THE PREPARATION OF MATERIALS. 29 



the mother liquors gave a distinct yellow. This indicates that sodium 

 has a decided tendency into the mother liquors, and hence can be sepa- 

 rated by fractional crystallization. 



Again, 1 per cent of lithium salt was added to potassic oxalate with 

 similar outcome. Rubidium and caesium were tested likewise. A sat- 

 urated solution of potassic oxalate and a salt of each of these metals was 

 made of such strength that the rubidium or caesium could easily be found 

 by means of the spectroscope. Each solution was evaporated until satu- 

 rated, and cooled ; and the crystals were whirled. The mother liquor gave 

 a test for the impurity, the crystals gave none. 



No other oxalates were seriously to be feared, because the insoluble ones 

 would appear at once and demand filtration, and soluble double oxalates 

 could hardly be isomorphous, and would therefore tend into the mother 

 liquors. Traces of calcic oxalate in fact appeared in the former fashion, 

 and traces of iron in the latter. 



In view of these satisfactory results, it was evidently only necessary 

 to crystallize the potassic oxalate often enough to make the separation 

 complete ; and since potassic oxalate has a fairly convenient change in 

 solubility with the temperature, the yield after a considerable number of 

 crystallizations is good, especially if pains are taken to carry out the pro- 

 cess systematically. 



Two very pure samples of oxalate were made, from two different sources. 

 As one source of material, a commercial sample prepared by Merck was 

 used. It had been labeled "Potassium oxalate, neutral, highest purity," 

 and contained as a matter of fact only the traces of calcium and iron 

 already referred to, but no lead or other discoverable impurity. A large 

 quantity of a solution of this salt was filtered hot into a platinum dish, 

 where it was five times systematically recrystallized, the crystals being 

 centrifugally whirled each time. 



Careful scrutiny in the spectroscope revealed in the product no other 

 spectroscopic lines than those of potassium, and careful qualitative tests 

 proved the absence of lead and iron. The salt dissolved without a trace 

 of residue in water, and was quite pure enough to serve as a starting 

 point for further operations, to be described under the heading "Potassic 

 bromide." 



A second sample of potassic material was obtained from Merck in the 

 form of the hydroxide, being marked "Potassium Hydrate, Chemically 

 Pure Reagent ; Conforms to the standards of Dr. Krauch." It was in 

 fact very pure, giving a perfectly clear solution both before and after 

 neutralization. It was neutralized with a specimen of oxalic acid which 

 had been carefully -purified by Mr. G. E. Behr, jr., for other work. This 

 acid contained no halogens, and left no residue on volatilization in plati- 



