and Laboratory Methods. 1247 



lead, and potassium interfere. The salts of the trivalent metals and of boric 

 acid prevent the formation of characteristic crystals. 



With simple salts of calcium the reaction is a beautiful one, leaving little to 

 be desired. 



Exercises for Practice. 



(See under Strontium.) 



IV. Potassium Ferrocyanide added to solutions of Calcium salts in the presence 

 of amtnoniujn chloride, gives rise to the formation of a Double Ferrocyanide of Potas- 

 sium and Calcium. 



CaClg + K^Fe(CN)6 = K2CaFe(CN)6. SHgO + 2KC1. 



Method. — To the drop of the solution of the substance ,] O ^\ 

 to be tested add a trace of acetic acid, then a moderate /^ ^'^ <^ j^ 

 amount of ammonium chloride, stir thoroughly and cause a ^^ n /!> /% 

 drop of a solution of potassium ferrocyanide to flow into the ^ I' o ^ ck:> O 

 test drop. Near the zone of union tiny rectangular and square y> O ° ^.V:^ 



plates are immediately precipitated (Fig. 48). ^ ^ V ^ 



Remarks. — In the presence of free mineral acids, first add ' vpvUo.otwnN 



ammonium acetate or sodium acetate in order to mitigate Fig. 48. 



their action. 



Concentrated solutions, with respect to calcium, lead to the precipitation of an 

 amorphous product. Too much ammonium chloride produces a like result ; but 

 the reagent alone, in the absence of the ammonium salt, unless added in consid- 

 erable excess, fails to yield a deposit of crystals. Barium gives large yellow 

 rhombs with the reagent without the addition of NH^Cl, while strontium fails to 

 yield a precipitate in either case. Potassium ferrocyanide is, therefore, some- 

 times useful in dealing with mixtures of the calcium group, but as a character- 

 istic test for calcium in simple salts it is of but little value. 



All elements forming insoluble or difficultly soluble ferrocyanides interfere, 

 and in most cases prevent the detection of calcium by the above method. 



Exercises for Practice. 



(See suggestions given under Barium.) 



V. The addition of Arsenic Acid to ammoniacal solutions containing Calcium, 

 precipitates Ammonium Calcium Arsenate. 



CaCl2 -i- H3ASO4 + SNH^OH = NH^CaAsO^. 6H2O + 2NH4CI + 3H2O. 



Method. — To the drop of the solution of the substance to be tested add 

 ammonium hydroxide in slight excess, and cause to flow into the test drop a drop 

 of an ammoniacal solution of arsenic acid. There is immediately produced a 

 heavy precipitate rapidly growing into large crystals belonging to the orthorhom- 

 bic system. These crystals of the double arsenate of calcium and ammonium 

 generally take the form of envelope-like crystallites, or if separating from dilute 

 solutions appear in hemimorphic forms like those of ammonium magnesium 

 phosphate, but of a greater size (Fig. 49). 



