1296 



Journal of Applied Microscopy 



has been added, it is well to bear in mind that there is danger of interference 

 from members of the magnesium group. 



Boric acid present in the test drop may prevent the formation of characteristic 

 crystals of barium oxalate. 



Although chlorides of iron and aluminum have, as has been stated, no dele- 

 terious iniiuence on the precipitation of the oxalates of calcium and strontium, 

 we meet in the case of barium with a most interesting and remarkable reaction. 

 Owing to the formation of a double oxalate, instead of the forms shown in Fig. 

 55, there are now obtained tufts and bunches of very long, fine, curving, hair-like 

 crystals of exceedingly characteristic appearance. The chemical composition 

 and formula of this compound is not yet clear. In order to obtain this interest- 

 ing compound, proceed as follows : To the test drop containing barium, add 

 ferric chloride in sufficient amount to impart a faint but distinct yellow color ; 

 then add a fragment or two of sodium or ammonium acetate ; stir. The yellow 

 should now have changed to a reddish tint. Into the drop thus prepared cause 

 a drop of oxalic acid to flow. Tufts and sheaves of very fine needles soon 



appear. The needles rapidly grow 

 longer and longer and soon begin to 

 curve in a most peculiar manner. 

 See Fig. 56. The presence of cal- 

 cium or strontium, or both, in even 

 large amount does not appear to have 

 any serious influence on the forma- 

 tion of this double oxalate of barium 

 and iron, save that its separation is 

 often somewhat retarded. In such 

 mixtures the oxalates of calcium and 

 strontium first appear in their usual 

 form, then after a time the hair-like 

 tufts of the double oxalate appear. 

 If the quantity of barium is quite 

 small, little rosettes of radiating 

 needles are obtained, separating near 

 the edges of the drop. 



Aluminum gives rise to the for- 

 mation of a similar product, but the crystal masses are colorless, while those of 

 the iron salt are light brown. 



Chlorplatinic acid interferes with the formation of barium oxalate in a manner 

 similar to iron and aluminum. Hence it is inadvisable to test a preparation 

 with oxalic acid for borium, which has already been tested for potassium. 



For a list of the elements with which oxalic acid may give a crystalline pre- 

 cipitate, see the list of reagents*. 



Exercises for Practice. 



Try the reaction of oxalic acid on salts of Ca, Sr, Ba, in neutal solution, first 

 *Jour. App. Micros. Ill, 8 1 8. 



Fig. 56. 



