CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF 

 HARVARD COLLEGE. 



THE OCCLUSION OF MAGNESIC OXALATE BY CALCIC 

 OXALATE, AND THE SOLUBILITY OF 

 CALCIC OXALATE. 



By Theodore W. Richards, Charles F. McCaffrey, and 



Harold Bisbee. 



Received Feb. 21, 1901. Presented March 13, 1901. 



For many years it has been known that magnesic oxalate is carried 

 down with calcic oxalate in the ordinary course of quantitative analysis. 

 This is only one case of the very general phenomenon of concomitant 

 precipitation, or occlusion, sometimes explained with the help of van't 

 Hoff's conception of "solid solution,"* and not widely understood, no 

 matter what name may be used. It is of considerable interest, both prac- 

 tically and theoretically, to obtain evidence concerning the mechanism 

 of this class of analytical irregularities. 



In a foregoing paper, f one of us has shown that occlusion is probably 

 the distribution of an undissociated substance between the solution and 

 the nascent solid. If this is the case, the amount of material occluded 

 should be directly proportional to the concentration of the undissociated 

 part of the substance in question. The application of this idea to the 

 present case of calcium and magnesium seemed capable of furnishing 

 a further clue to the theory of the general problem, as well as of pro- 

 viding a more satisfactory analytical method in this particular case. 

 Since the magnesium comes down with the calcium in the form of 

 oxalate, it is the concentration of the undissociated magnesium oxalate 

 in the solution which will determine the amount of the occlusion, if this 

 hypothesis be true. According to the law of mass action and the dis- 

 sociation hypothesis, this concentration may be increased by adding 

 either constituent ion in excess. It may be diminished by adding a 

 large excess of any other partially ionized substance, % especially those 



* Schneider, Z. phys. Chem. 10, 425 (1892). 



t Richards, These Proceedings, 35, 377 (1900). 



X Compare Arrhenius, Z. phys. Chem. 31, 198 (1899). 



