:;sj 



PROCEEDINGS OF THE AMERICAN ACADKMY. 



a- i lie precipitation of amnionic magnesic ])hosphate, require more time 

 for their completion, when much amnionic chloride is present.* 



If, as a matter of fact, the occlusion is proportional to the concentra- 

 tion of the uudissociated magnesic oxalate, additional amnionic oxalate, 

 even in the presence of amnionic chloride, ought to increase the weight 

 of the precipitate. The following experiments, similar to the last except 

 that in each case three grams instead of one of amnionic oxalate were 

 ii.M-d, were made to test this point. 



The Effect of Excess of Ammoxic Oxalate. 



Thus a trebling of the amount of oxalate present increased the error 

 by about a third of its previous value. At first one is surprised that the 

 increase is not greater; but it must be remembered that the oxalate was 

 added rather slowly, so that most of the precipitate was formed before 

 a large excess of oxalate was present. It is chiefly the concentration of 

 the magnesic oxalate present at the instant of precipitation, not the sub- 

 Bequent amount in contact with the precipitate, which influences the 

 distribution. When the precipitate has once appeared as a solid, the 

 action must be confined to the surface; lor diffusion into solids is ex- 

 ceedingly slow because of their rigid structure. 



A number of analyses were made in the hope of combining all the 

 circumstances which tend toward complete separation, and of eliminating 



all those which oppose it : but yet further difficulties arose. It seemed 

 probable that by gradual neutralization of an acid solution the calcic 

 oxalate might be precipitated in an environment containing as little mag- 

 nesic oxalate as possible, and thus be as \\'*^' as possible from this impur- 

 ity. Tlic mode of procedure was a- follows. To a mixture containing 

 it. 200 (■.,-.. 25 c.c. each of the calcic and magnesic solutions, were added 

 three grams of amnionic chloride. I 6 grams of oxalic acid, am! enough 



• Compare Ostwald, Scientific Foundations <>f Analytical Chemistry (Mac- 

 millan, 1895), p 186. 



