170 
IOWA ACADEMY OF SCIENCES 
Concerning the small amount of magnesium that may come down with 
the calcium; the usual washing of the precipitate with hot water might 
remove an amount of calcium equal to the weight of the precipitated 
magnesium, and this amount would be afterwards precipitated with the 
magnesium. One source of error might almost completely balance the 
other and a satisfactory result would thus be easily attained. However, 
we do not practice or advocate such a procedure. 
Treadwell-Hall in their work on quantitative analysis, page 71, call 
attention to an error quite often made by beginners. If all the calcium 
is not precipitated, it will come down with the magnesium and the 
total result will be too low. In order to separate the calcium 
from the mixture of the two phosphates, the precipitate after 
standing 12 hours is filtered, washed, dissolved in a small quantity 
of hydrochloric acid, and ammonia is added until the solution is alka- 
line. It is then strongly acidified with acetic acid, heated to boiling and 
precipitated with a boiling solution of ammonium oxalate. After 12 
hours, the calcium oxalate is filtered, washed with hot dilute acetJb acid 
and determined as calcium oxide. The filtrate is evaporated to dryness, 
the ammonium salts driven off, the residue dissolved in a little dilute 
hydrochloric acid and precipitated as usual. The reactions involved are 
represented by the following equations; 
3CaCk+2Na.HP04=Ca3 (PO4 ) .+4 NaCl+2HCl . 
MgNH4P04+3HCl=MgCk+NH4Cl+H3P04 
Gas (P04)2+6HCl=3CaCk+2H3P04. 
The acetic acid prevents any precipitation either of the calcium or 
magnesium until the ammonium oxalate is added to precipitate the cal- 
cium. If tiie magnesium ammonium phosphate is changed to the pyro 
phosphate, the acetic acid does not prevent the precipitation of the mag- 
nesium and the separation cannot be effected. 
The equation in this case is Mg +4HC1 2 MgCl 2 +H 4 P 207 . 
The equation is similar for calcium. 
The pyrophosphoric acid slowly adds water and becomes H^PoO^ -f H., 
O2H3PO4. 
The occlusion of ammonium oxalate by calcium oxalate. 
In a private communication from Dr. Gregory P. Baxter, of Harvard 
University, he states that he has not published the results of his re- 
search upon the occlusion of oxalic acid and ammonium oxalate by cal- 
cium oxalate and that his results though .somewhat unsatisfactory, 
pointed to no appreciable occlusion. Our test was as follows: We 
removed ammonia from a flask and Liebig condenser by distilling water 
through for an hour, or until the distillate gave no color with Nessler 
reagent. Then we added the well-washed calcium oxalate precipitate 
from a gram of the dolomite to the flask with 500 cc. of distilled water 
and 50cc. of alkaline permanganate solution. BOOcc. were distilled off 
and 50cc. of the distillate gave scarcely a trace of color with Nessler solu- 
tion. The same result was obtained whether the calcium oxalate was 
tested before drying, or whether it had been heated strongly wuth the 
blast lamp. The oxalate had been slowly precipitated from a boiling 
solution by constant stirring. 
