and Laboratory Methods. 1245 



//. Sa/ts of Calcium give with Oxalic Acid a difficultly soluble Calciiun Oxalate. 



CaCl2 + H2C20^ = CaC20^«H20 + 2HC1. 



Method. — A drop of a solution of oxalic acid is ^v^_ 



caused to flow into a drop of the solution to be tested. ^A J~^ 



The solution of the substance should be neutral or may // r-T, ^ f~~i 



contain a trace of free nitric acid. Calcium oxalate is '^^^j^ 



almost instantly precipitated in the form of tiny highly tb ^V 



refractive octahedra or rhombs. Often crosses and „ \^ '^%. 



more or less irregular bundles of minute needles are ^^ ^v C^ /S} 

 obtained (Fig. 46). "^ 



Re^narks. — The composition of the salt, with respect >pw.o.oi™„. 



to the amount of water of crystallization, varies accord- ^'^' '^^^ 



ing to conditions. It seems to be quite generally accepted that when precipitated 

 from neutral or alkaline solutions at room temperature the salt formed has the 

 formula CslC^O^* SHjO and is to be referred to the tetragonal system; while if 

 precipitated from hot neytral or acid solutions or from acid solutions at room 

 temperature there is obtained an oxalate of the formula CaC204» H2O, a mono- 

 clinic salt. This latter form of calcium oxalate seems also to result in the pres- 

 ence of an excess of oxalic acid. It follows, therefore, that with the conditions 

 which usually obtain, there may be precipitated either the salt with three mole- 

 cules of water of crystallization or the salt with only one molecule. 



Free nitric acid greatly retards the reaction, but the presence of a very little 

 of this acid gives rise to the formation of larger crystals (because of their being 

 more slowly formed), which are therefore more easily recognized. 



Calcium oxalate is insoluble in acetic acid and in sodium, potassium and 

 ammonium hydroxides, but is readily dissolved by the mineral acids. 



Strontium gives with oxalic acid an identical reaction, save that the crystals 

 of strontium oxalate are generally somewhat larger. 



Barium oxalate takes the form of fibrous bundles of needles and is not likely 

 to be mistaken for either calcium or strontium. 



Zinc under certain conditions may yield a zinc oxalate difficult to distinguish 

 from the oxalates of calcium and strontium. 



Magnesium oxalate will separate in forms not to be distinguished from 

 calcium oxalate if the test drop contains much acetic acid. 



Lead oxalate may also assume forms somewhat resembling those of calcium 

 oxalate, but after a short time these crystals grow into large, well developed prisms. 



Many other elements are also precipitated by oxalic acid. If such elements 

 are present in large amount they are apt to interfere. 



Owing to the minute size of the crystals, testing for calcium with oxalic acid 

 is not always satisfactory. As an offset to this disadvantage, chlorides of the 

 trivalent metals and boric acid have no effect other than a retardation of the 

 reaction. 



In the event of a precipitate of doubtful composition being obtained, draw off 

 the supernatant liquid, or separate by means of the centrifuge, add to the residue 

 a tiny drop of dilute sulphuric acid. Calcium oxalate is dissolved and in a few 

 seconds the characteristic crystals of CaS04» 2H2O make their appearance. 



