126 



On the Theoi 



y of Fractional 



Precipitation. 



Solution. 



Precipitate. 







NiS0 4 . 



OoS0 4 . 



NiS0 4 . 



CoS0 4 . 



Temp. 



6 f 



A. 



B. 



X. 



y- 







grm. 



grm. 











•1 



•9 



•1050 



•8105 



7-8 







•2 



•8 



•1980 



■6687 



6-7 







•3 



•7 



•2765 



•6215 



7-8 



•86 



•4 



•6 



•3510 



•5140 



7-8 



•92 



•5 



•5 



•4465 



•4420 



7-8 



•96 



•6 



•4 



•5295 



•3340 



7-7-5 



•84 



•7 



•3 



•5825 



•2615 



7-7-5 



1-15 



•8 



•2 



•7080 



•1760 



8-8-5 



•98 



•9 



•1 



•7865 



•0900 



7-7-5 



1-11 



e 

 Mean value of — 





= -97 









e' 





The value — ='97 indicates a very near equality possessed 



by C0SO4 and JSTiS0 4 to be decomposed by sodic hydrate. A 

 simple inspection of the experimental results shows this fact, 

 for it is seen that the ratios of the materials in the precipitates 

 are almost the same as the salts in the solutions. It is evident 

 therefore that if fractional precipitation were to be employed 

 for the separation of nickel and cobalt, the process would be 

 found as tedious (at least as sulphates and with sodic hydrate) 

 as that of the separation, say, of samaria from didymia. 



This near equality in precipitability is an interesting result 

 when it is remembered that the atomic weights of nickel and 

 cobalt are almost equal ; but, judging by other experiments 

 on precipitation, it does not seem that the precipitability or 

 basic power is related to the atomic weight. For instance, 

 the atomic weights of the three elements Sm, Di, and La are 

 150, 142*3, and 138*5 ; yet the separation of samaria from 

 didymia is a much more tedious operation than that of didymia 

 from lanthana, the precipitability of the first two oxides or 

 their basicities being more nearly equal than the second two, 

 didymia and lanthana. Another illustration of this fact is 

 shown by the six earths scandia, ytterbia, holmia, thulia, 

 erbia, and yttria, whose atomic weights are (R2O3), $c = 44'5, 

 Yb = 173,Ho = 162(?),Tm = 169-5(?),Er = 166,andY = 89'5; 

 but their separation from each other is a very tedious opera- 

 tion, indicating only slight differences in basic strength, Sc 

 and Yb being the least basic of the group, and Y the most 

 basic. 



Debus*, in his experiments on the fractional precipitation of 



* Ann. Ch. Pharm. vols, lxxxv., lxxxvi., lxxxvii. 



