 

 which is deposited together with Potassium chloride until p is reached. 

 Trom p, after removal of the heptahydrate, change would proceed through 

 Q to R. It is obvious that it would not occur along P H, as continued 

 concentration would involve the conversion of the heptahydrate into 

 hexahydrate, and would therefore merely condition a lag in the crystal- 

 lisation, supposing the heptahydrate were not removed. In like manner 

 change would not proceed along Q L, as concentration would involve a 

 gradual conversion of unremoved Potassium chloride into Carnallite. At 

 R the solution would dry up unchanged in composition. 



As a proof of the correctness of this method of interpreting the 

 diagram, the results may be quoted which were obtained by van't HofF on 

 concentrating a solution of equal molecular quantities of Potassium 

 sulphate and Magnesium chloride, i.e., 174-3 gm. K2SO4-I- 223-4 gm MgClj 

 6H2O. The use of such a solution is equivalent to starting in the plane 

 diagram from the origin, as the geometric convention followed involves one 

 of the salts being represented as a negative quantity of its reciprocal. 

 As the origin lies within the K^SO, field, the diagram shows that K2SO4 

 will be the first salt to separate, and that concentration will proceed along 

 the Magnesium chloride axis until the Schonite boundary is reached ; the 

 separation of Schonite will then set in. Provided the Potassium sulphate 

 be not removed, the course of change will now be along f m to Ai ; when 

 this is reached the deposition of Potassium chloride begins. 



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