336 ELEMENTARY CHEMICAL MICROSCOPY 



minutes will usually permit the crystals to develop and their 

 appearance will then be such as to avoid error. If, however, the 

 analyst is still in doubt he may proceed as follows: After allow- 

 ing sufficient time for the separation of almost all the calcium as 

 CaS04 • 2 H2O, draw off the supernatant liquor, add to the residue 

 a solution of ammonium carbonate, the crystals of calcium 

 sulphate will be dissolved and highly refractive rhombs and 

 grains of calcium carbonate will appear; these are easily found 

 by examining the preparation between crossed nicols. A high 

 power is generally required. 



A serious interference is that of the chlorides of the trivalent 

 metals. In the presence of these salts in large amounts it is 

 generally advisable to proceed thus: Add to the somewhat 

 dilute solution, ammonium acetate, heat to boiling, but avoid 

 long or violent ebullition, since in the latter case the precipitate 

 formed often refuses to settle. The clear liquid is then sepa- 

 rated from the precipitate (by drawing-off on the slide, filtration, 

 or by means of the centrifuge), concentrated if necessary, and 

 tested for calcium with sulphuric acid. 



Behrens states that calcium cannot satisfactorily be detected 

 in the presence of borates; this appears to be .true when only a 

 minute quantity of calcium is present with a high percentage 

 of boron and other salts; in such an event test by Method B: 



Strong mineral acids, in excess, so increase the solubility of 

 calcium sulphate as to require evaporation almost to complete 

 dryness before the crystals of this salt appear. The addition 

 of a fragment or two of sodium acetate or of ammonium acetate 

 is always necessary in such cases before the sulphuric acid drop 

 is allowed to flow in. This method of mitigating the action of 

 the free acids, also reduces the delicacy of the reaction because 

 of the formation of more soluble double sulphates of calcium and 

 sodium or ammonium. Hence the addition of an excess of a solu- 

 ble sulphate instead of sulphuric acid is not to be recommended. 



EXPERIMENTS. 



a. Try reaction, in the manner given above, on salts of calcium in neutral solution. 



b. Try the effect of precipitating in the presence of free HC1; then in the presence 

 of free HN0 3 . 



