and Laboratory Methods. 1379 



//. Potassium Atitwionate added to solutions containing Magnesium causes the 

 separation of Magnesium Pyro-antimonate. 



MgSO^ + HaKaSbgO- = HgMgSb.O^ . 9H2O + K2SO4.* 



Method. — First prepare an almost saturated solution of the reagent by heat- 

 ing a fragment with water. A drop of this solution is placed next the test drop, 

 and the two caused to unite. A dense amorphous precipitate is usually imme- 

 diately produced. After a time, crystals of magnesium pyro-antimonate appear, 

 generally near the circumference. The forms most frequently obtained are thin, 

 colorless, transparent hexagonal plates, and spherical masses more or less 

 crystalline in appearance (Fig. 67). Less often, short hexagonal prisms are seen. 



Re7narks. — The solution to be tested must be dilute and neutral. Free acid 

 not only interferes with the formation of characteristic ^ 



crystals, but also causes the reagent itself to yield an /— \ Q ■=■ 



amorphous precipitate. '^ C\ O ^ C^l 



The development of the crystals of magnesium pyro- ^^ 



antimonate is quite slow, and eventually they may attain yr^ Q sSto ^ ^ 

 a size of double or even triple that of those shown in \ j ^ ^^7\ r\ 



Fig. 67. 0^(fi^ 



Alcohol can be employed to hasten crystallization, but ^^ 



it is better to allow the preparation to take all the time it \p^;;^!otw«,. 



needs. fig. tiv. 



Lithium sometimes yields crystals not to be distinguished from those of 

 magnesium, more often circular disks and sperulites. 



Sodium (q. v.) gives fusiform crystals. 



Members of the calcium group are precipitated in an amorphous form, and 

 interfere with the test for magnesium. 



Ammonium salts should be absent. 



r 



Exercises for Practice. 



Try reaction on salts of Mg. 



Repeat the experiment in the presence of salts of NH^. 



Make a mixture containing Na and Mg ; test as above. 



Test a salt of Li. Try the effect of the reagent on salts of Zn and of Cd. 



Test a mixture of Mg and Zn. 



///. With Uranyl Acetate and Sodium Acetate. 



The method of applying the test has been described in Method III of Gluci- 

 num ; there, and under Sodium, Method II, the properties of the triple acetates 

 have been discussed in detail. 



The formula and appearance of the triple acetate of sodium magnesium and 

 uranyl will be found on page 9X5 of Vol. Ill, of this Journal. To this article, 

 and to that on Glucinum, the reader is referred for details as to methods of pro- 

 cedure, sources of error, etc. E. M. Chamot. 

 Cornell University. 



* See foot note, Sodium, Method V, Jour. App. Micros. Ill, 1900. p. 1048. 



